Chapter 7

Ultrasonic Testing
Ultrasonic testing is a versatile NDT method which is applicable to most
or non-metallic. By this method, surface and materials, metallic
internal discontinuities such as laps, seams.
voids. cracks. blow holes, inclusions and lack of bond can be
side. Ultrasonic testing utilizes high accurately evaluated from one
frequency acoustic waves generated by piezoelectric
transducers. Frequencies from l to 10 Mega Hertz (MHz) are typically used, although lower
or higher ranges are sometimes required for certain
applications. The resultant acoustic
wavelengths in the test material (depend on the ultrasonic wave velocity) are of the order of
one to ten millimetres. A highly directional sound beam is
transmitted to the test piece
through a suitable couplant. usually grease or oil like material. While various types of
instrumentation and display modes are feasible, the most widely employed is the pulse-echo
technique. with A-scan mode.
Since acoustic waves propagate effectively through most structural materials, but are
dissipated or reflected by inhomogeneities or discontinuities, measurement of the transmitted
and reflected energies may be related to the integrity, which is a function of the material
inhomogeneity and defect parameters. Ultrasonic test method provides quantitative information
regarding thickness of the component., depth of an indicated discontinuity, size of the
discontinuity etc.
7.1 BASIC PROPERTIES OF SOUND BEAM
7.1.J Sound Waves
All sound waves, whether audible or ultrasonic, are mechanical vibrations
involving movement
01 the medium in which they are travelling. Because of the relative
movement of the
Particles in the medium, the physical properties of the particles in the medium have to be
taken into consideration. A SOund wave may be transmitted through any material which
Denaves in an elastic manner. Ultrasonic waves are classified on the bas0S of the mode of
VIbration of the particles of the medium with respect to the direction of propagation of the
Waves, namely longitudinal, transverse, and surface waves.
(a) Longitudinal waves
e most common form of sound transmission is by longitudinal wave. In this form of wave
 )4 Practical Non-destructive Testing

mode, the oscillations occur in the longitudinal direction. Since compressional and dilational
Torces are active in it, it is also called compressional or dilational or pressure wave. In this
(ype of ultrasonic wave, alternate compression and rarefaction zones are produced by the
vibration of particles parallelto the direction of the propagation of the wave. Figure 7.1(a)
represents schematically a longitudinal ultrasonic wave.

DircctuOn of propagation
(a) Longitudinal wave

(h) Transyerse wave

Wave length

A1

Steel
Direction of propagation
ie Surface wave on stecl on the right, oscillation cllipse of a particle and sense of rotation
Fie, 7.1 Schematic representation of various modes of wave propagation

Because of its easygeneration and reception, this type of ultrasonic waves is most widely
used in ultrasonic testing. Almost all of the ultrasonic energy used for the testing of materials
onginates in this mode and then is converted to the other modes for special test applications.
This type of wave can propagate in solids, liquids and gases.
lh) Transverse or shear waves
In this Ivpe of wave. the direction of particle displacement is at right angles or transverse to
the direction of propagation. It is schematically represented in Fig. 7.1(b). For such a wave
to travel through a maternal. it is necessary that cach particle of the material is strongly
bound to its neighbours so that as one particle moves, it pulls its neighbour with it. thus
100 Practical Non-destructive Testing

be seen clearly. cannot be counteracted by stepping up the initial sound ener8y. This is
because. consequent to this, the 'grass' effect also increases simultancously. The only
remedy is to use lower frequencies, which sets alimit on the detectability of smallflaws.
Atenuation taking place in the material can be calculated using the following relationship:
A = Anea (7.13)
where Ag is the incident amplitude, A the value of amplitude after travelling a distance 'r"
and a the attenuation coefficient.
The cquation can be written as

rA
(7.14)
where the value of a is measured in nepers per mm.

7.3 INSPECTION METHODS

Ulrasonic testing is performed using one of the following procedures: (1) normal beam
pulse-echo, (2) normal beam through-transmission: (3) angle beam pulse-echo: (4) angle
beam through-transmission. Pulse-echo technique is most widely used for inspection of
components.

7.3.1 Normal Incident Pulse-Echo Inspection

The ultrasonic energy is coupled to the component being
inspected through a couplant
(usually oil or grease or glycerine) that transmits the ultrasound between the face of the
transducer and the surface of the component. When ultrasonic energy travels
through a test
sample and strikes a discontinuity, part of the energy will be reflected back and the
part propagates in the material in the forward direction. Also a refracted beam remaining
from the
discontinuity is available for interpretation. Ultrasonic energy that is reflected and
to the probe is the source of the defect indication shown on the returned
instrument screen (Fig. 7.3).
The sound energy that travelled completely through the test picce will be
giving the large back wall echo indication. Once excited in the material, reflected
at the end.
the ultrasonic pulse
will continue to reflect from the parallel surfaces, creating a multiple echo
display when the
Main bang First Second Transducer
(initial echo) back wall back wall
echo ccho Test Ulrasonic
picce beam

21
time-t

Fig. 7.3 Pulse-echo inspection of bar of length L.

