cotter ee vr NAVE FP RUPAGATION
_
Sound energy above the audible frequency of 16,000 Hz is
mechanical energy and Propagates through the material medi
by the elastic properties of the medium. Also, in-h
significantly modify and modulate the Propagation of
of mechanical energy, its propagation and its interact
It is a common experience that whenever a medium is disturbed by a force, the particles of the medium
are set into oscillation. The oscillation of the particles is either longitudinal or transverse or a combination
of both. In any of these types of oscillations, there is no bodily movement of the mass of the medium as
a whole; only the disturbance propagates. On the basis of particle displacement of the medium, ultrasonic
waves are classified as:
lomogeneities and discontinuities in the medium
these waves. Thus, ultrasonics is a study of a form
ion with the medium through which it Propagates.
° Longitudinal ‘waves ——_> = Direction of particle motion
particle Direction of disturbance
° Transverse waves —_—_ ttt Direction of particle motion
particle Direction of disturbance
i Direction of particle motion
e Compressional and
flexural waves
(Rayleigh waves
and Lamb waves) Direction of disturbance tT
66 Non-Destructive Test and Evaluation of Materials
h waves (also called surface waves): During propagation of these waves, particle oscitlatg,
Rayleigh waves (also ca
follows elliptical orbits as shown in Fig. 3.1.
Direction of Propagation. §——____—_»>
Fig.3.1 Rayleigh Waves (Surface Waves)
‘The major axis of the ellipse is perpendicular to the surface along which the wave moves; the minor
axis is parallel to the direction of wave motion. These waves travel along flat or curved surfaces of thick
Solids, The depth of these waves below the surface, with effective intensity, is of the order of a wavelength
only. These waves are used to detect flaws or cracks on or near the surface of test objects,
Lamb waves (also called flexural waves or plate waves): These waves are produced in thin metals
Whose thickness is comparable to the wavelength, These waves are complex in nature; elastic properties,
Structure, dimensions of the medium and cyclic frequency determine their propagation through a medium.
32a) and (b illustrate these waves, Symmetrical lamb waves have comprescional Particle displacement
along the neutral axis and elliptical particle displacement along the surface,
or Direction of propagation
FiB.32 (a) Symmetric Lamb Waves (b) Asymmetric Lamb Waves 3.1.1
cast Wa
Ww ve Propagati Ulrasonics 67
ih Then waves sy, gation
longitudinal op set ONE att
o oF in the after the oth .
er force the parte © direction, exe Tegular intervals, the particles, wheth i
Position of rest Particle of the medion en Petite motion. IF an infinite fer moving in the
relsove to the direction of at any tim near the point of application of fore: medium is subjected
neighboring particles snot PMPAEATION of th after the start of the disturbance depends ones coms
and eneray tance STURN near the on the disturbance. Asa result of the inpaloe sure pestio8
a similar manner. anges PI8*® from one ein ofthe applied fore, a diatbar muse tansmited by
The periodic foreeahg 2 Oth way, the energy een Ts second particle eee
i ces om element nl nfuences the tid in
i ill it
splacement
- the energy pa
vaves. In a gaseo
" ag aseous medium these travel only as a longitudinal wave; in
es, and in solids, they travel as longitudinal,
answer, surface or lamb waves,
in so far as ultrasonic testing is con
in solids. :
The velocit i
ity of propagation of various types of waves is as foll
lows:
oases)
«(Fllaa)
(087+1.120)[E 1 Le
S l+o p 2+)
emed, we i
e will confine our attention to the propagation of waves
E= youngs modulus
where C;, = longitudinal velocity
(p= volume density of the medium
Cy = transverse velocity
Cs = surface wave velocity ‘ion ratio
o=
Waves
variation in the
‘ical calculations,
3.1.2 Pressure and Intensity of Sound
ression and rarefaction. This cas pressure
variation of pressure is periodical. For all practi
ed and given by:
aod energy travels as waves of comp
ium as the waves travel through it. The
Toot mean square (rms) of the pressure is ust
mum pressure
P,,, = [© where Po is the maxi
fo
™ 2 i]
given by:
$8 Non-Destructive Test and Evaluation of Materials
- ‘ murce is
The intensity of sound at a point some distance from the so
w
I=
Al 5
where W’ = rate of energy propagation and A = area perpendicular to the direction of Propagation,
This intensity depends on the
* distance of the area from the source
* properties of the medium i
* orientation of the area with respect to the direction of energy propagation
In practice, it is difficult to measure the intensity, but easier to measure the pressure, Pressure i
elated to intensity and energy by the following relation:
2
1= Fm where p= density of the medium
pC
P
2
E- C= velocity of sound in the medium
Sound pressure, power and intensity are measured on a logarithm scale. The scale is designated ste
decibel scale and is a comparative scale. The level of two powers w, and w, is expressed on the decibel
scale as
4
10 logy (2)
This is the decibel level of w, above wy and is written as ‘dB’.
The decibel scale is applied to sound measurements by the following definitions:
w
(@ Sound pressure level = 10 60( * | with reference to wo
a L
(i) Sound intensity level = 10 logia| 7~} with reference to Jy
pe
ii) Sound pressure level = 10 logy (3) with reference to Py
3
P
= 20 lose (7
The usual reference levels used in the study of sound are:
Wy = 10°? watts
Jy = 107? watts/m?
P, = 0.00002 newtons/meter?
= 0.0002 Dynes/cm? = 2044p (micropascals)
: acl
Inan ultrasonic test system, signal amplitudes are measured as electrical voltages only and aie
Power is proportional to the square of voltage; therefore, the level of acoustic power is giV¢!
sti ee Ultrasonics 69
10 logio () with
13 Teference to Y= 29 log, (%)
10 |
Note: Sometimes the yy :
‘eperia a ‘
The two are related as, 1 leper = on base ¢ is used instead of the common logarithm base ‘10°,
“115 dB or 1 dB = 8.69 Neper.
3.1.3 Acoustic Impedance
This implies the resistance
ene " of i
specific impedance is defined au theme to the
Thus, specific impedance z = ply an
For plane harmonic waves PIV = pe
Passage of sonic energy through it. In acoustics, the
sure to particle velocity.
P= density of the medium
Y= particle velocity
a velocity of sound in the medium
= pe, Zs expressed in kg/m? i
where is expressed in kg/m? and c is expressed in ise elias
Z depends on the structure and metallurgical condition of te material, as these factors affect both p and c.
