CONTENTS

CHAPTER PAGE NO

1. Propagation of Ultrasonic Waves in Materials 1

2. Behavior of Ultrasound at material Interfaces 6

3. Ultrasonic Beam 12

4. Ultrasonic Probes 16

5. Ultrasonic Equipment 26

6. Instruments Linearity 32

7. Calibration and Standard Reference Blocks 34

8. Calibration 38

9. Selection of Ultrasonic Probes 49

10. Type of Discontinuities 52

11. Application 60

12. Defect Sizing 69

13. Irrelevant Indications 71

14. Techniques of Testing 73

15. Codes and Reporting 77

1
 CHAPTER - 1
PROPAGATION OF ULTRASONIC WAVES IN MATERIALS

Energy motion

When we touch a vibrating object, we can feel the vibrating. Vibration is a

back and forth movement of a mass from its rest position. The distance of a
mass moves move form its test position is called displacement. It is will
known that when an object vibrates, sound is emitted and, conversely when a
sound is produced by an object that the object will be in a state of vibration.
The vibration of the object causes each particle of the carrying medium to
vibrate and transfers part of the energy to its immediate neighbor. This sets
up a wave form. The most important point to be remember is that for
propagation of sound a medium, like in a vacuum, there will be no particles
and so no vibration and hence no wave form.

Velocity of Sound

The velocity of sound wave is the distance the wave front travels through the
medium in one second. The velocity is characteristics of the medium and is
dependent on the elastic properties and density of the medium. Velocity of
the ultrasound is expressed in meter per second.

Frequency

The number of cycles per second completed by each particle of a medium is

called as frequency. It is expressed in Hertz (cycles per second).

1 Mega Hertz (MHz) = 1 X 106 cps

Wave Length

This is the distance the wave front moves

through the medium during one cycle.
The wave length is a function of
frequency of the wave and sound
velocity of the medium in which the
wave is propagated. This relationship is
express as:

λ = v/f or v = f. λ
λ = wave length
f = frequency of sound in the medium

2
V = velocity of the sound in the medium, as a general rule we can say that
under normal testing conditions the minimum defect size capable of being
detected is λ/2.

Ultrasonic

Our ears are not sensitive to vibration at any frequency, but only respond to
vibration with in a certain frequency range. This range is normally taken to be
from 16 Hz to 20 kHz. Vibration with in this range are normally said to be in
the Audible Range (Sonic). Frequencies below 16 Hz are in the Subsonic
Range and frequencies above 20 kHz are termed Ultrasonic.

Modes of Propagation

Ultrasonic energy propagates through materials in several different “modes”.

The mode of the sound wave depends upon the manner in which the particles
within the carrying medium move relative to the direction of the wave
propagation.

There are four types of ultrasonic wave modes which can be propagated in
materials.

Longitudinal (Compressional)
Shear (Transverse)
Surface (Rayleigh)
Plate (Lamb)

Longitudinal

The particle motion in longitudinal waves is parallel to the direction of

propagation and rarefactions. This type of wave can be propagated in a
medium having volume,

3
Elasticity i.e. liquid, solid and gases and their velocity is highest.

Shear

The particle motion in shear waves is at right angle to the direction of

propagation.

Shear waves will not travel through liquid and gases because forces of
attraction between molecules are too small. This type of wave can only be
propagated in medium having a shear modulus i.e. solids only and their
velocity is approx ½ that of Longitudinal Velocity.

Surface

The particle motion is elliptical with major axis at right angle to the direction
of propagation. Surface waves travel along the surface and only penetrate the
medium to a depth of approximately one wave length. At this point the

4
energy is 1/25th of wave energy of the surface. Surface waves can follow
contoured surfaces wave can be easily dampened by foreign particle in their
path. They travel in solid only and velocity is app. 0.9 times that of transverse
wave velocity in the same material.

Plate

Plate waves produced in thin sheet material when beaming is done by an

angle probe at particular angle where the thickness of the sheet is app. one
wave length.

In this case the plate acts as a wave-guide through which symmetrical and
asymmetrical modes can simultaneously pass. They have varying velocity
which depends on thickness of material, angle of incidence and operating
frequency. They are used for testing lamination of thin plate and for
depending in cladded materials.

Use of the Wave Modes

The wave systems mostly used in the examinations of Welds, forgings and
castings are compression and shear. Compression waves are produced by
angle or shear wave probes.

Surface waves can only be used for the inspection of components with a very
good surface finish and should be used with care. It is sometimes possible for
shear wave probes with high refracted angles such as 70º or 80º to produce
surface waves. These can produce strong reflections which may give rise to
confusion if the operator is nor aware of their presence. Plate waves only be
used on very thin materials and are seldom applied to material.

5
 CHAPTER - 2
BEHAVIOUR OF ULTRASOUND AT MATERIAL
INTERFACES

Reaction of sound waves at a Boundary/ interface

When ultrasonic waves of either a compression or shear, strikes boundary or

interface of two dissimilar materials at 90º, then some of the wave energy will
be reflected and some will be transmitted across the boundary. The amount of
reflected and transmitted energy depends on the difference in acoustic
properties. I.e. acoustic impedance, between the adjacent materials. Acoustic
impedance is the product of the material density and the acoustic wave
velocity within the material.

Z=PXV
Where, Z is the acoustic impedance
P is the density, and
V is the applicable wave’s velocity

The ratio of the reflected and transmitted acoustic energy to that which
incident upon the interface is given by the following formula:

IR (Z2 – Z1)2
---- = R = -------------
IO (Z2 + Z1)2

IT 4Z2 X Z1
---- = T = -------------
IO (Z2 + Z1)2

Where,
IO = Incident wave energy
IR = Reflected wave energy
IT = Transmitted wave energy
Z1 = Acoustic impedance of Medium I
Z2 = Acoustic impedance of Medium II
R = Reflection Factor

As on example, let us consider sound waves coming from steel and striking
water.

6
Here we have

Z for steel = 45 X 106 Kg / m2 / sec

Z for water = 1.5 X 106 Kg / m2 / sec

Now
(Z2 – Z1)2 (1.5 – 45)2
R = ------------- = ------------- = 0.88 = 88%
(Z2 + Z1)2 (1.5 + 45)2

4Z2 X Z1 4 X 1.5 X 45
T = ------------- = --------------- = 0.12 = 12%
(Z2 + Z1)2 (1.5 + 45)2

Density Z 103 kg
Material V trans m/s V long m/s
kg/m3 /m2 /s
Aluminum 2700 3130 6320 17064

Aluminum Oxide 3600 5500 9000 32400

Bismuth 9800 1100 2180 21364
Brass 8100 2120 4430 35883
Cadmium 8600 1500 2780 23908
Cast Iron 6900 2200 5300 24150
Concrete 2000 0 4600 9200
Copper 8900 2260 4700 41830
Glass 3600 2560 4260 15336
Glycerin 1300 0 1920 2496
Gold 19300 1200 3240 62532
Grey Castings 7200 2650 4600 33120
Lead 11400 700 2160 24624
Magnesium 1700 3050 5770 9809
Motor oil 870 0 1740 1514
Nickel 8800 2960 5630 49540
Perspex (acrylic) 1180 1430 2730 3221
Polyamide 1100 1080 2620 2882
(Nylon)
Polyethylene 940 925 2340 2200
Polystyrol 1060 1150 2380 2523
Polyvinylchloride 1400 1060 2395 3353
(PVC Hard)
Porcellaine 2400 3500 5600 13440

7
Quartz 2650 0 5760 15264
Quartz Glass 2600 3515 5570 14482
Silver 10500 1590 3600 37800
Steel (Low Alloy) 7850 3250 5940 46629
Steel (Calibration 7850 3250 5920 46472
Block)
Tin 7300 1670 3320 24236
Titanium 4540 3180 6230 28284
Tungsten 19100 2620 5460 104286
Uranium 18700 0 3200 59840
Water (20º C) 1000 0 1480 1480
Zinc 7100 2410 4170 29607

In a steel plan parallel block of 100 mm thickness, when an ultrasonic wave is

propagated, it travels top the back of the material and returns. What we do
with UFD is to measure this short time at the specific speed of 5920 m/s i.e.
app. 6000 m/s.

Distance = 200mm
Time = ------------------ = ------------- = 33.3μ secs.
Speed (Velocity) 6000 meter

Now if there is a reflector which is at half way e.g. 50 mm, then the time taken
will be the half of 33.3 μ sec. It the reflections at these two different times, are
displayed on a linear scale from 0-10 divisions, then each division would be
3.33 μ sec (equivalent to a distance of 20mm or a thickness of 10mm) and
reflection of 16.66μ sec will be displayed at ‘5’ div. This is how our instrument
read distance of a reflector.

Reflection, Refraction and Mode Conversion

When ultrasonic waves strikes a boundary or interface at an angle other than

normal, the reactions of Reflection, Refraction and mode conversion will take
place. Refraction can be defined as the bending of or change in direction of
ultrasound where as mode conversion is the changing from one form of
particle vibration to another.

8
As an example, when longitudinal beam strikes a boundary of medium I and
medium II of two dissimilar materials an angle other than normal then for
each longitudinal wave there will be reflected and refracted components of
both longitudinal and share wave as shown in figure.

Snell’s Law expresses this change of angle and velocity thus.

Sini SinrL SinrS SinRL SinRS

------ = ------- = ------- = -------- = --------
V1L V1L V1S V2L V2S

Where

V1L = Velocity of longitudinal wave in medium I

V1S = Velocity of shear wave in medium I
V2L = Velocity of longitudinal wave in medium II
V2S = Velocity of shear wave in medium II
i = Incident Longitudinal wave angle
rs = Reflected shear wave angle
rL = Reflected longitudinal wave angle
Rs = Refracted shear wave angle
RL = Refracted longitudinal wave angle

9
 First Critical Angle

As the angle of incidence increases

the angle of refraction also increases.
The angle of incidence at which the
refraction angle of longitudinal wave
reaches 90º and travels parallel to the
interface is called first critical angle.
As an example when the incident
angle in Perspex reaches 27.6º, in steel
reaches 90º. At this angle shear wave
exist in the steel.
Second Critical Angle
If the angle of incidence is further
increased till the angle if refraction
for the shear wave mode is 90º, then
the angle of incidence, for which it
happens, is called second circle angle.
At this angle all the shear wave
energy is converted in to surface
waves. It the angle of incidence is
more than second critical angle, then
total reflection for both longitudinal
and shear wave modes takes place.

