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MONITORING STRUCTURAL INTEGRITY

BY ACOUSTIC EMISSION

A symposium
presented at
Ft. Lauderdale, Fla., 17-18 Jan. 1974
AMERICAN SOCIETY FOR
TESTING AND MATERIALS

ASTM SPECIAL TECHNICAL PUBLICATION 571

J. C. Spanner, editor
J. W. McEIroy, co-editor

List price $23.75

04-571000-22

AMERICAN SOCIETY FOR TESTING AND MATERIALS

1916 Race Street, Philadelphia, Pa. 19103
9 by AMERICAN SOCIETY FOR TESTING AND MATERIALS 1975
Library of Congress Catalog Card Number: 74-28978

NOTE
The Society is not responsible, as a body,
for the statements and opinions
advanced in this publication.

Printed in Baltimore, Md.

March 1975
Foreword
The symposium on Monitoring Structural Integrity by Acoustic Emis-
sion was presented in Ft. Lauderdale, Fla., 17-18 Jan. 1974. The
symposium was sponsored by Committee E-7 on Nondestructuve Testing,
American Society for Testing and Materials. J. C. Spanner, Westinghouse
Hanford Co., presided as symposium chairman. J. W. McElroy,
Philadelphia Electric Co., presided as symposium co-chairman.
Related
ASTM Publications
Acoustic Emission, STP 505 (1972), $22.50, (04-505000-22)
Contents
Introduction 1
Why Acoustic Emission--Why Not?--B. H. SchofieM 3
Acoustic Emission in the Frequency Domain--L. J. Graham and G. A. Alers l1
Acoustic Emission During Phase Transformation in Steel----G. R. Speich and
A. J. Schwoeble 40
Development of Acoustic Emission Testing for the Inspection of Gas
Distribution Pipetines--J. W. McElroy 59
Evaluating the Stability of Geologic Structures Using Acoustic
Emission--H. R. Hardy, Jr. 80
Acceptance Testing Welded Ammunition Belt Links Using Acoustic
Emission--P. H. Hutton 107
Industrial Use of Acoustic Emission for Nondestructive Testing--T. F.
Drouillard, R. G. Liptai, and C. A. Tatro 122
Industrial Application of Acoustic Emission Analysis Technology--D. L.
Parry 150
Establishing Structural Integrity Using Acoustic Emission--(7. F. Morais
and A. T. Green 184
Acoustic Monitoring Systems to Assure Integrity of Nuclear Plants--Raj
Gopal 200
Detection and Location of Flaw Growth in Metallic and Composite
Structures--M. P. Kelly, D. O. Harris, and A. A. Pollock 221
Acoustic Emission~A Bibliography for 1970-1972--T. F. Drouillard 241
General Index 285
STP571-EB/Mar. 1975

Introduction

A wide variety of nondestructive testing methods and procedures are

utilized during the fabrication of structures when the consequences of
failure are costly, constitute a hazard to the public, or both. In addition,
a final proof test (pressure test) is applied to most pressure vessels and
many pressurized systems. The consequences of catastrophic failure dur-
ing proof testing are often such that almost any method for reducing the
probability of failure is economically justified. Present acoustic emission
technology offers this capability and, in addition, provides a viable method
for evaluating the basic integrity of many other types of engineering struc-
tures. Numerous successful applications of acoustic emission during proof
testing of aerospace tanks, pressure vessels, and piping systems have been
reported in the literature of the past 15 years.
Acoustic emission is the transient elastic energy that is spontaneously
released when materials undergo deformation, fracture, or both. Efforts
toward utilizing this phenomenon in materials research studies, and for
nondestructive testing, have increased substantially in recent years.
Materials investigated have included both metals and nonmetals, although
most of the work published to date has been concerned with metallic
specimens or structures. Analogous studies have been conducted on geo-
logic materials (rocks, etc.), where the terms "microseismic activity" or
"rock noise" are often used in lieu of the term "acoustic emission."
The continued increase in the number of reported applications of
acoustic emission to monitor structural integrity influenced ASTM to
authorize this special technical publication to publish the papers presented
during an ASTM Symposium on Monitoring Structural Integrity by
Acoustic Emission. This symposium was held in Fort Lauderdale, Florida,
in January 1974, under the sponsorship of the ASTM E-7 Committee on
Nondestructive Testing, and was a sequal to an introductory ASTM
Symposium on Acoustic Emission which was held in December 1971. That
symposium was documented in ASTM STP 505.
The purpose of the 1974 symposium, and of this STP, is to present a
collection of papers selected to provide a representative coverage of
recent activities in applying acoustic emission to monitor the integrity of
engineering structures. It is significant that many of the speakers at this

Copyright91975 by ASTM International www.astm.org

2 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

symposium are among the leading U.S. experts in this new and rapidly
expanding area of technology.
The first few papers provide background information on the acoustic
emission method and its applications, and discuss specific characteristics
of the signals that are emitted by structural materials. The next series of
papers describe the techniques that were used, and the results that were
obtained, when commercial and developmental acoustic emission instru-
mentation systems were employed to monitor the integrity of a wide variety
of engineering structures and components. The last paper is a bibliography
containing 412 references on acoustic emission that were published during
the years 1970-1972.
This publication is intended to provide a permanent record on the
technological status of Monitoring Structural Integrity by Acoustic
Emission as it existed in early 1974. It is expected to be of value to
those who are actively engaged in this field, as well as to those with
structural integrity monitoring applications requiring the unique capabil-
ities offered by this relatively new nondestructive testing method.

J. C. Spanner
Manager, Nondestructive Testing Engineering,
Westinghouse Hanford Co., Richland, Wash.;
symposium chairman.

J. W. McElroy
Research engineer, Research Division
Philadelphia Electric Co., Philadelphia, Pa.;
symposium co-chairman.
B. H. Schofield 1

Why Acoustic Emission Why Not?

REFERENCE: Schofield, B. H., "Why Acoustic Emission-Why Not?,

Monitoring Structural Integrity by Acoustic Emission, ASTM STP 571, Ameri-
can Society for Testing and Materials, 1975, pp. 3-10.

ABSTRACT: The relative apathy of the industrial community to take advantage

of the significant benefits of acoustic emission is discussed against the
background of the current state of the technology. Examples of immediate
applications are noted. It is suggested that developing trends necessitate timely
initiation of industrial utilization and that such efforts and the experience gained
therein are a prerequisite to the realization of the technical benefits of acoustic
emission and the establishment of proper and adequate guidelines.

KEY WORDS: acoustics, emission, pressure vessels, defects, hydrostatic tests

The purpose of this paper and, undoubtedly the material presented in

many of the papers of this symposium will fortify this purpose, is to
encourage and promote more widespread practical utilization of the acous-
tic emission (AE) technology, at least in those specific areas where the
acoustic method has been shown to be effective and of technical and
economic value.
Background
Following the first comprehensive and continuing research studies in the
early ~950's, a number of proposals for the practical commercial and
industrial utilization of AE emerged. These applications related principally
to the determination of the integrity of pressure vessels under hydrotest. At
this early stage there was little commercial or industrial motivation to apply
the technique as it was almost entirely an art, known by a few, and what
instrumentation there was available appeared to be the typically disor-
ganized conglomerate of the eccentric researcher. However, it was not
long before equipment and systems were being produced and made gener-
ally available specifically for AE studies, and by the middle 1960's both
1 Manager, Consulting Services, Teledyne Materials Research, Waltham, Mass. 02154.

Copyright 91975 by ASTM International www.astm.org

4 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

technique and instrumentation had been developed to a reasonable state of

the art. A number of practical nondestructive testing (NDT) applications
had been successfully demonstrated, while versatile and sophisticated
instrumentation components and systems were developed contemporane-
ously to put the technique into practical use. Nevertheless, as we now
approach the mid 1970's it can be undeniably stated that here in this country
we find that relatively little industrial and commercial advantage has been
taken in benefiting from this new technology; the generic question seems to
be--Why acoustic emission? In the following, the author does not pretend
to fully answer this question but hopes to present a sufficient premise to
propose the question, Why not? in appropriate applications. Although
numerous cases could be cited showing the current applicability of AE as a
research tool, the emphasis of this discussion is confined to industrial
utilization.
Current Case for Acoustic Emission
The basis for the first question can be found quite readily, albeit, it
abounds in a mixture of cynicism and questionable technical logic, if not a
lack of common sense. Probably more important, however, is that the
prevalence of the question evidences the disappearance, to a large degree,
of technical entrepreneurship to explore innovation, but it also reflects
both the sophisticated complexities and subtleties involved in the
technical-business decision processes within large firms and industries.
For example, several years ago the author undertook a survey of a
particular large industry to determine the nature and magnitude of the
market thatcould and would utilize AE at its then present state of the art.
The specific acoustic application was the determination of the structural
integrity of large, heavy wall, expensive pressure vessels. The technique
involved the nondestructive testing in the manufacturer's shop prior to
installation. Results expected from these tests would be the detection of
structural defects and their propagation, if any, induced by pressure load-
ing; the accurate determination of the physical location of these defects
anywhere in the vessel; and a very high probability of precluding cata-
strophic failure of the vessel during the hydrostatic test.
The survey respondents showed a unanimous and authentic interest in
the AE method and acknowledged the existence of many applications
where the technique would not only be helpful but also where such informa-
tion was urgently needed, and no other tools were available to meet their
unique requirements. Nevertheless, coupled with this technical interest
and need was an overriding concern and indulgence with the limitations of
the technology and the possibility of some uncertainties or ambiguities in
the data. The source of these concerns was less related to any technical
SCHOFIELD ON WHY ACOUSTIC EMISSION 5

shortcomings than to the structured problems of decision making within the

respective firms--between the technical and the administrative executive
staff levels. If there are uncertainties and ambiguities in the data, how
would these be explained and resolved to the satisfaction of upper man-
agement?
Many of the modern, high-pressure, high-temperature vessels are con-
structed from new materials using new techniques and methods of fabrica-
tion, and the structures themselves are of increasing geometric complexity.
The engineering staff of the firm procuring the vessel is thereby faced with
many difficulties, some of uncertain or ambiguous technical basis requiring
decisions in design, manufacture, and operation. The addition of a rela-
tively unknown technology with its attendant problems and educational
burdens was not relished by the survey respondents, and it was a general
concensus of the respondents that they would be better off without the in-
formation obtained by the acoustic method. Technical questions, for
which difficult or uncertain decisions would have to be formulated to the
satisfaction of upper management, would be thereby eliminated. One may
be sympathetic to the sensitivities expressed, but it cannot be denied that
such a "head in the sand" philosophy is not technically acceptable and
could be financially disastrous in time. Further, this approach would not
appear to be justifiable on the basis of current AE technology. In the
following discussion the author expects to show, by selected examples,
that the advantages of AE outweigh still existing shortcomings and,
furthermore, that elimination of the latter can only be accomplished
through the practical experience gained in utilizing the technique on real,
full size, industrial "specimens."
As an example of the remarkable potential of the AE techniques, as
specifically related to pressure vessels, the results of a relatively recent test
program are noteworthy.
The vessel under test was about 16 in. in diameter, 4 ft long, and had a
wall thickness of 0.5 in. in the cylindrical section. The material was ASTM
A516 having a yield strength of about 70 000 psi. The prime purpose of the
tests was to study the influence of yield strength on the vessel failure. AE
tests were appended for whatever information could be gleaned and were
undertaken at the expense of the AE investigator. On-line computer
facilities were not utilized in these emission tests; hence, the data were
analyzed subsequent to the actual pressure testing rather than in real-time.
Figure 1 is a pictorial representation of the acoustic data prior to general
yielding of the vessel. Each data point represents a located source of
acoustic emission. Only a fraction of the total number of sources obtained
are presented in the figure; nevertheless, the overall pattern remains un-
changed. It is evident from the pattern that a line of clustered emission
6 MONITORINGSTRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

weld weld
seam seam
l ;

4X, 2X 48 in
e,% 9

i9
|e
le
I..~. 3X
I
I
J.

L approximate
location of
rupture

48.7 in Circumference
(15.5 in Ola.)

sensor locations
9 m i s s i o n sites

FIG. I----Emission sources located during hydrotest of vessel.

sources developed during the pressurization. The dense line of dots actu-
ally outlines, approximately, an artificial defect that had been machined
into the vessel prior to test and was the precise location where final fracture
eventually occurred. However, more interesting than the final appearance
of the emission pattern is the "pressure-time-emission location" pattern as
it developed. Emission sources began to appear in the center region of the
length of the defect and continued in activity as new emission sources
SCHOFIELD ON WHY ACOUSTIC EMISSION 7

progressed outwardly along the length of the defect line. Such a pictorial
display could be obtained by a computerized real-time display of the
emission sources and would show the early initiation and the dynamic
growth of the deformation pattern associated with the defect. In the case of
a defect which is acoustically active along its entire length, as in this case,
the emission pattern has the capability to show the geometric shape of the
defect and with sufficient discrimination could, in conjunction with addi-
tional emission parameters, provide growth rate data for a propagating
defect.
This is not to say that the specific type of defect, or other geometric
details, would be discernible, nor would the degree of severity necessarily
be assessable in terms of the ultimate response of the vessel. The previ-
ously discussed emission pattern could be, for example, produced by a
region of local plastic deformation, by a stringer of macroscopic metallur-
gical defects, or by a crack. To date no definitive and consistent charac-
teristics of the emission signal has been found that distinguishes defects in
such detail. Complementary methods such as ultrasonics,, radiography,
etc. would be essential to define the type of defect and its exact geometric
dimensions and metallurgical details. This is especially obvious for the case
where a relatively long crack may be acoustically active in only a small
region of the total defect, such as at the tip of the crack. The acoustically
inactive portions of the crack, of course, do not provide any information
concerning the total extent of the defect, and other complementary
techniques would be called for in such an instance.
There is, however, one notable emission characteristic indicative of
defect severity, in that the emission data do signal the onset of impending
failure. This is the distinct change in the emission rate, invariably accom-
panied by an increase in signal energy, from a relatively uniform, linear
count rate to an exponentially rising count rate at a given and identified
location in the vessel. An obvious requirement for observation of the
impending failure characteristic is, of course, the detection and observa-
tion of the emission rate prior to the onset of the unstable propagation of the
defect. A test of a vessel which at the outset contains a defect of critical
dimensions would be most difficult to evaluate but, with the proper
preparatory studies of the vessel material, could provide the quantitative
emission criteria, which coupled with the versatility of the high-speed
computer, could in turn provide the capability of detecting impending
failure even under such extreme conditions. The laboratory studies would,
~ofnecessity, have to be rather extensive and undoubtedly expensive. Not
only would it be necessary to establish emission reference data for the basic
material but also for various welding configurations as well as fracture
mechanics-emission experiments.
8 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

Nevertheless, the efficacy of the AE method is preeminently apparent.

What other method can identify the presence and location of an active
"defect" so quickly and efficiently. Consider for a moment the surface area
of a vessel represented by a diameter of 25 ft, and a length of 60 ft, spherical
heads, and several complex nozzle and other configurations. A full vol-
umetric NDT inspection of the vessel material is normally a formidable task
involving considerable expense. Consequently, in the usual case it is
necessary to exercise engineering judgment as to the extent of economi-
cally and technicaUy justifiable inspection, with some calculated probabil-
ity of risk that significant defects would remain undetected. The emission
method immediately shows economical and technical benefits in vastly
reducing, if not eliminating, this factor of ignorance since in essence the
total vessel is subject to surveillance. Without doubt one can approach the
evaluation of the integrity of a vessel much more confidently with the added
knowledge provided by the acoustic method.
The second subject I would like to pursue, but briefly, concerns the
controversy regarding the propriety of the hydrotestper se, and the practi-
cal solution offered by the use of AE. There is a school of thought that the
hydrotest itself may produce damage or worsen an already existing defect
condition and that the extent or probability of such damage or degradation
of the vessel integrity will be unknown and not subject to analytical deter-
mination. The acoustic method offers a unique means of detecting whether
or not such additional damage is induced by any given hydrotest. With the
exception of a vessel or structure that is undergoing general and gross
yielding, experience has shown that the propagation of a defect in the
vessel will produce detectable AE. The location of the source or sources
can be determined, and, if deemed necessary, close examination by com-
plementary techniques can be made. Clearly, if the vessel exhibits no
emission, or only a minor amount widely distributed over the vessel sur-
face, one can be confident that the hydrotest has not affected the vessel
adversely. For those cases wherein the hydrotest does produce structural
damage, the emission data will provide the information to locate the area of
such damage, as well as to provide a qualitative assessment of the extent of
the defect activity. For those circumstances where hydrotests are to be
conducted periodically after the vessel is in service, the advantages of
having AE data from the manufacturer's shop hydrotest and the initial
hydrotest following installation cannot be overstated. These two tests will
provide an invaluable reference for all subsequent emission surveys rela-
tive to defect areas and their significance to vessel integrity.
Certainly a philosophy not to conduct a hydrotest to assess existing
integrity for fear of producing additional damage in the vessel is merely
exchanging one form of ignorance for another. The availability of the
SCHOFIELD ON WHY ACOUSTIC EMISSION 9

acoustic method substantially mitigates such concerns, and the balance of

risks between conducting a hydrotest or eliminating the hydrotest is shifted
in favor of the test monitored by AE.
There are, of course, numerous additional considerations and examples
which could be cited in favor of industrial utilization of AE, and the papers
of this symposium are excellent examples. It should also be noted that the
American Society of Mechanical Engineers' (ASME) Code, to a limited
extent, has recognized the NDT potential of AE (Section XI), and, of
course, ASTM holding a second symposium in as many years and the
forming of the Subcommittee E07.04 has shown the presence of wide-
spread interest. It should be recognized by those in the pressure vessel
industry that development of codes and standards is inevitable and that
they will play an influential role within this industry. It is imperative that
personnel within the industry play a part in these developments and that
they contribute from a background of knowledge and experience with the
subject matter. It is through the knowledge gained from actual experience
with AE that appropriate and relevant codes and standards will evolve.
It is certainly not inappropriate in this day and age to also mention that
the public demands for increased safety of large pressure vessels, particu-
larly those in the nuclear industry, will also play an influential role, and the
impact of the public interest on industry will depend, to a large extent, on
industries' own initiatives. Nothing would be more detrimental to all than
the premature demand to foster an undeveloped technology on an inexperi-
enced, unprepared industry.
Lastly, I wish to note, not only the escalating interest in the emission
technique in foreign countries but also their rather zealous production and
placement of AE systems into diverse industrial applications. Without
doubt the experience being currently accumulated by these systems will
place these countries in a foremost position in this technology. What was
once essentially a U. S. monopoly in practical AE is rapidly disappearing,
and, considering the fact that this country has many of its large, complex,
and expensive vessels built in foreign plants, there should be keen interest
in advancing our own knowledge of the technologies that may well be in
common use in these countries in the near future.

Conclusion
Over the past 15 years there has been an evergrowing and accelerat-
ing need for not only improvement and advancement in our exist-
ing stock of NDT tools but also an urgent requirement for new methods
answerable to the increased complexities and demands of modern struc-
tures and standards. With the possible exception of the yet undeveloped
10 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

field of holography, AE is the only new tool that has appeared on the
technical scene and which holds a promise of meeting those modern needs.
Such promise has not been met with an enthusiasm in the utilization of
the technique. There appears to be a philosophy to await the ultimate and
full technical development of the method whereby ambiguities and uncer-
tainties will be eliminated; hence, until then, why use AE? Such an ap-
proach ignores the current state of the art for certain and particular applica-
tions and the present advantages that can be realized while imposing a more
demanding standard.
It is to be recognized that it is through the experience of utilization that
the full potential of AE will be developed. In the area of pressure vessel
integrity the method has no peer in terms of ultimate potential. Currently
AE offers significant practical benefits; hence, why not take advantage of
these and by so doing also produce the additional benefits which will
naturally develop as a consequence of intimate involvement with the
technique.
Undoubtedly, AE will find increased utilization and broader applica-
tion, accompanied by the development of codes and standards within these
uses. The rate of progress and the wisdom of the guidelines will depend, to a
large extent, on the participants in this development.
L. J. G r a h a m 1 a n d G. A . A l e r s ~

Acoustic Emission in the Frequency Domain

REFERENCES: Graham, L. J. and Alers, G. A., "Acoustic Emission in the

Frequency Domain," Monitoring Structural Integrity by Acoustic Emission,
ASTMSTP571, American Society for Testing and Materials, 1975, pp. 11-39.
ABSTRACT: A means for quickly and easily determining the broadband fre-
quency content of acoustic bursts as short as 20 /xs in duration has been
developed using a video tape recorder and a standard spectrum analyzer. It is
shown by examples from several tests on laboratory specimens and on large
structures that the frequency content of an acoustic burst is related to the
mechanism which produced it and is not affected substantially by the specimen
size or by mode conversion due to multiple reflections in the structure. The
frequency content of the burst can be changed in two ways, however: by the
frequency-dependent attenuation of the propagation medium and in the cases
where the medium is dispersive. Results of measurements on the effect of these
factors in a variety of structures are given. Although acoustic emissions from
many materials tend to be "white noise," several examples of acoustic emis-
sions and extraneous background noise bursts having distinctive frequency
spectra are given which suggest possibilities for discriminating true acoustic
emission signals from background noise on the basis of frequency content alone.

KEY WORDS: acoustics, emission, spectrum analysis, tape recorders, plastic

deformation, crack propogation, ultrasonic frequencies, transmission loss,
wave dispersion, Lamb waves

There is a twofold impetus for determining the frequency content of

individual acoustic emission (AE) bursts. The first one is for possible
identification of source mechanisms and for insight into the physical
parameters associated with their operation. These mechanisms include
dislocation motion, crack propagation, phase transformations, and twin-
ning [1-8]. 2 The second one is for identifying differences between the AE
generated by any of the effects just mentioned and those produced by other
extraneous noise sources [9-11]. This information is essential in some AE
triangulation applications where extraneous noises from the test environ-
Senior staffassociate and group leader, respectively, Science Center, Rockwell Interna-
tional, Thousand Oaks, Calif. 91360.
2 The italic numbers in brackets refer to the list of references appended to this paper.

11
Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 23:22:35 EST 2016
Copyright9 1975byby ASTM International
Downloaded/printed www.astm.org
University of Washington (University of Washington) pursuant to License Agreement. No further reproductions authorized.
12 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

ment are so numerous as to saturate the information handling capabilities of

the computer used to analyze the incoming signals. If the frequency content
of the signals and the noises are both known, filtering or other electronic
means can help to reduce the amount of irrelevant information early in the
data processing. Of these two general areas of study, the latter is of most
immediate technological interest. This paper will present results of some
studies in this area concerned with identifying differences in the frequency
spectra of noise from different sources.
Most mechanically produced resonant vibrations of structures and
laboratory test specimens occur in the low kilohertz frequency range. Early
studies of the frequency content of AE were limited to just this range
because of limitations in the techniques available at the time to frequency-
analyze transient acoustic bursts. Due to this limitation, the results ob-
tained were highly dependent upon the geometry of the specimen [12-14].
This tendency for the lower frequency modes of structures to be excited
mechanically can be advantageous, however. Mechanical signature
analysis of structures and machinery is an important technological area,
and real-time frequency analysis eqiupment covering the frequency range
to 50 kHz is available for this purpose [15]. Also, this low-frequency range
has been successfully utilized in AE studies of the fracture of fiber com-
posites where the resonant frequencies of the fibers excited during fracture
can be identified [16]. A third advantage, one that is more pertinent to the
present discussion, is that AE tends to be very broad banded in frequency
content while many mechanical components such as solenoids, gears,
cams, and bearings excite only the low-frequency components. This can
allow the discrimination of one against the other by simple electronic
means.
There are also other types of mechanically and hydraulically produced
noises which can have frequency components extending up into the low
megahertz frequency range, and examples of these will be given later.
These can be particularly troublesome in AE testing because they have
frequency components covering the same general frequency range as the
flaw-generated bursts. It is, therefore, desirable to be able to frequency
analyze these acoustic bursts in detail over a broad frequency range in
order to find characteristic features of their spectra which can be used to
distinguish them from AE. It is only recently that instrumentation has been
developed for the broadband frequency analysis of short duration, tran-
sient acoustic bursts. The three most promising methods are: (I) digital
conversion with computer analysis [7,9], auto-correlation techniques [2],
and (3) record and playback with a helical scan video tape recorder [3,17].
Of these methods, the latter has the advantage of being able to record every
AE event for later analysis at the discretion and convenience of the inves-
tigator.

Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 23:22:35 EST 2016
Downloaded/printed by
University of Washington (University of Washington) pursuant to License Agreement. No further reproductions authorized
GRAHAM AND ALERS ON THE FREQUENCY DOMAIN 13

The present studies extendover a two-year period during which a video

tape recorder and commercial frequency analyzer were used to determine
the frequency spectra of acoustic bursts from many different test situa-
tions. Means for determining the broadband response of the acoustic
transducers and for evaluation of associated broadband electronics were
also developed. A summary of the important results of this study which
previously have been points of conjecture are:
1. The frequency content of an AE burst is not substantially altered by
mode conversion during reflections at the boundaries of a solid structure.
2. The observed frequency spectrum of an AE depends both on the
frequency dependence of the acoustic attenuation and on the dispersive
character of the transmission medium between the source and the trans-
ducer.
3. Although AE in many materials tends to be nearly "white noise" at
least up to 2 MHz, several cases have been observed where there is a strong
structure in the frequency spectra.
4. Extraneous noise bursts can often be distinguished from flaw-
generated emissions by differences in their frequency spectra.

Experimental Method
A Sony video tape recorder intended for home use was modified for use
as an analog signal recorder [3]. The principal modifications made were to
eliminate extraneous synchronization signals required in the TV recording
format, to provide for internal synchronization, and to realign the FM
amplifier to increase its bandwidth and dynamic range. This instrument
was then used in conjunction with a broadband transducer, amplifier, and
spectrum analyzer system as shown in Fig. 1. A key feature of this recorder
is its "stop-action" capability which allows a repetitive playback of any
16.7 ms time interval of the recorded signal for steady viewing on an
oscilloscope or for presenting to a standard frequency analyzer (such as the
Hewlett-Packard Model 8552A/8553B). With synchronized electronic gat-
ing of this repetitive signal, any portion of the recorded signal as short as 20
/.~s in duration can be analyzed independently for its frequency content.
The frequency response of the recording and analyzing system should be
considered in two parts--the electrical and the acoustical. The component
limiting the electical response is the tape recorder which is down 3 dB at 3
kHz on the low end and 2.5 MHz on the high end, but with a useable
frequency range to 3 MHz. All other electronic components are considera-
bly more broadbanded. The acoustic response of the system is governed by
the transducer and its acoustic coupling to the specimen, so considerable
effort has gone into determining this characteristic. To do this, an acoustic
"white noise generator" (WNG) similar to that described by Chambers [1]
was built and is shown schematically in Fig. 2. It consists of a steel plate

MODIFIEDVIDEO z
TAPERECORDER o

r
0-3MHz BANDWIDTH
40dB DYNAMICRANGE
STOPACTIONONPLAYBACK

AW/DEBFAIER I O / ~
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FIG. l--Broadband system for recording and analyzing acoustic emissions.

~~ F SiCPOWDER

( ~ ~ . - ~ j ~ I~/~RGREASE
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FIG. 2--Acoustic "white noise" source f o r determining transducer response and f o r
acoustic attenuation measurements.

having a depression on one face in which fine particles of silicon carbide are
fractured continuously under the rotating action of a fused silica rod. A
specially built high-fidelity capacitor microphone mounted directly on the
steel plate was used to determine the acoustic output of this noise
generator. Its output voltage, shown in Fig. 3, exhibits a fairly smooth 1/f2
dependence upon being excited by the WNG. Since the voltage output of a
capacitor microphone is proportional to displacement amplitude, this de-
pendence is as would be expected for an acoustic source with a periodic
driving force of constant amplitude at all frequencies [18]. The response of
a typical piezoelectric transducer, also shown in Fig. 3, does not fall offas
i i i i I ' i i i

A
PIEZOELECTRIC

i 0
| | i a

1
FREQUENCY
|

(MHz)
I I I !

2
FIG. 3--Response o f two types o f transducers to a "'white noise" source.

Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 23:22:35 EST 2016
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16 M O N I T O R I N G STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

rapidly with increasing frequency since its response is more nearly propor-
tional to particle velocity in the acoustic disturbance than to displacement.
The particle velocity due to acoustic white noise as just defined has a 1/f
dependence. However, the internal mechanical resonances of the
piezoelectric element produce a very irregular response curve.
Examples of the responses of several piezoelectric transducers to the
WNG are shown in Fig. 4. In Fig. 4a, three laboratory-assembled transduc-
ers are compared which were made from about 3-mm (1/8-in.) diameter,
longitudinally poled PZT-5A of three different thicknesses. The choice of
one of these transducers for a particular application is determined by its
having the maximum sensitivity in the frequency range of interest. For
broadband testing the 1.1 MHz transducer has the overall greatest sensitiv-
ity. In Fig. 4b are shown the responses of this transducer and of two com-
mercial transducers (from Dunegan-Endevco) to allow a comparison be-
tween our method of determining transducer response and other methods
which are in common usage. In obtaining these response curves, it was
found that no special care needed to be taken in bonding the transducers
to the white noise generator beyond using normal ultrasonic coupling
techniques. Viscous oil or thin solid bonds couple the higher frequencies
slightly better than less viscous liquids such as glycerine or water, for
example.
A question might be raised concerning the suitability of using a continu-
ous white noise source for determining the response characteristics of
transducers intended for the detection of short-duration AE bursts. How-
ever, experience has shown that AE from many sources produce the same

I I I I I i i i l ~: , , i , I l i i , ' -

1.1 MHz PZT - 5A

.. 9...... 0.4 MHz P Z T - 5A ~
.> L ~,,~IL 1.1 MHZ P Z T - 5 A
...... coM,'
----,.,M.z,,z'r-oA-
"o : ~ 9
po .
v
ill

........

r
:E
9. ".."" "..............-..

I t I I I ! I I I I , , , , I , , , ,

1 0 1 2

FREQUENCY (MHz) FREQUENCY (MHz)

FIG. 4--Response o f several piezoelectric transducers to a "white noise" source.

transducer response as does the WNG. Two examples are shown in Fig. 5,
and others have been observed such as the deformation of 7075 aluminum
and 9Ni-4Co steel, and the slow crack growth in several polycrystalline
ceramic materials [5]. In Fig. 5a the AE was detected with a capacitor
microphone transducer, and in Fig 5b the AE was detected with a piezo-
electric transducer. In these figures, as in many of the figures in the
following sections, there are three curves. The solid curve is the transducer
response to the AE burst being analyzed. Its amplitude scale is shown rela-
tive to the dashed line which is the electronic noise level at the preamplifier
input (2/z V peak at 1 MHz). The dotted line is the response of the partic-
ular transducer used to the acoustic output of the WNG. It is shown super-
imposed on the AE frequency spectrum for comparison purpose, although
its amplitude may be an order of magnitude greater. In some cases it was
necessary to obtain the recorded AE data using a high-pass filter to keep
from saturating the amplifiers with the high-amplitude, low-frequency
components of the acoustic bursts. The frequency spectrum of Fig. 5a was
obtained using a 100-kHz to 3-MHz bandpass filter. Its effect is seen in the
droop in the low-frequency end of both the acoustic emission spectrum and
in the electronic noise spectrum. Similar effects will be seen in some of the
spectra presented later.
The similarity between the frequency spectra of the AE and the WNG in
Fig. 5 lends support to the use of the continuous white noise source as a
practical means of determining AE transducer response. It has also been

'i~ ! I I I I I I I !

"J :..~ (b)

I[
s ~..-
<

0
, , , , I

1
b....t
t'
.... 9 ..... "r-.... I

2
FREQUENCY (MHz)
(a) Plastic deformation of single crystal MgO using a capacitor microphone transducer.
(b) Plastic deformation of Ti-6AI-4V using a piezoelectric transducer.
FIG. 5---Examples of"white noise" acoustic emission bursts.

useful in determining the frequency-dependent acoustic attenuation in

structures by systematically changing the separation between the WNG
and the pickup transducer on the structure. Typical results of this type of
measurement for various structures are given later.
Spectral Analysis
In the previous section, emphasis was placed on the similarity between
the frequency spectra of AE from several materials and the frequency
spectrum of the continuous white noise provided by the WNG. We have
also observed several examples in which the frequency spectra of AE and
of background noises are not white noise and which in some cases are very
distinctive. These will be presented in the following paragraphs in order to
support the contentions made in the introduction and to illustrate the
usefulness of frequency analysis to AE technology.

Tests on Low-Alloy Steel

Acoustic emissions produced during tension tests of A533-B steel speci-
mens are shown in Fig. 6. Repeating the identification of the three curves
on each figure, the solid curve is the frequency spectrum of the AE, the
dashed curve is the electronic noise level of the preamplifier, and the dotted
curve is the transducer response to acoustic white noise. The frequency
spectra of all the AE produced throughout the first 95 percent of the test
were like the one in Fig. 6a. As necking of the tension specimen proceeded
there was a tendency for the spectra of many of the AE to take on more of a
white noise character as in Fig. 6b. Also, bursts started to appear which
had the frequency spectrum shown in Fig. 6c. This type of burst oc-
curred more frequently as neck formation progressed and the specimen
f'maUy fractured. The oscilloscope trace of Fig. 6e shows two AE bursts
which occurred within a few milliseconds of each other near the end of
the test and which had the quite different spectra shown in Fig. 6b and
c, respectively. This observation of two adjacent bursts with different
frequency content supports the contention that the spectral content at
high frequencies is not dominated by specimen resonances.
A significant observation regarding these bursts is that frequency
analysis of each 20/xs time increment within the ring-down time of the AE
results in the same frequency spectrum. This indicates that the burst of
elastic strain energy forming the AE does not change its frequency content
upon multiple internal reflections in the specimen. Subsequent studies on
various specimens showed that the spectra did not depend upon the speci-
men geometry except for details in the spectra at lower frequencies caused
by specimen resonances. These studies also identified the source
mechanisms of the low-frequency AE of Fig. 6c as crack extension and of
the high frequency AE of Fig. 6a as plastic deformation [8].

FIG. 6----Examples of distinctive acoustic emissions generated in A533-B low-alloy pressure

vessel steel.

The AE shown in the oscilloscope trace of Fig. 6f were recorded during

fatigue crack growth in the 15-cm-thick wall of a nuclear reactor pressure
vessel made from a low-alloy steel similar in composition and mechanical
properties to A533-B [19]. Their frequency spectrum is shown in Fig.
6d. Two points should be observed. First, the time duration of the AE
bursts in the thick-walled pressure vessel was only about 10 to 20/xs. This is
very short compared to laboratory test results on small specimens, pre-
sumably because in this structure there is no opportunity for alternate
acoustic paths to produce an apparent lengthening of the burst. Therefore,
these signals are probably related more closely to the time during which the
energy is actually released at the source than is typically seen in the
laboratory. Because of the large size of the structure, its resonances do not
appear on this time scale. The second point is that the frequency spectrum
of these AE's is very similar to the spectrum identified with plastic defor-
mation in A533-B steel in Fig. 6a in that the predominant energy content of
the AE is at higher frequencies. We have, therefore, identified tentatively
their source as the plastic deformation at the crack tip accompanying
fatigue crack growth, although laboratory tests on specimens of the reactor
material are needed to confirm this.

Fatigue Test of 2219-T87 Aluminum

The AE's in Fig. 7 were recorded during fatigue crack growth in a 1.2-m
by 1.2-m by 0.63-cm plate of 2219-T87 aluminum containing a machined-in
part-through crack near its center [11 ]. A five-channel electronic lock-out
system was used in monitoring the AE so that the source of the emissions
recorded could be positively identified as being in a small region of the
specimen around the crack. The Types 1 and 2 bursts were generated at the
location of the growing fatigue crack, with the Type 1 AE occurring more
frequently. With the transducer located 3 cm from the crack, the duration
of the AE ringdown was fairly short but increased as the transducer was
moved to a distance of 50 cm from the crack, as can be seen in the upper
right-hand picture of Fig. 7. The frequency spectrum of the AE remained
the same, however.
The Type 3 bursts occurred continuously over about one quarter of each
20 s duration fatigue cycle near the maximum load and originated in the
hydraulic load cylinder of the test machine. These are of unusually high-
frequency content and of very short duration even though the acoustic path
from their source to the transducer was several meters in length through a
steel bar, plate, clevis pin joint, and compression lap joint. Amidst these
Type 3 bursts there would occasionally be a burst which had a frequency
spectrum like that of the Type 2 burst. Their source could not be positively
identified since the lock-out module was turned off during these recording
periods. They are believed to be due to the fatigue crack growth, however.

Fatigue Test of 2024-T851 Aluminum

Figure 8 contains results of another fatigue test on a 0.5-cm-diameter by
10-cm-long bar of2024-T851 aluminum at 1 to 8 Hz. The majority of the AE
has the spectrum shown in Fig. 8a and appeared as in Fig. 8e. A 100-kHz
high-pass fdter was used while recording these emissions. Occasionally, a
burst having five times the ring-down time would occur. These had the
spectrum of Fig. 8b which nearly approaches the white noise spectrum
except for some lack in energy at low frequency. A 100-t~s duration burst
occurred repetitively on every cycle near zero load which had the spectrum
shown in Fig. 8c. This is assumed to be a mechanically produced burst
because of its short duration. The dominant acoustic noise which occurred
during the test was the hydraulic noise near maximum load. This had the
spectrum shown in Fig. 8d and appeared as in Fig. 8f. These results are
rather atypical and are presented for that reason. The noises produced by
other hydraulic machines more typically have strong spectral components
up to 300 to 500 kHz. Also, most mechanically produced acoustic bursts, at
least due to impact, produce bursts with spectral components only below
200 kHz.

0
Z

0e"

-r

Z
FIG. 7----Acoustic bursts observed during f a t i g u e o f 2219 aluminum. The Types 1 a n d 2 bursts were g e n e r a t e d at the f a t i g u e
crack a n d the Type 3 burst w a s p r o d u c e d by the loading machine. 1,o

FIG. 8---Examples of acoustic emission and machine noise produced during fatigue of a
2024 aluminum specimen.

Crack Growth in Alumina Ceramic

Controlled slow crack growth in polycrystalline ceramics produces AE
which in general are nearly white noise [6 ]. This is illustrated in Fig. 9 by the
Type 1 burst observed for a pure alumina ceramic. About 1 percent of the
AE appear on the oscilloscope with a much longer ring-down time, and
invariably these Type 2 bursts contain more of their energy at higher
frequencies. Similar results have also been observed for other ceramics.
These tests were made in a very quiet environment in three-point bending
under essentially dead-weight loading conditions so that misinterpretation
of the source of these bursts is not likely. One other type of burst that was
observed during these tests is illustrated by Type 3 in Fig. 9. These bursts
only occurred when the ceramic specimen had a roughness in its surface
exceeding about 50/xm, which would cause "punchout" impressions in the
surface ofa 125-/zm-thick Mylar shim used to cushion the loading points. It
should be emphasized that the large difference in waveform and in spectral
content of these acoustic bursts can not be attributed to specimen geometry
or transducer resonances, since they are all observed under the same
conditions, sometimes within milliseconds of each other. Their differ-

FIG. 9----Examples of acoustic bursts observed during slow crack growth in polycrystalline
alumina ceramic.

ences, therefore, must be due to their generation mechanisms. This is true

of all the frequency spectra illustrated but is particularly relevant to these
results because of the simplicity of the specimen and test conditions.

Examples of Other Extraneous Noises

Some further examples of extraneous background noises are shown in
Figs. 10 and 11. Electrical noise spikes, as in Fig. 10a, are very broad-
banded and show none of the characteristics of the acoustic response of the
transducer. Many types of mechanical impact noise are typified by the
spectrum in Fig. 10b which is for a burst produced by the meshing of metal
gears. The broadband continuous noise of Fig. 10c was produced by a
steam turbine. The three spectra shown are for three rates of steam flow
and illustrate the broadband nature of this noise. The multiple bursts shown
in Fig. 11 are believed to be due to a "stick-slip" friction mechanism caused
in this case by shear stresses produced by differential thermal expansion
across a mechanically coupled joint between two pieces of metal. Other
observations of similar acoustic bursts have been made during the tighten-
ing of bolts. Another example might be the Type 3 bursts of Fig. 7 which
were generated during the motion of the piston in a hydraulic ram. The
frequency spectra of the bursts in these three cases were all similar in that

FIG. IO----Examples of background noises recorded during acoustic emission tests.

they had either one or two prominent peaks in their spectra at relatively
high frequency.

General Comments on Spectral Analysis

It is obvious from the diversity of the frequency spectra in the foregoing
examples that the separation of extraneous noise bursts and A E by recog-
nizing differences in their frequency content is not simple but is certainly
feasible. Each test situation is different and will have to be analyzed
individually. The fact that there are differences between the spectra is
encouraging, however. These results can only hint at the scope of the future
use of spectral analysis in AE technology and in studies of the AE genera-
tion mechanisms. It is apparent that simple impulse models of AE genera-
tions which have been discussed [1,13,20,] do not predict all of the fre-
quency spectra which have been observed.
It should be pointed up at this time that most of the frequency spectra just
described were obtained for acoustic bursts which were recorded with the

FIG. 11 Multiple acoustic bursts believed to be due to "st&k-slip" friction.

transducer located on the specimens or structures close to the source of the

burst. In flaw-locating systems on large structures using triangulation
methods, the effect of the frequency dependence of the attenuation in the
acoustic path between the transducer and the source must also be consid-
ered. Some results obtained on typical structures toward providing this
information are presented in the following section.

Acoustic Transmission
The acoustic white noise generator and broadband transducer described
previously were used to determine the frequency dependence of the acous-
tic attenuation in various structures. The generator and pickup were acous-
tically coupled at various separation distances on the structures, and the
frequency spectra of the sound transmitted along the various paths were
obtained. From a series of these spectra for each of the acoustic paths the
loss in amplitude as a function of distance could be determined over a wide
frequency range.

Large Pressure Vessels and Other Structures

The attenuation was determined over several acoustic paths on the large
gas pressure vessels shown in Fig. 12. The inner vessels were A283 steel;
the dome ends, bands, and the large manway at one end were A212-B steel;

TRANSDUCER POSITION LONGITUDINAL

POSITIONS
OF WNG

~ U T S I D E

/
C,CU.FRENT,AL
E
48o,
o~,~S,D~__~~GE

. ,, POSITIONSOF WNG

18oo~s,o~~-~
t " 180~ i UTSIDE

~ 4,0 w

(a)

,'o~s%~C~ ~ ~ ~ j~
\\~~
~ WNG POSITIONS

[ II
S"
~
,,g
5-1/2"
L
I-
4r' J
-I t
(b)
F I G . 12--Gas p r e s s u r e v e s s e l s on w h i c h a c o u s t i c a t t e n u a t i o n m e a s u r e m e n t s w e r e made.

and the pipe flange at the other end was A 105 steel. In Fig. 13 are shown the
relative amplitudes of the acoustic noise at various frequencies which was
received by the transducer as the separation distance between the WNG
and the transducer was changed along the length of the pressure vessel. The
attenuation rate changed as the separation distance between the WNG and
transducer increased due to the so-called geometrical attenuation, which is
the result of the energy contained in the acoustic wave being spread over an
increasing area as the wave advances. In the pressure vessel the rate of
attenuation was constant beyond about 1.5 m (5 ft) due to a waveguide

L ' _ _ ' _ ' ' J ' ' ' w . . . . , . . . .

O" ~ ~ ~ 100 KHz.

- 9 ~ ~ ~ ~ m

~ ~ ~ ~ -'- ~ ~ ~ 1 5 0 K H z "O" ~

i "~0~ "~ 400 KH.~

_ - ~ . ~ --~-~.. --~ ...

- \ -

- \-..~. --~ ----...~-. -

~ - 850 KHz ~,,,

I , , , , I , , , , I , , , , I . . . . I . , ~ /
0 5 10 15 20 25

SEPARATION DISTANCE (FT)

FIG. 13---Signal loss as a function of distance and frequency along the outer surface of the
gas pressure vessel of Fig. 12a.

effect along the length of the vessel. The average rate of attenuation at 1.5
m (5 ft) and 7.5 m (25 ft) obtained from the straight line approximations to
the data of Fig. 13 are shown in Fig. 14. Included are the results of
measurements around the circumference of the vessel as well as along its
length which showed no difference between these two paths. This pressure
vessel was located outside and had many layers of paint to protect it from
oxidation. This apparently added to the attenuation because similar data on
a variety of large aluminum and steel plates and girders having no paint on
their surfaces show a similar shape to the frequency dependence but with
values in the range of 0.6 to 3.0 dB/m (0.2 to 1.0 dB/ft) at 1 MHz instead of
about 8.2 dB/m (2.5 dB/ft) [11 ]. One girder which was heavily oxidized had
about the same attenuation rate as the pressure vessel, however, suggest-
ing that either a damping or scattering material on the surface of a
structure is deleterious to the transmission of AE signals.
The relative circumferential position of the WNG and transducer on the
outer surface of the vessel was found to have no effect on the attenuation
rate, but putting the WNG on the inside wall of the vessel introduced an
additional attenuation as shown in Fig. 15a. The unusual frequency depen-

" , l l , l l l / ' l , ,
AT - N/o

~ 2 o
=.

o I I I I I I I I I
0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0
FREQUENCY (MHz}
FIG. 14---Acoustic attenuation in the gas pressure vessel o f Fig. 12a.

20
I I I I
m
INSIDE TO OUTSIDE
GAS PRESSURE VESSEL
AT 25 ft

oL/"
0
I
.2
I
.4
I
.6
I
.8 1.0
FREQUENCY (MHz)
(a)

4O
I I I

DOUBLE WALL
ii GASPRESSU.E
VESSEL m

9 AT 1$t BAND
O AT 2nd BAND
I"1 AT 3rd BAND

o0 I
.2
I
.4
1
.6
1
.8 1.0
FREQUENCY (MHz)
(b)
(a) From inner wall to outer wall.
(b) Across compression fit joint.
FIG. 15---Additional acoustic attentuation due to specific transmission p a t h s in gas pres-
sure vessels.

dence of this source of attenuation could influence the choice of the fre.
quency range for a flaw location system. For example, if it was desired to
monitor the growth of an inner surface crack with transducers located on
the outer surface of such a vessel, the frequency range between 100 to 500
kHz should be avoided if possible. Other considerations such as the fre-

quency content of the AE and extraneous noises and the normal attenua-
tion over the acoustic path may prohibit this choice. This example illus-
trates the necessity of knowing all of these factors when designing a
monitoring system.
Another significant source of attenuation was found on the pressure
vessel shown in Fig. 12b, which had thick steel strengthening bands com-
pression fit around its outer circumference. The additional attentuation of
the acoustic energy across the interface between the vessel wall and the
strengthening bands is shown in Fig. 15b. The successful monitoring of AE
where such an interface is in the path seems improbable.

Foam-Insulated Vessel
Another example of a structure which resulted in a very high acoustic
attenuation was a cryogenic liquid storage tank which had a foam insulation
material bonded to one side of the aluminum plate which formed the tank
[10]. The data of Fig. 16 were obtained with the WNG and transducer
bonded to one side of the aluminum plate and with the foam insulation
bonded to the opposite side. A large difference in the attenuation was
observed between spraying the material onto the plate to a thickness of 5
cm or preforming it in a slab of that thickness and then gluing it in place.
This experience and the experience with the strengthened gas pressure
vessel suggest that alternate construction methods should be considered
in the design of structures such as these if AE monitoring of the struc-
tures are anticipated.

Bearings and Shafts

Figure 17 illustrates acoustic transmission across a ball bearing journal.
The signal received by the transducer when it was bonded to the shaft is
compared in each case to the signal received when it was bonded to the
bearing housing. In Fig. 17a and b the bearing was dry and there was a side
load on the shaft of about 0.5 kg (1 lb) and 13 kg (30 l b), respectively. In Fig.
17c medium-weight machine oil filled the bearing housing, and in Fig. 17d
the oil had been allowed to drain out over a 20-h period so that only a light
film remained on the bearings. In both of the latter cases there was a 0.5 kg
(1 lb) side load on the shaft. This test illustrates that AE monitoring across
stationary mechanical linkage and of rotating machinery components
might be feasible if acoustic coupling through a grease or oil layer is
provided.

Effect of a Dispersive Medium

The examples just given of the acoustic transmission characteristics of
various structures were obtained using the WNG as a source of continuous

3.0 I I I I I I I I I I I I I -

1 / 8 in. T H I C K
2"219 A1
m

I I
t
8 dB/in.
t
10 dB/in.
--
2.0

=_ 5B ~.

