Designation: E650/E650M − 12

Standard Guide for

Mounting Piezoelectric Acoustic Emission Sensors1
This standard is issued under the fixed designation E650/E650M; the number immediately following the designation indicates the year
of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.
A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

1. Scope* 3.1.5 waveguide, acoustic—a device that couples acoustic

1.1 This document provides guidelines for mounting piezo- energy from a structure to a remotely mounted sensor. For
electric acoustic emission (AE) sensors. example, a solid wire or rod, coupled to a sensor at one end and
to the structure at the other.
1.2 Units—The values stated in either SI units or inch-
pound units are to be regarded separately as standard. The 3.2 Definitions:
values stated in each system may not be exact equivalents; 3.2.1 For definitions of additional terms relating to acoustic
therefore, each system shall be used independently of the other. emission, refer to Terminology E1316.
Combining values from the two systems may result in non- 4. Significance and Use
conformance with the standard.
4.1 The methods and procedures used in mounting AE
1.3 This standard does not purport to address all of the sensors can have significant effects upon the performance of
safety concerns, if any, associated with its use. It is the those sensors. Optimum and reproducible detection of AE
responsibility of the user of this standard to establish appro- requires both appropriate sensor-mounting fixtures and consis-
priate safety and health practices and determine the applica- tent sensor-mounting procedures.
bility of regulatory limitations prior to use.
5. Mounting Methods
2. Referenced Documents
5.1 The purpose of the mounting method is to hold the
2.1 ASTM Standards:2 sensor in a fixed position on a structure and to ensure that the
E976 Guide for Determining the Reproducibility of Acoustic acoustic coupling between the sensor and the structure is both
Emission Sensor Response adequate and constant. Mounting methods will generally fall
E1316 Terminology for Nondestructive Examinations into one of the following categories:
5.1.1 Compression Mounts—The compression mount holds
3. Terminology the sensor in intimate contact with the surface of the structure
3.1 Definitions of Terms Specific to This Standard: through the use of force. This force is generally supplied by
3.1.1 bonding agent—a couplant that physically attaches the springs, torqued-screw threads, magnets, tape, or elastic bands.
sensor to the structure. The use of a couplant is strongly advised with a compression
3.1.2 couplant—a material used at the structure-to-sensor mount to maximize the transmission of acoustic energy
interface to improve the transfer of acoustic energy across the through the sensor-structure interface.
interface. 5.1.2 Bonding—The sensor may be attached directly to the
structure with a suitable adhesive. In this method, the adhesive
3.1.3 mounting fixture—a device that holds the sensor in acts as the couplant. The adhesive must be compatible with the
place on the structure to be monitored. structure, the sensor, the environment, and the examination
3.1.4 sensor—a detection device that transforms the particle procedure.
motion produced by an elastic wave into an electrical signal.
6. Mounting Requirements
6.1 Sensor Selection—The correct sensors should be chosen
1
This guide is under the jurisdiction of ASTM Committee E07 on Nondestruc- to optimally accomplish the acoustic-emission examination
tive Testing and is the direct responsibility of Subcommittee E07.04 on Acoustic objective. Sensor parameters to be considered are as follows:
Emission Method.
Current edition approved June 15, 2012. Published July 2012. Originally size, sensitivity, frequency response, surface-motion response,
approved in 1985. Last previous edition approved in 2007 as E650 - 97(2007). DOI: and environmental and material compatibility. When a multi-
10.1520/E0650-12.
2
channel acoustic-emission examination is being conducted, a
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
subset of sensors with characteristics similar to each other
Standards volume information, refer to the standard’s Document Summary page on should be selected. See Guide E976 for methods of comparing
the ASTM website. sensor characteristics.