For example, when incident angle in Perspex reaches 57.2º, the refracted shear
wave in steel reaches 90º.

Thus between the first and second critical angle only shear waves exit in steel.

Example:

Application of Snell’s law to calculate the value of 1st and 2nd critical angle
when longitudinal wave travels from Perspex to steel.

Sinα1 Velocity of longitudinal wave in Perspex

------- = -------------------------------------------------------
Sinβ1 Velocity of longitudinal wave in steel

α1 = 1st Critical Angle

Given: Velocity of longitudinal wave in Perspex = 2730 meter/sec

Velocity of longitudinal wave in steel = 5900 meter/sec

10
 Β = 90º, Sinβ = sin 90º = 1

Sinβ x 2730 1 x 2730

Sinα1 = --------------- = ----------- = 0.4627
5900 5900

α1 = 27.5º

To calculate Second critical angle

Let, α2 = 2nd Critical Angle

Sinα2 Velocity of longitudinal wave in Perspex

--------- = -------------------------------------------------------
Sinβ2 Velocity of Shear wave in steel = 3245 meter/sec

Given: β2 = 90º, Sinβ2 = 1

Velocity of shear wave in steel = 3245 meter/sec

1 x 2730
Sinα2 = ------------
3245

α2 = 57.2º

Calculate the incident angle in Perspex to produce 70º shear wave in steel:

Given: Velocity of longitudinal wave in Perspex = 2730 meter/sec

Velocity of shear wave in steel = 3245 meter/sec
β = 70º, Sinβ = .9396

Sinα Velocity of longitudinal wave in Perspex

--------- = -------------------------------------------------------
Sinβ Velocity of Shear wave in steel

Sinβ x Velocity of longitudinal wave in Perspex

Sinα = ----------------------------------------------------------------
Velocity of shear wave in steel

0.9396 x 2730
Sinα = ------------------ = 0.7905
3245
Incinental angle α = 52º

11
 CHAPTER - 3
ULTRASONIC BEAM

Huygens Principe:
The pulse of sound emitted by a
crystal, travels into the material and
forms a path similar to the beam of
light from a flashlight. As it travels
it spreads and diminishes, and will
be reflected at an interface.
According to Huygens Principle
each point along the face of a crystal
can be considered a finite source,
radiating sound in a spherical
manner (first figure above).

With each expansion and contraction of the crystal, waves of positive and
negative pressure, respectively, are radiated from each point source. As the
waves radiate away from the crystal they will overlap and create areas of
interference. When two positive (or negative) pressure areas overlap, it is
referred to as constructive interference. When a positive and negative overlap
they will cancel each other out, and are referred to as destructive interference
(second figure below).

Near Field

Near Field is the region

directly adjacent to the
transducer and characterized
as a collection of high and
low pressure regions caused
by constructive and
destructive interference. The
near field is parallel and
contains areas of maximum
and minimum effects. This
can be demonstrated if a
series of small reflectors of
the same area, but varying
depths are scanned and a
graph of signal amplitude
against distance is plotted.
The end of the near field is
the point where the
amplitude is greatest.

12
Near zone calculation:

D2 D2 X frequency
Near zone = ------- or ---------------------
4Xλ 4XV

D = Transducer Diameter
λ = Wavelength
V = Velocity of test material

This formula covers the total distance from the transducer whether the beam
is developing in a plastic stand off wedge or not. Therefore, when calculating
the near zone of an angle probe, allowance must be made for the development
time in Perspex.

Far Field

Beyond the near field is the “far field”. Here the beam diverges, and is
referred to as beam spread. Through the far field the sound pressure disperses
according to the inverse square law. (i.e. the intensity is inversely
proportional to the square of the distance), and travels in straight lines
(rectilinear propagation).

Beam Axis

13
Beam Axis may be defined as the locus of points of maximum sound intensity
in the far field, in a beam of ultrasonic waves, and its geometrical
prolongation in to the near field.

Kλ Kλ
Half beam angle Sinα = --------- = ---------
D D
Where

D = Crystal diameter
f = Crystal frequency
V = velocity of sound in material
Λ = wave length
K = constant defining beam edge
1.2 for theoretical null
1.08 for 20dB down point (10% of peak)
0.88 for 10dB down point (32% or peak)
0.7 for 6 dB down point (50% of peak)

Sound attenuation in the Far Field

The amplitude of sound decreases in the far field. This allows a predictable
amplitude response from a discontinuity as a function of discontinuity depth
and reflective surface. Sound attenuation in the Far Field depends on several
factors.

Absorption: It is the loss due to mechanical interaction of the molecules, thus

causing frication and finally being dissipated in the form of heat.

Scattering: Sound is scattered by pores, boundaries or nonmetallic

precipitates or defects in the material.

14
Beam spread: As the pulse gets wider, due to beam spread, the sound
intensity at the beam axis is reduced.

The amount of sound attenuation within a material is governed by the

density, elasticity, and grain size and grain structure of the material. These
factors will be affected by alloying, heat treatment, working etc.

15
 CHAPTER - 4
ULTRASONIC PROBES

Generation of Ultrasounds
A search unit or Ultrasonic Probe contains one or more transducers
or crystals which converts electrical energy to Ultrasonic energy and
ultrasonic energy back to electrical energy. This property is termed
‘Piezoelectric’ effect.

Conversion of Ultrasonic energy (mechanical energy) to electrical energy is

termed as the direct Piezo electric effect and is used in ultrasonic receivers.
But when alternating bolt age is applied to the crystal, alternate expansion
and contraction (Vibration) of the crystal will occur which will produce a
Pulse of ultrasonic energy. This is the reverse Piezo-electric effect. It is used in
ultrasonic transmitter. A short voltage pulse of less than 10 micro sec or
voltage of 100 – 1000 v can oscillate the crystal to its natural frequency. This
depends on the thickness and material of the crystal.

Piezo – electric Materials for Probes

The three most common Piezo-electric materials used in ultrasonic probes are
Quartz, Lithium Sulphate and Polarized ceramics. The most common
ceramics are Barium Titan ate, Lead Metaniobate and lead Zircon ate Titan ate
(PZT).

Quartz

A natural and will known transducers are the quartz crystals. They may be X
– cut or Y – cut. The X – cut crystal produce a longitudinal mode and the Y –
cut produce a transverse mode.

Sr.no Advantage Sr.n Disadvantage

. o.
1 Good chemical, electrical and 1 Least efficient generator of

16
 thermal stability. acoustic energy
2 Very hard and wear resistant. 2 Suffers from mode conversion
interference
3 Insoluble in water 3 Requires high voltage to drive
it at low frequencies.
4 Resists ageing.

Lithium Sulphate

As an alternative to using naturally occurring crystals it is possible to “grow”

them synthetically. Lithium sulphate is one such crystal used in NDT. When
the crystal has grown it has inherent piezo-electric characteristics.

Sr.no Advantage Sr.no Disadvantage

. .
1 Most efficient receiver of 1 They are very fragile and
ultrasonic energy and are have low mechanical
intermediate as a generator of strength.
ultrasonic energy
2 Good resolution easily 2 Soluble in water.
damped
3 Low electrical impedance 3 Limited to use up to 165º F.
(73.8º C)
4 They do not age and are
affected very little by mode
conversion interference.
5 Operate well on low voltage

Polarized Ceramics

Certain ceramic materials which are ferroelectric elements (mechanical

deformation when external electrical voltage is applied) do not exhibit Piezo-
electric effect in their natural state. These ceramics consists of randomly
oriented domains which, when aligned, impart the piezo-electric effect. This
can be accomplished by heating this element to its curie temperature and
applying an electric field. The electric field is maintained across the element
while the temperature is lowered. The treated ferroelectric element now has
piezo electric capability.

Curie – Temperature: Curie temperature is the temperature at which a

piezoelectric crystal will lose its piezoelectric properties. It is essential that the
Curie temperature is not exceeded in use (see table below).

17
 Potassium
Lithium Barium
Quartz Zirconium
Sulphate Titanate
Titanate
Curie
550 75 115 340
temperature

Sl.no. Advantage Sl.no. Disadvantage

Most efficient generator of They have a tendency to
1 1
ultrasonic energy ‘age harden’
They operate well on low Suffer from some mode
2 2
voltage. conversion interference.
They are practically They are limited by
3 unaffected by moister and 3 relatively low mechanical
high temperatures. strength.

Material As transmitter As receiver As trans receiver

Quartz Very poor Fair Poor
Lithium Sulphate Poor Very good Good
Barium Titanate Poor Poor Fair
Lead Zirconate
Poor Poor Good
Titanate (PZT)

Probe classification

There are four main factors by which probes are classified:

™ Frequency
™ Refracted angle
™ Single or Twin Crystal
™ Crystal Diameter
Frequency
The most common application of the probes (testing of steel) requires a
resonant frequency of transducers ranging between between 2 MHz and 6
MHz. For different application probes with smaller frequencies up to 500 KHz
and with higher frequencies up to approx. 25 MHz in built.

Frequency is dependent on thickness of the crystal.

Velocity of crystal material λ

t crystal = ------------------------------------- = ---
2 X frequency 2

18
The frequency of the probe governs the wavelength of the sound wave in a
material. The lower the frequency the longer the wavelength and the better
penetration of the sound energy. The higher the frequency the shorter the
wavelength and although this means poorer penetration, the sensitivity to
small reflectors is increased. Higher frequency probes also have smaller beam
spread.

Refracted Angle

The refracted angle o a compression wave probe is usually 0º and so the angle
is never marked. The angle marked on shear wave probe is the refracted angle
of that probe in steel e.g. 45º, 60º and 70º. Surface wave probes are usually
marked with an S. The refracted angle is always measured with respect to the
normal and not with respect to the surface. Probes used for materials other
than steel would normally be marked with the material, e.g. 45º, Al 60º Cu.

Single or Twin Crystal

The probe is easily identified by noting its construction. A twin crystal probe
is in effect two separate probes in one housing. The purpose of twin crystal
probes is to examine for near surface defects or to test thin materials. The
dead zone problem experienced with single crystal probes is overcome by
containing the dead zone with in the Perspex shoe of the transmitting crystal.