Z
INSULATION
0 GLUED ON

z
k-

1.0

NO INSULATION
0 T I --I --I I I I ' I I9 I | "
0 200 400 600 800 1000 1200
FREQUENCY (kHz)
F I G . 16--Acoustic attenuation o f a l u m i n u m sheet having a f o a m insultation material on its
reverse side.

noise which could be monitored at various positions on the structures. It

was demonstrated earlier that the transducer r e s p o n s e to a continuous or a
pulsed white noise source was the same and that the f r e q u e n c y content of
an A E was not changed due to multiple reflections within a specimen.
Therefore, the results of the attenuation m e a s u r e m e n t s would be valid
when applied to A E monitoring.
There is an interesting case where analyzing the transmission of a con-
tinuous noise does not tell the whole story, h o w e v e r . In a dispersive m e -

~'f'...-"
...._ ... (") _ ]:
0
z
.-I
,~'~ .
". ............ o
z
0
PLATE CONTAINING .~
DEPRESSION / 9./... (b)
ROTATING SHAFT ~ ACOUSTIC t')
............ f NOISE GENERATOR

BEARING~
oo

:>
t3
r0.

LOAD -~ R

(d)
u,
"" x. ....
0
BEARING / ~ z

- A

0 1
FREQUENCY (MHz)
(a) Dry bearing, no load. (c) Oil-filled bearing, no load.
(b) Dry bearing, 13 kg side load. (d) After oil drained out, no load.
FIG. 17--Acoustic attenuation across a ball bearing journal.

dium the different frequency components of a broadband acoustic wave

propagate with different velocities so that after the wave has traveled some
distance the different frequency components are separated in space and
arrive at the transducer at different times. One example of such a medium is
sheet material where the thickness of the material is small compared to the
wavelength of the acoustic waves. The principal propagation modes in this
case are the symmetric and antisymmetric Lamb modes. The frequency
dependence of the group velocities of these modes depend on the elastic
properties and the thickness of the sheet. This dependence for a 1.6-mm
(1/16-in.) thick sheet of 2219 aluminum taken from tabulated computer
calculations [21] is shown in Fig. 18. A bulk longitudinal wave velocity of
6374 m/s and a Poisson's ratio of 0.345 were used in constructing this figure.
During a fatigue test o f a 1.8-m (6-ft) long by 0.3-m (l-ft) wide by 1.6-mm
(1/16-in.) specimen of 2219-T87 aluminum, broadband acoustic bursts were
recorded which appeared as shown in Fig. 19 at three positions 50 cm (20
in.) apart along the length of the specimen. The increasing separation of the
symmetric (SM) and the antisymmetric (AM) modes with distance traveled
is clearly seen in these oscillographs. With reference to Fig. 18, the fastest
wave component is the lowest frequency symmetric Lamb mode followed

6 ' I
I
2219 AL
VL := 6374 M/SEC
~ = 0.345
5 M,,.',.ETERS

SY..E'rR,C" \

v--

.J
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> 3
o.

ANTISYMMETRIC
YMMETRIC

1 ~ , I ,
1 2
FREQUENCY (MHz)
F I G . 18---4)ispersion curves for Lamb waves in sheet aluminum.

by the higher frequency components of this mode. It is this mode which

appears first as a small burst at the left side of the oscilloscope traces of Fig.
19. The fastest antisymmetric component is very broadbanded, containing
all frequencies above about 0.5 MHz. Most of the energy of the acoustic
wave is concentrated in this mode because of its broadbandedness, and this
results in the appearance of the high-amplitude signal in the oscilloscope
traces. This is followed by the slower, lower frequency components of this
mode. Observations of this same phenomenon have been described previ-
ously using a repetitive, low-frequency acoustic emission simulator as the
source of the acoustic signal [22].
The freqency analysis of successive 20/zs portions of the acoustic burst
recorded at the greatest distance from the source and labeled A,
B , C , 9 9 9 Y in Fig. 19 are shown in Fig. 20. The same qualitative behavior
as previously described can be observed. In B through F the higher fre-
quency components of the symmetric mode are seen to occur at later times,
and at K the broadband antisymmetric mode is seen to occur. This is
followed in L through W by the lower frequency, slower moving compo-
nents of this mode.
The source of the acoustic bursts can be determined in two ways from the
oscilloscope traces of Fig. 19. Knowing the distance between any two
observation points and the increase in the mode separation time at these
two points determines the distance of either point to the source and also
determines the difference in the reciprocals of the principal velocities of the
two modes. Alternatively, if the two principal mode velocities are known,
that is, the velocities of the broadband portion of each mode, then the
location of the source can be determined from the mode separation time on
any one of the oscilloscope traces [10]. The implication of this possibility is
that source location could be accomplished using only one transducer in
certain cases. In the test just described the source was 30 cm (12 in.) from
the nearest transducer location which was where the end of the specimen
was gripped for the fatigue test.

General Comments on Acoustic Transmission

In summarizing the results of the tests described in which the acoustic

transmission properties of structures were evaluated, two points should be
emphasized. When designing an AE monitoring system, a knowledge of the
frequency dependence of the attenuation in any acoustic path under con-
sideration is important in establishing the operating frequency range of that
system. The second point is that the observation and analysis of frequency
dispersion under special test conditions lends support to the conclusion
that under most conditions the frequency content of an AE brust is not
changed during transmission through the medium or upon mode conver-

z
c7

o
z

z
FIG. 19---Acoustic bursts after transmission through three distances in sheet aluminum
showing effect o f dispersion.

I | ! i ' I

C 0

(M A

Y
i J i
0 1 2 0 1 2
FREQUENCY (MHz)

FIG. 20----Frequency analysis o f successive 20 p~ increments o f the acoustic burst at the

bottom o f Fig. 19.

sion upon reflection at its boundaries. If a change in the frequency

content did occur, it would have been certainly observed under the condi-
tions of our tests.

Summary
A means for quickly and easily determining the broadband frequency
content of acoustic bursts as short as 20/.~s in duration has been developed
which will allow a rapid survey of the different types of AE and extraneous
background noise bursts in an AE test environment. This can be done early
in a test and provides information regarding transducer spacing and the
noise discrimination and sensitivity requirements of the electronic compo-
nents of a triangulation system.
The frequency content of an acoustic burst is related to the mechanism
which produced it and is not affected substantially by mode conversion
during multiple reflection in a solid. This is substantiated by the observa-
tion that in most cases the frequency content near the leading edge of an AE
is the same as at a time later in its ringdown. This ring-down time is
determined by the size of the structure and how many internal reflections
can be effected before the energy in the burst is dissipated. Bursts from a
few microseconds to a few milliseconds in duration have been observed. In
the special case of a dispersive medium, a change in the spectrum during
ringdown is observed and understood. It was also shown that the frequency
spectrum of an AE does not depend substantially upon the specimen size or
shape, although minor changes occur in the spectra at lower frequencies
due to excitation of specimen resonances. The studies of the acoustic
transmission characteristics of a structure have shown that frequency-
dependent attenuation and dispersion do affect the frequency content of an
acoustic burst which has propagated some distance through a structure.
Several examples of AE bursts with unusual frequency spectra have
been observed. A thorough study of two types of AE generated in A533-B
steel has resulted in the identification of the two types as being due to
plastic deformation and crack extension, respectively. Other unique
spectra have also been observed, but the identification of their sources
requires further study. The generalizations that mechanical impact noises
have low-frequency spectra and hydraulic noises tend to have more broad-
banded spectra have been demonstrated, but exceptions to these generali-
zations have also been shown. Examples of several distinctive types of
frequency spectra have been given which suggest that discrimination of
noise bursts from different sources can be accomplished in some cases by
frequency content alone.
In conclusion, broadband frequency analysis has been demonstrated to
serve a useful function in the AE technology. Its use in the study of the

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38 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

mechanisms governing the mechanical properties of materials is also

suggested.
Acknowledgments
We wish to acknowledge the valuable assistance of several key people in
the various divisions of Rockwell International in obtaining the results
described. These are J. S. Buck of Atomics International Division, F. E.
Sugg of Space Division, and J. F. Moore of B-1 Division. Parts of the work
were supported by the Naval Air Systems Command and by the Rockwell
International Interdivisional Technology Program.
References
[/] Chambers, R. H. and Hoenig, S. H., "New Techniques in Non-destructive Testing by
Acoustical and Exo-electron Emission," Semiannual Progress Report, ARPA Contract
F33615-68-C-1707, Sub-item IAA, April 1970.
[2] Ono, Kanji, Stern, Richard, and Long, Marshall, Jr., inAcoustic Emission, ASTM STP
505, American Society for Testing and Materials, 1972, pp. 152-163.
[3] Graham, L. J. and Alers, G. A., "Investigation of Acoustic Emission from Ceramic
Materials," AD 745000, Final Report, Naval Air Systems Command, Contract No.
N00019-17-C-0344, May 1972.
[4] Beattie, A. G. in Proceedings, Ultrasonics Symposium, Institute of Electrical and Elec-
tronics Engineers, Oct. 1972, pp. 13-17.
[5] Graham, L. J. and Alers, G. A. in Proceedings, Ultrasonics Symposium, Institute of
Electrical and Electronics Engineers, Oct. 1972, pp. 18-21.
[6] Graham, L. J. and Alers, G. A., "Acoustic Emission from Polycrystalline Ceramics,"
AD 754839, Final Report, Naval Air Systems Command, Contract No. N00019-72-C-
0382, Dec. 1972.
[7] Maxfield, B. W. and Cochran, R., Materials Evaluation, Vol. 31, No. 2, 1973,pp. 17-20.
[8] Graham, L. J. and Alers, G. A., Materials Evaluation, Voi. 32, No. 2, 1974, pp. 31-37.
[9] Sugg, F. E., "On-Board Checkout of the Structural Integrity of Cryogenic Tanks, 3rd
Quarterly Report, S-II Advanced Technology Study No. 4, North American Rockwell,
Space Division, 13 Aug. 1971.
[10] Graham, L. J., "Frequency Response Characteristics of Acoustic Emission System
Components," SCTR-73-6, Science Center, Rockwell International, Thousand Oaks,
Calif., March 1973.
[/1] Graham, L. J., "Frequency Response Characteristics of Acoustic Emission System
Components, Part II," SCTR-73-14, Science Center, Rockwell International, Thousand
Oaks, Calif., Sept. 1973.
[/2] Egle, D. M. and Tatro, C. A., Journal of the Acoustical Society of America, Vol. 41, No.
2, 1967, p. 321.
[/3] Stephens, R. W. B. and Pollock, A. A., Journal of the Acoustical Society of America,
Vol. 50, No. 3, 1971, p. 904.
[14] Dunegan, H. L. and Tatro, C. A. in Techniques of Metals Research, Vol. V, Part 2, R. F.
Bunshah, Ed., Interscience, New York, 1971, pp. 273--312.
[15] Sparks, C. R. and Wachel, J. C., Materials Evaluation, Vol. 31, No. 4, 1973, pp. 53-60.
[16] MuUin,J. V. and Mehan, R. L.,Journal of Testing and Evaluation, Vol. 1, No. 2, 1973,
pp. 215-219.
[/7] Reis, J. J., Research~Development, Vol. 23, No. 2, 1972, pp. 24-26.
[/8] Graham, L. J., "Acoustic Emission Transducer Characterization," SCTR-71-19, Sci-
ence Center, Rockwell International, Thousand Oaks, Calif., Dec. 1971.
[19] Graham, L. J., "Frequency Analysis of Acoustic Noises During Fatigue Testing of the
EBOR Nuclear Reactor Pressure Vessel," SCTR-72-6, Science Center, Rockwell Inter-
national, Thousand Oaks, Calif., April 1972.

[20] Tatro, C. A., "Acoustic Emission Sensors and Instrumentation," TID-4500, Lawrence
Livermore Laboratory, Livermore, Calif., 1973.
[21] Bradfield, G. in Notes on Applied Science, No. 30, National Physical Laboratory,
London, 1974, pp. 122-156.
[22] Fowler, K. A. and Papadakis, E. P. in Acoustic Emission, A S T M STP 505, American
Society for Testing and Materials, 1972, pp. 222-237.

Acoustic Emission During Phase

Transformation in Steel

REFERENCE: Speich, G. R. and Schwoeble, A. J. "Acoustic Emission During

Phase Transformation in Steel," Monitoring Structural Integrity by Acoustic
Emission, ASTM STP 571, American Society for Testing and Materials, 1975,
pp. 40-58.
ABSTRACT: Acoustic emission was monitored during phase transformations
that occur during cooling in a wide variety of steels. Acoustic emission was
generated during the formation of martensite but not during the formation of
ferrite, bainite, or pearlite. This observation is consistent with the rapid, diffu-
sionless, shear-like nature of martensite formation and the slow, diffusion-
controlled growth of ferrite, bainite, or pearlite. The martensite start tempera-
tures, and the temperature range of martensite formation determined by acous-
tic emission were in good agreement with those determined by metallographic or
dilatometric methods. The intensity of acoustic emission generated during
martensite formation decreased markedly as the carbon content of the steel
decreased, becoming nearly undetectable in a maraging steel. This decrease in
intensity correlates with a morphological change from large plate-shaped mar-
tensite units to smaller lath-shaped martensite units as the carbon content of the
steel is decreased.
KEY WORDS: acoustics, emission, phase transformations, monitors, crack
propagation

A c o u s t i c e m i s s i o n ( A E ) is d e f i n e d as t h e h i g h - f r e q u e n c y s t r e s s w a v e s
generated by the rapid release of energy that occurs within a material
during crack growth, plastic deformation, or phase transformation. This
e n e r g y m a y o r i g i n a t e f r o m t h e s t o r e d e l a s t i c e n e r g y a s in c r a c k p r o p a g a t i o n
o r f r o m t h e s t o r e d c h e m i c a l f r e e e n e r g y as in a p h a s e t r a n s f o r m a t i o n .
R e f i n e m e n t in t h e d e t e c t i o n s y s t e m s f o r A E h a s l e d t o a r a p i d l y a c c e l e r a t i n g
i n t e r e s t in its u s e f o r d e t e c t i o n o f c r a c k g r o w t h in s m a l l l a b o r a t o r y s p e c i -
m e n s , d e t e c t i o n o f i n c i p i e n t f a i l u r e in l a r g e e n g i n e e r i n g s t r u c t u r e s , w e l d

1 Research Laboratory, U. S. Steel Corp., Monroeville, Pa. 15146.

40
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SPEICH AND SCHWOEBLE ON TRANSFORMATION IN STEEL 41

monitoring, and studies of plastic deformation and phase transformations

in various materials [1,2]. 2
The"clicks" that occur during the formation of martensite in high-nickel
steel were reported by Frrster and Scheil in 1936 in one of the first studies
of AE in the literature [3]. However, from that time until only a few years
ago, no further work was done on AE generated during phase transforma-
tions in solids. Recently, several studies of AE during phase transformation
in metals have been reported. Liptai et al [4] have studied AE during
formation of martensite in gold-cadmium and indium-thallium alloys and in
cobalt and plutonium, during both heating and cooling. They also studied
the eutectoid transformation in a tin-cadmium alloy and showed that such a
diffusion-controlled transformation did not generate AE. Speich and
Fisher [5] have studied martensite formation in a 28Ni-0.1C steel which
transformed below room temperature. They correlated the acoustic emis-
sions with the number and size of the martensite plates and determined that
a number of plates are involved in each emission because of the autocataly-
tic nature of the martensite transformation. More recently, Ono et al [6],
using AE techniques, have determined the Ms temperatures of a number of
steels that transform below room temperature. However, a general study of
AE during formation of the various transformation products in steel has not
been attempted.
In the present work we have made a study of AE generated in a wide
variety of steels during transformation into ferrite, pearlite, bainite, and
martensite. Such a study was needed for a number of reasons. First, it
provided needed information on the mechanism of phase transformation
responsibility for strengthening steel. Second, since phase transformations
occur in the heat-affected zones of weldments, it is necessary to understand
AE arising from these causes before it can be used to detect microcracking
during welding operations [7]. Finally, phase transformations serve as a
controlled AE source and can be used to study the effect of different
variables on the propagation of the elastic waves that make up the AE
signal [8].

Experimental Procedures
Materials
The steels studied in this work included a series of AISI 4300 steels, an
AISI 52100 steel, an AISI 410 stainless steel, a maraging steel, several
iron-nickel alloys, an iron-carbon alloy, a line-pipe steel, and iron. Their
chemical compositions are given in Table 1. The AIS14300 steels, the AISI

2 The italic n u m b e r s in brackets refer to the list of references a p p e n d e d to this paper.

Steel C Mn P S Si Ni Cr Mo Co Cu Ti Cb AI N

Fe --C:
Fe 0.007 <0.01 0.002 0.005 0.016 0.003 0.007 . . . . . . 0.15 _-- 0.005 0.00
Line pipe 0.15 1.61 0.007 0.008 0.31 0.018 0.027 . . . . . . {).'3{) ... 0.02 0.006 ...
Fe-0.97C 0.97 0.009 0.001 0.021 0.037 0.032 0.005 0.001 . . . . . .
A1SI steels:
4330 0,32 0.80 0.001 0.010 0.29 1.80 0.85 0.24 . . . . . .
l,O 4340 0.41 0.72 0.002 0.010 0.26 1.90 0.82 0.28 . . . . . .
4360 0.64 0.74 0.003 0.013 0.27 1.85 0.86 0.29 . . . . . .
4380 0.83 0.80 0.003 0.013 0.27 1.85 0.86 0.29 . . . . . .
52100 1.00 0.36 0.008 0.016 0.28 0.11 1.47 0.02 . . . . .
Fe-Ni:
9Ni 0.10 . . . . . . . . . . . . 9.00 . . . . . . . . . . .
20Ni 0.01 ... 20.0 . . . . .
Maraging 0.003 0.02 (}.1~1 1).~)1)4 0.011 17.0 ... 4172 7.~ ...
M S I stainless:
410 0.11 0.57 . . . . . . . . . . . . 13.1 . . . . . . . . .

52100 steels, the Fe-20Ni, and the maraging steel permitted AE generated
during martensite formation to be monitored from essentially 0 to 1.1
weight percent carbon. This wide range of carbon content also permitted
the entire range of martensitic structures from lath to plate martensite to be
studied. These steels also have sufficient hardenability so that they trans-
form completely to martensite with the cooling rate used in this work
(6~
The iron, iron-carbon alloy, and line-pipe steel permitted AE to be
monitored during both ferrite and pearlite formation during continuous
cooling. Acoustic emission generated during isothermal formation of bain-
ite was monitored in the AIS1 4360 and 4380 steels.
Equipment
The furnace used in the present work is shown in Fig. I. Particular care
was exercised in the design of this furnace to eliminate all random noise
caused by heating and cooling of refractory elements. The specimen was

FIG. 1---Furnacefor acoustic emission studies at high temperatures.

enclosed in an evacuated fused silica tube to prevent oxidation. The fur-

nace was arranged so that it could be lowered away from the fused silica
tube to permit the specimen to cool continuously to room temperature.
Temperature of the specimen was monitored by a chromel-alumel thermo-
couple spotwelded to the specimen. Fused silica tubes rather than refrac-
tory tubes were used to insulate the thermocouple wires. When the speci-
men was to be cooled to room temperature, the vacuum was replaced with
helium, and the furnace lowered. The cooling rate in helium was 7~ at
800~ 1.5~ at 400~ and 0.4~ at 200~ In a few cases, the fused
silica tube was surrounded by a smaller furnace maintained at 400~ after
the main furnace was lowered. This permitted AE to be monitored during
isothermal formation of bainite.
Acoustic emission from the specimen was monitored by use of an exter-
nally mounted transducer and a stainless steel waveguide. The specimen
(3/8 by 1/8-in.-diameter) was welded to the stainless steel waveguide (26 by
1/8-in.-diameter) using a resistance wire welder; good welds and good
acoustic coupling were obtained in all cases.
The AE equipment was a standard commercial unit [9]. A block diagram
indicating the various electronic components is given in Fig. 2. The
piezoelectric transducer was also obtained from a commercial supplier [9].
It is a 150-kHz resonant frequency lead-zirconate transducer constructed
so that much of the background electrical noise was minimized. The trans-
ducer was coupled to the waveguide with a couplant grease. The waveguide
was expanded smoothly near the end so that the 1/2-in.-diameter trans-
ducer could be attached and yet allow all the acoustic energy to reach the
transducer. Electrical signals from the transducers were fed directly into
the preamplifier, which had a fixed amplification of 40 dB. Signals were
then passed through a bandpass filter (100 to 300 kHz) which removed
much of the background noise. Finally, the AE signals were further
amplified 20 to 55 dB with a variable broadband amplifier. These amplified
signals either were fed directly into an electronic counter or were shaped in
a digital envelope processor and then fed into the electronic counter. The
digital envelope processor makes a single envelope of signals resulting from
multiple reflections within the specimen, waveguide, and transducer. The
electronic counter operated with a fixed threshold of 1 V. The output from
the counter was fed into a digital to analog converter which gives a fixed
millivolt signal for a given number of counts. The output of the digital to
analog converter was used to drive the Y-axis of an X-Y recorder. The
thermocouple output was used to drive the X-axis of the same recorder to
obtain a plot of acoustic counts versus temperature. In a few cases, a reset
clock was used to obtain count rate data rather than total counts. Also, the
cooling curves of a few specimens were recorded using a separate millivolt
recorder.

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SPEICH AND SCHWOEBLE ON TRANSFORMATION IN STEEL 45

FROM I PREAMPLIFIE" I
TRANSDUCER I 4ode
II
BAND-PASS I
FILTER I
t oo-3ook.- I

I AMPLIFIER I
0-60 dB

I DIGITAL- I
I"O$CILLOSCOPE J ~ I ENVELOPE I
" ! PROCESSOR I

It
~ i C~176 I

X-Y
RECORDER I I~176
ANALOG
CONVERTER

FROM THERMOCOUPLE
FIG. 2----Blockdiagram of electronic components.

Total counts per unit volume which include multiple reflections will be
referred to as Nv. When these counts were digital-envelope-processed to
correct for multiple reflections, they will be referred to as N~ or the total
number of acoustic emissions per unit volume.
After cooling to room temperature, all the specimens were examined by
standard light metallography techniques to determine the morphology of
the various transformation products.

Results

Ferrite-Pearlite
Acoustic emission during ferrite and pearlite formation was monitored in
the iron, line-pipe steel, and Fe-0.97C alloy during cooling from 1000~
and the results are given in Fig. 3a. Total counts per unit volume, N,,, at an
amplification of 95 dB were recorded. Cooling curves for the same speci-
mens are given in Fig. 3b. A thermal arrest occurs at 900~ in the iron
specimen, indicating formation of ferrite. Similarly, a thermal arrest occurs
at 620~ in the Fe-0.97C specimen, indicating the formation of pearlite.
Comparison of the two sets of curves shows that no AE was observed

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46 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

TEMPERATURE, F
I000 I '~00 1400 1600 IGO0
0.02 I I I I i

95 dG (A) ACOUSTIC EMISSION

0.01 Fe
9. - /
( ( ( "7
- COOLING . 9 LINE PIPE

~[AT~I~ > ~Fe-O,97C 2

(G) COOLING CURVES

ioo

y-~ARL,ITE ~ y,a
t-
o
I I I I ~ I
500 G00 700 800 900 I000
TEMPERATURE, C

FIG. 3--Absence of acoustic emission during ferrite and pearlite formation in steel.

during the formation of either ferrite or pearlite. Examination of specimens

by light microscopy revealed an equiaxed ferrite grain structure in the iron,
and a fine pearlite structure in the Fe-0.97C alloy. Similar results were
obtained for the line-pipe steel, which transforms into a mixture of ferrite
and pearlite during continuous cooling. AE results obtained during heating
are also shown in Fig. 3a for the Fe-0.97C alloy. No AE was observed
during heating of any of the steels.

Bainite
Acoustic emission during bainite formation was monitored in specimens
of the AISI 4360 and 4380 steels that were transformed isothermally at
400~ by using a small furnace to surround the quartz tube after the
austenitizing furnace was lowered. Total counts per unit volume, N,,, at an
amplification of 90 dB for the AIS14360 specimen are shown in Fig. 4 along
with metaUographic determinations of the volume fraction of bainite
formed [10]. No AE was observed during bainite formation in this steel.
Similar results were obtained for the AISI 4380 steel.
Martensite
To investigate acoustic emission generated during martensite formation
in low-carbon steels, the maraging, 20Ni, and 9Ni steels, and the AISI 410

I I I 39oc~
'~E
O.OI - (A) ACOUSTIC EMISSION 90(115
I-- 4360
Z

s
,, ~
z 0
r
IO0 -- 18) PERCENTAGE TRANSFORMEO,~
~ 5o

IO 102 103 104 105

TIME, $
FIG. 4---Absence of acoustic emission during bainite formation in steel.

stainless steel were austenitized at 1000~ and continuously cooled in

helium at 6~ The hardenability of these steels is sufficient so that they
transform completely to martensite with this cooling rate. Total counts per
unit volume, N,,, at an amplification of 90 dB are given in Fig. 5. The AE in
the maraging steel and the 20Ni steel was just barely detectable, whereas
the 9Ni and 410 stainless steels had considerably higher levels of acoustic
emission.
No AE is observed until the martensite start temperature, Ms, is reached.
The total counts then increase with decreasing temperature because the
amount of martensite increases with increased degree of undercooling
below Ms [10]. A maximum in the total counts is reached eventually at
about 60~ below Ms where the austenite has completely transformed to
martensite. Similar results were obtained for the other steels as sub-
sequently discussed.
To investigate AE during martensite formation in medium-carbon steels,
the AISI 4300 steels with various carbon contents were austenitized at
1000~ and continuously cooled in helium at 6~ The hardenability of
these steels is sufficiently high so that they transform almost completely
to martensite with this cooling rate. Total counts per unit volume, N~, at
an amplification of 90 dB are given in Fig. 6. AE in these steels was
nearly the same as in the 9Ni steel, but it increased gradually with in-
creasing carbon content.
To investigate AE in a high-carbon steel, the AISI 52100 steel was
anstenitized at 1050~ and continuously cooled in helium at 6~ The

t~
)-
z
0

>
z

200 250 300 350

TEMPERATURE, C
FIG. 5--Acoustic emission resulting from martensite formation in low-carbon steels.

TEMPERATURE, F

2~ I
300
l 400
I ~5 o I 6oo
l 1--
90dB

o , Ix,._, . ~, ~ , ~ , ~
I00 150 200 300 350
TEMPERATURE, C
FIG. 6--Acoustic emission resulting from martensite formation in medium-carbon steels.

hardenability of this steel is sufficient so that complete transformation to

martensite occurred at this cooling rate. Total counts per unit volume, N,,,
at an amplification of 90 dB are given in Fig. 7A. The count rate, dN,,Mt, is
given in Fig. 7B. The rate of change of the total counts with temperature is
reflected in the counting-rate curve because the cooling rate is nearly
constant in this temperature range. The counting rate was low, just below
Ms, increased to a maximum at 50~ below Ms, and then decreased to a low
value at lower temperatures. The temperature at which no martensite was
detected was 120~ below Ms.

Ms Temperature
The Ms temperature of the steels can be determined by AE although
some uncertainty results because of the long tail present near Ms. The
simplest procedure was to extrapolate the straight-line portion of the AE
curve to the temperature axis. The temperature at which the straight line
intercepted the temperature axis was taken as the Ms temperature (see
Figs. 5 to 7). A similar scheme has been used by Brook et al [11]. The
M8 temperatures obtained in this manner are given in Table 2. In general,
TEMPERATURE~F
300 400
l I ~ I J
90 dB
12 .

~o X (A) TOTAL COUNTS

2 4
\

%
w
! (B) COUNT RATE

80.5

I00 150 200 250

TEMPERATURE, C

FIG. 7----Acoustic emission resulting from martensite formation in high-carbon steel.

T A B L E 2----Comparison of M s temperatures determined by acoustic" emission and by

metallograpic methods.

M~ (acoustic
Steel emission), ~ M~ (other work), ~

9Ni 328 360 (12)

20Ni 227 234 (13)
Maraging 238 . _ _

410 stainless 340 (12)

4330 345
4340 302 290 (12)
4360 21o 21o (1o)
4380 150 .
52100 167 140 ~ - ""
(1"2)
240 b
Austenitized at 1065 ~
b Austenitized at 843 ~
References at end of paper.

good agreement was found between the M, determined by AE and by

metallographic [10, 12] or dilatometric means [13]. The Ms temperatures of
the AISI 4300 steels are also shown in Fig. 8 along with those determined
for iron-carbon alloys [14]. The M, temperature of the 4300 steels was
lower than that for iron-carbon alloys of the same carbon content pre-
sumably because of the additional alloying elements present in these

~ I I I
80O
400

~. \\\ ~,-C,Ret 14

_
600
300

-- 4 5 0 0 STEELS 4 0 0 :Z"
:~ 200

I00 2OO

1 I I
0.2 0.4 0.6 0.8
CARBON, weight percent

FIG. 8--Mm temperatures of medium-carbon steels determined by acoustic emission.

steels. However, the dependence of the Ms temperature on carbon content

was similar to that reported for iron-carbon alloys.
The total counts per unit volume for 100 percent transformation to
martensite for the various steels was very sensitive to carbon content, as
can be seen in Fig. 9. The total counts from the very low-carbon maraging
steel and the Fe-20Ni alloy were very small but increased gradually be-
tween carbon contents of 0.1 to 0.8 percent and then increased sharply at
higher carbon contents. The AISI 52100 steel gave a total count that was
almost an order of magnitude higher than that for the AISI 4380 steel.
Since the total counts per unit volume previously discussed include the
counts caused by multiple reflections within the specimen, waveguide, and
transducer, a number of specimens were transformed and AE recorded
using digital envelope processing. Here, the envelope of the "ring down"
pulse is obtained and counted as one pulse. These digital-envelope-
processed counts represent true acoustic emissions and are referred to as
acoustic emissions per unit volume, N~. A comparison of the total counts
12
I

I0 ~ A 52100
0 4380
0 4360
-- V 4340
9 4330
V 9Ni
m 8 m 9 20Ni
E rn M A R A G I N G

~- gOde

2 - 0

o I I
0 0.5 LO
CARBON, weight percent

FIG. 9---Effect o f carbon content on acoustic emission f o r formation o f 100 percent

martensite in steel.

per unit volume, Nv, and the acoustic emissions per unit volume, Ng, at an
amplification of 90 dB for a 4380 steel is given in Figs. 10A and 10B. The
digital-envelope-processed counts were also recorded at amplifications of
85 and 80 dB, and the results are given in Fig. 10B. Digital envelope
processing at 90 dB reduced the total counts by a factor of 10, which
appears reasonable since this is about the number of ring-down reflections
observed in AE pulses when they are examined on a memory oscilloscope.
The decrease in the total counts with decreasing amplification was ex-
pected and reflects the 1 V threshold voltage of the counting system.
Pulses not amplified to this voltage are not counted.
A summary of the digital-envelope-processed results for 100 percent
martensite is given in Fig. 11 for different steels and different amplifica-
tions. It is clear that higher values of N~ were recorded at all amplifications
for the AISI 52100 steel than for the lower carbon steels. The lowest value
of N e was recorded for the maraging steel.

Metallography
The metallographic structures of the various steels after cooling from
1000~ to room temperature at 6~ are shown in Figs. 12A to 12F. The
microstructure of the iron specimen consisted of equiaxed ferrite (Fig.

TEMPERATURE, F
SSO 25o ~o
f !
4380 4380
(A) UNPROCESSED (B) DIGITAL-ENVELOPE
ROCESSEO

0.1~
90 dB

~
Q
O.IC

0.0~ " 85dB 1

iO0 1150 I00 150

TEMPERATURE, C

FIG. lO---Effect o f digital-envelope processing and amplification on total martensite

counts.

Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 23:22:35 EST 2016
Downloaded/printed by
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SPEICH AND SCHWOEBLE ON TRANSFORMATION IN STEEL 53

AMPLIFICATION , d8
80 70 8O 90 95
I I I I I

Z~ 52100 Zl
,0' m 0 4380
0 4360
V 4340
9 4330 ~ 0
[3 MARAGIN8

o
"~ i05 J 3

i
o>
Z

I I
iO3 i0 4
Ef/s I
FIG. I l--Effect of amplification on digital-envelope-processed counts for formation of lO0
percent rnartensite in various steels.

12A); the microstructure of the line-pipe steel consisted of acicular ferrite

and pearlite (Fig. 12B); the microstructure of the Fe-0.97C steel consisted
of fine pearlite (Fig. 12C); the microstructures of the 9Ni and maraging
steels consisted of small lath-shaped martensite units arranged into larger
block-like units (Fig. 12C andD); and the microstructure of the 52100 steel
consisted of large individual plate-shaped martensite units which contained
some microcracks (Fig. 12F). Since detailed studies of the substructure of
lath and plate martensite 3 using transmission electron microscopy have
been published elsewhere [15], no attempt was made to duplicate this work.
In general, the substructure of lath martensite consists of a high-density
dislocation network, whereas the substructure of plate martensite consists
of fine internal twins.
Discussion
Copious acoustic emission is generated during the transformation of
steel into martensite. In contrast, no AE is generated when steel transforms
a Common nomenclature for the lath-shaped martensite units formed in low-carbon steel is
lath martensite. The plate-shaped martensite units formed in high-carbon steels are
referred to commonly as plate martensite [15].

FIG. 12--Microstructures o f acoustic emission specimens: (a)iron, (b)line-pipe steel, (c)

Fe-0.gc, (d) 9Ni steel, (e) maraging steel, and (O AISI 52100 steel. (a), (b) xSO0; (c) to
(O x750.

into ferrite, pearlite, or bainite. This is not surprising since the growth of
martensite occurs in a diffusionless manner at nearly the shear velocity of
elastic waves in steel ( - l 0 s cm/s) [16,17]. Also, this transformation occurs
at low temperatures and releases a large amount of stored energy (-300
cal/mol). The rapid release of part of this energy in the form of elastic waves
is detected as AE. The attenuation of these elastic waves results in the
generation of heat.
In contrast, the tranformation of steel into ferrite, pearlite, or bainite
occurs at higher temperatures and involves slow, diffusion-controlled
g r o w t h ( ~ 1 0 -4 cm/s). Also, much smaller amounts of stored energy are
released [18-22]. Thus, it is expected that AE would not be generated by
such transformations. The present results are also in agreement with earlier
studies by Liptai et al L41 who found no AE generated during a eutectoid
transformation in a tin-cadmium alloy but found copious AE generated
during martensite formation in gold-cadmium and indium-thallium alloys,
and in cobalt and plutonium.

The good agreement between the martensite start temperature observed

by normal metallographic means or by dilatometry and those determined
by AE (Table 2) indicate the reliability of this method for determining Ms
temperatures. The AE method is much faster than the metallographic
method and much more sensitive than the dilatometric method. It appears
generally applicable to a wide variety of steels except for those with very
low hardenability.
The variation of the total acoustic emission for the martensite transfor-
mation with carbon content (Fig. 9) appears to be related to the morphology
of the martensite formed. The low-carbon steels (below 0.2 percent carbon)
all form lath martensite (Fig. 12). 4 This structure consists of small lath-
shaped martensite units (-0.25 by 2 by 50/~m) which are arranged into
larger block-shaped units [15]. In contrast, the high-carbon steels (above
0.8 percent carbon) all form plate martensite. This structure consists of
large plate-shaped martensite units ( - 5 by 50 by 50/xm diameter) [15]. For
medium-carbon steels (0.2 to 0.8 percent carbon) mixed lath/plate marten-
site microstructures are observed [15,23]. The shape of the curve in Fig. 9
simply reflects the amount of plate martensite in the steels. At 0.8 percent
carbon where the steels begin to transform completely to plate martensite
[15,23], a sharp increase occurs in the total counts.
The much greater acoustic emission accompanying the formation of
plate martensite in contrast to lath martensite may simply be caused by the
larger volume of the transformation units in plate martensite. The lath-
shaped units in lath martensite occupy a volume of 25 x 10-12 cm 3, whereas
the larger plate-shaped martensite units in plate martensite occupy a
volume of 4.5 • 10-9 cm 3. If the amplitude of the stress wave generated by
formation of martensite was proportional to the volume of the transforma-
tion unit, we would expect the voltage generated at the transducer would
also to be proportional to the volume of the transformation unit. Examina-
tion of Fig. 11 indicates that AE signals are detected from the 52100 steel at
amplifications about 1/50 those required for the maraging steel. The volume
ratio of lath and plate martensite for these steels is 1/160. More sophisti-
cated analysis of the relationship of the amplitude of the stress wave to the
size of the martensite unit will be needed before agreement with experiment
can be improved. Obviously, in addition to the size of the unit, the
magnitude and direction of the shear strains are involved.
Alternative explanations of the increased acoustic emission, other than
morphological changes, such as the lower transformation temperatures of
high-carbon martensites or microcracking in high-carbon martensites can
be eliminated. The transformation temperature of maraging steel (227~ is
4 An exception to this rule is high-alloy steels such as 28Ni-0.1C steels which form plate
martensite.

nearly the same as that of the 52100 steel (167~ yet, hardly any AE is
observed in the maraging steel in comparison with that found in the 52100
steel. Although microcracking was found in the 52100 steel as reported by
other investigators [24,25], no microcracking was observed in plate mar-
tensite formed in the 28Ni-0.1C steel studied in our earlier work [5]. Yet,
AE even greater than that found in the 52100 steel is observed in this steel.
The copious AE accompanying the formation of plate martensite in 28 to 30
percent nickel, low-carbon steels [3,5] emphasizes that martensite mor-
phology and not carbon content is the primary cause of the increased AE.
The variation of the AE in low-alloy steels with carbon content simply
reflects the effect of carbon content on morphology.
Another possible explanation of the much smaller AE in low-carbon
lath-martensite structures compared to that in high-carbon plate-
martensite structures is a lower martensite growth rate. Owen et al
[26] argue that because of the rapid decrease of the Ms temperature with
small carbon additions in the case of lath martensite, its growth rate is
controlled by the requirement that carbon moves with the interface in the
form of a Cottrell atmosphere. This results in growth velocities for lath
martensite much lower than the velocity of growth for plate martensite
and could account for the lower AE. Additional research will be required
to differentiate between this explanation and that based on simple dif-
ferences in the size of the martensite units.
The total acoustic emissions per unit volume should be comparable to the
number of martensite plates in the specimen. Because of the large number
of steels studied here, it was not possible to make the detailed quantitative
metallographic studies used in our earlier work [5]. However, using an
average martensite plate volume determined by metallography for the
52100 steel of 4 . 5 • 10-9 cm z, the total number of martensite plates should
be ~ 2 • 108 cm -3. The total acoustic emissions per unit volume observed
for this steel is only 3 x 10e cm -3. Each emission thus must involve about
60 plates. This is not surprising because of the autocatalytic nature of the
martensite transformation [27]. Earlier work based on more detailed metal-
lography indicated that about 15 plates were involved in each AE [5].
Similar techniques cannot be applied to lath martensite because the
amplitude of the signals from most of the martensite units are too small to
detect.
It is clear that if AE is to be used to detect flaws during welding opera-
tions, considerable caution must be exercised so that the AE arising from
formation of martensite in the weld heat-affected zone is not misinterpreted
as microcracking. The decrease of the martensite AE with lower carbon
contents may make it possible to avoid this problem in some of the lower
carbon steels.

Summary and Conclusions

Acoustic emission is generated w h e n steels transform into martensite but
not when they transform into ferrite, pearlite, or bainite. This is attributed
to the rapid ( - 1 0 5 cm/s), diffusionless, shearlike growth of martensite.
Ferrite, pearlite, and bainite, which exhibit slow (--10 -4 cm/s), diffusion-
controlled growth, are not e x p e c t e d to give rise to acoustic emission.
Acoustic emission m a y be used to determine the M~ t e m p e r a t u r e of
steels. Such m e a s u r e m e n t s are in good a g r e e m e n t with both metallographic
and dilatometric determinations of the Ms t e m p e r a t u r e s of alloy steels. This
technique is much m o r e sensitive than other techniques and can be used to
examine the transformation m e c h a n i s m in great detail.
Acoustic emission during martensite formation is m a r k e d l y sensitive to
carbon content. L o w - c a r b o n maraging steels generate an e x t r e m e l y low
level of acoustic emission when they transform to martensite, whereas
high-carbon steels generate a v e r y high level of acoustic emission. This is
attributed to the different morphologies of martensite present in these
steels. In steels with less than 0. l p e r c e n t carbon, lath martensite is formed
exclusively, and the small volume of the martensite units results in a very
low level of acoustic emission. As the carbon content is increased, plate
martensite is formed in increasing a m o u n t s , and the larger v o l u m e of these
martensite units n o w results in increased acoustic emission. T h e acoustic
emission continues to increase rapidly up to 1.0 p e r c e n t carbon where plate
martensite is formed exclusively.

References
[1] Liptai, R. G., Harris, D. O., Engle, B. B., and Tatro, C. A., International Journal of
Nondestructive Testing, Vol. 3, 1971, pp. 215-275.
[2] Liptai, R. G., Harris, D. O., and Tatro, C. A. in Acoustic Emission, ASTM STP 505,
American Society for Testing and Materials, 1972, pp. 3-10.
[3] F6rster, F. and Scheil, E., Zeitschriftftlr Metallkunde, Vol. 9, t936, pp. 245--47.
[4] Liptai, R. G., Dunegan, H. L., and Tatro, C. A., International Journal of Nondestructive
Testing, Vol. 1, 1969, pp. 213-221.
[5] Speich, G. R. and Fisher, R. M. inAcoustic Emission, ASTMSTP505, American Society
for Testing and Materials, 1972, pp. 140-50.
[6] Ono, K., Schlotthauer, T.C.,and Koppenaal, J. J., UCLA Report Eng-7334, University
of California, Los Angeles, May 1973.
[7] Hartbower, C. E., Reuter, W. G., Morais, C. F., and Crimmins, P. O. in Acoustic
Emission, ASTM STP 505, American Society for Testing and Materials, 1972, pp. 187-
221.
[8] Brown, A. E. and Liptai, R. G. inAcousticEmission, ASTMSTP505, American Society
for Testing and Materials, 1972, pp. 318-331.
[9] Dunegan-Endevco Corporation, Livermor,e, Calif.
[10] I-T Diagrams, Isothermal Transformation of Austenite in a Wide Variety of Steels,
United States Steel Corporation, Pittsburgh, Pa.
[11] Brook, R., Entwistle, A. R., and Abraham, E. F.,Journal lron and Steel lnstitute, Vol.
195, 1960, pp. 292-298.

[/2] Greninger, A. B. and Troiano, A. R., Transactions of the American Society for Metals,
Vol. 28. 1940, pp. 537-562.
[13] Yeo, R. B. G., Transactions of the Metallurgical Society, American Institute of Mining,
Metallurgical, and Petroleum Engineers, Vol. 227, 1963, pp. 884-890.
[/4] Greninger, A. B., Transactions of the American Society for Metals, Vol. 30, 1942, pp.
1-26.
[15] Krauss, G. and Marder, A. R., Metallurgical Transactions, Vol. 2, 1971,pp. 2343-2357.
[/6] Bunshah, R. F. and Mehl, R. F., Transactions, American Institute of Mining, Metallur-
gical, and Petroleum Engineers, Vol. 197, 1953, pp. 1251-1258.
[/7] Mukherjee, K., Transactions of the Metallurgical Society, American Institute of
Mining, Metallurgical, and Petroleum Engineers, Vol. 242, 1968, pp. 1495-1501.
[18] Aaronson, H. I., Decomposition of Austenite by Diffusional Processes, Interscience
Publishers, New York, 1960, pp. 388-529.
[19] Mehl, R. F. and Hagel, W. C., Progress in Metal Physics, Vol. 6, 1956, pp. 74-134.
[20] Speich, G. R. and Cohen, M., Transactions of the Metallurgical Society, American
Institute of Mining, Metallurgical, and Petroleum Engineers, Vol. 218, 1960, pp.
1050-1059.
[21] Goodman, R. H., Matas, S. I., and Hehemann, R. F., Transactions of the Metallurgical
Society, American Institute of Mining, Metallurgical, and Petroleum Engineers, Vol.
227, 1963, pp. 651--658.
[22] Hehemann, R. F., Kinsman, K. R., and Aaronson, H. I., Metallurgical Transactions,
Vol. 5, 1972, pp. 1077-1084.
[23] Speich, G. R. and Leslie, W. C., Metallurgical Transactions, Vol. 3, 1972, pp. 1043-
1054.
[24] Marder, A. R. and Benscotter, A. O., Transactions Quarterly, American Society for
Metals, Vol. 61, 1968, p. 293.
[25] Grange, R. A., Transactions, American Society for Metals, Vol. 62, 1969, pp. 1024-
1027.
[26] Owen, W. S., Schoen, F. J., and Svrinivasan, G. R.,Phase Transformations, American
Society for Metals, Cleveland, Ohio, 1970, pp. 157-180.
[27] Magee, C. L., Phase Transformations, American Society for Metals, Cleveland, Ohio,
1970, pp. 115-156.

Development of Acoustic Emission Testing

for the Inspection of Gas
Distribution Pipelines

REFERENCE: McElroy, J. W., "Development of Acoustic Emission Testing for

the Inspection of Gas Distribution Pipelines," Monitoring Structural Integrity by
Acoustic Emission, ASTM STP 571, American Society for Testing and Mate-
rials, 1975, pp. 59-79.

ABSTRACT: This paper describes how a utility became interested in acoustic

emission testing for the survey inspection of its buried gas pipelines. Several
weld failm'es emphasized the need to develop inspection techniques to monitor
these pipelines in order to locate weld flaws in high-stress regions to permit
prompt weld repairs and assure system integrity. It was found that a weld must
have a certain metallurgical flaw and must be located in a high-stress region in
order to precipitate an inservice oxyacetylene weld failure in a pipeline. It was
found that metallurgical flaws in high-stress regions can be located by acoustic
emission tests.
This paper describes the field development of the acoustic emission testing
technique and the correlation of the field results with the laboratory results. It
was found that the stress levels imposed by either internal pressurization or by
external loading in the field tests are sufficient to cause critical flaws to emit and
be located. Then replacement or reparation of the critical flaws located in this
manner would restore the integrity of the pipeline. Discussed in the paper are the
other nondestructive correlations such as radiography and correlations through
tension tests, hydrostatic tests, bend tests, and fatigue tests.

KEY WORDS: acoustics, emission, pipelines, nondestructive tests, crack prop-

agation, gas pipes, residual stress

Between the mid-nineteen-teens and mid-nineteen-fifties, approxi-

mately 500 miles of steel pipe, 4 in. in diameter and larger, were installed by
oxyacetylene joint welding techniques in what is now the Philadelphia
Electric Company's gas distribution system (Fig. 1). This is approximately
Research engineer, Research Division, Philadelphia Electric Co., Philadelphia, Pa.
19101.

59
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Copyright9 1975byby ASTM Intemational
Downloaded/printed www.astm.org
University of Washington (University of Washington) pursuant to License Agreement. No further reproductions authorized.
60 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

FIG. 1--1911 style of oxyacetylene welding.

20 percent of the total steel mains in the system. Though the quality of many
of these welds would be questioned by today's standards, the incidence of
failure has been small, averaging about one a year for the past seven or eight
years. These failures have emphasized the need to develop an inspection
technique to monitor these pipelines for the purpose of locating weld flaws
in high-stress regions.

Pipelines Stresses
To better understand the mechanism of failure, a study of the stresses
associated with underground gas pipelines was conducted. The major
stresses are residual, long-term cyclic from changes in ground temperature
and short-term cyclic from heavy vehicular traffic. Some residual stress is
introduced into a pipeline at the time of installation. Over a period of years,
the residual stress is increased by soil settlement or movement. The general
range of residual stress was found to be from 3000 to 20 000 psi oriented in
random directions. The thermal stresses during a mild winter can be ex-
pected to contribute 5000 psi tension, whereas a severe winter contributes
7200 psi. The maximum stress recorded for fully loaded five axle sand
trucks over smooth pavement was 1000 psi coaxial with the pipe at the top
side. Rough pavement was found to increase the magnitude of stress up to
200 percent.

