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UT LEVEL-2 Part-1

Module-1
Sound Modes

Presented by
N.Kuppusamy

Singapore Chapter

NDT HORIZON
1
24-Feb-07
By: N.Kuppusamy

Introduction
This module illustrates the Basic Modes of Sound.
Ultrasonic testing uses high frequency sound energy
to conduct examinations and make measurements.
Sound is produced by vibration or oscillation (Back
and forth movement).
EXAMPLES OF OSCILLATION

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24-Feb-07
By: N.Kuppusamy
Displacement

Vibration is defined as the displacement of mass


about its rest position. It is given by the formula:

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24-Feb-07
By: N.Kuppusamy

Basic Principles of Sound


Sound is produced by a
vibrating body and travels in the
form of a wave.
Sound waves travel through
materials by vibrating the
particles that make up the
material.

The pitch of the sound is determined by the


frequency of the wave (vibrations or cycles Sounds

completed in a certain period of time).


Ultrasound is sound with a pitch too high to be
detected by the human ear.

4
24-Feb-07
By: N.Kuppusamy
Sound Spectrum
Frequency
Description Example
Range, Hz
Infrasound
0-20 Earth Quake
Infrasonic
Audible sound Human
20-20,000 Speech , Music
Hearing Range
>20,000 Ultrasound Bat, Quartz crystal

5
24-Feb-07
By: N.Kuppusamy

States of matter and its structure


Generally [at least as for as we are concerned] matter
exists in three states

Other states include: Plasma state (ionized state of matter), Quark state (A state where the
Proton, & Neutron decompose to quarks)

6
24-Feb-07
By: N.Kuppusamy
7
24-Feb-07
By: N.Kuppusamy

Wave Parts

Introduction to Waves
Wave Parts
The Anatomy of a
Wave and online quiz

8
24-Feb-07
By: N.Kuppusamy
Wave parts

Introduction to
Waves
Wave Parts
The Anatomy of a
Wave and online
quiz

9
24-Feb-07
By: N.Kuppusamy

Basic Principles of Sound


The measurement of sound waves from crest to crest determines its
wavelength ().
Wavelength and Amplitude
The wavelength is the distance
between the "crests" of two waves
that are next to each other. The
amplitude is how high the crests are.
Transverse wave

Compression wave

Wave length is determined by the following relation:

Wave length = Velocity / Frequency

10
24-Feb-07
By: N.Kuppusamy
Basic Principles of Sound
Since the sounds are traveling at about the same
speed, the one with the shorter wavelength will go
by more frequently; it has a higher frequency, or
pitch. In other words, it sounds higher.

Strings

Amplitude is Loudness
The size of a wave (how much it is "piled up"
at the high points) is its amplitude. For sound
waves, the bigger the amplitude, the louder
the sound.

11
24-Feb-07
By: N.Kuppusamy

Basic Principles of Sound


The time is takes a sound wave to travel a distance of one complete
wavelength is the same amount of time it takes the source to execute
one complete vibration.

The sound wavelength


is inversely proportional
to its frequency. ( = 1/f)

The velocity of Longitudinal, shear and surface waves are fixed for a
given material. The velocity of sound in each material is determined
by the material properties (elastic modules and density) of that
material.

12
24-Feb-07
By: N.Kuppusamy
Basic Principles of Sound
Several wave modes of vibration E
are used in ultrasonic inspection. VL =
U
The most common are
E = Youngs modulus of elasticity
longitudinal, shear, and Rayleigh
(surface) waves and Plate (Lamb) U = material density
waves.

Longitudinal /
Compression
Waves

Longitudinal waves are waves in which the motion of the particles in the
medium is in the same (or opposite) direction to the wave propagation.
In longitudinal waves, the particles of the medium move back and forth
creating regions of high and low density (or high or low pressure).
It exists in all material forms (Solid, Liquid and Air)
13
24-Feb-07
By: N.Kuppusamy

Longitudinal Waves

Longitudinal Waves The animation


shows a one-dimensional longitudinal
plane wave propagating down a tube. The
particles do not move down the tube with
the wave; they simply oscillate back and
forth about their individual equilibrium
positions.

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24-Feb-07
By: N.Kuppusamy
Wave Propagation & Particle

Water waves are an example of waves that involve a combination of


both longitudinal and transverse motions. As a wave travels through
the waver, the particles travel in clockwise circles.
The radius of the circles decreases as the depth into the water
increases. The movie below shows a water wave traveling from left
to right in a region where the depth of the water is greater than the
wavelength of the waves.

15
24-Feb-07
By: N.Kuppusamy

Wave Propagation & Particle Motion


Shear / Transverse Waves: In a transverse wave the particle
displacement is perpendicular to the direction of wave propagation.
Waves on a string are transverse waves. The animation below shows a
one-dimensional transverse plane wave propagating from left to right.

G
Shear wave velocity for a given VT =

material is nearly 50% of


U
longitudinal velocity in that material.
G = Shear modulus of material
It exists only in solid mediums. U = material density

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24-Feb-07
By: N.Kuppusamy
Wave Propagation & Particle Motion
Rayleigh surface waves are the waves with both longitudinal and
transverse motion found in solids.
The particles in a solid, through which a Rayleigh surface wave passes,
move in elliptical paths, with the major axis of the ellipse perpendicular to
the surface of the solid.
As the depth into the solid increases the "width" of the elliptical path
decreases.
Rayleigh waves are different from water waves in one important way. In a
water wave all particles travel in clockwise circles. However, in a Rayleigh
surface wave, particles at the surface trace out a counter-clockwise
ellipse, while particles at a depth of more than 1/5th of a wavelength trace
out clockwise ellipses.
Its velocity is approximately 90% of shear wave in a given material

Rayleigh waves are reflected from a sharp


edge or corner. But, it continues to travel
around smooth curvatures and rounded
corners.

Rayleigh wave
motion

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24-Feb-07
By: N.Kuppusamy

Wave Propagation & Particle Motion


Rayleigh or surface waves

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By: N.Kuppusamy
Lamb waves
If a surface wave is introduced into a material that has a thickness equal to
three wavelengths, or less, of the beam, a different kind of wave results. The
material begins to vibrate as a plate; i.e., the wave encompasses the entire
thickness of the material.
When this occurs, the normal rules for wave velocity along the plate break
down. The velocity is no longer dependent upon the type of material and the
type of wave. Instead, we get a wave velocity that is dependent on the
frequency of the wave, the angle of incidence, and, of course, the type of
material
There are two general types of lamb (or plate) waves depending on the way
the particles in the material move as the wave moves along the plate.

Symmetrical & Asymmetrical Lamb Waves

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24-Feb-07
By: N.Kuppusamy

Lamb waves Symmetric


Each type of Lamb wave has an infinite number of modes that the wave may attain.
These modes, too, are dependent on the three factors of the frequency of the wave,
the angle of incidence, and the material.
These modes are differentiated by the manner in which the particles in the material
are moving.

N.Kuppusamy
20
24-Feb-07
By: N.Kuppusamy
Lamb waves Asymmetric

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By: N.Kuppusamy

Basic Principles of Sound


Ultrasonic reflections from the presence of
discontinuities or geometric features enables detection
and location.
The velocity of sound in a given material is constant and
can only be altered by a change in the mode of energy
or change of part temperature.

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By: N.Kuppusamy
Temperature and the speed of sound
Temperature is also a condition that affects the speed of sound.
Heat, like sound, is a form of kinetic energy. Molecules at higher
temperatures have more energy, thus they can vibrate faster.
Since the molecules vibrate faster, sound waves can travel more
quickly. The speed of sound in room temperature air is 346
meters per second. This is faster than 331 meters per second,
which is the speed of sound in air at freezing temperatures.
The formula to find the speed of sound in air is as follows:
Sound Temperature

v = 331m/s + .6m/s/C * T
v is the speed of sound and T is the temperature of the air. One
thing to keep in mind is that this formula finds the average speed
of sound for any given temperature. The speed of sound is also
affected by other factors such as humidity and air pressure.

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24-Feb-07
By: N.Kuppusamy

Interactive sites which allow you to observe and manipulate


transverse and longitudinal waves. Each site offers its own
uniqueness

JAVA APPLET Wave Types Transverse and Longitudinal - This


java applet let you visualize the difference between transverse
wave and longitudinal wave.
Transverse Wave and Longitudinal Waves this interactive site
allows you to examine both types of waves
Longitudinal, Transverse and Mixed Type Waves this site allows
you to examine and manipulate both types of waves and a mixture
of both waves

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24-Feb-07
By: N.Kuppusamy
Theory of Ultrasonic Testing
Module-2
Sound Properties
Presented by
N.Kuppusamy

Singapore Chapter

NDT HORIZON
1
24-Feb-07
By: N.Kuppusamy

Frequency
Sounds can be low or high. Sounds can be low like a growling
tiger or high like a chirping bird. This characteristic of sound is
called pitch or frequency. Objects which vibrate faster produce a
higher frequency, and objects which vibrate more slowly produce a
lower frequency.
The frequency of a sound is equal to how many times it vibrates
each second. Vibrations per second are measured in Hertz (Hz).

An object that vibrates 1 time each second would have a frequency


of 1 Hertz (Hz).

An object that vibrates 5 times each second would have a


frequency of 5 Hertz (Hz).

2
24-Feb-07
By: N.Kuppusamy
Surfing the Waves
Imagine that you are floating on a surfboard, and waves
are going past you. As each wave passes, you rise and
fall.
The frequency in this case is the number of times per
second you bob up and down. (Obviously, it will be less
than once per second with ocean waves, so the frequency
in this case will be a less than one Hertz.)

Ocean Frequency

3
24-Feb-07
By: N.Kuppusamy

Basic Principles of Sound

Ultrasonic waves are very similar to light waves


in that they can be reflected, refracted, and
focused.
Sound requires a medium to vibrate (propagate)
whereas light doesnt.

Because Electromagnetic radiation is a


combination of oscillating electric and
magnetic fields moving through a medium
perpendicular to each other through
space and carries energy from one place
to another.

4
24-Feb-07
By: N.Kuppusamy
Basic Principles of Sound
Reflection and refraction occurs when sound
waves interact with interfaces of differing
acoustic properties.
In solid materials, the vibrational energy can
be split into different wave modes when the refraction
wave encounters an interface at an angle
other than 90 degrees.
The angle of reflection and refraction are
governed by Snells law.

Reflection and
Refraction

Echo

5
24-Feb-07
By: N.Kuppusamy

Reflection and Refraction


Sin i = Vi
Snells Law:
Sin r Vr

Both reflection and refraction are governed by Snells law and it holds true for both
longitudinal and shear waves.

Reflection : Angle of Reflection is equal to incident angle.