Crystal Diameter

The diameter of the crystal may be marked on the probe and is important
since it affects the length of the near zone and also the beam spread. The
larger the diameter the longer the near zone but the smaller the beam spread.

Characteristics of ultrasonic Probes

Sensitivity
Sensitivity of an ultrasonic test system is the ability to detect the smallest
discontinuity. Sensitivity refers to the amplitude of the electrical voltage
generated by the probe depending on the magnitude of ultrasonic signal
impinging of its surface. Hence, it determines how small a defect can be
detected. Sensitivity of a probe is measured by the amplitude of its response
to the energy reflected from a standard discontinuity (e.g. flat bottom hole in
a standard reference block).

Resolution
Resolution refers to the ability to separate echoes from two or more targets
which are close together in depth. An example would be the front surface
echo and the signal from a small discontinuity locating just under the surface.

19
A long pulse has poor resolving power.
Very short pulses are desirable for high
resolution.

Acoustical damping results in shorter pulses

and thus better resolution. This requires
board band width; this requires board band
width, low Q probes.

f0
Q (mechanical Quality Factor = -------
f2-f1

where f0 – central resonant frequency, f2 –

frequency above f0 where sonic amplitude is
70% of that at f0, f1 – the frequency below f0,
where sonic amplitude is 70% of that at f0
and f2 – f1 is the band width.

20
Near field effects

Amplitude of waves reflected from defects located within the near zone is
affected by the diffraction pattern and the location of the defect within the
near zone. The amplitude of the reflected wave may experience at least one
maximum and one minimum as the distance from the probe increases in the
near field. Therefore, it is difficult to determine the size of defects located in
the near field by the amplitude of reflected waves. Length of this near fields is
given by.

D2
N = -----
4λ
Where, D = probe diameter
λ = wave length

Dead zone

The crystal of the probe is energized by short electrical pulses of a particular

pulse duration and pulse repetition frequency. When the probe is acting as a
transmitter and receiver, the transmission continues until the pulse dies dies
out and then only it can act as a receiver. Also even after the electrical pulse is
out off, the crystal, because of its inertia continues to vibrate (ringing). By the
time the pulse dies out, or the crystal stops vibrate the sonic energy travels
into the material and the distance traveled by the sound energy with in the
pulse duration period is known as ‘dead-zone’. When a flaw lies within this
zone, (represented by the board initial pulse signal from this will not be
received and hence, cannot be detected. To minimize dead-zone, sort pulse
can be used and the internal damping of the probe may be improved.

Construction of Probes
Normal Probe
Electrodes are fitted on both face of the
crystal and wires from these electrodes
lead to the connector socket of the probe.
Sometimes damping material is also
applied on the interface of the crystal.
Usually a wear-plate made from material
of known acoustic properties attached to
the outer face of the crystal which
protects the crystal from wear and tear.
Probes from immersion testing will be
fetted in a water tight body (housing). To
reduce wear and tear of normal probe, a
protective cover is provided for some
probes.

21
These probes provide better contact on rough surfaces, are more adaptable to
the surface and consequently give more uniform coupling at less pressure.
One drawback in some cases is the fact that transmitted pulse and the echoes
are somewhat broader and defects near the surface are difficult. Also there
will be slight loss of sensitivity.

Angle Probe

For an angle Probe, the crystal is mounted on a Perspex wedge as shown in

the figure so that the beam of sound reaches the bottom flat surface of the
probe at an angle that will produce the desired angle of refraction in the
material tested.

The longitudinal wave after

refraction from the interface changes
its mode into transverse wave. The
reflected sound beam in the Perspex
wedge is damped with a suitable
damping material. The angle marked
on the probe in the refracted beam
angle in steel. For other materials,
correction of this angle is to be made
accordingly. Due to wear and tear of
the sole of an angle probe, the beam
exit point and the angle of refracted
beam may change. In order to avoid
this change and possible damage to
the crystal, the probe can be fitted
with a new sole.

TR Probe

Long delay blocks and separate

probes for transmission and
reception are used in TR probe to
avoid the undesirable effects of dead
zone. The crystals are placed on
Perspex wedges so that the sound
beam from the transmitter enters the
receiver after reflection in the
material. An acoustical barrier is
provided between the two sections of
the probe to avoid any cross talk.
Testing and wall thickness
measurement of very thin sections
can be done with this type of probe
with very high accuracy. (Better near
surface resolution).

22
Focused beam Probes

Spherically and cylindrically ground acoustical lenses are commonly added to

immersion type probes. Cylindrically ground lenses focus the sound beam to
a line and spherically ground lenses focus the beam to a point sensitivity from
a focused beam increases resulting from the increase in intensity.

Probes come in many shapes, sizes and physical characteristics. Single

element probes act as transmitter and receiver or only as transmitter /
receiver. In a double element probe one is a transmitter and the other is a
receiver both mounted I the same casing.

Probes with contour corrected soles

When testing specimen with small diameter (like rods, pipes etc) ordinary
probes do not provide adequate contact. To ensure that sufficient sound
energy is transmitted into the specimen the probes are fitted with a sole of
suitable shape. The matching sole is permanently fixed in to the probe.

23
The effects of transducer diameter and frequency on sound propagation

Transducer Diameter
Greater the Diameter ……………… Smaller the beam spread
Greater the Diameter ……………… Grater the out put of energy
Greater the Diameter ……………… larger the near zone

Probe Frequency
Higher the Frequency ……………… Smaller the beam spread
Higher the Frequency ……………… Larger the near zone
Higher the Frequency ……………… Smaller the dead zone
Higher the Frequency ……………… Lower the penetrating power

Couplants
In order to transmit the ultrasonic energy from probe to the test sample the air
has to be totally expelled as we know that the air is a bad conductor and good
reflector of the energy.

A Couplant is used to couple the probe to the face of the work piece. There
are varieties of couplants. A proper Couplant considering following point
must be used:

1. Surface finish of the test piece

™ Relatively smooth – water with wetting agent such as glycerin.

™ Rough surface / vertical surface – used thick oil or grease

Couplant should make full contact and stay on the surface but is
easy to apply and remove.

2. Temperature of test surface

™ Special paste for high temperature measurements

24
3. Harmless to the specimen
™ For carbon steel water may promote corrosion. A non corrosive
agent should be used.

4. An acoustic impedance of ideal Couplant should be between that of

crystal and work piece, values approaching that of work piece. Finally
the Couplant should be consistent and give effective results.

25
 CHAPTER - 5
ULTRASONIC EQUIPMENT

Ultrasonic Equipment

Ultrasonic equipment is produced by many manufactures. It is possible that

they use different terminology for equipment controls and their function.
Here we will use common terms for identification purposes. Remember, even
though the same control has several different terms used to identify it, its
basic function will remain the same.

Block Diagram

There are fie basic blocks

1. The clock / the timer / synchronizer

2. The transmitter / sender / pulser
3. The receiver / amplifier
4. The display unit

Clock Timer

The clock timer provides constant rate of pulses to be used as a coordinator

for all other function. The number of these pulses in a unit of time is varied by
the pulse repetition rate control. The clock timer provides electrical energy to
both the transmitter and the sweep generator.

Transmitter

The transmitter will transmit a high voltage pulse of short duration to the
piezoelectric element in the transducer and also provide a reference indication
(called as initial Pulse) on display unit through receiver/ amplifier. The start
of the initial pulse is referred to as electrical zero and is distinguished from
acoustical zero. The location of the initial pulse is not used for calibration
purpose, because a time lag exists between the display of the initial pulse and
ultrasound entering the part. This lag is referred to as the “electrical zero to
acoustical zero time lag.”

Sweep Generator

This makes the electronics in the display unit to move (sweep) from left to
right at a selected speed forming a sweep line. As it also represents time, it is
known as time base. The waveform used to generate sweep line is called the
“Saw tooth” waveform.

26
Receiver

The receiver in the instrument senses the relatively low voltage created when
the piezoelectric element in the transducer converts received mechanical
energy to electrical energy. The receiver will amplify, filter, and rectify the
electrical pulses and pass them to the display unit.

Cathode ray tube (CRT)

The CRT is a device similar to a picture tube in a Television set. The CRT
consists of a base connector for attaching it to the circuitry of the instrument,
an electron gun that generates a stream of electrons, a tube for focusing and
accelerating the electron stream, vertical and horizontal deflection plates that
deflect the electron stream, producing a vertical and horizontal display, and a
phosphorus coated viewing screen that will illuminate when bombarded by
the electron stream.

Equipment control

Each control on an ultrasonic instrument is in some way connected to one of

these circuits and provides a specific function. The clock timer operation is
varied by the pulse repetition rate control.

Pulse repetition rate

Changing the pulse repetition rate will affect the amount of time between
transmitted pulses. A sufficient amount of time between pulses I necessary to
allow ultrasound to travel through the specimen being inspected. Spurious
indications may be encountered if the pulse repetition rate is too high for a
given material type and thickness. Repetition rate controls can be found in
several different forms: fixed, variable by means of switches, or variable by
means of the material range control.

Pulse Energy

The transmitter may be affected by several controls. Some instruments have a

pulse energy control that changes the voltage applied to the piezoelectric
element. The voltage may range from 200 volts up to and in excess of 1200
volts, depending on the instruments capabilities. Higher voltage excitation
pulses will result in deeper material penetration.

Damping (Pulse Length)

27
A very common control found on instruments is the damping or pulse length
control. This control will change the duration of the pulse applied to the
piezoelectric element. Increasing damping will decrease the ringing.
effect of the transducer, thus increasing near surface resolution and, at the
same time, decreasing depth of material penetration.

Gain control

The receiver circuit is affected by several important controls. In order to make

relative discontinuity size comparison, the instrument must be capable of
providing a calibrated adjustment for indication amplitude. This is
accomplished by the use of a calibrated gain control. The gain control is
divided in to step.

dB increments, usually in the form of a fine and coarse control. The coarse
control adjustable in 20 dB increments and fine control adjustable in 2 dB
increments. The gain control does not increase or decrease the ultrasound
intensity in the part being examined. The gain control only affects the
amplifications of the receiver output. The decibel is used as a relative unit of
measure for sound intensity in Ultrasonic testing. The sound intensity in a
part being examined is controlled by the transmitter circuit.

