345589879-Ultrasonics-Demo.ppt

Ultrasonic Testing of materials :
A nondestructive method of examining materials by introducing ultrasonic
waves into, or onto the surface of the part being examined and determining
various attributes of the material.

Ultrasonics is the production of sound waves above the frequency of human
hearing [ above 20,000 cycles or 20 KHz per second ] and can be used in a
variety of applications such as, distance measurements, proximity
detectors, movement detectors, liquid level measurement. Ultrasonic is
used in medicine and robotics, security devices, laboratory and industrial
cleaners and a host of other applications. Bats use ultrasonic echo location
to find it’s flying path, search and catch insects. Bats operate in the
frequency range of 50 – 150 KHz.

Ultrasonic Testing :
High frequency [ 0.5 to 15 MHz ] ultrasound waves [ mechanical
vibrations ] are introduced into a material to detect changes in material
properties. A piezoelectric transducer is excited with a pulsating voltage
to generate ultrasound waves in the test material. The sound is
reflected back from something--either the back side of the part or from a
flaw--depending on what is in the material. When it reflects back, the
signals are detected, displayed and interpreted to determine the
thickness of the metal or the flaw that’s inside the metal.
Flaw - An imperfection or discontinuity that may be detectable by
nondestructive testing and is not necessarily rejectable.

Common uses of Ultrasonic
testing :
Surface and subsurface defects in
many materials including metals,
plastics, and wood. To measure the
thickness of materials and
otherwise characterize properties of
material based on sound velocity
and attenuation measurements.
Major use ;
Thickness measurements
Corrosion mapping
Metal cracking
Bonding
Plates
Pipes
Forged products
Cast products
Rolled products
Welded joints
Concrete

Advantages :
Depth of penetration for flaw detection or measurement is superior to other
methods.
Ultrasonic testing has the advantage of detecting discontinuities with access
to only one side of the test specimen.
Fast response time, Permits high speed automatic testing in ultrasonic
systems.
Accurate determination of imperfection position and estimation
of imperfection severity
Provides immediate information
Minimum part preparation is required.
Very small imperfections can be detected
Method can be used for much more than just flaw detection.
Ultrasonics units are portable, battery operated, radiation free and can
inspect deeper and may reveal flaws that may not be detected with
radiography such as Crack and Lack of Fusion.
Discontinuity - A lack of continuity or cohesion; an intentional or
unintentional interruption in the physical structure or configuration of a
material or component.

Disadvantages :
Provides indirect indication, discontinuities can not be identified directly.
Surface finish and roughness can interfere with inspection.
Requires full scanning of entire test area.
Requires a coupling medium which makes recording difficulties.
Thin parts may be difficult to inspect.
Reference standards are often needed.
Discontinuities must be intercepted perpendicularly.
Linear defects oriented parallel to the sound beam can go undetected.
Less sensitive to smaller flaw like porosity and slag fragments.
Conventional techniques does not provide permanent record of test signals.
Test reliability depends on operator’s skill and attention.
Skill and training required is more extensive than other techniques.
Major disadvantage of ultrasonic is too much operator depending, as no
record is left. However, in the last few years, new technologies like Time of
Flight diffraction and Phased Array techniques, have made UT recordable
and replacement of radiography by UT is now possible under circumstances
stated in various Codes.

Ultrasound waves are high frequency mechanical vibrations traveling
through a medium, which may be a solid, a liquid, or a gas. These waves
will travel through a given medium at a specific speed or velocity, in a
predictable direction, and when they encounter a boundary with a different
medium they will be reflected or transmitted according to simple rules. This
is the principle of physics that underlies ultrasonic flaw detection.

Ultrasonic testing :
High frequency sound waves are sent into a material by use of a
transducer. The sound waves travel through the material and are received
by the same transducer or a second transducer. The amount of energy
transmitted or received and the time the energy is received are analyzed to
determine the presence of flaws. The reflections from discontinuities and
the back wall are detected and displayed as information of the test object.
Separate indications for flaws and the back wall are displayed. Changes in
material thickness, and changes in material properties can also be
measured.
Reflector - An interface at which, ultrasonic beam encounters a change in
acoustic impedance and at which at least part of the energy is reflected.

