The document discusses non-destructive testing (NDT) methods, focusing on ultrasonic testing. It describes the basic principles of ultrasonic testing including how ultrasound is generated and transmitted through materials. The key NDT methods covered are visual testing, liquid penetrant testing, magnetic particle testing, eddy current testing, and radiography. The document also discusses ultrasonic testing techniques such as pulse-echo and through transmission, as well as factors that affect ultrasonic inspection such as frequency, wavelength, attenuation, and probes.

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2. 2
• 2 types of testing methods, Destructive testing & Non-
Destructive testing
• Destructive testing-After testing the material looses its
functionality, e.g. Chemical analysis, mechanical properties
etc.
• Non-Destructive testing-The soundness of material is
tested without affecting its functionality.
• NDT methods enhaneces the reliability of the
components/product at manufacturing stage or in service.

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The use of noninvasive
techniques to determine
the integrity of a
material,
component or structure
or
quantitatively measure
some characteristic of
an object.
i.e. Inspect or measure without doing harm.

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• Visual inspection (VT)
• Liquid penetration (PT)
• Magnetic particle testing (MT)
• Eddy current testing (ET)
• Radiography (RT)
• Ultrasonic testing (UT)

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• Flaw Detection and Evaluation
• Leak Detection
•Dimensional Measurements
•Structure and Microstructure Characterization
•Estimation of Mechanical and Physical
properties
•Stress (Strain) and Dynamic Response
Measurements
•Material Sorting and Chemical Composition
Determination

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There are NDT application at almost any stage
in the production or life cycle of a component.
•To assist in product development
•To screen or sort incoming materials
•To monitor, improve or control
manufacturing processes
•To verify proper processing such as heat
treating
•To inspect for in-service damage

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• A liquid with high surface wetting characteristics is applied
to the surface of the part and allowed time to seep into
surface breaking defects.
• The excess liquid is removed from the surface of the
part.
• A developer (liquid/powder) is applied to pull the
trapped penetrant out the defect and spread it on the
surface where it can be seen.
• Visual inspection is the final step in the process. The
penetrant used is often loaded with a fluorescent dye
and the inspection is done under UV light to increase
test sensitivity.

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The part is magnetized. Finely milled iron particles coated with
a dye pigment are then applied to the specimen. These
particles are attracted to magnetic flux leakage fields and will
cluster to form an indication directly over the discontinuity.
This indication can be visually detected under proper lighting
conditions.

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Eddy current testing is particularly well suited for detecting surface cracks but
can also be used to make electrical conductivity and coating thickness
measurements. Here a small surface probe is scanned over the part surface
in an attempt to detect a crack.

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FREQUENCY RANGE
(Hz)
DESCRIPTION EXAMPLE
0 - 20 Infrasound Earth quake
20 – 20,000 Audible sound Speech, music
> 20,000 Ultrasound Bat, Quartz crystal

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•Sound frequency above human audible range is called
ultrasonic sound
•The ultrasonic principle is based on the fact that solid
materials are good conductors of sound waves & reflected at
interfaces.
•Whereby the waves are not only reflected at the interfaces
but also by internal flaws (material separations, inclusions
etc.)
•A considerable amount of information about the part being
examined can be collected, such as the presence of
discontinuities, part or coating thickness; and acoustical
properties can often be correlated to certain properties of the
material.

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•RESOLUTION:-The ability to detect nearby small
defects separately
•SENSITIVITY:-The ability to detect the smallest
defect
•ATTENUATION:-At a grain boundary the wave is
partly absorbed, partly scattered & partly
transmitted, so the sum of absorption & scattering
is the attenuation of the wave
•ACOUSTIC IMPEDANCE:-The resistance offered by
the medium to he propagation of sound wave
through it
•HASH:-At times when the material is very coarse
the waves are reflected from the grain boundaries &
we get small echoes which are known as hashes or
noise

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

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•The measurement of sound waves from crest to
crest determines its wavelength (λ).
•The sound wavelength
is inversely proportional
to its frequency. (λ = 1/f)
•Several wave modes of
vibration are used in
ultrasonic inspection.
The most common are
longitudinal, shear, and
Rayleigh (surface) waves.
•The velocity of sound in
a given material is
constant and can only be
altered by a change in
the mode of energy.