3.2 REFLECTION, REFRACTION, DIFFRACTION,
MobE CONVERSION AND ATTENUATION
3.2.1 Reflection
The Snail's law of reflection, as applicable to light rays, is applicable to acoustics, provided that the
dimensions of the reflecting medium are large compared with the wavelength. The law may be stated as:
(a) The incident ray, the reflected ray and the normal, at the point of incidence, lie in one plane.
(b) The angle of incidence is equal to the angle of reflection as shown in Fig. 3.3.
MN = Reflecting surface ZAOB = i= Angle of incidence
AO = Incident beam ZBOC Angle of reflection
OC = Reflected beam A
OB = Normal at the point of incidence :
A
3.2.2 Refraction
Sound waves incident obliquely on the boundary separ-
ating two media, where the velocities of propagation 2° 0 .
different, undergo an abrupt change in direction. This
Phenomenon is known as refraction. The laws governing Fig. 3.3. Reflection
the phenomenon of sound refraction are similar to aa
‘plicable to light waves. The laws may be stated as: 70 Non-Destructive Test and Evaluation of Materials
(a) The incident ray, the normal tothe refracting surface at the point of incidence and the reacts
ray lie in one plane. 8
(b) The sine of the angle of incidence bears a constant A :
ratio to the sine of the angle of refraction. which is :
equivalent to the ratio of the sound velocities in the eS :
media concerned. Figure 3.4 illustrates this ice.
MN = Refraction surface mn
+
zone Near zone| Far zone
Distance from the generator Divergent beam far-zone
@ )
Fig. 3.8 Pressure Variation in the Forward Direction from the Surface of a Piezo-Electric Plate
The length of the near field is determined by locating the peak of the final maximum pressure. This is
determined by making m = 0 in the relationship. Hence, we have length of the near zone =
For D> A, length of the near zone = D/4A
The number of maxima and minima is given by:
M=D/i.
In the far zone or Fraunhoffer zone, the sound waves travel as a diverging beam. This zone is
interference free. In this zone, the intensity of sound decreases E
as the square of the distance from the piezo-electric plate. cl
From the viewpoint of ultrasonic testing, the far field is of A
interest to us. The sound beam diverges in the far field, as |
the distance increases as shown in Fig, 3.9. ‘B
It may be seen that the pressure amplitude is maximum y D
along the axis OP, but with increasing distance, it keeps 3N fA
decreasing. Also, the pressure amplitude reduces when
observed away from the axis. The zero pressures for distances
N, 3N and 6N are at A, B, C, D, E and F, The divergence
Profile of the beam is obtained by joining the zero pressure i
points, viz. O, A, C, E and O, B, D, F, etc. The angle that this line makes with the axis of the beam 'S
called the angle of divergence. The angle of divergence is defined as:
nie CA
sol
Fig. 3.9 Divergence and Pressure
Variation in the Far Fiel cv / Ultrasonics 75
sin = Ty ft frequeney
diameter of gencrator
7= angle of divergence
V= velocity of sound
constant that assumes the following values
is 2 rhe pressure at any point along the axis OP
tion C= 0.44 for 70% pressure point
0.56 for 50% pressure point
= 1.08 for 10% pressure point
Al various pressure Points across the beam cross-
's taken as 100% pressure point,
1.20 for 0% pressure point
The 70% pressure point corresponds to 30% reduction in pressure and is referred to as the 3 4B drop
pint. Similarly, the 50% and 10% pressure points are refered to asthe 6 dB and the 20 dB drop poing
respectively.
The values of C mentioned are valid for small values of A/D (small values of divergence) and circular
generators. If the generator is not circular, the relation is not accurate. In
sessed experimentally,
The ultrasonic field variation can be controlled by suitably adjusting the diameter and frequency of
the ultrasonic generator. Using a large diameter transducer and high frequency can reduce divergence to
asmall value, which is desirable for ultrasonic testing. However, large diameter generators also increase
thenear zone (also called zone of confusion). Therefore, in practice, divergence is kept within a tolerable
range by a compromise between diameter and frequency of the generator.
such cases, divergence is
3.4 Prezo-Evectric Erect
The word ‘piezo’ means pressure and piezo-electric effect implies pressure electricity. Coan nanally
occurring crystals like quartz and tourmaline show piezo-electric property. The estas ae jected
to mechanical vibration, produce electrical pulses ina perpendicular direeion = phen Si ena
are subjected to high frequency electrical pulses, dimensional se is ol serve ins pore xtc
Tra, Caniuous impinge earl phenomenon Ia sound Wave, i alematng
This shows that the piezo-electric effect is a reversi me
xpansion a eaorecsiont impinges on the piezo-electric plate, fees eS eran
Voltage with the frequency of the wave. The generated voltage 7 roport pal we de agotnds of yt
Pressure, Thus, a direct piezo-electric effect is used to receive ultrasoune,
used for generating ultrasound.
Some piezo-electric materials like qua
Commercially used piezo-electric material r
Phosphate, lithium sulfate, lead niobate, potassium
jae irconate titanate, etc.). eiPolyervata
ae eee a om solutions under controlled conditions. Polycrystalline ceramics are
'ynthetic crystals are grown frol
ieee roachiieve piezo-electric property, These ceramics
a intering at high temperature 0 M1 ential (~ 5 KV). The crystal domains
rote ng ed ig ey ing pa” S Tcl oat
ign with he eh cd romain in that condition even ae the Vif the erystal is heated above thi
Which polarization is achieves
i el in ture. But most of the
aline and rochell salt occur in nat
me . tne synthetic compounds such as ammonium dihydrogen
* aasiurm dihydrogen phosphate and polycrystalline ceramics N
36 NomDesmuctive Test and Evaluation of Materials
trie property. Natural crystals also have their own ¢
remperature, the erystal loses its piezo-el ave .
t ations ‘The following table shows some of the properties of popularly used piezo-electrig cya
temperature. The following 1
ommon properties of piezo-electric materials
TABLE 3.1 Some ci
i Property Quart: (SiO") saihien Sate parti Tree ri
Density (emer) er | 2.06 54 a
Acoustic velocity (m sec) 5740 5460 5100 ‘000
‘Cue temperature CC) 376 130 120 19035
Acoustic impedance (N.s'm*.10°) 15.2 M2 27 30
3.5 ULTRASONIC TRANSDUCERS AND THEIR CHARACTERISTICS
Ultrasonic transducers (or probes or search units) are devices to generate and receive ultrasound. For
non-destructive test purposes, piezo-electric elements of suitable dimensions are used to generate the
complete range of ultrasonic frequencies at all levels of intensities. The transducers convert electrical
energy into mechanical energy (vibration) and vice-versa, as explained earlier.