Ultrasonic waves are propagation of mechanical energy through oscillatory
motion of the particles in a medium.
Frequency : All sound waves oscillate at a specific frequency, or number of
vibrations or cycles per second, which we experience as pitch in the familiar
range of audible sound. Human hearing extends to a maximum frequency of
about 20,000 cycles per second [ 20 KHz ], while the majority of ultrasonic
flaw detection applications utilize frequencies between 500,000 and
10,000,000 cycles per second [ 500 KHz to 10 MHz ]. At frequencies in the
megahertz range, sound energy does not travel efficiently through air or other
gasses, but it travels freely through most liquids and common engineering
materials.
Velocity : The speed of a sound wave varies depending on the medium
through which it is traveling, affected by the medium's density and elastic
properties. Different types of sound waves will travel at different velocities.
Material Longitudinal Shear
Steel 5950 3230 in meters / sec
Aluminum 6320 3080
Water 1485 ------
Perspex 2730 1430
Brass 3830 2050

Wavelength:
Any type of wave will have an associated wavelength, which is the distance
between any two corresponding points in the wave cycle as it travels
through a medium. Wavelength is related to frequency and velocity by the
simple equation
λ = c / f
where
λ = wavelength
c = sound velocity
f = frequency
Wavelength is a limiting factor that controls the amount of information that
can be derived from the behavior of a wave. In ultrasonic flaw detection, the
generally accepted lower limit of detection for a small flaw is one-half
wavelength. Anything smaller than that will be undetectable. In ultrasonic
thickness testing, the theoretical minimum measurable thickness is one
wavelength.

Ultrasound parameters
Wavelength : one complete oscillation of a vibrating particle.
Frequency : No of complete oscillation of the particle per second.
Velocity : The distance, sound energy travels in one second.
Amplitude - The vertical pulse height of a signal, usually base to peak.
Display

Ultrasound behavior :
Ultrasound gets Scattered by very small reflectors [ near or larger than
wavelength ] with a resultant loss of energy. Scattering is random reflection
of sound energy from grain boundaries and similar microstructure.
Diffraction occurs at ends of a larger reflector which may be detected and
used for flaw measurements.

Steel grains under very
high magnification.
Sound is scattered by
these grain boundaries
when their size
approaches the
wavelength. Sound is
scattered with significant
loss of energy and
produce base line noise .
Scattering - The
dispersion, deflection, or
redirection of the energy
in an ultrasonic beam
caused by small
reflectors in the material
being examined.

Graphite noodles in cast iron scatter ultrasound which produce noise and
loss of penetration.
Scattered energy - Energy that is reflected in a random fashion by small
reflectors in the path of a beam of ultrasonic waves.

Coarse grained material disperses ultrasound by random reflection from
grain boundaries with a reduction in penetration depth. This produces
noise on the baseline, serious loss of back reflection and indistinguishable
signal from smaller flaws.
Penetration depth - The maximum depth in a material from which usable
ultrasonic information can be obtained and measured.
Back reflection - Indication of the echo from the far boundary of the
material under test.

Grain refinement by heat treatment reduces ultrasound attenuation and
may permit the examination.

Attenuation of ultrasound :
The distance that a wave of a given frequency and energy level will travel
depends on the material through which it is traveling. As a general rule,
materials that are hard and homogeneous will transmit sound waves more
efficiently than those that are soft and heterogeneous or granular. Three
factors govern the distance a sound wave will travel in a given medium:
beam spreading, attenuation, and scattering. As the beam travels, the
leading edge becomes wider, the energy associated with the wave is spread
over a larger area, and eventually the energy dissipates. Attenuation is
energy loss associated with sound transmission through a medium,
essentially the degree to which energy is absorbed as the wave front moves
forward. As frequency decreases, beam spreading increases but the effects
of attenuation and scattering are reduced. For a given application,
transducer frequency should be selected to optimize these variables. The
amplitude of ultrasonic wave decreases as the propagating distance
increases. The amplitude of ultrasonic wave which has propagated the
distance of x is represented as
V(x)=V0 e -ax
where, a is the attenuation coefficient of a material.

Materials that cannot be tested ultrasonically include anything that can not
transmit ultrasound or scatter energy. Coarse grained material such as
copper, cast iron, austenitic stainless steel disperses ultrasound by
random reflection from grain boundaries.
Coarse grain material produce random noise and it becomes impossible to
interpret actual flaw signals.
Noise - Many undesired signal (electrical or acoustic) that tends to
interfere with the reception, interpretation, or processing of the desired
signal.
Base line - The time of flight or distance trace (horizontal) across the A-
scan CRT display (for no signal condition).

A Scan testing [ time / distance – amplitude display ] :
A piezoelectric transducer generates ultrasound energy within the test
specimen. The transducer can also convert mechanical energy coming back
from the specimen into electrical energy. Therefore, a transducer can both
send and receive energy. The returning energy can be transformed in an
electrical impulse which can be displayed on a CRT monitor in form of
echoes, thus allowing the identification of flaws within the specimen or
directly showing the thickness.
Echo - Indication of reflected energy.

For complete examination, the probe is moved over the entire test surface.
The back wall signal is monitored along with any new signal appearing
before the back wall. Any significant drop in back wall signal height or
appearance of a new signal are to be interpreted for the possible presence
of a discontinuity along the sound travel path.
Scanning - The movement of a search unit relative to the test piece in order
to examine a volume of the material.

Display
Ultrasound travels through the
material and If the material is sound,
the energy travels up to the back
surface and is reflected and returns to
the transducer. A back wall echo is
displayed which represent the
thickness.