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PROPERTIES OF LONGITUDINAL WAVES
•Propagates by compression & rarefaction of the particles in the
direction of travel
•Can propagate in solid, liquid & gases
•It has the highest velocity & longest wavelength among all types
of waves
•It has lesser attenuation than shear waves
If the particles oscillates in the direction parallel to
the propagation of wave than it is said to be a
Longitudinal wave

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Direction of oscillation
LONGITUDINAL WAVE
Direction of propagation

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If the particle oscillates in the direction
perpendicular to the propagation of wave then it
is said to be a Transverse or shear wave
PROPERTIES OF TRANSVERSE WAVES
•Propagates by lateral shear at right angle to
the direction of wave travel
•Can only travel in solid mediums

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

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•Ultrasonic waves are introduced into a material where
they travel in a straight line and at a constant speed until
they encounter a surface
•The probe has a piezoelectric crystal in it which is
excited by extremely short electrical discharge, causing it
to vibrate & produce Ultrasonic waves
•The probe is coupled to the surface of the test object
with a liquid or coupling paste so that the sound waves
from the probe are able to be transmitted into the test
object.
•The operator then scans the test object, i.e. he moves
the probe evenly to and fro across the surface. In doing
this, he observes the instrument display for any signals
caused by reflections from internal discontinuities

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•At surface interfaces some of the wave energy is
reflected and some is transmitted.
•The amount of reflected or transmitted energy can
be detected and provides information about the size
of the reflector.
•The travel time of the sound can be measured and
this provides information on the distance that the
sound has traveled.

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Medium 1 Medium 2
Incoming wave Transmitted wave
Reflected wave
Interface
The reflection takes place at an interface due to
difference in acoustic impedance of the two
mediums

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Sound travel path
Work piece
Flaw
Probe
s

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•Not only the back
surface reflect the sound
wave, the same is done
by every change in the
composition of the
material
•A flaw too reflects the
sound wave
•Depending on the size
of flaw the back wall
echo amplitude changes

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•Change in depth of flaw changes the position
of defect echo on screen
•When the flaw is near the probe the defect
echo is near the initial pulse
•As the flaw changes its position its defect
echo also moves accordingly.
•However the defect echo would always lie
towards the left of the Back wall echo

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• As the size of flaw increases the amplitude of
the defect echo also increases
•And if the amplitude of the defect echo
increases the amplitude of the back wall echo
will decrease
•If the flaw is capable of reflecting the whole
sound wave then in such a case there would be
no Back wall echo

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• Ultrasonic testing is a very versatile inspection method,
and inspections can be accomplished in a number of
different ways.
• Ultrasonic inspection techniques are commonly divided
into three primary classifications.
• Pulse-echo and Through Transmission
(Relates to where reflected or transmitted energy is
used)
• Normal Beam and Angle Beam
(Relates to the angle that the sound energy enters the
test article)
• Contact and Immersion
(Relates to the method of coupling the transducer to
the test article)

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Pulse echo
technique
Through transmission
Immersion testing

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• In pulse-echo testing, a transducer sends out a pulse of
energy and the same listens for reflected energy (an
echo).
• Reflections occur due to the presence of discontinuities
and the surfaces of the test article.
• The amount of reflected sound energy is displayed
versus time, which provides the inspector information
about the size and the location of features that reflect the
sound.

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Digital display showing
signal generated from
sound reflecting from
back surface.
Digital display showing the
presence of a reflector
midway through material,
with lower amplitude back
surface reflector.
The pulse-echo technique allows testing when access to only one
side of the material is possible, and it allows the location of
reflectors to be precisely determined.