Refraction : Angle of refraction is a function of incident angle and velocity ratio


between incident and refractive mediums.

i = incident angle i r
r = reflected angle Medium 1
r1 = refracted angle
Medium 2 r1

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24-Feb-07
By: N.Kuppusamy
Reflection

When a longitudinal wave is reflected inside the


material, the reflected shear wave is reflected at a
smaller angle than the reflected longitudinal wave.

This is due to the fact that the shear velocity is less


than the longitudinal velocity within a given material.
Reflection and
Sound Reflection
i1 = r1 i1 r2 r1 Refraction

i1 > r2 Medium
r2 < r1

7
24-Feb-07
By: N.Kuppusamy

Refraction
Refraction is the bending of waves when they enter a medium where their speed is
different. Refraction is an important phenomena with in ultrasound. This property is
used to generate shear wave in the second medium.

As a toy car rolls from a


Fast Slow Hard floor onto carpet,
Medium Medium It changes direction
As a column of
marching troops Because the wheel that
crosses from a fast Hits the carpet first is
medium to a slow Slowed down first.
medium,
the direction
Visualizations
of march
of Reflection
changes
Slow
Fast medium
Concrete Swamp medium

Another visualization of refraction can come from the steering of various types of
tractors, construction equipment, tanks and other tracked vehicle. If you apply the right
brake, the vehicle turns right because you have slowed down one side of the vehicle
without slowing down the other.

8
24-Feb-07
By: N.Kuppusamy
Refraction
Refraction takes place at an interface due to the different velocities of the acoustic
waves within the two materials.

When a longitudinal wave is refracted into a material, the refracted shear wave
angle is smaller than the refracted longitudinal wave.

This is due to the fact that the shear velocity is less than the longitudinal velocity
within a given material.

Please remember that some of the wave energy is always reflected at the interface

i
Medium 1
r1 < r2
Medium 2 r1 r2
L-wave

Shear wave

9
24-Feb-07
By: N.Kuppusamy

Mode Conversion
When sound travels in a solid material, one form of wave energy can be
transformed into another form.

For example, when a longitudinal waves hits an interface at an angle,


some of the energy can cause particle movement in the transverse
direction to start a shear (transverse) wave.

Mode conversion, occurs when a wave encounters an interface between


materials of different acoustic impedance and the incident angle is not Mode conversion1
normal to the interface.

Mode conversion can occur in both reflective and refractive mediums.

Mode conversion occurs every time a wave encountered interface at an


angle, ultrasonic signals can become confusing at times
Mode conversion

10
24-Feb-07
By: N.Kuppusamy
Diffraction, Scattering and Reflection
When a wave encounters a reflector within a medium, one of
the following occurs:
Diffraction occurs when the sound wave length is larger
than the reflector size (this condition prevails at the
edges of a discontinuity) [O > Reflector size].
Scattering occurs when the sound wave length is about the
same size of reflector [O # Reflector size].
Reflection occurs when the sound wave length is smaller
than the reflector [O < Reflector size].

11
24-Feb-07
By: N.Kuppusamy

Diffraction
Diffraction: the bending of waves around small* obstacles and the
spreading out of waves beyond small* openings. (* small compared to the
wavelength)
When a wave encounters a point reflector (small in comparison to a wave-
length), the reflected wave is re-radiated as a - spherical wave front.
When a plane wave encounters the edges of reflective interfaces, such as
near the tip of a fatigue crack, specular (mirror like) reflections occur along the
"flat" surfaces of the crack and cylindrical wavelets are launched from the
edges.
Their redirection into the path of subsequent advancing plane waves results in
incident and reflected (scattered) waves interfering, i.e., forming regions of
reinforcement (constructive interference) and cancellation (destructive
interference).

A plane wave is one in which quantities vary only with the distance along a certain
direction, and with the time.

12
24-Feb-07
By: N.Kuppusamy
Diffraction

Sound If you were outside an open


waves door, you could still hear
because the sound would
spread out from the small
opening as if it were a localized
source of sound.

Sound Diffraction
source Around post
Diffraction
Past small
Suppose you bought a opening
concert ticket without looking
at the seating chart and wound up
sitting behind a a large post. You
would be able to hear the concert If you were several wavelengths
quite well because the wavelength of sound past the post, you would
of sound are long enough to bend not be able to detect the presence
around the post. of the post from the nature of the
sound.

13
24-Feb-07
By: N.Kuppusamy

Diffraction of Sound

Important parts of our experience with sound involve diffraction. The


fact that you can hear sounds around corners and around barriers
involves both diffraction and reflection of sound. Diffraction in such
cases helps the sound to "bend around" the obstacles. The fact that
diffraction is more pronounced with longer wavelengths implies that
you can hear low frequencies around obstacles better than high
frequencies, as illustrated by the example of a marching band on the
street.
Another common example of diffraction is the contrast in sound from
a close lightning strike and a distant one. The thunder from a close
bolt of lightning will be experienced as a sharp crack, indicating the
presence of a lot of high frequency sound. The thunder from a distant
strike will be experienced as a low rumble since it is the long
wavelengths which can bend around obstacles to get to you. There are
other factors such as the higher air absorption of high frequencies
involved, but diffraction plays a part in the experience.

14
24-Feb-07
By: N.Kuppusamy
Critical Angles
There is an incident angle at which the angle of refraction
of the longitudinal wave is 90 degrees (i.e.,parallel to
surface). This is called First Critical Angle.
The incident angle at which the angle of refraction for the
shear wave is 90 degrees, is known as the second critical
angle.
At this point, all of the wave energy is reflected or
refracted into a surface following shear wave or shear
creep wave.
Slightly beyond the second critical angle, surface waves
will be generated.

15
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By: N.Kuppusamy

Creep Waves
At the first critical angle of
incidence, much of the acoustic
energy is in the form of an
inhomogeneous compression wave,
which travels along the interface
and decays exponentially with
depth from the interface.

This wave is sometimes referred to as a "creep wave." They


are similar to water waves.
Because of their inhomogeneous nature and the fact that
they decay rapidly, creep waves are not used as extensively
as Rayleigh surface waves in NDT.
However, creep waves are sometimes useful because they
suffer less from surface irregularities and coarse material
microstructure, due to their longer wavelengths, than
Rayleigh waves.

16
24-Feb-07
By: N.Kuppusamy
Attenuation
Sound waves decrease in intensity and amplitude as they
travel away from their source, due to geometrical spreading,
scattering, and absorption.
Loss of energy due to absorption and scattering is known as
attenuation and it is measured in dB/m or dB/mm.
This loss is proportional to the grain volume in the material
and inversely proportional to the wavelength (1/O the beam.
It is also expressed in nepers (Np) per unit length.
1 dB/cm = 8.686 NP/cm.

17
24-Feb-07
By: N.Kuppusamy

Fine and coarse grained steel


at the same magnification
Fine grained steel Coarse grained steel

18
24-Feb-07
By: N.Kuppusamy
Grains
Grey iron Spheroidal graphite iron

19
24-Feb-07
By: N.Kuppusamy

Attenuation
A decaying plane wave is expressed as:

In this expression A0 is the amplitude of the propagating wave at


some location. The amplitude A is the reduced amplitude after
the wave has traveled a distance z from that initial location. The
quantity is the attenuation coefficient of the wave traveling in
the z-direction. The dimensions of are nepers/length, where a
neper is a dimensionless quantity. e is Napier's constant which is
equal to approximately 2.71828.
The units of the attenuation value in nepers/length can be
converted to decibels/length by dividing by 0.1151. Decibels is a
more common unit when relating the amplitudes of two signals.

20
24-Feb-07
By: N.Kuppusamy
Attenuation
Attenuation is generally proportional to the square of
sound frequency. Quoted values of attenuation are often
given for a single frequency, or an attenuation value
averaged over many frequencies may be given. Also, the
actual value of the attenuation coefficient for a given
material is highly dependent on the way in which the
material was manufactured.

Thus, quoted values of attenuation only give a rough


indication of the attenuation and should not be
automatically trusted. Generally, a reliable value of
attenuation can only be obtained by determining the
attenuation experimentally for the particular material
being used.

21
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By: N.Kuppusamy

Attenuation
Generally defined as loss of amplitude over the distance
traveled in total transit time (i.e., 2T in pulse echo testing)
There are many factors which accounts for the amplitude
loss. The amplitude loss due to beam divergence has to be
taken into account when calculating attenuation in the far
zone.
i.e., Amplitude difference = Beam spread - Attenuation
Generally in the far zone, doubling the distance reduces the
back echo by half or 6dB due to beam spread.
? Attenuation in the far zone (i.e., when the NF is < thickness)
dBdifference 6
= dB/inch or dB/m
2T
Attenuation in the near field (i.e., when the NF is > thickness)
dBdifference
= dB/inch or dB/m
2T

22
24-Feb-07
By: N.Kuppusamy
Attenuation
Attenuation can be determined by evaluating
the multiple backwall reflections seen in a
typical A-scan display like the one shown in
the image.

The number of decibels between two adjacent signals is


measured and this value is divided by the time interval (or
distance) between them.
This calculation produces a attention coefficient in decibels
per unit time Ut (or dB per unit distance). This value can be
converted to nepers/length by the following equation.

0.1151 Where v is the velocity of sound in meters


D Ut per second and Ut is decibels per second
v
23
24-Feb-07
By: N.Kuppusamy

Geometrical Spreading
Inverse Square Law
As one moves further from a source of spherical waves, the amplitude of the
sound at your location gets less. The intensity I is the power W in the wave
divided by the area A over which it is spread: I = W/A or W/4 r2
Where, A = 4 r2.
In the absence of absorption,
the intensity of spherical sound
waves decays as 1/r 2
The amplitude (sound pressure)
of a traveling simple spherical
wave is proportional to the
square-root of its intensity.
Therefore in the absence of
absorption, the pressure
amplitude of spherical sound
waves decays as 1/r .

24
24-Feb-07
By: N.Kuppusamy
Scattering
Microscopic reflections in directions other
than its original direction of propagation is
called Scattering.
Scattering losses are greatest where the
wavelength is less than one-third the grain
size.
Scattering is a more difficult problem, than
absorption and occurs when the ultrasonic
High scattering
beam encounters small, randomly oriented
reflectors in the material.
These reflectors may be grain boundaries,
microscopic voids or particles that scatter
the incoming wave.

Scattering Low scattering

25
24-Feb-07
By: N.Kuppusamy

Scattering
Scattering can make the trace unreadable, and cause
discontinuities to be missed.
As scattering is caused by a multitude of small reflectors, the trace will display
a random collection of small peaks, which together may be so large as to make it
difficult to distinguish real discontinuities within this noise.

The presence of a small amount of grass at the base of


the trace is generally an indication that the sound S/N
energy is coupled to the test object.
Once this grass exceeds about 10% full screen height
(FSH), however, it is known as material noise and makes
discrimination difficult between natural scattering and
discontinuities. Normally, you need to have a signal to
noise (S/N) ratio as high as possible, and at least 3:1 for
reliable detection.