Fatigue Tests
Pipe sections containing a weld were removed from selected locations on
the older oxyacetylene welded pipe systems. Some were cut into test
specimens. Prepared test coupons and whole pipe sections were tested to
failure on fatigue life testing machines imposing stresses of the magnitude
found to exist in high-stress regions of the system. Examination of the
fracture surface from fatigue life test specimens showed similar patterns
to field weld failure specimen we had examined. We concluded that the
field weld failures were due to fatigue.
The fatigue process occurs in three stages. 2 The first stage is crack
initiation. This complex stage includes the period in which microstructural
changes occur. Crack propagation is the second stage. The crack grows
with each stress cycle leaving a path of transcrystalline structure (Fig. 2).
Eventually the net section becomes too small to support the load, and an
almost instantaneous failure occurs leaving a path ofintercrystalline struc-
ture (Fig. 2).
Since the transcrystalline structure was the only structure that sustained
cyclic loading, Fig. 3 plots the fatigue life versus transcrystalline area. It
would seem reasonable to assume that those data points in the figure that
fall within a narrow band around the mean would be suitable for distribu-
tion service. However, those that fall below the general pattern would be of
a critical nature, and these welds should be considered unacceptable.
Critical metallurgical flaws causing zero cycle failures were identified as
pre-existing cracks. These cracks were found by means of radiographic and
destructive examination of the fracture face. These welds were observed to
be well into the second stage of fatigue, about to enter the third.
Since acoustic emission (AE) testing can detect the first stage of fatigue
and warn of impending failure, it was decided to run a field test to see if it
could be adapted for use as a survey method.

Acoustic Emission Field Tests

First Field Test

The first test was conducted on a 1000-ft section of 16-in.-diameter line.
Six 30-kHz transducers were placed on the pipe at 180-ft intervals. The
emissions were monitored as the pressure in the line was increased from 0
to 100 psi with nitrogen pressure. Figure 4 shows a block diagram represen-
tative of the equipment used in monitoring the pipeline. After being
amplified through the transducer preamplifier-amplifier arrangement, the

2 Mason, S. A. and Hirschberg, M. H., The Rote o f Ductility, Tensile Strength and
Fracture Toughness in Fatigue, FranklinInstitutePublication,PermagonPress, Philadelphia,
Pa.

z
-4
0
z
Q

..q
C

7"
Q
W__

W
-r

0
C

t~
0
7"

FIG. 2 - - T e n s i l e c o u p o n s s h o w i n g t r a n s c r y s t a l l i n e a n d i n t e r c r y s t a l l i n e f r a c t u r e areas.

FIG. 3--4?ouponfatigue test results.

emission signals are processed and sent to the time analysis computer.
Here the time difference of arrival of the emission signal at adjacent
transducers is computed and memorized for locational analysis at the
conclusion of the test. a
This test showed that neither traffic noise, nor noise of gas flowing during
pressurization, would interfere with AE monitoring. Signal attenuation
was considerably higher in this pipe than expected; however, signal trans-
mission appeared adequate to survey sufficiently long sections during a
3 Parry, D. L., Industrial Application of Acoustic Emission Analysis Nondestructive Test-
ing Technology, Exxon Nuclear Company, Richland, Wash., Jan. 1974.

D1gltal PJanual
Mag/~oe Input
Acoustic I
Transducers I [-?
Printer
[] i ~ Dlsplay
gltal I I--"1
cation ~
D
D e.t>b
A~ollfie~ ~ ~
Oiscr1~ nator
Ploter
3
IIII * ' Chart
Illl Dlsplay
Visual
Scan
System

~. Magnettc
Signal Conditioningand TapeRecord
AnalystsCircuitry Storage

FIG. 4----Block diagram of the monitoring equipment used in the field.

single pressure cycle to make the technique feasible. It was found that
previous operating pressures must be exceeded in order to produce suffi-
cient emissions.
Several prime emitters were located in the test section (Fig. 5). Two were
excavated and found to be welds. One was a butt weld of two sections of
straight pipe, and the other was a miter joint weld. Only the butt weld could
be cut into tensile coupons. Several of these coupons, when tested in a
tensile machine, were found to fail at the loading condition defined in the
fatigue tests and, therefore, could be considered failures at zero cycles
(Fig. 3).

Second Field Test

The second test was run on a section of 4-in. pipe about 2000 ft long. This
pipe section was instrumented with eleven transducers spaced about 200 ft
apart. Two types of tests were conducted, the first being a heavy vehicle

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MCELROY ON INSPECTION OF GAS DISTRIBUTION PIPELINES 65

FIG. 5--AT amplitude data between two transducers for the pressure test during Field
Test 1.

experiment where a heavy truck was driven down the road with one set of
wheels kept as nearly as possible directly over the pipe. The purpose of
using the heavy vehicle was to determine if it would be possible to induce a
stress in the pipe by external mechanical loading from earth shift. This
induced stress would be more representative of the stresses that cause
failures experienced in the field.
The second phase of this test was conducted using pneumatic pressure as
the stressing medium. As with the first test, bottled nitrogen was used to
raise the pressure to 100 psi.
The AE activity during the pneumatic test was very low and did not
indicate significant crack growth at any point in the line. Only on a bridge
(200-ft section of test) did the emission levels reach significance; however,
the noise caused by the wind masked the emission sites. Signals were also
obtained by stressing the pipe by traversing the underground section of the
pipeline with the heavy vehicle. Figure 6 shows the data between two
transducers for this test.
Although significant crack growth was not indicated, three welds pro-
ducing some emission activity during pneumatic loading and one producing
activity during heavy vehicle loading were excavated and the welded pipe
sections removed from the line. Three welded sections were chosen at
random from the bridge section. All seven of these weld specimens were
fatigue tested under the loading conditions defined earlier.
In confirmation of the field test results, which indicated that no signifi-
cant flaws existed in the underground section of the test, the number of
cycles required to fail the four welds tested was above the average (Points
1, 2, 3, and 4A on Fig. 7). Visual examination of the specimens (Fig. 8, 2B),
reveals little or no corrosion after more than 50 years of service and more
than 90 percent weld penetration.

FIG. 6---AT amplitude data between two transducers for the heavy vehicle test during Field
Test 2.

FIG. 7---Fatigue data for whole pipe specimens from Field Test 2 and Field Test 3.

0
z
"7

g,
0
z

0
z

FIG. 8--Fracture areas Jkom two specimens f r o m Field Test 2.

The three welds from the bridge fractured below the average number of
cycles (Points 4, 7, and 8 on Fig. 7) correlating with the significant emission
levels observed. Figure 8 (7B) shows the fracture area for one of these
bridge specimens. It can be seen that specimen 7B had 20 percent weld
penetration.
Third Field Test
A third test was run on 1600 ft of 12-in. steel main. Fourteen sensors were
located along the line. Again, both pneumatic and heavy vehicle tests were
run. The level of emission activity for all tests run on this section of line was
low. However, two sources from the pneumatic test (Points 1 and 3 of Fig.
7), two from the truck test (Points 5 and 6), one from both (Point 2), and one
for control (Point 4) were chosen for excavation. Upon excavation, it was
found that all six sources were located at welds. All of these welds were cut
out and fatigue tested.
The results of the fatigue testing correspond with the results found in the
field. The level of emission activity in the field was in general low, and the
specimens performed well in the fatigue tests (Fig. 7). All the specimens
identified by emission activity during the field test failed at a much lower
number of cycles than the control specimen, which produced no emissions
during the field test.

Fourth Field Test

A fourth test was run on 6600 fl of 12-in. gas line. This line was tested in
two sections, each about 3300 ft long. The test differed from previous tests
in that a small hole probe was used (Fig. 9) for remote attachment of the
sensor to the line. This eliminated the need for digging a hole large enough
to allow a man to instrument the pipe directly.
Again both pneumatic and heavy vehicle stressing was used. A total of 35
emission sites were detected and located. Six of the most significant were
excavated and found to be at welds. Selection of the most significant is
made by comparing the number of events occurring at a given location on
the pipeline to the base average number of events occurring along the
pipeline. If the number of events occurring at that location is many times
the base average number of events the emission site can be considered to be
significant (Figs. 5 and 6). Two control welds from this line were also
removed for further testing. Six of the eight welds were removed, capped,
and prepared for hydrostatic testing. The two remaining whole pipe sec-
tions were stressed by bending to a maximum stress of 2000 psi.
The failure pressures of the six correlate with the emission activity
observed during the field test (Fig. 10). Those specimens which were
observed as significant failed at much lower pressures than the control
welds. The two specimens involved in the bend test did not fail.

FIG. 9--Small hole probes.

Conclusions of the Field Tests

AE testing to locate faulty welds appears to be feasible. The hoop
stresses involved in pneumatic stressing of lines appear to be high enough
to cause movement of critical flaws in the oxyacetylene welds and to
produce emissions. However, the compression and tensile stresses in-
duced by heavy vehicle stressing appear to be adequate to cause the more
critical flaws to emit. Figure 7 shows that except for the welds from the
bridge (Points 4, 7, and 8) on Test 2, the weld detected by the truck (Point 3)
failed at a number of cycles less than those indicated by the pressure test.
Figure 7 shows that the welds indicated by the truck in Test 3 (Points 5 and
6) again failed at a lower number of cycles than the ones indicated by the
pressure test. In Test 4, Fig. 10 shows that the welds indicated by the truck
(Points 1 and 5) failed at lower pressures than the welds indicated by the
pressure. In addition to its economic advantage, heavy vehicle loading
appears to be the most advantageous stressing medium due to the kind of
stressing involved.
Laboratory Results
The specimens from Test 4 were monitored for emissions during destruc-
tive tests in the laboratory. The purpose of these tests was to see if there

FIG. lO----Failure data for whole pipe specimens from Field Test 4.

was a relationship between weld emission activity under typical field

stressing conditions and the ultimate strength of the piping. Five 215-kHz
transducers were used during the hydrostatic tests. Three were placed
circumferentially around the weld to locate emission sites, and the remain-
ing two were placed equidistant from the weld as coincidence monitors to
assure that the emissions were coming from the vicinity of the weld (see
Fig. l la). In conjunction with the preamps, amplifiers, filters, and dis-
criminatory functions, a AT digital printer and two X - Y recorders were
used.

TRANSDUCER LOCATIONS
04

~
2 3
0 0 0 CTION
WELD SEAM

05
BOTTOM TOP BOTTOM

FIG. 1 la--Sketch of transducer locations.

Figure 1lb shows the AT amplitude at 600 psi along the weld seams of the
six specimens used during the hydrostatic testing. The most significant
growth can be seen in Specimen 1 which produced the most emission
activity during the field test by heavy vehicle loading. The least emission
activity occurred in Specimen 7, a control specimen. Specimen failures
occurred at locations predicted by their AE activity (Table 1). Specimen 1
failed at the weld at - 4 0 / ~ s between transducers 1 and 2 and +95 /zs
between 2 and 3 (10/~s = l in. along the weld seam and the 0 reading is
midway between the transducers). Specimen 2 failed off the weld at +50/~s
between transducers 1 and 2. Specimen 3 failed off the weld at +20/.r
between transducers 1 and 2. Specimen 5 failed at the weld at +20/~s
between transducers 2 and 3. Specimen 6 failed off the weld - 5 0 p~s
between transducers 2 and 3. Specimen 7 failed off the weld at +70/.~s
between transducers 2 and 3. It can be seen that there is a correlation
between emission sites and failure locations.
A correlation can also be seen between the radiographs and AE data from
the welds. Figure 12 compares the radiograph interpretation and the AE
interpretation along the weld seam of Specimen 1. Wherever a crack
existed on the radiograph, a large AT amplitude was also located. This
figure shows that lack of penetration is not as critical a flaw as an existing
crack. Only under a severe lack of penetration did any other AT amplitude
build up.
Figure 13 exhibits the event counts for all the specimens versus pressure
during the hydrostatic tests. In general, at a given pressure the weaker
welds produce more counts than the stronger welds. At 600 psi this
assumption is true for all the specimens. It should be noted in this figure
that the emission sites located during the heavy vehicle field test failed first
in the weld. The emission sites located during the pressure field test did not
fail in weld but in corrosion pits. This indicates that pressure tests may not

z
0

='4
e"

0
e-
..q

o)
0
z

FIG. 1 lb--AtT amplitudes along the weld seams f o r specimens f r o m Field Test 4.

Field Results Laboratory Results

Field Emission Activity Most Significant

% of Base Average Event Count Emission Site Failure Site Failure Characteristics
Speci- Number of Emissions at Typical Along Weld Along Weld
men Field Stress Seam Between Seam Between Pressure,
No. Heavy Vehicle Pressure Type of Stress Level AT,/xs Transducers AT,/xs Transducers Location psi
810 95 hoop at 1580 psi 33 +95 2 and 3 +95 2 and 3 at weld 600
',,4 -40 1 and 2
310 375 hoop at 1580 psi 2 +50 1 and 2 +50 1 and 2 off weld 2000
150 275 hoop at1580 psi 12 +20 1 and 2 +20 1 and 2 off weld 3700
500 285 hoop at 1580 psi 20 +20 2 and 3 +20 2 and 3 at weld 1750
50 50 hoop at1580 psi 4 -73 2 and 3 -50 2 and 3 off weld 7700
50 50 hoop at 1580 psi 12 none (control) +70 2 and 3 off weld 8500
on 250 flexuralatl000psi 484 -7 1 and 2 no failure no failure
bridge
not
stressed 350 flexural at 1000 psi 789 +90 1 and 2 no failure no failure

FIG. 12---Comparison between radiograph indications and acoustic emission indications.

create sufficient field stress on the pipe to cause all weld flaws to emit AE's.
It also suggests that the flexural stresses induced by heavy vehicle loading
causes significant emission from weld flaws.
Table 1 shows the emission activity at typical field stress levels for both
field testing and laboratory testing. The emission activity in the field is
described as a percentage of the base average number of emissions occur-
ring along the pipeline. The emission activity in the laboratory is in the form
of event count. If the field emission activity from the pressure test is used to
determine which emission site is most critical, Specimen I or 2, the wrong
decision could be made. The emission site corresponding to Specimen 1
had 95 percent of the base average number of emissions occurring along the
pipeline, and the emission site corresponding to Specimen 2 had 375 per-

FIG. 13--Total counts as a function o f pressure for specimens from Field Test 4.

cent. The decision made from the pressure test would be that Specimen 2is
more critical than Specimen 1. However, if the hevy vehicle field data
were used, the 810 percent from Specimen 1 would be judged more critical
than the 310 percent from Specimen 2 which is in agreement with the failure
data. The cause of this discrepancy may be the stress involved. Pressuriza-
tion creates hoop stresses that tend to be in the same direction as the plane
in which the critical flaws lie. Heavy vehicle loading creates stresses
perpendicular to the plane in which critical flaws lie.

In additional to the six welds tested hydrostatically, two were tested by

bending. The same monitoring equipment with the same sensitivities was
used in this portion of the laboratory tests. The pipe section was supported
at the two ends and loaded vertically at the weld. The purpose of these tests
was to determine the effect of hoop stresses and flexural stresses on the
generation of AE's. Table 1 compares the event counts at the field stress
level for both types of stress.
The maximum pipe stress recorded in earlier heavy vehicle loading tests
was 1000 psi. The magnitude of this flexural stress on a pipe will vary
depending upon type of road, depth of pipe, temperature, etc. A flexural
stress range of 0 to 1000 psi was applied to Pipes 8 and 9. The pressure range
in a pressurization field test is 0 to 100 psi. Therefore, hoop stresses from
the pressure range 0 to 100 psi (0 to I580 psi hoop stress) were applied to
Pipes 1,2, 3, 5, 6, and 7. The advantage for using flexural stresses is obvious
from looking at the event count at field stress levels in Table 1. This
advantage is confirmed by comparing the field emission activity with the
event count obtained in the laboratory tests (at the field stress level). More
significantly, these data also show that the AE field activity results can be
used to locate weak welds with a high degree of accuracy.
If heavy vehicle loading were to be used in the field, a study of the Kaiser
effect would be necessary. The Kaiser effect is the immediately irreversible
characteristic of AE phenomenon resulting from an applied stress. The
effect results in little or no AE until previously applied stress levels are
exceeded. A heavy vehicle can only stress a given point on the pipe once
during a run, and this stressing cycle takes place in a short period of time.
According to the Kaiser effect, it is only during the first stressing that
significant emissions are produced. This is very undesirable since the
stressing rate during AE monitoring should be sufficiently slow to allow the
detection of sufficient emissions for flaw site location. This procedure
allows for a buildup of emission events at a given AT location.
There is evidence, however, that despite limitations caused by the
Kaiser effect, repetitive heavy vehicle loading continues to cause emis-
sions in weld flaws. The plot of pressure versus emission count rate, Fig. 14
(bottom) confirms the Kaiser effect for a line pressure test. This is expected
since pressurization allows growth. Figure 14 (top), load versus emission
count rate, shows that the Kaiser effect does not appear to be valid for a
second run of the same bend test. This fact allows the heavy vehicle to
repeat its runs over the pipe to allow a buildup of emission events at critical
regions. Figures 15a and b are prints of radiographs of welds found in the
field. The weld shown in Fig. 15a did not emit during the heavy vehicle field
test, but the weld shown in Fig. 15b produced emissions with each run of
the heavy vehicle loading.

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MCELROY ON INSPECTION OF GAS DISTRIBUTION PIPELINES 77

FIG. 14----Comparison of the Kaiser effect for two different types of stressing.

FIG. 15----Prints of radiographs.

FIG 15---Continued.

Conclusions
Several oxyacetylene weld failures have emphasized the need to develop
inspection techniques to monitor the welded gas mains on our system in
order to locate weld flaws in high-stress regions to permit prompt weld
repairs and assure system integrity. Failures occur due to a combination of
stresses, flaws, and material characteristics which were present in the
pipeline when it was constructed, and static and dynamic stress conditions
which occur after construction. It was found that a weld must have a certain
metallurgical flaw and must be located in a high-stress region in order to
precipitate inservice oxyacetylene weld failure in a pipeline.
AE testing was found to be a feasible technique to locate the flaws,
usually existing cracks. Cracks were found to emit more frequently than
any other flaw in oxyacetylene welds during field and laboratorytests (Fig.
12). The stressing means preferred in AE field tests is the bending moment
caused by the heavy vehicle loading. There may be certain areas where this
loading may be impossible; however, pneumatic pressurization can be

substituted satisfactorily. By removing the critical flaw located by AE

testing, the integrity of the gas distribution system can be reconfirmed.
Acknowledgments
I would like to thank Exxon Nuclear for the field work they performed. I
also acknowledge Trodyne and NDT International for the necessary acous-
tic emission equipment for the laboratory work. I would also like to thank
R. W. Whitesel for his assistance and his encouragement to the project.

Evaluating the Stability of Geologic

Structures Using Acoustic Emission

REFERENCE: Hardy, H. R., Jr., "Evaluating the Stability of Geologic Struc-

Pares Using Acoustic Emission," Monitoring Structural Integrity by Acoustic
Emission, ASTM STP 571, American Society for Testing and Materials,
1975, pp. 80-106.
ABSTRACT: This paper describes the application of acoustic emission to
the study of geologic structures. Evaluation of the overall mechanical stabil-
ity of large-scale geologic structures such as underground and open-pit
mines, highway and waterway cuts, petroleum reservoirs, and underground
gas storage reservoirs is extremely complex. Acoustic emission appears to
be one of the most suitable techniques available for such purposes.
A brief review of the associated literature is presented along with a
description of a mobile monitoring facility developed by the writer for field
use. Two current field projects, associated with underground gas storage
and longwall coal mining, presently underway by the writer are described.
KEY WORDS: acoustics, emission, microseisms, rock mechanics, mining,
gas storage, slopes, hydrofracturing, geologic structures, rocks, tests, stabil-
ity, rock bursts

In the discipline of rock mechanics the major effort relative to the

application of acoustic emission (AE) 2 techniques has been associated
with field studies on geologic structures, that is, structures composed
of, and located in, geologic materials. Unfortunately until recently
(circa 1960) results of such studies have been of limited value. An
earlier paper by the writer [1] 3 describes in considerable depth the

1 Professor of mining engineering and director, Rock Mechanics Laboratory, Depart-

ment of Mineral Engineering, College of Earth and Mineral Sciences, The Pennsylvania
State University, University Park, Pa. 16802.
The terms microseismic activity, rock noise, seismo-acoustic activity, subaudible
noise, elastic shocks, and micro-earthquake activity are also utilized by workers in
various geologically oriented disciplines such as mining, civil engineering, etc.
3 The italic numbers in brackets refer to the list of references appended to this paper.

80
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HARDY ON STABILITY OF GEOLOGIC STRUCTURES 81

general application of acoustic emission techniques in rock mechanics

(material behavior, model tests, and field studies) and includes an
extensive literature review (some 90 references) on this subject. The
present paper will concentrate mainly on acoustic emission field studies
carried out since 1965, and in particular those involved in the evalua-
tion of the stability of geologic field structures. The application of AE
techniques in the study of such structures will first be briefly reviewed;
development of a suitable mobile monitoring facility will then be de-
scribed, and finally the preliminary results of two current Penn State
AE field studies will be discussed.
Review-of Acoustic Emission Field Studies
Historically, AE studies associated with geologic materials were in-
itiated in order to evaluate the stability of underground mining opera-
tions and as a method for predicting the occurrence of violent under-
ground disturbances such as rock and coal bursts. During the late
1930's and early 1940's Obert and Duvall [2--6] showed that, in the
laboratory as well as in the field, the AE rate increased greatly as the
specimen or structure became more highly loaded. Conversely, as
equilibrium was reached, after a structural failure or a reduction in the
applied load, the rate decreased. In other words, the AE rate appeared
to be a factor indicative of the degree of instability of the structure.
With the exception of a few other isolated basic studies, the early work
of Obert and Duvall has provided the basis for the majority of the
geologically oriented AE field studies carried out in North America. A
recent paper by the author [1] discusses in some detail a number of the
early AE field studies. In this selection a number of the current appli-
cations of AE in the evaluation of the stability of geologic structures
will be described.

Underground Mining Applications

In the late 1930's and early 1940's government agencies both in the
United States and Canada became involved in AE studies related to
underground mining. At about the same time similar studies became
active in Europe and Asia. These programs were initiated as a result of
difficulties experienced in mining at increasing depths or in highly
stressed zones, the most spectacular of these being the sudden violent
failure of mine structures known as rock bursts [7]. Such studies
continued in a relatively routine and uneventful manner until the early
1960's when more sophisticated techniques for monitoring underground
AE activity were investigated, and in particular techniques for accurate
source location were developed. During this period Cook [8] developed

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82 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

a refined monitoring system for use in the Witwatersrand gold mining

area. The system was capable of recording the outputs of up to 16
sensors on magnetic tape for a continuous period of 25 h. Normally
eight sensors were utilized each being connected into two channels of
the recording system, the sensitivities of which differed by a factor of
30. In this way events having a wide range of energies could be
recorded. As a preliminary investigation had indicated that a large
proportion of the AE energy in the mining area under study occurred in
the frequency range 20 to 50 Hz, the frequency response of the overall
monitoring system was restricted to approximately 15 to 300 Hz. In
terms of monitoring facilities developed more recently, such a system
would be termed "narrowband."
In order to determine source locations underground it is necessary to
know the velocity of propagation in the associated material. Cook
determined this by detonating two or three pounds of explosive at
known locations and monitoring the arrival of the resulting stress
waves at each of the sensors. Velocities determined in this manner
were found to be accurate to within ___5 percent, and it was estimated
that the detonation locations could be determined (using the recorded
data) to an accuracy of -+ 10 ft.
The later work of Blake [9,10], Blake and Duvall [11], and Blake and
Leighton [12,13] are of particular interest since their monitoring
facilities were "wideband" compared to most earlier instrumentation.
The system developed by Blake and Leighton [12] was designed to
have a flat frequency response in the range of 20 to 10 000 Hz. It is
interesting to note these authors state that "the frequencies generated
by rock noise contain many high-frequency components." In contrast,
Cook [8] indicated that the majority of the events occurred in the
frequency range 20 to 50 Hz. This seeming disagreement is further
evidence of our limited appreciation of the overall frequency spectrum
involved in acoustic emission phenomena associated with geologic
materials.
Blake and Leighton [12] utilized commercially available piezoelectric
accelerometers as AE sensors. In use these were cemented to the walls
of boreholes drilled in various underground locations. The output of
each sensor was connected to a preamplifier located in the borehole
itself, the signal from which was transmitted by cable to a post
amplifier and to one channel of a 7-channel FM magnetic tape recorder
located at a central monitoring location. Using an array of at least five
sensors, studies have been carried out in a number of hard rock mines.
Data recorded on magnetic tape were processed by re-recording it on a
multichannel oscillograph to determine a series of travel time differ-
ences. These data, along with propagation velocity data obtained in the

mine, in a manner similar to that described by Cook earlier, were used to

calculate source locations [13]. They estimate the accuracy of such
locations to be within _+I0 ft. Blake and Leighton concluded that, in
hard rock mines, broadband monitoring provides much more quantita-
tive information about the behavior of a rock structure than can be
obtained using traditional narrowband facilities. It is their opinion that
"when used regularly by experienced personnel, it can become a valu-
able engineering tool in detecting, delineating and estimating the stabil-
ity of potential failure zones in rock structures."
Although a number of AE studies are underway in Europe, the
majority of these have been associated with coal mines. Exceptions are
those in Sweden 4 and early studies in East Germany [14].
To date relatively few AE studies have been conducted in North
American coal mines partly due to the fact that most active mines are
relatively shallow (approximately 500 ft and hence do not suffer from
high-stress conditions), and due to the existence of strict laws as-
sociated with the use of electrical equipment in such mines. At present
however, two coal oriented projects are in progress supported by the
U. S. Bureau of Mines. The first is an in-house project 5 underway in a
rock burst prone Rocky Mountain coal mine south of Denver, Col-
orado. Here sections of the mine have been instrumented with velocity
sensitive AE sensors (geophones). Monitoring facilities located outside
the mine are similar to those used earlier by Blake and Leighton [12] in
hard rock mine studies with the exception that the system is operated
narrowband (90 to 180 Hz). Results to date have been very encouraging
in that it has been possible to define in advance potential zones of
instability.
The second coal mine project is being carded out by the writer as
part of a research project sponsored by the U. S. Bureau of Mines.
Studies are underway in a longwall coal mine in central Pennsylvania to
investigate the feasibility of using AE techniques to locate potential
zones of instability around coal mine workings. This field study in-
volves monitoring the AE generated by working mines during their
normal operation using surface sensors located in shallow boreholes
positioned over the working area of the mine. This study is unique in
the fact that measurements are made from the surface rather than
underground. This approach provides several advantages, including the
fact that there are no electrical limitations on the monitoring system, and
that the study will in no way interfere with normal mine operations. A more
detailed discussion on this project is included later in this paper.

4 H. Helfrich, Terratest AB, Bromma, Sweden, personal communication.

5 F. Leighton, U. S. Bureau of Mines, Denver, Colo., personal communication.

In contrast to most North American coal mines, many European coal

mines are relatively deep (approximately 2000 ft) and therefore suffer
from high stress conditions, and in many cases frequent rock bursts. At
present extensive underground AE studies are underway in Poland
[15-20], Czechoslovakia [21], and Russia [22]. Limited underground AE
studies are underway in West Germany [23]; however, an extensive
study of coal mine rock bursts in the southern Ruhr valley (Bochum
area) is presently underway at the Ruhr University 6 using surface
mounted transducers. When completed, their system will involve a
three station array with distances between stations of the order of
miles. Each station contains displacement sensors mounted in three
orthogonal directions. Sensors, with resonant frequencies of the order
of 2 Hz, are utilized and AE events are recorded on magnetic tape at
each station. Arrangements for a radio or telephone data link between
stations is presently under consideration.
Surface Mining Applications
In the last few years research in the field of slope stability associated
with open pit mining has increased rapidly. AE techniques appear to
provide a useful tool for monitoring slope stability, and research by the
U. S. Bureau of Mines has contributed a great deal to the development
of this technique. The design and installation of AE monitoring equip-
ment for slope stability studies presents a number of unique problems
which are discussed by Broadbent and Armstrong in a recent paper
[24].
Paulsen et al [25] have utilized AE techniques to study slope stability
in an open-pit mine at Boron, California. They sum up the situation by
stating that a plot of the AE with time provides a graphic picture of
what is going on in the Boron open-pit. An increase in activity over
and above the normal background probably indicates that a potential
slope stability problem exists. A decrease in activity indicates stabiliza-
tion may be being achieved, whereas an accelerating activity rate indi-
cates failure may be imminent.
During a recent study at Kennecott's Kimbley pit, near Ruth,
Nevada, extensive AE studies were undertaken as part of the routine
monitoring of the pit slope stability during slope steepening [26]. In this
study AE sensors were installed inside two adits (horizontal shafts)
driven into the pit wall, as well as in the pit wall itself. Cables from
these were connected to a mobile monitoring facility located on the
surface behind the pit slope. Due to the high ambient noise generated
by the mining operation itself, measurements were restricted to the
6 H. Baule and A Cete, Geophysics Institute, Ruhr University, Bochum, Germany,
personal communication.

times between shifts and on weekends when mining facilities were

inactive. During the Kimbley study the pit slope was steepened from
about 45 to about 60 deg. Although the new slope was considered to be
a stable one, AE studies were included in the test program as a safety
measure as well as to investigate the correlation, if any, with slope
angle. During slope steepening, changes in AE rate were erratic but
appeared to be related to the development of temporary stress concen-
trations occurring during mining. Following completion of the 60-deg
slope the AE rate dropped to a low value indicating the new slope
configuration was stable.
Petroleum and Natural Gas Applications
AE has wide field application in the petroleum and natural gas
industry. The study of hydrofracturing being one application of consider-
able importance. Here fluids are injected under pressure into low per-
meability strata with the purpose of fracturing these strata, and increas-
ing their permeability and porosity. Such techniques are commonly
used to stimulate a poorly producing oil or gas well or to increase the
capacity of an underground gas storage area. Aside from information
obtained from surface monitoring of injection pressure and volume,
observation well measurements, and examination of rock core drilled
after the hydrofracturing, little is really known in regard to the fractur-
ing process that is going on perhaps 5000 to 10 000 ft below surface.
A number of workers are giving consideration to the utilization of AE
techniques to monitor the initiation and propagation of underground
fractures associated with hydrofracturing. For example, studies pres-
ently underway by Overbey and Pasini 7 are concerned with the de-
velopment of techniques for determining the location and orientation of
underground fractures developed during hydrofracturing.
At present the writer is directing a project [27] which involves the
use of AE techniques to study the stability of underground gas storage
reservoirs (basically zones of porous rock surrounded by impermeable
cap rock). This project is supported by the Pipeline Research Commit-
tee of the American Gas Association, and will be discussed in detail
later in this paper. Additional studies are also under consideration in
which the stability of large cavities located in salt, and commonly
utilized for storage of pressurized gases, will be investigated using
similar techniques.
Civil Engineering Applications
AE techniques appear to be gaining increased attention in civil en-
gineering oriented projects. For example one of the earlier applications
7 W. K. Overbey and J. Pasini HI, U. S. Bureau of Mines, Morgantown, W. Va.,
personal communication.

of this technique was that of Crandell [28] who employed it as a safety

monitor for use in tunneling projects. More recently Beard [29] lists a
number of tunneling projects where simple AE monitoring devices have
been used with great success. Consideration has been given to using
the technique for investigating leakage of water reservoirs and the
stability of earth filled dams. s
Cadman et al [30] describe an automated monitoring system for
studying landslides. Goodman and Blake [31-33] have investigated slope
stability associated with potential landslide areas and unstable highway
cuts. They determined that there was a definite correlation between the
estimated state of slope stability and the observed AE rate. They also
noted that no AE's were observed in rock cliffs and steep rock cuts
subjected to frequent rock falls. According to Goodman and Blake [33],
the observed activity in soft landslide materials was found to be in the
audio frequency range and appeared to originate within a distance of
100 ft from the AE transducer. Furthermore their studies indicated that
the source of AE events could be located in rockslide areas, but such
location was not practical in soft landslide areas, due to the extreme
attenuation of high-frequency signals and spatial variation of propaga-
tion velocity inherent in such materials.
Other civil engineering problems such as those associated with the
underground disposal of radioactive wastes, acid mine water, and other
undesirable liquids by injection into deep boreholes (drilled to depths
well below the water table) may well be investigated using AE techniques.

Other Applications
In recent years geophysicists concerned with the prediction and
causes of earthquakes have become increasingly interested in the study
of AE [34,35], since it is felt that the background pattern of AE may
provide important information relative to future earthquakes.
Consideration has also been given recently to the use of AE
techniques in a variety of other geologically oriented applications in-
cluding investigation of the stability of glaciers, snow avalanche warn-
ing systems, flame front location in underground coal gasification
studies, and stability of underground compressed air storage facilities.
Penn State Research Program
Since 1970 the Rock Mechanics Laboratory at The Pennsylvania
State University has been involved in AE studies associated with the

a R. M. Koerner and A. E. Lord, Drexel University, Philadelphia, Pa., personal

comminication.

evaluation of stability in geologic structures. To date the major field

effort has been concentrated on two projects, namely:
1. Study of underground gas storage reservoirs, with the purpose of
developing techniques for evaluation of the mechanical stability of such
structures as a function of storage pressure and duration.
2. Study of the feasibility of using surface mounted AE sensors for
investigating the mechanical stability of underground coal mines.
Both projects are related directly to the economics and safety interests
of the gas and coal industry, and as such are particularly appropriate in
light of the current energy shortage.
Before discussing these projects and the associated instrumentation it
is important to briefly consider the frequency spectrum of anticipated
AE from large geologic structures. The writer has shown in an earlier
paper [l] that when dealing with geologic materials AE's in the fre-
quency range of at least 10 Hz to 500 kHz have been observed;
however, as the distance from the AE source to the point of detection
increases, this range decreases dramatically. The dominant frequency
spectra depends of course on the overall sensitivity of the monitoring
system, and the attenuation characteristics of the media; however, field
studies by other workers suggest that for distances on the order of a
few thousand feet relatively little energy would be expected in the
frequency range above l0 kHz. However, the monitoring system de-
veloped at Penn State was designed to cover a wider range, making it
possible to initially scan the overall frequency range in order to select
the most suitable frequency band for final measurements.

Monitoring Facilities
In order to carry out field studies at gas storage and coal mine sites,
it has been necessary to develop a suitable mobile monitoring facility.
Figure 1 illustrates a block diagram of this facility. The electronic
system is housed in a large air conditioned camper van and has been
designed to operate from ll0 VAC line voltage, an associated motor
generator, or a d-c battery supply. The battery supply and motor
generator are located in a small trailer unit; the monitoring system,
therefore, can be operated completely independent of commercial
power, and studies may be carried out in remote locations.
The monitoring system has facilities for simultaneously recording the
output of 14 AE sensors (S). During recording, the output of each
sensor is amplified by a preamplifier (PR) followed by a post amplifier
(A). The resulting signal then passes through a filter unit (F) and to one
channel of the tape recorder. The tape recorder has been equipped with
both direct and FM electronics so that signals from dc to 600 kHz may

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88 M O N I T O R I N G STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

TAPE RECORDER
SANGAMO-3614

"" -- r-- Ps-,v Ps~ ~, I~ '61 III

--
as- t ..
i
J1...,c*"l.._'..'_l--.L.r m
L)i-
~ k-f_
III
i i i
T ~91 . . . . . J. . . . J: [ - = ' ~ '~1
; ; ,J_ l , L~oOltoc,tvoict I
IMOTORGENERATOR1 r-I/OvA.-C------§ I' 1
[ , - - - - - , I _.a._ ~sv,~ , _1 ~ 4 |/ l
I _ ! - - C

T.,.. o.,* - o_.

J 2 2 0 6 P - 207 ~ D-3
S -ROREHOLE SENSOR ( D-4
PR- PREAMPLIFIER (ITHACO 144L) J D-5
1
A- POSTAMPLIFIER(][THACO 454) J
F- FILTER (ROCKLAND I100) L- ps_1~r
D- DRIVER AMPLIFIER(HONEYWELL liE)
PS- POWER SUPPLIES A/El/C/D- POWER SWITCHES
MP- METER PANEL RC-RECORD INPUTS
RA- RADIO(SPECIFICPRODUCTSWVTR) PS- PLAYBACK OUTPUTS
IX:L- OIGITALCLOCK(NEUTRONIC 2SO) -~--PATCH PANiaL
VS - VOLTAGE SENSING CIRCUIT ( ~ - POWER CONNECTOR

FIG. 1--Block diagram of Penn State acoustic emission field monitoring facility.

be recorded and played back if necessary. It should be noted that in

most studies, however, filters have been set to reject frequencies above
10 kHz. In recent tests this limit has been reduced to 2.5 kHz and
lower.
The facility was designed to monitor and record field data for later
detailed analysis; however, a visicorder-type ultraviolet recorder has
been incorporated in the facility to provide visual display and prelimi-
nary analysis of data in the field. A timing unit, standardized against
the U. S. Bureau of Standards radio station WWV, is also included in
the monitoring system. This unit provides continuous coded time sig-
nals to one channel of the tape recorder so that the time of occurrence
of all AE events will be accurately known.
Figure 2 shows an overall view of the monitoring facility. The main
electronics are mounted in an antivibration rack, as shown in Fig. 3,

FIG. 2--Mobile monitoring facility.

located in the van. The preamplifier units are normally located close to
the sensors. Considerable effort was expended to develop a highly
reliable facility. Since an initial two to three month break-in period,
during which a number of refinements were introduced, the facility has
been essentially trouble-free. A detailed report describing the design
and development of the mobile monitoring facility is available [36].

Sensors and Installation Techniques

Figure 4 illustrates various methods investigated for mounting AE
sensors at different field sites. During early studies at gas storage sites
sensors were attached directly to wellhead facilities. Results from these
studies were inconclusive, and further investigation of this potentially
useful and inexpensive technique is planned for the future. In general
surface mounting of sensors is the least satisfactory due to the high
attenuation of the surface media (usually soil). Type D installation
utilizing a borehole probe has proven to be the most successful; how-
ever, it is unfortunately also very expensive. Type B installation in
which sensors are buried in shallow holes has had limited success;
however, a modification of this installation, Type C, in which sensors
are cemented at the bottom of 15 to 20-ft-deep boreholes drilled
through the soil into bedrock, has proved more satisfactory.
At present, two basic types of sensors are utilized--geophones 9 (vel-
ocity gages) and accelerometers. ~~ The latter are utilized in the
g Geospace Type GSC-IID (Marsh Case), Model M-4, resonant frequency: 14.0 _+ 0.5
Hz, frequency response: 30 to 3000 Hz (relatively flat).
i0 Endevco, Model 2219E, mounted resonant frequency: 16 000 Hz -+ l0 percent,
frequency response (-+2 percent): 2 to 3000 Hz.

FIG. 3----Overall view of monitoring electronics.

borehole probe and were used early in the gas storage study directly on
the wellhead surface facilities. Geophones are utilized in all installa-
tions where sensors are buried or cemented in holes. Figure 5 shows a
typical geophone unit mounted in a waterproof marsh-type case. These
units have been extremely reliable and a number mounted permanently
in extremely wet ground have preformed reliably now for a period of
over one year.
Figure 6 illustrates the principle of the borehole probe. It contains a
rubber-walled inflation chamber which can be expanded by externally
applied gas pressure. An AE sensor (accelerometer) located inside the
chamber is bolted through the rubber wall to a sensor shoe on the
outside surface. In use, the probe is located at the desired location in
the test borehole and then pressurized. This forces the sensor shoe
tightly against the borehole wall, effectively clamping the attached
accelerometer in position. The borehole probe has been used with great

///;",I,7 _1~~ IGEOPHONE

I
SPIKE
' ///4' /, .:::
/ i-;7 H

i .' ,: :....;
/ :~ -'.
(A) SURFACE MOUNTING
I :% .:]'

(B) SHALLOWBURIAL

/ ///// ~; i r l i , 5 / I / / /

~ A N D
I , ".". / ,, LED
9:. : /

(C) DEEPBURIAL
"--I~ /
~176176176
(D) BOREHOLE PROBE
FIG.4--Various methods o f mounting acoustic emission sensors at field sites ( S =
sensor, P A = preamplifier, and J B = junction box).

success at a number of locations, including one test site where it was

used at the bottom of a 300-ft borehole containing 150 ft or more of
water. In recent installations a support rod, attached to the lower end
of the probe (see Fig. 6) and bearing on the bottom of the borehole, has
been utilized to support the probe. This arrangement eliminates any
tendency for the probe to slip vertically in the hole after pressurization.

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92 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

FIG. 5--I~'pical geophone (velociO' gage) installed in marsh-type case.

Figure 7 shows two views of the Mark II model borehole probe de-
veloped for use in BX (2 a/a in. diameter) diamond drillholes.

Analysis Techniques
As has been noted earlier the prime purpose of the mobile monitoring
facility is to record field data for later analysis. To date the majority of
such analysis has been done "manually." Figures 8a and b illustrate
the system used. Initial editing of the field data is carried out as shown
in Fig. 8a. Here data recorded on Tape Recorder 1 in the field are
played back through amplifiers and filters into either an ultraviolet re-
corder, Tape Recorder 2 or both. Such play-back may be accomplished
for up to seven channels simultaneously. The ultraviolet recorder pro-
vides permanent chart records (hard copy) of the field data which may
be examined for the presence of specific types of AE events or in order
to study ambient background characteristics. At this stage in the
analysis the tape fottage of each feature of interest is logged for future
reference. Normally when operating directly into the ultraviolet re-
corder the replay tape speed is increased considerably over the original
recording speed in order to compress the recording in time and thus
provide a convenient length of hard copy. When a set of AE events of
particular interest are noted on the hard copy then the appropriate
sections of tape are replayed at different speeds and at various filter
settings in order to study these events in more detail. For example, this
arrangement has been utilized to carry out simple frequency analysis of
specific signals by replaying these for a number of filter settings and

IS SENSOR
-- I V-I N=
PA- PREAMPLIFIER i V_2F~ 'u1
N2 - NITROGEN GAS SUPPLY

PRESSURE ~
L(0-200
oo-rpu.r PSI)
PR E ( ~ U ~ R E G U L A T O R
i
I

II
LINE REEL~:ToPRESSUREINFLATING'[~ILiINE i v-4~
v-SVENT INFLATING UNIT
I
UNIT
: NYLONCABLE
9=P---INSERTION ROD ELECTRICAL CABLE
TO MONITORING FACILITY

\ \ \ \ ~ ,
\
\ \ \ \ N "% "% "% "%'~ CABLE REEL ANYLON
\
\
\ \ BOREHOLE ~J F ~- ~ CABLE

~
\
\ \
\

,,~[,
,, ~,k RUBBER ~AC~ET
\ ,-'lU
-I1["
!- SENSOR SHOE BOREHOLE
\
.~1[- INFLATIONCHAMBER

PROBE
t .Jr_ J-

FIG 6---Principle of operation and installation of acoustic emission probe.

measuring the change in amplitude of the signal on the ultraviolet

recordings.
The circuit in Fig. 8a also provides a convenient means of actually
"editing" the original field data so that only specific sections are
re-recorded on Tape Recorder 2 for further anlysis. For example sec-
tions of data containing signals due to known culteral noise (for exam-
ple, trucks and cars, mine machinery, electrical transients, etc.) may be
eliminated. Such editing is a prerequisite for later manual or computer
analysis.
For more detailed analysis of field data the system illustrated in Fig.
8b is employed. Here field data re-recorded on Tape Recorder 2 during

FIG. 7---Mark H model acoustic emission borehole probe (support rod and associated
adapter not shown). (a) Probe a s s e m b l e d ready for use. (b) Borehole probe dissembled
showing component parts (1 = rubber bladder, 2 = probe body, 3 = connecting rod
yoke, 4 = pipe adaptor, 5 = probe end section, 6 = preamplifier, 7 = plug, 8 = plug nut,
9 = bladder clamp nut, 10 = bladder sleeve, II = a c c e l e r o m e t e r , and 12 = sensor shoe).

editing may be analyzed by a variety of techniques. It should be noted

that until recently (when Tape Recorder 2 was acquired) the output of
Tape Recorder 1 was played directly into the analysis system without
prior editing. The manual analysis system incorporates the following: (a)
a transient recorder, so that selected AE signals may be "captured"
and recorded on the x-y plotter; (b) a real-time frequency analyzer, so
that either individual AE signals, groups of such signals, or ambient
background signals may be analyzed for frequency content and the
resulting frequency spectra recorded on the x-y plotter; and (c) a digital
counter and associated printer, so that the total number of acoustic
events above a specific magnitude (defined by trig level set), or the rate
of occurrence of such events during a specific interval may be re-
corded. It should be noted that the manual analysis system allows only
one channel of data to be processed at a time.
At present a computer based analysis system, utilizing the Penn State
Hybrid computer facility, is under development. A simple block dia-
gram of this system is shown in Fig. 8,:. Here edited field data (7
channels) is played directly into the analog section of the computer,

i ~ORDER~AMPt~F,LTr"
TAPE I ~

TAPE J
RECORDERI
NQ 2 J
A. EDITING CIRCUIT

. •TRANSIENT
RECORDER I - ~ I
X-Y
I RECORDERI-DtAMPJ-HFILTp"
TAPE I , - ~ /REAL T'ME.I
, - - - ,
~'1 FREQUENCYt "-"
I I P'~
NO. 2 |ANALYZER J

COUNTERH PRINTERI
B. MANUAL ANALYSIS SYSTEM

TAPE
IR ECOR DE R
NO. 2 COMPUTER~ A/D i .1TA~E I
/

DIGITAL L JJCONTROL~ P R O G R A M
COMPUTERr
1 "1
LI
CRT J
1 JLINE J
"1PRINTER I
C. HYBRID COMPUTERANALYSIS SYSTEM
FIG. &--Block diagrams of data analysis systems.

analog processing (if necessary) is carried out, and the resulting signals
are digitized and placed on digital tape within the computer. This
digitized data may then be processed in a variety of ways, and the
results displayed on a cathode ray tube (CRT), line printer, or other
output device. To date, programs for interfacing the field data to the
system, and for frequency analysis of selected sections of data have
been developed and "de-bugged." At present a program for amplitude
distribution analysis is under development.
Underground Gas Storage Project
Since 1966, a continuing research program, supported by the Ameri-
can Gas A s s o c i a t i o n (AGA), has b e e n u n d e r w a y by the R o c k
Mechanics Laboratory at Penn State. This program is concerned with
the optimization of pressures in underground gas storage reservoirs,
namely, development of analytical and experimental techniques for
evaluating the mechanical stability of such structures. Prior to 1970,
this program mainly involved analytical and laboratory model studies.
AE techniques were employed first in the laboratory studies in order to
define initial and ultimate failure of the test models. For this purpose
sensors, consisting of semiconductor strain gages bonded to small brass
plates, were attached to the models prior to jacketing and testing under
triaxial loading conditions. Details of the laboratory test procedures and
results obtained have been described in a number of recent papers
[27,37-40].
Field oriented studies in which the feasbility of using AE techniques
to monitor the stability of full-scale underground gas storage reservoirs
were initiated in 1970. Field measurements were first carried out in the
summer of 1972 at a gas storage reservoir in northern Pennsylvania.
These have continued along with field studies initiated at a northern
Michigan storage site in the spring of 1973. Figure 9 illustrates dia-
grammatically an instrumented gas storage field site. Here, a number of
AE sensors installed in boreholes and shallow surface holes are used to
monitor acoustic activity in the storage reservoir area. By suitable
analysis of the AE data, it is hoped to determine: (a) degree of overall
reservoir stability and (b) location and intensity of any major sources of
instability. The areal extent and depth of most reservoirs, as well as the
intrinsic AE background level, introduce a number of experimental
difficulties. Many of the studies carried out to date have been as-
sociated with the d e v e l o p m e n t and modification of experimental
techniques to overcome these difficulties and ensure that meaningful
field data are obtained.
Since the summer of 1973 an extensive study has been underway at

I1 ~- BORE HOLE
Itl
".. ", imam
CASED ~ .~ \\ I
BOREHOLE ~ \ | 1]~

cAP ~oc~ ~ ~- ' ~ "

~ . 9 . ~"".:."~":"~".'...... . "~ X

FIG. 9-4nstrumented gas storage field site.

the Michigan field site. During this study, AE signals were detected by
a borehole probe, shallow burial geophones, and geophones cemented
into boreholes drilled 20 ft deep into bed rock. The monitoring facilities
and techniques described earlier in this section were utilized. The main
object of these studies was to study in detail the signals originating
from the reservoir while it was being pressurized to full capacity.
Injection was carded out over a two week period in September. During
the injection period, AE measurements were made at some 20 test
periods ranging from 25 to 165 min in length; following this, measure-
ments were made at intervals of one week and three weeks. Such
measurements will continue at intervals of approximately one month
through the spring of 1974.
To date, only very preliminary analysis has been carried out on the
recent Michigan data; however, a number of low frequency AE signals
were positively identified. Figure 10 illustrates the expanded form of
one such signal, monitored by the borehole probe located at a depth of
approximately 300 ft, showing both P- and S-wave components. The
low frequency character of the signal (approximately 20 Hz) is evident
from the figure. Considering only data monitored by the borehole

Z
0

i - , , i .....
t-
9 It A,
J
I

V V -- V ~IU r 9 9 9 9 I~ I l l 9 II L ~ 9

i ~, I I' |i I
I Id qMI I I i

:,'I ' .... ,[ ' ' ;

I P-WAVE -- - ~ " ' S-WAVE 9
- - I
_ _ !
O
C

N
=,,
FIG. lO.--Typical low frequency acoustic emission signal detected at the Michigan gas
storage site (recorded using an accelerometer in a horehole probe and monitoring system o
z
bandwidth 0]'0 to 2.5 kHz).

probe, it is estimated that, at the relatively low resolution used in the

preliminary analysis, background activity was of the order of 5 to 6
events per hour. Following major increases in wellhead pressure, par-
ticularly at pressure levels of the order of 1000 psi, AE rates were
found to jump to as high as 50 events per hour. When the reservoir
pressure became stable either after a pressure rise or drop, the AE rate
fell to a value close to original background level (approximately 5 to 6
events per hour). More detailed analysis of these data will be carried
out during the next few months.
In general, the field studies carried out to date (particularly those at
the Michigan test site) indicate that detectable AE signals are generated
from pressurized underground gas storage reservoirs. Those signals
positively identified have been of relatively low frequency (=20 Hz)
which is realistic when it is considered that transducer to reservoir
distances are on the order of a few thousand feet. The major problem
in the analysis of field data is associated with the presence of a strong
background noise level.
Longwall Coal Mine Project
Since 1970, a detailed field program, supported by the U. S. Bureau
of Mines, associated with the m e a s u r e m e n t of AE generated by
longwall coal mining operations has been underway by the Penn State
Rock Mechanics Laboratory. The monitoring facilities described earlier
in this section have been employed. Figure 11 illustrates the sensor

J U l_ILlL_I~_IU l_Jl_~LJ~_ll_1U U L.I U L J L J W U I IUL.IUL

oo
O0
O00EDt:~ QCnC )rTDOQ~dl C)O00 ~ D C7t:200000
nn nmn n n m ml nnnnm n n m m nn
/-'e'.
MONITORING--/--5 L SCALE d
FACILITY / r 400' -i
I 9 -SURFACE GEOPHONES I
-- MINING DIRECTION
9- ~

FIG. I l----Sensor array over a Iongwall coal mine in central Pennsylvania (distances
between transducers 1 to 4 are approximately 125 ft).