*A Summary of Changes section appears at the end of this standard

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States

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 E650/E650M − 12
6.2 Structure Preparation—The contacting surfaces should 6.3.7 When bonds are used, the possibility of damaging
be cleaned and mechanically prepared. This will enhance the either the sensor or the surface of the structure during sensor
detection of the desired acoustic waves by assuring reliable removal must be considered.
coupling of the acoustic energy from the structure to the sensor. 6.3.8 The use of double-sided adhesive tape as a bonding
Preparation of these surfaces must be compatible with the agent is not recommended.
construction materials used in both the sensor and the structure. 6.4 Mounting Fixture Selection:
Possible losses in acoustic energy transmission caused by 6.4.1 Mounting fixtures must be constructed so that they do
coatings such as paint, encapsulants, loose-mill scale, weld not create extraneous acoustic emission or mask valid acoustic
spatter, and oxides as well as losses due to surface curvature at emission generated in the structure being monitored.
the contact area must be considered. 6.4.1.1 The mount must not contain any loose parts of
6.3 Couplant or Bonding Agent Selection: particles.
6.4.1.2 Permanent mounting may require special techniques
6.3.1 The type of couplant or bonding agent should be
to prevent sensor movement caused by environmental changes.
selected with appropriate consideration for the effects of the
6.4.1.3 Detection of surface waves may be suppressed if the
environment (for example, temperature, pressure, composition
sensor is enclosed by a welded-on fixture or located at the
of gas, or liquid environment) on the couplant and the
bottom of a threaded hole. The mounting fixture should always
constraints of the application. It should be chemically compat-
be designed so that it does not block out a significant amount
ible with the structure and not be a possible cause of corrosion.
of acoustic energy from any direction of interest.
In some cases, it may be a requirement that the couplant be
6.4.2 The mounting fixture should provide support for the
completely removable from the surface after examination. In
signal cable to prevent the cable from stressing the sensor or
general, the selection of the couplant is as important from an
the electrical connectors. In the absence of a mounting fixture,
environmental standpoint as it is from the acoustical stand-
some form of cable support should be provided. Care should be
point.
taken to ensure that the cable can neither vibrate nor be moved
6.3.2 For sensors that are primarily sensitive to particle easily. False signals may be generated by the cable striking the
motion perpendicular to their face, the viscosity of the couplant structure and by triboelectric effects produced by cable move-
is not an important factor. Most liquids or greases will work as ment.
a couplant if they wet the surfaces of both the structure and the 6.4.3 Where necessary, protection from the environment
sensor. For those few sensors which are sensitive primarily to should be provided for the sensor or sensor and mounting
motion in the plane of their face, very high-viscosity couplant fixture.
or a rigid bond is recommended. 6.4.4 The mounting fixture should not affect the integrity of
6.3.3 The thickness of the couplant may alter the effective the structure being monitored.
sensitivity of the sensor. The thinnest practical layer of 6.4.4.1 Permanently installed mounting fixtures must be
continuous couplant is usually the best. Care should be taken constructed of a material compatible with the structure. Pos-
that there are no entrapped voids in the couplant. Unevenness, sible electrolytic effects or other forms of corrosion must be
such as a taper from one side of the sensor to the other, can also considered when designing the mounting fixture.
reduce sensitivity or produce an unwanted directionality in the 6.4.4.2 Alterations of the local environment by the mount,
sensor response. such as removal of the insulation, must be carefully evaluated
6.3.4 A useful method for applying a couplant is to place a and corrected if necessary.
small amount of the material in the center of the sensor face, 6.4.5 The mounting fixture should be designed to have a
then carefully press the sensor on to the structure surface, minimal effect on the response characteristics of the sensor.
spreading the couplant uniformly from the center to the outside 6.5 Waveguides—When adverse environments make direct
of the sensor face. contact between the sensor and the structure undesirable, an
6.3.5 In some applications, it may be impractical to use a acoustic waveguide may be used to convey the acoustic signal
couplant because of the nature of the environment (for from the structure to the sensor. The use of a waveguide inserts
example, very high temperatures or extreme cleanliness re- another interface with its associated losses between the struc-
quirements). In these situations, a dry contact may be used, ture and the sensor and will distort, to some degree, the
provided sufficient mechanical force is applied to hold the characteristics of the acoustic wave.
sensor against the structure. The necessary contact pressure 6.5.1 An acoustic waveguide should be mounted so as to
must be determined experimentally. As a rough guide, this ensure that its surface will not contact any materials that will
pressure should exceed 0.7 MPa [100 psi]. cause signal damping in the waveguide.
6.3.6 Great care must be taken when bonding a sensor to a 6.5.2 If acoustic waveguides are used when acoustic-
structure. Surface deformation, that can be produced by either emission source location is being performed, the extra time
mechanical loading or thermal expansion, may cause a bond to delay in the waveguides must be accounted for in the source
crack, peel off, or, occasionally, destroy the sensor. Bond location program.
cracking is a source of acoustic emission. A compliant adhesive
may work in some cases. If differential expansion between the 7. Verification of Response
sensor, the bond, and the surface is a possibility, a suitable 7.1 After the sensor(s) are mounted on a structure, adequate
bonding agent should be confirmed by experiment. response should be verified by injecting acoustic signals into

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the structure and examining the detected signal either on an 7.3 Post Verification—At the end of an acoustic emission
oscilloscope or with the AE system to be used in the exami- examination, it is good practice to verify that all sensors are
nation. If there is any doubt as to the sensor response, the still working and that there have been no dramatic changes in
sensor should be remounted. coupling efficiencies.
7.1.1 The test signal may be injected by an external source
such as the Hsu-pencil source, or a gas jet (helium or other 8. Report
suitable gas), or by applying an electrical pulse to another 8.1 Any report of the mounting practice should include
sensor mounted on the structure. For a description of these details of the sensor mounting fixture(s), surface preparation
methods see Guide E976. method, and the couplant that was used.
7.2 Periodic Verification—On an extended acoustic emis-
sion examination, it may be desirable to verify the response of 9. Keywords
the sensors during the examination. Verification should be 9.1 acoustic emission; acoustic emission sensors; acoustic
performed whenever circumstances indicate the possibility of a emission transducers; AE; bonding agent; couplant; mounting
change in the coupling efficiency. fixture; waveguide

SUMMARY OF CHANGES

Committee E07 has identified the location of selected changes to this standard since the last issue
(E650 - 97(2007)) that may impact the use of this standard. (June 15, 2012)

(1) Changed designation to be a combined units standard. (3) Changed values in 6.3.5 to the correct combined units
(2) Added new 1.2 to include combined units standard state- format.
ment.

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