 Ulrasonic Testing 0!

large. As there is coninuous attenuaton of thc sound energv in the

the amplitude of the echoes deereases
7.32 Normal incident
Through-Transmission
Testing
Mant a ime. pulse -ccho technique may not provide required test information. This may
r hen a law 0r otheT allomaly does not provide a suitable reflection surface or where
aentation or locaton of the MaW which is not favourable for detecion using single
Also, highly atlenuating matenals are oflen tested with through-transmission techniquc.
Hgure 4 shows the probe artangement for though-transm1ssion techniquc. The technique
ten uscd in large castings and highly attenuating materials.

Transducer Transducer
transmmitter recever

Fig. 7.4 Through-transmission inspection of bar of length L

7.3.3 Angle Beam Pulse-Echo Testing

Angle beam transducers provide access to areas that are inaccessible to normal beam probes.
Figure 7.5 shows the angle beam test arrangement. Angle beam inspection is accomplished
with shear wave probes. Angle beam test applications are explained in detail in section 7.5.
Shear wave Echo from
To flaw detector
transducer defect-A

Shear
wave

Fig. 7.5 Angle beam test arrangement

734
Criteria for Probe Selection
Dehe factors that affect the probe selection are flaw senstivity, beanm divergence.
penctration, resolution etc.
73.5 Flaw Sensitivity
The
frequcney of the probe Is Onc of the most important factors to be considered when the
102 Practical Non-destructive Testing

minimum deteetable flaw size is of concern. Wihile

there are anumber of other
that affect the flaw sensitivity, the
detectability flaw is a direct function of the
of a
which varies inversely with the frequency. Most favourable detection paramclcts
the flaw somew hat larger than the wavelength. As the
flaw, the likelihood of detection decreases
conditions e v ,
wavelengtn becomes larger than
considerably. In general, it has been found t
wavelengl,
using a particular frequency probe. defect size of the order of half of its wavelenoth
detected.

1.3.6 Beam Divergence

Divergence is a function of diameter and frequency of the
in the material. Hence selection of the proper probe is probe,forand sound wave veloct
inspection. essential aSsuring a satisfactor
7.3.7 Penetration and Resolution
Attenuation plays an important role in ultrasonic testing in a variety of
it account for the loss of signal height for equal reflectors at increasingways. Not only does
probe. it is also a useful diagnostic tool for several distances from the
affecting sources of attenuation are beam spread and types of inspection. Two of the most
frequency probes will minimise the attenuation and scattering. In general, the use of lower
that the longer wavelength pulse is less maximise the penetration due to the fact
affected by scattering at grain
frequency probes, then. are said to have greater ability for boundaries etc. Lower
What is an advantage on the one hand is penetration.
often a disadvantage on the other. Lower
frequency probes have lower resolution, i.e. decreased
reflectors. Therefore higher frequency probes are ability to resolve closely spaced
chosen for better resolution.
7.4 TECHNIQUES FOR NORMAL BEAM INSPECTION
It must be
emphasized that the basic of an ultrasonic flaw
display information. Interpretation of therole detector is to
data must come from qualified inspectors. obtain and
interpretation offers great potential provided correct data have been furnished to the Automatic
In order to demonstrate the
basics of ultrasonic inspection, discussion is instrument.
of a few examples. made with the help

7.4.1 Fatigue Cracks

FLgure 7.6 shows the normal beam testing of
assumed to be planar in shape, with boundaries wellbars tor latigue cracks. Fatigue cracks are
echoes on the screen. With length Land time for adetined,
pulse
providing sharp distinct reflected
the defect present at a distance a, would retlecting at the bar end echoe of
appear at time (a/L)I on the screen M.ltinla
echoes of the flaw ccho would also appear. If the
probe IS moved
defect echo would disappear, while the back echo remains. For a away
Irom
circular the location the
flaw, moving the probe in a circular fashion around the outer extremity of shaft withofa. pla
the end
should generate a pattern of rising and falling flaw echo ihat will
indicate the approvimate
Size and shape of the flaw. Where access to a suitable inspection location is availahla t
useful to confirm the prescnce and shape of a law by approCnng " Irom a
direction.
different
 Ulrasonic Testing |03

Flaw echo Back ccho

Multiple
cchos
Crack

Fig. 7.6 Typical pulse-echo response from a planar reflector (fatigue crack)

7.4.2 Inclusions, Slag, Porosity, and Large Grain Structure

Many manufactured products contain internal defects that give reponse quite differently
than that seen for a fatigue crack. Such defects are inclusions, slag and porosity to name a
few. Figure. 7.7 shows the screen appearance and the general reflection behaviour of such
defects. In these situations, where there is no flat planar reflector, the scatter at the defect
may be sufficient to destroy the back echo signal. In this event, the loss of the back echo may
be more informative.

First defect ccho

Second defect echo

b
Slag

() (b)
(a)
Fig. 7.7 (a) Screen appearance from slag, inclusions or porosity (b) reflection
pattern for the above type of defects

I4.3 Thickness Measurement: Corrosion Detection

uasonic transit time measurements are conveniently used for determination of thickness
pping. tubing, and pressure vessels. Thickness measurement, of course, is crucial in the
Pevention of failures caUsed by corrosion. Since the longitudinal wave spced is eSsentially
tant tor a given engineering materials, changes in material thickness may be determined
baccurately using the position of the back echo obtained from conventional normal
beam inspection.
CXhess measurement is based on the transit time comparison, i.c., reference transit