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100 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

array utilized in a recent study over a central Pennsylvania coal mine.

In this figure the crosshatched section is the mined area, and the
position of the longwall (area being mined) is shown by the heavy line.
A variety of sensor installations, similar to those discussed earlier (see
Fig. 4), have been employed during the coal mine project; however, in
the study illustrated here, only surface mounted geophones (see Fig.
4a) were used. Figure 12 illustrates AE signals detected by the five
sensors during a selected 60 s period. It is interesting to note that Sen-
sors 2 and 3 are directly over the area being mined and exhibit
maximum activity; Sensor 4 is over the unmined area (solid coal);
Sensor 1 is over the edge of the mining area. Sensor 5 is approximately

FIG. 12--Acoustic emission signals detected during a 60 s period over a longwall coal
mine (recorded using geophones (velocity gages) and monitoring system bandwidth 0 to
2.5 kHz).

300 ft from the mining area, over a development area mined considera-
bly earlier, and exhibits minimum activity.
During these measurments, filter limits in the monitoring system
were set at 0 to 2.5 kHz. Figure 13 illustrates a number of typical AE
signals detected during a 2 h monitoring period. In Fig. 13a the near
coincidence of events M, N, and O indicate that they are a result of the

. . . . . . . . . . t . . . . . . . . t..................... :..

. . . . . M

2- ,; -. ..... :rI.~.,:T-_T- 3:; _--~--..:~_.! i

-'-- 1' "
I

1212_~.- ~ " - " -r

...........................................

. . . . O! . . . . . . . ? ..................
II . . . . . . I . . . . . . . . . . ' .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . _ L _ - ......

i . . . . . . . . t ........... t . . . . . . t .... ~ ...... i

(A) (B)

I..i~ 05 S E C - - - ~
, t-t-i" . . . . . . . . I~, , ,~i , , n-~J~l
i4, i'i"i- I . I'
!I1!!
i:.it
~I l" ill l t Iii~J-~!llltti
ITi'-/1 IFTilfilillilri~
I I I..Ii Iltlll!ltJ[llll~
Iltti IIIIit!11!!11111
~ ! I -i.I .~:tlll~iillll!l
: : ' : r
H4 IttttJtt,,,,,,
l ! ! i_t
~[ i ' l i l i l l i ' , , ,

:l I i':LI:t 1I.M:i,LId_~;'~
~i~ :ii~
(D) (E)
(C)
FIG. 13--Typical acoustic emission signals observed over a Iongwall coal mine (re-
corded using geophones (velocity gages) and monitoring system bandwidth 0 to 2.5 kHz).

same underground instability. Figure 13b shows the same events with
data played back at a slower tape speed in order to display the detailed
character of the signals. Figures 13c, d, and e illustrate a number of
other typical AE signals observed during the study.
A simple f r e q u e n c y analysis study (as described earlier under
"analysis techniques") was carried out on a number of AE signals and
on sections of typical background. Results of this study are shown in
Fig. 14. It is apparent that the background covers a wide frequency
range. In contrast however, typical AE events have their major energy
in the range 160 to 560 Hz with a pronounced peak around 320 Hz.
Magnetic tapes containing the field data from the study were also
processed using sections of the manual analysis system shown in Fig.
8b. In particular, the number of signals on each monitoring channel
having amplitudes well above the general background (defined by trig-
ger level set) were counted and recorded on a digital printer. This
analysis was carried out for a series of 10 min intervals over the total
test period. The results are shown graphically in Fig. 15. The fact that

1.0 I I I I I I I I | I I I I I I

0.8 ::::::::::::::::::::::
0.6

0.4
z

o ' i I t t ' I 1 1 t I t I
I I I I i I i I I I

0.8

0.6

0.4

0,2
* I

0 1 1 1 1 I t I I i I i J I I
400 560 720 880 1040 1200
FREQUENCY - Hz

FIG. 14--~requency spectra o f typical acoustic emission signal (A) and background
(B).

I f I I I I I I | I I I I |

1.~ LOCATION
- 4
20C

0 J t_,_, ,... _
e-~

g . ~ LOCATION
-3
,,=, 40(]
Q.
W
2oc
z

g G
r~
Ir 4oc LOCATION - 2

Z
ta 20C
b.I

IOC LOCATION - I

:i
0
1 t
20
i 1 ] ! I
40 60 80 I00 120
I

MONITORING TIME-MINS
l
LOCATION - 5
I I I 1 I
140

FIG. 15----Rate o f acoustic emission activity at different sensor locations during test
period.

the major AE activity occurs at Locations 2, 3, and 4, immediately

over the mine workings, is further evidence that it is directly associated
with mining. There is no positive explanation for the pronounced peaks
which occur at approximately 50 min. They are probably associated
with mining operations; however, no underground observations were
made during the study.
AE studies associated with longwall coal mining are continuing. To
date, measurements at the mine site have been carried out at four
different locations. A detailed analysis of the data is presently under-
way.
Discussion
This paper has reviewed the use of AE as a tool for evaluating the
stability of geologic structures. Studies by a number of workers have
indicated that this technique has wide potential application in the fields
of mining, petroleum and natural gas, and civil engineering. The de-
velopment, by the writer, of a mobile AE monitoring facility has been
described, and its application to studies of underground gas storage
reservoirs, and longwall coal mines has been discussed.

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104 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

At present a number of factors limit the usefulness of AE in such

applications, namely:
1. Difficulty in separating AE signals from ambient background
noise.
2. Inability in most geologic structures to obtain an equivalent un-
loaded condition.
3. Large dimensions of most geologic structures and resulting at-
tenuation (usually highly frequency dependent) of signals with distance
from source.
4. Electrical and mechanical difficulties in instrumenting such struc-
tures.
5. Difficulty in source location due to anisotropic propagation veloc-
ity characteristics of geologic materials.
In spite of these limitations, AE has proven increasingly useful in
such applications in recent years. With the development of better
monitoring facilities, improved transducers and field installation
techniques, and development of computer based analysis methods, the
successful application of AE techniques to geologic structures should
increase manyfold.
Acknowledgments
The major financial support for the field studies conducted at Penn
State described in this paper has been provided by the Pipeline Re-
search Committee of the American Gas Association (Project PR-12-43),
and the U. S. Bureau of Mines (Projects G0101743 and G0144013).
Development of the mobile facility for field monitoring and AE was
supported jointly by the American Gas Association and the U. S.
Bureau of Mines. Assistance in construction of the facility provided by
members of the Penn State College of Earth and Mineral Sciences MI
Shop is greatly appreciated.
The author would also like to express his thanks for the assistance i
provided by other members of the Mineral Engineering Department at
Penn State, in particular, Dr. A. W. Khair and Dr. R. Y. Kim, research
associates; E. Kimble, research aide; and W. Comeau, M. Gopwani, G.
Mowrey, and L. Beck, graduate assistants.
References
[1] Hardy, H. R., Jr., in Acoustic Emission A S T M STP 503, American Society for
Testing and Materials, pp. 41-83, 1972.
[2l Obert, L., "Measurement of Pressures on Rock Pillars in Underground Mines. Part
I.," RI 3444, U. S. Bureau of Mines, 1939.
[3] Obert, L., "Measurement of Pressures on Rock Pillars in Underground Mines. Part
II," RI 3521, U. S. Bureau of Mines, 1940.
[4] Obert, L., " U s e of Subaudible Noise for Prediction of Rock Bursts," RI 3555, U. S.
Bureau of Mines, 1941.

[5] Obert, L. and Duvall, W. I., " U s e of Subaudible Noise for Prediction of Rock
Bursts. Part If,'" RI 3654, U. S. Bureau of Mines, 1942.
[6] Obert, L. and Duvall, W. I., "'Microseismic Method of Predicting Rock Failure in
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1945.
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Wiley, New York, 1967.
[8] Cook, N. G. W. in Proceedings, Fifth Symposium on Rock Mechanics, Minneapolis,
1962, Pergamon, New York, 1963, pp. 493-516.
[9] Blake, W., Rock Burst Mechanics, Quarterly of Colorado School of Mines, Vol. 67,
No, 1, 1972,
[10] Blake, W., "Rock Burst Research at the Galena Mine, Wallace, Idaho," TPR 39,
U.S. Bureau of Mines, 1971.
[11] Blake, W. and Duvall, W. I., Transactions, Society of Mining Engineers, Vol. 244,
1969, pp. 288-290.
[12] Blake, W. and Leighton, F. in Proceedings, Eleventh Symposium on Rock Mechan-
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gineers, 1970, pp. 429-443.
[13] Leighton, F. and Blake, W., "Rock Noise Source Location Techniques," RI 7432,
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[14] Buchheim, W., "Geophysical Methods for the Study of Rock Pressure in Coal and
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[15] Neyman, B. and Zuberek, W., "Seismoakustische Forschungen zum Gebirgschlag-
problem," Freiberger Forschungshefte C 225, Geophysik, 1967, pp. 44--61.
[16] Neyman, B., Grabis, Z., Trombik, M., and Zuberek, W., "Results of Investigations
in the Bobrek Mine on Pressure and Rock Bursts Using Seismo-acoustic Methods,"
Prace GIG Komunikat nr 464, Katowice, 1969, Slask.
[17] Wierzchowska, Z., "Fundamentals and Methods System for Conducting Microseis-
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Przeglad Gdrniczy, Vol. XXVII, No. 11, 1971, pp. 503-511.
[18] Zuberek, W., "Analysis of the Course of Seismoacoustic Activity in the Case of
Rock Burst Provoked by Shotfiring," Prace GIG Komunikat nr 533, Katowice, 1971.
[19] Neyman, B., Szec6wka, Z., and Zuberek, W., "Effective Methods for Fighting Rock
Bursts in Polish Collieries," Proceedings, Fifth International Strata Control Confer-
ence, London, 1972.
[20] Neyman, B., Sokolowski, H., Trombik, M., and Zuberek, W., "Stationary and
Portable Seismoacoustic Apparatus," Bezpieczehstwo Pracy w Gdrnictwie, 1972, in
press.
[21] Stas, B. et al in Proceedings, Twelfth Symposium on Rock Mechanics, Rolla, 1970,
American Institute of Mining, Metallurgical, and Petroleum Engineers, 1971, pp.
109-119.
[22] Antsyferov, M. S., Ed., Seismo-Acoustic Methods in Mining, Consultants Bureau,
New York, 1966.
[23] Braliner, G., "BeK~impfung der Gebirgsschlaggefahr: Entwicklung yon Betriebsver-
fahren 1963-1971," Gliickauf-Betriefsbiicher, Band 16, Essen, t973, pp. 66-68.
[24] Broadbent, C. D. and Armstrong, C. W. in Proceedings, Fifth Canadian Rock
Mechanics Symposium, Toronto, 1968, Department of Energy, Mines, and Re-
sources, Ottawa, 1969, pp. 91-103.
[25] Paulsen, J. C., Kistler, R. B., and Thomas, L. L., Mining Congress Journal, Vol.
53, 1967, p. 28.
[26] Wisecarver, D. W., Merrill, R. H., and Stateham, R. M., Transactions, Society of
Mining Engineers, Vol. 244, Dec. 1969, pp. 378-385.
[27] Hardy, H. R., Jr., and Khair, A. W., "Applications of Acoustic Emission in the
Evaluation of Underground Gas Storage Reservoir Stability," Proceedings, Ninth
Canadian Symposium on Rock Mechanics, Montreal, 1973, in press.
[28] Crandell, F. J., Journal, Boston Society of Civil Engineers, Jan. 1955, pp. 39-59.

[29] Beard, F. D., Civil Engineering Vol. 32 No. 5, 1962, pp. 50-51.
[30] Cadman, J. D., Goodman, R. E., and Van Alstine, C., "Research on Subaudible
Noise in Landslides," Geotechnical Engineering Report on Investigation, NSF Grant
GK 109, 27 June 1967, Department of Civil Engineering, University of California,
Berkeley,
[31] Goodman, R. E. and Blake, W., "Microseismic Detection of Potential Earth Slumps
and Rock Slides," Report No. MT-64-6, Institute of Engineering Research, Univer-
sity of California, Berkeley, 1964.
[32] Goodman, R. E. and Blake, W., "An Investigation of Rock Noise in Landslides and
Cut Slopes," Felsrnechanik and lngenieurgeologie, Supplement II, 1965.
[33] Goodman, R. E. and Blake, W., "Rock Noise in Landslides and Slope Failures,"
Highway Research Board, 44th annual meeting, Washington, D. C., 1966.
[34] Oliver, J., Ryall, A., Brune, J. N., and Slemmons, D. B., "Micro-Earthquake
Activity Recorded by Portable Seismographs of High Sensitivity," Bulletin, Seis-
mological Society of America, Vol. 56, 1966, p. 899.
[35] Watanabe, M., "The Occurrence of Elastic Shocks During Destruction of Rocks and
Its Relation to the Sequence of Earthquakes," Geophysical Papers Dedicated to
Professor Kenzo Sassa, Kyoto University, Kyoto, Japan, 1963.
[36] Hardy, H. R., Jr., and Kimble, E. J., Jr., "Design and Development of a Mobile
Microseismic Monitoring Facility," Internal Report RML-IR/72-20, Department of
Mineral Engineering, The Pennsylvania State University, 1972.
[37] Hardy, H. R., Jr., et al, A Study to Evaluate the Stability of Underground Gas
Storage Reservoirs, American Gas Association, Inc., Arlington, Va., June 1972.
[38] Hardy, H. R., Jr., Khair, A. W., and Kimble, E. J., Jr., "Application of Model
Studies to the Optimization of Pressures in Underground Gas Storage Reservoirs,"
Presented at Fourth Bi-Annual Gas Technology Symposium, Society of Petroleum.
Engineers, American Institute of Mining, Metallurgical, and Petroleum Engineers,
Omaha, May 1972.
[39] Khair, A. W., "Failure Criteria Applicable to Pressurized Cavities in Geologic
Materials Under In-Situ Stress Conditions," Ph.D. thesis, Department of Mineral
Engineering, The Pennsylvania State University, 1972.
[40] Siskind, D. E., Hardy, H. R., Jr., and Alexander, S. S., International Journal of
Rock Mechanics and Mining Sciences, Vol. 10, March 1973, pp. 133-150.

Acceptance Testing Welded Ammunition

Belt Links Using Acoustic Emission

REFERENCE: Hutton, P. H., "Acceptance Testing Welded Ammunition Belt

Links Using Acoustic Emission," Monitoring Structural Integrity by Acoustic
Emission, ASTM STP 571, American Society for Testing and Materials,
1975, pp. 107-121.
ABSTRACT: The objective of the work described in this paper was to
provide a reliable method for 100 percent acceptance testing resistance
welds used in the fabrication of 40-mm automatic weapon ammunition belt
links. Links contain rounds of ammunition and are coupled together to form
a belt which is automatically fed into the weapon. If one of the links in a
belt fails, the immediate result is a jammed weapon. Four projection resis-
tance welds (two on each side) are used to assemble the link, and weld
quality can vary widely. Conventional nondestructive test techniques cannot
distinguish a sound resistance weld from a weak one in the post-weld
condition. This work has shown, however, that by applying a moderate
proof load to the weld joint, weak welds can be detected by acoustic
emission. The technique is being adapted to production application with an
expected inspection rate of 1200 links per hour in its initial version. This
work is representative of many applications of acoustic emission to measure
component structural integrity that can be accomplished within existing
state-of-the-art technology.
KEY WORDS: acoustics, emission, nondestructive tests, welded joints,
ammunition belt links

" S t r u c t u r a l i n t e g r i t y " is u s u a l l y t h o u g h t o f in the c o n t e x t o f large

c o m p l e x s t r u c t u r e s s u c h as b u i l d i n g s , b r i d g e s , aircraft, etc. It is e q u a l l y
s i g n i f i c a n t , h o w e v e r , w h e n a p p l i e d to a s i n g l e s y s t e m c o m p o n e n t .
T h e s e c o m p o n e n t s are s t r u c t u r e s in t h e m s e l v e s , a n d t h e i r i n t e g r i t y is
vital to s y s t e m o p e r a t i o n . T h i s d i s c u s s i o n c o n c e r n s o n e s u c h c o m p o n e n t .
P r o d u c t i o n o f large q u a n t i t i e s o f i t e m s s u c h as m e c h a n i c a l s y s t e m

1 Senior development engineer, Pacific Northwest Laboratories, Battelle Memorial

Institute, Richland, Wash. 99352.

107
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108 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

components imposes a challenging problem of determining if the

finished components meet quality requirements. If the consequence of
failure of one component is critical, inspection must approach 100 per-
cent. This, of course, introduces the problem of method and cost to
accomplish 100 percent nondestructive inspection.
A specific case in point is links used to make up ammunition belts
for automatic weapons. The particular links of concern in this discus-
sion are for a 40-mm aircraft weapons system. Link fabrication in-
volves making four simultaneous projection resistance welds, and the
integrity of these welds is a primary factor in link structural integrity. If
one of these welds fails in service, often the immediate result is a
jammed weapon. Since many thousands of these links are used, assur-
ance of 100 percent high quality links is a significant problem.
Inspection by destructive testing of random specimens on a batch
basis has been used for want of a better method. Since failure of a
single component (link) is critical in this case, random specimen testing
is not satisfactory. Alternate approaches, however, which provide
higher assurance of integrity are very limited. Conventional nondestruc-
tive testing methods (radiography, ultrasound, eddy current, etc.) are of
little value in measuring the quality of a resistance weld. Mechanical
proof loading is one potential method for 100 percent inspection which
has been tried by the Weapons Command. This met with limited suc-
cess because loads sufficient to fail a weak weld often permanently
distort sound links.
A unique combination of moderate mechanical proof loading with
simultaneous monitoring of acoustic emission (AE) from the link is an
approach which has been successfully demonstrated and is the subject
of this paper.
Background
The AE phenomenon is becoming quite well known, and its use for
nondestructive evaluation of structural integrity is increasing. The
phenomenon p e r s e will not be discussed here other than to point up
that concerted investigation of AE began about 1950 [1-3]. 2 Since that
time, it has been the focal point of considerable effort to better under-
stand the phenomenon and develop specialized instrumentation and
techniques to apply it for nondestructive evaluation of material and
structural integrity [4--8].
A good reference on AE technology through 1971 is provided in the
American Society for Testing and Materials [9]. Also, comprehensive
bibliography of publications on AE is included in this publication.
The italic numbers in brackets refer to the list of references appended to this paper.

The work described in this paper does not represent a major ad-
vancement of AE technology. Its significance lies in the fact that it is
an example of the many specific applications of AE that can be made
within the framework of existing technology.

Link Proof Test by Acoustic Emission

Problem
As mentioned earlier, the weld joint in these ammunition links is the
primary factor in link integrity. Figure 1 shows the link as used in
making up an ammunition belt. The link is fabricated from approxi-
mately 1/32-in.-thick steel strip. The strips are preformed into link
halves which are then joined by two projection welds on each side
where the sections overlap.
All four welds on a link are made simultaneously. With the various
parameters that must be controlled to produce a good resistance weld,
there is significant potential for producing some faulty welds in mass
producing links. The contrast between a good and a bad weld is
presented in Fig. 2. In the good weld, the strips are used to produce a
joint nugget which is usually stronger than the base material in tension,
that is, the base material will tear before the weld will break. With a
poor weld, the joint interface is literally just stuck together. A moder-
ate force, particularly impact, will fail the joint at the interface.
A preliminary feasibility investigation showed that the poor welds
produced more AE under load than did the good welds. This was true
even though the load was insufficient to fail a poor weld. It appears

FIG. 1---Forty m m ammunition belt links.

FIG. 2--Typical good and bad link welds,

that the AE is being generated by preliminary cracking of a weak weld

under proof loading. The decision was thus made to develop a link
proof test which utilized an existing mechanical link tester to stress the
links and use AE analysis to detect poor welds.

Proof Test Development

The mechanical link tester is shown in Fig. 3. The link test fixtures
(Fig. 4) position the link for testing as shown in Fig. 5. A horizontal air
cylinder loads the link weld sections in shear, and the vertical cylinder
adds a bending load through a spreader linkage. This simulates the
service load imposed on the link. The machine is hand loaded and is
capable of testing 20 links per minute.
A block diagram of the laboratory system used to monitor and
analyze AE is presented in Fig. 6. The instruments are shown in Fig. 7.
At the outset, AE was sensed directly from the link by attaching a
sensor to each link before testing. This was the simplest and most
direct method for confirming feasibility. Once feasibility was dem-
onstrated, it was necessary to consider a monitoring technique that
would be compatible with production testing. The first and most crucial
step was to modify the sensing method since attaching a sensor to each
link is obviously not practical for quantity testing. The sensing function
was transferred to the link spreader arms. Piezoelectric crystals were
embedded in the spreader arms as shown in Fig. 8. A recess was
machined in the edge of each arm, and a crystal was mounted in the

FIG. 3---Proof test m a c h i n e 40-ram ammunition links.

recess using a room temperature curing epoxy. Simultaneous monitor-

ing of link emission using these sensors and one mounted on the link
confirmed that the arm sensors do detect essentially all of the informa-
tion available directly from the link. It was concluded that a spreader
load of about 60 lb produced the best results. At a 60 lb spreader force
on the link, the link-spreader arm interface pressure is about 5000 psi
which appears to provide good coupling of acoustic information across
the interface.
The nature of the acoustic information detected is shown in Fig. 9.
The top photo shows the total information obtained during the first 0.3 s
of the spreader cycle for three different links. About the first 0.024 s of
this information is primarily noise. The mechanical process for stress-
ing the links is very noisey. The sliding action of the spreader
mechanism produces a wide frequency range noise which cannot be
electronically filtered out without also filtering out the AE information.
The approach chosen to overcome this problem is to gate out the noise
from moving parts of the link stressing mechanism and monitoring for
AE during static stressing of the link after the spreader arms have
reached full displacement. See also Fig. 10. Testing has shown the

FIG. 5---Link installed for test.

112
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Downloaded/printed by
University of Washington (University of Washington) pursuant to License Agreement. No further reproductions authorized.
z
e-

7
0
Z

c
Z
..~
0
Z

r-
z

FIG. 6 - - L a b o r a t o r y a c o u s t i c e m i s s i o n s y s t e m f o r link p r o o f test.

FIG. 7---Instrument system to analyze acoustic emission.

emission during this period to be indicative of link weld quality. Ini-

tially, the data were analyzed for periods up to 2 s after full displace-
ment of the spreader arms. It was determined subsequently that equally
good results could be obtained With an analysis period of 0.25 s, and
thus increase the process capacity of the AE system.
Evidence of the need for precise timing in this technique for over-
coming noise interference was obtained inadvertently. A microswitch
was installed to reference monitor system turnon to operation of the
spreader arms. Subsequent testing produced rather poor correlation

FIG. 8--Sensor mounted in spreader arm.

FIG. 9----Selection of significant data.

between acoustic information and link weld quality. It was ultimately

determined that the particular microswitch used would close momentar-
ily on initial setdown of the vertical head and trigger the monitor
system about 0.015 s early (Fig. 10). This had the effect of including so
much machine noise that the contrast of information from a good and a
bad link was obscured. This problem was remedied easily by using a
different microswitch orientation.

Results
A form of signal energy analysis has produced the best results in this
work. This consists of electronically integrating for the area under the
half-wave rectified envelope of the emission signal in terms of volts
amplitude and time duration. One unit is chosen to be an area equiva-
lent to 1 V for 50 tzs. In the laboratory development phase, the number
of energy units measured from a given link was totalized with a digital

FIG. lO---Detail acoustic data from link.

counter and recorded manually. Some of the results obtained are given
in Table 1.
The rate of success in identifying links with good welds as opposed
to those with bad welds has been 85 to 90 percent using the laboratory
system. The faulty indications are attributed .to improper triggering of
the monitor system. This has been confirmed in several cases where
the basic data were preserved on magnetic tape so that it could be
reanalyzed. The prototype system with more precise adjustment on the
triggering microswitch is expected to overcome this problem.
Prototypic System
A prototypic system for production application has been fabricated.
As the block diagram of this system (Fig. 11) shows, the prototype is a
simplified version of the laboratory monitor system. The same basic

I--- MONITORPOINTS "I"

1 ACOUSTICEMISSION c
1 _ _ I I I ENERGYANALYZER
I ELECTRONICI ' ~ . ~ - ' ~ DEFFERENTIAL[ .L ~ I AMPLIFIER'FILTER[ .L ['~GATE ] .L I SUr~V,ATIONOUTPUT ---'lb OUTPUTACTUATES Z
PULSER I ~ I PRE,~PL,,ER~ ~ I'--*1 G~,NAD~,'O ~ CLOSEDO.I SEC~ IOVFOR]OOUNffS LINKEJECTIONSYSTEM
[OVOUT ~ ~ GAIN: 250 ~ I T [ FILTER:IOOKHzHit ' A ' [ FULLSCALE 0
Z
' ' ' I ~ T / ADJUSTABLETHRESHOLD
:E
MICROSWITCH I [ / / 0"75-2"0V P'P m
NORMALLY'OliN I J /D E '~
CLOSEDBYARM [ CLOSEDMICROSWITCHCLOSESELECTRONICSWITCHAN GAT
[ SPREADERPLUNGERI OPENMICROSWITCHRESETSSUMMATIONCOUNTERANDOPENSELECTRONICSWITCH
3:
NOTE: 3:
t-
THIS SYSTEMOPERATESIN CONJUNCTIONWITHTHEMECHANICAL z
LINKTESTERFABRICATEDBYTHEARMYWEAPONSCOMMAND.THE -4
MECHANICALTESTERPROVIDESNECESSARYPROOFLOADINGOFTHE
LINK. Z
m
FIG. 1 l---Acoustic emission system for M16A2 link proof test.
e-
Z
p~
u~

",4

TABLE l--Typical link weld proof test results using acoustic emission.

Total
Link Energy
No. Units Weld Condition from Destructive Testing

B-234 17 all good

B-235 81 all good
B-236 198 2 bad inner welds
B-237 551 1 bad outer weld
B-238 90 all good
B-239 33 a l bad outer weld
B-240 743 2 bad welds on one side
B-241 104 1 bad outer weld
B-242 412 ~ all good
B-243 53 all good
B-244 310 2 bad welds on one side
B-245 58 all good
B-246 122 1 medium and 1 bad weld on one side
B-247 76a 1 bad outer weld
B-248 32 all good
B-249 335 2 bad welds on one side
B-250 330 1 bad outer weld
B-251 240 I bad outer weld

a Inconsistent values attributed to improper triggering of analyzer system (assumes

> 100 units denotes a bad weld).
NOTE: Inner welds designate those at the end of the inner overlap section.
Outer welds designate those at the end of the outer overlap section.

analysis functions are retained, but, having established the various

parameter values within a narrow range, the functions can be per-
formed more simply. The analyzer produces a d-c voltage proportional to
the total AE energy measured. The system sensitivity is adjusted so
that a 10 V energy analog output will represent the division point
between a good and a bad link weld. If a given link generates enough
emission to produce a 10 V analyzer output, the link will be rejected. If
the value is less than 10 V, the link will be accepted. The selective
eject function is performed rather easily with the mechanical tester. It
was designed to eject the links into the reject or accept container based
on whether or not the spreader arms could displace the link weld
section sufficiently to contact a limit switch. The limit switch is now
replaced by an electronic switching function which is to be controlled
by the output voltage of the emission analyzer.
Calibration of the monitor system is accomplished by introducing an
artificial signal into the spreader arms where it is detected and pro-
cessed by the monitor system. The most suitable of several methods tried
is to generate a signal in the spreader arms using a transducer installed
in the arms the same as the AE sensors. A 10 V, 10/~s pulse is fed to

FIG. 12--Calibration.

FIG. 13--Prototype acoustic emission monitor for link welds.

these transducers from a pulse generator. The resulting signal as de-

tected by the monitor system is shown in Fig. 12. This is a reproduci-
ble signal which is a reasonable simulation of the real data, and it is
quite simple to generate. The signal can be used to effectively calibrate
system gain, threshold setting, and energy analysis.
The prototypic system is packaged in a standard, 12 wide IM bin
with exception of the calibration pulser and the preamplifier as shown
in Fig. 13. The pulser and preamp are both powered from the IM bin
power supply. The prototype system is now undergoing final testing.
The emission monitoring technique developed can accommodate a
maximum link test rate of about 450 links per minute per station.

Summary
This paper describes the development of a test technique and pro-
totypic equipment to apply AE for evaluating the quality of resistance
welds in 40-mm ammunition belt links. The work is significant in two
respects: (l) AE is the only currently available nondestructive test
method that could perform the needed function and (2) this work is a

specific example of the many beneficial applications of AE that can be

accomplished within existing state-of-the-art technology.

Acknowledgments
T h e a u t h o r w i s h e s to r e c o g n i z e t h e s i g n i f i c a n t c o n t r i b u t i o n o f J. F.
D a w s o n , T. J. D a v i s , a n d G. D. S h e a r e r to t h e w o r k d i s c u s s e d . This
w o r k w a s s p o n s o r e d b y t h e U . S. A r m y W e a p o n s C o m m a n d , R o c k
Island Arsenal.

References
[1] Mason, W. P., McSkimin, J. H., and Shockley, W., Physical Review, Vol. 73, No.
10, May 1948, pp. 1213-1214.
[2] Kaiser, J., "Untersnchnngen iiber das Auftreten Gerauschen Beim Zugversuch,"
Ph.D. thesis, Technische Hoschule, Munchen, t953, pp. 43--45.
[3] Schofieid, B. H., Bareiss, R. A., and Kyrala, A. A., "Acoustic Emission Under
Applied Stress," Technical Report 58-194, ASTIA Document No. AD155674, Wright
Air Development Center (USE), 30 April 1958.
[4] Hutton, P. H., "Acoustic Emission Monitoring for Continuous Crack Detection in
Nuclear Reactor Pressure Boundaries," BNWL-1597, Battelle-Northwest, Richland,
Wash., Nov. 1971.
[5] Jolly, W. D., "The Use of Acoustic Emission as a Weld Quality Monitor," BNWL-
SA-2727, presented at the American Society of Mechanical Engineers 24th Annual
Petroleum Engineering Conference, Tulsa, Okla., 21-25 Sept. 1969.
[6] Romrell, D. M., "Acoustic Emission Monitors Crack Growth in Ceramics,"
BNWL-SA-3064, presented at the 1970 Spring Conference of the American Society
of Nondestructive Testing, Los Angeles, Calif., 10 March 1970.
[7] Parry, D. L., "Acoustic Integrity Analysis in the Oil and Chemical Industry," Jersey
Nuclear Co., the 37th Midyear Meeting of the American Petroleum Institute's
Division of Refining, 9 May 1972.
[8] "In-Service Inspection Program for Nuclear Reactor Vessels," SwRI Project 17-
2440, Biannual Progress Report No. 6, Vol. I, Southwest Research Institute, San
Antonio, Tex., Jan. 1972.
[9] Acoustic Emission, ASTM STP 505, American Society for Testing and Materials,
May 1972.

Industrial Use of Acoustic Emission

for Nondestructive Testing

REFERENCE: Drouillard, T. F., Liptai, R. G., a n d T a t r o , C. A.,

"Industrial Use of Acoustic Emission for Nondestructive Testing," Monitoring
Structural Integrity by Acoustic Emission, ASTM STP 571, American Soci-
ety for Testing and Materials, 1975, pp. 122-149.

ABSTRACT: The industrial use of acoustic emission is presented. Emphasis

is given to the characteristics of acoustic emission as a unique nondestruc-
tive testing method for use in the evaluation of the structural integrity of
materials, components, and engineering structures. Several applications of
acoustic emission testing are discussed.

KEY WORDS: acoustics, emission, nondestructive tests, pressure vessels,

crack propagation, plastic deformation, phase transformations

The use of acoustic emission (AE) to characterize and evaluate a

material or engineering structure under load is exciting much interest in
the scientific and engineering communities. It is one of the first nonde-
structive testing (NDT) methods to provide a means of evaluating struc-
tural integrity by the detection of active flaws that may ultimately
cause failure of the material or structure. Sources of AE which gener-
ate stress waves in material include local dynamic movements, such as
the initiation and propagation of cracks, twinning, slip or plastic defor-
mation, sudden reorientation of grain boundaries, bubble formation
during boiling, or martensitic phase transformations. The stresses in a
metallic system may be well below the elastic design limit, and yet the
region near a flaw or crack tip may undergo plastic deformation and
fracture from locally high stresses, ultimately resulting in premature or
catastrophic failure under service conditions. The industrial use of AE,
1 Senior engineer, Rocky Flats Division, Dow Chemical U.S.A., Golden, Colo. 80401.
Staff, Directors Office, Energy and Resource Planning, and Head, Mechanical En-
gineering Materials Test and Evaluation Section, respectively, Lawrence Livermore
Laboratory, University of California, Livermore, Calif. 93550.

122
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DROUILLARD ET AL ON NONDESTRUCTIVE TESTING 123

particularly as applied to the assessment of structural integrity, could

lead to great economies in testing high-performance components with
the added advantage that the serious flaw cannot be missed, since it
would be a principal contributor to an AE record.
Figure 1 is a graphic illustration of examples of various uses of AE
and how the technique has been used increasingly since 1966 to assess
structural integrity. Although the figure is not meant to be totally
inclusive of all applications of AE techniques, it does indicate the many
facets of application of the technology and also the increasing uses of
AE. Most uses, of course, involve plasticity and fracture of materials.
This paper will discuss the industrial use of AE for NDT quality
assurance programs. Also discussed will be some of the concepts and
considerations important in applying AE and some of the problems
associated with introducing AE into a production atmosphere. Various
examples will be presented along with some information about how to
effectively use AE on the production line. This discussion will address
itself to the testing of relatively small engineering structures and pres-
sure vessels and to testing situations where real-time interpretation and
evaluation of test data are requisite.

New Methods for Nondestructive Testing

Have you ever thought, " I f I only had it to do over again." The
authors feel that many of us are having that second chance with AE.
Some 20 years ago many of us had the experience of pioneering in the
field of ultrasonic testing. Equipment was crude and always seemed
inadequate for the job. There were no experts or sources of authority
on the applications of ultrasonics. Learning was primarily by cross
correlating ultrasonic results with radiography, then the backbone of
NDT, and by actually probing suspect areas and exposing defects or
discontinuities.
In the past few years AE has emerged from a well researched
technology to a promising new tool for NDT. Much like ultrasonics,
AE started out with equipment limitations. Those of us who were
working in the field before commercial equipment was available had to
build our own systems from component instruments that were designed
for other purposes. These systems were limited in capability. Noise
was our worst enemy. Probably most all of us can look back at our
early test results and wonder what was real emission and what was
noise. Early equipment was of the vacuum tube type which limited
frequency and gain with a tradeoff for noise. Today solid-state elec-
tronics has virtually eliminated instrument limitations. A number of
instrument manufacturers provide a wide variety of commercially avail-

1972

TENSILE TESTS
1970 PRESSUREVESSELTESTS
FRACTURE MECHANICSTESTS
STRUCTURALEVALUATIONS
FLAW LOCATION
1SS8 ~ F I E L D ENVIRONMENTS
INSTRUMENTATION
INNOVATIONS
STRESSCORROSIONTESTING
FILAMENTARY COMPOSITES
TENSILE TESTS PRODUCTIONMONITORING
PRESSUREVESSELTESTS COMPOSITESHELL
FRACTUREMECHANICSTESTS STRUCTURES
STRUCTURALEVALUATIONS PHASETRANSFORMATION
FIELD ENVIRONMENTS DETECTION
TRANSDUCERINNOVATIONS STRESSRUPTURETESTING
INSTRUMENTATIONINNOVATIONS PIGMA WELD MONITORING
STRESSCORROSIONTESTING ELECTRIC RESISTANCE
FILAMENTARYCOMPOSITES WELD MONITORING
PRODUCTIONMONITORING WELD MATERIAL
COMPOSITESHELL STRUCTURES QUALIFICATION
PHASE TRANSFORMATIONDETECTION LASERGLASS DAMAGE
STRESSRUPTURETESTING THRESHOLD
ONS VERY SMALL PARTS
QUALITY ASSURANCE
LASER WELD MONITORING
LASER SYSTEMINTEGRITY
ELECTRON BEAM WELD
MONITORING
SCREWTHREAD FAILURE
PREDICTION
HIGH EXPLOSIVECRACKING
TUBE PINCH~0FF

FIG. 1--Growth in the use o f acoustic emission techniques for the materials research
and assessment of structural integrity.

able equipment. Technical services are available from many well qual-
ified people, some pioneers in the field of AE. A valuable source of
information is the Acoustic Emission Working Group (AEWG) meet-
ings which provide a forum for the exchange of information and infor-
mal discussion of current work in the field of AE. Also there is a
wealth of technical information in the open literature. Included in this
publication is a bibliography of most all of the literature on AE pub-
lished during 1970 through 1972. These provide the NDT engineer with
almost unlimited resources in equipment, technical assistance, and in-
formation for the application of AE as a tool for NDT.
The determination of stress inten~ty by AE monitoring has been
readily demonstrated, and the correlation between acoustic emission
and stress intensity factor is clear [I-5]. 3 Much effort has been expended
in the last few years in attempting to ensure the safe operation of
high-performance components. Meticulous searching for flaws by con-
ventional NDT techniques is conducted on these components, and,
once located, additional time is expended determining the size and
shape of the flaw. Then an analysis is made using pertinent information

3 The italic numbers in brackets refer to the list of references appended to this paper.

such as defect size, shape, and location pertaining to the local stress
intensity. These findings are compared with the critical stress intensity
factor for the material, derived from specialized specimen testing, and
with material properties, stress levels, and geometry to reach a decision
on whether or not the part or structure can be used safely.
Application of A E testing could lead to great savings in evaluating
components. AE emission is not an approximation, but rather it is the
actual detection of fracture events as they o c c u r under known applied
stress conditions. Many discontinuities presently considered to be un-
acceptable defects based on ultrasonic and radiographic standards have
little or no effect on structural integrity. The added consideration and
advantage of A E testing is that a serious flaw cannot be overlooked,
regardless of its size, since it is indeed an active or unstable flaw and is
therefore a principal contributor to the A E record. After all, a discon-
tinuity is a defect only if it can grow and eventually cause failure. If a
defect grows and gives off AE, when properly instrumented that emis-
sion can be detected.

Basic Concepts of Acoustic Emission

The basis for most conventional N D T methods is the detection of a
perturbation in the interaction o f a defect or a discontinuity with a
directed beam of some sort of energy such as X or gamma radiation,
ultrasound, thermal energy, microwaves, and so forth. Similarly, A E
involves the interaction of a defect and energy that is used to interro-
gate the integrity of a material or structure, the energy being a strain
field. H e r e the energy or strain field interacts with the defect itself;
hence, the defect acts as a meter of information about itself, assessing
its own part in the evaluation of structural integrity of the material or
component. In A E the defect plays a very active role rather than the
passive role it usually plays during evaluation by conventional N D T
methods. In other words, if the defect is large enough or oriented in
such a way as to be affected by the strain energy during the loading of
a material or a structure the defect will play an active role in heralding
its own abuse. If the defect is not affected by the strain field, it will not
be an active emitter o f AE; hence, the defect is in a stable condition
and will not affect the structural integrity of the material being tested.
Herein lies a new concept in NDT. Thus, AE is a dynamic test, limited
to the detection of an active flaw during a change in the stress field
around the flaw. It cannot detect static flaws. This is why A E must be
applied during tests which stress the material or structure, for exam-
pie, p r o o f testing of a pressure vessel and tension or bend testing of a
weld joint. So in a sense, an AE technique requires that a defect act as

its own assessor in that only active crack growth or crack initiation
mechanisms will be sources of AE. Those which do not interact in such
a manner as to generate AE might not be or probably are not important
to the structural integrity of the material.
AE detection is nondirectional. Most defects act as point source
emitters, radiating acoustic energy in spherical wavefronts. Thus, a
sensor can be placed most anyplace on a structure and detect emissions
produced anywhere in that structure. This is converse to other methods
of NDT which depend on directing a beam of energy through a pre-
scribed path in the part under test. Consequently, by using AE, a
structure can be monitored to detect emission producing events, in
many cases, without prior knowledge of their existence or location.
Because AE data describe volumetric deformation and fracture proces-
ses, we have perhaps one of the most useful tools to study, in real-
time, dynamic processes and local transient instabilities.
AE is an extremely sensitive test method. To give some idea of its
sensitivity, relative to the more familiar NDT methods, the minimum
detectable crack size for ultrasonics, radiography, and eddy-current
techniques is about 10 -3 in.; for strain gages, about 10-6; and for AE,
about 10 -12 in. [6]. Thus, the dynamic range of sensitivity to events
that can be detected by most basic commercially available AE systems
extends from gross events that produce audible signals to microevents
such as dislocation movements.

Application to 100 Percent Inspection

In recent years, much effort has been expended in developing 100
percent N D T inspection procedures. Whole industries have evolved to
meet the demands of scanning 100 percent of the surface of test parts
with the narrow energy beams required to locate small flaws. Although
various combined efforts have met with considerable success, testing of
this type is very expensive, and few consumer products can be in-
spected with this degree of thoroughness. AE testing can quite often be
used in production line situations where 100 percent inspection is desir-
able. If a part or material is stressed to a suitable level and monitored
by AE techniques, a critical defect will play an active role by being the
source of A E that will attest to the structural integrity, or lack of
integrity, of the part. The results of such tests may not be definitive in
terms o f flaw size and location; however, such tests can be readily
incorporated into the production line with an entirely different goal in
mind. One might consider the goal to be one of determining whether
the part is the same or different from a part that has been judged
acceptable by other techniques and procedures. The A E signature or

activity of each part, when properly recorded, is unique to that part.

Deviations from the normal signature can be used to reject a part or to
assign it to a more detailed study by conventional NDT methods.
Again, the noteworthy feature of AE activity is the active role that
an existing flaw plays in the presence of a stress or strain field. Flaws
which are oriented such that they do not emit AE activity are stable
and static in nature and, hence, do not detract from the structural
integrity of the material or part being tested. Conversely, active flaws
trumpet their abuse and growth by emitting AE; hence, they are self-
proclaimed prophets of impending failure. Such testing philosophies can
be used in proof testing, periodic proof testing, or monitoring during
the service life of a component. In recent times an increased effort has
been directed toward testing and quality assurance programs in indus-
try. It is said that a good engineer is a cautious engineer because he
understands the laws of nature, the first of which is Murphy's law. This
law states, " I f anything can go wrong, it will." Sir Isaac Newton
pointed up that "the harder you push, the faster it goes," but it was
Mr. Murphy who informed us that "the harder you push, the quicker
you'll break your pusher." There is a perversiveness in nature which
almost always assures that disaster lurks in new designs. Physicians
have a motto, P r i m u m non nocere which means, "First do no harm."
It would be nice if each engineer could glance up from his design board
and see this injunction just before he releases drawings to the shop.
Paper designs are fine, but engineers must be sure that all considera-
tions are taken into account. This necessitates some type of testing
program of the mhterial or component, a procedures which is becoming
very expensive and yet more widely used.
The authors consider the field of AE as a tool for NDT with more
enthusiasm than was experienced in the introduction of ultrasonic test-
ing some 20 years ago. As in the success of any supply and demand
situation, AE testing has rapidly developed to a point where it is now
ready for application as a NDT tool, and the market or need for such a
test, because of fabrication tolerances, new high-strength and more
exotic materials, and the demand for greater performance from en-
gineering structures, is here and now.

Considerations in Setting Up an Acoustic Emission Test

In order to effectively set up a meaningful AE test there are a
number of steps that must be considered.
Objectives
What is the purpose of the test? What emission producing events are
to be detected? Is source location required? Where will the test be

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128 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

conducted? What is the environment in which the test will be per-

formed?
When will the test be performed, at what stage of fabrication or
service life? It is important to plan during the design stages when to
use AE most effectively so that allowances can be made in the fabrica-
tion sequence. For example, a pressure vessel should be monitored
during the initial proof pressure test to establish a baseline record
against which subsequent proof test emission records can be compared.

Mechanics of Materials and Structures

The materials and geometry of the part or structure to be tested must
be analyzed. How do the materials in the structure emit? Are they
brittle materials (good emitters) or ductile materials (poor emitters)?
All of the possible source mechanisms of AE from the materials and
joints in the structure must be determined. Various known sources of
AE which are related to the metallurgical properties of materials in-
clude dislocation and slip movement, twinning, plastic deformation,
sudden grain boundary reorientation, stable crack growth, Luder's line
propagation, and phase change which may be martensitic transforma-
tion or liquid/solid transformation. Other sources of AE may be elec-
tronic sources, such as radio frequency (RF) noise and electronic sys-
tem noise. A third source of AE is mechanical noise, such as impact of
two bodies, friction from rubbing surfaces, cavitation or bubble col-
lapse, boiling or bubble formation, fluid flow (either by gas or liquid),
gas fill (which includes expansion and impingement), and grip noise
caused by deformation and slipping.
It must be decided at what locations emissions are most likely to
occur such as high-stress areas. Once this is decided the part or
structure can be properly instrumented and environmentally prepared
so that only relevant emissions are detected and nonrelevant ones can
be excluded.
A means of stressing the part must be selected, for example, pres-
surization, tensile or compressive loading, and thermal stressing. What
is the maximum load that can be applied without causing plastic defor-
mation, yet be sufficient to accomplish the test? It must be remem-
bered that AE testing is nondestructive to acceptable parts but may be
destructive to defective ones.