The ability to get a good S/N ratio is important, but should be


approached with caution.
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By: N.Kuppusamy
Absorption
Absorption: The conversion of sound to other forms of energy.
Absorption occurs when the ultrasonic energy is physically converted
into heat within the material.
Energy is taken from the beam, so of course the returning signals have
less energy, and appear smaller on the UFD screen. This can generally
be overcome by increasing amplification to compensate for the losses.
As the frequency is lowered and the wavelength becomes greater than
the grain size, attenuation is due only to damping of the wave. In
damping losses, wave energy is lost through heat due to friction of the
vibrating particles.
Absorption is used to advantage in medical ultrasonic therapy, which
intentionally produces considerable amounts of heat in human tissue to
aid in recovery from injury

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24-Feb-07
By: N.Kuppusamy

Approximate attenuation
characteristics

Attenuation Range
Metals Non Metals Max Testable Thickness
at 2 MHz

Low Attenuation Cast and Wrought Aluminum Glass,


1 10 metre
Up to 10 dB/m Wrought Steel Porcelain
Cast Steel, SG Iron
Medium Attenuation Perspex,
Wrought Copper, Brass, 0.1 1 metre
10 100 dB/m PVC
Lead
High Attenuation Porous Ceramics,
Grey Iron 100 mm
> 100 dB/m Rocks

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24-Feb-07
By: N.Kuppusamy
Dealing with Absorption and Scatter
Increasing amplification may help to overcome
absorption
Although the material is difficult to test, proper
attention to the attenuation characteristics can result in
a valid test.
The first reaction to dealing with attenuating materials is
generally to increase the gain (amplification) of the
instrument to compensate for the energy loss.
This will compensate for basic absorption, but will not
help when faced with scattering. Lower frequencies also
act to reduce absorption effects.

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24-Feb-07
By: N.Kuppusamy

Dealing with Absorption and Scatter


Increasing amplification does not help with scatter
With scattering, much of the scattered beam will be sent back
to the receiver and will be detected, giving rise to an
apparently random set of indications, (material noise), often
referred to as grass (or hash in American terminology).
If excessive amplification is used, the grass becomes
excessive, and the screen display becomes unmanageable.
A similar effect occurs when driving in fog putting the
headlights on high beam results in the driver being dazzled
by the reflections from the fog droplets, and does not improve
visibility.

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Dealing with Absorption and Scatter
Frequency selection will increase tolerance of scattering
As attenuation is greater at short wavelengths (high
frequencies), high attenuation materials are usually
examined using low frequencies, typically 1 to 2 MHz.
Some experimentation may be required to find the
optimum frequency, by progressively decreasing the
frequency until a usable frequency is found.
To continue our analogy of driving in fog, using a lower
frequency is like using fog lights that operate with a
lower optical frequency that is, a colour closer to the
red end of the visible spectrum.

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24-Feb-07
By: N.Kuppusamy

Typical maximum test ranges for compression mode


Frequen Fine Grained Coarse Grained SG
Grey Iron
cy Steel Steel Iron
(mm)
(MHz) (mm) (mm) (mm)

5 200 100 100 25


2 3,000 750 1,000 250
1 5,000 1,500 1,500 400

These are typical ranges. In practice, maximum range will depend


on the probe design, equipment, pulse strength, probe diameter and
specific material grain structure.
For shear waves, which have approximately half the wavelength,
the maximum shear wave ranges are approximately equal to a
compression wave of twice the frequency in the table above. For
example 2 MHz shear has a similar test range to 4 MHz
compression.
The improved penetration at low frequencies is obtained at the
expense of reduced sensitivity to smaller discontinuities
32
24-Feb-07
By: N.Kuppusamy
Dealing with Absorption and Scatter
Increased pulse energy can sometimes help when
testing longer ranges
Some instruments are able to produce a longer duration
pulse to put more energy into the test piece. This facility may
be useful in dealing with materials of moderate attenuation,
but suffers from a similar response to excess amplification.
Increased pulse energy also results in a loss of resolution.

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24-Feb-07
By: N.Kuppusamy

Scattering puts practical limits on examination

The higher the attenuation of a material, the lower


the maximum thickness that can be reliably
examined.
Australian standards AS1065 (forgings) and
AS2574 (castings) place upper limits on the
attenuation of the material under test.

34
24-Feb-07
By: N.Kuppusamy
Acoustic Impedance
Sound travels through materials under the influence of sound
pressure. Because molecules or atoms of a solid are bound
elastically to one another, the excess pressure results in a wave
propagating through the solid.
The acoustic impedance (Z) of a material is defined as the
product of density (p) and acoustic velocity (V) of that material.
Z = pV
Acoustic impedance is important in
1. the determination of acoustic transmission and reflection at
the boundary of two materials having different acoustic
impedance
2. the design of ultrasonic transducers.
3. assessing absorption of sound in a medium.

35
24-Feb-07
By: N.Kuppusamy

Amount of Energy Reflected


The reflected energy in terms of Pressure
(Amplitude) is the difference divided by the Z 2  Z1
sum of the acoustic impedances of the two R
materials. Z 2  Z1
100% + R = Transmission

The reflected energy in terms of intensity


(power) is the square of the difference 2
divided by the sum of the acoustic impedances Z 2  Z1
of the two materials. Note that Transmitted R
Sound Energy + Reflected Sound Energy = 1 Z 2  Z1
Amplitude Intensity for Reflection only.
T + R = 100%

Applet for Energy transmitted


36
24-Feb-07
By: N.Kuppusamy
Reflection and Transmission Coefficients (Pressure)
Ultrasonic waves are reflected at boundaries where there are
differences in acoustic impedance, Z. This is commonly referred to
as impedance mismatch. The fraction of the incident-wave intensity
in reflected waves can be derived because particle velocity and
local particle pressures are required to be continuous across the
boundary between materials.
Formulation for acoustic reflection and transmission coefficients
(pressure) are shown in the interactive figure below. Different
materials may be selected or you may alter the material velocity or
density to change the acoustic impedance of one or both materials.
The red arrow represents reflected sound, while the blue arrow
represents transmitted sound.

Applet for energy transmitted

37
24-Feb-07
By: N.Kuppusamy

Amount of Energy Transmitted

`
The reflected energy in terms 2Z 2
of Pressure (Amplitude) is given T (1  R)
by Z 2  Z1

The amplitude is no longer true to say that T=100-R. Under certain


circumstances there may be transmission of more than 100% and it is not
important from which side the wave approaches the boundary because Intensity
and Amplitude are still connected through Z

The reflected energy in terms of


intensity is given by ` T (1  R )
4 Z 2 .Z1
Z 2  Z1 2
Applet for energy transmitted

38
24-Feb-07
By: N.Kuppusamy
Negative Reflection Coefficient
(Reflection from a HARD boundary)
When R is negative (-), which indicates phase reversal after
reflection
As the wave pulse approaches the fixed rigid end,
the internal restoring forces which allow the wave
to propagate exert an upward force on the end of
the string.
But, since the end is clamped, it cannot move.
According to Newton's third law, the wall must be
exerting an equal downward force on the end of
the string. This new force creates a wave pulse
that propagates from right to left, with the same
speed and amplitude as the incident wave, but
with opposite polarity (upside down).

At a fixed (hard) boundary, the displacement remains zero and the reflected wave
changes its polarity (undergoes a 180o phase change)

39
24-Feb-07
By: N.Kuppusamy

Positive Reflection Coefficient


(Reflection from a Soft boundary)
When R is positive there is no phase reversal takes place
after reflection
When a sound wave approaches a soft boundary
(metal-fluid), The soft boundary permits it to move
upward. The net vertical force at the free end is zero.
The reflected wave pulse propagates from right to left,
with the same speed and amplitude as the incident
wave, and with the same polarity (right side up).

At a free (soft) boundary, the restoring force is zero and the reflected wave has the
same polarity (no phase change) as the incident wav

40
24-Feb-07
By: N.Kuppusamy
Other Mediums
From high speed to low speed From low speed to high speed
(low density to high density) (high density to low density)

Tension
Speed of waves on a string
Density
41
24-Feb-07
By: N.Kuppusamy

Energy Reflected/Transmitted
Note that the energy reflected at a water steel interface is 0.88 or 88%.
0.12 or 12% is transmitted into the component. If reflection and
transmission at interfaces is followed through the component, and loss
by attenuation is ignored, a small percentage of the original energy
returns to the transducer.
Assuming acoustic energy at the transducer is 100% and energy
transmitted into a component at a water steel interface is 12% as
discussed above. At the second interface (back surface) 88% or
10.56% would be reflected and 12% transmitted into the water. The
final interface would allow only 12% of 10.56 or 1.26% of the original
energy to be transmitted back to the transducer.

42
24-Feb-07
By: N.Kuppusamy
Variation of Acoustic Pressure with angle

Variation of acoustic pressure


with angle of reflection or
refraction during immersion
ultrasonic inspection of Aluminum.
The acoustic pressure of the
incident wave 1.0 arbitrary unit.
Points A and A correspond to the
first critical angle, and point B to
the second critical angle for this
system.

43
24-Feb-07
By: N.Kuppusamy

Relative Amplitude in Steel

Longitudinal, Shear and Surface Wave Modes


with Changing Plastic Wedge Angle.

The picture shows the first critical angle in plastic for steel is
approximately 30 degrees; the second critical angle is approximately 56
degrees.
Incident angles useful for shear-wave NDI fall between the two critical
angles. The amplitude of Shear wave increases with incident angle while
the longitudinal wave amplitude decreases.

44
24-Feb-07
By: N.Kuppusamy
Relative Amplitude
Partition of acoustic energy at
water steel interface.
The Reflection coefficient, R,
is equal to 1-(L+S).
Where, L is the transmission
coefficient of Longitudinal
wave and S is the transmission
coefficient of Shear wave.

45
24-Feb-07
By: N.Kuppusamy

Summary
Attenuation occurs by absorption and scattering.
Absorption can often be managed by use of lower frequency,
increased pulse energy or additional amplification.
Scattering is managed by using lower frequencies and minimizing
the beam path length where possible.
The decibel (dB) notation is a convenient way of measuring and
comparing echo amplitude over a very wide range.
Attenuation properties may be expressed as attenuation
coefficients (dB/mm), and are influenced by metallurgical
condition, homogeneity and probe frequency.

46
24-Feb-07
By: N.Kuppusamy
Test of Reasoning
You are testing some forgings and you suddenly notice
that there are lots of small, apparently irrelevant indications
on the screen. Your more experienced fellow technician
says its just grass and to turn the gain up. What is your
colleague referring to, and should you blindly follow his
advice?
You have been injured in a football match and youre
having ultrasonic treatment at the physiotherapist. Are your
tissues mainly absorbing or scattering the ultrasonic
waves?