Display
Ultrasonic display :
If a discontinuity exists on the path of
the ultrasonic energy, a part of it will
be reflected. The energy reflected
from the flaw and that from the back
surface, having traveled different path
lengths shall be indicated on the CRT
screen by the two different echoes on
the time base.

Display
Ultrasonic display :
Flaw being closer to the probe, the
flaw signal shifts to the left of the
screen. Back wall signal remains at
the same location because the
thickness is unchanged, with further
reduction in signal amplitude.

Display
The generated waves travel through the material in the form of beam
resembling a solid cone diverging steadily from the source of generation. In
a flawless object, energy reflected from the back wall is received by the
probe if the front and the back surface are parallel and the back wall echo is
produced. The red zone is the focus of the ultrasound beam where sound
pressure is maximum. Back surface - The end of a reference block that is
opposite the entry surface.

Display
A flaw lying in the path of the ultrasound beam, reflects some of the energy
thereby reducing the energy reaching the back wall, which results in a loss
of amplitude of the back wall signal.
Back surface - The end of a reference block that is opposite the entry
surface.

The CRT Screen divisions where test signals are displayed :
The CRT screen is graduated in 50 small equal divisions, divided into
10 major groups. By positioning known back wall echo signals at appropriate
scale divisions, different test ranges are produced.
Graduations on the time base scale when calibrated with reference to the
material under examination shall therefore readily give us the depth of flaw as
well as thickness of the material.

CRT Display :
Natural test signals are radio frequency type and have a serrated look. The
signals are rectified to smooth looking positive going signal for easy
interpretation.
Evaluation - A review, following interpretation of the indications noted, to
determine whether they meet specified acceptance criteria.

Unrectified Display :
Natural test signals are radio frequency type and have a serrated look. The
signals are rectified to smooth looking positive going signal for easy
interpretation.
Indication - Evidence of a discontinuity that requires interpretation to
determine its significance.

CRT Display :
Natural serrated signals are rectified to smooth looking positive going signal.
Initial pulse - The response of the ultrasonic system display to the transmitter
pulse.
Indication - That which marks or denotes the presence of a reflector.

1 2 3
Reading a CRT display :
Test range : 100 mm [ 1 small division for a 100 mm range is,
100 / 50 divisions = 2 mm ]
1. Initial pulse [ scale zero ]
2. Flaw signal at 78 mm [ 39 X 2 ]
3. Back wall signal at 100 mm [ 50 X 2 ]

Signal amplitude :
The ultrasonic signal is the voltage of the signal displayed on the CRT in
terms of vertical deflection.
Height of an echo signal is proportional to the area of a reflector. The
difference in signal height is measured in decibel [ dB ]
The decibel is not a measuring unit like Volt, Ampere or any other unit, but it
is a ratio between a reference unit and the measured value.
The dB difference between two signal levels are
dB = 20 log [ sig 1 / sig 2 ]
If one signal is twice the other, the ratio is 2 : 1
log of 2 = 0.3 hence dB difference is 20 X 0.3 = 6.
The difference in dB will be the same whatever the initial setting on the
gain control of the flaw detector.
The gain control in the flaw detector is calibrated in dB. When signal
changes are considered, doubling the echo height causes a 6dB increase.
For signal changes, a numerical ratio of 1.25 is 2dB, 2 is 6 dB, 4 is 12 dB, 5
is 14dB, 10 is 20dB, 100 is 40dB, and so on.

Sound Waves :
are propagation of mechanical energy through a medium. Sound waves in
solids can exist in various modes of propagation that are defined by the type
of motion of the particles involved. Longitudinal waves and shear waves are
the most common modes employed in ultrasonic flaw detection. Surface
waves and Lamb waves are also used depending on applications.

Longitudinal [ or compressional ] Waves :
Longitudinal wave is produced when mechanical force acts perpendicular to
the test surface. Longitudinal wave propagates by pushing the particles
toward the path of propagation. Wave propagates through compression and
rarefaction of the particles. Particle displacement is parallel to the direction
of the wave propagation. In solids, the particles do not move away from its
original position, but oscillate around it’s rest position. Longitudinal wave has
highest velocity among all waves and lower attenuation. All ultrasound
waves for material testing are generated in the longitudinal mode. Straight
beam probe generates these waves.

A longitudinal or compressional wave is characterized by particle motion in
the same direction as wave propagation, as from a piston source. Audible
sound exists as longitudinal waves only. Displacements of the particles
produce zones of compression and rarefaction.
Display

Expansion of the compressed zone
produces more compression zones. A
series of compression and rarefaction
transfer energy from one end to the
other end of the object.
Display

Display
Particle movements in longitudinal waves

When the mechanical force acts at an
angle to the test surface, shear waves
also known as transverse waves are
produced if the material is solid in nature.
A shear wave is characterized by particle
motion perpendicular to the direction of
wave propagation. The particles move up
and down with respect to it’s rest position,
and apply pull on adjacent particles.
Display

In a shear wave, the particles move up and down, pulling other particles with
it. This is possible only in solids, where the particles are locked by inter
atomic forces.
Shear waves can not be generated in liquids and gasses.