Classification of Defects and Failure Modes

A clear definition of failure must be made. Usually failure can be
defined by one or more of the following conditions; (a) excessive
change in shape due to plastic deformation, either due to localized

yielding or because of gross yielding of the entire structure, (b) de-

velopment of a leak path through the structure, or (c) loss of structural
integrity by collapse, fracture, or rupture. Is failure due to overload,
fatigue, stress corrosion, hydrogen embrittlement, faulty design, etc.?
It is necessary to have an understanding of the types of defects
associated with the structure under test, how these defects can affect
structural integrity, and how these defects will emit under stress. What
defects are inherent in the base materials? What defects are associated
with the joining processes, with forming processes such as forging,
machining, etc., and with heat treatments involved in the fabrication
and service life of the structure? What defects can be produced in base
material by joining processes, heat treatment, incorrect machining, and
forming practices?
Failure modes and emission characteristics representing defective
conditions must be known and emission signatures established. For
example, if the purpose of the test is to prevent catastrophic failure of
a pressure vessel during proof pressure testing, signatures of all failure
modes for that particular vessel design must be experimentally deter-
mined by actual burst testing mockup vessels that are fabricated under
close control to study variables such as material variations, forming
and machining effects, and welding procedures. Figures 2 and 3 show a
comparison of AE records from two similar pressure vessels that were
pressurized to rupture. Figure 2 is the signature of a vessel in which
failure occurred in base material, while Fig. 3 is the signature of a
vessel that failed in a more brittle manner in the heat-affected zone of
the waist weld.

z . 1"51
_oua= t

"='-2;EO.5F/
.

,~ o/

i i i i ii ~
BURST AT 1280 psig " 7
c/) x

o i i i i i i I
5. 4 5 6 7 8 9 I0 II 12 13
HYDROSTATIC PRESSURE, psig x I00

F I G . 2--Typical acoustic emission o f a pressure vessel that f a i l e d in base metal.

o_o
~
I
i
BURST
q
AT1095
i i i
psig~
i

- o

HYDROSTATI
PRESSURE,
C psigxI00
FIG. 3--Typical acoustic emission record o f a pressure vessel that failed in the
heat-affected zone of the waist weld.

Data Evaluation
Acceptance and rejection criteria must be established. What degree
of acoustic activity can a flaw exhibit before there is concern about
structural integrity? What system gain and signal conditioning must be
used to detect emissions produced by events of concern?
The AE test engineer or operator must be given authority to "call the
shots" during the test. He must have prior knowledge of the stress and
environmental limitations under which he can perform the test. For
example, in testing a pressure vessel he should be able to stop, hold,
reduce, and reapply pressure to verify relevant from nonrelevant
(noise) emissions. He should be able to recycle, that is, repeat the load
or pressure cycle several times to determine the trend towards more or
less emission activity. And by all means he must have the authority to
stop and abort a test when catastrophic failure is imminent, particularly
if failure means loss of life or property.
Finally, disposition must be designated for the part under test ex-
hibiting unacceptable AE activity. Can the part be repaired or does it
go to the scrap pile? If component parts or a defective structure can be
salvaged or if repairs can be made it then becomes necessary to locate
the source of emission. This will dictate the test procedure and extent
of instrumentation. Will subsequent testing by other NDT methods be
used to further evaluate or disposition the part?

Data Presentation
For most testing situations one or more of the following methods of
data presentation may be utilized:
1. Plotting summation of all A E events on an X - Y recorder. Either
linear or logarithmic plots can be made, depending on the type of final
stage amplifier used.
2. Plotting count rate on an X - Y recorder. Most c o m m o n l y used is
linear count rate at some convenient reset time interval. Count rate
plotted from a logarithmic t a c h o m e t e r has been effectively used to
display data where log compression over up to four decades is required
[7]. The log tachometer has no reset time as do digital count rate
systems, rather it has an averaging time constant that can be set to
various time intervals. Equivalent to the log tachometer plot is the log
frequency plot. This is accomplished by using a combination of a
Hewlett-Packard 5210A f r e q u e n c y meter and an H-P 7563A logarithmic
voltmeter/amplifier to provide a logarithmic-related d-c output voltage.4
3. Plotting root mean square (rms) voltage of A E events on an X - Y
recorder [8,9]. Either linear, logarithmic, or squared rms voltage can be
plotted, depending on the type of voltmeter used. F o r example, a
Hewlett-Packard 3400A rms voltmeter provides a d-c output voltage
proportional to the meter deflection or input voltage; an H-P 7562A
logarithmic voltmeter/converter produces a d-c output voltage in a
logarithmic relationship to the true rms value of an a-c input voltage or
to a d-c input voltage; and a Ballantine Model 323 true rms voltmeter
provides a d-c output which is a squared function of the input voltage.
4. Listening to the audible sound of emissions over a speaker. To do
this the raw signal is h e t e r o d y n e d with a carrier-wave from an oscil-
lator to produce an audible signal.
5. Visual observation of the emission signals on an oscilloscope
screen to evaluate their ringdown characteristics.
6. Amplitude distribution analysis may be considered to assess the
evaluation analysis [21,33].
7. Triangulation and source location techniques may be used to lo-
cate the source of A E activity [10-12,20,24,33,].
The most useful information plotted in a typical A E test such as
p r o o f testing a pressure vessel is the summation of AE events and the
rate at which they occur as a function of some engineering parameter;

4 No specific intent is made to recommend any one manufacturer's equipment. The

authors cite certain instruments simply because they have successfully used these in-
struments for the described purpose. Equivalent instrumentation of different manufactur-
ers may perform equally well.

104 _,
. . . . . . . . . . . . 'BuR~'r.,

Io a
I-'I"

!.!,o,
o

~ 2

0
0 I 2 :3 4 5 6 7 8 9 I0 II 12 13 14 15
HYDROSTATIC PRESSURE, psi9 X 102

FIG. 4----Comparison o f linear root mean square voltage, total emission, a n d log count
rate as a f u n c t i o n o f pressure f o r a pressure vessel taken to burst at 1470 psig (Run 561).

for example, changes in stress, such as load or pressure, or as a

function o f time. Figure 4 shows a comparison of three methods o f
plotting A E from a pressure vessel taken to burst. The lower plot is
linear rms voltage or rms emission amplitude (Nr,,s); the middle plot is
summation or total emission (~N); and the upper plot is log count rate
from a logarithmic t a c h o m e t e r with a 1 s averaging time constant (/Vtoo).
The method or methods of plotting these data are selected to best
display the events as they happen, so that real-time interpretation can
be made as the test progresses.

Applications of Acoustic Emission

The most practical approach to applying an effective nondestructive
testing program in the manufacture of a high-quality engineering struc-
ture or weldment is to first evaluate incoming materials. This assures
that only good materials enter the production stream. Next is to inspect
material after a forming operation and welds after the welding operation
to assure that the operations did not introduce defects into the struc-
ture. Finally, an evaluation of the finished produce must be made to
guarantee quality and reliability to the c u s t o m e r and to minimize the
manufacturer's liability in the event of accident or failure of the prod-
uct in service.

Monitoring Vessels During Pressure Testing

One of the most common uses of AE is to monitor small (as opposed
to nuclear reactor size) pressure vessels during a proof pressure test or
during pressurization to destruction. This is an excellent way to
evaluate base material, the weld joint, and the fabrication process.
Destructive burst tests can be run during the design and development
stages to facilitate selection of materials and joining process and to
optimize weld joint configuration and welding procedure. Once the
pressure vessel design and fabrication techniques have been established
and put into production, the finished product can be monitored with
AE during proof pressure testing to establish its integrity and reliability
as a pressure vessel.
The simplest technique of monitoring a pressure test with AE is with
a single transducer to detect all emission activity during pressurization.
This technique merely detects emission events but provides no informa-
tion as to location of the emission source. A simple technique of
locating the general source of emission in zones in the vessel is by
coincidence gating. Figure 5 shows a block diagram of an AE system,
including two coincidence gates, which can be used effectively on a
symmetrical pressure vessel up to some 3 ft in diameter [10]. The two
coincidence gates discriminate and identify indications from the waist
weld area and from around the inlet/outlet tube joint of the pressure
vessel. Assuming the inlet/outlet tube is located at some place off the
pole of the vessel, a transducer array could be utilized as shown in Fig.
6. The two transducers located at the poles form a coincidence (W) to
detect any indication originating in a band around the waist of the
vessel. The three equal spaced transducers (one of which is common to
both coincidence circuits) around the inlet/outlet tube form a coinci-
dence (T) to isolate any indication from the tube joint. All other indica-
tions from the vessel outside of the two gated areas cause no coinci-
dence.
Assuming a weldment is designed and made properly, that is, the
weld is stronger than base material, if the vessel is pressurized to burst,
failure would be expected to occur in the base material--most likely in
the heat-affected zone of the waist weld joint or in some high-stress
area in the base material well away from the weld. The AE signatures
for such burst tests (Figs. 2, 3, and 4) would be representative of failure
of "healthy" pressure vessels of this particular design. If a vessel was
defective in any way, due to faulty material, weld joints, or fabrication
techniques, failure would occur premature to that of a "healthy" ves-
sel, yet produce a similar AE signature. Thus, the premature emission
would provide warning that fracture is occurring, and the slope of the

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134 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

PRESSURE
TERMINATION }
(50Q) I
~'~OSCLLOSCOPEI
I I
Nrm.l I1
I ~1~ STRIP-CHART 1~1

1 -~ v, oc / /
"11
.uo,o / __.1 RMS L \ J, PRESSURE I

ACOUSTIC
i
TRANSDUCER| 'I R ,.'~.-_--_ R
.~ I , ,OUTPUT
2~ T O T A L I Z E R ( T ) [ ( ~1
w v I No. 2 ~ ~,
MONITOR
(out)

T-3 ~-~ICOINCIDENCE[-~ VOLTMETERRMS~ . ~

I -1 v. DC
T-1 (out)
I
Mo, l;o.l__.l
(out) - I I-
RMs tL
[ vOLTMETER
ICOINClDENCE I '
T-4 ~ GATEW I
I GATE W
y~
d BRIDGE
BALANCE
v I AMPLIFIER
PRESSURE
TRANSDUCER
ISAM-1I
Legend J--GA
ER
-- Ohms.
V -- Volts.
Nrm$ Emissionamplitude(toot meansquare),
[PRE
URE
]~N - Summationof emissions.
-- Squarewavepulse.

FIG. 5--Block diagram o f acoustic emission system with two coincidence gates.

TRANSOUCER
~ LET/OUTLET TUBE

II ~ I

2
13

IST WELD
4

FIG. 6--Transducer array on a pressure vessel instrumented with two coincidence

circuits to discriminate acoustic emissions from the tube joint area (Transducers 1, 2,
and 3) and the waist weld area (Transducers 1 and 4).

curve could be extrapolated to predict imminent failure, in some cases

within 5 percent of the failure pressure. Figure 7 shows the AE record
from a pressure vessel that failed at approximately 50 percent of the
expected burst pressure of a healthy vessel, yet at less than proof
pressure of the required certification test. Figure 8 is a plot of the
coincidence gate output showing that essentially all emission came
from the waist where, indeed, failure did occur. This illustrates the
effectiveness of a proof pressure test in detecting defective product and
preventing catastrophic failure. It is easily seen that imminent failure
was approaching and that the test could have been stopped prior to
failure, thus facilitating repair or salvage of parts and material.

1.5 ~'~.~_..__~..
z GAS FILL NOISE
o 9

L=.II.-
=3o.5 I \ I,!
= ~ ""URS~
~ i z ~ 4 s e "}l s
/

i-
"I
-0 I 2 3 4 5
_ J 6 7 8
GAS PRESSURE, pslg x I 0 0

FIG. 7---Acoustic emission record o f a defective pressure vessel that failed in the
heat-affected zone o f the waist weld at approximately 50 percent o f the design burst
pressure.

I&l
r
z
Ir
Q
r
----.
O -
u i i i i i i i i
O I 2 ~ 4 5 6 7

r-,

(J i
o I ~ ~ ~ ~ ~
GAS PRESSURE, psigx 1 0 0
F I G . 8----Coincidence plots o f acoustic emission activity in the waist weld area ( W ) and
in the inlet~outlet tube joint area ( T ) during pressurization to failure o f a defective
pressure vessel (acoustic emission f r o m burst test plotted in Fig. 7).

Once it has been established that AE can be effectively used to

monitor a proof pressure test for the given material and design involved
and that a high degree of reliability has been established, it may be
advantageous because of cleanliness requirements and corrosion condi-
tions to pressurize a vessel with gas rather than some liquid medium.
Thus, gas proof pressure testing could be utilized safely without fear of
catastrophic explosion.
Often it is important to know precisely where AE signals are originat-
ing from in a pressure vessel or structure. Randomly located, single
event microcracks can occur in many materials under stress. These
cleavage cracks occur in highly stressed or embrittled grains and termi-
nate and arrest in more ductile, tougher surrounding grains. Unless one
of these microcracks continues to propagate it is not considered seri-
ous. If, however, one continues to grow and approach critical crack
size it can be of significant interest to have such knowledge when
evaluating test data. Therefore, it may be considerably more meaning-
ful to have the capability to determine the location of the source of AE
within a structure. The literature cites many techniques for flaw loca-
tion in large planar surfaces and large vessels such as nuclear reactor
pressure vessels [11-23]. Several papers discuss techniques, show
transducer arrays, and present computer programs for flaw location on
thin-wall spherical vessels [24] and nonspherical vessels [25], both
surfaces of revolution.
One particular condition that has been experienced by the authors in
testing weldments is the case where the only warning of premature,
catastrophic failure of the weld is the audible sound of emission. The

audible sound of emission is a very important and valuable means of

analyzing AE data because of the spectral content from various emis-
sion producing events. The path through which AE's have to travel to
reach the transducer can alter the spectral content of the sound packet
if there are any changes in material properties that can affect sound
transmission characteristics. In simple structures such as a one-piece
tension or fracture specimen there is no significant spectral information
simply because there is no change in material properties of the speci-
men between the source of emission and the transducer to alter or filter
the spectral content of the sound packet. Therefore, the spectrum from
specimen to specimen would be the same. However, when a weld joint
is located in the sound path the structure becomes complex. The weld
acts as a filter that can alter the sound transmission characteristics in
as many ways as there are variables and types of defects associated
with that particular welding process base material combination.
It has been observed in aluminum welded beryllium fracture speci-
mens [26] that both continuous and burst-type emissions are produced.
Normally these data are sufficient to plot against some engineering
parameter such as load to produce a characteristic signature by which
failure of the material can be predicted. In this test it was found that
conventional techniques of displaying test data were inadequate. The
audible sound of emission was the only warning that the load on the
specimen was within 10 percent of failure load. Normally, all of the
emission produced by these aluminum welded beryllium fracture
specimens is associated with dynamic processes in the beryllium and
not in the aluminum weld. The weld is generally tough and ductile and
any emission producing process going on in the aluminum does not
produce detectable emission at the 90-dB system gain that was used.
During tension testing of several specimens, failure occurred at about
60 to 70 percent of the expected ultimate failure load. In both cases the
test operator noted an abrupt and persistent drop in pitch of the audible
sound of emission events just prior to failure. Based on the audible
signal he called out failure of the third specimen within 5 percent of the
failure load. The fourth specimen he unloaded after the drop in pitch
and prior to failure.
This condition was found to be associated with fracture in the
aluminum weld and was not related to base metal. Metallographic
analysis of several of these weld joints (one that failed and the one that
was unloaded prior to failure) showed that two conditions were pres-
ent: A gross amount of porosity throughout the weld as shown in Fig. 9
and embrittlement of the weld metal. The porosity is believed to have
been produced by dissolved hydrogen or hydrocarbon contamination of

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138 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

FIG. 9----Cross section of weld joint in beryllium fracture Specimen 617-3 showing
excessive porosity and root crack in the aluminum weld deposit (as polished, •

the weld wire, probably drawing compound that was not properly
cleaned off the wire.
Two possible causes of embrittlement investigated were an unusually
high silicon content (40 to 50 percent) from the weld wire (normal
silicon content of the wire is approximately 12 percent) and interstitial
embrittlement. Porosity in this type of weld is not uncommon and will
not, by itself, cause failure. However, embrittlement in addition to
porosity provided a situation where random brittle fracture of ligaments
between pores could occur as load increased. The majority of such
ligament fractures occur at the root of the weld where the greatest
tensile stress is produced. Within 10 percent of the failure load proxi-
mal fractured ligaments at the root abruptly started to coalesce (Fig.
10), producing the.drop in pitch of the audible signal. The porous
condition of the weld reflects, scatters, diffracts, and mode converts
the higher frequency ultrasonic waves. Applying Huygen's principle,

FIG. lO---Details of crack tip at the root of embrittled and porous aluminum weld in
beryllium fracture Specimen 617-3 showing unfractured ligaments between pores (as
polished, xlO0).

each pore becomes a point source radiator for frequencies of approxi-

mately the same wavelength as the pore diameter. Thus, only the lower
frequency waves can propagate through the weld to the transducer--
thus the drop in pitch.

Monitoring Welding Processes

A number of people have reported successful application of AE
techniques for monitoring various welding processes [27-31], including
submerged arc, gas tungsten arc (GTA), pressurized inert gas metal arc
(PIGMA), metal inert gas (MIG), and electron beam (EB) processes.
Defects formed during welding, as well as post weld cracking and
martensitic phase transformation can be detected. Figure 11 shows a

0.5 m
_= z
o S t a r ~ .. I Stop 0

Q
I .... .I ....... I _
t-
in 10 20 30 40 50 60 70 80 90 100 110
..I
t.,
=Z
<~
z
O 0
..<

uJ
U~
=E (b)
r
1.0 B 0
C
CRACKS
CRACKS
R
r~

0.5
0
z

10 20 30 40 50 60 70 80 90 100 t 10

T I M E (seconds)
FIG. 1l---Acoustic emission records of linear root mean square voltage plotted as a function of time for a defect-free PIGMA
weld in beryllium Specimen AE-220) (a) and f o r Specimen AE-225 in which cracking occurred (b).

comparison of AE records for a " g o o d " PIGMA weld and one exhibit-
ing crack formation during welding and post-weld cooldown. The
4-pass butt welds were made in 8-in.-diameter, 0.250-in.-thick wall
beryllium pipe sections using aluminum filler metal. Cracking was in-
duced by inserting short segments of copper wire into the root of the
weld groove.
In addition to detecting defect formation, AE emission can be used
to record the signature or noise of the welding process which is the
summation of all emission producing processes. Welding and metallur-
gical processes can include liquid-to-solid and solid-state phase trans-
formations, expulsion of droplets of molten weld metal, cavitation and
sloshing of the molten pool, arc noise, slag formation and cracking, and
deformation processes due to thermal expansion and contraction in the
joint area. These emissions are a function of welding parameters--
voltage, amperage, wire feed, and travel speed--and therefore should
produce a signature that can be related to the characteristics of the
weld joint configuration, materials, and welding process. Any deviation
in the welding process due to malfunction of equipment, power tran-
sients, material defects and changes in material properties, such as
wrong alloy, will affect the AE signature and facilitate immediate cor-
rective action.
For noisy welding processes where noise is due to the RF generated
by the arc, a differential transducer can be used to minimize arc noise.
For stable arc welding processes, a single ended transducer provides
greater sensitivity.
A unique application of AE and a welding technique is the determi-
nation of crack susceptibility of pressed powder beryllium [32]. It is
difficult to establish uniform weld parameters for joining pressed pow-
der beryllium because of its inconsistent material properties. Each
pressing, or log, is inherently nonhomogeneous, exhibiting throughout
the log variations in density, chemistry, and mechanical properties.
Also, there is variation in properties of material made to the same
specification by different manufacturing processes.
The purpose of this program was to determine by AE the susceptibil-
ity to cracking of 0.250-in.-thick beryllium specimens subjected to a
thermal shock. To accomplish this the specimen is clamped between
two flat plates, each containing a circular relief hole. The clamping
blocks provide both restraint and a heat sink to the specimen. Welding
torch access to the specimen is through the relief hole in the top plate
with an electrode-to-work distance of 0.040 in. Figure 12 shows the
clamping fixture with the GTA welding torch in place.
The specimen is subjected to a welding environment for a short

FIG. 12-----Clamping fixture u s e d to hold beryllium coupon in determining crack suscep-

tibility by the acoustic emission p r o d u c e d as a result o f a thermal shock. The GTA
welding torch a n d acoustic emission sensor are s h o w n in place.

period of time while monitoring the process for AE. The AE produced
during the process is caused by gross crack propagation and is a
measure of the weldability and fracture toughness of the beryllium
material. Gross cracking usually starts several seconds after the arc is
shut off and lasts about 10 s. All of the cracking takes place usually in
less than 30 crack increments.
Figure 13 shows the test setup including the rack of instrumentation.
Figure 14 shows a block diagram of the AE system. AE's are detected
with a single ended transducer, since the GTA welding process in-
volves an extremely stable arc which causes no acoustic noise. Signals
are amplified for a total system gain of 60 dB and bandpassed 100 to

FIG. 13--Test setup and acoustic emission instrumentation used in the crack suscepti-
bility test o f beryllium coupons.

AUDIO1
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X-Y-Y'
RECORDER
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FIG. t4---Block diagram o f acoustic emission system used to determine crack suscep-
tibility o f beryllium coupons.

300 kHz. Desirable beryllium material has good fracture toughness and
will produce a star shaped crack which is confined to the weld crater as
shown in Fig. 15a. This crater cracking produces no AE at 60 dB gain
as shown in Fig. 16. In beryllium having poor fracture toughnesswthe
crater crack will propagate through the heat-affected zone and cause
gross cracking in the specimen as shown in Fig. 15b, producing a series
of discrete AE events as shown in Fig. 17.

Summary
Acoustic emission is an extremely sensitive, dynamic method of
nondestructive testing. It detects signals from real events when they
occur. The real "name of the game" is to interpret emissions as to the
mechanisms that generated them and then analyze this information in
relation to the entire structure. From this type of analysis one can get
detailed information about how a specific structure is failing. This in
terms of fracture mechanics provides us with one of the most useful
and reliable tools to evaluate the integrity of engineering structures.
AE monitoring during proof testing of a structure or pressure vessel,
guided by fracture mechanics principles, can ensure that there are no
active flaws to cause failure during the service life of that structure or
pressure vessel. Therefore, AE provides us with a new concept in
nondestructive testing--not merely a means of detecting discontinuities
and the traditional NDT concept of accept or reject based on size,
location, and distribution of these discontinuities---but a decision based

FIG. 15----Beryllium weld test coupons showing crater cracking in Specimen WT-70 (a)
and gross cracking in Specimen WT-71 (b).

01 i ~-- ; ~-- ~

I / I I I I I
TIME (5 seconds per division)
FIG. 16--Acoustic emission record of crater cracking in beryllium weld Specimen
WT-70.

on whether a flaw is active or in a state of arrest. It must be r e m e m -

bered, a structure cannot fail unless a flaw is present and grows. When
that h a p p e n s and the structure is properly instrumented, A E activity is
detected. The detection of this flaw growth, along with a knowledge of
h o w the material or structure fails, then provides us with a tool to
detect incipient failure in that structure.
F o r a m o r e c o m p l e t e b a c k g r o u n d in the fundamentals and b r o a d
application of A E technology the reader is referred to Refs 33 through
48.

i 2 f--

~8 j , , , , , I |
TIME ( 5 seconds per division)

FIG. 17--Acoustic emission record of gross cracking in beryllium weld Specimen WT-71.

Acknowledgment
T h i s w o r k w a s p e r f o r m e d u n d e r t h e a u s p i c e s o f t h e U . S. A t o m i c
Energy Commission.

References
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[2] Dunegan, H. L. and Harris, D. 0., Experimental Techniques in Fracture Mechanics,
A. S. Kobayashi, Ed., SESA Monograph No. 1, Society for Experimental Stress
Analysis, 1973, Chapter 3, pp. 38-75.
[3] Dunegan, H. L., Harris, D. O., and Tetelman, A. S., Materials Evaluation, Vol. 28,
Oct. 1970, pp. 211-227.
[4] Dunegan, H. L. and Tetelman, A. S., "Non-Destructive Characterization of
Hydrogen-Embrittlement Cracking by Acoustic Emission Techniques," Technical
Bulletin DRC-16. Dunegan Research Corp., Livermore. Calif. (now
Dunegan/Endevco, San Juan Capistrano, Calif.), 1969; Engineering Fracture
Mechanics, Vol. 2, 1971, pp. 387-402.
[5] Tetelman, A. S., "Acoustic Emission and Fracture Mechanics Testing of Metals and
Composites," UCLA-ENG-7249, University of California, School of Engineering and
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[6] Tatro, C. A. in Acoustic Emission, ASTM STP 505, American Society for Testing
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[7] Brown, A. E. and Liptai, R. G. in Acoustic Emission, ASTM STP 505, American
Society for Testing and Materials, 1972, pp. 318-331.
[8] DrouiUard, T. F., "Presentation of Acoustic Emission Test Data from Welded
Beryllium Fracture Specimens," RFP-1617, Dow Chemical Co., Rocky Flats Divi-
sion, Golden, Colo., 2 Nov. 1970; paper presented at the 6th Acoustic Emission
Working Group Meeting, Argonne, II1., 12-13 Nov. 1970.
[9J Hamstad, M. A. and Mukherjee, A. K., "The Dependence of Acoustic Emission on
Strain Rate in 7075-T6 Aluminum," UCRL-74260, Lawrence Livermore Laboratory,
Livermore, Calif., 14 Nov. 1972; paper presented at the 3rd Society for Experimental
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[10] Sewall, N. R. "Acoustic Emission Coincidence Detector for Monitoring High Re-
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Livermore Laboratory, Livermore, Calif., 7 March 1973; paper presented at the 19th
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[11] Compton, M. R., Materials Evaluation, Vol. 31, July 1973, pp. t21-127.
[12] Cross, N, O. et al in Acoustic Emission, ASTM STP 505, American Society for
Testing and Materials, 1972, pp. 270-296.
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Pressure Vessels," Publication 70-PET-31, American Society for Mechanical En-
gineers, New York, 1970.
[14] Green, A. T., Dunegan, H. L., and Tetelman, A. S., "Nondestructive Inspection of
Aircraft Structures and Materials via Acoustic Emission," Technical Report
DRC-107, Dunegan Research Corp., Livermore, Calif. (now Dunegan/Endevco, San
Juan Capistrano, Calif.), Sept. 1970.
[15] Hatano, Hajime, Yoshida, Yasunori, Sagehashi, Ichitaro, and Niwa, Noboru, "De-
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[16] Hoff, M. in 1972 Ultrasonics Symposium Proceedings, Catalog No. 72 CHO

708-8SU. Institute of Electrical and Electronics Engineers, New York, 1972, pp.
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[17] Hoark, C. R., "Structural Integrity Acoustic Emission Monitoring," ADN Report
09-01-72.2, Grumman Aerospace Corp., Advanced Development Program, Bethpage,
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[18] Jolly, W. D., Davis, T. J., and Sheff, J. R., "Develop and Evaluate the Coincident
Detection-Location System," Task 10, Project IV, In-Service Inspection Program for
Nuclear Reactor Vessels, R. D. Wylie, Ed., SwRI Project 1%2440, Biannual Prog-
ress Report No. 7, Southwest Research Institute, San Antonio, Tex., 25 Aug. 1972,
pp. 149-195 (NTIS: PB-212-144).
[19] Jolly, W. D. and Parry, D. L., "Field Evaluation of the Acoustic Emission System,'"
Task 4, Project IV, In-Service Inspection Program for Nuclear Reactor Vessels, R.
D. Wylie, Ed.. SwRI Project 17-2400, Biannual Progress Report No. 7, Southwest
Research Institute, San Antonio, Tex., 25 Aug. 1972, pp. 97-143 (NTIS: PB-212-144)
[20] Kelly, M. P. and Bell, R. L., "Detection and Location of Flaw Growth in the EBOR
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Capistrano, Calif., 1973.
[21] Nakamura, Yosio, Materials Evaluation, Vol. 29, Jan. 1971, pp. 8-12.
[22] Vetrano, J. B. and Jolly, W. D., Materials Evaluation, Vol. 30, Jan. 1972, pp. 9-12.
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117-124.
[24] Blake, H. S. and Davenport, C. M., "Acoustic Flaw Triangulation on a Thin
Spherical Shell: An analytical Solution," Y-1806, Union Carbide Corporation, Oak
Ridge Y-12 Plant, Oak Ridge, Term, 19 Dec. 1971.
[25] Sackschewsky, V. E. and Lott, L. A., "Acoustic Emission Source Location on a
Nonspherical Surface of Revolution," RFP-2090, Dow Chemical U.S.A., Rocky
Flats Division, Golden, Colo., 19 Nov. 1973.
[26] Drouillard, T. F., "The Sound of Imminent Failure" Dow Chemical U.S.A., Rocky
Flats Division, Golden, Colo., Dec. 1972; paper presented at the 10th Acoustic
Emission Working Group Meeting, Pasadena, Calif., 6-8 Dec. 1972.
[27] Jolly, W. D., Materials Evaluation, Vol. 28, June 1970, pp. 135-139, 144.
[28] Chance, R., "Acoustic Emission In-Process Electron Beam (EB) Weld Inspection
Technique," Note No. ADN 09-01-70.4, Grumman Aerospace Corp., Advanced
Development Program, Bethpage, New York, No. 1970 (NTIS: AD-884-729
(USGO)).
[29] Drouillard, T. F., "Crack Detection in PIGMA Welding of Beryllium," Dow Chemi-
cal U.S.A., Rocky Flats Division, Golden, Colo., April 1971; paper presented at the
7th Acoustic Emission Working Group Meeting, Atlanta, Ga., 20-21 May 1971.
[30] Hartbower, C. E., Reuter, W. G., MorNs, C. F., and Crimmins, P. P. in A?oustic
Emission, ASTM STP 505, American Society for Testing and Materials, 1972, pp.
187-221.
[31] Prine, D. W. in Proceedings of the U.S.-Japan Joint Symposium on Acoustic
Emission, English Volume, Japan Industrial Planning Association, Tokyo, Japan,
1972, pp. 171-199.
[32] Drouillard, T. F., "Determination of Crack Susceptibility of Beryllium by Acoustic
Emission," ETDL-100-73-1. Dow Chemical U.S.A., Rocky Flats Division, Golden,
Colo., 25 June 1973; paper presented at the 1l th Acoustic Emission Working Group
Meeting, Richland, Wash., 18-20 July 1973.
[33] Acoustic Emission, ASTM STP 505. American Society for Testing and Materials,
1972.
[34] Dunegan, H. L. and Tatro, C. A. in Techniques of Materials Research, Chapter 12,
R. F. Bunshah, Ed., Vol. 5, Part 2, Interscience Publishers, New York, 1971.
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Chapter 1, R. S. Sharpe, Ed, Academic Press, New York, 1970, pp. 1-30.
[36] Engle, R. B. and Dunegan, H. L., "Acoustic Emission: Stress Wave Detection as a
Tool for Nondestructive Testing and Material Evaluation," UCRL-71267, Lawrence

Radiation Laboratory, Livermore, Calif., 13 Sept. 1968; International Journal of

Nondestructive Testing, Vol. 1, June 1969, pp. 109-125; Physics and Nondestructive
Testing, W. J. McGonnagle, Ed., Voi. 2, Gordon and Breach, New York, 1971, pp.
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[37] Fisher, R. M. and Lally, J. S., Canadian Journal of Physics, Vol. 45, 1967, pp.
1147-1159.
[38] Frederick, J. R., Materials Evaluation, Vol. 28, Feb. 1970, pp. 43--47.
[39] Gieske, J. H., Transactions, International Standards Association, Vol. 11, 1972, pp.
24-30.
[40] Hartbower, C. E., Morais, C. F., Reuter, W. G., and Crimmins, P. P., "Develop-
ment of a Nondestructive Testing Technique to Determine Flaw Criticality," Techni-
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1972 (NTIS: AD-747-225).
[41] Liptai, R. G. in Composite Materials: Testing and Design, ASTM STP 497,
American Society for Testing and Materials, 1972, pp. 285-298.
[42] Liptai, R. G. and Harris, D. O., Materials Research and Standards, Vol. 11, March
1971, pp. 8-10, 44.
[43] Mehan, R. L. and Mullin, J. V., Journal of Composite Materials, Vol. 5, April 1971,
pp. 266--269.
[44] Romrell, D. M., Materials Evaluation, Vol. 30, Dec. 1972, pp. 254-258.
[45] Romrell, D. M. and Bunnell, L. R., Materials Evaluation, Vol. 28, Dec. 1970, pp.
267-270, 276.
[46] Tatro, C. A., "Acoustic Emission Related to Nondestructive Testing," UCRL-73441
Revision l, Lawrence Livermore Laboratory, Livermore, Calif., 22 Sept. 1971; paper
720175 presented at Society of Automotive Engineers' National Automotive En-
gineering Congress and Exposition, Detroit, Mich., 10-14 Jan. 1972.
[47] Tatro, C. A., "Evaluation of the Integrity of a Structure Using an Acoustic-
Emission-Instrumented Proof Test," UCID-16084. Lawrence Livermore Laboratory,
Livermore, Calif., 18 July 1972.
[48] Tatro, C. A., Liptai, R. G., and Moon, D. W., "Acoustic Emission from Formation
and Advancement of Cracks," UCRL-74607, Lawrence Livermore Laboratory,
Livermore, Calif., 5 March 1973; paper presented at the International Conference on
Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys,
Unieux-Firminy, France, 10-16 June 1973.

Industrial Application of Acoustic

Emission Analysis Technology

REFERENCE: Parry, D. L., "Industrial Applications of Acoustic Emission

Analysis Technology," Monitoring Structural Integrity by Acoustic Emission,
ASTM STP 571, American Society for Testing and Materials, 1975, pp.
150-183.
ABSTRACT: Acoustic emission analysis technology as a nondestructive
testing tool is being applied in industry to a wide variety of structures. Its
acceptance, however, has been more rapid in industries where it is applied
to metallic pressure containment structures. These applications include
pipelines, piping systems, storage vessels, pressure vessels, and even com-
plex pressure containments such as the primary coolant systems of large
nuclear power plants.
The application techniques used and a description of typical results ob-
tained during field applications provides additional insight into a new power-
ful testing tool that is rapidly gaining acceptance throughout the industry.

KEY WORDS: acoustics, emission, nondestructive tests, pressure vessels,

pipelines, nuclear power plants, steel structures, tanks (containers)

Less than a decade ago acoustic emission (AE) analysis was primar-
ily a research subject. Today it has developed into a testing tool which
is rapidly expanding the capabilities of the nondestructive testing
(NDT) field.
The technology, which in this paper will be referred to as acoustic
emission analysis, detects and analyzes minute AE signals generated by
discontinuities in materials under applied stress. Proper analysis of
these signals can provide information on the location and structural
significance of the detected discontinuities.
AE analysis provides the following distinct advantages over tradi-
tional NDT techniques such as ultrasonics and radiography:

i Manager, nondestructive testing services, Exxon Nuclear Company, Inc., Richland,

Wash. 99352.

150
Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 23:22:35 EST 2016
Copyright9 1975byby ASTM International
Downloaded/printed www.astm.org
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PARRY ON INDUSTRIAL APPLICATIONS 151

1. It provides a complete integrity analysis of a structure during a

fast, one-step test.
2. It is a dynamic testing tool in that it measures the response of a
discontinuity to imposed structural stresses.
3. It can detect and evaluate the structural significance of flaws which
may be inaccessible to the traditional NDT techniques.
4. It requires only limited access and downtime for the requalification
of inservice structures.
5. It can be used to limit the maximum pressure during pressure
testing of containment systems and prevent failure if the structure
contains a significant discontinuity.
AE analysis is now being used in the United States, Europe, and
Japan for the nondestructive evaluation of structures ranging in com-
plexity from simple fluid transmission pipelines to the complex primary
containment systems of large nuclear power plants. The technology is
equally applicable for the acceptance testing of new structures or the
requalification of in-use structures. Its acceptance as a NDT tool is
now being evidenced by a special American Society of Mechanical
Engineers (ASME) ad hoc committee which is formulating application
and acceptance standards for AE analysis technology.
In the last three and one half years alone Exxon Nuclear Company,
Inc.'s nondestructive test services organization has tested 83 major
industrial structures using their AE systems. These tests have been
conducted on such structures as pipelines, heat exchangers, storage
tanks, petrochemical pressure vessels, nuclear pressure vessels, and
the entire primary coolant systems of large nuclear power plants.
This paper discusses the field application of AE analysis, and de-
scribes specimens of the results that have been obtained by application
of the technology for the integrity analysis of large, complex industrial
structures.

Discussian
AE analysis techniques can be applied to a wide variety of structures
and materials [1-7]. ~ The technique has been applied by imposing stress
on materials such as wood, plastic, fiberglass, concrete, and, of course,
metals. The emission producing mechanisms in each type of material is
different, but the result is the same; pulses of energy are generated.
The generation of AE usually requires that a stress be applied to the
structure undergoing test. It need not be a large amount of stress nor
does it generally need to be a specific type of stress. Beamed structures
can be loaded, torsional stress can be applied to rotary shafts, thermal
2 The italic n u m b e r s in brackets refer to the list of references a p p e n d e d to this paper.

stresses are applied to materials during welding, or pressure induced

stress can be applied to pressure containment systems.
This paper deals primarily with metallic pressure containment struc-
tures which can be stressed either pneumatically or hydrostatically.
Exxon Nuclear, however, has tested other types of structures utilizing
both thermal stresses and mechanical loading.
Structures that are stressed for the first time will emit acoustic
signals from all structural discontinuities, including deformation emis-
sion due to minor yield in localized regions of high-stress concentra-
tion. Immediate reloading of a large structure will produce 50 to 75
percent less emission and only from discontinuity regions. Additional
repeated loadings will normally produce approximately 10 percent" of
the original amount of AE; this AE release generally begins at approx-
imately 90 percent of the maximum stress level previously applied.
Some investigators have chosen to call this phenomena the "Kaiser
effect" [8]. However, the Kaiser effect relates more specifically to
irreversible dislocation movement. In a nondefective material under
stress, dislocations will move until pinned, thus producing a relatively
irreversible emission source.
In a large structure small minor discontinuities may grow under a
certain amplitude of externally applied stress until opposing internal
regions, such as the plastic zone around the leading edge of a micro-
crack, match the stress levels imposed by external mechanisms; this
produces a temporary irreversible emission source. However, cyclic
stress application will cause an opening and closing of the larger cracks
which result in incremental crack growth. This crack growth produces
acoustic signals on each cycle. In addition, acoustic signals are pro-
duced by surface-to-surface movement of the crack and possibly vari-
ous other mechanisms.
The end result is an output of AE's from the more prominent discon-
tinuities near the top peak of each stress cycle which is imposed on a
large structure. These signals, just as in a standard one cycle stress
test, can be utilized to determine the presence and relative significance
of defects.
It has also been noted on large structures that a short period of
relaxation of structural stress will allow a partial recovery of the
structure's AE activity. For example, vessels that have been operating
at high continual stress loadings, and then shut down for four to five
days will, upon restressing, produce AE from the majority of structural
discontinuities regardless of their size or significance. The vessel acous-
tic signature produced by restressing usually represents a total acoustic
energy release value of within 15 percent of that experienced during the
initial preservice pressurization.

AE analysis techniques, therefore, may be equally applicable for the

integrity analysis of new structures or the requalification of in-use
structures. The only major variant is the method or stress technique
best suited for evaluation of the structure.
Test Systems
This paper describes the AE analysis system and application tech-
niques used by Exxon Nuclear in field service testing of industrial
structures. However, most acoustic analysis test systems function by
using the same basic principles. AE's are detected by the placement of
acoustic transducers on the surface of the structure to be tested, the
detected signals are amplified and analyzed to determine the severity of
the defects and their locations.

Detection and Signal Conditioning

Figure 1 shows a block diagram of Exxon Nuclear's ACOUST 3 sys-
tems. Acoustic signals detected by transducers located on the structure
are amplified by special low-noise, high-gain preamplifiers. These sig-
nals are transmitted through signal cabling to the main unit, where
additional signal conditioning takes place. The detection and signal
conditioning circuitry may be operated at several select frequencies
ranging from 30 kHz to 2.5 mHz. One hundred kHz are utilized for the
testing of most pressure containment structures.
After conditioning, the signals are processes through two analysis
systems. These systems are the energy release system, and the signifi-
cance and location computer systems.
The energy release system monitors the amount of acoustic energy
per unit stress being released from a structure and displays this signa-
ture for visual scan on a chart recorder. In actuality, several energy
release circuits are utilized on complex structures; each circuit
monitors different components of the structure, that is, pressure con-
tainment system pump casings, piping, valves, pressure vessels, etc.
The energy release circuitry is regarded as an early warning system
that can provide immediate warning of possible "significant" crack
growth. Figure 2 is an example of a signature showing the growth of a
defect which is considered "significant" to the integrity of the structure
undergoing test. The signature gives warning that a source or sources
within the structure are producing AE's at an unstable rate. Normally,
the occurrence of such a signature would require that the application of
stress be halted while a cross check with computer data is made. The
computer data display the location of the energy source and its relative
magnitude.
s Trademark of Exxon Nuclear Company, Inc.

~ - ~ ~ ~ j Signal 0
zm
unctioi Box Conditioning
f ,t
m
Time Source z
Q
Analysis Location
C~puter C~puter,
J ~
Io-ol ~ m
Release Digital TapeUnit l Output
Structure Under
Test Circuit
oc -

Analog Tape O

Unit z

v9
Display
FIG. I--Block Diagram of Exxon Nuclear ACOUST systems.

FIG. 2--Acoustic energy release pattern showing significant defect growth.

The computer significance and location analysis is actually accom-

plished by two computers, the time analysis computer (TAC) and a
digital minicomputer. The TAC is a statistical computer which utilizes
the input signals from the detection channels to determine the differ-
ence in the time-of-arrival (at) of the AE's at various tranducers. The
At data generated by various AE sources are accumulated in the com-
puter memory until they have become statistically valid. At this point
the At information is fed into the digital minicomputer which has been
programmed with information concerning structural geometry, exact
transducer locations on the structure, and signal transmission, ve-
locities.
Defect locations and relative significance are computed by special

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F I G . 3--Typical computer m a p o f a pressure vessel showing six defects analyzed over a

pressure range f r o m I00 to 122 kg/cm 2.

probability programs and displayed on a computer map (printout) which

is a scaled geometrically accurate layout of the test structure. The
computer maps display source location and significance by probability
numbers ranging from " 9 " for the highest probability, to ~ for the
lowest. An energy factor is displayed with the printout to allow a
significance rating of data to be made. Figure 3 is a typical computer
map of a pressure vessel showing six defects analyzed over a pressure
range from 110 to 122 kg/cm ~. This map indicates that there were six
defects emitting during this stress range. The most prominent source is
designated as a " 9 " , the next most prominent in a 9-in.-diameter area
designated by " 8 " , " 8 " , " 7 " ; there are four locations designated as
" 7 " which are the least prominent emitters.
Computer maps such as these are summarized as they come from the
computer by use of a scaled transparency of the structure under test.
Figure 4 shows the summary map for the test from which the Fig. 3
example was taken. It can be noted that the computer summary shows
all physical attachments and weld seams associated with the vessel.
Upon completion of a test the computer summary shows all defects
that emitted during the test; it tells the stress range during emission and
their relative significance to structural integrity.
In the case where a significant defect is detected during the test, the
TAC output is utilized as a first step confirmation of the energy release
signature, that is, the energy associated with the defect is computed.
The minicomputer probability map is utilized to cross check both the
TAC and energy release analysis.

Auxiliary Systems
In addition to the data analysis circuitry of the ACOUST systems,
visual and audio scan circuitry are e m p l o y e d to aid the system
operator. All signals are placed on analog magnetic tape for the pur-
pose of post-analysis of the test, should it be required, and also to
provide a permanent record of the test. Digital magnetic tapes are used
to store all computer derived data.
Figures 5 and 6 show the type of units used by Exxon Nuclear in
field service application of acoustic analysis technology.

Application Techniques
The application of the ACOUST system for the integrity analysis of
large structures requires that a coordinate system be established for
location of transducers and the referencing of pertinent physical fea-
tures of the structures. For example, in preparation for testing a large
vessel, engineering prints and tape measurements are utilized to estab-

Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 23:22:35 EST 2016
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158 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

, Circumference D
X Reference
TOP HEAD

1 7: 3 4

~ g Lugs

xo xd
b
t

/
~ A

Bracket-

- - ~ v v v v v ~ v .

xd
"//////
,,-~8 xc
/ /~av / /, "/////,l~'J/////, ~ Skirt

xd xd
11 12 #11~ 113 14
xd ~ _ _xcL __ 9 xd_ -
v
~d
v ~
.w .~ v. .

BOTTOM HEAD

LEGEND - - - ~ = defect areas

xd = transducer
~ = weld seams

F I G . 4 - - C o m p u t e r s u m m a r y m a p transparency o f vessel layout.

FIG. 5--Type of units used in field service application.

lish a coordinate system. After establishment of the coordinate system,

transducer locations are chosen, and their position is referenced to the
coordinate system. Transducer coordinates and vessel geometry are fed
into the computer program.
The number of transducers is governed by the complexity of the
structure and not its size. On a simple structure, such as a large
cylinder, only four transducers may be required to analyze the entire
cyclinder. However, if the cylinder contains numerous nozzles, weld
attachments, and internals, then additional transducers will be required
to provide precise defect location capabilities. This necessity becomes
obvious when it is recognized that each nozzle represents a void in the
cylindrical shell which acoustic signals must travel around in traversing
the vessel. In Fig. 7, it is evident that if only Transducers l through 4
are used there will be deviations in transmission path lengths as the
signals travel from the flaw to the transducers. This path length devia-
tion will cause inaccuracy in flaw location. However, if four more
transducers are added in the nozzle region, represented as Transducers
5 through 8; these four transducers will observe no perturbation in
signal transmission path length, and precise flaw location analysis will

FIG. 6---Type of units used in field service application.

be possible. It is reiterated then that the number of transducers re-

quired to analyze a structure depends upon its complexity rather than
its size. Normally 12 to 16 transducers are required to analyze a large
chemical or nuclear pressure vessel. However, there are exceptions
such as a boiling water nuclear reactor pressure vessel which contains
up to 24 nozzles and, accordingly, may require up to 24 transducers.
Once the transducer positions are selected, a small area at each
location is buffed free of scale or corrosion, and a transducer is at-
tached by either epoxy or by magnetic mounts.
Preamplifiers are positioned adjacent each transducer (within 15 ft),
and signal cables are connected to transmit the detected AE signals
back to the unit.
Once the system instrumentation is completed, calibration begins.
Calibration requires that simulated acoustic signals be induced into the

-Nozzles

FIG. 7--Vessel layout.

structure. These signals are of known energy content and are utilized to
determine transducer sensitivity and total system detection and defect
location capabilities.
The vessel coordinate survey, transducer positioning, hookup of elec-
tronics, and system calibration usually require one to two days on a
simple structure and three to four days on a more complex structure
such as large nuclear or chemical vessels. It may take up to 14 days to
prepare for conducting an integrity analysis of the entire primary cool-
ant system of a large nuclear power plant. However, it should be
understood that the setup and calibration of an acoustic analysis system
can be conducted simultaneously with the conduct of normal operation
or maintenance. Schedule delays are not incurred normally in the
application of AE emission analysis for the qualification of new struc-
tures or the requalification of in-use structures.
Acoustic data acquisition and analysis start with the application of
structural stress and continue through the structural loading period. On
pressure containment systems this period is described by a hydrostatic
or pneumatic stress ramp up to maximum pressure.
Computation of defect significance and locations is conducted
throughout the test. On completion of the test the computer summary
map may be used to define the position of various structural discon-
tinuities, the stress range through which they emitted acoustic signals,
and their significance to structural integrity.
The on-line analysis is supplemented normally by a post-analysis
period using magnetic tape recorded data. The post-analysis is con-
ducted to cross check and confirm all data results obtained during the
test.

Test Results
NDT in industry, using AE analysis, encompasses many types of
structures and structural materials. However, the tests chosen for dis-
cussion in this paper are confined to various types of containment
systems such as pressure vessels, storage vessels and tanks, piping
systems, and nuclear power plant primary coolant systems. Ninety
percent of the service tests conducted to date have involved these
types of structures.