A transducer essentially consists of a case, a piezo-electric element, backing material, electrodes,
connectors and protection for the piezo-electric
element from mechanical damage. Figure 3.10
Coaxial —>t”
shows the essential elements of a transducer “°°” Casing
assembly. ‘Signal connector
A casing is the housing within which various
elements are contained. It is metallic or molded ee
plastic. When the piezo-electric element is subjected matetal
to electrical impulses, it vibrates or ‘rings’ for a
long time. For non-destructive testing, along period Elec
Ground ——+»
of vibration is undesirable as it adversely affects ,S"Und- ma
defect resolution capability. To prevent excessive oe
ringing, highly attenuating materials (called backing .
materials) ae bonded to the back face of the piezo. Fig. 3.10 Elements of a Transducer Assembly
electric element. Backing materials consist of a mixture of ‘graphite, powdered metals (e.g. tungsten) and?
metal oxide of random grain size, Wear resistance of the crystal can be increased without sacrificing
resolution and sensitivity by the use of a thin layer of aluminum oxide or boron carbide.
3.5.1 Types of Transducers
Normal Beam Transducers
These
and Mate iee {or contact testing and immersion testing. Transducers generat, tans!
transducer and Fig. 409 — normal to the test surface, Figure 3.11 shows a normal beam cont
electric element is ~~ shows an immersion transducer, In the immersion type of testing, the piez”
‘nade completely waterproof and a grounding electrode is provided in the font f° Signal connector
Backing
material
Electrode
| —=Selrode
Normal Beam with Wear Plate
Fig 31
Angle Beam Transducers
Ulirasonies 77
Coaxial
45 connector
Casing
Signal connector
Backing I
material “|| jhe Ground
connector
Crystal—{_|
Fig. 3.12 Immersion Search Unit
hese are contact type transducers that transmit and receive longitudinal waves at an angle to the test
raterial surface, During the transmission of the wave, the longitudinal wave is mode converted to a
‘hear or surface Wave on entering the material.
During reception, the shear or surface wave is
mode converted back to the longitudinal wave. Figure
3,13 shows the essential elements of an angle beam
wransducer.
‘The transducer is similar to a normal beam probe,
except that a wedge cut at an appropriate angl
atached to the normal beam transducer.
le is
Apart from those mentioned, various types of
transducers, in different sizes and frequencies, have
been developed for specific inspection applications.
Some of these are discussed next.
Oual Element Transducers
Coaxial
>
‘connector
Signal connector
lnthis type, the transmitter and receiver elements are
this.
Focused Transducers
es transducers are designed to concentrate
ine, 8 into a small area. This improves
redugee 7 Semsitivity and resolution and also
lens of the effect of acoustic noise. An acoustic
2 nom eetermined focal length is attached 10
Porateg beam probe, Sometimes it is incor-
Congo transducer facing. The focusing
Cylindrical or spherical. While examining
Separated with a cork-divider. Figure 3.14 illustrates
Transmitter element.
‘Acoustical barrier —}
(cork)
Casing
Ground
connector
Electrode
Wedge}
Crystal
Ds
Fig. 3.13 Angle Beam Contact Search Unit
Coaxial connectors
Receiver element
Plastic blocks
LS
3.14 Dual Element Transducer 78 Non-Destructive Test and Evaluation of Materials
. sing is used. Spherical focusing concentrates the sound bes...
eee ae towed while examining near surface defects. Figure 3.15 illustra
cone. Spherical foc
transducers.
ee
arr
oir
Water
. Surface contour: The disturbance caused by reflection and refraction can make it difficult to
interpret the indications in contoured parts with complex geometry. In such cases, it is helpful to
examine the component from two opposite sides, wherever possible. Duplicate measurements at
symmetrical points can bring out the similarities or dissimilarities. Often, secondary echoes appear,
which are not due to any flaw, as shown in Fig. 3.19A.
These secondary echoes appear beyond the back-wall echo. Sometimes, a divergent beam
traveling a slightly longer, slanted path than the axial beam in the material gives rise to such
echoes. These are called ‘Ghost echoes’ and need not create confusion to the operator, as they
appear beyond the back echo.
Triangular reflections: While testing solid cylindrical or spherical components with a normal
probe, additional echoes may be generated due to a longitudinal wave returning to the crystal
after more than one reflection at the side walls. Or before entering the crystal the longitudinal
wave gets mode converted to a transverse Wave, which is mode converted back to a longitudinal
wave after another reflection.
Either of these cases can occur de
test section, as shown in Fig. 3.19.
pending on the angle of incidence and the diameter of the
A Back echo
~ without side reflections
Signal
B|
due to side ‘reflections
NOP | sora]
c
a
4
Lb
Ke
sion due (0 Geometry
due to side reflections
‘and mode conversion
Fig. 319A. Secondary Reflec
Fig. 3.19 Triangular Reflections 82 Non-Destructive Test ‘and Evaluation of Materials
4. Fillets and holes: When a cylindrical part with @ threaded hole is scanned from the end
aathnce, a stant beam falling on the sidewall, after reflection from the threaded surface, produsn
surface, a sl roe
aked indication, as shown in Fig. 3.20. | ,
mping inside the hole with a wire and observing
e
a multi-pe:
This possible to check this problem by dar
damping of the echo. a ;
5. Mode conversion duc to geometry: Cylindrical components with shoulders and fillets give 5
to mode-converted echoes as follows: Tise
fillet echo as Fig. in 3.21.
« Aslant beam hitting the fillet directly results in a
Aslant beam hitting a shoulder is mode-converted as a transverse wave and reaches the opposit
ite
fillet, Here, the wave is again mode-converted and reaches the probe, resulting in an echo
This phenomenon of mode conversion at angular incidence needs to be understood clearly, *
A 80
that there are smaller chances of misinterpretation of echoes.
PAN
Po
“Oo Fm
Fig. 3.20 Reflection from a Threaded Hole Fig. 3.21 Reflection due to Fillets
* Mode conversion can occur in components with a threaded hole as shown in Fig. 3.22.