Reject control
A reject control is provided in the receiver circuit to suppress low amplitude
indication that may appear on the CRT from such things as reflected
ultrasound created by coarse grained material. Caution is advised when using
this control because it can affect the vertical linearity of the instrument.

Band width
Each transducer emits a range of frequencies. A 2.25 MHz transducer, for
example, will be a band of ultrasound over a range of frequencies centering
around 2.25 MHz. An instrument capable of receiving and tuning to a specific
frequency is considered a narrow band instrument. Ultrasonic instruments
classified a narrow band instrument. Ultrasonic instruments classified as
being broadband are capable of receiving a wide range of frequencies.
Instrument band width is selected, for the most part, by the type of material to
be inspected and the sensitivity required.

Distance Amplitude correction (DAC)

A distance amplitude correction (DAC) control is also offered on some
instruments. This control will electronically compensate for attenuation over a
given material thickness. This electronic compensation is used to produce
equal amplitude indications for equal reflectors at different depths. Any form
of distance amplitude correction (DAC) control is part of the receiver circuit.
After amplification, and before filtration and rectification, the signals are still
in a radio frequency (RF) from. Display of an RF signal on a CRT leads to

28
difficulty in interpretation because both the positive and negative voltage
components would be presented. To ease interpretation, the RF signal is
rectified in the video amplifier circuit before being displayed on a CRT.
Focus / Intensity

Display controls include focus, intensity, and vertical and horizontal sweep
movement.

Delay Control

Two controls essential to the calibration of the sweep are the delay and range
controls. The delay control will move the display uniformly across the
viewing screen. This allows positioning of indication from calibration
reflectors to desired positions on the sweep.

Range Control

The range control provides the necessary expansion and compression for the
sweep. This function is necessary for compensation and calibration to a
specific material velocity and thickness. Both the range and the range and the
delay controls are part of the sweep generator circuit.

Gating Controls

Some instruments also provide gating controls. These controls are used to
monitor a specific portion of the sweep. In an indication appears in the gated
area and its amplitude is above (positive gating), or below (negative gating),
the set level, an audible visible or recording type device will respond. Gating
is used to draw attention to, or record, discontinuity conditions within the
material being inspected. In the negative mode, a gate may be used to monitor
a back surface indication. The loss of Couplant in this case would cause gate
response.

29
 The Decibel – Measurement of Sound Intensity

In electrical power the unit Bel on a logarithmic base has always been used to
compare the ratio of two powers.

This unit is always expressed by the common logarithm of the ratio of two
quantities.

An instrument whose power is increased from P1 to P2 is expressed as:

P2
Power level or gain = log10 -------- Bels
P1

The unit Bel was found to be too large for certain systems and subdivided by
10 to give the unit decibel (dB) thus;

P2
Grain = log10 -------- Bels
P1
Since the UFD measures voltages and not power and power is proportional to
the square of the voltage, the gain or amplification can be expressed thus:

V2

30
 = 20 Log10 ----- (removal of the square)
V1

h2
dB = 20 Log10 ------
h1

Here, h2/h1 is ratio of echo heights.

The attenuation or amplification is measured on a logarithmic base, thus the

difference in decibels will be the same whatever the initial setting on the gain
or attenuator control of the flaw detector.

31
 CHAPTER 6
INSTRUMENT’S LINEARITY

Code Requirements

Almost all codes that utilize ultrasonic as an inspection method have specific
requirements concerning the performance of the instrumentation used during
an ultrasonic examination. These requirements and performance tolerances
vary from one code to another but their basic remains the same, to assure
accurate and linear data presentation.

Tolerance specified in this lesson generally reflect the requirements of ASME

Section V. They are for guidance and may or may not satisfy a particular code
requirement.

Linearity

Instruments linearity is to be checked at the beginning of each period of

extended use or on periodically basis. Linearity can be best defined as the
signals occurring after equal time interval (e.g. multiple echoes from back
wall) should appear with equal spacing from CRT.

The horizontal linearity of an instrument can be determined by placing a

straight beam transducer on a suitable calibration block and adjusting delay
and sweep control to display multiple echo patterns. Instrument reject control
should be off for all linearity checks. These echoes should then be adjusted as
close as possible to their respective scale divisions.

The relationship between the displayed thickness and the true material
thickness is used to verify horizontal linearity. For example, if a 25mm
calibration block was selected for the check, each of the ten echoes should
represent 25mm of material thickness and should be precisely located at 1, 2,
3,4,5,6,7,8,9 and 10 respectively.

The tolerance is + or – 2% of the full screen width in order to provide

adequate linearity.

Also procedure may require that this check be performed at each range
stetting on the instrument or each range setting to be used during the
examination. A nonlinear horizontal sweep will provide incorrect depth data
during an examination.

Vertical Linearity

The vertical linearity of an instrument can be assessed by displaying two

indications (1st & 2nd BWE) on X – axis. This can be accomplished by coupling

32
an straight beam transducer on calibration block to get echoes bear mid
position on X-axis. The echoes should be adjusted to 100% and 50% full screen
height by use of the instrument gain control. After these adjustments have
been made, the transducer must be held steady and remain free of movement
throughout the rest of the check. To do this, generally, special prove with
small calibration block fitted (e.g. KK N-23) is used. The relationship between
the 100% and the 50% full screen height echoes is a 2 to 1 ratio (100/50=2).
This ratio is then observed as the gain control of the instrument is decreased
in 2dB increments to the point where the first echo is at 10% full screen height.

Maintaining a 2 to 1 ratio at echo gain setting is evidence of vertical linearity.

A typical allowance for error in vertical linearity is 5% of full screen at each
gain setting. Instrument error in vertical linearity will result in inaccurate
amplitude response relationships from discontinuities or reference reflectors.

Amplitude (Gain / attenuator) control linearity

To check the accuracy of this control, a transducer is placed one suitable

calibration block to produce on echo at horizontal mid screen. With this echo
adjusted to 80% full screen height, increase and decrease the gain control for
various echo heights. This should be within the tolerances. Proper amplitude
control linearity will ensure accurate dB comparisons form one discontinuity
to another over the full screen height of the CRT. Records of these linearity
checks should be maintained and be traceable to correct its linearity error.

If an instrument is incapable of conforming to the linearity requirements of a

code, it will require internal electronic adjustment to the instrument to correct
its linearity error.

Other checks like assessment of Dead Zone, assessing overall sensitivity of the
probe and amplifier, checking resolution are accomplished with V1 block and
they are covered in Level I.

Echo set at % of dB change Echo limit % of

FSH FSH
80% - 6 db 32 to 48%
80% - 12 dB 16 to 24%
40% + 6 dB 64 to 96%
20% + 12 dB 64 to 96%

33
 CHAPTER 7
CALIBRATION AND STANDARD REFERENCE BLOCKS

Standard reference blocks

In Ultrasonic testing, discontinuities are usually compared to a reference

standard. The standard may be one of many reference blocks or sets of blocks
specified for a test. A typical flat bottom hole block (FBH) which can be used
as a reference block is shown below.

Defect free material. A flat bottom hole provides a known area of reflection
perpendicular to the sound beam.

Three commonly used sets of standard FBH reference blocks are

1. Area amplitude blocks.

2. Distance amplitude blocks
3. ASTM basic sit of area and distance amplitude blocks

Area amplitude blocks provide standards for discontinuities of different sizes

at the same depths. Distance amplitude blocks provide standard for
discontinuities of the same size at different depths.

A. The ASTM basic set consists of ten 2” dia blocks that have ¾”, deep,
flat bottomed holes drilled in the center at one end. One block has a
3/64” dia. FBH and a metal distance of 3” from the test surface to the
flat bottomed hole. The next seven blocks each have a 5/64” FBH, but
metal distances are ⅛, ¼, ½, ¾, and 1½, 3 and 6 inch from the test
surface to the FBH. The two remaining blocks each have an 8/64” FBH
and metal distance of 3: and 6”.

34
In this basic set, the three blocks (No. 3, 5 & 8) with 3” metal distance provide
the area amplitude relationship and the seven blocks with the 5/64” dia. FBH
and varying distances provide the distance – amplitude relationship.

METAL DISTANCE, INCHES ⅛ ¼ ½ ¾ 1-½ 3 3 3 6 6

FBH DIA IN 64 THS INCH 5 5 5 5 5 3 5 8 5 8

B. Alcoa series ‘A’ Reference Blocks

The Alcoa series ‘A’ area amplitude blocks shown in figure 45

consists of eight blocks 3, ¾” ling and 1 15/16” square. The hole diameters
vary from 1/64” to 8/64”. These blocks are numbered to correspond with the
diameter of the holes i.e. no. 1 block – 1/64” dia hole, no 2 block – 2/64” dia
hole etc. up to no. 8 block -8/64” dia hole.

C. Alcoa Series ‘B’ Reference Blocks

35
The Alcoa series ‘B’ or hitt distance amplitude blocks consistence of 19, 2
inch/dia blocks. All the blocks have ¾”deep flat bottom hole drilled at the
center of the bottom surface. The hole diameter is the same for blocks in one
set. Sets of blocks with 3/64” or 5/64” or 8/64” dia hole are available. The
material distance in each set are 1/16”, ⅛”, through 1” in one eight-inch
increments and 1 ¼” through 5. ¾” in half inch increments.

Another type of calibration block is the IIW block (international institute of

welding). It provides the following:

36
Verification of known distance and angular relationships, verifies transducer
angle and beam exit point, checks resolution, sensitivity and beam
penetration power. In short it may be called a system evaluation block.

A miniature calibration block as shown below also can be used for range and
sensitivity calibration for normal and angle probes.

37
 CHAPTER 8
CALIBRATION

Range Calibration of Normal Probe

To accurately determine how deep a reflector is located in a material or to

perform precise material thickness measurements, it is essential that the
horizontal display of an instrument be calibrated to a distance that
corresponds to a specific material thickness and velocity. This is accomplished
be the use of instrument delay and range controls.

By placing a straight beam transducer on a known material of known

thickness (for example, 25mm of steel), indication will appear on the cathode
ray tube (CRT). The indications correspond to the elapsed time it took for the
ultrasound to travel from the front to the back surface and return to the
transducer. In order to calibrate the range of the instrument, these indications
must be positioned on the time base so that as equal material thickness is
represented between each. In this example the back echo and the first
multiple are used from the 25mm calibration block to calibrate the range to
100 mm of sound travel.