Pressure Vessels
AE analysis is being extensively used for the qualification of new
pressure vessels and for the requalification of inuse pressure vessels.
Preservice acceptance tests of petrochemical or nuclear pressure

'~hr4
12:8oo*.~.JJ'~ 1

-<
0
z
-7
~_E~/CT o,F,ee~7"oe 9 ~s. o o o ,~'~zs. r~

/v2,4TC~l~d" S :

t-
e- r (wo~z~$),- J-J~t-f /I '~/o~,~Y
O
Z

FIG. 8--Chemical pressure vessel.

vessels or both are conducted normally at the manufacturing plant in

conjunction with the acceptance hydrostatic test. The vessel is located
usually in the production shop in a readily accessible position. The
vessel is surveyed, transducer positions are selected, and the transduc-
ers are mounted. Instrumentation is hooked up and calibrations com-
pleted. These operations can be conducted in conjunction with normal
manufacturing preparations for hydrotest. However, some complica-
tions can arise from the manufacturer's requirements to heat (using gas
heaters) and rotate the vessels up to the time of test. These types of
complications can be overcome by locating transducers on regions of
the vessel not exposed to the flames from the gas heaters. Calibration
and checkout of instrumentation is conducted during a short period
where vessel rotation is slowed or stopped. During this period the
signal cables are connected, and the calibrations are conducted. Then
the cables are disconnected so that rotation can be resumed.
Figure 8 illustrates a chemical pressure vessel tested under the condi-
tions just described [9]. Fourteen transducers were used to instrument
the vessel as shown in the vessel layout of Fig. 4. The vessel was
approximately 39 ft tangent to tangent with an inside diameter of 8 ft, a
wall thickness of 3.3 in.; with 0.2 in. of weld overlay cladding on the
inner surface. The vessel was constructed of A387 Grade C steel and
contained seven penetrations. The hydrostatic test involved a steady
stress ramp of 5 kg/cm 2 per min up to a maximum pressure of 122
k g / c m 2.
Figures 3 and 9 show two of the six computer maps generated during
the test. Figure 4 is the computer summary map transparency of the
vessel (in layout form). Twelve discontinuity regions were identified on
the vessel, eleven of these discontinuities were analyzed as Grade 1;
Source 7 was analyzed as a Grade 2. 4
Customer confirmation of these sources was obtained following the
test by the use of dye penetrant, visual inspection, X-ray records, and
ultrasonics.
The majority of the discontinuities were identified as fillet weld
cracks on external appendages; some extending 4 in. in length. Source
7 was identified as a weld slag inclusion 8 mm in length and 18 mm
below the surface; this discontinuity was analyzed and classified as a
Grade 2 which was within the acceptance levels for the vessel. It
should be noted that Source 7 was detected during one of the first
computer analysis in the stress range from 12 to 23 k as shown in Fig.
9. Its presence was observed throughout the test.

4 An explanation of Exxon Nuclear's grading levels are defined in the Appendix.

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;i::
:;.:~ii;
!:!;:3
~ii;
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:9;:IE:;iii:~:i:
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~:~~.:;,f!!;
f!!/!:!!;
G ~:!;~:
3~!!;
f!i,.3
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il#i!~
I-I[iZ4T? ( D~::fr~::~`~ ;:.::Z"~:F~`~
F::~``HT:'F`:~L.f f[`~ v~`.:H~~`.``~`H.]`~r:~;:F1~`

FIG. 9---Computer map showing pressure data point for range 12 to 23 kglcm 2.

The vessel was analyzed by AE analysis and confirmed by standard

NDT techniques; it was found to be of high integrity. It contained only
one discontinuity of Grade 2 level which was within the acceptance
levels of the vessel.
The use of AE analysis for the requalification of inservice vessels has
definite advantages over standard techniques such as ultrasonics and
radiography. For example, requalification of a catalytic reactor vessel
used in the oil industry would normally require the following:
1. Take vessel out of service.
2. Remove insulation.
3. Open vessel and remove catalyst.
4. Clean inner surface.
5. Scaffold interior and exterior surfaces.
6. Scan welds with ultrasonics.
This operation is expensive in terms of catalyst lost and unit down
time.
The simultaneous requalification of two such vessels using AE
analysis was accomplished as follows [10].
1. Six-inch "plugs" of insulation were removed from the vessels,
and acoustic transducers were installed.
2. System signal cable was connected and the system calibrated.
3. Both vessels were heated to near operating temperature and pres-
surized using hot flowing hydrogen (an operational gas).
4. Acoustic analysis of both vessels was accomplished during the
pressurization and the vessels declared free of significant discon-
tinuities.
5. The vessels were ready to be placed back in immediate service.
Inservice vessels are requalified normally by AE analysis during a
pneumatic or hydrostatic pressurization to operating pressure or 10
percent above. However, in certain cases where a vessel or piping
system cannot be taken out of service, a cyclic stress technique can be
used to evaluate acoustically the integrity of the structure. This tech-
nique requires instrumenting the vessel during operation. After calibra-
tion and checkout are completed, cyclic stress is applied to the vessel
by repeatedly dropping the pressure as low as possible and then in-
creasing it back to the operating pressure level. The majority of the AE
data is acquired normally during the top 10 percent of the stress cycle
as explained in the "Discussion" section.
Acoustic source information is collected during each cycle; These
data may be analyzed in an identical manner to that used for a straight
one-cycle hydrostatic or pneumatic test.
A classical example of the potential of this method for integrity

950
t~J

Cycle 13 ~e \ s
tx:
>-

I I ~ Acoustic I:(
t~J

~" { . I I Sig nature . LO

0
950-

Cycle 12 .~e
O~

coust,c JlJl/ ,.=,

A J Signature ,r ~4~.j/}

tJJ

Cycle 11 ~ k,9~ j

Acoustic .;~
J.J~1 Signature dd ~./14 tJJ

FIG. lO---Acoustic signature of cyclic failure.

analysis of a vessel is demonstrated by a recent test for the evaluation

of the capabilities of AE analysis [11].
In this test, AE analysis was applied to an experimental pressure
vessel which contained four concealed defects. The primary purpose of
the test was to assess the capability of the technology to detect and

Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 23:22:35 EST 2016
Downloaded/printed by
University of Washington (University of Washington) pursuant to License Agreement. No further reproductions
168 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

FIG. 11--Failed pressure vessel.

locate defects and predict impending vessel failure under the stress
conditions imposed by a cyclic hydrostatic pressurization.
One of the defects was detected only in the early stages of the test;
however, the remaining three defect regions were observed throughout
the test. Two of these three were identified as primary sources during
the final pressure cycles to failure; one of which caused vessel failure.
Vessel failure was predicted two cycles before it actually occurred. The
acoustic energy release signatures allowing this prediction are shown in
Fig. 10. Note that more acoustic data are present at lower pressures
during each successive cycle; also note the rapid increase in energy
release rate during the last 10 percent of each cycle.
Figure 11 shows the vessel and one of the uncovered defects follow-
ing the test.

Storage Vessels and Tanks

Storage vessels and tanks are often periodically requalified for ser-
vice by a random ultrasonic inspection of sections of various structural
welds. This technique leaves some doubt that all of the welds are in the
same condition as the sections that were chosen to be scanned.

I
0
z
-7
t~
t-
4 - m

'" I
I 2
t'l

I ....
J z
i
FIG. 12---Hortonspheroid sphere storage tank.

AE analysis provides a means of requalifying such structures by a

100 percent analysis of all welds. Such a test is illustrated by the
requalification of a Hortenspheroid storage tank measuring 142 ft in
diameter and 52 ft in height [12]. This vessel, a sketch of which is
shown in Fig. 12, was surveyed and instrumented with 20 transducers
as shown in Fig. 13. The vessel which contained 25 ft of fluid was first
stressed by filling slowly to the 29 ft level; this fill operation placed
stress primarily on the vessel walls. The vessel roof was next stressed
by the injection of 12.5 psig of air. AE data were detected, processed,
and analyzed through both fill and pressurization stages.
During the test, 16 minor discontinuities, as shown in Fig. 13, were
detected, located, and classified as Grades 1 and 2, that is, insignificant
to the integrity of the structure. Table 1 describes each discontinuity
location. The majority of the discontinuities were attributed to minor
weld defects and in some cases support bracket and flange movement.
Accordingly, the vessel was requalified for service.

Pipeline Analysis
A NDT problem common to many industries is the integrity analysis
of buried pipelines. AE analysis provides a quick, accurate, and effi-
cient means of evaluating miles of buried pipelines. Exxon Nuclear, to
date, has conducted nine pipeline tests using AE analysis.
These lines have included a wide variety of types such as radioactive
waste transfer lines, diesel fuel oil transfer lines, and natural gas trans-
mission and distribution lines.

T A B L E l--Discontinuity locations.

Source Phase of Test Location Description

S1 Phase I circumferential weld junction
$2 Phase II circumferential weld
$3 Phase I and II buckle in tank
$4 Phase II circumferential weld
$5 Phase I and II fire line support brackets
$6 Phase II circumferential weld junction
$7 Phase I ladder support
$8 Phase II circumferential weld
$9 Phase II buckle in tank at weld
S10 Phase I circumferential weld
S11 Phase II circumferential weld junction
S12 Phase II circumferential weld
S 13 Phase I circumferential weld joint
S14 Phase I circumferential weld joint
S15 Phase I circumferential weld
S16 Phase I and II possible flange movement

~nF~cRE~'Ti ~.

• 20
/
I I I
Legend
xd : t r a n s d u c e r
( ~ : Source

FIG. 13--Minor discontinuities in Hortonspheroid sphere storage tank.

FIG. 14---Special pipeline acoustic probe.

AE analysis techniques were utilized in these tests for both crack

and leak detection. The ability of the systems to detect and locate
minute leaks is phenomenal as demonstrated by a recent test on a 1400
ft section of buried diesel fuel line. Five detectors applied to the line
were able to detect and locate a 1-fta/h leak when the line was pres-
surized with air.
The number of transducers required to test a pipeline is dependent
upon the quality of the pipe, that is, grain size, type of joining welds,
etc. On old lines joined by poor quality butt welds, transducer spacing
may have to be as close as 200 ft; however, on high quality lines with
full penetration welds, transducer spacings can be as great as 1000 ft.
The ability of AE analysis to evaluate the integrity of a pipeline was
demonstrated on a 6600-ft section of 12-in. low pressure gas distribu-

tion line [13]. Attenuation measurements on this butt welded line,

which had been buried over 50 years ago, indicated the need for
transducer spacings of 230 ft.
To instrument the line the customer bored 2-in. holes through the
covering earth every 230 ft. A special pipeline acoustic probe (Remote
Acoustic Probe3), as shown in Fig. 14. was placed down each hole and
into contact with the pipe surface. Surface contact for good acoustic
coupling was made by a special sharp tip transducer located on the end
of each probe. Signal cables were run to each probe from a centrally
positioned mobile system.
The line was tested in two phases for comparison of stressing tech-
niques. The first phase required pressurizing the line to 120 psig using
nitrogen; during this pressurization acoustic data were accumulated and
analyzed. The second phase involved overhead loading of the line by
stressing with a heavy vehicle. Figure 15 shows the vehicle stressing
the line (which was buried 4 to 6 ft) by driving its length with one set
of wheels directly above the line.
Thirty-five emission sources were detected and located in the 6600-ft
pipeline. Figure 16 shows an example of a TAC display locating these
types of sources. All were analyzed as insignificant to the integrity of
the line under the imposed stress conditions.
Following the test the pipeline was excavated at the locations where
the six most prominent sources were indicated. Pipe welds were found
at each of the six locations. A section of pipe containing each weld was
removed from the line and capped for hydrostatic pressurization. Two
other weld sections not indicated by acoustic analysis as containing
defects (sources) were removed and capped for hydrotest as control
specimens.
These eight sections were cyclic stressed to failure in the laboratory
by the customer. All six welds that were indicated by AE analysis to
contain discontinuities failed after a relatively low number of cycles, as
compared to the two control sections [13].
After a series of such tests the customer concluded that AE analysis,
as applied during both pressurization and overhead loading of buried
pipelines, could detect and accurately locate defects in joining welds.
Acoustic analysis technology is now planned for use by this customer
in evaluating hundreds of miles of gas distribution pipelines.

Nuclear Plant Primary Coolant Systems

Perhaps the most complex use of AE analysis today is its application
for the integrity analysis of the primary coolant systems of large nu-
clear power plants. These systems contain large thick-wall, multinozzled

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174 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

FIG. 15--Heavy vehicle stressing the line.

FIG. 16---Defective welds in 180 f t section o f low-pressure gas line.

pressure vessels, three to four large pumps and steam generators, a

large pressurized vessel, valves, and hundreds of ft of thick-wall high-
pressure piping.
The preservice and inservice qualification of such systems requires
thousands of manhours of effort using ultrasonics and surface inspec-
tion techniques. AE analysis can provide an analysis of the complete
system during the perservice hydrostatic test or upon repressurization
for approach to p o w e r after shutdown maintenance and refueling
periods.
Figure 17 represents one loop of a four loop plant showing acoustic
transducer locations on pumps, valves, piping, and the reactor pressure
vessel. Note that each component is instrumented with four to six
transducers. These transducers are required for precise location of
structural discontinuities detected during the test.
Normally, 60 to 80 acoustic transducers and signal conditioning
channels are required for the integrity analysis of the complete primary
coolant system of a nuclear plant.
Figure 18 shows typical transducer locations on the reactor vessel of

P HEAO

A,,~....~_~//ht6" 4 ' '

gTEAm
GE~/ERA~'(>R ~ I@ERCTOR VERSEL

~Bt'5-~O

-- V/~

LEGEND
A# - ACOUST-~i~rransducer Positions
B# = ACOUST:I~B)rransducer Positions

FIG. 17---Plant showing acoustic transducer locations on pumps, valves, piping, and
reactor pressure vessel.

a pressurized water reactor plant (PWR). Transducer locations are

governed by the accessibility to the vessel. Accessibility is normally
available at the main nozzles and on the bottom head of the vessel.
The top head of such a vessel is a complicated structure containing
up to 60 penetrations as seen in Fig. 19. The top head is instrumented
with five to six transducers and is treated as a separate component.
This is required because it is isolated acoustically from the remainder
of the vessel by seal rings.
Figure 20 shows a top view of a complete PWR primary system.
Sixty transducers were used to conduct an integrity analysis of this
system during the preservice hydrotest [14]. (Not all transducers are
shown in Fig. 20). The transducers are positioned such that they may
be used to analyze each component of the system as well as the
attached piping. For example, Transducer 54 on the steam generator

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J
19 13 19 '§ 19 i ,~ 19 t a e ,e 19 '~
9
@ 19 ,~ 19 , ,
19 t ,

0
Z
-7
C7

I1h24 r

5z
FIG. 18----Reactor p r e s s u r e v e s s e l t r a n s d u c e r l o c a t i o n s .

"4
"4

FIG. 19---Nuclear reactor closure head.

~ssurizer

Pump12A / ~ . ~ ip11A

>
.<
;team m- m/ o i, ~ #~ ,,'| Steam C)
G ~enerator Z
9enera
eneror,, i 11 Z
:7
r-

Pump12Bq ~ ip 11B
c~

5
Z
&m
PIG. 20---Nuclear power plant primary coolant system transducer placement.

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180 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

and Transducer 85 on the coolant pump are used in conjunction with

other transducers to analyze the steam generator and pump volute.
However, they also are used to analyze the main coolant piping which
connects the two units.
The primary difference between the testing of a single structure and
the testing of a nuclear plant primary coolant system is that the nuclear
system contains multiple structures with multiple configurations all of
which have to be tested simultaneously. The entire system has to be
analyzed as a whole. This becomes obvious when it is realized that
acoustic signals appearing at the reactor vessel nozzle may not be
originating from the nozzle; they may be coming down the connecting

270u
900 101oi0 30 10

FLANGE

Ss 2~~

10_ 10
~32
t2 LOOPLEG
WELD
(s34

2D_
[z5 $28
L
20

WELD

30- --30
BOTTOM
HEAD WELD

1 l I
10 2G i0 4O 50 60

FIG. 2 1 - - N u c l e a r vessel c o m p u t e r s u m m a r y map.

50 40 30 20 i0

I I I I I
MAN MAN
WAY~ WAY

s41
10 10

COLDLEG COLDLEG
PUMP PUMP

I
50
I
40
I
30
I
20
I
10

FIG. 22--Steam generator 12 view inverted from computer map.

piping from a discontinuity in the pump or steam generator, or perhaps

from the pipe itself.
Each system component, therefore, has to be analyzed in conjunc-
tion with other system components that are associated with it. An
example of this is shown by Figs. 21 and 22. Acoustic Source $27
located on the reactor vessel steam generator nozzle and Source S1
located on the steam generator hot leg nozzle are both actually caused
by emissions generated by discontinuities located in the hot leg piping
connecting the reactor vessel and steam generator.
Additional complications that are imposed in the AE analysis of a
nuclear plant are the generation of AE's from component mounts,
flanges and bolts, internal structures, insulation drag, and fillet weld
cracks in attached brackets and supports. All of these sources have to
be recognized and identified.
Figure 21 shows the nuclear vessel acoustic sources defined during
the preservice testing of the primary system of an 880 MW nuclear
plant. Sources $23 and $27 were caused by discontinuities in attached
piping, Sources S21, $22, $24, $25, $26, $29, $33, and $34 were
attributed to minor weld discontinuities, for example, Source $25
showed up on X-ray as minor slag inclusions over a length of several
inches.
Sources $28 and $30 were areas of insulation drag as identified by
AE signal characteristics. Source $26 was cross correlated by trans-
ducers on the bottom head of the vessel as shown in the computer
summary transparency Fig. 23. This source was not defined with stan-

900 2700 900

60 50 0o 40 30 20 I0
] a I -'-',, ] 18~176
i I

10-, ~10

I I I I I
60 50 40 30 20 10

FIG. 23--Reactor vessel bottom head view inverted from computer map.

dard nondestructive techniques but is suspected to be a small area of

cladding disbonding.
The potential value of using AE analysis for the preservice and
inservice integrity analysis of nuclear plant primary coolant systems is
obvious. It is complete; it is rapid and it is accurate.

Conclusions
Acoustic emission analysis technology has application in a wide vari-
ety of industries for the integrity analysis of an even wider range of
structures and structural materials. Its acceptance, however, has been
more rapid in industries where it is applied to metallic pressure con-
tainment structures. These structures include pipelines, piping systems,
storage vessels, pressure vessels, and even complex pressure contain-
ments such as the primary coolant systems of large nuclear power
plants.
The results being obtained by this technology are showing it to be a
powerful new tool which has added a new dimension to the NDT field.

APPENDIX

Grade 1
An acoustic source (defect or discontinuity) which is a minor emitter and is
"insignificant" to structural integrity.

Grade 2
An acoustic source which is a predominate acoustic emitter, representing a
definite defect or discontinuity, but the presence of which will not cause
structural failure under the imposed stress conditions.

Grade 3
A "significant" defect which will grow to critical size if stress or loading of
the structure is continued.

References
[1] Pollack, A. A. and Smith, B., Non-Destructive Testing, Vol. 5, No. 6, Dec. 1972,
pp. 348-353.
[2] Hutton, P. H., "Proof Testing Ammunition Belt Links by Acoustic Emission,"
BNWL-SA-4522, Battelle-Northwest, Richland, Wash., March 1973.
[3] Green, A. T., Lockman, C. S., and Steele, R. K., Modern Plastics, Vol. 41, July
1964, pp. 137-139.
[4] Green, A. T., "Stress Wave Emission and Fracture of Prestressed Concrete Reactor
Vessel Materials," DRC-71-3 Dunegan Research Corp., Livermore, Calif., May 1971.
[5] Hardy, H. R., Jr., in Acoustic Emission, ASTM STP 505, American Society for
Testing and Materials, 1972, pp. 41-83.
[6] Knill, J. L., Franklin, I. A., and Malone, A. W., International Journal of Rock
Mechanics and Mining Services, Vol. 5, 1968, pp. 87-121.
[7] Harris, D. O. and Dunegan, H. L., "Acoustic Emission Testing of Wire Rope,"
De-72-3A, Dunegan/Endevco, Livermore, Calif., Oct. 1972.
[8] Kaiser, J., "Uentersuchungen fiber das Auftreten Gerauschen Beim Zugversuch,"
Ph.D thesis, Technishe Hochschule, Munchen, 1950, and Arkiv fi~r das
Eisenhiittenwesen, Vol. 24, 1953, pp. 43---45.
[9] Parry, D. L., "NDT-Acoustics Demonstration Tests, Bilbao, Spain," XN-108, Feb.
1973, Richland, Wash.
[10] Parry, D. L., "NDT-Acoustics Integrity Analysis of Two Catalytic Reactor Ves-
sels," XN-200, Jan. 1974, Richland, Wash.
[11] Parry, D. L., "Techical Report to Rolls-Royce and Associates for NDT-Acoustics
Testing of Culcheth Pressure Vessel," XN-129, May 1973, Richland, Wash.
[12] Parry, D. L., "NDT-Acoustics Test of Hortonspheroid Vessel," XN-109, Feb. 1973,
Richland, Wash.
[13] McElroy, J. W., "The Development of Acoustic Emission Testing for the Inspection
of Gas Distribution Pipelines," Philadelphia Electric Company, Philadelphia, Pa., Jan.
1974.
[14] Parry, D. L., "NDT-Acoustics Preservice of Calvert Cliffs Nuclear Power Plant Unit
I," XN-149, Sept. 1973, Richland, Wash.

Establishing Structural Integrity

Using Acoustic Emission

REFERENCE: Morais, C. F. and Green, A. T., "Establishing Structural

Integrity Using Acoustic Emission," Monitoring Structural Integrity Using
Acoustic Emission, ASTM STP 571, American Society for Testing and Ma-
terials, 1975, pp. 184--199.

ABSTRACT: Acoustic emission techniques have advanced from laboratory

use to applications such as establishing the quality of pressure containment
and many other structures. The authors discuss the statistical relationship
between acoustic emission data and the structural integrity of a series of
production pressure vessels. In this application, acoustic emission data
obtained from the first 25 percent of a required hydroproof test determined
the bursting strength of the vessel. Additional information contained in the
data is shown to establish the mode of structural degradation.
At least seven ways of utilizing acoustic emission data to establish the
quality of pressure vessels are presented. Use of acoustic emission tech-
niques to detect and locate a growing defect in a structure, while under a
relatively severe environmental condition, is presented.
Various methods of data processing are described and the relations to
parameters which may indicate the degree of quality of the structure are
shown. Data from materials such as glass-reinforced plastics, concrete,
glass, and metallics are presented. References to less well known efforts
show the universal nature of the technology.

KEY WORDS: acoustics, emission, pressure vessels, parameters, plastic

deformation, hydrostatic tests

The advent of filament-wound structures in the early 1960's required

the development of unique methods for verification of structural integ-
rity. This was particularly true in the area of nonhomogeneous rocket
motor cases. Early in the development of these motor cases it was
apparent that the required hydrostatic proof test was no guarantee of
remaining structural adequacy. Motor cases which had passed hy-

Project engineer and general manager, respectively, Acoustic Emission Technology

Corporation, Sacramento, Calif. 95815.

184
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Copyright9 1975byby ASTM International
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MORALS AND GREEN ON ESTABUSHING STRUCTURAL INTEGRITY 185

drotest successfully failed catastrophically during pressurization upon

rocket test firings. Motor cases which were not hydrotested also failed
during test firings. One leading project engineer, commenting on the
requirement for hydroproof test, observed that for this particular in-
stance "virginity was no proof of virtue."
In early 1961 the authors began a program to determine the relation-
ship between the " s o u n d s " emanating from the rocket motor case
during pressurization and the residual structural integrity of the
chamber. The program results showed the existence of a distinct rela-
tionship between the acoustic emissions and the residual integrity of the
chamber. The measured acoustic emission (AE) amplitude (measured
as the root mean square (RMS) of the total signal) during the proof
pressure hydrostatic testing is inversely related to burst pressure. In
addition the amplitude and frequency components of the AE data were
analyzed to show that two events were significant to establishing
chamber integrity. These were glass-filament laminar motion (that is,
interlaminar shear) and glass-filament strand fracture. Both events were
detected as AE data, each having distinct amplitude and frequency
components. The integration of the AE data resulting from these
events, over the hydrostatic test pressure increment, allowed a predic-
tion of the chamber burst strength from the developed relationship
between RMS AE (integrated) and burst pressure.
In recent years AE techniques have been used as a nondestructive
test method for providing early and ample warning of impending fail-
ure, detecting and locating hidden flaws, and detecting AE associated
with subcritical flaw growth. More generally, acoustic emission data
have been used in the following ways to establish the quality (that is,
structural integrity) of structures:
1. Detection of propagating defects or flaws.
2. Detection and location of propagating defects of flaws [1 ].2
3. Detection of gross or localized yielding.
4. Determination of pressure vessel burst strength.
5. Early warning of impending failure [1 ].
6. Determination of effective welding process parameters.
7. In situ monitoring of welding processes.
Each of these applications is a means of using AE techniques to assure
the quality or structural integrity of structures or both.
Filament Wound Pressure Vessels
The data acquisition system utilized accelerometers bonded directly
to the chamber as AE sensors, amplifiers, a magnetic tape recorder,
2 T h e italic n u m b e r s in brackets refer to the list of references a p p e n d e d to this paper.

0.9

0.8

~l 0,7

i 5 0.6

0.5

0.4
BURST PRESSURE('PSIG): 1714-b00
A E R~S (INTEGRATED)
,'~ 0,3

=
0.2
=

o i i L i i i
1200 1300 1400 1500

BURST PRESSURE CPSlG)

FIG. 1---Integrated acoustic emission amplitude versus chamber burst pressure.

and an analog computer. Similar portable systems were utilized at

various chamber manufacturer's facilities throughout the United States.
Up to five channels of data were processed through an analog compu-
ter in order to perform the integration of the AE data versus pressure
in real time. The AE integrated value was then correlated with the
burst pressure value of the particular chamber. During the course of
this production run, approximately 1 out of every 16 chambers fabri-
cated was tested to failure [2-5]. The relationship established between
the cumulative AE signal level and the burst pressure is shown in Fig.
1. The AE signal level (cumulative RMS) data were obtained over the
pressure increment from 20 to 60 percent of the maximum "prooftest
pressure level. Figure 2 illustrates the AE RMS (integrated) data versus
hydroproof pressure for a single AE sensor. Additional analysis deter-
mined that the integration could have been reduced to include the AE
data between only the 20 to 40 percent of hydrotest pressure level with
little loss in accuracy. It may be seen that between the 20 and 40
percent of full-scale hydrotest pressure the AE RMS (integrated) value
attains approximately 85 percent of its value at 60 percent of the
pressure. A statistical analysis of the data was used to develop the
mathematical relationship shown in Fig. 1. Production chambers, only
hydrotested, were monitored by this AE method, and the AE RMS
(integrated) value from a portion of the full hydrotest pressure range
was used to predict that chamber's burst pressure.

_ 40

z -- 30

_ 20

--|0

400 500 600

I I I
HYDROTESTPRESSURE(PSIG)

FIG. 2---4ntegrated acoustic emission amplitude versus hydrotest pressure.

An additional feature of the technique developed was its sensitivity

in detecting manufacturing process changes. The AE data shown in
Fig. 3 illustrate that a change in chamber material (upper and middle
curve) and a new manufacturing mandrel design (middle and lower
curve) were both defined clearly.
Similarly, frequency spectrum analysis of the AE data and further
detailed laboratory research clearly identified at least two of the modes
of chamber degradation; these being interlaminar motion between the
layers of glass rovings and fracture of discrete glass filaments. Figure 4
indicates the variations in the AE data which describe these events.

Metallic Pressure Vessels

The objective of these tests was to demonstrate the use of AE as a
nondestructive test method for providing early warning of impending
failure of A508 grade B, reactor steel, model pressure vessels. Minor
objectives were to demonstrate the ability to detect AE associated with
subcritical flaw growth [6].

Test Procedure
A block diagram of the test setup used in these investigations is
shown in Fig. 5. Three cylindrical pressure vessels were tested. They
were fabricated by welding end caps on to a machined section 7.7 in.

Z
6~
I I I I I I I I I

0 lO0 200 300 400 pre5~u~e,PSIG600 700 BOO 900 945

C~
~10.0 2
i, I H TYPE: PRODUCTION GLASS: S-994
I~I ~III . ~ , s. 75o796 DOILY: E-H~S, 1009, 3M

~O.l
F-

-(
8
| I I I I I I I I ),
0 I00 200 300 400 500 600 700 800 900 945 0
p r e s s u r e , PS]G C
lO.O
im~ lh , TYPE: PRODUCTION GLASS: S-994
i ,III IIL..I SN 7508N4 . DOILY: E-HTS, I 0 0 9 , 3M
1,O_
0
z
O.1

I I I
Ibo zob 3;0 4'00 50;
pressure,
ps~OO 700 ado 900 94s

FIG. 3----Acousticem~sion versus p r e s s u r e f o r each o f t h r e e m o t o r cases.

i o ~ \ , -
/ .'--LL____.~..~ .~ )..I_~
~

o X

g | g

~ ----=.j~. 0 s m
?

i ~ 1 7 6

0 0 0 0 0 C~ 0 ""
0 0 0 0 0 0

(ZH) ADN3nb3~NOISSIW39IISAOOV

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190 MONITORINGSTRUCTURALINTEGRITYBY ACOUSTIC EMISSION

J
RESET CLOC1

v '~
I RAMPGENERATORI
- -

PREAMP
y_OT_ i I TO,ALIZER
SCOPE
TEKTRONIX
I
Iro3s I
XECORDER
FIG. 5--Acoustic emission system block diagram.

long, 0.850 thick, and 5.52 in. outside diameter, each machined from
the middle of a shell forging of A508 steel. A circular arc flaw, 1.18 in.
long, 0.072 in. opening, and 0.23 in. deep was machined with a milling
cutter in the central portion of the vessels. The flaw was oriented in the
longitudinal direction. The bottom of the flaw was then electron beam
(EB) welded and exposed to an electrochemical process which caused
the EB weld to crack. As a result of this cracking a sharp crack front
was produced, which extended the overall crack depth to 0.33 in.
Two of the vessels were tested at a temperature of 130~ and the
other at 0~ A light weight oil was the pressurization medium used for
both temperatures. The pressure was recorded by manually reading a
pressure gage located in the control room. The pressure was controlled
by the opening and closing of a hand value on an air operated inten-
sifier.
With minor variations, a single test consisted of the following steps.
1. Initial incremental pressurization with continuous AE monitoring
to reach a given pressure step level and hold period. The hold periods
would vary depending upon the time it would take to monitor the strain
gages. After reaching a maximum pressure level and hold period, the
pressure was released,
2. A second incremental pressurization with continuous AE monitor-
ing to reach a new maximum pressure level and subsequent release of
the pressure.

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MORALS AND GREEN ON ESTABLISHING STRUCTURAL INTEGRITY 191

3. A third incremental pressurization with continuous AE monitoring

until failure of the vessel was imminent. The pressure was then main-
tained constant at this peak value until the AE subsided and the strain
gage reading was recorded. The pressure was then increased to failure.

Discussion of Results
The results obtained during these tests were conclusive regarding the
detection of AE from subcritical crack growth and crack instability.
Each of the three vessels underwent at least one repressurization
cycle. The total amount of emissions during repressurization was very
small or nonexistent. This absence of emission upon repressurization
was to be expected due to the irreversible nature of AE.

Vessel A1-C Tested at 130~

The first pressurization was 0 to 9000 psi and was accomplished
in 1500 psi steps. The second pressurization was from 0 to 15 000 psi in
1500 psi steps. The third pressurization was from 0 to 15 000 psi in 3000
psi steps. Pressure was then increased in 1000 psi steps until 29 000 psi
was reached, at which time the pressurization was increased to produce
strain steps of 0.2 percent. The data from these pressurizations are shown
in Fig. 6.

106

VESSEL VIAIC
MATERIAL 508
GAIN 86dB
BANDPASS - . I - .3MHz
DIFFERENTIAL TRANSDUCER
TEST TEMP. 130 F

J
----- FIRST PRESS CYCLE
X X SECONDPRESS CYCLE
THIRD PRESS CYCLE
SUMMATION OF ALL CYCLES

r i i i ~ i | i i ! i i ! J !
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

PRESSURE(PSI X 103)
FIG. 6---Summation o f acoustic emission versus pressure (composite) Vessel AI-C.

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192 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

105

VESSEL A|C
86dB
. I - .3NHZ

/
f
J
4.5 6I ;.s ~ 0' 3' 4:s 6~s r
PRESSURE (PSI X 1000)

FIG. 7a--Summation ofacoustic emission versus pressure to 9000psi Vessel Al-C.

lO 5
VESSEL A1C
86d8
.1-.31r

,J
/

/3 /4 z[ z~ ~r 2h 2~
P~EssuRE CPSI x 10001

FIG. 7b---Summation o f acoustic emission versus pressure 23 to 29 000 psi Vessel A1-C.

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MORALS AND GREEN ON ESTABLISHING STRUCTURAL INTEGRITY 193

VESSEL A1C
86dB
.1 - .3NHz

FAILURE

,j_-----

i
32.2 PRESSURE (PSI X 103)

FIG. 7c--Summation o f acoustic emission versus pressure 32 200 psi to failure Vessel
A1-C.

Replotted AE data obtained during pressurization are shown in Figs.

7a, b, and c. These data were replotted as a function of pressure due to
the step pressurization process used. At the 19 000 psi hold period,
increase in cumulative counts was observed. This observation remained
true for each of the subsequent 1000 psi loadings and hold periods up
to 29 000 psi.
Continuous emissions were observed starting at the 22 000 psi pres-
sure level. Prior to this point, all emissions were of the burst type. This
early continuous emission is thought to be indicative of the intervessel
wall yielding. From 22 000 to 29 000 psi both types of emission were
observed with less and less burst type as the vessel pressure neared the
29 000 psi level. At this point, burst emission gave way to the continu-
ous emission only. This would be indicative of the pressure vessel
undergoing gross plastic deformation.
It was also observed that the continuous emission decreased at each
higher pressure step beginning with 30 000 psi, at which point the burst
type of emission started increasing. This became more evident at the
31 300 psi pressure step. See Figs. 7b and c.
At pressure step level 32 000 psi, it was evident that the vessel was

nearing failure. It was predicted to fail within the next strain level
(pressure) step (normal pressure loss during each of the hold periods
was approximately 700 psi). The next strain level step was n e v e r
reached. At 32 000 psi, the vessel failed without reaching the maximum
pressure previously obtained.

Vessel A1-B Tested at O~

The vessel was submerged in an ethylene-glycol-water solution. Dry
_ce was used as a cooling medium. The sensor was attached in the
same manner used for the elevated temperature (130~ tests. The
cumulative emission data from this test were replotted as a function .of
pressure and are shown in Fig. 8.
The total A E counts from this vessel were approximately 47 percent
lower than for Vessel V1-A1-C and 56 percent lower than for Vessel A1-D.
A few possible reasons as to why this lower value was obtained were: (1)
pressurization medium was oil and would stiffen at the 0~ test temperature
which may have an attenuating effect upon the A E signal; (2) glycol-water
solution may tend to have the same effect as just described but to a much
less degree; (3) acoustical couplant used during the 0 to 22 000 psi portion of

106

VESSEL A1 - B

NATER[AL ASO8B

GAIN 84de

BANDPASS . 1 o , 314HZ

DIFFERENTIAL TP,ANS O0CER

TEST TEMP. OOF

i a i i i i i J f
4 8 1Z 16 20 24 28 3Z 36
PRESSURE(WSZX 1000)
FIG. 8---Summation o f acoustic emission versus pressure (composite) Vessel A1-B.

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MORALS AND GREEN ON ESTABLISHING STRUCTURAL INTEGRITY 195

this test was found to be frozen, no couplant was used after that time; and
(4) possible differences in the sensor sensitivity at 130~ and 0~ From the
continuous emissions it was observed that the inside of the vessel was
yielding at approximately 20 000 psi. At 29 000 to 30 000 psi, general
yielding occurred and the visual pressure gage indicated a loss of 10 000 psi
during the 5 min hold period. The vessel was then repressurized but only
reached a maximum pressure level of 29 500 psi. At this point, the test
engineer suspected a leak in or at the pressure vessel. AE did not show any
indication of a leak at or in the vessel. The leak was found at the pressure
relief valve in the control room.
Again, the vessel was repressurized (19 000 to 30 000 psi). It was
observed on the oscilloscope that the continuous emission was passing its
peak. During the entire yielding process (continuous emission) oscillos-
cope observation showed that the burst type of emission never disap-
peared. This would lead one to surmise that the crack continued to grow
during yielding which was not the case for Vessel AI-C. (This can be
readily seen by comparing cumulative emission plots of Vessel A 1-C to this
vessel.)
Upon reaching 33 000 psi, continuous emission was nonexistent, and the
burst type emission rate increased with a much larger amplitude. It was
noted during the hold period at 29 000 psi that the emission generally
continued shortly after the beginning of the hold period.
The pressurization line froze at the vessel during hold periods which
necessitated clearing the system. This was accomplished by opening and
closing the pressure relief valve to produce a shock to the pressure system.
As a result, this would decrease the start of the new maximum pressure step
to a lower pressure level, normally anywhere from 8000 to 10 000 psi lower
than the maximum pressure achieved during the previous pressure step.
After reaching 35 000 psi and holding until AE had subsided, the frozen
pressure system was then shocked and cleared. Repressurization com-
menced at 22 000 psi, and AE data began at 31 500 psi. This apparent lack
of irreversibility should be a subject for further investigation.
The AE data showed that the vessel was nearing failure at 33 000 psi. The
test engineer was notified of impending failure at 35 000 psi. The vessel
fractured at approximately 36 800 to 40 000 psi. The visual pressure gage
was oscillating between these values, and therefore an exact pressure value
was unavailable.

Vessel A 1 - D Tested at 130~

Pressurization Cycle 1 was 0 to 15 000 psi in 3000 psi steps with a hold
period at each pressure step level. The repressurization cycle was 0 to
32 300 psi in 5000 psi steps until 15 000 psi after which the pressure steps

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196 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

106

VESSEL V1A1 - O
HATERIAL ASOBB
GAIN B4dG
B.l#. lOOK-3OON
DIFFERENTIAL TRANSDUCER

TEHP. T30~F ~ FAILURE 1 HIN.

OF HOLD

HOLDING PRESS

/
H
/
i

FIRST LOADING ~.. -''0" I

PRESSURE (pSI X 103)

FIG. 9---Summation o f acoustic emission versus pressure (composite) Vessel AI-D.

were variable with a hold period at each pressure step level. Each hold
period would depend upon the time it would take to read and record the
strain data.
The cumulative emission data from this test were replotted as a function
of pressure and are shown in Fig. 9. AE data obtained during pressurization
are shown in Figs. 10a and b. As indicated in Figs. 10a and b, increased
cumulative emission counts were observed during the pressurization
period. Increase in cumulative counts during the hold periods started with
the 22 500 psi hold.
This vessel behaved in much the same manner as the previous 130~ test
vessel in the following ways: (1) increase in cumulative counts during the
hold periods started with the 22 500 psi hold; (2) continuous emission was
observed at 21 000 psi (believed to be due to yielding from the interwall as
in the autofrettage process); (3) peak gross yielding occurred at the 29 000
to 30 000 psi level; and (4) continuous emission decreased and became
nonexistent after reaching 36 000 psi. It was also observed that the rate and
amplitude of burst type emissions increased at the 31 000 psi level, and it
became evident that the vessel was nearing failure. The test engineer was
then notified of the impending failure at 32 300 psi at this point, pressuriza-

105
VESSELA1D
84dB
.1 - .3HHz
130~

/
i
i I
6 ~19 I~ [4 01 |01 ~5 16 17 %921 22,5 2527.S 28 28~5
~( 60 HINUTES
PRESSURE(PS! X IOS)

FIG. lOa----Summation o f acoustic emission versus pressure 0 to 28 5000psi Vessel A1-D.

5
I0
VESSEL AID
84dB
.1-.31~z

z
1 FAILURE

START OF HOLD
f /
/

i i i i i i

28.6 29.25 31 3"1.7 32.0 32.3

PRESSURE(PSI X 1C3)

FIG. lOb----Summation o f acoustic emission versus pressure 28 600 to 32 200 psi Vessel
A1-D.

tion was stopped and locked off. The vessel failed approximately 1 min
later.

Concrete Materials
During this investigation, a group of compressive concrete test cylinders
of three aggregate types was tested to failure under standard ASTM test
procedures. Standard cylindrical concrete compressive test specimens
were compressively loaded to failure in a hydraulic test machine, and the
acoustic emission date obtained from sensors attached either to the
specimen itself or the test machine platens were recorded on magnetic tape
and processed for the analysis and program report. In brief, the results
illustrated that the AE data from concrete is an indicator of failure
processes. Early warning of total compressive failure and preliminary
correlation with the material modulus were obtained. [7].
Amplitude distribution information was obtained for each of the nine
compressively loaded specimens. A statistical distribution showing the
probability of larger signal levels occurring has been plotted from the
average of the three specimens of each aggregate type. Figure l l shows the
cumulative probability plot for AE recorded during the tests. For compara-

I ! I I" I

O 188 ~eries - 3 ea - Limestone Mid. E

@ 19~ Seriea - 3 ea - Gra~acke ~w E

196 Srries - 3 ea - Chert High E
1.0

~~ 1.0

5 10 15 20 v 25

R e l a t i v e A c o u s t i c Emission S i g n a l S t r e n g t h dB
FIG. ll-----ProOabili~ofx > X versus re~tive A E s~nalstrength.

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MORALS AND GREEN O N ESTABLISHING STRUCTURAL INTEGRITY 199

tive purposes, the probability of larger signals occurring is inversely related

to the modulus of the concrete. Degradation of concrete strength is denoted
frequently by changes in modulus; thus, AE techniques may provide a
means of determining structural integrity of concrete structures in situ.

Conclusions
The AE data gathered on these programs, each encompassing different
materials, have been shown useful in providing a measure of the structural
integrity of the monitored item. With limited test specimens the AE data are
useful as a comparative evaluator. However, with a larger test specimen
population the AE data have shown itself to be capable of quantitative
analysis.
The data processing methods, as utilized in these efforts, are
straightforward and relatively simple to implement. Newer more sophisti-
cated methods hold promise for continued success as the technology
advances.

References
[1] Steele, R. K., Green, A. T., and Lockman, C. S., Method and Apparatus For
Nondestructive Testing Pressure Vessels, U.S. Patent 3.545,262, Dec. 1970.
[2] Green, A. T., Lockman, C. S., and Haines, H. K., Acoustical Analysis o f Filament-
Wound Polaris Chamber, Report 0672-01F, Aerojet-General Corp., Sacramento,
Calif., Sept. 1963.
[3] Lockman, C. S. and Green, A. T., Accelerometer Techniques as Applied on Polaris
First Stage A3 Production Chamber Hydrotests, Report 75-466, Aerojet-General
Corp., Sacramento, Calif., July 1964.
[4] Lockman, C. S., Polaris A3 Hydrostatic Proof-Pressure Accelerometer Data Results,
Memo 9633:SP023, Aerojet-General Corp., Sacramento, Calif., July 1966.
[5] Green, A. T., Lockman, C. S., and Steele, R. K., "Acoustic Verification of Structural
Integrity of Polaris Chambers," Society o f Plastic Engineers, Jan. 1964.
[6] Derby, R., First Intermediate Vessel Test, Report ORNL-4855, Heavy Section Steel
Technology Program, Oak Ridge National Laboratory, April 1973.
[7] Green, A. T., Stress Wave Emission and Fracture o f Prestressed Concrete Reactor
Vessel Materials, Report 4190, Aerojet-General Corp., Sacramento, Calif., June 1969.

Acoustic Monitoring Systems to Assure

Integrity of Nuclear Plants

REFERENCE: Gopal, Raj, "Acoustic Monitoring Systems to Assure Integrity

of Nuclear Plants," Monitoring Structural Integrity by Acoustic Emission,
ASTM STP 571, American Society for Testing and Materials, 1975, pp.
200-220.
ABSTRACT: A comprehensive program is underway at the Westinghouse
Electric Corporation to develop and evaluate acoustic monitoring systems
as a nondestructive testing tool for nuclear power plants that will assure
structural integrity with minimum interference to normal power plant ac-
tivities. A prime requirement for instrumentation for nuclear reactors is that
it should function reliably in severe environments for several years. Sensors
and instrumentation for source location and display developed for this
application are described. Results of tests performed in major test facilities
for vessel flaw growth, vessel rupture, and pipe rupture under various
operating conditions are summarized. Evaluation performed in nuclear
power plants both during hydro tests and operation of plant is presented.
Laboratory tests for studying relationships between acoustic emission and
fracture mechanics technologies are presented along with a summary of
wave propagation results. Measurements, to date, indicate that an acoustic
monitoring system has practical applications in nuclear power plants for
improving safety and availability of plants.

KEY WORDS: acoustics, emission, nuclear power plants, nuclear reactors,

instruments, pressure vessels, monitors

Inservice monitoring of the integrity of nuclear reactor pressure ves-

sels and pressure boundaries using acoustic emission (AE) activity
appears to hold considerable promise for ensuring safe operation of a
plant with minimum interference to normal power production. This
technique is a new, nondestructive testing (NDT) tool, which with
further experience and development, should provide a valuable addition
to the presently accepted methods of testing.

Manager, Instrument Systems Development, Westinghouse Nuclear Energy Systems,

Pittsburgh, Pa. 15230.

200
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GOPAL ON ACOUSTIC MONITORING SYSTEMS 201

Increased demands on improved safety are being put forth by en-

vironmentalists and regulators on nuclear plants. An astronomical
amount of engineering and financial resources, both private and public,
have been and are continuing to be used to develop, design, and build
systems which alleviate the adverse consequences of component and
equipment failures. Another approach to safety is to reduce the proba-
bility of the unwanted event to an acceptably low value through
monitoring of certain components and equipment for abnormal perfor-
mance during operation. Acoustic monitoring systems may be em-
ployed to reduce significantly the probability of any major failures during
operation of a plant. The systems are designed to monitor critical
components and equipment for abnormal performance during operation.
The detection of abnormal conditions is at the incipient level which
then provides ample time for corrective action before the situation
deteriorates to a significant failure condition.
Improvements in availability, required by plant operators based on
economic considerations, can also be achieved by reduction of inspec-
tion requirements and preplanning of inspection areas through effective
utilization of surveillance systems. Vessel inspection intervals, as well
as the areas to be inspected, can be selected based on AE activity.
Development of dependable surveillance systems represent a great
opportunity to increase the safety and economy of nuclear power
plants. An inherent advantage to the approach proposed here is the fact
that these systems contribute in a positive way to plant economics,
both in operating costs (improved availability) and capital cost (poten-
tial reduction in standby safety systems), as well as the improvement in
safety.

Background
Surveillance of pressure boundary integrity by AE techniques may
enable on-line detection of developing cracks and flaws and detection
of small leaks of the coolant in critical areas.
Zones of AE activity can be located in large structures by using a
moderate number of widely spaced sensors, and measuring the relative
arrival times of AE pulses at these sites. Source location is possible by
computation based upon relative signal arrival times to three or more
known locations.
Practical implementation of this monitoring technique in nuclear
power plants requires that the instrumentation meet a number of severe
requirements. For example, the sensor must be mounted on the reactor
pressure vessel where it will see high temperature and high radiation
for extended periods of time. All other instrument components housed

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202 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

within the reactor containment vessel will also be subject to relatively

high temperature, humidity, and radiation. In addition, the AE monitor-
ing system must be able to detect and locate the AE sources in the
noisy environment of the working reactor. This means that the fre-
quencies used for detection must be above the frequencies of the
acoustic background noise and below the frequency where severe
sound attenuation begins to occur in the pressure vessel material.
Acoustic Emission and Fracture Mechanics
The ability of the large steel pressure vessels that contain the reactor
core and its primary coolant to resist fracture constitute an important
factor in ensuring safety in the nuclear industry. In nuclear reactor
pressure vessels, some regions (beltline) are subjected to irradiation
embrittlement. The complexity of any safety analysis is increased by
the effects of irradiation damage.
Fracture mechanics technology has advanced to the stage where it is
of direct engineering value for the prevention of fast fracture in vessels
and piping of the type employed by the nuclear industry. Utilization of
fracture mechanics technology requires information and data in three
areas; material properties, stresses existing in the structure, and defects
in the structure. Expressions relating these three factors for the particu-
lar components and defect geometry of concern are also prerequisites
for the successful use of the technology. The total useful life under
cyclic loading conditions, as in a nuclear power plant, is dependent
upon the rate of growth of flaws from subcritical to a critical size.
Detection of subcritical crack growth is an important aspect of any
surveillance program. Thus, AE surveillance and fracture mechanics
technology complement each other and they provide a potential tool to
decrease the probability of failure in nuclear power plants.
Westinghouse Program on Acoustic Emission
A comprehensive program to develop AE technology for application
in preventing pressure boundary rupture of nuclear power plants is
underway at Westinghouse.
The functional requirements are:
1. Detect and locate onset of flaw growth in the pressure boundary.
2. Characterize flaw growth.
3. Evaluate effects of rate of growth and flaw size on planned in-
spections and allowable operating modes.
Major phases of the program are shown in Fig. 1.
This paper will summarize Westinghouse accomplishments in
1. Acoustic emission instrumentation developments.

DETECT I LOCATE EVALUATE

I
Source Sen s o p I Operator-Comparison Of
Actual Rate And
Size To Limits.