47
24-Feb-07
By: N.Kuppusamy

Points to Ponder
Why does attenuation increase with probe frequency?
How would you expect the attenuation of compression and
shear waves of the same frequency to compare?
Why is the sunset red in colour?
(Higher energy waves attenuated by the atmosphere due
to increased viewing distance)

48
24-Feb-07
By: N.Kuppusamy
Phase
Phase relates the vibration to time. When two vibrations are
in phase, it is called constructive phase (peak peak or valley
valley). Both waves augment each other and resultant wave is
more in amplitude.
When two vibrations are in opposite phase (peak valley), they
cancel out each other and the resultant amplitude is zero.

CONSTRUCTIVE DESTRUCTIVE DIFFENTIAL


INTERACTION INTERACTION INTERACTION

49
24-Feb-07
By: N.Kuppusamy
Theory of Ultrasonic Testing
Module-3
Decibel

Presented by
N.Kuppusamy

Singapore Chapter

NDT HORIZON
By: N.Kuppusamy

Decibel Notation
The unit of Sound is Bel, which is much bigger
quantity for normal use. Therefore we use smaller
unit called decibel (dB).
In ultrasonics the attenuation characteristics of a
given material are expressed in terms of an
attenuation coefficient which has units of decibels
per metre or dB/mm, so we need to understand
decibel notation.
If you are not familiar with logarithms, now would be
a good time to learn about them.

By: N.Kuppusamy Event Horizon


The ultrasonic flaw detector uses
decibels to measure attenuation
The most immediately obvious means of measuring
the relative pressure of the sound wave is through
its echo amplitude.
If one echo has an amplitude of 100% FSH and
another has an amplitude of 50% FSH, the first
can be said to have twice the acoustic pressure of
the second.

By: N.Kuppusamy Event Horizon

Need of Smaller Unit, dB


In ultrasonics we need to work over a very large range of
amplitudes. While it is easy to compare large screen heights, it
is difficult to compare small screen heights.
If we want to compare a 10% echo with a 5% echo, the
readability of the screen makes it impossible to make an
accurate comparison is it 4%, 5%, or even 6%? The inaccuracy
of such a comparison is too large.
To improve the useable range, most UFDs are equipped with a
calibrated gain control (sometimes called an attenuator in the
US) to allow more accurate comparisons. The gain control is
calibrated in decibels (dB).

By: N.Kuppusamy Event Horizon


Decibel notation is used for comparing
signals conveniently over a wide range

The Bel is a unit for


comparing the power of two
signals by measuring their
ratio.
W1
If we measure two signals
and they have powers W1
1bel log
and W2 respectively, the bel W2
is a convenient way of
comparing them.

By: N.Kuppusamy Event Horizon

Comparing signals (contd)

The result of this calculation is the relative power in Bel.


The decibel (dB) unit is one-tenth of a Bel, so any
measurement expressed in decibels will be ten times the
same measurement expressed in Bels:

W
1decibel 1db 10 log 1
W2

By: N.Kuppusamy Event Horizon


Comparing signals (contd)
In ultrasonics we are concerned with
measurements of sound pressure, not
dB 10 log
p1
2

power. So we need an expression of p2 2


decibels in terms of pressure. As
2
power is proportional to the pressure p
squared, we have: dB 10 log 1
p2
When comparing two amplitudes on
the screen, the amplitude is a p
dB 20 log 1
measure of sound pressure. p2

To determine the dB equivalent, measure each amplitude,


find the ratio, take the log, then multiply by 20.

By: N.Kuppusamy Event Horizon

Comparison of two amplitudes


Amplitude 1 Amplitude 2 Ratio dB =
log(A1/A2)
%FSH %FSH (A1/A2) 20log(A1/A2)

100 10 10 1 20
100 50 2 0.3 6
100 25 4 0.6 12
80 40 2 0.3 6
50 25 2 0.3 6
100 1 100 2 40
100 0.1 1000 3 60

Some interesting points from this table are:


Large
Many
The
A
Many
6
20dBvariations
dB
UFD
dB
UFD
signal
values
signal
units
unitsin
isof amplitude
isone
have
have
any
onethat can
coarse
signal
that
fineisis be
steps
twice
is easily
steps
10not
times2measured
inanother
an
in absolute
dB
20
another,accurately
intervals,
dBandintervals,
measurement
isand
also
which using
is commonly
awhich a it is
commonly
calibrated
corresponds
always
used
corresponds
in
value gain
relativecontrol.
ultrasonics.
into
to Ifof
ultrasonics.
ratios
to
a ratio
some for
of instance,
other
10:1
1.25:1 you want
between
reference,
between to
thethe
eg.accurately
coarse
the
steps. compare
response
steps. a very
from a
strong (100% FSH) and very weak (1% FSH) signal, you can simply adjust the
backwall or drilled hole.
calibrated gain for each signal so that the signal reaches the same screen
height. Then measure the gain difference to give an accurate comparison.
By: N.Kuppusamy Event Horizon
Some interesting points from this table are:
1. The dB values of any signal is not an absolute measurement it
is always relative to some other reference, eg. the response
from a backwall or drilled hole.
2. A 20 dB signal is one that is 10 times another, and is a
commonly used value in ultrasonics.
3. A 6 dB signal is one that is twice another and is also commonly
used in ultrasonics.
4. Many UFD units have coarse steps in 20 dB intervals, which
corresponds to ratios of 10:1 between the coarse steps.

By: N.Kuppusamy Event Horizon

Some interesting points from this table are:


5. intervals, which corresponds to a ratio of 1.25:1 between the
steps.
6. Large variations in amplitude can be easily measured
accurately using a calibrated gain control. If for instance,
you want to accurately compare a very strong (100% FSH)
and very weak (1% FSH) signal, you can simply adjust the
calibrated gain for each signal so that the signal reaches the
same screen height. Then measure the gain difference to
give an accurate comparison.
The use of dB is common in many other applications. We
often see the silencers on noisy equipment being given a
noise reduction rating. For instance, if it has a rating of 40
dB, the noise power reduction is 100 fold; if the rating is 80
dB, the noise reduction is 10,000 fold.

By: N.Kuppusamy Event Horizon


Self Test
1. If the noise reduction rating on a compressor is 80
dB (10,000:1) and you want to double the noise
reduction to 20,000:1, how many additional decibels
of noise reduction would you need?
a. 6 dB
b. 20 dB
c. 40 dB Answer: a
d. 80 dB 20log(20,000) = 86.02 dB,
therefore an extra 6 dB is
needed.

By: N.Kuppusamy Event Horizon

Decibel value
The decibel value of a signal is positive if
greater than the reference and negative if
less than the reference
When the amplitude in question is greater than
the reference, it is said to have a positive gain
relative to the reference. When the amplitude is
less than the reference, it is said to have a
negative gain (or a positive attenuation) relative
to the reference.

By: N.Kuppusamy Event Horizon


Example
If you have a reference signal at 50% and an
unknown signal at 100%, the unknown signal is said to
have a positive gain of 6 dB.
If you have a reference gain of 50% and an unknown
signal of 25%, the unknown signal is said to have a
negative gain of 6 dB, or an attenuation of 6 dB.

By: N.Kuppusamy Event Horizon

Some typical dB ratios relative to 100% FSH

Attenuation or Attenuation or
Amplitude Amplitude
Negative Gain Negative Gain
(%) (%)
(dB) (dB)

100 0 40 8.0
95 0.5 32 10.0
89 1.0 25 12.0
84 1.5 20 14.0
79 2.0 16 16.0
71 3.0 12.5 18.0
63 4.0 10 20.0
56 5.0 3.2 30.0
50 6.0 1.0 40.0
45 7.0 0.1 60.0
0.01
By: N.Kuppusamy Event Horizon 80.0
Practice
Establish an echo from a convenient back wall
and adjust the gain such that the signal is at
100% FSH.
Make sure the suppression (reject) is turned off.
Note the gain setting (dB).
Reduce the gain a total of 20 dB in 2 dB steps
and note the screen height for each step.
Compare the theoretical and actual screen
heights.

By: N.Kuppusamy Event Horizon

Quick Decibel Calculations


It is possible to calculate many dB equivalents if you
know that 6 dB represents a ratio of 2:1 and 20 dB
represents 10:1. The trick is to realize that addition
of decibel values corresponds to multiplication of
ratios, and subtraction of decibel values corresponds
to division of ratios. For example, to determine the
ratio equivalent to 12 dB, we note that 12 = 6 + 6.

By: N.Kuppusamy Event Horizon


Quick Decibel Calculations
Changing to ratios, the 6 dB value becomes 2, and
the addition becomes multiplication. We therefore
have a ratio equivalent of 2 times 2 = 4. That is, 12
dB means a ratio of 4:1, or a quadrupling with
respect to some reference value.

Decibels : 12dB 6  6
p p p
Ratios : 2 x 2 4

By: N.Kuppusamy Event Horizon

Quick Decibel Calculations

Here is another example, where we find that 14 dB is


equivalent to a ratio of 5:1.

Decibels : 14d 20dB  6dB


p p p
10 y 2 5

By: N.Kuppusamy Event Horizon


Work out the following examples

Table of decibel and ratio breakdowns

dB dB Breakdown Ratio Breakdown Ratio


12
14
8
30
26
-8
-12

By: N.Kuppusamy Event Horizon

Table of decibel and ratio


breakdowns

dB dB Breakdown Ratio Breakdown Ratio


12 6 dB + 6 dB 22 4:1
14 20 dB 6 dB 10 2 5:1
8 20 dB 6 dB 6 dB 10 2 2 2.5:1
30 6 dB + 6 dB + 6 dB + 6 dB + 6 dB 2 2 2 2 2 32:1
26 6 dB + 20 dB 2 10 20
-8 6 dB + 6 dB - 20 dB 2 2 10 0.4:1
-12 -6 dB - 6 dB 22 0.25:1

By: N.Kuppusamy Event Horizon


Readability problems with stepped gain
control
Most analogue UFD units have a fine stepped gain
control, in which the gain can be adjusted only in
steps of 2 dB. This is rarely a practical limitation, but
it may make it difficult at times to measure
accurately.
With experience, you will become competent in
interpolating between steps and improve your
accuracy. You should soon be able to estimate gain to
an accuracy of 1 dB, and with further experience, to
read to 0.5 dB. Most digital instruments read gain
with a much greater precision.

By: N.Kuppusamy Event Horizon

Practical Measurement of Attenuation


It is important to make
attenuation measurements
in the far zone
We will talk about near and
far zones in the next task,
but for now:
Attenuation
In the near zone, the Measurement

ultrasonic response is
erratic and it is not
possible to make reliable
comparisons.

In the far zone, the ultrasonic response is predictable and


sound pressure can be predicted more accurately.