Aer A
L '
8 t
err 8 t
Y
Fig. 3.22 Reflection from Bore Wall
In position 4.
, the angle of incidence ji
In position the antic ar ety 's 45° and after reflection, the beam retraces its pal
mode-converted and travels i cies is 61° for a steel component, The longitudinal wave gels
travels back to the bore surface. j 28 & tansverse wave, The converted transverse Wa
prt te probe The resuhing finer itis mode-converted as a longitudinal wave an
te echo . is located further A
es resulting due to shape and Same rie eee
he part, it helps to:
© Mark the echo positi
‘Positions. Materi: a 5
are from possible defects or mat and geometrical considerations suggest whether the re
flection’ 10,
6M
« study 2 symmetrical location on the sam
angle probes. where possible, to confirm
|, Make accurate measurements of the loc
composites: influence ultrasonic attenuation
gepth to which an ultrasonic test can be carr
of the reflected signal from defects located at different
“ ny
aerial characteristics: Material properties lik
MMi structural in-homogencities resulting f
Ultrasonics 3
€ test speci
men oF o
n findings a symmetrical component, Use
ation of the echo
Paes
ie density clastic modulus, met
AY-up and cure operations
and acoustic im
ied out. Attenuat
tallurgical structure
in fiber-reinforced
pedance. Attenuation determines the
on loss also influences the amplitide
vo materi ee depths
impedance of two materials on cither side of an interface determine ee neTAl, The acoustic
transmission of ultrasonic energy from one
The reflected and transmitted energy depends on the rat
the impedance of the first material. The sou
impedance mismatch increases.
Flaw characteristics: Ultrasonic reflection
and acoustic properties of the flaw. The
amplitude of reflection. A flaw filled with
with organic matter or oxide inclusion gi
determines the
ext i
medium to the other Me of efestion and
Eo io of impedance of the second material to
Transmitted through the interface decreases as the
from a flaw depends on the size, shape, orientation
acoustic properties of the flaw also determine the
air produces a good reflection, whereas a flaw filled
ives a weak signal. In the former case, impedance
mismatch between the material and the air inside the flaw is high; in the latter case. itis low.
Further, a flaw with a smooth surface reflects well while a flaw with a rough surface reflects
weakly.
‘Any flaw with a good reflecting surface, oriented at right angles to the beam direction produces
a signal with good amplitude. Unfavorable orientation of large flaws does not allow the beam to
reach the back-wall. This situation may lead to the absence of a flaw as well as a back-wall
signal. Angle probes are used in such a situation,
‘Acoustic coupling: In a direct contact test, the degree of acoustic coupling depends on the surface
finish and the acoustic impedance of the intervening medium. Oils of different viscosities are
used here. Thicker oils or greases are used both for vertical and horizontal surfaces of the
components. The use of a different couplant can cause a variation in results even for similar tests.
ona given sample. To minimize this, probin;
using shear wave probes with a thick fluid as
due to accumulated couplant in front of the probe.
. False indications: These are caused due to noise or rever
an apparent discontinuity moving in a ho
equipment located nearby. This indication can
1g conditions must be standardized. Sometimes, while
couplant, there may bea faint surface wave indication
eration in the test specimen. Sometimes,
y due to other heavy-duty
rizontal plane is probabl h
‘a defect indication, as itis
be distinguished from
imegular and not synchronized with the time base. ees
The indication due to reverberation is cause
high pulse frequency. This occurs when th
pulse before the next pulse is transmitted.
Synchronized with the sweep line or the
Completely but can be reduced to the poi
pecomes possible by reducing the gain to th
Interpretation of indications: All types of d
Produce echoes, the amplitude of which
impedance. The information available on th
ti
t that a Corrs
h depends, amon
material with low
attenuate the initial sound
ontinuity indication,
cannot be eliminated
valid discontinuity
d where an
he test material does not
‘The result is apparent di
h a disturbance
get interpretation of the
ime base. Such
ne required level.
efects, disconunl
g other t
1, in-homogeneities,
ir specific acoustic
ssentation is:
ities OF material
hings, on th
e CRT during the test by A-scan pres ~y
haractrizing the size of the discontinuity in the path gr
the
84 Non-Destructive Test and Evaluation of Materials
The amplitude of the signal cl
ultrasonic beam. ;
The loss of back reflection amplitude.
© Location of discontinuity from the scanning surface.
Various 4-scan indications corresponding to particular defects can be characterized as follows:
}. Spherical defects (gas hole type): An echo from a single defect with dimensions greater than
selected standard defect size, is sharp and clear. A group of gas holes gives rise to an
representing a superimposed multi-peaked signal and the trace appears jagged.
>. Volume defects of irregular shape (inclusion type): Non-metallic inclusions give rise to
cho
ir an ec
trace that is forked or broadened (where resolution is poor). When the probe is rotated around 2
defect, the echo does not disappear, but its shape varies.
3. Sharp linear defects (crack-like): Linear defects like cracks give a sharp and clear signal, Thy
amplitude can vary when the probe is moved around the defect. For long cracks, the amplitude
decays when the probe is moved in a circular path with the flaw in the center.
4, Lack of penetration in welds: Echoes from this type of defect give a clear and sharp signal. The
signal disappears when the probe is moved round in a circle with the defect at its center. When
lack of penetration occurs at the root, the maximum reflected signal occurs at half the skip distance
from the weld center. If the flaw echo remains stationary over a long distance of the scan, the
lack of penetration is considerable. If the probe is moved in the direction perpendicular to the
axis of the weld and the echo disappears rapidly, then the defect is small and shallow. Often, the
study of echo formation and its movement over the CRT screen, while manipulating the probe,
provides useful information.
3.7 ULTRASONIC TESTING
3.7.1 Basic Methods and General Considerations
Allitrasonic testing depends on the nature of the product, its manufacturing process, the surface condition,
geometry and accessibility of the scanning area.
There are three basic test methods commonly used in industries: pulse echo, through transmission
and resonance.
