Ultrasonic testing uses piezoelectric transducers to generate and detect ultrasound for non-destructive testing of materials. The transducer converts electrical energy into mechanical vibrations to transmit ultrasound, and vice versa to receive reflected ultrasound. Different frequencies are used depending on the material, with higher frequencies providing better resolution but poorer penetration. Ultrasound travels through materials as compression or shear waves at velocities dependent on the material. Reflections from discontinuities are detected and analyzed to evaluate materials for defects.

1. NON –DESTRUCTIVE TESTING(RME-080)
BASICS OF ULTRASONIC TESTING
Prepared By : Rohit Sahu
Assistant Professor (ME)
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DEPARTMENT OF MECHANICAL ENGINEERING
Disclaimer: The materials provided in this presentation and any comments or information
provided by the presenter are for educational purposes only. Nothing conveyed or provided
should be considered legal, accounting or tax advice.

2. CONTENT
 Introduction
 Nature of Sound
 Generation of Ultrasound

3. INTRODUCTION
.
 Ultrasonic techniques are very widely used for the detection
of internal defects in materials, but they can also be used for
the detection of small surface cracks. Ultrasonic are used for
the quality control inspection of part processed material, such
as roiled slabs, as well as for the inspection of finished
components. The techniques are also in regular use for the in-
service testing of parts and assemblies.
 Sound is propagated through solid media in several ways and
the nature of sound will be considered first.
*Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD

4. NATURE OF SOUND
 Sound waves are elastic waves which can be transmitted through both fluid
and solid media. The audible range of frequency is from about 20 Hz to
about 20 kHz but it is possible to produce elastic waves of the same nature
as sound at frequencies up to 500 MHz.
 Elastic waves with frequencies higher than the audio range are described
as ultrasonic. The waves used for the non-destructive inspection of
materials are usually within the frequency range 0.5 MHz to 20 MHz.
 In fluids, sound waves are of the longitudinal compression type in which
particle displacement is in the direction of wave propagation; but in solids,
they are shear waves, with particle displacement normal to the direction of
wave travel, and elastic surface waves can also occur. These latter are
termed Rayleigh waves.
Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD
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5. WAVE VELOCITY
Table 1 Velocities of longitudinal sound waves
Medium Velocity Ve (m/s)
Air* 330
Water 1430
Oil 1740
Aluminum 6190
Copper 4600
Magnesium 5770
Steel 5810
Perspex 2730
Polyethylene 2340
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*At atmospheric pressure and 15°C.
The velocity of shear waves, Vs , within a solid is roughly half the longitudinal wave
velocity
The velocity of Rayleigh waves, Vr, in a solid is about 90 per cent of that of shear
waves
*Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD
Velocity of Longitudinal Waves in different medium given in table 1

6. WAVE LENGTHS
 The wavelength, λ, is related to the frequency and wave velocity as;
V = λf
where f is the frequency. Table 2 gives the wavelengths of sound in
various materials at several frequencies.
.
It will be seen from Table 2 that an ultrasonic beam with a frequency of
10 MHz will be capable of detecting defects in steel of sizes greater
than 0.58 mm, but only defects larger than 4.65 mm would be observed
if a frequency of 1.25 MHz were used.
*Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD
6

7. WAVE LENGTH OF SOUND
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*Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD
Table 2 Wavelengths of sound (compression waves) in some materials
Material ;>.. (mm) for frequency (MHz) of
1.25 2.5 5.0 10.0
Air 0.26 0.13 0.066 0.033
Water 1.14 0.57 0.286 0.143
Oil 1.39 0.70 0.35 0.175
Aluminium 4.95 2.48 1.24 0.62
Copper 3.68 1.84 0.92 0.46
Magnesium 4.62 2.31 1.16 0.58
Steel 4.65 2.32 1.16 0.58
P11rspex 2.18 1.09 0.55
Polyethylene 1.87 0.94 0.47