Plant Staff--Evaluation Of
Propagation II i DProcessor
ata Display
Inspection
Requirements
I (When, Where}.
I

FIG. 1--Various phases o f acoustic emission technology.

2. Evaluation of major test facilities.

3. Evaluation in nuclear plants.
4. Evaluation in laboratory facilities.

Acoustic Emission Instrumentation System

An AE instrumentation system primarily consists of a matched and
balanced s e n s o r - c a b l e - a m p l i f i e r s u b s y s t e m which permits the
piezoceramic sensor's low-level electrical output signal to travel with
relatively little attenuation or interference over a long distance and a
data processor for source location and analysis.
Sensor and Electronics
Significant breakthroughs in sensor design and signal transmission
have been made by Westinghouse to meet the requirements for reactor
pressure vessel acoustic surveillance. A sensor that is expected to
operate for long periods of time (years) in the high-temperature, high-
radiation, and high-humidity environment has been designed. The sen-
sor acoustically couples directly to the metal structure that is being
monitored, eliminating the need for waveguides, and it can drive long
lengths (500 ft or more) of cable without a preamplifier.

FIG. 2--Westinghouse acoustic emission sensor.

The sensor is designed to be part of a matched and balanced passive

electrical transmission system. The sensor c r y s t a l ' s impedance is
matched to the impedance of a transmission line, and this line imped-
ance is then matched to a high input impedance preamplifier. This
system is able to drive several hundred feet of cable between the
crystal and preamplifier with very little signal attenuation.
Figure 2 shows the nuclear reactor grade severe environment AE
sensor.
The sensor is hermetically sealed and meets the following environ-
mental specifications:
Temperature: +550~ at the face of the transducer and +300~ at the
connector
Pressure: 100 psia
Humidity: I00 percent
Radiation: 500 R/h
Vibration: 0.03 in. double amplitude --+0.003 in. in the 10 to 30 Hz
range and 1.0 g (0 peak) - 0 . 1 g in the 30 to 500 Hz range
Shock: 15 g (11 ms, halfsine)
The sensor is made to be easily coupled acoustically to any structure
using a minimum of surface preparation. All that is required is to press
the sensor face against the structure's surface using a moderate com-
pressive force, which is supplied by a spring. The spring force is
transmitted to the sensor's face through a retaining ring which snaps
into a special groove located near the face. The mounting fixture is
attached to the test structure either mechanically, using C-clamps or
large hose clamps, or magnetically with a group of magnets having a
total rated holding force of about 280 lb.
Figure 3 shows the electroacoustic sensitivity of a sensor compared to
a primary standard lithium sulfate sensor. The sensor is calibrated by
comparing its output signal against that of a primary standard. Both
sensors are acoustically loaded by a medium that is representative of a
nuclear reactor pressure vessel, and both are exposed to the same
sound field. The primary standard has a flat response and a sensitivity
of -124 dB referenced to 1 V per microbar. The sensitivity of the
sensor is 34 dB above that of the standard at a frequency of 500 kHz;
that is, its equal to about -90 dB referenced to 1 V per microbar.
The AE sensor system presently uses a RG-22B/U cable. This cable
has a 95 ohm characteristic impedance, a propagation velocity equal to
65.9 percent of free space velocity, and a 1 MHz attenuation constant
of 0.24 dB per 100 ft length.
In the signal-conditioning electronics, special care is taken to ensure
that the preamplifier first stage has as small a noise figure as possible.

Amplitude (db)

- 2 0 dbx m

-30

-40

-50

~70

L Response 10 KHz Spectrum

-80 Of Standard Analyzer
No. 20979 Bandwidth
-90 I

I I I I I I
100. KHz 300. KHz 500. KHz 700. KHz 900. KHz 1.1 MHz

Frequency
FIG. 3---Acoustic emission transducer sensiticity calibration.

All subsystems are compatible with each other for impedance-matching

and for achieving a high signal-to-noise ratio. A bandpass filter with
sharp cutoff characteristics (>50 dB/octave) is used to pass only fre-
quencies in the range of 300 to 700 kHz. The filtered and amplified
signal then is led into a discriminator which produces a digital pulse for
analog signals above a threshold voltage level. The threshold level is vari-
able and generally set at a level slightly into the background noise.
Digital pulses are fed into the analysis system.

Analysis System
AE signal totalization, source location, and some source display are
performed in CAMAC modules and a mini computer. The analysis
system along with an optional rack containing a cathode ray tube
(CRT) display and a hard copy unit installed at a nuclear plant is shown
in Fig. 4. This system is capable of monitoring an entire vessel with 20
or more channels by arranging transducers in zones made up of four
transducers each. From the measured differences in time of arrival of

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206 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

FIG. 4--Westinghouse acoustic emission monitoring system.

the acoustic pulses, the known location of the transducers, and a

known (empirical) velocity of pulse propagation, the computer performs
calculations to locate and display any flaws that may be present in the
vessel being monitored. A modular approach to system design has been
followed using a CAMAC-type interface so that a complete system may
be configured and easily modified or updated for a given requirement.
Location and totalization systems are configured in modular forms to
accommodate multisensor inputs. The location module contains cir-
cuitry for addressing, interrupt requests, and buffer registers to store
data for input to the mini computer. A module contains eight identical
circuits.
A totalization module contains eight identical counter circuits to
monitor eight sensor inputs. Each circuit has a 24-bit counter as-
sociated with it to register a possible 224 acoustic events. The counter
output signals are fed to data selectors which decode the bindary data
for presentation to the PDP-11 mini computer via the CAMAC data-
way.
A mini computer is used to process the acoustic data and drive the
CRT display. The computer processes the data, supplied by the

CAMAC location modules, using a hyperbolic triangulation technique

to find flaw locations. Once a location is found, the " h i t " is scaled and
plotted on the CRT, and also stored in a buffer for further analysis
offline. A flat pattern of the reactor vessel outline is traced on the CRT
by the computer, with each hit plotted on the fiat pattern to enable
easy reference on the actual vessel. The hard copy unit can be used to
keep a permanent record of the hits as they appear on the flat pattern,
along with corresponding X, Y values in reference to a known location
on the vessel. Hard copies can also be made from a display of total
counts for each channel that is displayed on the CRT on demand.
Cassette storage of the total counts per channel can be used to provide
rate information as these data are logged on a periodic basis, usually
once an hour.
Measurement Programs in Test Facilities
Of prime importance in determining the worth of AE monitoring is
testing the detection of emitted acoustic signals against natural back-
ground noise. Several tests were performed in major test facilities to
obtain information on the nature and magnitude of acoustic signals
emitted by reactor pressure vessels and piping having intentional de-
fects leading toward catastrophic failure.

Nondestructive Test Method Evaluation Facility (NDTF) Tests

The NDTF testing was performed under the Edison Electric Institute
Research and Development Program for Inservice Inspection of Nu-
clear Reactor Pressure Vessels [1]. 2 This test facility provided the
proper environment for determining the capability of various NDT
systems to detect existing or developing flaws in a pressure vessel
operating in environments typical of those found in present-day, large
nuclear power plants.
The facility was constructed by modifying the experimental beryllium
oxide reactor (EBOR) vessel which was 11 ft in diameter, 26 ft high,
and had wall thickness varying from 3 to 8 in.
A flaw was introduced into the pressure vessel or appurtenances, and
the area was then stressed hydrostatically or by mechanical means to
provide failure sites for evaluation of acoustic and other NDT equip-
ment.
The filling, circulating, and pressurizing system provided the normal
sounds associated with water flow and operating equipment. Additional
noise similar to that in large nuclear power plants was provided by
external generators to evaluate the capability of the acoustic system

2 The italic numbers in brackets refer to the list of references appended to this paper.

being tested to detect and locate the various flaws in the noisy envi-
ronment typical of operating reactors.
The significant conclusions resulting from testing at the EBOR test
facility are:
1. AE technology is applicable for use in monitoring the integrity of
the primary system boundary of a nuclear reactor.
2. AE's from flaws in the vessel and fatigue bars welded to the
vessel could be detected in the presence of simulated normal reactor
background noise and at elevated temperatures.
3. AE sources resulting from flaws both in the interior and exterior
of the test vessel were successfully detected and located.

Vessel Rupture Tests

Intermediate size reactor vessel rupture tests performed under the
Heavy Section Steel Technology program at Oak Ridge National
Laboratory [2] have been monitored for acoustic activity during pres-
surization until rupture.
The test specimen was a cylindrical forging of 6-in. carbon steel. The
outside diameter was 39 in., and, after welding heads on the top and
bottom, it was about 8 ft tall. The vessel was installed upright in a
concrete cavity which was sealed prior to the start of the test.
The significant results of three tests are summarized as follows:

Summary of Vessel Rupture Tests

Test
No. Test Conditions Results

1. Single flaw: 8 in. long, 2 in. deep, all four channels showed exponential
130~ increase in activity; location system
showed location of flaw growth
2. Two flaws: one in base metal and all our channels showed activity; weld
other in weld--both 8 in. long, 2Y2 flaw acoustic emission activity was
in. deep, 75~ greater; triangulation showed growth
near both flaws--higher concentration
near the top of weld flaw and great-
est increase from 26 000 psi to rup-
ture at 26 450 psi at the weld flaw
3. Three flaws: A.0 ~ weld exterior, B. eight channels were used; exponental
base metal exterior, C. 18& weld increase in counts was not predomi-
interior, 19&F nant; location s h o w e d initial flaw
growth to be higher at C than at A
and B; however, towards end of test
(from 29 000 to 32 000 psi) significant
growth was observed at Flaw A where
eventual failure occurred

Pipe Rupture Tests

Westinghouse has participated in tests at the Atomic Energy Com-
mission sponsored pipe rupture test facility at San Jose [3] to measure
AE signals indicative of the failure of reactor grade pipes. In the initial
two tests significant electrical and mechanical noise problems were
encountered. However, true AE burst signals correlated qualitatively
with defects observed by ultrasonic techniques. Noise problems were
solved by appropriate selection of electrical and mechanical isolation
techniques for the third test. Excellent data were obtained during this
test.
The test specimen was a 6-in. Schedule 40, Type 304SS elbow, one end of
which was welded to a short vertical stainless pipe which was bolted to the
test stand. The other end was welded to a short horizontal section which in
turn, was welded to a carbon steel section connected to the hydraulic
loading mechanism.

Acoustic E m i s s i o n - - M i l l i o n s Of Counts

Rupture
3.6

3.2 Transducer A
i 1.2
2.8 m

Transducer A 1.0
2.4 m

2.0 m .8
Transducer B
1.6 Thru Wall Crack .6
1.2 m

.4
.8 m

.4 m
~mm
.2

I I I I I I I I I I I I
0 4 8 12 16 20 24 28 32 36 40 44

Loading Cycles In Hundreds

FIG. 5---Fatigue cycles versus total counts.

The test plan called for cyclic pressure and flexural loading at a
frequency of 12 cpm.
Periodic inspections were carried out with ultrasonic detectors.
After initial failure of the specimen, cycling was continued without
pressurization for additional AE data acquisition.
The results of the tests are:
1. The quiescent noise of the transducer-amplifier system was very low.
2. The physical appearance of the signals were the burst type emission
with a sharp leading edge. They appeared to come in packets.
3. Even in the presence of electrical and mechanical noise sources
during the first two tests, qualitative correlation was observed between
true AE signals and flaws observed by ultrasonic techniques.
4. Cumulative counts from a channel as a function of number of cycles is
plotted in Fig. 5 which reflects the classical data pattern for the fracture of a
stainless specimen.

Leak Detection System Tests

The goal of an acoustic leak system test is to develop a system that will
detect "through wall" leaks from the reactor coolant system boundary of
less than 0.02 gal/min during plant operation even in the presence of larger
leaks in less critical areas such as pump seals.
A series of tests were conducted at Westinghouse Forest Hills facilities
to determine the properties of continuous AE's generated from pressurized
water leaks. Special piping was mounted on top of an autoclave (Fig. 6).
Water leaks were analyzed and data recorded for conditions varying in
pressure and leak flow rates. The results indicate that the acoustic
monitoring system is capable of detecting small leaks on the order of 0.02
gal/min or smaller, as shown in Fig. 7.

Plant Tests
Westinghouse has pioneered the development of AE instrumentation
systems for use in severe environments such as found on nuclear power
plants. Extensive conceptual and feasibility tests have been carried out in
the following locations:
1. Saxton Reactor.
2. Robinson Unit 2.
3. Turkey Point Units 3 and 4.
4. Ginna.
5. Prairie Island Unit 1.
6. Beaver Valley Unit 1.
Significant results obtained using recent instrumentation are covered in
the following sections.

Leak Simulator
2.0 In. Schedule 80 Pipe r ..... : Steam Capture

Transducer L. . . . .

FIG. 6---Leak detection s y s t e m test.

Signal Level
A b o v e B a c k g r o u n d N o i s e (DB)

100 m
Water T e m p e r a t u r e 5 7 5 ~
Approximate Pressure
2 0 0 0 PSI
~S

/:
90 m

80 m

70 m

#.
f
60

I I I
0.01 0.015 0.02 0.023

L e a k R a t e - - G a l l o n s Per M i n u t e
FIG. 7---Acoustic signal level at various leak rates.

Primary System Attenuation Tests

The attenuation test consisted of introducing an acoustic signal at a point
on the vessel and measuring response characteristics at several points on
the system. An isometric view of a plant primary system is shown in Fig. 8.
With signal injection at the top flange of a typical reactor vessel,
attenuations measured at various points are summarized in Table 1.
Surface discontinuities, for example, welds increase the attentuation of
the acoustic wave.
The characteristic wave shape of the simulated acoustic wave changes
when the signal is introduced on the vessel interior and detected by exterior
transducers. The sharp leading edge, present for waves generated on the
exterior, is not as predominant in waves originated from the interior.
Attenuation on the head structure was high, but signals could be detected
by transducers mounted on the lifting lugs.

Hydro Tests
Acoustic monitoring of in-plant hydro tests have been made on two
nuclear plants.

3 Hidden

9 Transducer Location

FIG. 8--1sometric view of primary system.

TABLE 1--Primary system attenuation.

Point Location Signal Level

1 Top flange of vessel; 0 deg 0 dB

2 Top flange of vessel; 180 deg from point - 14 dB
3 Inside vessel 1 in. below 0 deg nozzle -26 dB
4 Pump to vessel pipe 0 deg; 8 ft from vessel -34 dB
5 Bottom of vessel -37 dB
6 Steam generator to pump piping -45 dB

Typical installation of transducers is shown in Fig. 8. The location of the

pressure vessel transducers was dictated in large measure by accessibility.
The biological shield restricts access to the circumference except at the
nozzle area and bottom of the vessel.
The ventilating shroud limits top head access. The compromise locations
were control rod ports and head lifting lugs as the outermost available
points.
Signals typically encountered are classified into three groups:
1. Long-duration signals with a sharp leading edge and decaying
envelope characterize acoustic emissions from flaw propagation and stress
relieving of welds.
2. Signals with a slow rise time and a decaying envelope are typically
associated with a mechanically induced signal.
3. Spikes are typically associated with electrical interference. Confirma-
tion of this fact is accomplished by noting zero time difference between
different channels.
A planar layout of the reactor, Fig. 9, is used for location. The vessel
shell is examined by using transducers on the nozzles and bottom of the
vessel. The nozzles are shown in a planar view to establish minimum path
lengths. The nozzle transducer position is rotated 90 deg into the plane of
the pressure vessel outer surface. Total minimum path lengths between
transducers on the bottom of the pressure vessel and nozzle are obtained by
adding the nozzle minimum path to the pressure vessel shell minimum path.
Figure 9 also shows a typical triangle used for source location.
Location of sources in the bottom head is identical to that used for the
cylindrical portion except that it involves calculation of angles and time
lengths of the spherical triangle formed by the great circle paths connecting
transducer locations. Although transducer location was dictated by
accessibility, this did not interfere with source location.
The results show that reactor vessels were relatively quiet, acousti-
cally, during hydrostatic testing, with few large amplitude bursts from
different locations near welds. This phenomenon is probably caused by

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i.......... .i .I . . . . . . . I
hso*J~" " " " 127o0 " "~-1o* ~ " " 1~* " " J:,8o*l
0
z

I i QI zO
L Outlet I Inlet Outlet Nozzle I U
'I/ Boundary Inlet F

.A./.A. A A. ~

o
C
I
R

o
Location Of High-Level z
Legend: ~ ~ Burst Source
[] Transducer ~ / 9
Weld Line T -- Location Of Low-Level
Burst Source

FIG. 9--Prairie Island pressure vessel layout and source locations.

redistribution of residual stresses since only a few bursts were observed

during the test.

Measurements During Plant Operation

Results of background noise readings taken at various temperatures are
summarized in Table 2.
The background noise decreased significantly with increase in tempera-
ture.
Signal propagation characteristics measured with simulated acoustic
signals showed little variation at ambient (no noise) conditions and normal
operating condition as shown in Table 3.

Laboratory Testing
Acoustic Emission and Fracture Mechanics Program
At Westinghouse work has recently started on a joint acoustic emission
and fracture mechanics program. This work is being pursued to develop the
relationship that exists between AE activity and the magnitude of the stress
intensity factor, K, or the crack emitting the sound. The stress intensity
factor is a fracture mechanics parameter that describes crack severity in
terms of crack size and the stresses acting on the structure. By knowing the
relationship between AE activity and the stress intensity factor for
different materials, geometrics, and loading arrangements, it should be
possible to determine to some degree flaw severity by "listening" on the
surface of the structure with appropirate instrumentation. This can
possibly be done even when the flaw site is completely hidden--the only
requirement being that there be a good acoustic transmission path between
the flaw site and the transducer location.
The study of the relationship Between AE and fracture mechanics is still
in its infancy. Some of the pioneering work by Tetleman [4] has shown that
the AE count rate dn/dt is proportional to the stress intensity factor raised
to the fifth power; that is, dn/dt = C1K 5. Consequently, small changes inK
lead to rapid increases in count rate. In this manner, it is possible to
associate a critical count rate with a critical K-value (Kc), and hence with
the onset of failure.
AE monitoring of the fatigue testing of compact-tension (CT) speci-
mens made of A533B pressure vessel material has been recently initiated
at Westinghouse. Westinghouse AE instrumentation is being used.
Several 2T-CT specimens (that is, 2 in. thick) and 4T-CT specimen (4 in.
thick) have been tested. To prevent extraneous noise, such as rubbing
noises generated at the specimen load pin interface, undersize load pins
were made and encased in electrical shrink fit tubing. In addition, glass

Reactor Condition Root Mean Square Noise at Typical Locations

Temperature, ~ Pumps Botton, V Inlet Nozzle, V Outlet Nozzle, V Head, V Steam Generator, V
tO
..d
O~ 70 0 0.035 0.035 0.035 0.035 0.038
250 2 0.53 0.29 0.42 0.2 0.122
350 2 0.34 0.19 0.275 0.115 0.097
530 2 0.095 0.048 0.066 0.041 0.054

T A B L E 3--Effect of temperature on signal attenuation signal levels, volts peak to peak.

Channel 70~ 350~ 530~

Bottom center 15 15 20
Shell 0 deg 8 . 8
Shell 180 deg 7 "4' 7

tape was laid on the specimens on all areas where the specimen might
touch the load clevis.
The testing was conducted in conventional test machines in the fatigue
laboratory using hydraulic actuators and electromechanical servo control
systems.
This setup worked well and ensured that only noise generated at the
fatigue crack tip reached the AE instrumentation.
A system gain of 60 dB, a pulse width of 10/xs, and a discriminator setting
of 80 mV was selected for the tests based on previous experience. For the
latest tests, a " g a t e " was built for the system which disabled the
discriminator and totalizer near minimum load and thus permitted data to
be collected separately on signals emitted at or near the maximum test load.
This was done because acoustic signals occurring at maximum load were

Crack Length (Inches) Thousands Of Counts

l
2.8 ! 280
< / I
2.4 I 240

2.0 m
200
i+
1.6 160

1.2 120

.8 I 80

.4 40

I I I I I
40 60 80
Load Cycle (104)

FIG. lO.--Crack length and gated counts versus cycles for 4-in. specimen.

considered more meaningful. When this gate was used two discriminator
and totalizers were used, one being gated and one not.
Fatigue cycling was conducted in a tension-tension mode with an R ratio
of 0.1, that is, Pmi. = 0,1Pmax and a sinusoidal waveform A test frequency
o f 1 Hz was used t h r o u g h o u t all testing e x c e p t in some selected cases
where a f r e q u e n c y o f 10 Hz was used to initiate the fatigue crack.
Typical results are shown in Figs. 10 and 11. An a p p a r e n t relationship is
seen between counts and crack length.

A c o u s t i c Emission Wave Propagation and W a v e M o d e D e t e c t i o n Studies

On its travel through a structure such as a pressure vessel, from the point
of origin to the point of detection, the waveform of an AE burst becomes
distorted. First of all, the intensity of the signal diminishes as the wavefront
diverges from the source because of spreading loss, absorption, and
reflection from obstacles. Also, the w a v e f o r m ' s temporal structure
changes because the relative energy distribution between compressional,
shear, and surface waves is constantly shifting due to reflection and mode
conversion at the surfaces and because o f sound velocity differences. A
detailed understanding of the manner in which the waveform distorts is

Crack Length (Inches) Acoustic Emission Counts (Units Of 10 4)

1.6
I
I
1.4 I

1.2
/, u

1.0 I

.8 /
~e
.6 m

~ o~
.4 I

.2
J

I I I I I I I
2 3 4 5 6 7

Load Cycles (Units Of 10 4)

FIG. 11--Crack length and gated counts versus cycles for 2-in. specimen.

necessary if information measured at the transducer face on the structure's

surface (far-field data) is to be intelligently related to what is happening at
the crack tip (near-field data), and to what is measured on fracture
mechanics specimens in the fracture laboratory (reverberant data).
In order to study such distortion and predict location of the crack from
the signature itself, controlled tests have been run on an 8-in.-thick by
7-ft-wide by 12-ft-long steel plate. It was found that the waveshapes of the
received signals differed markedly depending on whether the plate was
excited acoustically on the top or the edge. For top side excitation, the
received signal waveform was independent on the spacing between the
sound source and receiving transducer, with the waveform primarily
composed of a single surface of Rayleigh wave of short-time duration.
For edge excitation, the received waveform comprised a group of
individual pulses, with the number of pulses increasing as the spacing
between the sound source and receiving transducer increased. The
multiple pulses were caused by reflections and mode conversions of
acoustic signals striking the top and bottom surfaces of the plate.
Test data, for both major surface and edge excitation, were confirmed
theoretically. A computer program was written to predict the shape of the
received acoustic pulse group for the thick flat plate case.

Conclusions
Results obtained from various phases of the program demonstrate:
1. Availability of a direct mounted transducer and a data processing
system suitable for nuclear plants.
2. Feasibility of identifying various stages of failure in pipes and vessels
such as flaw initiation, growth, and leak in the presence of noise.
3. Feasibility of locating flaws within a few inches.
4. Ability of systems to monitor plants on a continuous basis.
5. Feasibility of correlating AE events to crack growth under controlled
condition on specimens and test vessels.
6. Problem areas in characterizing AE signals measured at distances
from source.

Acknowledgments
In Westinghouse Electric Corporation, principal investigators are: W. C.
Leschek and C. F. Petronio in the transducer and electronic area; M. A.
Lebeda and P. J. Hite in analysis and display systems; A. F. Schmidt, T. R.
Sanders, and J. Craig in measurements; and V. J. McLoughlin and J. Craig
in laboratory tests. Excellent cooperation and participation of personnel at
various plants and test facilities are gratefully acknowledged.

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220 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

References
[I] "In-Service Inspection Program for Nuclear Reactor Vessels," Edison Electric Insti-
tute Project Biannual Report 1 to 6 by Southwest Research Institute, San Antonio,
Tex.
[2] "Heavy Section Steel Technology Program," Semi-Annual Reports, Oak Ridge Na-
tional Laboratory, Oak Ridge, Tenn.
[3] "Reactor Primary Coolant System Rupture Study," GEAP-10207, Atomic Energy
Commission Research and Development Quarterly Reports by General Electric Co.,
Atomic Power Equipment Department, San Jose, Calif.
[4] U.S.-Japan Joint Symposium on Acoustic Emission, July 1972, Proceedings.

Detection and Location of Flaw Growth

in Metallic and Composite Structures

REFERENCE: Kelly, M. P., Harris, D. O., and Pollock, A. A., "Detection and
Location of Flaw Growth in Metallic and Composite Structures," Monitoring
Structural Integrity by Acoustic Emission, ASTM STP 571, American Society
for Testing and Materials, 1975, pp. 221-240.

ABSTRACT: The application of a multichannel real-time acoustic emission

location system to the detection and location of structural defects in three
different materials and geometries is presented. Test results from welded steel
pressure vessels demonstrate that incremental crack growth during pressuriza-
tion can be accurately located and assessed. Additional results from graphite-
epoxy honeycomb structures indicate that acoustic emission is also a viable tool
in detecting and locating impending failure in composite materials.

KEY WORDS: acoustics, emission, pressure vessels, crack propagation, com-

posite materials, failure, honeycomb structures

Acoustic emission (AE) is the term applied to the impulsively produced

elastic waves produced by a material subjected to stress. Plastic deforma-
tion and the nucleation and growth of cracks have been found to be two of
the primary sources of AE from metals. The use of AE techniques to
ascertain the integrity of pressure vessels has been extensively covered in
the literature [1-16]. 3
AE techniques can be used to locate defects in structures, in addition to
detecting them. Source locations are determined by placing arrays of
sensors on the part and comparing the relative arrival time of the stress
wave at the various sensor locations. Most of the previously published
work on AE locational techniques [1,7-16] have been restricted to metallic

1 Field test manager and director of research, respectively, Dunegan/Endevco, San Juan
Capistrano, Calif. 92675.
2 Dunegan/Endevco UK, Royston, England.
a The italic numbers in brackets refer to the list of references appended to this paper.

221
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Copyright9 1975byby ASTM International
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222 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

structures, and the location of the source has been usually determined by
post-test analysis.
The purpose of this paper is to present test results obtained with a
recently developed real-time source location system. The applicability of a
multichannel location system to the detection and location of structural
degradation in three independent tests will be discussed:
1. Graphite-epoxy honeycomb panel punch tests (performed in conjunc-
tion with Lockheed Missiles and Space Company, Inc., Sunnyvale,
California).
2. Cylindrical steel pressure vessel test (performed in conjunction with
the Dutch Acoustic Emission Working Group at the KEMA facility in
Arnhem, The Netherlands).
3. Spherical steel pressure vessel test (performed in conjunction with
CEA-Saclay, France).
All source location work reported to date has been on isotropic and
homogeneous materials, such as metals and glass. The results to be re-
ported on the graphite-epoxy honeycomb are the first presented for such a
complex material and demonstrate the applicability of source location
techniques to this important class of materials.

Instrumentation
A block diagram of the 10-channel source location system used in these
tests is presented in Fig. 1. Several features and components of the system
are worth noting:

Transducer
Differential transducers were used in these tests because of the immunity
to electromagnetic interference afforded by the differential design.

Audio Monitor
The audio monitor heterodynes the high frequency signal from the signal
conditioner down into the audio range to provide an audible measure of the
AE activity.

Delta-T Interface
When an AE event has occurred, the delta-T interface determines the
transducer arrival sequence, measures the corresponding arrival times,
and stores the event information in a data buffer in computer memory.

Computer
The computer performs validity checks on the incoming data, calculates
X- Y coordinates of the source, and prepares the event data for display and
printout.

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TRANSDUCERS SIGNAL
CONDITIONERS
c~
I ~,,Ig,oJ PR~AMPS j
CLOCK
E-1
TELETYPE
AT READER/ PUNCH
LOGIC&
'N H I - INTERFACE
CO U T E R ~

DUAL CRT
CRTINTERFACE DISPLAY

I AUDIO
'1I
SELECTOR MIXER COUNTER RESET
CLOCK
II
I AUDIO I X-Y
RECORDER
RAMP
MONITOR GENERATOR

FIG. l--Block diagram of computerized location system. I*0

FIG. 2----Photograph of source location system.

X-Y Display Scope

The cathode ray tube (CRT) provides an instantaneous display of the
location of each source in relation to the position of the sensors involved. In
lieu of the normal display, a histogram, indicating the source activity by
area and first sensor excited, can be chosen.

Teletype
Besides serving as the input device between the operator and system, the
teletype printer-punch combination provides a hard copy output of each

event for test analysis. The arrival sequence, corresponding arrival times,
test time, and X- Y coordinates are output for each event. The paper punch
allows the operator to either duplicate the test results at a later date or
recalculate the X- Y coordinates using a new calibration value, if desired.

Calibration
The source location system is "self-calibrating" since the wave velocity
of the material is measured automatically by the system in a special calibra-
tion run prior to testing. A piezoelectric transducer used as a pulser is
attached to the structure next to one of the pickup sensors and excited by an
electrical signal to simulate an AE event. The closest transducer detects the
event immediately and enables three counters which are disabled as the
other sensors are hit. This technique provides the system with the max-
imum time of flight between neighboring transducers for use in calculating
X- Y coordinates during the test. Figure 2 shows a photograph of the source
location system.

Graphite-Epoxy Honeycomb Panel Tests

This work was performed to assess the applicability of AE techniques to
the detection and location of localized failures in composite materials.* A
48-cm-square panel of 1.3-cm-thick honeycomb with graphite-epoxy
facesheets was loaded locally to destruction in several places by forcing a
1.3-cm-diameter flat-ended punch into the facesheet. Local failure oc-
curred by fiber breakage and delamination of the facesheet (which had
several crossplys), and debonding between the facesheet and core. A
45-kN-capacity Instron test machine which produced a load-time plot of
the test was utilized.
A square monitoring section with 41-cm spacing between 140-kHz res-
onant frequency transducers was incorporated for this work. The punch
was indented on one side of the honeycomb, and tests were performed with
transducers mounted on both sides of the sheet in order to determine the
effect of the honeycomb material on stress wave transmission.
Test results from two punch tests (one with the punch and transducers on
the same side, and one with the transducers on the opposite side) are
presented in Figs. 3 and 4. (The square pattern of bright spots are the sensor
locations.) These photos include all emission recorded during the test up to
final failure, which occurred when the 1.3-cm-diameter punch penetrated
the first graphite-epoxy facesheet. The scatter of sources appears to be
excessive until you consider that the structure is designed to spread the
load (and hence the damage) over a wide area. A comparison of the teletype

4 Work performed in conjunction with Lockheed Missiles and Space Company, Inc.,
Sunnyvale, Calif.

test printout with the load-time chart indicated that emission occurred in
short duration bursts just prior to major load drops in the load-time curve. 5
Figures 5 and 6 illustrate the AE activity just prior to major load drops in the
two tests just described. These photos indicate that scatter is less severe
during actual localized destruction than during the course of an entire
punch test. From these results it is reasonable to assume that the damage is
indeed spread over a much larger area during a total load cycle.
These tests results demonstrate that AE source location techniques can
be used to detect and locate incipient failures in composite materials (at
least composites with multiple layers of fibers to minimize anisotropy).

Cylindrical Pressure Vessel Test

A hydrostatic test on a cylindrical pressure vessel of API 0.SL grade X60
steel was monitored.6 The material treatment and chemical composition of
the pressure vessel material are listed in Table 1.
The vessel was a double-ended cylinder 2.79 m in length with a diameter
of 1.22 m. Several defects in the form of brittle welds, sawcuts, and eloxed
(electrodischarge machined) notches were introduced into the vessel, as
shown in Fig. 7. A software routine for use with cylindrical structures was
incorporated to calculate source coordinates during the test. Interlocking
equilateral triangular arrays of 140-kHz resonant frequency transducers
spaced 130 cm apart were used. Figure 8 illustrates the CRT presentation
for a development of the cylinder along with all weld, nozzle, and defect
locations.
Table 2 includes the maximum pressure attained, system gain, and the
number and identification of artificial defects for the three pressure cycles.
Figure 9 represents the source location display for simulated AE intro-
duced at the five artificial defect areas which generated emission during one
or more pressure cycles.

T A B L E l---Chemical composition and material treatment f o r pressure vessel used in

K E M A test.

Material treatment: API 0.5L Grade X60

Hot rolled 1.5% cold expanded
Chemical composition, %: C = 0.20 S = 0.012
Mn = 1.28 AI = 0.034
Si = 0.26 V = 0.106
P = 0.006 Cb = 0.005

5 A drop in the load curve would represent failure in a full-scale structural test, but for this
work the load was continued until the punch penetrated the face sheet.
+ Test performed in conjunction with Dutch Working Group on Acoustic Emission at
KEMA facilities, A r n h e m , The Netherlands.

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KELLY ET AL ON DETECTION AND LOCATION OF FLAW GROWTH 227

FIG. 3---Location results for punch test on honeycomb. Results for entire test to destruction
(Test 1)are shown. Transducers and punch on same side of panel.

FIG. 4---Location results for punch test on honeycomb. Results for entire test to destruction
(Test 2) are shown. Transducers and punch on opposite sides o f panel.

FIG. 5--Emission from honeycomb panel just prior to load drop in Test 1.

FIG. 6---Emission from honeycomb panel just prior to load drop in Test 2.

Pressure
0
Z

5
\ Z

Z
~7
9ooo .. ~ 250
6
r
2790 ~.
L 5
Z
All dimensions in m m
o
= Brittle weld (Smitweld, Reno
35 AB). Dimensions 100xl5mm.
a n d 5 0 x 1 5 mm.
-8- = E.C. s p a r k e r o s i o n n o t c h e s ,
a b o u t 3 3 % of w a l l t h i c k n e s s ; 0
l e n g t h 60mm, r a d i u s n o t c h
0.06 m (c.o.d measurements)
= Groove by grinding

FIG. 7--Cylinder vessel showing artificial defects.

D
Z
SPARK
:ROS ION
IOTCHES L5 CM Z

[8 C M
)ZA
Wozzle

m
L5 CM

5
D

15 CM
WELD

0
Z

tOW /"
lore: The various natural welds are indicated by dashed lines. /
Groove

FIG. 8--CRT presentation of cylindrical vessel with flaws, nozzle, and welds.

FIG. 9 - - C R T display o f location of simulated emission introduced at the defects in the

cylindrical vessel.

FIG. I ~ R T display o f emission sources for cylindrical pressure vessel Cycle 1.

FIG. 11--CRT display of emission sources for cylindrical pressure vessel Cycle 2.

Photographs of the CRT test data for Cycles 1 and 2 are presented in Figs.
10 and 11. Emission from Defect 1 is evident in both pictures, and most of
the other sources are associated with the many natural welds in the struc-
ture. No emission was received from either of the eloxed notches (Defects 7
and 8). The first 2 cycles were performed with gains of 90 and 84 dB which
turned out to be higher than necessary or desirable. This accounts for much
of the scatter observed in Figs. 10 and 11.
The computer program requires that the transducers be arranged in a
series of interlocking equilateral triangles. In order to cover the vessel with
ten sensors, it was necessary to make some of the triangles nonequilateral
which adversely affects locational accuracy.
The emission from the area indicated by the letter P in Figs. 10 and 11
initiated from an area where two preamplifiers were sitting on the vessel.
The preamplifiers were removed for the third cycle and emission from that
area ceased. A post-test ultrasonic inspection indicated there were no
defects in this region; therefore, the lack of emission during the third
pressurization can be attributed to the removal of the preamplifiers rather
than the Kaiser effect.
The third pressure cycle is divided into three photos: 0 to 2.3 MN/m 2, 2.3
to 2.8 MN/m 2, and 2.8 to 3.7 MN/m 2. Figures 12-14 show the effect of crack
extension during these cycles. The CRT was cleared of all current sources
at 2.3 and again at 2.8 MN/m z. Figure 12 shows cracking initiated in Defect

FIG. 1 2 ~ C R T display o f emission sources fi~r cylindrical pressure vessel Cycle 3 (0 to 2.3
MN/m z).

FIG. 13-----CRTdisplay o f emission sources for KEMA pressure Cycle 3 (2.3 to 2.8 MN/m2).

FIG. 14~CR T display o f emission sources for KEMA pressure Cycle 3 (2.8 to 3.7 MN/m 2).

2 prior to 2.3 M N / m 2 with scattered emission f r o m Defects 3 and 4. Defect 4

had substantial crack growth during the 2.3 to 2,8 M N / m 2 (Fig. 13) period
with a few scattered events from the remaining flaws. In the final period
(2.8 to 3.7 M N / m 2, Fig. 14) light activity was detected f r o m several of the
defect areas.
The source locations in these p h o t o s (Figs. 12-14) correlate v e r y well
with the pulser generated A E simulated flaw data shown in Fig. 9. A post-
test ultrasonic and d y e - p e n e t r a n t inspection (reported in R e f 17) substanti-
ated the crack growth in each of the brittle welds as well as the absence of
crack extension in the eloxed notches. The vessel was not taken to failure.

Spherical Pressure Vessel Test

A test conducted in conjunction with C E A Saclay in France was per-
formed on a welded spherical pressure vessel made of A M M O steel. The

TABLE 2----System gain, maximum pressures attained and number o f defects in test o f
cylindrical vessel.

Max Pressure Number of Identification

Cycle Attained, MN/m~ System Gain, dB Defects of Defects
1 0.65 90 3 1, 7, 8
2 3.7 84 3 1, 7, 8
3 3.7 74 8 1 to 8

TABLE 3----Chemical composition and mechanical properties of AMMO steel.

Chemical composition, %: C= 0.12

Mn = 1.20
Mo = 0.45
Cr = 0.05
V = 0.07
S = 0.015
P = 0.007
Mechanical properties: tr u l t i m a t e s t r e n g t h = 579 M N / m z
tr yield s t r e n g t h = 469 M N / m 2
% r e d u c t i o n in a r e a = 2 4 %

chemical composition and mechanical properties of the AMMO steel, as

reported in Ref 18, are listed in Table 3.
The pressure vessel was 918 mm in diameter with a wall thickness of 4.7
mm. A full-thickness sawcut 205 mm long was machined in the parent metal
and sealed on the inside to contain the pressure.
The vessel was instrumented with eight 750-kHz resonance frequency
transducers spaced 58 cm apart. A diagrammatic representation of the ves-
sel with transducer locations is shown in Fig. 15. A routine developed for
testing spheres was used to calculate source coordinates during the test.
Figure 16 illustrates the CRT presentation for the sphere along with all flaw,
weld, and nozzle locations.
Table 4 shows the pressure and total system gain for each of the pressure
cycles which were recorded on punched paper tape.

Nozzle
Equatorial
weld 3

Flaw

Transducers

~ Repaired flaws

FIG. 15---Position of flaw and transducers in Saclay vessel.

Flaw

if' I Zone 3 I" ~, I Zone 5 ]

z~176 /
5 I & 7 ~ Equatorial weld

FIG. 16--CRT presentation of sphere with flaw, nozzle, and weld.

The crack extremities (simulated with a pulser) are indicated by the two
bright spots in Fig. 17.
Although extension of the flaw was visually observed during each pres-
sure cycle, a relatively small amount of emission was detected from the
defect area. Therefore, the total system gain was gradually increased from
76 to 90 dB during the course of the test. Photographs of the CRT display
during several cycles are shown in Figs. 18-20. Note that emission from the
weld and nozzle is evident in e v e r y photo, but source activity from the
defect area is nonexistent in some cases. Figure 20 represents the emission
recorded during the final pressure cycle, which resulted in severe bulging at
the defect and leaking of the pressure vessel.
Because crack growth was difficult to detect in the presence of welds and
nozzles, a laboratory investigation was initiated to study the A E charac-
teristics of A M M O steel. Tests were conducted on flawed and unflawed
tension specimens of both the parent and weld materials. Figure 21 pre-
sents the results of a test on a fatigue precracked specimen of the base

TABLE 4----Total system gain and maximum pressure attained for recorded pressure cycles
in Saclay test.

Cycle Max Pressure Attained, MN/mz Gain, dB

1 2.0 76
2 2.1 76
3 2.5 76
4 2.5 80
5 2.5 86
6 4.0 to (burst) 90

FIG. 17--Flaw boundaries in spherical pressure vessel.

FIG. 18--CRT display o f emission sources for spherical pressure vessel Cycle l.

FIG. 1 9 ~ r ' R T display ~f emission sources Jor spherical pressure vessel Cycle 4.

FIG. 2O--CRT display of emission sources for spherical pressure vessel cycle 6 (final cycle).

4 , . , , , , 0.40

[~S140, 100dB, 0.1-0.3MHz

~.~ 3 0.30

~u 0.20 i

o.1o

. . . . 0
0 2 4 6 8 i0 12

strain, %
FIG.21---Acoustic emission stress strain plot for fatigue precracked specimen of AMMO
steel.

material. It is evident from this graph that the material (AMMO steel) is
very " q u i e t " even at a gain o f 100 dB. The system locational accuracy is
demonstrated by the exceptional nozzle and equatorial weld emission
detected during several of the pressure cycles.
The lack of emission from the parent metal defect, even though the defect
was observed to be growing, points up the possible danger of performing
source location tests without a knowledge of the AE characteristics of the
material. Some quiet materials do exist, in which case a lack o f emission
does not necessarily imply the absence of defects.

Conclusions
The test results presented show that A E source location techniques
provide accurate information on the location o f defects in a variety of
materials and structures. The three tests presented were for widely varying
conditions and materials, yet the results showed (except possibly for parent
metal defects in A M M O steel) that standard A E location techniques pro-
vide a powerful tool for the verification of integrity of pressure vessels and
other structures. The lack of emission from the parent metal defect in the
AMMO steel vessel points up the desirability of performing an A E charac-
terization of an unfamiliar material prior to testing a structure of that
material.

References
[1] Green, A. T., Nuclear Safety, Vol. 10, No. 1, Jan.-Feb. 1969, pp. 4-18.
[2] Hartbower, C. E., Climent, F. J., Morals, C. F., and Crimmins, P. P., "Stress-Wave
Analysis Technique Study on Thick-WalledA302B Steel Pressure Vessels," NASA CR
101887, National Aeronautics and Space Administration, Wasington, D.C., July 1969.
[3] Harris, D. O. and Dunegan, H. L. in Testing for Prediction of Material Performance in
Structures and Components, ASTM STP515, American Society for Testing and Materi-
als, 1972, pp. 158-170.

[4] Green, A. T. and Dunegan, H. L., "Acoustic Emission Analysis of Crack Propagation
and Fracture in Pressure Vessels and Pressure Vessel Materials," Technical Report DC
72-1, Dunegan/Endevco, San Juan Capistrano, Calif., presented at First International
Conference on Structural Mechanics in Reactor Technology, Berlin, Sept. 1971.
[5] Dunegan, H. L. in Proceedings, Second International Conference on Pressure Vessel
Technology, Part II, American Society of Mechanical Engineers, San Antonio, Tex.,
Oct. 1973, pp. 635-642.
[6] Code, R. R., "The Use of Acoustic Emission to Improve Motor Case Structural Reliabil-
ity," presented at American Institute of Aeronautics and Astronautics-Society of Au-
tomotive Engineers 9th Propulsion Conference, Las Vegas, Nev., Nov. 1973, AIAA
Paper 73-1258.
[7] Waite, E. V. and Parry, D. L., Materials Evaluation, Vol. 29, No. 6, June 1971, pp.
117-124.
[8] Vetrano, J. B. and Jolly, W. D., Materials Evaluation, Vol. 30, No. 1, Jan. 1972, pp.
9-12.
[9] Cross, N. O., "Acoustic Emission Techniques for Insuring Safe Hydrostatic Tests of
Pressure Vessels," Paper 70-PET-31, American Society of Mechanical Engineers.
[10] Cross, N. O., Loushin, L. L., and Thompson, J. L. inAcoustic Emission, ASTM STP
505, American Society for Testing and Materials, 1972, pp. 270-296.
[11] Gopal, R., "Final Report of Westinghouse Participation in AE Tests at the EBOR Site,"
EEI-TVA Rp79 Addendum Report, May 1973, pp. 49-88.
[12] Jolly, W. D., Reinhart, E. R., and Ying, S. P., " A E Experiments at EBOR," EEI-TVA
Rp79 Addendum Report, May 1973, pp. 120-123.
[13] Kelly, M. P. and Bell, R. L., "Detection and Location of Flaw Growth in the EBOR
Nuclear Reactor Vessel," Dunegan/Endevco Report DE-73-4, San Juan Capistrano,
Calif., Feb. 1973.
[14] Bentley, P. G., Burton, E. J., Cowan, A., Dawson, D. G., and Ingham, T. in
Proceedings, Second International Conference on Pressure Vessel Technology, Part II,
American Society of Mechanical Engineers, San Antonio, Tex., Oct. 1973, pp. 643-654.
[15] Chretien, N., Bernard, P., and Barrachin, B. in Proceedings, Second International
Conference on Pressure Vessel Technology, Part II, American Society of Mechanical
Engineers, San Antonio, Tex., Oct. 1973, pp. 655-667.
[16] Birchon, D., Dukes, R., and Taylor, J. inProceedings, Second International Conference
on Pressure Vessel Technology, Part II, American Society of Mechanical Engineers, San
Antonio, Tex., Oct. 1973, pp. 669-684.
[17] Dufour, L. B., "Report Concerning Acoustic Emission Localization Experiments Per-
formed at KEMA on 9 and 10 Oct. 1973," No. IV-3617-74.
[18] Tomachevsky, E. G., "Essai de Functionment du Localisateur a 10 Voies Dunegan-
Endevco 1010," Note Technique EMT 73-254, FE4111.4.03.

Acoustic Emission A Bibliography

for 1970-1972

REFERENCE: Drouillard, T. F., "Acoustic Emission: A Bibliography for

1970-1972," Monitoring Structural Integrity by Acoustic Emission, ASTM
STP 571, American Society for Testing and Materials, 1975, pp. 241-284.
ABSTRACT: The bibliography includes nearly all references in the literature on
acoustic emission published during the three-year period, 1970 through 1972.
Included in the 412 references are several for each of the associated technologies
including: signature analysis, boiling detection, cavitation, leak detection,
seismology, and rock mechanics. Information has also been obtained from eight
abstracting and indexing services searched in compiling the bibliography. The
bibliography has been arranged alphabetically by author and is cross referenced
with a list of approximately 400 authors. Also included is a subject index.
Technical articles listed were published in some 90 different journals and in 8
different languages.
KEY WORDS: acoustics, emission, bibliographies, indexes (documentation)

Acoustic emission (AE) is a new and rapidly developing technology for

materials research and nondestructive testing (NDT). In materials re-
search, deformation and fracture processes studied as sources of emission
include: dislocation pile up and break away, slip, twinning, Luders line
formation, martensitic phase transformation, stress corrosion and stress
corrosion cracking, and crack initiation and propagation. As an NDT tool,
acoustic emission testing is used to evaluate structural integrity and, in the
surveillance of pressure vessels and structures, to detect incipient failure
during proof loading, periodic overloading, and in-service monitoring.
Since the amount of literature on AE being published has increased at an
exponential rate, a bibliography of current literature is timely and valuable
to those persons already working in the field of AE, as well as to those just
coming into the field.

Senior NDT engineer, Rocky Flats Division, Dow Chemical U.S.A., Golden, Colo.
80401.

241
Copyright by ASTM Int'l (all rights reserved); Fri Jan 1 23:22:35 EST 2016
Copyright 9 1975byby ASTM International
Downloaded/printed www.astm.org
University of Washington (University of Washington) pursuant to License Agreement. No further reproductions authorized.
242 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

The bibliography has been organized to include the literature on AE

published in the form of technical reports, journal articles, technical pre-
sentations, proceedings, doctoral and master theses, patents, and bound
volumes during the three-year period 1970 through 1972. Included in the
412 references are several references for each of the associated tech-
nologies involving AE phenomena. These include: signature analysis, boil-
ing detection, cavitation, leak detection, seismology, and rock mechanics.
With few exceptions, copies of all reference material have been obtained by
the author for the purpose of accuracy and to establish their availability.
In addition to references cited in AE literature, the following abstract and
index services were searched in compiling the bibliography: Nuclear Sci-
ence Abstracts, Metals Abstracts, The Engineering Index, Physics Ab-
stracts, Chemical Abstracts, Corrosion Abstracts, Scientific and Techni-
cal Aerospace Reports, and Dissertation Abstracts. Another valuable
source of information has been the Acoustic Emission Working Group
(AEWG) meetings at which members and guests informally discuss their
current activities in the field of AE and report on work that will be reported
in future publications.
The bibliography is arranged alphabetically by the first author, then the
second author, etc., and finally by title. Each reference is numbered; these
numbers provide a cross reference to the Author Index and Subject Index.
Each reference lists all of the known publications and presentations in
which a paper or report has been included. Approximately 400 persons are
listed in the Author Index. Technical articles were published in approxi-
mately 90 different journals and 8 different languages: English, Russian,
JapAnese, German, French, Italian, Portugese, and Dutch. Some refer-
ences, however, are not technical articles but consist of editorials or
staff-written journal articles or new briefs which are included because they
represent a part of the literature.
Several references do not report directly on AE, but discuss material that
borders on or directly relates to AE and, therefore, are considered a
valuable part of the bibliography.
Names of all journals are given in full and are followed by the CODEN, a
five-character code intended to identify and facilitate machine handling of
journal titles.
Many reports and translations are available to the public from the Na-
tional Technical Information Service (NTIS), Springfield, Virginia 22151.
Most of the listed report references are so designated and can be ordered by
referencing the report number or the accession number appearing in brack-
ets [ ].
Comments or recommendations for additions which may have been
omitted, or other improvements to the bibliography would be welcomed.