2 Pulse echo test method: Here, short pulses of ultrasonic waves are transmitted in the materi
under test. These pulses are reflected from discontinuities in their path or from any boundary of
the material. The reflected waves (or echoes) are received by the same transducer and are dispose!
on the CRT, which provides the following information:
. Te relative size of the discontinuity in terms of the amplitude of the signal displayed 0"
* The depth ofthe discontinuity on the CRT time base scale, which is appropriately calibrated
terms of known material thickness
$ mel i vor of the WAVE
ea this method, a single transducer is used both as transmitter and receiver of the
climes (Wo transducers are used, one as transmitter and the other as receiver i rtht Ultrasonics 98
‘The main advantage of this method i fe
TE ie Of this method is that only one
ating and the method is capable ot providing a a eee of the ext objets require
red for
ngand the m size ns well
jowever, a limitation is that the i pat teoah local nual
i he material immediately below th areal ae cae
transducer contact surface
thin the near zone ot be examines 88 the appropriate shoe is attached to the
wit le priate delay sh
lelay shoe is
vducer in contact testing n
transducer in contact testing or a suitable length of water e
ing. Figure 3.23 illustrates this method ‘ater column is provided in immersion
te
tes
CRT
_—————— on
Test specimen | \rq, 4
um
Flaw == ee crt
@ = Transmitted echo
b= Flaw echo * °
= Back echo B
Flaw
Fig. 3.23 Pulse Echo System
_2¢ Through transmission method: Two transducers are used here, one as transmitter, the other as
receiver. Short pulses of waves are transmitted into the material. The test method requires access
to two nearly parallel surfaces of the test object. The receiver transducer is aligned properly with
the transmitter transducer on the opposite side of the test object to pick up the ultrasonic waves
passing through the material. ‘The soundness or quality of the test material is evaluated in terms
of energy lost as the ultrasound travels through the material, The presence of a discontinuity is
indicated by variations in the energy amplitude. A significant reduction in energy ‘amplitude indicates
adiscontinuity. The main disadvantage of this methods its inability to locate the defect. Misalignment
of the search unit can also create an int Yblem. Figure 3.24 illustrates the test system.
erpretation prol
‘An advantage of the through transmission system is better near surface resolution.
cnt
Sound
crt
duction
amplitude
Flaw
Fig. 3.24 Through Transmission System Ultrasonics 87
ransmitted echo
law echo
Incident angle
62 Refracted angle
ig-50" Irmersion Angle Beam Method, Using Longitudinal Waves
1 is important to appreciate that shear waves cannot be used in a fluid: ”
wae are used and introduced into the medium at an angle, with ten eee
saves entering the medium get mode-converted as shear waves at an angle. After reflection ‘rom a
{pieet or boundary, the transverse wave gets mode-converted and travels back to the transducer as a
jongitudinal wave. —
In the through transmission immersion technique, the specimen is immersed in a liquid couplant,
usually water. A separate transmitter and receiver are axially adjusted through manipulators. Ultrasonic
energy is transmitted into the specimen, which is mounted on a special fixture, for easy adjustment. Any
defect in the path of the ultrasonic beam causes a shadow and hence, a reduction in the intensity of the
beam. Figure 3.29 illustrates the test system. _,
Further, to reduce the difficulty in interpretation, the water path, that is the distance between the
transducer and the front surface of the specimen, should be selected such that the ultrasonic transit time
in the liquid column is greater than the ultrasonic transit time between the front and back surfaces of the
test material.
cAT
Tank —4
Water ;
Front
surface
R
= itted echo
Stecinen _| a= Transmit
a b= Front surface echo
c= Flaw echo (Reduction in amplitude)
Support Back surface
ansmission Immersion System
Fig, 329 Through Tra 88 Non-Destructive Test and Evaluation of Materials
Angle Beam Testing
‘An ultrasonic beam is transmitted into the test specime!
ce, To achieve this, the piezo-electric element
le. The flat edge
mat an
angle to the test surfa ‘aan
is mounted on a plastic wedge at the desired angt
of the wedge is placed on the test surface as shown in Fig. 3.30.
When the angle of the incident beam is other than normal to
the test surface, refracted longitudinal and shear wave components
are produced due to mode conversion. Longitudinal waves are
originally produced in the wedge but it is possible to have either
Tongitudinal or shear waves in the test specimen. Both may be —
present at the same time depending upon the angle of the wedge. an Meio a
‘As the angle of incidence is increased to the second critical aan
anale, when the shear wave travels paraleltothesurface, surface py
wave mode is developed in the medium. This wave can penetrate "383-30 Angle Beam Technique
the medium to the extent of one wavelength. This wave pattern
is known as ‘surface wave’ or ‘Rayleigh wave’ and its velocity is about 90% of the velocity of shear
waves. These waves are used for detecting surface discontinuities in the contact mode of testing, The
waves follow the contour of the test specimen around
fillet radii and other irregular surface features. Figure 3.31
illustrates this.
In very thin sheets, the angular incidence of the sound
Test specimen
Transducer
beam and mode conversion at the interface produces plate Test specimen
or lamb waves. The velocity of these waves depends on
the type of material, the frequency, and the velocity in Fig. 3.31. Surface Wave Testing
the wedge material, the angle of the wedge and the plate
thickness. Various applications of these waves are given in Table 3.2.
TABLE 3.2. Application of shear, surface and plate waves
Testir
fing Method Typical Applications
Shear i
waves Inspection of welds, plates, pipes, tubing and complex geometry
forging and castings
Surface waves e |
Inspection of surface defects (e.g, heat Weatment cracks, fatigue
Cracks, tool marks, stress raisers, etc.)
Detection of laminations in thin materials, lack of bonding in
composite materials
Plate waves
3.7.2 Testing of Products
Castings
stings is limi foncat i
sion ral i of semay ° the detection of large isolated discontinuities such as voids,
application of ultrasonics to eastinge en CisPetsed coarse porosity and coarse grain structure. The
coarse grain structures. The inspec ned due to their size, shape, thickness, surface roughness amd
testing cast blooms for primary and veo at c*Stines with parallel sides with ultrasonic is simple, 38
condary piping for cropping before further operations.