Display
Particle movements in shear waves.

Wave length of ultrasound depends on the frequency and velocity of sound,
in the medium through which it is traveling.
Velocity = frequency X wavelength

Reflection at an interface of two materials :
When ultrasound hits an interface of two mediums, part of the incident
energy is reflected back into the incident medium. The remaining energy
will be transmitted through.
Interface - The boundary between two materials.

Transmission at an interface of
two materials :
A sound beam that hits an
interface at perpendicular
incidence will be transmitted
straight through. When the sound
beam hits the interface at an
angle, the transmitted beam will
be refracted and undergo mode
conversion.
Mode - The type of ultrasonic
wave propagating in the
materials as characterized by the
particle motion (for example,
longitudinal, transverse, etc.).

Transmission at an interface of two materials :
A sound beam that hits an interface at perpendicular incidence will be
transmitted straight through. When the sound beam hits the interface at an
angle, the transmitted beam will be refracted and undergo mode conversion.

Reflection at an interface of two materials :
The amount of energy reflected, or reflection coefficient, is related to the
relative acoustic impedance of the two materials. Acoustic impedance is the
resistance to sound propagation which is a material property. It is defined as
density multiplied by the speed of sound in a given material. For any two
materials, the reflection coefficient as a percentage of incident energy /
pressure may be calculated through the formula
Reflection % = [ (Z1 – Z2) / (Z1 + Z2) ] 2 X 100.
Where,
Z1 = acoustic impedance of first material
Z2 = acoustic impedance of second material
For the metal / air interface, the reflection coefficient approaches 100%.
Virtually all of the sound energy is reflected from a crack or other
discontinuity in the path of the wave. This is the fundamental principle that
makes ultrasonic flaw detection possible.
Only about 1% of the generated energy finally returns to the probe.
Acoustic impedance in kg / meters2 / sec of
Steel is 45, Aluminum 17, Water 1.48, perspex xx.x

Couplants : is a material [ usually liquid ]
that facilitates the transmission of
ultrasonic energy from the transducer into
the test specimen. Couplant is generally
necessary because the acoustic
impedance mismatch between transducer
front face, air and the test specimen, is
large and, therefore, nearly all of the
energy is reflected and very little is
transmitted into the test material. The
couplant displaces the air and makes it
possible to transmit more sound energy
into the test specimen so that usable
ultrasonic signals can be obtained from the
test part. In contact ultrasonic testing a thin
film of oil, grease, glycerin, Propylene
glycol or water is generally used between
the transducer and the test surface. In
immersion testing, water column or water
bath conducts ultrasound into the test
material.

Immersion testing : The test setup for
ultrasonic testing in immersion is a water
tank, in which the test piece and the
ultrasonic transducer are immersed. As the
water serves as a coupling fluid between
probe and test piece, the ultrasonic
transducer need not to be driven in direct
contact to the test piece surface. This way
it is possible to mount the transducer onto
a multi-axis probe driving unit and to
automatically test complex, preferably axis
symmetric parts. All inspections, regardless
of the shape of the test piece, can be
performed at a high speed and with less
man power. Ultrasonic testing in immersion
has a wide range of applications but
generally limited to laboratory conditions.
Couplant - A substance used between the
search unit and test surface to permit or
improve transmission of ultrasonic energy.

Reflection of ultrasound :
ultrasound is highly directional, and at test frequencies used for flaw
detection, are well defined. A sound beam that hits an interface at
perpendicular incidence will reflect straight back. When the sound beam
hits the interface at an angle, will reflect forward at the same angle. The
angle of reflection equals the angle of incidence.

Refraction :
Sound energy that is transmitted from one material to
another, bends in direction. A beam that is traveling
straight will continue in a straight direction, but a beam
that strikes an interface at an angle will be bent
according to Snell’s Law :
Sin 1 V1
-------- = -----
Sin 2 V2
Where,
Sin 1 = incident angle in first material
Sin 2 = refracted angle in second material
V1 = sound velocity in first material
V2 = sound velocity in second material

Reflection and Refraction of ultrasound :
When ultrasound travels from one medium to another medium at an angle to
the interface, both the reflected and refracted beams split into longitudinal and
shear wave modes.

Transmission of ultrasound :
When sound travels from water to steel, 88% of the incident energy reflects
back into water. The remaining 12% energy enters steel.
Acoustic impedance [ Z ] of steel = 45, water = 14.5
Reflection % = [ (45 – 14.5) / (45 + 14.5) ] 2 X 100 or 88%.