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Digital display
showing received
sound through
material thickness.
Digital display
showing loss of
received signal
due to presence of
a discontinuity in
the sound field.

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• In normal beam testing, the sound beam
is introduced into the test article at 90
degree to the surface.
• In angle beam testing, the sound beam is
introduced into the test article at some
angle other than 90.
• The choice between normal and angle
beam inspection usually depends on:
- The orientation of the defect – the
sound should be directed to produce
the largest reflection from the feature.

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• Transducers are manufactured in a variety of forms, shapes and
sizes for varying applications.
• Transducers are categorized in a number of ways which
include:
•- Contact or immersion
•- Single or dual element
•-Normal or angle beam
• In selecting a transducer
for a given application, it
is important to choose the
desired frequency,
bandwidth, size, and in some cases focusing
which optimizes the inspection capabilities.

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•Probes whose beams
are normal to the
surface are called
straight-beam
probes.
•Normal probes
transmit and receive
the sound waves
normal to the surface of
the test object.
•Angle probes transmit and receive the sound
waves at an angle to the surface of the test object.
•Probes whose beams enter at an angle are
called angle-beam probes.

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• Flaw detectors are
instruments designed
primarily for the inspection
of components for defects.
• However, the signal can be
evaluated to obtain other
information such as material
thickness values.
• Both analog and digital
display.
• Offer the user options of
gating horizontal sweep and
amplitude threshold.

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• Information from ultrasonic testing can be
presented mainly in three formats.
 A-scan
 B-scan
 C-scan

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• B-scan presentations display a
profile view (cross-sectional) of a
test specimen.
• Only the reflector depth in the
cross-section and the linear
dimensions can be determined.
• A limitation to this display
technique is that reflectors may be
masked by larger reflectors near
the surface.

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• The C-scan presentation displays a plan type view of the
test specimen and discontinuities.
• C-scan presentations are produced with an automated data
acquisition system, such as in immersion scanning.
• Use of A-scan in conjunction with C-scan is necessary when
depth determination is desired.
Photo of a Composite
Component
C-Scan Image of
Internal Features

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• Sensitive to both surface and subsurface discontinuities.
• Depth of penetration for flaw detection or measurement is
superior to other methods.
• Only single-sided access is needed when pulse-echo
technique is used.
• High accuracy in determining reflector position and
estimating size and shape.
• Minimal part preparation required.
• Electronic equipment provides instantaneous results.
• Detailed images can be produced with automated
systems.
• Has other uses such as thickness measurements, in
addition to flaw detection.

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• Surface must be accessible to transmit ultrasound.
• Skill and training is more extensive than with some other
methods.
• Normally requires a coupling medium to promote transfer
of sound energy into test specimen.
• Materials that are rough, irregular in shape, very small,
exceptionally thin or not homogeneous are difficult to
inspect.
• Cast iron and other coarse grained materials are difficult
to inspect due to low sound transmission and high signal
noise.
• Linear defects oriented parallel to the sound beam may go
undetected.
• Reference standards are required for both equipment
calibration, and characterization of flaws.

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54. 54
•A sound beam can be roughly divided into a
convergent (focusing) area, the near-
field, and a divergent (spreading) part, the
far field
•The length N of the near-field (near-field length)
and the divergence angle is dependent on the
diameter of the element, its frequency and the sound
velocity of the material to be tested
•The center beam is termed the acoustic axis.

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•Calibration is a operation of configuring the
ultrasonic test equipment to known values. This
provides the inspector with a means of comparing
test signals to known measurements.
•Calibration standards come in a wide variety of
material types, and configurations due to the
diversity of inspection applications.
•Calibration standards are typically manufactured
from materials of the same acoustic properties as
those of the test articles.

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• To select the appropriate NDT method we
must consider the following
 Thickness of material to be tested
 Location of discontinuity
 Type of material to be tested
 Orientation of discontinuity
 Type of defect