Ultrasonic testing of ¢
inclusions, cracks and roost products Ultrasonics $9
fg ous generally have uniform
square section havi ect a round
332 80 aie jmen, if th pine t and causing. signal withoe ge ot Fectangular Figure
Fe ae ee a acteet happens to he Fela RY ambiguity in interpretation
Mile ination NO eal petluetion in the back signal amatneye ie beam it may not give
fiom aitferent aes feed e explore the pre Head mlitude. Such cases require scans
¢ and rectangular aaa and to assess its nat 3
uch SqUATE phowslilie aeaniot © scanned in two directions prefer: iy Mn ana sie. Normally,
other. Figure ifeaion fran ° In Of a round section of a wrought maar Dr eae
zn defect indication shows un when the probe is in position B. The deface ne oss Dut
re si position 4, Beene the Dlane of the defect is parallel to the sonic heamy nee en the
Ou cover a “a yo al
fhe scan path should cover at least 120° (preferably 180°), as indicated bean’ In such cases,
probable orientations of defects. : cated by the arrow, to cover all
Bcometrical sections
anger ini
%
Flaw
(0)
Fig. 3.32 Simple Regular Section
Fig. 3.33 Round Section
Forgings of complicated shape, it may not be possible to scan 100% of the surface. In such cases,
the best way is to test the billet thoroughly for any defect -
and, in a multistage forging operation, to test the product
4 convenient stages. In the final forged condition, only k
A
T
Some areas may be amenable to an ultrasonic check. 2
Figure 3.34 shows the cross section of a forging where
Ticks originate from the inner radius, invariably at an angle Flaw
OF 45° to the lug. To detect this type of erack, a 45° angle a
beam probe is used, as shown. In the absence of a crack, a jd}
ulasonic beam follows a path as shown at and aler :
"flection from the inner wall, may not return 0 oe a= Transmitted echo
'Ucer, resulting in no signal. When a crack is Pere b= Flaw echo
ultrasonic beam is intercepted by the crack, retraces Ils Testing of Selected Location
th and produces a defect signal as indicated in B. Fig. 334 90 Non-Destructive Test and Evaluation of Materials
Figure 3.35 shows an actuator housing, which
is subjected to fluctuating hydraulic pressure,
Teading to fatigue failure. Fatigue cracks appear
as shown at B. Here, the geometry of the
component permits the use of a 60° angle probe
to detect any crack in its initial stage of formation.
In the absence of a crack, as shown at position
‘A, there is no defect signal. In the presence of a
crack, when the probe is positioned at B, the
ultrasonic beam is intercepted by the crack,
resulting in a defect signal. This type of angle
beaming of critical areas is found to be useful in
the detection of defects initiated during service.
Weldments
Fig. 3.35 Testing of Selected Locations of a Housing
Welds are ultrasonically tested either by the straight beam or the angle beam technique. The angle beam
technique is commonly used, as the straight beam testing requires weld beads to be ground to get a fat
scanning surface, Apart from being expensive grinding of weld beads is not always permissible from
design considerations. Welds are usually tested at frequencies 1, 2, 2.5 and 5 MHz with varying angles
like 35°, 45°, 60°, 70° and 80°. The procedure adopted is as follows:
(a) Determination of skip distance and half skip distances. Skip distance S is the distance between
‘two nodes and is given by S = 2s tang,
where = refracted angle, S= skip distance and t = specimen thickness
(b) Two lines are drawn from the central axis of the weld bead, one at a distance of S/2 and the other
at a distance S, parallel to the weld bead.
(c) Now the angle probe is moved for skip distance S to half skip distance S/2, with a swivel motion
of the probe at ‘A’. When the probe is at S'the top of the weld section is scanned; when itis t
2, the root of the weld is scanned; in between S and S/2, the intermediate weld sections are
scanned. Any defect in the weld section will intercept the ultrasonic beam and make it retrace is
path, thereby causing a signal on the CRT screen. Figures 3.36 and 3.37 illustrate this.
SET STE?
Transducer 4 ASY
Transducer }«—s—> As
I Node Node. fi “ “ ‘Specimen
{| 0 rit
rl?
Fig.3.36 Path of Shear Wave
kip distance
Fig. 3.37 Testing of Welding Testing of Fillet Welds
Ultrasonic testing is employed extensiy
method. However, due to the eo,
Utirasonies 94
‘ely for butt welds,
vely fort . but some fillet welds can also be tested by this
mplexity of joints, all fil
Figure 3.38 shows the testing of
a double fillet weld, usi
the weld. Scan 4 shows a multiple reflection pattern from the horizontal member; § ir
all echo; scan C indicat ‘
sound weld with a weak back w:
and a weak back echo, Figure 3
ransmitted echo
Flaw echo (Weld)
Immersion Testing of Welded Tube
Figure 3.39 illustrates the immersion testing of.
weld tubes. In weld testing, it is usually desirable
to generate shear waves in the test object. To
achieve this, the transducer is tilted through an
angle of incidence between 15° and 33°. In shear
Wave testing, a small and poorly defined reflection
for the entry surface and a strong reflection from
the crack in the test specimen are shown on the
CRT screen.
Much of the sound wave is reflected from the
Surface. A fraction of the sound wave that enters
‘38 shows the testing of a
Back surface echo (Plate)
jack surface echo (Weld)
ict welds are not amenable to
ultrasonic testing,
ing a normal probe from a hori
zontal member of
can B indicates a
ld, showing a good defect echo
ingle fillet weld using an angle probe.
les a defective wel
€)
= Transmitted echo
b= Flaw echo
3.38 Fillet Weld Testing Using Angle Probe
Probe
assembly
CRT
Front surface
Flaw
‘Sound
Fig, 3.39 Immersion Testing of Welded Tubes 92. Non-Destructive Test and Evaluation of Materials
the specimen travels around the circumference until it
strikes a discontinuity that will cause a reflection. In
areas where no discontinuity is present, only a small
front reflection is shown.
Two Probe Method (Pitch Catch)
Weld defects perpendicular to the plate surface and weld
axis or defects whose plane is parallel to the weld axis
and perpendicular to the plate surface arc cxamined
using two probes. Figure 3.40 shows the typical
location of two such defects detected and evaluated by
a pair of probes 7,73 and T,T,. This type of testing
requires both probes to be coupled ona guiding Fig. 3.40. Pitch Catch M
3 Meth
mechanism for effective working ® Two dnl Probes cr Using
Resonance Method for Testing Thickness
A test component is said to be in resonance when its thickness is an integral multiple of half wavelengths
of the applied frequency. Continuous compression waves of variable frequency are transmitted into th
material to establish a standing wave, creating :
the condition for resonance. Figure 3.41 shows
the standing wave pattern due to resonance.