By: N.Kuppusamy Event Horizon


To measure the relative attenuation:

Calculate the approximate near 2


zone length (N) of the probe by D
applying the formula:
N
4O
Where:
N is the near zone length in meters (mm)
D is the probe crystal diameter in meters (mm)
is the wavelength in meters (mm)

By: N.Kuppusamy Event Horizon

To measure the relative attenuation..


Using either an immersion or contact set up, display two or
more backwall reflections on a parallel-sided sample of the
material as shown. Use backwalls beyond three near zone
lengths (3N), unless this is impossible due to the material
characteristics.
Display the first backwall at 100% screen height.
Note the extra gain required to bring the next backwall to
100% screen height. Record this extra gain (g1).
Note the thickness between the backwalls (d)

g1
Attenuation Coefficient D
2u d
By: N.Kuppusamy Event Horizon
To measure the relative attenuation..
Example
For a 10 mm/2 MHz zero L-probe, calculate the near zone:

c 5900
O 0.00295m 2.95mm
f 2 u106
D2 10 u10
N 8.5mm
4O 4 u 2.95
Attenuation

For a 25 mm thick test object, first


backwall is approximately three near zones

By: N.Kuppusamy Event Horizon

To measure the relative attenuation..

Example (contd)
First backwall is set at 100% FSH
Gain is adjusted to bring the second backwall to 100% by adding (let us
say) 2 dB
Difference in gain (g1) = 2 dB
Distance between backwalls (d) = 25 mm
Attenuation coefficient = 2 / 25 2 = 0.04 dB/mm = 40 dB/meter

By: N.Kuppusamy Event Horizon


Do yourself

Measure the attenuation of your V1 block (AS2083 block1) for


your probe frequency in the 25 mm direction (through the
thickness of the block), then in the 100 mm direction (across
the width of the block).
Do you get the same answer in both directions? Discuss your
results.
Your customer has three machined samples. One has very high
attenuation, one is medium, and the third is very low. Your
customer thinks one was made from steel plate, one was a grey
iron casting, and the other was an SG [Spheroidal Graphite -
Ductile Iron] casting. How can you help the customer sort
them?

By: N.Kuppusamy Event Horizon

Points to Ponder

1. Why do we divide by 2 when calculating the attenuation


coefficient?
2. Can you see some shortcomings with this technique?
3. How could you make it more accurate?
4. Why do you get a different answer in different
directions when testing the V1 block? (there may be
more than one reason)

By: N.Kuppusamy Event Horizon


Applications of Attenuation Measurements

Measurements can tell whether a material can reasonably be examined


If a material has excessive attenuation, it may not be possible to effectively
examine it, particularly in thick sections. Some standards place limits on the
attenuation characteristics of materials, and if the attenuation is too high,
it may be necessary to carry out corrective heat treatment, or to place
qualifications on the results of the examination.
Attenuation measurements can check heat treatment processes
Attenuation increases with increasing metallurgical grain size. Excessive
grain size is often an undesirable property and may be uneven through the
section. Relative attenuation measurements are quite simple and quick to
make, and can be used to check that heat treatment has been effective.
Attenuation can also be used to discriminate between SG iron and Grey Iron
castings.

By: N.Kuppusamy Event Horizon

Applications of Attenuation Measurements

Comparing attenuation can ensure consistent test sensitivity


Calibration blocks are generally made from ideal fine-grained
materials. If the test is done on a different material, the
examination may be carried out at an incorrect sensitivity due
to the attenuation difference between the calibration block
and material under test. For example, this results in a loss of
sensitivity when testing higher attenuation materials.
Standards such as AS2207, ASW D1.1 give guidance for using
attenuation measurements to compensate for losses in
sensitivity due to attenuation variations.

By: N.Kuppusamy Event Horizon


Spot weld testing using attenuation

Resistance spot-weld testing uses attenuation to evaluate weld quality


There are thousands of spot welds in the thin metal sheets in the average
motor vehicle. These were traditionally tested by measuring the force
required to pull apart a test weld. This is not a very scientific test and has
recently been challenged by an ultrasonic method that can determine much
more about the weld quality.

Spot weld

By: N.Kuppusamy Event Horizon

Nugget Weld Examination Procedure

The examination is carried out with a very high frequency, typically 20 MHz, and
a very small probe with a flexible water filled membrane to conform to the weld
profile. The display can result in four types of responses:
If there is a large weld nugget (good weld), there is a series of backwalls
corresponding to two metal thicknesses. The entire beam passes through the
nugget. There is, however a very steep decay in the backwall pattern, as the
weld nugget is of higher attenuation than the sheet steel. At the high frequency
used, this high attenuation is quite obvious by the rapid echo decay.
If the weld nugget is undersize, there is a similar pattern to the larger weld
nugget, but some intermediate echoes occur as all the sound does not travel
through the weld, due to the unfused area around it.

By: N.Kuppusamy Event Horizon


Nugget Weld Examination Procedure
If the metal surfaces are bonded, but there is no effective
weld nugget, the display will be the same as (1) above. But in
the absence of the weld nugget, the decay pattern will not be
as steep. This is called a cold shot, and although there has
been an ineffective bond, sound can go through the interface.
If there is complete lack of fusion, there will be a display of
backwalls corresponding to one metal thickness.
This is a very useful application of attenuation to distinguish
between a satisfactory weld nugget (higher attenuation) and
an unsatisfactory cold shot bond (lower attenuation).

By: N.Kuppusamy Event Horizon

Summary
Attenuation occurs by absorption and scattering.
Absorption can often be managed by use of lower
frequency, increased pulse energy or additional
amplification.
Scattering is managed by using lower frequencies and
minimizing the beam path length where possible.
The decibel (dB) notation is a convenient way of measuring
and comparing echo amplitude over a very wide range.
Attenuation properties may be expressed as attenuation
coefficients (dB/mm), and are influenced by metallurgical
condition, homogeneity and probe frequency.

By: N.Kuppusamy Event Horizon


Theory of Ultrasonic Testing
Module-3A
Coefficients &
Couplants
Presented by
N.Kuppusamy

Singapore Chapter

NDT HORIZON

Introduction
In this section you will learn about immersion testing and
understand all about reflection and transmission in more detail
just what does happen when an ultrasonic beam strikes an
interface? This is vital for understanding ultrasonic tests.
The things you will need to know to do this task are:
reflection and transmission at interfaces
principles of immersion testing
how to set up an immersion test
specific instrumentation for immersion testing
focused probes
automated scanning and recording systems
other applications of immersion testing.

1-Nov-05 N.Kuppusamy
Interfaces
An interface is a boundary where two different materials meet
So far, you have examined waves travelling through one medium. What
happens when a sound wave strikes an interface between different
materials?
In general, when sound waves come to an interface, some of the sound will
be reflected, and some will be transmitted, or pass through the interface.
A similar situation occurs with light waves when you look in a shop window.
You will see the objects in the shop (transmitted light) as well as your own
reflection (reflected light).
You may have noticed some offices use striped mirrors, which the
customer cannot see through because they see a mix of reflected and
transmitted light which they cannot interpret, while staff in the office
only see transmitted light and can see the customer quite clearly.
The most common interfaces we encounter during ultrasonic testing are
metal-to-water and metal-to-air. We also encounter Perspex-to-metal
interfaces in probe design and use. There are also applications where we
examine metal-to-metal bonds, and even vulcanised rubber bonds.

1-Nov-05 N.Kuppusamy

Interfaces

Some interfaces you will encounter include:


the far wall of a test object (metal-to-air interface)
a void in a casting (metal-to-gas-interface)
a slag inclusion in a weld (metal-to-non-metal interface)
a void filled with water (metal-to-water interface)
a crack filled with oil (metal-to-oil interface)
a shrink fit (a mix of metal-to-metal and metal-to-air
interfaces, depending on the quality of the shrink fit)
the far wall of a pipe filled with water (water-to-metal and
metal-to-air interfaces).

1-Nov-05 N.Kuppusamy
An interface occurs where there is a change in
acoustic impedance

An interface is formed where different materials meet, but what do we


mean by different? We need a property of the materials to let us
quantify how sound waves will behave at an interface. This property is
the acoustic impedance and it is a measure of the resistance to sound
propagation through a medium.
The formula for calculating acoustic impedance is very simple:

Z U uc EuU
Where:
Z is the acoustic impedance (kg/m2s)
is the density (kg/m3)
c is the acoustic velocity (m/s)
E is Youngs modulus (Newtons/m2)

1-Nov-05 N.Kuppusamy

Interface ...

Now we can describe an interface in a much more scientific way.


An interface is a zone where there is a change in acoustic
impedance.
The junction between weld metal and parent metal of the same
acoustic impedance is therefore not an interface, unless the
junction is discontinuous (e.g. has cracks or other physical
defects).
An atomic junction between two dissimilar metals is an interface.
Conversely, two different metals would not have an interface if
their acoustic impedances happened to be identical.
For water, the acoustic impedance is approximately 1000 1483
= 1.48 106 kg/m2s.

1-Nov-05 N.Kuppusamy
Check Your Progress

Density and acoustic compression velocity in


Calculate the acoustic impedance of various materials
steel.
Compression
Density ()
Answer: 45.4 106 kg/m2s Material
(kg/m3)
Velocity (cc)
(m/s)
Calculate the acoustic impedance of Aluminium 2,700 6,320
Perspex. Steel 7,700 5,900
Answer: 3.2 106 kg/m2s Perspex 1,180 2,730
Water 1,000 1,483
Mercury 13,600 1,450
Rubber-
1,200 2,300
vulcanised
Tungsten 19,100 5,460
Air 0.1290 345

1-Nov-05 N.Kuppusamy

Acoustic Impedance

Note that you will soon meet a concept known as attenuation.


Dont confuse attenuation with acoustic impedance, as these
terms and their meanings are quite different.
Acoustic impedance is of vital importance in the reflection and
transmission of sound at interfaces. Consider an ultrasonic wave
travelling through one medium, which strikes an interface with
another medium at normal incidence. When the beam strikes the
interface, some of the energy will be transmitted across the
interface and some will be reflected back.
We can use the acoustic impedance to predict the relative
acoustic pressures and energies of the reflected sound and the
transmitted sound. But what is acoustic pressure?

1-Nov-05 N.Kuppusamy
Reflection and Transmission
Acoustic pressure
Relative acoustic pressure is the property we record when measuring
signal amplitude in ultrasonic testing. Compression waves propagate by
fluctuations in pressure, so a wave will cause local variations in pressure
as it passes. It is these pressure variations that are detected by the
piezoelectric transducer and converted to an electrical signal, which is
then displayed on the UFD screen. When we measure the strength of
signals in ultrasonics we are comparing their sound pressures.
The acoustic pressure can be expressed as: P = Z x A
Where:
P is the acoustic pressure
Z is the acoustic impedance
A is the amplitude of particle vibration caused by the sound wave

You will not need to actually calculate absolute acoustic pressures.