8. NATURE OF SOUND CONT ..…
 The frequency of sound chosen to inspect a component
is very important. High-frequency sound, for example 5
MHz for contact testing applications, would provide good
sensitivity, which is a term referring to an ability to
detect small material imperfections, called
discontinuities.
 This same sound frequency would also have good
resolution, which is a term used to describe the ability of
a testing system to display closely-spaced
discontinuities separately on the instrument screen.
*Source Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT Common Methods,
Professor Robert A Smith published by the British Institute of Non-Destructive Testing
8

9. NATURE OF SOUND CONT.
 The sound at 5 MHz cannot penetrate through the
coarse grained material due to a scattering effect called
attenuation, or loss of ultrasonic energy.
 To test this coarse-grained material, low-frequency
sound such as 1 MHz would be selected. Sound at 1
MHz would have good penetration but poor sensitivity
and resolution.
 As mentioned above, the objective of many NDT
applications is to look for the presence of
discontinuities.
.
*Source Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT Common Methods,
Professor Robert A Smith published by the British Institute of Non-Destructive Testing
9

10. NATURE OF SOUND CONT ....
 A discontinuity can only be called a defect it if exceeds
specific acceptance standards. If we have two cracks of
the same length but in different components, one may
be acceptable but the other may not, therefore only one
crack can be called a defect.
 We now must look in more depth at the nature of
ultrasound, which is at the heart of the method
*Source Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT Common Methods,
Professor Robert A Smith published by the British Institute of Non-Destructive Testing
10

11. GENERATION OF ULTRASOUND
The transducer is capable
of both transmitting and
receiving sound energy.
Ultrasound is generated with a transducer.
A piezoelectric element in the
transducer converts electrical
energy into mechanical
vibrations (sound), and vice
versa.
Source: Web

12. GENERATION OF ULTRASOUND
 Sound generation – the piezoelectric effect
 The piezoelectric effect is utilized, which is a property of certain crystals to convert
electrical energy into mechanical energy and vice versa – as you seen the fig. below.
Fig. Probes are often called transducers because they convert energy from one form to another, ie electrical energy to sound energy and sound
energy to electrical energy.
 Certain crystalline materials show the piezo-electric effect, namely, the crystal will
dilate or strain if a voltage is applied across the crystal faces. Conversely, an
electrical field will be created in such a crystal if it is subjected to a mechanical
strain, and the voltage produced will be proportional to the amount of strain.
*Source Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT Common Methods, Professor Robert A Smith published by the British
Institute of Non-Destructive Testing
*Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD
12

13. GENERATION OF ULTRASOUND CONT.
 Piezo-electric materials form the basis of electro-mechanical transducers.
These may be naturally occurring, artificially manufactured or grown in
solution. Naturally occurring crystals, such as quartz, are rarely used these
days because man-made ceramic materials tend to give much better
properties in terms of sound generation, resolution, etc. A typical ceramic
material is barium titanate, lead metaniobate and lead zirconate, are used
widely.
 When quartz is used the disc is cut in a particular direction from a
natural crystal but the transducer discs made from ceramic materials
such as barium titanate are composed of many small crystals fused
together, the crystals being permanently polarised to vibrate in one plane
only.
.
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*Source Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT Common Methods, Professor
Robert A Smith published by the British Institute of Non-Destructive Testing.
*Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD.

14. GENERATION OF ULTRASOUND CONT.
 An ultrasonic wave incident on a crystal will cause it to vibrate,
producing an alternating current across the crystal faces. In some
ultrasonic testing techniques two transducers are used - one to
transmit the beam and the other acting as the receiver - but in very
many cases only one transducer is necessary. This acts as both
transmitter and receiver.
 Ultrasound is transmitted as a series of pulses of extremely short
duration and during the time interval between transmissions the
crystal can detect reflected signals.
*Source Non-Destructive Testing (NDT) – Guidance Document: An Introduction to NDT Common Methods,
Professor Robert A Smith published by the British Institute of Non-Destructive Testing.
*Source- Non-Destructive Testing by Barry Hull & Vernon John Published by MACMILLAN EDUCATION LTD.
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