Two things to remember are:

1. Never use the initial pulse for calibration purposes because of the
electrical to acoustical zero time lag, and
2. Always use the leading edge of the indications for alignment, never the
trailing edge.

When calibrating, the initial pulse can be identified by lifting the probe. The
initial pulse will be the signal that appears first in time. The leading edges of
back surface reflections and multiples are used. They are representative of the
total elapsed time necessary for sound to travel from the front surface of the
material to the back surface and back to the front again.

The main objective is to make the ‘X’ Axis represent a desired material
thickness. The time base is graduated in equal distance increments, much the
same as a tape measure. Typically, 10main divisions each divided 5 sub-
divisions marked and numbered on the face of the CRT. This system of
dividing the X – Axis can change from on instrument to another, but its
purpose remains the same.

One range calibration is not suitable for every inspection encountered. For
example, a 100mm range sweep would not be suitable for inspecting 200mm
material.

Sequence for Range Calibration:

38
 1. Identify the material type to be inspected and select a calibration block
with equal acoustical properties. In some situations, the actual
component material will be used for calibration block.

2. Set the range control of the instrument on a setting compatible to the

100mm of material that is to be displayed.

3. Couples a straight beam probe to a calibration block (25mm of steel, for

example) and observe the indications displayed.

4. Adjust the instrument again control as necessary and identify the

initial pulse (remember; do not use the initial pulse for calibration
purpose).

5. Using the delay control, adjust the leading edge of the back reflection
from the 25mm steel calibration block to 2.5 main divisions.

6. Using fine range control, adjust the leading edge of the third multiple
to 7.5 main divisions.

7. Repeat steps 5 and 6 as necessary to position the first back and third
multiple precisely at 2.5 and 7.5 respectively. (Some instruments
provide delay and range control lockable to prevent inadvertent
movement during the examination; they should be made use of).

The CRT is now displaying 100mm of steel. Note that the leading edge of the
initial pulse will not be precisely aligned at 0, this is of no concern. To
calibrate for a 50mm range, the same steps are used except that one back and
second multiple are used. This holds true as long as the calibration block used
remains of 25mm steel. A change from one calibration block thickness to
another will always have to be allowed for during calibration. Just about any
material thickness can be calibrated for provided a suitable calibration block
is available. (Range calibration lower than calibration block thickness is nor
possible).

Various materials will have their own corresponding ultrasonic velocity. Any
time an instrument is calibrated on one material type and then used to inspect
a different type, the sweep will be out of calibration.

Either the range can be converted for other than steel material if the velocity is
known or we can calibrate the range in other material using steel block.

Conversion from 100mm calibrated range in steel to plastic:

Test Range in steel = 100mm

Longitudinal Velocity in steel = 5920 m/s

39
Longitudinal velocity in plastic = 2730 m/s

Velocity in Plastic
Test range in plastic = Range in steel X ---------------------------
Velocity in steel

Test range in plastic = 46.12mm

(Note: if the sound velocity of the material in which the conversion is to be

made form the sound velocity of material used for calibration and becomes
higher for higher velocity)

Calibration the instrument with the aid of steel calibration block for a range of
50mm in Perspex

Thickness of the steel calibration block = 25mm

2730
Equivalent beam path in Perspex = ------- x 25 mm = 11.5mm
5920

Test range in Perspex (10 main Div.) = 50mm

Scale factor (one main Div.) = 5mm

11.5mm
Scale position for 1st BWE = ----------- = 2.3 main divisions
5

Scale Position for 2nd BWE 4.6 main divisions

Scale Position for 3rd BWE 6.9 main divisions
Scale Position for 4th BWE 9.2 main divisions

Adjust first and third echo with delay and range control respectively.

With this method even the longitudinal range can be converted into
Transverse wave and vice versa (later it can be seen as how normal probes are
partially used for calibration of range for transverse wave/angle probe
calibration.

Range calibrations Test Range (TR) = thickness of calibration block (t)

1. Set the range control of the instrument on a setting compatible to range

to be displayed.
2. Adjust the initial pulse near zero with delay control (need not be very
accurate).

40
 3. Couple a straight probe to a calibration block and observe the
indications displayed.
4. Adjust the 1st BWE at 10 main div. with range control.
5. Shift 1st BWE to div. zero with delay control.
6. Adjust 2nd BWE at 10 main div. with fine range control.
7. Repeat steps 5 and 6 and ensure that the echoes coincide with the exact
calibration marks.
8. Shift 1st BWE at 10 main div. with delay control only.

Note: If a narrow width block with longer depth is used, e.g. 100mm depth of
VI block you will find there are many geometrical echoes between BWE1,
BWE2. You should be able to identify the 2nd BWE properly. Steps 2 and 4 will
generally bring the 2nd BWE after completion of step 5 to somewhere between
9 to 10 main div.

Example: Using 91mm thickness of VI Block adjusts 100mm TR

Steps:

1. The difference between the starting point and the end point (105 – 80 =
25mm)

2. Calibrate range of 25mm with suitable block (0-25mm)

3. Calculate which BWE can be seen in above range. (4th BWE

corresponds to 100mm

4. Work out the screen reading for this echo (100 – 80 = 20mm on 25mm
FSR will be at 8 main div.)

5. Shift the pattern with delay control till 4th BWE from 25mm thickness
block is at eighth divisions.

Partial Range calibration cab be made in terms of calibration block thickness

such as 0.2t-1.2t or 0.5t-1.5t. For this calibration it is not necessary to measure
the thickness of the block.

Angle Probe:

Probes which transmit their sound beams at an angle are known as angle
beam probes because they send and receive the sound pulses into and from
the test specimen at a certain angle to the surface.

Most of the standard angle beam probes transmit and receive transverse wave
having a particle displacement perpendicular to the propagating direction.

41
When testing with angle beam probe, the sound travels from the crystal in the
probe through the plastic delay block to the surface of the test specimen and is
refracted at the interface probe front to test specimen. A very large percentage
of parts tested using angle beam probes are made of steel.

That is way the angles marked on the angle probe are with the transverse
refraction angle it delivers in the steel. The most commonly available angles
are 45º, 60º & 70º. Special angles like 35º & 80º are also available.

As for as the frequency is concerned angle beam probes do not have the
variety we find in Normal Probes. The main reason for this is that high
frequency transverse waves already undergo a very high attenuation due to
scatter at the grain boundaries. Because of this, the range of angle probe is
very low.

With the angle probes we do not get reflection from back wall in plane
parallel test piece. The reflection is received either from the quadrant or from
the corner and no doubt from the reflector lying perpendicular to the beam.

So for calibration we need special blocks like VI or V2 or semi circle. The

curved surfaces of reference blocks serve as reference reflectors. In this the
echoes used for calibration travel the same distance regardless of the
refraction angle of the probe. After the calibration of sound path (‘V’ path) the
echo for the discontinuity does not give any immediate information regarding
the location. It is only after the multiplication of the Beam Path (BP) with the
sin & cos of the refraction angle β that we get surface distance / projection
distance (PD) and depth (d).

The values derived are with respect to a particular point on the probe
radiating maximum energy called as BEAM INDEX / EXIT POINT / BEAM
EXIT and at a correct angle (the marked angle may deviate during use). So
these tests first to be carried out.

Checking Beam Index

Maximize an echo from quadrant; the probe point coinciding with radius
focal point marked in the calibration block is correct Beam Index. Instead of
marking this point, the distance from the front of the probe to this point is
found and called as ‘X’ – value of the angle probe. Generally for Kraut Kramer
miniature probes it is 12mm but can very during use. It is better to do the
calibration prior to checking the angle of the probe for no misleading
indications.

Checking Angle

42
On VI or VII block the angles are marked for checking the angle of the probe.
But this can only help you in rough assessment if the angles deviate. The exact
calculation of angle will help you to precisely locate discontinuities.

(Note: The angles are in steel, if you have to use the blocks subsequently on
any material whose velocity is different than steel, the angle in that particular
material needs to be calculated).

Example: Calculate the angle in aluminum for standard 70º angle probe.

There are many ways of calculating correct angles of the probes, but we will
solve the examples with VI.

Correct angle Measurement

After the instrument is calibrated, direct the beam to the Perspex/steel

interface by keeping the probe at the particular angle engraving mark on VI,
peak the echo, and note the correct distance. Imagine we are using 45º and
distance on calibrated screen observed is 78mm.

70
Angle of the probe = Cos-1 ----------- mm = 47.26º
BP + 25

If the instrument is not calibrated then, measure the distance from the front of
the probe to the edge of the block,

Projection distance scaled + ‘X’ value of Angle Probe – 35mm

= tan -1 -----------------------------------------------------------------------------------
70mm

43
44
 a. Range Calibration of Angle probe using VI Block

First the probe is position to direct the beam towards 25mm radius. Echo of
maximum amplitude is obtained and positioned to read 2.5 on horizontal
scale. Now the probes turned in the opposite direction and a maximum
amplitude echo in obtained from 100 mm radius (fine) are so adjusted to
make these two echo indications position at 2.5 and on horizontal scale.

The scale from 0 – 10 on CRT now corresponds to 100mm. (1 small scale

division = 2 mm). Range of 150 of 150 mm shown in above.

Other ranges can also be calibrated using multiple echoes as reference. When
the probe is facing 25mm radius side the first echo indication comes after the
beam travel of 25 mm and the 2nd and 3rd etc. after additional travel of 125
mm each i.e. the echoes will be at 25, 150, 275 etc. when the probe is facing the
100 m radius side, the echo indications position at 100, 225, 350 etc.

In both the above positions the echoes will be displaced by 125 mm between
them. This is due to the probe crystal positioning i.e. it can only receive
signals which are normal to it.

b. Range calibration of Angle Probe using IIW V2 block

This block has two arcs of radius 25mm and 50 mm. It is light in weight,
facilitates range calibration for normal and angle probes, determination of
beam exit point and angle for 45º, 60º and 70º probes and has a side drilled
hole 5mm or 1.6 mm dia.