Signals you will be measuring in ultrasonics are always relative measures
of the acoustic pressure, and are recorded in terms of either screen
height or decibels.

1-Nov-05 N.Kuppusamy

Reflection and Transmission

The reflection coefficient is a measure of reflected sound


pressure
The reflection coefficient (R) tells us what fraction of the
incoming wave pressure is reflected back from an interface.
For example, if the incident sound pressure is 100 units, and
the reflection coefficient is 0.2 (20%) then the reflected wave
will have a pressure of 20 units.
The reflection coefficient can be calculated from the acoustic
impedances of the two materials. We will do this now for the
simplest case of square, or normal incidence where the incoming
wave strikes the interface at ninety degrees.

1-Nov-05 N.Kuppusamy
Square incidence
The incident wave approaches and strikes an interface at square incidence
(0). It has a pressure of S.

The interface is a zone in which there is a change in acoustic impedance


Z 2  Z1
The reflected wave has a pressure of Rx S, R=
Z 2  Z1
The transmitted wave has a pressure of Tx S, 2Z 2
=
T=1-R Z 2  Z1
For sound travelling from medium 1 with acoustic Z 2  Z1
impedance Z1 to medium 2 with acoustic impedance Z2 R ( Square Incidence)
Z 2  Z1
The transmission coefficient is a measure
of transmitted sound pressure
The transmission coefficient (T) is the ratio
of the transmitted wave pressure to incident
wave pressure.
Adobe Acrobat
Document
2Z 2
Square Incidence
Z 2  Z1
1-Nov-05 N.Kuppusamy

The steel / water interface

A very common interface in ultrasonics is from steel to water.


Lets calculate the reflection and transmission coefficients for
square incidence.
For sound travelling from steel to water:

Z1( steel ) 5900 u 7700 45 u106 kg / m 2 s

Z 2 ( water ) 1500 u1000 1.5 u106 kg / m 2 s


1.5  45  43.5
R  0.935
1.5  45 46.5
2 u 1.5 3
T 0.065
1.5  45 46.5

1-Nov-05 N.Kuppusamy
What do these coefficients mean?

For sound travelling from steel to water, the sound pressure of


the wave reflected back into steel is 93.5% of the incident wave.
Dont worry about the negative sign of the reflection coefficient
it signifies that positive pressures at the interface in the
incident wave become negative pressure in the reflected
wave and vice versa. This is called a phase change and will be
discussed later.
For sound travelling from steel to water, the pressure of the
wave transmitted across the interface into the water is 6.5% of
the incident wave.

1-Nov-05 N.Kuppusamy

Check Your Progress


Calculate the reflection and transmission coefficients for sound
travelling from water to steel.
Answer: R = 0.935, T = 1.935

To summarise, when the beam strikes the interface, some of the


sound pressure will be transmitted across the interface, and
some will be reflected back. The only time when no pressure will
be transmitted across the interface is when the other side is a
vacuum. For practical purposes however, a metal-to-air interface
is an almost perfect reflector.

1-Nov-05 N.Kuppusamy
The transmission coefficient can be greater than 1.0
In the question earlier, the transmission coefficient looks odd at
first. How can there be a greater pressure transmitted than was
incident in the first place? This is because it is not pressure
that is conserved across the interface, but energy. It is common
to have a transmission coefficient greater than 1.0.
This situation is similar to a transformer, where we can achieve a
higher voltage at the output of a transformer, but the total
energy output is always the same as the energy input.
There is, however, a simple relationship between reflection and
transmission coefficients. The total pressure on the incident
side is equal to the sum of the incident wave pressure and the
reflected wave pressure. The incident pressure can be taken as
1.0 (100%).
Thus, incident wave pressure + reflected wave pressure =
transmitted wave pressure, or: 1+R = T

1-Nov-05 N.Kuppusamy

Energy Coefficients

2
So far, we have calculated the Z 2  Z1
reflection and transmission
R ( Energy )
coefficients in terms of the Z 2  Z1
pressure of the waves. It is
also possible to calculate them 4 Z 2 u Z1
in terms of energy. T 2
( Energy )
Z 2  Z1
In this situation, the total energy is the same on both sides
of the interface, so we can say:

1-Nov-05 N.Kuppusamy
Check Your Progress
Calculate the energy reflection and transmission coefficients for
sound traveling from steel to water.

Answer: R = 0.87, T = 0.12

1-Nov-05 N.Kuppusamy

Comparing the pressure and energy conventions

The pressure convention is like measuring the voltage across a


transformer and can give a positive or negative coefficient of
reflection, as well as an increase in pressure across the
interface
The energy transmission is like measuring power across a
transformer, and will always give a positive reflection
coefficient. There is always a conservation of energy across the
interface.
You will mainly use the pressure conventions, as they relate more
to screen height as a measure of acoustic pressure.
Both can be used and you should be aware of them and be able to
calculate the coefficients for both cases.

1-Nov-05 N.Kuppusamy
Couplants
Although we are dealing with immersion testing for which the principal
couplant is water, it is important to consider couplants generally. In ultrasonics,
a couplant, as the name suggests, joins or couples the probe to the test object.
Transmission coefficients explain why we need couplant
If the ultrasound wave emerges from the probe into air, there will be
very low transmission and very high reflection, meaning very little of
the signal will enter the test piece. Remember that air has an acoustic
impedance of almost zero. If you want to couple the probe to the test
piece, it is necessary to eliminate the air interface. The most
convenient couplants are liquids such as water or oil.
For contact testing, a surface layer of couplant is used, which
displaces the air between the probe and test piece. Water is commonly
used as a couplant, and is often thickened with a cellulose paste to give
better application to surfaces. Oil or grease can also be used where
there is a risk of any adverse corrosion effect from using water based
couplants. The couplant thickness in contact testing is usually very
small, about 0.1 mm.
For immersion testing, the probe and the object are immersed in
water with a significant water gap in between. This is very convenient
for automating a process, and will be the key to this task.

1-Nov-05 N.Kuppusamy

The ideal couplant has particular properties

A couplant can be any viscous material liquid, semi-liquid or paste


that:
wets the surface of the probe and test object
is non toxic and non corrosive
can be applied and removed easily
has an acoustic impedance somewhere between the probe
and test object, although this is not generally possible
is homogeneous and free of bubbles that would scatter the
beam
is sufficiently viscous to prevent flow off the test surface
allows easy movement over the test surface.

1-Nov-05 N.Kuppusamy
Some common liquids make good couplants
Water is the cheapest and most abundant couplant, but may
need detergents added to wet the surface, or methyl cellulose
to act as a thickening agent to retain it on the surface. It may
also be necessary to add rust inhibitors when water is used.
Oils and greases are used where water is unsuitable they also
stay on the surface longer and do not evaporate as quickly from
warmer surfaces
Glycerine is the most favourable liquid for acoustic impedance
properties, and may be mixed with water if required.
Mercury is theoretically a very good couplant due to its high
acoustic impedance, but is neither practical nor safe to use.

1-Nov-05 N.Kuppusamy

Review
Here are some important points to remember.
For waves striking an interface at right angles:
An interface is a boundary at which there is a change in acoustic impedance.
Sound meeting an interface at right angles will be partly transmitted across
the interface, and partly reflected by it.
The sound pressure and energy of the reflected and transmitted waves can
be calculated if the acoustic impedances are known.
The greater the difference in acoustic impedance values of the two media,
the greater the amount of reflection and the lesser the amount of
transmission and vice versa.
The pressure transmission coefficient can be higher than 1.0 - that is, the
transmitted pressure can be higher than the incident pressure.
The pressure reflection coefficient can be positive or negative. A negative
coefficient signifies a change of phase. Transmission coefficients are always
positive. For pressure, 1 + R = T.
The energy reflection coefficient can only be positive, so does not indicate
any phase change. Transmission coefficients are always positive. For energy
coefficients R + T = 1.
In ultrasonic testing a liquid couplant is placed between the probe and test
object to maximise sound transmission across the interface.
1-Nov-05 N.Kuppusamy
Practice

Set your zero compression probe to give a backwall reflection


from the 25 mm thickness of the IIW block (V1 Block). Set this
echo as close as you can to 100% full scale height (FSH). While
maintaining the echo, wet your free hand with some oil or water
and dab it exactly opposite the probe. Every time you touch the
opposite side, you should see the backwall dip slightly, about 5%.

1-Nov-05 N.Kuppusamy
Theory of Ultrasonic Testing
Module-4
Flaw Detector

Presented by
N.Kuppusamy

Singapore Chapter

NDT HORIZON
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The Ultrasonic Flaw Detector

The flaw detector consists of a number of key components. They


are designed to operate in Through-Transmission/Pulse-Echo
modes. In this chapter you to learn about the flaw detector and
understand what the various controls do.

Things you will need to learn:


1. the basic block diagram of the UFD
2. how the flaw detector works
3. the controls on a flaw detector
4. enhancements to improve the performance
5. comparison of digital and analogue UFDs
6. matching the impedance of probes and UFD
7. how to review data from various suppliers.

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Parts of the Ultrasonic Flaw Detector

The UFD is made up of six basic elements

1. Timer: controls the rate at which pulses are generated. The rate
at which the timer operates is called the Pulse Repetition
Frequency (PRF). In some instruments the user can control this,
while in others it is automatically adjusted by the UFD to suit the
range.
2. Pulse generator: generates a spike of instantaneous voltage when
triggered by the timer.
3. Probe: converts the voltage spike to a mechanical sound wave. The
wave is generated at the resonant frequency of the transducer.
The probe also reconverts the received mechanical sound wave to
an electrical image of the sound wave.

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Parts of the Ultrasonic Flaw Detector


4. Sweep generator: sends an electron beam across the CRO
(cathode ray oscilloscope) at a constant speed, by applying a
voltage between the side plates of the CRO.
5. Amplifier: amplifies the received signal from the
transducer. There may also be other processing of the
signal such as rectification.
6. CRO or Digital display: shows the received wave form. In
American literature, the CRO may be called the CRT
(cathode ray tube).

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The operation of a flaw detector and probe is repeated
sequence of steps Block Diagram

The timer signals the pulse generator that it is time to send a pulse.
At the same time, it also signals the sweep generator that a pulse is
being sent, and:

1. The pulse generator sends a spike to the transducer, around 300 V,


which converts the spike to a mechanical sound wave that commences
its journey from the transducer.
2. At the same time, the sweep generator sends an electron beam on its
journey across the CRO.

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The operation

3. The electron beam leaves the left side of the CRO at the same instant that
the sound wave leaves the transducer. The UFD and probe wait while the
sound pulse travels through the material and is reflected back, returning to
the probe. The returning sound wave reaches the transducer, which
Block Diagram immediately reconverts it to an electrical signal in the milli-volt range.
4. The weak electrical signal from the transducer is received by the amplifier
and amplified in accordance with the gain applied. Other processing, such as
rectification may also be applied at this stage.
5. The amplified and processed signal is applied to the top and bottom plates of
the CRO, by which time the electron beam has travelled some of the distance
across the screen. At that point, the image of the received sound wave is
displayed on the trace, indicating its amplitude, shape, and transit time. Note
that the transit time is the time taken to do the round trip to the reflector.