If fis the fundamental resonant frequency
and Vis the velocity of the wave in the test
material, then
8
tea
Vv
f=5 ode
SIS
| Harmonic He i
ee oe larmonic Ml Harmonic
[The thickness of the component ean be
calle of tne compones
ulated from this relationship. Since resonance can occur at any of the harmonic frequencies and the
difference between any two adj ies gi
ly two adjacent hi 5 Eee!
Pees z cent harmonies gives the fundamental frequency, the following relationship
Fig. 3.41 Standing Wave Pattern due to Resonance
oe ae
Susi - In)
where.) =
fn +) = resonant fr *
aonant frequency at the (1 +1)" harmonic and f, = resonant frequency atthe"
Direct contact as well as immersi
Resonance methods are u:
and detection of gross defect
fonfiescnanes : aor
msion resonance methods are used for thickness determination
ckness gauging, corrosion inspect s osiles
ed ; ae ing comp
ts in thin materials, pectlog. bom teste 3.8 INTERPRETATIONS AND GUIDELINES FoR
ACCEPTANCE/REJECTION
Interpretation implies analyzing the in
amplitude of the echo signals and correlati
component.
3.8.1 Determination of the Size and Shape of Flaws
The appearance of a flaw echo depends upon the size, nature and orientation of the flaw within the
large if its cross-section is larger than that of the sonic beam at the flaw
location. The size of such a flaw i i
more than the actual flaw size, due to beam divergence. Realistic flaw mapping is possible only when the
Orientation of the flaw is normal to the sonic beam.
A small flaw. whose size is smaller than the diameter of the probe
crystal, is determined by comparing
the flaw signal amplitude with the signal amplitude from a known s'
ize target.
ee ee Ultrasonics 101
some ee ee i shape of the flaw can be obtained by observing the shape and height
ho dynamics) on the CRT screen when th i pri
ofthe echo when the probe is manipulated. A steep rise of the ech
11 amplitude, generally, is indicati eee ere
‘en for asmal ly. is indicative of a lamellar flaw: this hay
: : : ; this happens when the flaw surface
ic perpendicular fee ie beam. If the plane of the flaw is not perpendicular, the echo formation is
giow and gradual. If the flaw is of irregular shape, it results in a complex and multi-peaked signal. By
robe manipulation itis possible to align the sound beam in a preferred direction, Echo formation and the
Frange of its shape due to sonic beaming, for linear and irregular flaws, is illustrated in Fig. 3.50.
—<—— 100% reference curve
for 5/64” 9 F.B.H. target
Echo amplitude —>
(b) Depth —>-
Fig.3.50 Echo Dynamics for Different Flaws Fig. 3.51 Flaw Assessment by Distance-Amplitude
Curve Method
Determination of Flaw Size (Flaw Smaller than Probe Diameter)
Distance-amplitude Curve Method
If the acceptance limit is, say, a 5/64" diameter flat bottom hole, then a minimum of three blocks of 5/64”
diameter flat bottom hole with different metal distances are selected. With a specific gain setting, a readable
target echo as well as a back echo are obtained. As the target distance increases, the echo amplitude
decreases and if the peaks of the echoes are joined, a curve results as shown in Fig. 3.51. This curve is
known as the 100% reference curve of the distance-amplitude curve.
If the echo from an unknown flaw crosses this curve,
then the flaw is more than 5/64"; if it is below this curve,
the flaw size is less than 5/64”.
6
DGS Diagram (AVG Diagram) Method 5
4
3
Initial
‘echo
A set of curves developed by Kraut Kramer for flaw
evaluation, connecting the distance, gain and the size of
known disc-like flaws, are quite useful in the comparative
evaluation of discontinuities.
On the CRT screen, both defect and back-wall signals 4
are obtained. An arbitrary reference level on a vertical
scale, say 3, as shown in Fig. 3.52 is selected. The back Depth range —>
echo height is brought to this reference level and the gai pip. 3.57 Flaw Assessment by DGS Method
__Reference line __ | / Materials
102 Now Destructive Test and Evaluation of Mat
cl le comes to the same reference leve}
e flaw echo amplitud ‘ ne re
v creased until the a i value *G", From the C4
value is noted. Now the gain fference in these two gain values gives the gain al oe car
aU ra a fthe flaw ‘a’ is known; by dividing this w i he near 78
° i of depth of z amnel "
ce a Paice of the flaw in terms of the near zone, ra sy" emis
the probe, the tional horizontal avis ofthe dingram as shown inf rar mere
: fi i calet value: .
This is plotted al ne the diagram, corresponding to the calculated vi Mya tt he
anata Row sce", The pratt ofthe ttionalized Maw size °S' an the crys ame
! ea is normal to the beam axis
jth diagram is applicable only when the An is norma oe eam
ite hewiswstmed taboo ype. These diagrams are drawn for materials
cstumed to be of dise type.
and the flaw is assum x y —
Cae coumlenion ch nis depani is don cto eliminate the probe characteristics. Hence, irrespective
his diagram is done to
The normalization of this .:
of the probe, the diagram can be used universally.
rationalized flaw size *S
30)
400.1
a4
4 60
10, Ga
2a
100
or 02 03° 05 1.0 3456 810 20 30 40 60 80
Fig. 3.53 DGS Diagram
Correction for Curvature and Transfer Loss
Where the surface is curv
with flat crystal can be
is small, either curved
the standard flat bottom
is as follows:
Fo nents
‘od and the curvature is regular as in cylindrical objects, the testing of coe is
done provided the convex surface has a radius of 225 mm or more. es
Crystals or flat erystals with curved shoes suitably shaped can be ical approae
hole calibration blocks are adopted for the curved surface, the practical
A flat bottom hole block is selected, whose total hei
The
der test.
; ight is equal to the diameter of the part um
« probe is positioned away from the flat bottom hol
«ig adjusted SUC
le to get the back echo. The gain is adjuste ”
Ultrasonics 103
that this back echo comes to a readable height on the vertical range of the CRT screen; the gain value is
poted. Now the curved partis scanned with this gain, The back echo is found to be less than the earl :
eight or sometimes there is no back wall echo at all, duc to transmission loss at th
. r s 18S le curved entr
surface. The gain Is slowly increased until the back echo from the curved part attains the same h mt
The difference in these two gain values is due to curvature, which is what is required for comipaiaate
the transmission loss. Hence, any curved part can be tested and evaluated after drawing the distance
amplitude curve with flat reference blocks and then increasing the gain value by the compensation gain
value.