Normal Incidence :
For perpendicular incidence, the direction and the wave mode in the
second medium is the same as in the first medium.
Angular Incidence :
the refracted beam splits into longitudinal and shear waves. The angle of
the longitudinal component is larger than the shear waves. When the angle
of incidence increases, the refracted angles also increases.

First Critical angle :
The increasing incident angle reach a point, when the longitudinal wave is
refracted along the surface and produces creeping waves which travel
immediately below the part surface.
Second Critical angle :
with further increase in incident angle, at double the first critical angle, the
shear wave is also refracted along the surface and converted to surface
waves traveling along the part surface.

First critical Angle :
Incident angle in Perspex for first critical angle with Steel ;
Given, longitudinal wave velocity in perspex 2.73 X 106 mm / sec,
longitudinal wave velocity in steel 5.95 X 106 / sec.
Sin I velocity in perspex
------- = ------------------------
Sin R velocity in steel
velocity in perspex X Sin R
Sin I = ------------------------------------
velocity in steel
I = Sin – [ ( 2.73 X 1 ) / 5.95 ]
Sin – [ .46 ] or 27.60
Critical angle - The incident angle of the ultrasonic beam beyond which a
specific refracted wave no longer exists. Normal incidence (also see straight
beam) - A condition in which the axis of the ultrasonic beam
is perpendicular to the entry surface of the part under examination.

Surface Waves in water :
The floating object moves up and down at it’s own place, and the energy
propagates along the surface in the form of waves.

Surface Waves in water :
The particles movement in a
surface wave is elliptical. The
waves propagate,
perpendicular to the particle
vibration.

Surface Waves :
Surface waves are known as Rayleigh waves which travel on and just
beneath the surface of a material, penetrating up to a depth of
approximately one wavelength. Below one wavelength, the energy drops
to only 4% and there is no possibility to detect any defect at this depth.

Display
Particle movement in surface waves

Testing for surface cracks with a surface wave probe.
As the name suggests, surface waves [ or Rayleigh waves ] travel along
the surface of components, penetrating to a depth in the order of one
ultrasonic wavelength. These waves propagate along the surface, follows
smooth curve, travel with low attenuation, and is reflected from defects at
or very near the surface. Surface waves are sensitive to surface condition
and will be attenuated by excess couplant left on the surface. Since
energy is concentrated in the surface region, small blemishes on the
surface can give rise to spurious indications. The inspection surface
requires excess couplant or dirt to be removed.

Display
When a surface wave probe is used in pulse-echo mode, it is suitable for
the detection of surface breaking flaws, provided that the beam direction is
normal to the plane of the flaw, sound will be reflected back to the
transducer. Surface wave probes can also be used in transmitter -receiver
mode, such that when the signal detected by the receiver is weakened or
totally disappears, it signifies that a surface breaking flaw lies between
them.

Display
A variety of reflectors can be used for set up of surface wave
inspections. Electrical discharge machined notches, saw cuts, chiseled
notches, and drilled holes can be used. Suggested surface wave
standards are the side-drilled holes or the
notch in the IIW block when the search unit is placed on the large
front or back surface of the block. The reflected signal from one of
the holes or the notch can be compared with the reflected signals from
discontinuities in test parts.
Signals should be compared at equivalent travel distances (distance
from search unit to reference standard reflector equal to distance from
search unit to test part discontinuity

Lamb wave : Plate waves, can be propagated only in very thin metals.
Lamb waves are the most commonly used plate waves in NDT. Lamb
waves are a complex vibrational wave that travels through the entire
thickness of a thin material. Propagation of these waves depends on
density, elastic, and material properties of a component, and they are
influenced by a great deal by selected frequency and material thickness.
With Lamb waves, a number of modes of particle vibration are possible,
but the two most common are symmetrical and asymmetrical. The
complex motion of the particles is similar to the elliptical orbits for surface
waves. This technique can detect crack and lamination in thin strips.
Long range ultrasonics is a method which use lamb waves to test pipes
upto 50 meters in length for gross defects.

Testing for surface cracks with a surface wave probe.
Surface wave propagate along the surface and is reflected from defects at
surface.

Display
Critically refracted longitudinal waves / Creeping Waves :
The angle of incidence of ultrasonic beam at perspex / steel surface
necessary for producing LCR waves in the specimens is estimated as sine
inverse of the ratio of longitudinal ultrasonic velocity in perspex to that in
steel specimens. This angle has been estimated to be about 27.23.
Creeping waves are high angle compression waves, which propagate
immediately beneath the inspection surface. They are used for a number of
applications where surface-breaking or very near-surface planar flaws need
to be detected. As creeping waves propagate, they interact with the
inspection surface causing secondary shear waves to be emitted. This
continuous transfer of energy, from one wave mode to another, means that
creeping waves are attenuated rapidly and inspection is only effective over a
relatively short range [ 40 - 50mm / 1.6 - 2.0“ ]. For this reason they are
normally used to inspect specific areas such as the toe of welds, where the
probe can be placed in close contact with the area of interest. Unlike true
surface waves, creeping waves are relatively insensitive to the condition of
the inspection surface and do not require excess ultrasonic couplant or dirt
to be removed.