When the probe is facing the 25 mm radius side the first echo indication
comes after the beam travels 25mm and the 2nd and 3rd etc. after additional
travel of 75 mm each i.e. the echoes will be at 25, 100, 175, 250 etc. when the

45
Probe is facing the 50 mm radius side, the echo indications position at 50, 125,
200 etc.

TR Probes

Two normal probes, one acting as a transmitter and the other as a receiver are
housed in one casing. These probes are specially used to detect near surface
defects as the delay blocks used in these probes and individual functioning of
probes practically eliminate the undesirable effects of dead zone. For range
calibration with TR probe, two know thickness is necessary because of the
angle of incident beam and beam divergence because multiple echoes less
prominent. Also interference echoes from split transverse waves (resulting
from mode conversion at the reflecting back surface of the specimen) can
appear behind the 1st back wall echo. These echoes will appear always behind
the 1st back wall echo as the velocity of transverse waves is less than the
velocity of longitudinal waves. Often it is not possible to distinguish the 2nd
back wall echo from the interference echo.

Set the instrument in the T-R mode. If we have two blocks of steel, thickness 5
mm and 10mm, first the probe is placed on 5mm block and the back wall echo
is adjusted to position at 5 on horizontal scale of CRT and then on 10 mm
block. The back wall echo for 10mm block is adjusted to read 10 on the
horizontal scale. Further adjustments in delay and range (fine) controls are
necessary till the CRT indications read 5 and 10 respectively when placed on 5

46
mm and 10mm thick blocks. Now the range or 0-10 on the horizontal scale is
calibrated for 0-10mm steel.

It may be noted that in the through transmission mode, the first indications
occurs after the beam travels the metal path in one direction and subsequent
indications occur after traveling double the distance.

Sensitivity Calibrations

The defects detected, located and probably identified by ultrasonic test should
be evaluated with respect to a known standard to have any meaning.
Obviously depending on the application of the part tested defects may be
classified as acceptable or reject able or reject able. Because of the variety of
materials, sizes, shapes, manufacturing processes involved, it is difficult to
have a uniform acceptance standard. Therefore, sensitivity level at which
ultrasonic tests are to be carried out differs accordingly.

To arrive at the sensitivity level of test, the following information is necessary-

1. Size and nature of defects expected.

2. Probable location of defects.
3. Orientation in the part tested.

The ideal method is to determine the sensitivity level at which testing is to be

carried out, is to have mock-ups of the parts to be made (preferable material
of the same metallurgical properties) with know defects (size, mature and
orientation) to be duplicated to the minimum acceptable level.

Usually flat bottom holes, side drilled holes, notches, ‘U’ or ‘V’ shaped and
buttress type are used for this purpose. Depending on the radius of the flat
bottom hole, it presents a know area of reflection perpendicular to the beam
path. Side drilled holes are usually preferred for angle beam testing because
they present the same area of reflection to any incident angle. The three types
of notches mentioned above can duplicate surface defects and the angle of the
‘V’ notch can be selected to match the probable orientation of the expected
defect.

The size of these artificial defects essentially depend on the application of the
part being tested and are usually given by the relevant acceptance standards.
However when testing a part, due consideration should be made for the echo
amplitude reduction with increasing distance. For this purpose, a distance
Amplitude correction curve (DAC Curve) is made using the echo amplitude
of the same reflector area at different distances within the testing range and
plotting them on the CRT screen itself (amplitude on ‘Y’ axis and distance in
the ‘X’ axis.)

47
48
 CHAPTER – 9
SELECTION OF ULTRASONIC PROBES

The proper selection of ultrasonic test system mainly depends on probe and
secondly on the test instrument.

Probe Selection

A probe should be able to indicate discontinuities from the work piece. So the
probe must comply with the conditions dictated by the work piece. Now we
know that there are following variables with the probe.

Frequency

1. Determined by the permeability: (admitting passing of energy) of the

material. This is:
a. Dependent on gain structure of the material. Coarse Grain
structure (cast iron) – High attenuation, grass (stray echoes)
produced on CRT at high gain, difficult to identify defects – use
low frequency (0.5 to 1 MHz)
b. Inclusion of foreign particles – (non metallic inclusions in
forgings, or porosity in sintered material)
c. Materials with different velocities in different directions.
(Anisotropic material)

All above conditions contribute to High attenuation, grass (stray echoes) are
produced on CRT at high gain. It is difficult to identify defects. Use low
frequency 0.5 to 1 MHz, another way to use highly damped probes called as
“wide band” probes.

2. Penetration: Longer depths can be penetrated by low frequency. This

is best judged by the presence / absence of BWE with reserve gain.

3. Orientation of Planner defects: If the defect cannot be struck

perpendicularly by the sound beam then at high frequency with
narrow beam, the flow detection is much more difficult, as the
reflection on longer reaches the receiver probe. Use of low frequency
with wide beam will help.

4. Hidden surface flaw due to the shape: The use of the low frequency
with broad beam might have to be used for hidden flaw behind a step
when normal transmission from face cannot be reached.

5. Resolving Power: Higher the frequency better the resolution

49
 6. Flaw Extn. Accuracy: Better with high frequency due to narrow beam
spread

7. Sensitivity: The ability to detect small flaw-better with high frequency.

8. Near surface defects: High at high frequency (narrow IP) or use TR
probes.

9. Near zone: high at high frequency – control by small dia probe.

Size

1. Use larger diameter for more power handling capacity and also for
faster inspection unless the limitation do not come due to work piece
dimensions and flaw size being evaluated in near Fields.

2. Smaller size for curved surfaces and limited scanning areas, but not at
low frequencies due to wide beam spread.

Angle

1. Thickness of the material: Use lower angle for thick materials to work
on lower beam paths for low losses and higher angle for lower
thickness.

2. Weld preparation angle: To hit fusion face perpendicularly.

3. Mode Conversion: In steel avoid 60 º angle probes for right angled

reflections.

4. Limiting Angle: For circumferential scanning in Pipes and Tubes.

5. Beam displacement: Use closer to 45º

6. Geometry: May limit particular angle

Mode of Operation

Single – normally preferred except for near surface defect location or low
thickness measurement.
Double – Detection of near surface flaws and for wall thickness measurement.

Note: wall thickness can be measured with single probe with multiple
Echo sequence or using delay line.

Beam Direction

50
Should be chosen such that expected defects offer defects offer the maximum
amount of reflection by beaming the surface of the flaw in perpendicular
direction, shape, geometry, and surface condition may limit the possible
direction. There are many special probes, which may not have been covered
in this chapter.

51
 CHAPTER – 10
TYPE OF DISCONTINUTIES

In order to perform the ultrasonic testing, the proper probe and instrument
must be selected. The choice depends upon the type orientation and location
of defects.

If the operator does not have the knowledge of the defects as detailed above,
then to cover the entire test sample in different directions and to look for the
defects at any location with the unknown orientation will be a task next to
impossible.

The discontinuities can be divided in to three categories:

a. Inherent: are related to the melting, solidification and casting processes

and are present from the starting stock (e.g. blow holes, piping, seams,
inclusions)

b. Processing: Are related to various manufacturing processes, such as

matching, forming, extruding, rolling, welding, heat treating and
plating.

c. Service: Are related to the various conditions, such as stress corrosion,

fatigue and wear.

The type of defects detailed here are with respect to ultrasonic testing and
major manufacturing process.

Forging / Rolling / Extrusion

Let us first briefly review the process of manufacture as this would give us
information on certain inherent defects which take the deferent from and
name during above manufacturing processes.

At first during steel making process the raw materials like from ore etc. are
melted in the furnace. The impurities (Slag/ Non Metallic Inclusion – NMI) in
the molten metal being lighter float on the surface of the molten metal. The
molten metal is poured into moulds to form INGOT.

Ingot

1. Non Metallic Inclusions (NMI): Irregular shape due to the impurities

in raw material.

2. Porosity: Rounded gas cavities, due to the entrapment of escaping

gases.

52
 3. Pipe / Center line piping: In form of pipe caused due to insufficient
material to fill up the cavities created by contraction of molten metal.

All these defects are generally observed on or near the top surface. Knowing
the likely origination of discontinuities in INGOT at top face, this is avoided
by cropping (cutting) and then it is know as BLOOM.

Slabs and Billets: Are smaller processed sections of blooms

Rolling: Billet in stretched to reduce thickness & increase length to produce

bars or reduced in thickness and increase in width and length to from Plates.

Plate

a. Lamination: Large planner and parallel to the surface due to heavy

rolling of NMI and oxidized porosity and raw material with ‘Pipe’
defect and spreading in all direction but mainly in the direction of roll,
found mostly in the centre.

Rolled Products (Bar Stock)

a. Stringer: Inclusions in Longitudinal direction due to stretching of NMI

b. Intermittent/Elongated Cavities: Due to the Porosity

53
 c. Pipe/ Centre Line Piping: Smaller around and longer with pipe shape
due to the pipe

All these defects are inherent of raw material used for processing and likely
to be more severe at the end, which is the top surface of Ingot.

d. Seam: Surface Cracks in direction of rolling due to the surface cracks in

INGOT and improper rolling or defect in rollers. (May look spiral if
Bar is formed with billet because of angular roller orientation).

Forging

a. Forging lap: folding of metal on the surface observed in closed die

forging due to mismatch of mating surfaces of dies or poor die design.
Always open to surface.
b. Forging bursts/Cracks: Surface is internal (generally in centre) due to
the rupture caused by forging or under heated stock.

Extrusion: The forming of part by forcing it through die. The inherent bar
stock discontinuities, will be present in the formed part. Cracks may be
introduced in extrusion process if the metal flow is not proper.

Pipe: (Welded)

a) Lamination: Due to plate used as raw material.

54
 b) Lack of Fusion: Called as “seam” appear on inner or outer surface due
to faulty welding process.

Pipe: (Seamless)

a. Seam: On outer surface due to bar stock used as a raw material.

b. Slugs: Metal buildup on the inner surface of the pipe during piercing
operation.

c. Grinding Cracks: Processing discontinuity, due to excess heat during

grinding operations. The orientation is at perpendicular to the rotation
of grinding wheel.

Heat Treatment crack: No specific direction but are most likely to be at any
sharp areas such as corners, fillets etc. and at junctions of light & heavy
sections, due to stresses built up in process, if not relieved properly, will give
rise to cracks.

Fatigue Cracks: Service discontinuity, through fatigue (repeated use or

overloading). Mostly open to surface but can be subsurface if propagating
from internal discontinuities – occur crosswise to the direction of stress
movement.

Casting:

Blow holes: round gas cavities at or below the surface due to air entrapment.