Most of the engineering components do not have a
surface finish comparable to that ofa calibration block.
Therefore, after calibration with a standard block, when
the probe is transferred on to the test component, there
will be some energy transmission loss on the
component surface. For this transfer loss, correction
can be applied. As explained earlier, a calibration block
equal to the thickness of the part under test is taken
and multiple echoes are obtained by placing the probe
on the calibration block away from the target hole. Depth range
Curve A is thus obtained. With the same gain setting,
the part under test is scanned and multiple echoes are
obtained. If the peaks are joined, Curve B is obtained
(Fig. 3.54).
Though material, gain and other conditions are similar, there is a transfer loss, resulting in a drop in
echo amplitude, Now the gain is slowly increased such that Curve B overlaps Curve A and the difference
in gain to bring Curve B to Curve A level is noted. After plotting the distance-amplitude curve on the
calibration block, the gain is increased by a value equal to the transfer loss and the component is tested
at this corrected gain value. Similarly material attenuation correction can be applied and different materials
tested with available blocks.
4
Transfer loss,
Echo amplitude —>
Fig. 3.54 Transfer Loss Correction
Guidelines for Acceptance/Rejection
Afier assessing the size of a defect, a final verdict as to acceptance or rejection is required. Guidelines
for the acceptance of critical, stressed engineering components are:
(a) For forgings and other primary members that are finished or semi-finished a single echo amplitude
more than or equal to the one obtained from a 2 mm (5/64") diameter flat bottom hole is not
acceptable ; ;
(b) For any defect giving an amplitude indication greater than a 1.2 mm (3/64") diameter flat bottom:
hole is accepted and the estimated defect is recorded
(c) A stringer type of discontinuity giving a continuous indication with an amplitude greater than that
given by a 1.2 mm diameter flat bottom hole over a length exceeding 12.5 mm is unacceptable.
(d) Multiple discontinuities giving indications greater than tht given by a 1.2 mm diameter lat bottom
ae ae considered acceptable provided the minima separation between them is 25 mm
(c) Over and above clauses (8) "(g), ifa defect indication is found to break into a surface or hole on
the finished part, the defect is unacceptable ~
104 Non-Destructive Test and Evaluation of Materials
ra reas onent should be marked with a pencil and surrounded with Suitable Paint. The
Raised feats ould be marked on the scanning surface of the component as “X". The size Of the
aes ed above ‘X" in mm. The depth of each defect should be marked below +: if
Seatac arn fora 1.2 mm diameter defect ata depth of 19. mm from the scanning surface
mm. A typical marking for a
12
x
19.0
Guidelines for Defect Evaluation in Heavy Engineering Applications
In heavy engineering applications involving huge components, large defects ce toleabl) Comparing
the echo to that of an artificial flat bottom hole is neither useful nor necessary. In such cases, the general
approach to evaluate the defect is to compare the defect echo amplitude to that of the back echo and
express this in percentage. A typical classification of defects for heavy forging is given in Table 3,4,
TABLE 3.4 Guidelines for evaluation of defects in heavy engineering applications
[Component Defect Nature and Sonic Extent of Defect
| Ciessification Indications Permissible
| Isolated Distributed |
5 aaa
| Maximum height of Maximum number of
| Class 1 both flaw echo and Not permitted defects permissible over
back echo is 100% a specific length and the
minimum separation
distance benween wo |
permissible defects are |
as agreed upon between
the parties concerned
Class 1 Maximum height of Maximum height of
Flaw echo: 75% Flaw echo: 100%
Back echo: 25% Back echo: 100%
Chass 11 Maximum height of Maximum height of
Flaw echo: 100% Flaw echo: 75%
Back echo: 50% Back echo: 25%
Class 1: Highly stressed dynamic components (e.g. steam turbines)
Class I: Medium stressed components (e.g, hydraulic Components, die blocks, ete.) ag
Class III: Low stressed general engineering components (e.g. mill rollers and general engineering
Guidelines for Acceptance/Rejection of Welds
Welding defects are assessed by con
is show!
Feference block made of the same material as that of the job. A typical reference block is sho"
Fig. 3.55,
A ndard
"paring the defect signal with the signal obtained from a stmt, where b=
beam path
Length of the block determined by
the angle of the search unit and the
T= Thickness of the block depending on
the job thickness,
D= Depth of the side drilled hole
(normally 1'4")
d= Hole diameter varying from 3/32" to
3/8" depending the job thickness
calibration for Angle Probes
The basic calibration hole is 3/8" diameter for any
thickness greater than 1". As an alternate to the hole,
a‘V" or square notch of a known depth (usually a
percentage of the thickness of the test material) can
be made on the test part in a non-critical area. After
the testing is over, the notch can either be weld
corrected or left as it is, depending on the design requirement.
With either the hole or the notch, the
distance-amplitude curve can be plotted
|
Ultrasonics 495
oe lt
L 7!
ae N\
Basic calibration hole
diameter
Fig. 3.85 Calibration Block for Weld Testing
with the angle beam probe. The search
unit is positioned on the calibration block
so as to beam the side-drilled hole as
indicated in Fig. 3.56.
The search unit is moved such that
the maximum response is obtained at 3/
8th of the beam path position (one beam
T
14T
T] T
1
path corresponds to one skip distance
probe movement) and this echo amplitude
is brought to a readable height by
adjusting the gain. The peak of the echo is marked
on the CRT screen. Similarly, the probe is placed
at 5/8th and 7/8th beam path positions to get the
maximum response and the peaks are marked.
“The marked points are joined to form the primary
reference curve. This curve is known as the
100% primary reference line and the line joining
the 50% amplitude points of the echoes is known
was the 50% reference line. After this, the gain
setting should not be disturbed. For practical
purposes, as agreed between the contracting
viges, any bole diameter or notch size is defined
Pithe limit of acceptance, Any defect giving a
aS sponse more than 50% level is recorded and
Fig.
3.56 Angle Beaming of Side-Drilled Hole
asin
s/eth 7/81
Fig. 3.57 DAC Curve for Angle Beaming —~
any indication more than 100% can be consi unacceptable. If the surface roughness of the cal;
idered
b it from tha fers est part, transfer loss correction should be applied as disc
lock is different from ation,
106 Non-Destructive Test and Evaluation of Materials
earlier.
Se i i ve can be dravn using the side-