The cycle from steps 1 to 5 is occurring at a rate of around 500 times per second (500 Hz).
This cycle rate is called the pulse repetition frequency (PRF).

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BLOCK DIAGRAM

Block Diagram

24-Feb-07 7
CRT
N.Kuppusamy

Points to Ponder

1. Why is the trace brighter at higher PRFs?


2. Why is the trace duller when using shorter ranges?
3. What will be the effect of a standoff block on the time of
entry into the test object?
4. What will happen if the clock is set too fast and the pulse is
sent before the previous one has died away? Would you expect
this effect to be more obvious in high or low attenuation
materials?

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Digital displays are becoming increasingly
popular

Many modern UFDs are now digital, and the


analogue CRO screen has been replaced by the
digital display of a computer screen. The digital
display allows much greater flexibility in recording
the trace, but loses some of the real time speed of
an analogue CRO.

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Using the UFD Controls

The block diagram


describes the function of
the components. We will
now consider how the
functions of the various
components are managed
through the controls. UFD controls

No review of controls can consider every possible control available,


so we will discuss those most commonly used in the order that they
will most probably be required.

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The Range Control

The range button controls the sweep rate;


This allows the screen to display the required test range. Obviously, if
the test requires only 100 mm of range, it is pointless displaying 1000
mm and trying to interpret signals in the left hand 10% of the display.
The range is expanded or contracted by varying the rate at which the
sweep generator moves the electron beam across the screen. For a
very long range, e.g. 5 metres, the electron beam will, relatively
speaking, sweep very slowly and will appear much brighter. For a short
range, the beam will sweep very quickly across the screen, and spend
most of the time waiting for the next sweep.
Range is normally adjusted in coarse steps with the coarse range
control, and in fine steps with the fine range control. Most equipment
will indicate the coarse step settings, e.g. 10, 100 or 1000 mm.

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The range button control

These apply only to compression waves in steel, so when using


shear wave probes the 100 mm setting would correspond to
approximately 50 mm. The fine range setting allows
continuous adjustment within the coarse ranges, as well as
calibration for other ranges, modes and materials.
Analogue instruments require the range to be set by using a
calibration block of known thickness. Some digital
instruments allow the range to be keyed in by specifying the
range required and the acoustic velocity, but need to be
verified with a calibration block.

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Zero (Delay) Control
The zero control allows synchronization to the material zero
Often, there is significant distance between the transducer and the test object,
with a corresponding delay. For example, there are delay blocks in contact
probes, and water paths in immersion testing. For this reason, the electrical zero
(the point at which the probe is energised) is not the same as the material zero
(the point at which the beam enters the test material). The zero control allows
the material zero to be set at zero distance on the screen. This is done by
electronically delaying the start of the sweep generator, so that the material zero
is displayed at the left of the UFD screen.
The zero control may also be useful when inspecting within a limited area of the
range. For example, it may be useful to set the range at 100 200 mm, with the
100 mm set at the screen zero to look at a particular indication occurring at 150
mm. This can also be very useful for improving accuracy when thickness testing
thick materials.

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Gain Control
The gain control determines the amount of amplification applied to the
screen display

The gain in most portable instruments is calibrated in coarse steps, e.g. 20


dB, and fine steps, e.g. 1 or 2 dB. Many digital instruments can set gain in
even smaller increments. The gain is the degree of amplification of the
amplifier and is applied equally to all indications on the trace. Most
instruments have a maximum gain of around 120 dB (an effective
amplification of 106 i.e. 1 million). Some instruments additionally have an
uncalibrated gain control which is useful for adjusting an echo to a precise
screen height, perhaps 80% when setting test sensitivity.
In some instruments, the fine gain settings will have slightly more electrical
noise than the coarse gain settings. It will generally be preferable to use the
maximum possible coarse gain and minimum fine gain to get the same total
gain. If in doubt, check your instrument experimentally.

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Gain Control
For most applications, it is important that the amplifier can
faithfully amplify signals over the required range of
frequencies used. Such amplifiers are called broadband
amplifiers. Some amplifiers can be set preferentially amplify a
limited range of frequencies these are called narrow band
amplifiers and may be used in special applications.
Note that some UFDs, especially Japanese and American, use
the gain in the opposite sense, and call it an attenuator. There
is no mystery in this, 6 dB of attenuation is just minus 6 dB of
gain and vice versa. Just be careful that you are aware of the
convention in the equipment you use.

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Suppression (Reject) Control

The suppression (reject) is used to deduct some of the


amplification
Closely related to the gain control is the suppression control.
The gain control allows the user to multiply and divide the
amplification, applying it equally to all reflectors. Commonly,
suppression operates by subtracting amplification by the
same %FSH from every indication.

Reject

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Suppression (Reject) Control
This has the result that if 10% of suppression is applied to the display, all
reflections will be reduced by 10% FSH. If you have indications of 100%, 50%
and 10% and apply 10% suppression, the indications will drop to 90%, 40% and
zero respectively.
There is often a temptation to apply suppression when the trace is showing a high
degree of material noise when testing coarse-grained materials. If you do this,
the amplifier is no longer linear, and will not amplify all indications by the same
amount, so there is a risk of missing small important indications. The presence of
low level grass on the screen is your reassurance that there is sound entering
the test object. It is preferable to learn to work with a small amount of material
noise on the screen to get this reassurance. The best ultrasonic professionals will
always operate with significant material noise on the screen.
Although this problem has been addressed in some later equipment designs, use
suppression only as a last resort, and do a simple linearity check each time you
use any equipment to prove to yourself that the suppression is off.

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Pulse Energy
Pulse energy can be modified slightly to combat attenuation
In some equipment, the strength of the pulse can also be
increased. This is done by either applying a stronger or longer
duration pulse. For highly damped probes, a stronger input pulse
may be achieved in some equipment with a tone burst
generator, which applies an alternating voltage to drive the
transducer harder at its resonant frequency.
This may give extra penetration range in difficult materials, but
will result in a loss of resolution. Like suppression, it should only
be used as a last resort.

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Single /Twin Selector

The single/twin switch selects the type of


probe to be used
The UFD needs to be set for either single or twin
crystal operation. In single crystal operation, the
probe is connected to both the pulse circuit and
the amplifier. In twin crystal mode, the
transmitting crystal is connected to the pulse
circuit and the receiver crystal is connected to
the amplifier.

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Pulse Repetition Frequency (PRF)

Pulse Repetition Frequency (PRF) can be adjusted in some UFDs


PRF controls the rate at which the pulses are generated. If the pulse
repetition frequency is too low, there are too few sweeps across the
screen, and the trace is very faint. A high pulse repetition frequency
is also needed when testing at higher speeds, or there is a risk that
the volume of material will not be fully scanned.
If the PRF is too high, a situation can arise where one pulse has not
fully died away before the next pulse is transmitted. The oscilloscope
does not know which reflected pulse relates to which transmitted
pulse, and random ghost echoes can appear on the screen.
In most portable equipment, the PRF is controlled internally by the
range control.

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Signal Processing

Pulse shaping controls can


make the pulse shape more
user friendly

Signal Processing

The raw pulse that is received by the amplifier is an unrectified


sine wave. Unless it is important to have an unrectified trace, most
traces are rectified for ease of interpretation. There is also some
smoothing applied to the trace to make it easier to interpret.

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Use of Monitors
Monitors (gates) can select a section of the trace for special attention
A monitor is set to read a specific part of a trace that is of particular
interest, for example between zero and the first backwall echo. The limits of
the monitor range are set, together with a threshold above which it is
required to record. Subsequently, whenever a reflection occurs in the area of
interest, data is exported. Depending on the instrument design, typical data
might be:

a yes/no that an echo has occurred in the monitored


area and has exceeded the set threshold to activate
an alarm
the amplitude of the reflection
Gate
the amplitude and range of the reflection
the complete ultrasonic trace in digital form for
subsequent analysis.

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Distance Amplitude Correction (DAC)

Distance Amplitude Correction


accounts for attenuation variations
As the pressure of the reflected
beam decreases with distance, the
amplitude of reflected echoes from
identical reflectors in the far zone
will decrease with increasing beam
path length.
DAC

Distance amplitude correction (DAC) allows this variation to be corrected


by the UFD, by either drawing a DAC curve or applying additional time
corrected gain (swept gain) to echoes at various beam paths to display
them all at a consistent screen height. The amount of DAC applied will
depend on the material and the type of reference discontinuity used.

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Point to Ponder
Would the DAC curve for a series of backwalls
look similar or different to the DAC curve for a
series of small disc reflectors, such as flat-
bottomed holes. Why?

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Comparison of Digital and Analogue Oscilloscopes

Analogue and digital oscilloscopes have significant differences


Traditional analogue oscilloscopes will have a green trace and engraved
calibration marks (graticules) on the screen. These are generally in multiples of 5
or 10 to allow convenient calibration of the time base. The rate of response is
instantaneous, and because the frequency of sweep of the electron beam across
the screen is very high, typically 500 traverses per second at a PRF of 500 Hz,
the path of the electron beam will look like a continuous line.
Analogue oscilloscopes will also have a number of internal controls for adjusting
focus, astigmatism, and alignment with the graticule. You will not normally be
required to adjust these controls, but be aware of them if your trace looks blurred
or misaligned.
One of the main properties to watch with oscilloscopes is that they should be
linear in their response. Tests for this performance will be described in the task on
Calibration.

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Comparison of Digital and Analogue Oscilloscopes

Digital oscilloscopes are becoming increasingly popular. Digital displays


construct the trace mathematically by sampling the analogue signal and
constructing a trace from the sampled points. The more sampled points, the
more the digital trace looks like an analogue trace. If the number of sampled
points is low, (in order to speed up the sampling process), the display looks
less like an analogue display. The equipment can send its display to a
conventional computer screen for viewing.
The screen markings are contained within the screen display, and
experienced users of analogue oscilloscopes will note the slower response
and update time of digital displays. They are also less able to resolve many of
the subtleties possible with an analogue display, but no doubt as faster
equipment becomes available, these differences will narrow.

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Advantages of Digital oscilloscopes

Their principal advantages are:


The user can program settings for later use to give
greater reproducibility.
The ability to store settings as a test record.
Traces can be saved for subsequent processing and
review.
The test can be rerun off site with changed settings.