Display
Creeping wave probes are a special type of Transmitter – Receiver
probes, which generate longitudinal waves at angles between 70° and 90°
in the test material. These waves, commonly known as creeping waves,
propagate parallel to the surface of the test piece; a shear wave beam is
also generated, which radiates at an angle of about 33°. Creeping wave
probes are suitable for detection and sizing of flaws close to the surface
like deep IGSCC (intergranular stress-corrosion cracking). Creeping
waves are unaffected by liquid drops, welding spatters or other materials
on the surface. However, the working range is short because of the steep
energy decay. Usually, the most sensitive point, the so-called "focus" is
located just in front of the probe itself. Nominal focus distance ranges up
to 20 mm and the maximum useful range is typically 45 mm.

Display
Attenuation of ultrasound :
The amplitude of ultrasonic wave decreases as the propagating distance
increases. The amplitude of ultrasonic wave which has propagated the
distance of x is represented as
V(x)=V0 e -ax
where, a is the attenuation coefficient of a material.
Attenuation - A factor that describes the decrease in ultrasound intensity
with distance. Normally expressed in decibel per unit length.

Display
Ultrasonic Probes / Transducers :
There are two major types of transducers : contact and immersion.
Contact : As the name implies, contact transducers are used in direct
contact with the test surface. Contact transducers, utilize a coupling
material, such as water, glycerin, grease, engine oil, wall paper paste,
methyl cellulose etc to prevent air gaps from resisting ultrasound
transmission in to the test material. The coupling medium must be non
corrosive. Except immersion type, all other transducer operate in contact
with the test object.
Immersion : Immersion transducers are designed to couple sound energy
into the test piece through a water column or water bath. They are used in
automated scanning applications and also in situations where a sharply
focused beam is needed to improve flaw resolution. These transducers are
longitudinal wave type with normal incidence. The transducer is angulated to
produce refracted angular beams inside the test object. It is not possible to
transmit shear wave in water.

Normal probes are used for detecting defects parallel to the outside surface
of a part.

Ultrasonic Probes :
Normal Incidence : They
introduce sound energy
perpendicular to the
surface, and are typically
used for locating voids,
porosity, and cracks or
delaminations parallel to
the outside surface of a
part, as well as for
measuring thickness.
Angular incidence : They
introduce sound energy at
an angle to the surface, and
are typically used for
locating discontinuities that
are neither parallel nor
perpendicular to the test
surface.

Angle probes are used for detecting discontinuities that are neither parallel
nor perpendicular to the test surface.

Piezoelectric Crystals :
Piezoelectric materials are used for
generating and receiving ultrasound.
Certain materials such as Quartz becomes
electrically charged when mechanical force
deforms its shape.
It is also possible to deform it’s shape by
applying electrical signal.
This property of the disc is used for
generation and detection of ultrasound waves
in test materials.
Modern ultrasonic probes use artificially
produced ceramics which is polarized to
develop better piezoelectric properties. The
ceramic material is non conductor, hence
both the faces are coated with silver to make
electrical connections.

Piezoelectric effect :
piezoelectric materials becomes electrically charged when mechanical
force acts on it’s surface. Piezoelectric disc is utilized for detection of flaws
when reflected waves applies deforming force on the disc’s surface.
Ultrasonic probes use artificially produced ceramics which generates
ultrasonic waves in the test material with better efficiency than quartz.

Reverse Piezoelectric effect :
The thickness of a piezoelectric disc changes when an electric field is
applied on to it’s surface. When the electrical polarity is reversed, the
deformation reverses.

A Triggering high voltage electrical pulse of short duration is applied to
the piezoelectric disc to force it into rapid oscillation :
The oscillating crystal surface, when in contact with a medium,
produces mechanical vibrations in the medium.

Expanding and contracting movement
of the front surface of the
piezoelectric element, which is in
contact with a material, produces
successive compression and
rarefaction in the medium which
transfers mechanical energy from one
end to the other end.
Display

Generation and Reception of Ultrasound :
Ultrasonic testing relies on the transducer to generate and receive
ultrasound. The ceramic piezoelectric crystal produces mechanical
vibrations that pass through the part and also change the returning pulse
echo from mechanical vibrations back into electrical signal so that the
detector can display these signals.

Composite elements :
Are produced to reduce impedance mismatch between the transducer and
the test part, thereby an increase in ultrasound transmission.
An array of active piezoelectric rods are embedded into a passive ceramic
polymer structure known as the 1-3 piezo-composite structure. Their
properties depend on the ceramic and polymer properties and on the
microstructure itself .
Composite materials have a high coupling coefficient that confers a high
sensitivity and signal to noise ratio [ + 10 to 30 dB compared to
conventional ceramics ]. The lower and adjustable acoustic impedance
allows a higher energy transfer in water, and a lower reverberation level on
the front face for immersion testing applications.