U n d e r a second project, acoustic emission literature through 1969 is being

researched and compiled. The receipt of papers or reports or entries for
inclusion, or other pertinent information would be appreciated.

Acknowledgments
In compiling the bibliography the cooperation of many persons was
required. Sincere appreciation is expressed to the many authors who so
generously responded to requests for copies of their reports and papers.
Particular thanks go to F. J. Laner, technical editor, R o c k y Flats Division,
Dow Chemical U S A for her many helpful suggestions in preparing the
manuscript and encouragement to compile such a complete bibliography,
to I. L. B o w e n for her extensive secretarial assistance, and to the library
staff of the R o c k y Flats Division of D o w Chemical U S A for their persistent
efforts in helping to obtain much of the referenced material. In addition,
appreciation is expressed to W. A. Dowden, C. P. Merhib, M. J. Pettigrew,
P. P. Savage, and J. C. Spanner who have compiled extensive bibliog-
raphies and generously forwarded copies which served as cross checks for
the current compilation. Finally, grateful acknowledgment is extended to
R. G. Liptai for his encouragement to prepare a subject index.
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DROUILLARD ON BIBLIOGRAPHY 271

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272 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

Author Index to Bibliography

Reference No. Reference No.

A Borsheim, G . L . 356
Agarwal, A. B . L . 1 Boultinghouse, K . D . 40, 241
Alers, C . A . 119, 120, 121 Bradshaw, J . E . 239
Allen, C . C . 2 Brigman, G. H, 41
Amano, M. 3 Brindley, B . J . 42, 43
Anderson, R . T . 4 Brown, A . E . 44, 75
Anderson, T . T . 5, 6, 7, 110, 145, Buchman, P. 45
309 Bukhalenkov, V . V . 253
Ando, Y. 175 BunneU, L . R . 46, 323,324, 325
Appl, F . J . 82, 254 Bunshah, R. F. a 78
Burnup, T . E . 26, 27
Arii, M. 8
Burton, E . J . 26
Arington, M. 326
Ashkin, M. 9 Bustin, W . M . 47
Astleford, W . J . 10
Averbukh, I . I . 1 I, 12, 13 C
Capuano, S. 23
B Carpenter, S . H . 204, 205
Bader, R . M . " 154 Carver, W, N. 128
Baird, B . C . 14 Chambers, R . H . 48
Balderston, H . L . 15 Chance, R. 49
Ballard, D . W . 16 Chegorinskaya, O . N . 13
Baranov, V . M . 252 Chentsov, V . P . 39
Barkhoudarian, S. 17 Chow, R. 370
Barron, K. 18 Chretien, J . F . 50
Bartle, P . M . 19 Chretien, N. 50, 51, 52
Barton, J . R . 227 Cochran, R. 246
Bazaikin, V . I . 87 Compton, M . R . 53,212, 213
Beatson, C. 20 Cooke, F . B . 327
Beattie, A . G . 21, 22, 314 Code, R . R . 54, 333
Beaumont, P. W . R . 90, 91 Corvin, I. a 104
Belisario, G. 23 Cothren, R . K . 330
Bell, R . L . 24 Cowan, A. 26
Belland, R . E . 203 Cox, B. 55
BeUosillo, S . B . 25 Craig, J . I . 148
Benson, D . A . 203 Crecraft, D . I . 144
Bentley, P . G . 26, 27, 222, 223 Crimmins, P . P . 56, 133,158,159,
Bentzen, F . L . 28, 29, 30 160, 161, 162,
Bergey, K . H . 82 163, 164, 165
Bernard, P. 51 Cross, N . O . 57, 58, 59
Bill, R, C. 31, 32 Crowe, J . C . 46, 60, 275, 356
Birchon, D. 33, 34 Curtis, G . J . 61
Bjeletich, J . G . 262
Blake, H . W . 35
Blalock, P . J . 36
D
Boiko, V . S . 37, 38 D'Annessa, A . T . 62, 63
Bolotin, Yu. I. 39 Darwish, F. A . I . 156
Borhaug, J . E . 255 Dau, G . J . 64

a Denotes editor or coordinator of report.

b Denotes editor or coordinator of report and author.
DROUILLARD ON BIBLIOGRAPHY 273

Reference No. Reference No.

Davenport, C. M. 35 Gerberich, W. W. 111, 160, 220
Davis, T. J. 65,213,214 Gieske, J. H. 112
Davis, W. T. 347, 348 Gillis, P. P. 113, 114
Dawson, D. G. 27 Goddard, D. J. 115
Deadwyler, R. 148 Gopal, R. 116
DeAtley, E. 66 Gradinar, V. V. 13
DeKanter, J. C. F. 67 Graft, K. F. 92
DeLacy, T. J. 4 Graham, L. J. 117, 118, 119,
DiPietro, A. 23 120, 121
Donat, R. C. a 154 Grate, T. A. 145
Dowden, W. A. 68 Green, A. T. 76,122,123,124,
Dreiman, N. I. 69 125, 126, 127,
Drobot, Yu. B. 39 128, 129, 130,
Drouillard, T. F. 70, 71, 72 131,132,133,355
Dukes, R. 33 Greshnikov, V. A. 39
Dunegan, H. L. 73,74,75,76,77, Grigory, S. C. 134, 396
78,79,80,81,85, Gusakov, A. A. 39
129, 130, 152, Guz, I. S. 87, 88, 135
153, 154, 198,
233 H
Haborak, R. C. 136
E Hagemaier, D. J. 137, 138, 139,
Egle, D. M. 82, 254 140, 141, 142
Eisenblatter, J. 83, 84 Haigh, C. P. 143
England, J. 33 Hall, E. T. 144
Engle, R. B. 85, 235 Halverson, S. L. 145,309
Ewing, R. C. 86 Hamstad, M. A. 146, 147
Hanagud, S. 148
F Hardy, H. R., Jr. 149, 150, 151
Feiertag, T. H. 340 Harrington, R. M. 262, 263
Felbeck, D. K. 1, 102, 328 Harris, D. O. 77,152,153,154,
Finkel, V. M. 87.88, 135 155, 156, 234,
Fisher, R. M. 353 235,236, 237
Fitz-Randolph, J. M. 89, 90, 91 Harrison, R. P. 42, 43
Fontana, M. G. 92 Hart, P. E. 46
Forestier, R. 93 Hartblower (sic), C. E. 158
Foster, R. G., Jr. 331 Hartbower, C. E. 131, 132, 133,
Fowler, K. A. 94, 95, 96, 240 157, 159, 160,
Francis, P. H. b 227, 235 161, 162, 163,
Frederick, C. L. 97, 98 164, 165,318
Frederick, J. R. 1, 32, 99, 100, Hartman, W. F. 166, 167, 168
101, 102, 256, Hatano, H. 169, 170, 171,
328 172, 173, 174,
Freund, L. B. 103 175, 176, 177,
Fritsche, G. R. 104 178, 282,283
Fry, D. N. 105 Heald, P. T. 290
Fuji, T. 106, 107 Heide, W. 84
Fujimura, T. 108 Hiles, L. A. 314
Hisakawa, H. 257
G Hoener, R. F. a 154
Gaffney, P . G . 228, 373 Hoenig, S. A. 48
Galkov, V . S . 225 HolT, M. 179
Garber, R . I . 37, 38 Horak, C. R. 180
Gatfi, A. 109 Horiuchi, R. 173, 174
Gavin, A . P . 6, 110, 145 Hsu, C, 7
274 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

Reference No. Reference No.

Hunter, C . W . 371 Kurosawa, S. 175
Hutton, P . H . 181, 182, 183. Kusenberger, F. N. 227
184, 185, 186, Kutkin, I. A. 87.88
187, 188. 189,
190, 191, 192, L
193, 194, 195
196, 197, 380 Lankford, J., Jr. 227
Latham, F . G . 280
Laura, P . A . 228, 373
Lawrence, F . V . 313
Ievlev, I. Yu. 253 Lewis, R . E . 263
Ireland, D. R. 198 Li, S . T . 229, 230. 231
Ishikawa, K. 3 Lindholm, U.S. a 235
Isono, E. 199, 200, 201 Liptai, R . G . 44.232,233,234,
Iwaya, R. 360 235, 236, 237
Lloyd, D.J. 362
J Lockman, C . S . 355
Jaffe, E. H. 288 Long, M., Jr. 285
James, D. R. 202, 203,204, Lord, A. E,, Jr. 224
205 Loushin, L . L . 59
Jaramillo, R. A. 22 Lyle, F. F., Jr. 2
Johari, O. a 104 Lynnworth, L . C . 238, 239, 240
Jolly, W. D. 193,206, 207,
208, 209, 210, M
211,212, 213, Magnani, N . J . 241
214, 215, 379, Mah, R. 242, 313
38O Martin, G. 261
Jost, H. 84 Martin, R . L . 243
Maxfield, B . W . 244, 245,246
K Mazzio, V . F . 26:5
Kamio, A. 216 McCauley, B . O . 247, 268,269,270
Kamm, H. W. 217 McClung, R . W . 248
Kammerer, C. C. 359 McFaul, H . J . 141, 142
Kanno, A. 218 McGonnagle, W . j . n 85, 229, 233
Karvinen, J. R. 6 Mehan, R . L . 109, 249, 265
Kasatkin, B. S. 219 Melekhin, V . P . 250, 253
Katz, Y. 220, 221 Merz, M . D . 221
Keiser, H. D. 412 Meyer, J . A . 397
Kerns, G. E. 354 Miannay, D. 52
Kim, R. Y. 151 Miglionico, C . J . 241
Kimpara, I. 361 Mikhodui, L . I . 219
Kirby, N. 26, 222, 223,280 Milne, A . R . 251
Kishi, T. 173, 174 Miloserdin, Yu. V. 252
Klot, R. V. 84 Mints, R . I . 250, 253
Knauss, P. L. 75 Mitchell, J . R . 82, 254
Koerner, R. M. 224 Mitchell, J . S . 255
Kolomiichuk, B. N. 219 Mitchell, L . D . 256
Konovalov, E. G. 225 Miyairi, M. 257
Kortov, V. S. 311 Mogi, K. 258
Kraska, I. R. 217 Molodtsov, K . I . 252
Krivenko, L. F. 37, 38 Moon, D. 141
Krivulya, S. S. 37, 38 Moore, J . F . 259, 260, 261
Kryter, R. C. 105 Morais, C . F . 159, 161, 162,
Kugler, A. M. 25O 163, 164, 165
Kuhn, B. A. 155,226 Mori, T. 107
DROUILLARD ON BIBLIOGRAPHY 275

Reference No. Reference No.

Morton, T . M . 262, 263, 350 Prine, D. W. 310
Moskal, F . J . 359 Proskurin, V. Yu. 311
Muenow, R . A . 264
Mukherjee, A . K . 147 R
Mulcahey, T . P . 7
Radon, J. C. 307, 312
Mullen, C . V . 29, 30
Radziminski, J. B. 313
Mullin, J . V . 249, 265
Ramakrishnan, V. 230, 231
Munse, W . H . 313
Rathbun, D. K. 314
Musiyachenko, V . F . 219
Reiman (sic), K. J. 315
Reimann, K. J. 6
N
Reinhardt, W. W. 316
Nakamura, Y. 266, 267, 268,
Reinhart, E. R. 2. 302,303
269. 270, 377
Reis, J. J. 317
Nakasa, H. 271,272,273
Reuter, W. G. 159, 160, 161,
Nepomuceno, L . X . 274
162, 163, 164,
Newman, D . R . 275
165,318
Nichols, R. H., Jr. 276
Rice, R. W. a 46
Nielsen, A. 277,278,279,280
Richards, C. M. 389
Niwa, N. 171, 172, 173,
Robinson, D. L. 299, 300
174, 175, 176.
Robinson, E. Y. 319
177, 178, 281,
Robinson, J. C. 105
282, 283
Rollins, F. R., Jr. 320
Noone, M . J . 109
Romrell, D. M. 321,322, 323,
324,325, 371
O
Roth, B. G. 155
Oaks, A . E . 284 Rothwell, R. 326, 386
Ogasawara, M. 200 Rummel, W. D. 327
Ono, K. 285 Russell, J. E 230, 231
Onoe, M. 286, 287
Onusic, H. 274 S
Ord, R . N . 194
Owens, J . S . 62, 63 Sagehashi, I. 176, 177, 178,
Owston, C . N . 288 282, 283
Sankar, N. G. 328
Satoh, I. 360
P Sawyer, F. B. 389
Palmer, I . G . 42,289, 290 Saxe, R. F. 329,330, 331
Palmer, M . N . 291 Scherba, E. S. 359
Papadakis, E . P . 96, 240 Schildreth, F. H. 411
Parker, J. 27 Schliekelmann, R. J. 332
Parks, J . T . 142 Schliessmann, J. A. 54, 333
Parry, D . L . 195,215,292, Schmidt, P. M. 73, 334
293, 294, 295, Schneider, S. j.a 46
296, 297, 298, Schofield, B. H. 335,336,337,338
299, 300, 383 Schroeder, E. C. 10
Patch, D . R . 240 Schuldies, J. J. 339
Phillips, D . C . 90, 91 Schuyler, D. R., II 340
Phillips, J . W . 301 Schwenk, E. B. 405,409
Pickett, A . G . 302, 303 Scott, C. C. 17
Pollock, A . A . 304, 305,306, Scott, I. G. 341
307, 308,312, Sharpe, R. S. a 194
357 Shearer, G. D. 405,409
Ponter, A . B . 143 Sheff, J. R. 214
Price, C . C . 6 Shepard, R. L. 240
Primak, W. 309 ShortaU, J. B. 342
276 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

Sides, W. H., Jr. 331 Vanderveldt, H. H. 228,372,373,374

Siegel, E . J . 343, 344 Veach, C. L. 268, 269, 270,
Singh, J . J . 345,346, 347,348 375, 376, 377
Smith, B. 308 Vetrano, J. B. 193, 196, 197,
Smith, K . A . 349 378,379,380,381
Smith, S. 350 Volkov, V. V. 219
Snow, R . S . 73,334 Volodarskii, A. Ya. 88
Spanner, J . C . 351
Sparks, C . R . 352 W
Speich, G . R . 353 Wachel. J. C. 352
Staehle, R . W . 354 Waite, E. V. 382, 383
Steele, R . K . 355 Wang. Y. J. 151
Stefanko, R. 151 Warren, R. H. 33, 34
Steffens, R . W . 356 Watwood, V. B. 371
Stephens, R. W . B . 357, 391 Wells, D. 384
Stem, R. 285,358 Wells, T. W. 313
Strauss, B . M . 104 Whiting, A. R. 397
Sugg, F . E . 359 Williams, J. A. 115
Suzuki, T. 360 Williams, P. G. 385
Wilshaw, T. R. 386, 410
T Wingfield, P. M. 34, 387,388
Takehana, M. 361 Winn, W. H. 389
Tangri, K. 362 Witt, F. J.~ 134, 210
Taniguchi, N. 201 Witt, P. A., Jr. 381,390
Tatro, C . A . 78,233,235,236, Wood, A. H. a 154
237, 363, 364, Woodward, B. 391
364, 365,366 Worme[i, J. C. 356
Tetelman, A . S . 77,79,80,81,90, Wright, R. E. 392
91,130, 154, 156, Wullaert, R. A. 198
198, 367,368, Wylie, R. D. b 2, 10, 28, 29, 30,
369, 370 190, 191, 192,
Thompson, J . L . 59 198, 206, 207,
Tomizawa, M. 257 208, 209, 212,
Tomoda, Y. 273 213,214,215,
Toronchuk, J . P . 362 302. 336, 378,
Townsend, W . C . 371 393,395,401
Tran, Q. 374
Trapp, W.J.Q 154 Y
Tsang, S. 260, 261 Yamamoto, E. 257
Tsaryuk, A . K . 219 Yarwood, C. P. 342
Ying, S. P. 134. 303,394,
U 395, 396, 397
Udagawa, T. 200, 201 Yoshida, Y. 177, 178

V Z
Vainberg, V . E . 1 I, 12, 13 Zurbrick, J. R. 398
DROUILLARDON BIBLIOGRAPHY 277

Subject Index to Bibliography

Numbers indicate reference number.

Accelerometer, 6, 24.78, 92, 133, 159, 176, Ball drop test, 69, 216, 218, 257
249, 280, 345, 355,361,364,411 Barium titanate
Acoustic emission, crystal, 45
abstract only, 9, 122, 123,369 transducer, 87, 88
general information, 25, 34, 39, 51, 52, 64, Bauschinger effect, 78, 101, 102, 328
67, 76, 78, 79, 80, 85, 99, 122, 123, Bearings, detection of incipientfailure in, 14,
127, 140, 149, 170, 182, 186, 194, 15,243,352,411
195, 199, 234, 235, 236, 274, 304, Bend testing, 73, 89, 92, 121, 132, 200, 201,
305, 306, 316, 335, 336, 351, 358, 220, 241,257,306, 339
363,364, 366, 372,398,400 Berylco (beryllium-copper alloy), 340
historical review, 25, 67, 78, 128, 149, 165. Beryllium, 70, 71, 76, 78, 85, 133
199, 234, 235, 236, 305, 335, 336, Bibliography of acoustic emission literature,
351,400 50, 68, 165, 342, 351,385
survey article, 20, 66, 67, 106, 338, 341, Bimetal specimen, 13
351,358, 372, 393,411 Boiling detection, 5, 6, 7, 105, 110, 143,329,
terminology, 98,400, 402 330, 331,391
Adhesive bond strength (see also Bond Bond strength, 91,97, 112,305,324. 325,326
strength), 61, 79, 305, 332 Boron/aluminum composite, 320
Aerospace, 259, 359 Boron. epoxy composite, 15, 89, 90, 91, 142,
Aircraft, 130,141,142,180, 194,254,266,268 249
Alumina, 46, 109, 119, 226, 324, 325,406 Brass, 166, 168
Aluminum alloys, 1, 15, 31,48, 54, 56, 76, 78, Brass/mercury embrittlement couple, 115
79, 80, 85, 96, 99, 101, 111, 120, Bridges, testing of, 264, 308
130, 147, 153, 154, 159, 161, 164, Brittle fracture, 55, 57, 59, 83, 84, 87, 104,
166, 168, 170, 173, 174, 181, 211, 124, 199, 301,305, 306, 307, 324,
215, 225, 235, 259, 260, 262, 268, 325, 363
271, 272, 305, 312, 340, 343, 344, Bubble formation (see Boiling detection)
345, 346, 348, 364 Burst-type emission, 55, 235,360
Aluminum-copper-magnesium alloy, 173,
174
Aluminum-magnesium alloy, 173, 174 C
Aluminum-nickel alloy fiber-reinforced Cable (see Wire rope)
aluminum composite, 156, 368 Cadmium, 343,344
Aluminum titanate, 392 Calcite, 37
Aluminum-zinc alloy, 115 Calibration
Ammonium dihydrogen phosphate (ADP) of acoustic emission system, 44, 69, 94, 95,
transducer, 78, 364 192, 216, 218
Amplitude of transducer, 24, 44, 78, 92, 94, 95, 117,
discrimination, 345 192, 310
distribution, 120, 121,170, 174, 190, 269, Carbon/epoxy composite, 249, 265, 288
308 Ceramic, testing of, 46, 112, 119, 120, 121,
Amychometer, 410 183, 316, 323, 324, 325, 339, 392,
Anechoic chamber, 93 406
Anodize coating, 78, 79, 80 Closure, end cap, weld testing (see Encapsu-
Arkansas nuclear-one pressure vessel, 401 lation)
Astroloy, 63 Cobalt, 233
Audible sound of emission, 67, 71, 72, 78, 93 Coincidence gating, 67, 208. 214, 330, 375
278 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

Complex structures, testing of, 72, 96, 136, Diffusion-controlled phase transformation
264, 266, 293,294, 300, 340 (see Phase transformation, nuclea-
Composite material, 4, 56, 76, 79, 80, 89, 90, tion-and-growth)
111, 141, 142, 146, 156, 232, 235, Discontinuous yielding, 166, 167, 168, 202,
249, 265, 288, 314, 319, 320, 326. 205
342, 361,367 Dislocation, 31,32, 37, 38, 55, 67, 78, 85,100,
Compression test, 326 101, 102, 113. 114. 170. 173, 174,
Computer (used in source location), 35, 53, 202, 205, 253, 328, 362
86, 163, 174, 177, 179, 207, 208, break away. 202, 205
212,213,266, 295, 359, 382 density. 202, 205
Connecticut Yankee nuclear reactor, 198 pile-up, 328
Continuous emission, 55,235,360 Ductile fracture, 15, 83, 84, 111,124,289,305
Conrete, 76, 79, 80, 126, 129, 230, 231,264,
384 E
Copper, 31,76, 101,168,271,272,340, 362 Earthquake, 170, 235,258
Copper-aluminum-nickel alloy (martensitic Elastic
phase transformation), 253 deformation, 67,250
Correlation analysis, 262, 285,350,352 modulus, 124, 126, 228
Corrosion, 123,289, 294 Electromagnetic transducer, 244, 245,246
Count rate, 202,205,341,348, 369, 394, 396 Electron beam (EB) welding, 49, 97
Coupling, acoustic. 44, 45, 97,207,217,239, Electroslag welding, 8
245, 247, 345,364 Elk River nuclear reactor. 299
Crack Embrittlement
detection, 62, 63, 70, 77, 81,82, 84, 87, 88, hydrogen, 78, 79, 80, 81,153,235,369,399
92. 103, 107. 125, 126, 129, 133, irradiation, 271
135, 156, 158, 159, 160, 162, 164, liquid metal, 115
165, 181, 184, 196, 197, 201,216, Emission
217, 219, 220, 226, 235,241,242, burst-type, 55. 235, 360
251, 254, 262, 266. 305, 318, 323, continuous, 55,235, 360
324, 325, 346, 347, 348, 392 Encapsulation (end cap closure weld test-
growth rate, 115, 119, 121, 135, 220, 262, ing)
307, 348 radioactive isotopes, 275,356
initiation, 67,201,227, 368 waste, 60
instability. 160. 163,319, 367 End cap weld testing (see Encapsulation)
propagation rate (incrementally), 154, 158, Energy (acoustic) determination, 22, 354,
165, 179, 312, 348, 350 357,367
size, 347 Environment of test specimen, 55, 125, 161,
susceptibility test, 62, 63,324 220, 354, 369
velocity (see Crack growth rate) Etch pit examination, 202
Cracking, heat treat, 62, 63, 183 Etching, emission from, 337
Creep, 85, 101, 151,235, 343, 344 Experimental beryllium oxide reactor
Cross-coupling (transducer), 95, 96 (EBOR), 10, 29, 118,192,208,209,
Cryogenic temperature, testing at, 76, 133, 214, 215, 296, 302
159, 164,303,305, 307, 312, 359 Extensional wave, 95, 96
Cyclic loading (see Fatigue)
F
I) Failure prediction, 71,74, 81,82, 83, 85,101,
103, 125, 126, 152, t54, 155, 163,
Data, acoustic emission test 174, 175, 193, 194, 278, 279, 295,
presentation, 44, 59, 67, 92, 235,317,364 296, 320, 338, 339
processing, 158, 194, 235,364 Fatigue, 19, 23, 48, 56, 67, 73, 74, 76, 77, 78,
Deformation, 31, 114, 202, 205 79,80, 82, 101, 118, 121, 123, 130,
elastic, 67,250 136, 152, 154, 161, 163, 174, 175,
plastic, 67, 77,170, 174, 199,202,205,235, 180, 181, 183, 184, 185, 186, 188,
250, 256, 344 193, 199, 210, 214, 227, 235, 242,
Differential transducer, 44, 75, 95 254, 256, 257, 259, 260, 261,262,
DROUILLARD ON BIBLIOGRAPHY 279

266, 268, 280, 288, 313, 320, 334, Grain

337, 345,346, 348, 350 boundary, 31, 101, 115
crack, 153, 154, 174, 175,293,399 size, 3l, 32, 76, 100, 102,167,170, 173,174,
Ferroelectric crystal, 45 337
Fiber composite material, 79, 80, 89, 90, 91, Graphite/epoxy composite. 76, 141, 142
111, 141, 142, 156, 232, 235, 249, Grip noise (in tensile specimens), 364
288, 314, 319, 326, 361
Fiber fracture, 111, 142, 156, 249, 265,314.
319 H
Fiberglas reinforced plastic composite, 361 Heat-affected zone, failure in, 71
Filament-wound pressure vessel, 22, 99,146, Heat treat cracking, 62, 63, 183
232, 314 Heat treatment, 13, 46, 63,167,271,282,289
Flaw High flux isotope reactor (HFIR), 105
criticality, 56, 86, 132, 161, 163 High temperature
location (see Source location) testing, 188, 238, 380
Flexural wave, 96 transducer, 5, 6, 7, 95, 110, 145,206, 239,
Flow noise. 6, 10, 189, 190, 191, 209, 214, 240, 309, 315
215, 380 Historical review of acoustic emission (see
Forming process monitoring, 182, 187 Acoustic emission, Historical re-
Fracture, 71, 72, 221,367,368 view)
brittle, 124, 199, 220, 301, 305, 306, 307, Honeycomb, 130
324, 325 Hydraulic noise, 10, 189, 190, 191,380
ductile, 15, 83, 84, 111, 124, 289, 305 Hydrogen embrittlement, 78, 79, 80, 81. 153,
mechanics, 54, 59, 67, 74, 130, 163, 227, 235,369, 399
235,290, 305,318, 363,367, 385 Hydrostatic pressure test, 28, 57, 58, 59, 86,
toughness, 54, 74, 81, 85, 133, 158, 221, 150, 174, 177, 178, 182, 186, 193,
235, 241,269, 305,306, 326, 410 215, 292, 295, 296, 300, 334, 355,
toughness specimen, 124, 129, 154, 220, 383, 401
222,318
Frequency
analysis, 5, 48, 67, 87, 96, 117, 118, 119, I
120, 121, 135, 166, 170, 187, 225,
Ice, 148, 159. 251
246, 255,257,329, 330, 357,411 Impact test (ball drop), 69, 216, 218, 257
range of, in acoustic emission, 235, 305, In-service inspection, 3, 19, 57, 59, 116, 131,
354, 363,364 158, 163, 174, 175, 186, 193, 254,
Friction, 12, 41, 82, 269, 320 261, 264, 278, 279, 292, 324, 325,
welding, 97 338, 352,379, 380, 390, 393
Fusion line, failure in, 71 Incipientfailure, 14, 15, 74, 83,108, 131,159,
181,243,300, 320, 325,349, 352
G Inconel, 340
Gallium, 115 Indium-thallium alloy, 21, 78, 233
Gallium/aluminum embrittlement couple, Instrumentation,40, 44, 47, 56, 65, 67, 78, 92,
115 116, 128, 136, 161, 163, 174, 175,
Gas 176, 179, 184, 201,206, 207, 208,
tungsten arc (GTA) welding, 60, 97, 157, 209, 212, 213,222, 233, 235,242,
159, 164, 275, 334, 356, 357 250, 252, 255, 277, 286, 310, 312,
underground storage reservoirs, 150 317, 335, 338, 342, 355,358, 359,
Geologic material, 123, 149, 151,235,351 363, 364, 366, 375,376, 377, 378,
Glass, 46, 56, 120, 121, 132, 226, 272, 273, 384, 387, 388
326, 410 Iron, 340, 370
filament, 142, 146, 36l Iron, silicon, 85,219, 362
pressure vessel, 99 Irradiation effect (see also Embrittlement,
Glass/epoxy composite, 232, 235,326 Irradiation and Radiation dam-
Glossary of acoustic emission terms, 98 age), 202, 205,303,309, 399
Gold, 78, 225 Irreversibility (see also Kaiser effect), 77, 78,
Gold-cadmium alloy, 44, 233,246 154
280 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

K N
Kaiser effect, II, 76, 77. 99, 108, 126, 130, News brief (journal article), 403, 404, 405,
174, 175, 200, 283 406, 407,409, 411
Nickel alloy, 340
Noise, 5, 11,27,76.82,85,101,118,120, 128,
L 159, 204, 266, 268, 277, 310, 320,
Lamb wave, 95, 96. 170 345, 360, 364, 384, 390, 395, 401
Laser welding, 97 electrical, 11, 59, 65, 108, 360
Lead, 343,344 flow, 6, 10, 189,190, 191,209,214.215,380
metaniobate transducer, 95 hydraulic. 10, 189, 190, 191,380
zirconate titanate (PZT) transducer. 78. mechanical, 11, 59, 82,220, 254, 266, 268,
85, 95, 117, 233,345. 364 269. 320, 350, 360
Leak detection, 17, 47, 291,294, 298, 299 reactor, 118, 189, 190, 191, 192,206,208,
Liquid metal embrittlement, 115 209. 214, 302, 331. 380. 395, 401
Liquid metal fast breeder reactor (LMFBR), Noise generator, 117
5, 6.7, 17, 315, 329 Noise simulator, 10, 28, 192, 206, 214, 215,
Literature survey, 19, 39, 92, 142, 351 302, 391
Lithium Nondestructive
fluoride single crystal, 78,202, 205 evaluation (NDE) (see Nondestructive
niobate transducer, 6, 95, 110, 145, 309, testing)
315 inspection (NDI) (see Nondestructive test-
sulfate transducer, 78, 84, 92 ing)
Loading device. 40, 78, 92,277. 364, 384 testing (NDT), 4.16, 60, 67, 79, 80, 83, 85,
Location of acoustic emission source (see 112, 123. 137, 138, 139, 140, 141,
Source location) 142, 144, 157, 158, 159, 161, 162.
Log tachometer, 44 163, 170, 174, 177, 182, 217, 223,
Longitudinal wave, 95, 96, 245 224, 227, 229. 230, 231,248, 261,
Lucalox, 46, 109, 119 268, 269, 274, 275, 284, 293, 294,
Luders line formation, 85, 93, 173, 174 295, 298. 302, 306, 327, 332, 333,
335, 340, 347, 355, 356, 361, 363,
365, 379, 383, 385. 390, 393
M Nondestructive test method evaluation facil-
ity (NDTF), 28, 29, 30,302
Magnesia, 119, 120, 121 Novolac, 249
Magnesium, 285 Nuclear reactor, 28, 116, 145, 183, 184, 185,
oxide (see Magnesia) 189, 190, 191, t92, 206, 207, 208,
Magnetic tape recorder, 171, 180, 394 209, 240, 252, 278, 279, 292, 299,
Magnetostrictive transducer, 94, 95, 96, 239 303,322,329. 331. 391,395
Maintenance inspection, 14, 15, 349 pressure vessel, 3, 42, 83, 118, 183, 184,
Martensite, 219, 353 185, 194, 198, 199, 215, 289. 293,
Martensitic phase transformation, 19, 21,22, 294, 299, 300, 335, 336, 338, 379,
44, 71, 76, 78,233,244, 246, 253, 380, 383,393,401
353,385 Nucleation-and-growth phase transforma-
Mechanical test, 55, 78, 120, 121,339 tion, 233,253
Medicine, 123
Mercury (liquid metal embrittlement), 115 O
Metal forming, emission from, 182, 187
Metal inert gas (MIG) welding, 97, 159, 164 On-line testing (see In-service inspection)
Mica, 272 Oxide film, cracking of, 43, 78, 79, 80
Microcrack process, 368, 370
P
Microphone (transducer), 117,120, 121,361,
364 Passive pressure transducer, 78
Microseismic activity, 18, 150, 151,235 Patent, 47, 75, 291,355
Microstrain, 1, 31 Periodic overload testing, 67, 74, 76, 152,154
Mine (excavation), 149, 182 Periodic testing, 77, 152, 153, 154. 158, 200,
Mobile dislocation density, 202 255,279, 293,371
DROUILLARD ON BIBLIOGRAPHY 281

Phase transformation Quiet tensile testing machine, 155, 203,364,

martensitic, 19, 21, 22, 44, 71, 76, 78,233, 384
244, 246, 253,353,385
nucleation-and-growth, 233,253 R
Phase transition, 45 Radiation damage, 129, 145, 192, 198, 239,
Piezoelectric transducer, 45, 75, 95, 110, 117, 271
145,240, 309, 363,364 Railroad, 257, 310
Pipe testing, 193,196, 197,200.293,294,295, Rate, count, 202,205,341,348,369,394,396
39O Rayleigh wave, 87, 88, 95, 96
Pipeline testing, 47, 295 Reactor
Plasma arc welding, 97 noise (see Noise, Reactor)
Plastic, 13 nuclear (see Nuclear reactor)
deformation, 67, 77, 170, 174, 199, 202, pressure vessel, 27, 42, 53, 84, 108, 126,
205, 235, 250, 256, 344 131
zone, 221,227, 289, 290 surveillance, 116
Plate (structural member), 8 Reactors
Plexiglas. 87, 88, 135 Arkansas nuclear-onepressure vessel, 401
Plutonium, 233 Connecticut Yankee nuclear reactor, 198
Polarization, crystal, 45 Elk River nuclear reactor, 299
Pop-in, 235 experimental beryllium oxide (EBOR), 10,
Portevin-le-Chatelier effect, 166, 167 29, 118, 192, 208, 209, 214, 215,
Precrack, 76, 78, 83,84.85,124,161,210,318 296, 302
Preheat (weld), 201 high flux isotope (HFIR), 105
Preload, 11, 76, 78, 169, 289, 340, 360, 364 liquid metal fast breader (LMFBR), 5, 6, 7,
Pressure test, 146, 149, 150, 179, 209, 277, 17,315,329
355 San Onofre pressurized water reactor, 189,
Pressure vessel testing, 26, 27, 33, 42, 43, 53, 190, 191, 192, 215, 303
54, 57, 58, 59, 67, 73, 76, 79, 80, 84,
Recorder, 161
86,99, 104, 116, 125,126, 129, 131, Recovery of emission behavior, 289
134, 146. 153, 158, 159, 164, 174, Ren6 41 alloy, 62, 63, 79, 80
175, 177, 178, 182, 185, 186, 193, Residual stress, 100, 101,227, 337,410
194, 199, 200, 215,218, 223, 235, Resistance spot welding, 97, 144, 211,225,
277, 278, 279, 280, 290, 293, 294, 363,405,409
295, 297, 298, 300, 316, 336, 338, Resonant frequency, 12, 117, 352, 354
346, 355, 359, 371, 381,382, 383, Ring-cracking, 226
390, 393,397,407 Rochelle salt transducer, 78, 364
Pressurized inert gas metal arc (PIGMA)
Rock
welding, 70 burst, 182, 235,258
Prestressing, 129, 200, 230, 251 mechancis, 149, 235,258
Previous maximum load (see Kaiser effect) testing, 18, 150, 235, 258
Proof testing, 19, 43, 54, 67, 72, 73, 74, 77,
Rocket
152, 153, 154, 155, 158, 174, 177, chamber, 178
186, 200, 235, 254, 268, 279, 308, motor case testing, 54, 333
316, 333,365,385,401 Root mean square (rms) voltage (emission
Prototype reactor surveillance system signal), 44, 71, 147
(PRSS), 206, 207, 208, 209, 212, Rope, wire (see Wire rope)
213,215 Rotating machinery, testing of, 36, 255,264,
Pulse height analysis, 44, 174, 202,205,250, 349, 352
273,312, 329, 330, 341,357
Pyroceram, 46
S
PZT (see Lead zirconate titanate)
San Onofre pressurized water reactor, 189,
190, 191, 192, 215, 303
Q Scanning electron microscope (SEM), 104
Quality control, 51, 193,230, 264, 278 Scout rocket motor, 284
Quartz transducer, 78, 92, 117 Scratch test, 386, 410
282 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

Sea ice, 148,251 303, 305, 307, 310, 311,312, 313,

Sensitivity, limit of, 363, 364 318, 333, 334, 340, 353, 354, 360,
of transducer, 364 368, 370, 393,396, 399
Sensor, acoustic or ultrasonic, (see also Strain
Transducer types), 5, 6, 7, 24, 33, gage, 133, 150, 364
44, 45, 75, 78, 84, 92, 110, 116, 128, hardening, l, 167, 168
145, 161, 163, 194, 359 rate, 76, 78, 85, 124, 147, 204, 205, 311
Shear transformation (see Phase transforma- Stress
tion, martensitic) corrosion, 15, 19, 67, 78,124, 153,165,235,
Shear wave, 95, 96, 245 318, 385
Signal conditioning, 33, 44, 78, 85, 92, 128, corrosion cracking, 40, 55, 78, 79, 80, 161,
159, 194, 204, 208, 359, 364 165, 184, 193, 194, 199, 220, 241,
Signal-to-noise ratio, 350 293,318, 354, 369
Signature analysis, 36, 59, 187,255,279, 305, intensity, 54, 56, 76, 77, 78, 80, 81, 85, 92,
312, 349, 352, 355, 411 99, 125, 129, 134, 154, 161, 163,
Silica, fused, 46 165, 174, 177, 181, 198, 235, 241,
Silicon 262, 269, 305, 307, 345, 347, 369,
iron, 85, 219, 362 394
nitride, 119 Stress-wave analysis technique (SWAT),
Simulator, acoustic emission, 94, 95, 96, 180, 125, 133, 157, 163,235, 318
302 Structural integrity, 34, 59, 60, 127,136, 153,
noise, 10, 28, 192,206, 214, 215,302,391 158, 174, 180, 182, 183, 194, 195,
Single-edge notch (SEN) specimen, 56, 78, 232, 234, 235,259, 260, 278, 296,
85, 92, 161,227, 235, 318 297, 335, 347, 351, 365, 381, 387,
Slag, weld. 97, 181,310 388, 393
Slip, 31, 32, 78, 85, 100, 101, 102, 202, 205, Subcritical crack (flaw) growth, 153, 158,
250, 259, 260, 337 235, 367
Sodium chloride single crystal, 202, 205 Submarine, 162
Soil testing, 123,224 Submerged arc welding, 97, 107, 211,310
Source location, 27, 33, 35, 53, 57, 58, 59, 86, Surface wave (see Rayleigh wave)
97, 126, 128, 136, 155, 158, 159, Surveillance of structures, 116, 278, 279
163, 164, 169, 170, 171, 174, 177,
178, 179, 184, 193, 194, 196, 207,
208, 212, 213, 214, 215, 222, 266, T
267, 268, 278, 279, 293,294, 295, Tachometer (see Log tachometer)
296, 297, 298, 299, 300, 308, 338, Tape record, 155, 171, 180, 249, 363
355,359, 363,382, 383,385,408 Temperature (affect on acoustic emission),
Spectrum, acoustic, 23 129, 130, 134, 143, 159, 196, 209,
Spectrum analysis, 5, 48, 78, 87, 88, 96, 119, 211, 215, 238, 247,307, 311, 318,
120, 121, 132, 135, 143, 187, 190, 392, 396
225,285,352, 357, 411 Tensile
Spherical vessel, (see also Thin walled ves- specimen, 11, 23, 54, 56, 78, 83, 84, 132,
sel), 382 134, 159, 164, 198,269, 318, 340
Spinel material, 109, 119 testing, 71, 76, 78, 79, 80, 89, 93, 104, 108,
Spot welding, (see also Resistance spot weld- 124, 129, 133, 147, 152, 156, 161,
ing), 97, 144, 363,405,409 166, 169, 170, 173, 174, 176, 193,
Stacking fault energy, 167 200, 203, 204, 210, 220, 228, 235,
Stainless steel, 60, 97, 111,196, 211,271,272, 242, 257, 271,272, 273, 277, 282,
340 283, 301, 303, 311, 313, 314, 340,
Steel, 8, 15, 23, 26, 42, 43, 54, 56, 73, 76, 79, 343, 346, 360, 361,394, 396
80, 81, 84, 85, 92, 93, 99, 104, 107, Test fixture, impact, 69
108, 120, 130, 132, 153, 154, 161, Thermal shock test, (see also Crack suscep-
162, 169, 170, 173, 174, 197, 198, tibility test), 324, 325,406
200, 201, 210, 219, 221, 222. 227, Thin walled vessel, 35, 125
235, 256, 269, 271, 272, 273, 277, Thorium-yttrium oxide, 324, 325
278, 279, 280, 282, 283, 289, 290, TGS ferroelectric crystal, 45
DROUILLARD ON BIBLIOGRAPHY 283

Tin, 76, 412 W

cry, 78,235, 236, 412 Wave mode, 95, 96
Tin-cadmium alloy, 78 compressional (see Longitudinal wave)
Titanium alloy, 56, 76, 79, 80, 120, 125, 130,extensional, 95.96
132, 136, 161,220, 235. 305, 340 flexural, 96
Transducer calibration, 24, 44, 78, 92, 94, 95.
lamb, 95, 96, 170
117, 119, 310 longitudinal, 95, 96, 245
Transducer types, (see also Sensor), 117, Rayleigh, 87, 88, 95, 96
159, 170, 206, 235, 239, 240, 247, shear, 95, 96, 245
342, 363,364 surface (see Rayleigh wave)
accelerometer, 6, 24, 78, 92, 133,159, 176, transverse (see Shear wave)
249, 345, 355, 361,364, 411
ammonium dihydrogen phosphate (ADP), Wave Propagation, 96. 170, 172, 196, 238
78, 364 Waveguide, 5, 7, 95, 159, 206,207,208,215,
barium titanate, 87, 88 239, 353,391
differential, 44, 75, 95 Wedge opening loading (WOL) specimen,
electromagnetic, 244, 245, 246 129, 133, 154, 198, 303
high temperature. 5, 6, 7, 95, 110, 145,206,
239, 240, 247, 309, 315 Weld
lead metaniobate, 95 cracking, 107, 162,165,181, 183,185, 195,
lead zirconate titanate (PZT), 78, 85, 95, 199, 201,211,216, 219. 310, 322
117, 233,345,364 damage, 130
lithium niobate, 6, 95. 110, 145, 309, 315 defects, 62, 107, 159, 181, 211, 310, 313,
lithium sulfate, 78, 84, 92 333
magnetostrictive, 94, 95, 96, 239 delayed (post weld) cracking, 107, 157,
microphone, 117, 120, 121,361,364 162, 165, 201, 211, 216, 219, 310,
passive pressure, 78 387, 388,389
piezoelectric, 45.75, 95, 110, 117,145,240,
Welding, process monitoring, 13, 19, 49, 60,
309,363,364 70, 79, 80, 97, 107, 144, 181, 184,
quartz, 78, 92, 117 185, 186, 193, 195, 211, 216, 225,
rochelle salt, 78, 364 235, 275, 295, 310, 321,322, 356,
Transformation, phase, 19, 21, 22, 44, 71,76, 385, 387, 388, 389, 405,409
78, 233,244, 246, 353,385 Welding process
Triangulation, (see also Source location), 35, electron beam (EB). 49, 97
53, 54, 58, 59, 86, 96, 159, 164, 171,
electroslag, 8
178, 194,207, 355, 382,407 friction, 97
Tungsten inert gas (TIG) welding (see Gas gas tungsten arc (GTA), 60, 97, 157, 159,
tungsten arc (GTA) welding) 164, 275,334, 356, 357
Twinning, 32, 37, 38, 55, 71,76, 85, 100,235, laser, 97
236, 250, 253,328, 337, 412 metal inert gas (MIG), 97, 159, 164
plasma arc, 97
U pressurized inert gas metal arc (PIGMA),
70
Ultrasonic welding, 97 resistance, 144, 211,225
Unflawed tensile specimen, 78,129, 132, 159, resistance spot, 97, 144, 211,225,363,405,
164, 198. 235, 252, 313,346 409
Unload emission, 32, 78, 99, I01, 102, 328 spot, 144
Uranium, 76, 235 submerged arc, 97, 107, 211,310
Uranium-niobium alloy, 241 tungsten inert gas (TIG) (see Gas tungsten
arc)
V ultrasonic, 97
Vanadium, 271 White noise generator, 117
Vibration, 349, 352, 391,411 Wire rope, 152, 228,373,374
detection of. 20, 255 Wood, 79, 80, 264
Video tape recorder, 120, 121,317 Work hardening, 85, 235, 271
284 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMISSION

Y Z
Yield, 78, 85, 101, I02, 202, 256, 283, 360, Zinc, 78, 85, 170. 202, 205, 250
367, 396 Zinc-aluminumalloy, 115
strength, 76, 198 Zircaloy, 55,207
Yielding, discontinuous, 166, 167, 168,202,
205
General Index
STP571-EB/Mar. 1975

General Index
A Defects, 5
Destructive testing, 69
Alumina, 22
Aluminum alloys 2024-T851, 20 E
Aluminum alloys 2219-T87, 20, 33
Ammunition belt links, 107 Earthquake prediction, 86
Attenuation, 30, 63
F
B
Fatigue crack, 19, 20, 33
Bainite, 46 Fatigue testing, 61
Bearings and shafts, 30 Ferrite-pearlite, 45
Beryllium, 141 Flaw detection, 221
Beryllium, aluminum welded, 137 Fracture, 215,218
Bibliographies, 241 Frequency analysis, 92
Boreholes, probes, 82, 89, 97 Furnace, 43
Broadband frequency analysis, 11,
83 G

C Gas pipelines, 59
Gas storage, underground, 96
Ceramics, alumina, 22 Geologic structures, stability, 80
Civil engineering applications, 85 Geophones, 97, 99
Coal mines, 99
Concrete, 198 It
Crack propagation, 18, 22, 40, 59,
122, 141, 225, 234 Hydrostatic testing, 8, 71,186,208,
Cyclic stress, 65, 76 212,226
Honeycomb structures, 225
D

Data analysis, 94, 131, 153, 162,

186, 205,222 In-service inspection-reactor
Data display, 88 vessels, 207
Data evaluation, 130 Industrial usage, 122, 150
287

Copyright91975by ASTMInternational www.astm.org

288 MONITORING STRUCTURAL INTEGRITY BY ACOUSTIC EMMISSION

Iron, 40 R
Iron-nickel alloy, Fe-20Ni, 46, 51
Residual stress, 59
K Rock bursts, 81
Rock mechanics, 80
Kaiser effect, 76

L
Sensor design, 203
Lamb waves, 33 Signal conditioning, 153,204
Leak detection, 211 Signal display, 205,222
Signal energy analysis, 115
M
Signal recording, 18, 44, 70, 82, 87,
100, 131, 157, 224
Ms temperature, 49 Signal transmission, 44, 82, 87,153,
Martensite, 46 162, 203
Microstructure, 52 Slope stability, 85, 86
Mining, underground monitoring, 81 Spectrum analysis, 18, 19, 24, 187
Stainless steels
N 304 SS, 209
Noise, 13 410, 46
Nuclear power plants, 200 Steel
Nuclear reactors, cooling systems, A105, 26
173,212 A212-B, 25
Nuclear reactors, in-service moni- A283, 25
toring, 200, 211 A516, 5
A533-B, 18, 19, 215
P A1S1 4300, 47, 50
A1S1 4360, 46
Plastic deformation, 18 A I S 1 4380, 46
Petroleum and natural gas applica- A1S1 52100, 47, 51
tions, 85 AP1 0.5L-X60, 226
Phase transformations, 40, 45 French AMMO, 234
Pipe rupture, 209 Iron-carbon alloy, 46
Pipelines, 59 Steel, maraging, 46
Pipelines, welds, 59, 76 Steel pipe-lines, 59, 170
Pressure vessel rupture tests, 208 Stress analysis, 124
Pressure vessels, 5, 19, 25,133,162, Surface mining, 84
175, 184, 187, 200,226, 234
Pressure vessels, filament wound, T
185
Pressure vessels, foam insulated, 30 Tanks (containers), 168
GENERAL INDEX 289

Tape recorders, 13, 82, 87,100, 115, W

185 Wave dispersion, 11
Teletype, 223 Wave mode detection, 218
Transmission loss, 11 Wave propagation, 218
Tunneling, 86 Weld monitoring, 59,109,137,139,
TV recording, 13 141, 190

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