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Reading the UFD Screen

Digital displays have made reading


the beam path much simpler
Reading the distance on the screen is
relatively simple with most digital
equipment. There is generally a
larger choice of options for
calibration, and the screen is directly
marked with an easily readable grid.
Analogue displays need thought in
selecting the range

1. Main Division
2. Sub-division

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Reading the UFD Screen
Analogue instruments, however, can be a little more difficult to read
accurately. Most analogue instruments are marked with 10 major divisions,
each of which has 5 minor divisions, giving a total of 50 minor divisions. This
is quite uncomplicated when using a range of 0 - 100 mm, as each division
represents 2 mm and is easily read. Similarly, ranges of 0 - 50 mm and 0 - 10
mm are also easily read.
In numerous thickness testing applications, many of the readings are in the
range of 10 - 20 mm, which is beyond a calibration of 0 - 10 mm. Using 0 - 50
mm will result in an unacceptable loss of precision. A popular range for
thickness testing is 0 - 25 mm, which is easier to read correctly. In this
case, each major division represents 2.5 mm and each minor division
represents 0.5 mm, giving a readability of 0.25 mm with experience and
practice. With this range, the best accuracy that can be achieved in
thickness testing is therefore around 0.2 to 0.3 mm.

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UFD Screen
A legacy from Imperial units is the habit among some operators of calibrating
to a range of 0 - 125 mm (0 - 5 in). This makes the interval between major
divisions 12.5 mm and the interval between minor divisions 2.5 mm, and has
often resulted in reading errors.
You do not have to set the left-hand side of the screen to zero
One of the often-quoted reasons for using an unusual range is that the
indications sought occur just outside a more conventional range. For instance,
if there are likely to be indications to be assessed at 120 mm, why not move
the delay and set the range to 100 - 150 mm and read more accurately? This
will put the key indications nearer the centre of the screen, where linearity is
generally best, and indications are easier to see.
There is no right answer to selecting the correct range. The judgment needed
to make the best selection will come with practical experience.

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Check Your Progress

1. You decide that the ideal range for testing would be 0-250 mm and
calibrate accordingly. The range is calibrated in 10 major divisions,
each with five minor divisions. What does each major division now
represent?
a. 10 mm
b. 20 mm
c. 25 mm
d. 50 mm

Answer: c - 25 mm

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Check Your Progress

2. What does each minor division represent?


a. 2.5 mm
b. 4 mm
c. 5 mm
d. 7.5 mm

Answer: c - 5 mm

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Check your progress

3. You get an indication at the 7th major division, what


distance does it represent?
a. 150 mm
b. 175 mm
c. 180 mm
d. 200 mm

Answer: b - 175 mm

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Impedance Matching of Probe and IFD

So far we have represented the probe as a simple piezoelectric plate


comprising a slice of transducer cut to half a wavelength, and coated
with a conducting material. The crystal therefore acts as a capacitor
(C0). The cables that connect to it have a small resistance (RS). At
resonance, this acts as a capacitor, as seen by the UFD.
For optimum energy transfer to the probe, the UFD and the probe
should have similar electrical impedance at the probe frequency.

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Impedance Matching of Probe and IFD

This matching is achieved in practice by inserting an inductance (L0) in the


probe, connected in parallel with the transducer. Most probe manufacturers
include the required matching coil within the probe. If this is not done, it may be
necessary for the user to buy or make a coil to match the probe to the UFD.
The coil does not need to be installed within the probe, but will work as long as
it is connected across the receiver connections.
Matching may also be needed if non-standard coaxial cable is used, or if the
coaxial cable is very long. At higher frequencies, changing the cable type may
also adversely affect the impedance matching.
Note that this electrical impedance matching is quite unrelated to the acoustic
impedance of the materials under test.

24-Feb-07 35

N.Kuppusamy

Batteries
Portable UFDs require rechargeable batteries

There are two basic rechargeable battery types in use:


1. Lead Acid: These are generally called dry acid batteries. They are heavier than equivalent
NiCad batteries, but charging is relatively uncomplicated. Control systems for charging are
also simpler as the battery develops significant voltage as it is charged. The charger can sense
the battery voltage and switch off at a predetermined level. The batteries can also be trickle
charged and topped up after a slight discharge.
2. Nickel/Cadmium: NiCad batteries are lighter and more readily available, but need careful
management to retain battery life and charge. If they are recharged after partial discharge, they
may develop a memory and have reduced capacity. Top up charges can therefore damage the
battery. It is preferable to try and completely discharge a NiCad battery at every use, and then
recharge completely. It is much more difficult to measure the state of charge of a NiCad
battery, as the voltage drops only slightly with discharge. Most NiCad batteries are charged on
a time basis, making it all the more important to avoid partial discharge.

24-Feb-07 36

N.Kuppusamy
Your Task
You are progressing very well, and the company wants to buy you a new flaw
detector. How will you make a decision on what you need? What features are
important? What tests will you do to check that the equipment is
satisfactory? The three scenarios you will address are:
Purchase of a basic low cost, portable, battery operated analogue flaw
detector that can be used for general purpose contact testing work using a
variety of probes.
Purchase of a portable battery operated digital flaw detector for contact
testing. You may want to record some of the data for subsequent reporting
and processing.
A stand-alone piece of equipment that will be used for a fixed immersion
testing setup at a large forge shop to inspect a range of products including
wheels, shafts and complex forgings. Extensive data recording will be
required. This can be digital or analogue, depending on which equipment will
deliver the best outcome.

24-Feb-07 37

N.Kuppusamy

Your Task
You will need to:
Understand what controls are available for ultrasonic instruments and
what they do.
Consider which controls are important to you for your needs.
Review the data available from various suppliers at their web sites.
Think about some of the practical issues such as weight, size,
connectors, battery types and chargers as well as the ability to work off
both mains and batteries.
Think about the probes you use and their bandwidth. Be sure you have
enough bandwidth in the amplifier.

24-Feb-07 38

N.Kuppusamy
Data Presentation

Ultrasonic data can be collected and displayed in a number of different


formats. The three most common formats are know in the NDT world as
A-scan, B-scan and C-scan presentations. Each presentation mode
provides a different way of looking at and evaluating the region of
material being inspected. Modern computerized ultrasonic scanning
systems can display data in all three presentation forms simultaneously.

24-Feb-07 39

N.Kuppusamy

Data Presentation
A-Scan Presentation
The A-scan presentation displays the amount of received ultrasonic
energy as a function of time. The relative amount of received
energy is plotted along the vertical axis and elapsed time (which
may be related to the sound energy travel time within the material)
is display along the horizontal axis. Most instruments with an A-
scan display allow the signal to be displayed in its natural radio
frequency form (rf), as a fully rectified rf signal, or as either the
positive or negative half of the rf signal. In the A-scan presentation,
relative discontinuity size can be estimated by comparing the signal
amplitude obtained from an unknown reflector to that from a known
reflector. Reflector depth can be determined by the position of the
signal on the horizontal sweep.

24-Feb-07 40

N.Kuppusamy
Data Presentation
In the illustration of the A-scan presentation to the right, the
initial pulse generated by the transducer is represented by the
signal IP, which is near time zero. As the transducer is
scanned along the surface of the part, four other signals are
likely to appear at different times on the screen. When the
transducer is in its far left position, only the IP signal and
signal A, the sound energy reflecting from surface A, will be
seen on the trace.

As the transducer is scanned to the right, a


signal from the backwall BW will appear latter
in time showing that the sound has traveled
farther to reach this surface.

24-Feb-07 41

N.Kuppusamy

Data Presentation

When the transducer is over flaw B, signal B, will appear at a


point on the time scale that is approximately halfway between
the IP signal and the BW signal. Since the IP signal
corresponds to the front surface of the material, this indicates
that flaw B is about halfway between the front and back
surfaces of the sample. When the transducer is moved over
flaw C, signal C will appear earlier in time since

the sound travel path is shorter and signal B


will disappear since sound will no longer be
reflecting from it.

24-Feb-07 42

N.Kuppusamy
B-Scan Presentation (Data Presentation)

The B-scan presentations is a profile (cross-sectional) view of the a test


specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is
displayed along the vertical and the linear position of the transducer is
displayed along the horizontal axis. From the B-scan, the depth of the reflector
and its approximate linear dimensions in the scan direction can be determined.
The B-scan is typically produced by establishing a trigger gate on the A-scan.
Whenever the signal intensity is great enough to trigger the gate, a point is
produced on the B-scan. The gate is triggered by the sound reflecting from the
backwall of the specimen and by smaller reflectors within the material. In the B-
scan image above, line A is produced as the transducer is scanned over the
reduced thickness portion of the specimen. When the transducer moves to the
right of this section, the backwall line BW is produced. When the transducer is
over flaws B and C lines that are similar to the length of the flaws and at similar
depths within the material are drawn on the B-scan. It should be noted that a
limitation to this display technique is that reflectors may be masked by larger
reflectors near the surface.

24-Feb-07 43

N.Kuppusamy

C-Scan Presentation (Data Presentation)

The C-scan presentation provides a plan-type view of the


location and size of test specimen features. The plane of the
image is parallel to the scan pattern of the transducer. C-scan
presentations are produced with an automated data
acquisition system, such as a computer controlled immersion
scanning system. Typically, a data collection gate is
established on the A-scan and the amplitude or the time-of-
flight of the signal is recorded at regular intervals as the
transducer is scanned over the test piece. The relative signal
amplitude or the time-of-flight is displayed as a shade of gray
or a color for each of the positions where data was recorded.
The C-scan presentation provides an image of the features
that reflect and scatter the sound within and on the surfaces of
the test piece.

24-Feb-07 44

N.Kuppusamy
C-Scan Presentation (Data Presentation)

High resolution scan can produce very detailed images.


Below are two ultrasonic C-scan images of a US quarter. Both
images were produced using a pulse-echo techniques with
the transducer scanned over the head side in an immersion
scanning system. For the C-scan image on the left, the gate
was setup to capture the amplitude of the sound reflecting
from the front surface of the quarter. Light areas in the image
indicate area that reflected a greater amount of energy back
to the transducer. In the C-scan image on the right, the gate
was moved to record the intensity of the sound reflecting from
the back surface of the coin. The details on the back surface
are clearly visible but front surface features are also still
visible since the sound energy is affected by these features
as it travels through the front surface of the coin.

24-Feb-07 45

N.Kuppusamy

Types of Ultrasonic Testing Methods


1. Through Transmission Testing Method
2. Pulse-Echo Testing Method
3. Resonance Testing Method

To be covered in separate chapter

24-Feb-07 46

N.Kuppusamy
Pulser-Receivers
Ultrasonic pulser-receivers are well suited to general purpose
ultrasonic testing. Along with appropriate transducers and an
oscilloscope they can be used for flaw detection and thickness
gauging in a wide variety of metals, plastics, ceramics, and
composites. Ultrasonic pulser-receivers provide a unique, low-cost
ultrasonic measurement capability.

24-Feb-07 47

N.Kuppusamy

Pulser-Receiver
The pulser section of the instrument generates short, large
amplitude electric pulses of controlled energy, which are

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