Composite crystals :
The 1-3 structure of the composite avoids
radial vibration modes. This performance
directly benefits to the beam pattern and
pulse shape.
Composite materials can be mechanically
focused. This property allows the
manufacturing of cylindrical, spherical or
curved transducers without using acoustic
lens. Lens attenuation is avoided and allows
a more predictable beam pattern.
Composite materials also have a higher
mechanical resistance, that confers to the
transducers a higher resistance to
mechanical shocks, vibrations, temperature
constraints and pressure constraints.
Signal-to-noise ratio - The ratio of the
amplitude of an ultrasonic indication to the
amplitude of the maximum background
noise.

Normal probe : Typical transducers for ultrasonic flaw detection utilize an
active piezoelectric element ceramic, composite, or polymer. When this
element is excited by a high voltage electrical pulse, it vibrates across a
specific spectrum of frequencies and generates a burst of sound waves.
When the element is vibrated by an incoming sound wave, it generates an
electrical pulse. The front surface of the element is usually covered by a
wear plate that protects it from damage during contact testing.

Damping the crystal vibrations : The back surface of the element is
bonded to backing material [ usually tungsten powder in araldite ] .The
damping material attached to the back of the crystal mechanically damps
the vibration and shortens it’s ringing time. Sharper signals are produced
with an increase in echo resolution.
Crystal - The piezoelectric element in an ultrasonic search unit. The term is
used to describe single crystal piezoelectric as well as polycrystalline
piezoelectric, such as ferroceramics. Resolution - The ability of ultrasonic
equipment to give simultaneous, separate indications from discontinuities
having nearly the same range and lateral position with respect to the beam
axis.

Damping the element improves resolution. Resolution is the ability to
produce separate signals from discontinuities which are located very close
to each other at nearly the the same depth.

Piezoelectric Ceramic elements : Lead
Zirconate Titanate [ PZT ] and Barium
Titanate are most common. Barium
Titanate is the most efficient ultrasound
generator. Lithium Sulphate is the best
receiver but hygroscopic in nature. Lead
Zirconate Titanate has the best overall
generating – receiving performance. Other
elements are Lead Meta Niobate,
Polyvinylidene difluoride etc.

Wear Plates : Mostly Aluminium Oxide ceramic discs bonded to the front
surface of the active element is used as rubbing face which protects the
soft silver coated surface from wear during contact testing. The piezo
element is protected as long as the wear plate is undamaged.

Normal probe with 24 mm diameter active element. A Lemo type connecting
cable which connects the probe to the flaw detector and replaceable plastic
front face protective membranes such as Polyethlethylketone . A thin layer of
oil is used between the front face and the plastic membrane.

Normal probes, 24 mm and 10 mm element [ crystal ] size. The front ring
holds the replaceable plastic membrane in place.

Probe connectors :Delay line transducer. This contact transducer contains a
plastic wedge between the transducer and the part being measured.
Basically, it eliminates the near field.

A normal beam [ Longitudinal wave ] probe being used for flaw detection :
This method is called contact testing which uses a coupling medium between
the probe and the test part. Because sound energy at ultrasonic frequencies
does not travel efficiently through gasses, a thin layer of coupling liquid or gel
is normally used between the transducer and the test piece.

Normal probes for immersion testing have a
beam focusing lens attached to the face of
the probe. Immersion transducers don’t come
into contact with the component under
examination. Instead, they operate within a
liquid. The watertight housing eliminates the
chance of air pockets affecting results. The
probe and the object is immersed in a water
tank. This method is mainly used for thin
objects, small objects with shapes and
objects with complicates shapes. Immersion
technique mostly uses C scan recording
which records a plan view of the test object.

Advantages :.
The immersion technique provides a means
of uniform coupling
Quarter wavelength matching layer increases
sound energy output
All immersion transducers, except paintbrush,
may be focused spherically (spot) or
cylindrically
focal length concentrates the sound beam to
increase sensitivity to small reflectors
Applications :
Automated scanning
On-line thickness gaging .
High speed flaw detection in pipe, bar, tube,
plate, and other similar components
Time-of-flight and amplitude based imaging
Thru-transmission testing

Normal probes for immersion testing have a beam focusing lens attached to
the face of the probe. Immersion transducers don’t come into contact with
the component under examination. Instead, they operate within a liquid. The
watertight housing eliminates the chance of air pockets affecting results.
The probe and the object is immersed in a water tank. This method is mainly
used for thin objects, small objects with shapes and objects with
complicates shapes. Immersion technique mostly uses C scan recording
which records a plan view of the test object.