Gas porosity: Rounded gas cavities at or blow the surface due to the
entrapment of escaping gases.

55
Shrinkage Cavities: In form of pipe in heavy sections at change of cross
sections caused due to insufficient material to fill up the cavities created by
contraction of molten metal.

Hot Tear: Cracks at change in cross section, sharp corners, and mould cores,
due to restrain from free contraction during cooling.

Inclusions: Non-metallic particles like sand etc. due to erosion of the mould.

Cold Shuts: Hot metal over solidified metal or intersecting surfaces at

different temperature

Welding defects
Process discontinuities the process is similar to casting, only the heat input is
localized. Most of the defects are due to the technique of heat input and
solidification of weld metal.

1. Porosity: Round cavities caused due to entrapment of gases during

solidification.

56
2. Slag Inclusion: Due to entrapment of flux form the coated electrodes
or flux materials. Tungsten inclusions are formed when tungsten
electrode is used in particular welding process.

3. Cracks: Due to stresses developed during welding. May be in welds

(Longitudinal or Transverse) and in parent material in heat affected
zone (HAZ), due to lack of ductility or presence of base metal defects.

4. Lack of Fusion: Lack of joint, may be between parent metal & weld or
in weld beads

57
 5. Lack of penetration: Inability to penetrate the root of the weld joint.

6. Ex-cess Penetration: Also called as burn through and defined as the

penetration beyond the root of the weld caused by the Excessive Heat
Input.

7. Lamellar Tearing: Fractures appearing like horizontal terraces linked

by vertical walls are due to loading in the thickness direction of rolled
product and NMI in elongated form which lead to short transverse
ductility. It occurs often in HAZ and runs parallel to the weld fusion
boundary. (Generally observed in ‘T’ joints).

The following are the surface defects and can be viewed during Visual
inspection.

8. Undercut: Is a continuous or intermittent groove on the base metal

adjacent to the weld toe.

58
 9. Root Cavity: Is a concave groove at the root.

10. Weld Reinforcement: Represents the metal on the face of weld in

excess of that necessary for the specified weld sizes.

With the information of discontinuities type, likely location and orientation,

let us now turn to the actual testing applications.

59
 CHAPTER – 11
APPLICATION

Thickness Gauging

Ultrasonic is being widely used in thickness gauging involved in chemical

and fertilizer plants, ships, pipe lines, storage tanks etc. although a digital
read – out thickness meter can be used for this purpose, using an ultrasonic
Flaw Detector can provide additional information which will avoid the
possible errors while using a thickness meter.

Whenever shall (compared to beam size) defects are encountered, the

presentation on a flaw detector shows a defect indication and a back wall
indication. Using a thickness meter will only show the depth of the defect
moreover, a heavily corroded surface will show a broader indication on the
CRT whereas this information can not be obtained with a thickness meter.
Both the flaw detector and the thickness meter use the pulse echo method
with single crystal probes as transmitter and the other as receiver.

The instrument is calibrated in the usual manner for the distance required.
The gain setting is adjusted to receive an echo of adequate height. Gain may
be required to be increased on the actual job, as in the case of badly pitted
surface, while calibrating the distance axis, care has to be taken to ensure that
test block has the same velocity as that of the test specimen.

Testing for Laminations in Plates

A normal probe is brought into contact with the face of the plate through a
Couplant. The height of the back wall echo is kept at a fixed amplitude (50%
or above F.S.H.)

Any drop in back wall echo is considered to be caused by defective area.

Usually the plate is scanned along grid lines only and wherever the back wall
echo falls beyond a set level or when a defect indication is present, the
adjacent area is also scanned to determine the area of defect. Sometimes angle
beam technique is also used to detect the presence of defects which are not
oriented parallel to the surface.

Bond Testing

Ultrasonic test can be used to detect lack of bond when any two metals are
bounded by various methods including brazing. The plane where a lack of
bond is expected is known prior to the test. When two different materials
bond is expected is known prior to the test. When two different materials are
bonded, the interface itself (even when there is a sound bond) may reflect
some energy and in some cases flaw detection is not at all feasible.

60
The reflectivity ‘R’ of an interface depends on the acoustic impedances of the
two media.

Where, Z1 and Z2 are acoustic impedances of material 1 and 2 materials 2

respectively.

When the reflectivity is zero, a good bond does not give any reflection. Here
detection of lack of bond is similar to conventional flaw detection. When
reflectivity is very high, large reflections are obtained even from a place were
there is good bond and it becomes difficult to distinguish between good bond
and lack of bond.

Casting

The lower range of testing frequencies is generally applied to castings. It is

very difficult to test many coarse grained castings ultrasonically because of
the extreme scattering resulting from reflections at the grain boundaries.
Some alloys of brass, stainless steel, titanium and cast iron belong to this
group. Brass alloys cast under condition that control the grain growth can be
tested at 1 and 2.25 MHz because of the small uniform grain structure. Cast
lead can be tested at 1 MHz if it contains 6% antimony. Pure lead is a poor
transmitter of ultrasonic vibrations and even low frequencies are attenuated
within a few inches. Occasionally, when attenuation is too high, through
transmission method can be used instead of the pulse-echo method.
Shrinkage cavities, blow holes, inclusions and cracks can be detected.

Forgings

Most forging of a uniform size and shape are good objects for ultrasonic
testing. This applies to brass alloys, carbon steel, stainless steel, aluminum,
magnesium, titanium and nickel alloys. Flaking, forging bursts or thermal
cracks can be detected by ultrasonic test. Frequency in the range of 1 – 5 MHz
is suitable for test. Moreover, in forged parts, discontinuities may be flattened
out parallel to the test surface and thus present the best orientation for
reflecting ultrasonic beam.

Examination of the Weldment

Before the weld is examined using compression and share wave probes, the
parent material both sides of the weld must be checked using a compression
probe which cover the area to be scanned by angle probe. It is unlikely that

61
there will be access to both surfaces of the plate and thus it is standard
practice to use a twin crystal probe.

The purpose of this scan is to check for the presence of any laminar inclusion
and to give an accurate measure of plate thickness. The presence of laminar
inclusions may prevent the complete inspection of the weld or give false
indications.

Weld root Examination:

If the weld surface is smooth or has been ground off then it is advisable to run
a compression probe over it. Most welds, however, are not smooth enough to
examine with a compression probe and must be checked with angle probes.
The root is the most difficult part for the welder and it is usual to check this
area before examining the body of the weld.

For root scan we normally chose from 45º, 60º, 70º and sometimes 80º proves,
to have the shortest beam path length to the root and our choice is limited by
the condition of the weld cap. On thinner thickness it may not be possible to
position a 45º or 60 º probes so that the half skip beam point at the root gap,
without the toe of the probe riding up on to the weld cap. For welds with cap
in place we can make the following recommendations about probe angles for
various wall thicknesses for the root scan.

Parent material thickness probe angle

6.15mm 60º or 70º
15.35mm 60º or 45º
Over 35mm 45º

Shear wave weld body examination:

62
After the root area is checked then the main body of the weld is examined,
particular attention being paid to the fusion faces. To ensure complete
coverage of the weld it is necessary to scan the weld from at least two sides
using angle probes with a minimum of two different angles.

The initial choice of probe angle for the body scan depends upon the weld
preparation angle. The angle should be chosen to meet any lack of side wall
fusion at right angle for maximum response. For this purpose probe angle
shall be 90º - ø/2, where ø is the weld preparation angle. Figure below shows
the outer limit of the scan, which positions the probe so as to produce full skip
distance to the nearest edge of the weld cap that means that the probe index is
at a distance from the weld centre line equal to full skip plus half the cap
width.

To check the coverage a simple way is to take a scale drawing of the weld
preparation and superimpose on it the probe angle and scan pattern that the
technician has used.

Whilst the technician is checking the wild parallel to the weld preparation he
moves the probe in a square or zig – zag pattern. It is important that to
achieve full coverage that the probe is not moved literally more than the
probe width most codes call for a 10% overlap.

63
Scan for Transverse Defects:

Most of the technician’s time is spent examining the weld parallel to the weld
preparation looking for defects such as ICP slag, lack of fusion etc. if this
examination is complete then it is essential that he also examines the weld for
transverse defects. This is particularly important in submerged are welds in
excess of 20 mm thickness which may contain chevron cracking.

A submerged is weld can be examined by placing a 45º share wave probe

directly on the cap.

With a manual metal are weld it is not possible, to scan directly on the cap
and the scan for transverse defects is usually carried out by running the probe
on the parent material and aiming the probe obliquely to the weld.

If a defect is detected it may aid the interpretation if the wild cap is ground
flush in that area.

Some other common weld joint preparation and scanning techniques are
shown below.

64
Flaw Location: The flaws can be located with respect to the probe position
and depth from the scanning surface can be determined using the following
equations.

65
 S = 2tTapβ = Full-skip If B > ½ V Path
B = 2tSecβ = Beam-Path Surface distance = S2 = BSinβ
(V Path) depth = d = 2t-Bcosβ
B = B1+B2

Interpretation of Ultrasonic Indication in Weld Testing

Planner defects like cracks, lack of fusion and penetration etc. tend to produce
a higher amplitude echo. When the beam is striking perpendicular to these
defects, the echo indication is sharp. For lack of fusion and cracks, when the
angle of approach is changed, there will be considerable difference in echo
height.

Special defects like porosity reflect only a small amount of sound. Hence echo
height would be small and will remain unchanged when the angle of
approach is changed. Inclusion of an irregular shape will give rise to an echo
which is broad with many peaks of comparatively low height and changes the
pattern when the approach is changed.

The interpretation of ultrasonic test indications require good experience and

this experience can be gained only when the defect echo pattern is recorded
and later correlated to the actual defects by destructive or other methods of
confirmative tests.

66
A through knowledge of the weld geometry, welding procedure and probable
nature and orientation of defects which may be present, is required before
ultrasonic testing of welds is done.

A few examples of echo indications from weld defects are shown below.

Lack of penetration can be identified by sharp echo while scanning from both
sides of the weld the beam path of which corresponds to the thickness and
angle involved. When the probe is moved along the length of weld, this
indication remains at the same position on the screen.

When there is an excess penetration, depending on the shape of the root

surface a strong echo may be present. Similar effect can occur when there is
excess reinforcement on the outer surface also.

67
When there is a mismatch between the plates welded, a strong which
indication may be present when scanned from the lower side of the joint and
no indication, when scanned form upper side.

A concave root may also produce sharp echo indication on the screen.

Inclusions of an irregular shape will give rise to a broad echo with many
peaks of comparatively low height.

68
 CHAPTER – 12
DEFECT SIZING
Defect Sizing

It should be borne in mind that in UT, the echo height shows us the reflected
energy. The echo height to be representative of area of reflector, the centre of
the reflector should be on beam axis living perpendicular such as FBH.

The natural reflector in a material is unlikely to be of a nature of FBH so when

a defect is sized it should be clearly understood that it is equivalent, flaw size
(EFS) and not actual flaw size.

Defects can be said to be small or large. Large reflector is the one whose
diameter is greater than the beam cross section at the testing range. Small
reflector is encircled at the testing range by the beam cross section.

Large Reflector

Half Value / 6db drop Method

Mostly used for plotting of lamination boundaries in plate testing. The echo
indication as compared to the maximum indication decreased by 6 dB if half
of the sound beam strikes the reflector and half of it passes by. If by moving
the probe, we look for the 6dB drop points, then the axis of the Beam (center
line of the probe) points directly to the edge of the reflector.

69
20 dB drop Method

This defector measuring method is used where defect size approximates to, or
is longer than beam diameter, and the defect is parallel to scan surface. The
specimen is scanned and the prone centre is used to plot, on the surface, the
indicated dimension of the defect at the point where the amplitude drops by
20dB. By calculating the beam diameter at the depth of the defect its
approximate dimensions can be determined. The approach is commonly used
to size defects in weldments and is more accurate than the 6 dB drop method,
which relies on planar flaws to be accurate.
Dimension of defect = A – B

Where A = plotted Dimension

Where B = Beam diameter at depth of defect

Small Reflector

Equivalent Reflector size: Blocks with FBH to compare echo eights.

DGS diagram: Distance, Gain & Size method is also based on FBH.

DAC curve: Generally for a small reflector, instead of sizing a defect, practical
method used is “go / No go” indication. That is drawing distance Amplitude
correction (DAC) curve on the screen called as Reference Level. Any
indication above this is unacceptable.

70
 CHAPTER – 13
IRRELEVANT INDICATIONS

Irrelevant Indications

The echoes presented on CRT are not always the discontinuities echoes.
Because of Mode conversion and some other reasons, there are likely to be
undesired indications which cannot be avoided. So the operator should be
able to differentiate between desired and undesired (irrelevant) indications.
They are classified in different groups.

Electrical Interference

Noise appears on the screen – can be caused due to faulty coaxial cable or by
the defective amplifier of UFD at high gain. In mains operated UFD, it might
be additionally caused due to the electrical furnaces or welding equipment
being connected to the same power line. All these are easily identifiable and
thus can be neglected during testing.

Probe Indications

Loose crystal can cause prolonged ringing / widening Initial pulse /

increasing dead zone. In angle probes refection from plastic wedge can be
seen near IP, they will remain on the screen even if probe is lifted.

Surface of the material

In TR probe cross talk echo is an example of this type of irrelevant indication.

The surface waves traveling in all directions from normal probe on a fine
surface, can be reflected from the edge and produce an echo and be identified
by pressing an oily finger in the path of surface wave.

Refraction

In NP when the probe is coupled to a narrow & long work piece, the side wall
generates additional echoes due to mode conversion from longitudinal to
transverse and back to probe as longitudinal wave, these echoes appear after
the 1st BWE.

71
Shape of the work piece

In the specimens of different shapes with fillets and corners, or in rectangular

specimens with internal bores, it might be difficult to predict the irrelevant
indications. To differentiate them, examine the work piece from then one
surface and with different angle probes.

Material Structure

It plays an important role while testing. A large forging with low losses (very
fine grained) may show ‘Ghost echoes’. Castings (not heat treated) show a
coarse structure giving a noise on the screen. In certain materials the direction
of the sound beam with respect to the direction of oriented grain structure
also needs to be considered.

Couplant

Couplant flaw indications may be noticed when using surface wave probes or
angle probes with a high sensitivity.
Most of irrelevant indications outlined above ate inherent in Ultrasonic test
method itself and there may not be special improvements to overcome them.
So one must be able to test the work piece effectively by neglecting these
irrelevant indications.

72
 CHAPTER – 14
TECHNIQUES OF TESTING

Techniques

There are two basic Ultrasonic Testing techniques

Contract Testing
Immersion Testing

Contract Testing

In this the transducer is placed in direct contact with test specimen with a thin
liquid film used as a Couplant. On some contact type of probes, plastic
wedges, wear plates, or flexible membranes are attached over the face of the
crystal.

Immersion Tasting

1. In this method both the test piece and the probe are totally immersed
in water.
2. Few important points to be remembered about this testing are

3. Mostly water is used as a Couplant.

4. Because of total immersion of test piece and probe, good coupling is

ensured.

5. Probe should be water tight as completely immersed I water.

6. Generally st. beam probes are used for angle beam testing. Any desired
angle can be performed through manipulation and control of the
sound beam direction.

7. Mount on carriage assembly for ‘X’ & ‘Y’ movement of perpendicular

orientation to the test surface.

8. The water to steel interface, generate extra echo at the entry face of
work piece know as ‘Entrance of entry Echo”. This peaked when the
probe is properly oriented. There is fairly wide gap between initial
pulse and the front surface reflection (Entry Echo).

9. Minimum water gap should be such that no water multiples appear

between the expectancy range of the work piece. (Because of water to
steel velocity ratio being 1:4, the water gap must be more than ¼
thickness of test piece. Advisable to keep ¼ t + ¼ “.

73
 10. If water gap is small, then the multiple of entrance echo would appear
between entry surface and back surface indication making
interpretation difficult.

11. Difficult to test large part as the size of the tank and accessories will
have to be too large to accommodate work piece.

Data Presentation
Information in UT can be presented in three different formats

1. A Scan
2. B Scan
3. C Scan

A Scan
Most of the UFD for standard application use CRT showing the amplitude of
the echo signals on the vertical ‘Y’ direction and the distance of the
corresponding reflector are represented on the horizontal ‘X’ direction.

74
Relative discontinuity size can be determined by comparing amplitude from
know size reference reflector and displayed indication amplitude. Depth of
the reflector can be easily determined by be position on a calibrated
horizontal base line.

B Scan

This will display a cross sectional or elevation view of the test object on a long
persistent CRT. In this the probe movement is mostly displayed in the X
direction while the distance of the reflector is in the Y direction.

C Scan
This presentation method displays discontinuity areas relative to the plan or
top view to the test object. Mostly the presentation is in the form of paper
recorders to provide permanent records.

75
However, the depth information is not available. The pen movement of the
recorder and the prove movement are synchronized and scanning is
performed mechanically.

Methods

Pulse – Echo: Also called as refection method. In this both single or double
transducer is placed on one side of the specimen and the presence of a defect
is indicated by the reception of an echo before that of BWE. The exact location
and some idea of defect size can be assessed.

Through Transmission: The transmitting and receiving probes are on

opposite sides of the specimen. They are to be in the same axis.

The presence of defect is indicated by the reduction of received echo. It is not

possible to get the depth of defect.

Resonance: Is a characteristic of a vibration body, and a condition of

resonance exists whenever the thickness of material equals half wavelength or
multiples of sound in that material. Earlier thickness measurement was done
by this method using continuous waves. But now a day’s thickness measuring
instruments like “D” meters use Pulse Echo Method only.

76
 CHAPTER – 15
CODES AND REPORTING

Normally all the documents give an outline of:

1. What equipment to use?

2. What scanning procedure to follow?
3. What sensitivity to use?
4. What recording level to use?
5. What defect sizing system to use?
6. What to accept and what to reject?
7. What do we report?

The standard would describe:

1. Equipment
Performance standards are defined for the flaw detectors. The probe
frequency to be used and the range of angles with may be used, are also
defined.

2. Scanning Procedure
The number of scans to be used, and whether they are compression wave
(straight beam) or shear wave (angle beam) scans, is defined as will as the
coverage of material volume to be achieved, and the overlap between scans.

3. Sensitivity
The procedure for setting test sensitivity is related to a basic reference block
made from the same material and of similar thickness and surface condition,
to the work pieces. The distance Amplitude correction (DAC) Curve
represents the reference level at various depths in the specimen. The initial
test sensitivity is then set at twice the reference level (i.e. you add another 6
dB of gain).

4. Recording level
Using the test sensitivity described locates defect indications. The sensitivity
has to be adjusted back to reference level (6 dB down). The defect indications
are then compared with the DAC reference levels. All indications grater than
20% of that reference level must be investigated to determine shape, identify
and location of the defect because cracks are unacceptable regardless of the
signal amplitude. In any case all the indications grater than 50% of reference
level are to be recorded on the Inspection report.

5. Defect Sizing system

Although the method to be used to size defects is not specified in each
standard, most of the acceptance criteria depend upon defect length and a 6
dB drop method of sizing.

77
 6. Acceptance/Rejection Criteria
These are specified in many cases and are generally related amplitude (in
excess of reference level) and defect length (as a function of specimen
thickness). Any indication form a “crack like” defect is unacceptable.

7. Reporting
Even those standards which to not give acceptance criteria (Leaving these to
contractual agreement) do specify reporting level indications in excess of 50%
of the reference level. The other important items to be included in each report
are also specified.

Summary
Most standards describe performance characteristics for ultrasonic
equipment. These are well known to the equipment manufacturer. The
parameters for sensitivity, defect sizing, reporting levels and acceptable levels
are much more of a problem to the Inspector. Some standards and
specifications aim at producing standardization and repeatability of results.
These are the ones which set sensitivity, recording and reporting levels.
Usually these are based on DAC curves, or DGS diagrams, and Known
reference targets.

Reporting
There are several types of Report forms available. There is no fixed pattern as
to how a report should be. But the report should contain the minimum
following information:

1. Description of the component tested.

2. Type of Ultrasonic Flaw Detector
3. Type of Probe (Frequency, Size and Angle)
4. Couplant
5. Calibration blocks and reference standard used
6. Sensitivity setting
7. Standard or specification used
8. Scanning coverage
9. Place of Testing
10. Operator’s name and date
11. Result

78