Wheel probes provide a simple approach to
the scanning of large structures where an
immersion system may not be feasible. The
wheel probes are designed to use standard
ultrasonic immersion transducers or linear
array ultrasonic immersion transducers.
Single element wheel probes allow the user
to rapidly gather B-scan images of the
structure allowing easy detection of many
defects. The tire used with the wheel probe
is manufactured from rubber matched
acoustically to water. The use of water
coupling inside the wheel allows the wheel
to be used with standard immersion
transducers. A range of fittings and manual
buggies complete the range allowing quick
and easy scanning in either manual or
automatic modes.

The Cathode Ray Tube [ CRT ] which displays the test signals on the front
face, a phosphor coated screen. The heated filament in the electron gun
emits electrons which are focused on the screen to produce an illuminated
spot. The spot can be deflected along X and Y directions by application of
signals on the X and Y plates.
Base line - The time of flight or distance trace (horizontal) across the A-
scan CRT display (for no signal condition).
Sweep - The uniform and repeated movement of an electron beam across
the CRT.

The horizontal and vertical plates deflects the electron spot across the
screen. The fast moving spot display the electrical signals in X and Y plane.
A part of the electrical pulse is fed into the vertical plates of the CRT
simultaneously with the same being applied to the transducer. This gives the
transmission echo on the time base which is the starting point for measuring
time of interval on the calibrated scale. The returning signals coming from
the test part are applied to the vertical plates which deflects the spot
vertically to produce echoes.

The instruments which display the ultrasonic test information are known as
Ultrasonic Flaw Detectors. Three major components of an ultrasonic
system, are the transducer that generates ultrasound, the pulser -receiver
which acts as communicator between the transducer and display and a
screen to display test signals. The pulser provides excitation pulses to drive
the transducer, and the receiver provides amplification and filtering for the
returning signals coming from the part through the transducer. Pulse
amplitude, shape, and damping can be controlled to optimize transducer
performance, and receiver gain and bandwidth can be adjusted to optimize
signal-to-noise ratios. The display may be a CRT, a liquid crystal, or an
electro luminescent display. The screen will typically be calibrated in units
of depth or distance. Multicolor displays can be used to provide interpretive
assistance.

The ultrasonic transducer emits a pulsed sound beam, which is transmitted
through a coupling medium into the test piece. Boundary faces, like cracks,
inside the test piece reflect the sound pulse back to the transducer. The
transducer at once changes this mechanical energy into electrical energy
which after amplification is routed through different electronic circuits. It
ultimately reaches a vertical plate of cathode-ray tube in the form of
unidirectional voltage. This is indirected by a pipe on the time base which is
incorporated in between the horizontal plates of CPT to reckon the time of
travel of the Ultrasonic energy into the material.

The basic controls which are used to setup an ultrasonic flaw detector for
the examination are ;
on – off switch, focus, mode control, gain control, range and delay controls,
reject control, gate controls. Battery operated machines also provide a
battery charging socket and battery status indicator. The machine
automatically switches off when the battery is drained.

In addition to on – off, the switch selects between a low output – high
resolution or a high output – low resolution mode.
The mode switch selects single or double probe operation.
In the single mode, both the probe sockets are identical. In the dual mode,
one socket is only transmitter while the other is only receiver.
Display

The coarse range selects the operating range of the flaw detector, which is
normally 10 – 50, 50 – 250, 250 – 1000 and 1000 – 5000 mm, when using
a longitudinal wave probe. When a shear wave probe is used the ranges
become approximately half.
The 20 dB control has 0, 20, 40, 60 dB settings. Each step above the
0 step, amplify the existing signals by 10 times.
Display

The gate controls select a portion of the calibrated range to monitor
ultrasonic signals. A signal located in the gated region triggers an alarm
and a ‘ LED ‘ in the detector. Gate - An electronic means of selecting a
segment of the time range for monitoring or further
processing. The
reject control can be used to suppress lower amplitude noise signals which
interferes during the testing.
Reject or suppression – A control for minimizing or eliminating low
amplitude signals [ electrical or material noise ] so that larger signals are

The focus control sharpens the CRT trace for better resolution.
The 2 dB step gain control has 20 steps of 2 dB each. Each step amplifies
the existing signals by 1.25 times.
dB control - A control that adjusts the amplitude of the display signal in dB
units.
Display

Fine range and delay controls are multi turn controls used to adjust the
calibration signals to appropriate scale divisions. The delay control is used
to set the first calibration signal. The range control is used to set the second
calibration signal. The delay control can be used to shift the signals across
the CRT screen without disturbing a calibrated range.
Display

Calibration refers to the act of evaluating and adjusting the precision and
accuracy of measurement equipment. In ultrasonic testing, several forms of
calibration must occur. First, the electronics of the equipment must be
calibrated to assure that they are performing as designed. This operation is
usually performed by the equipment manufacturer and will not be discussed
further in this material. It is also usually necessary for the operator to
perform a "user calibration" of the equipment. This user calibration is
necessary because most ultrasonic equipment can be reconfigured for use
in a large variety of applications. The user must "calibrate" the system,
which includes the equipment settings, the transducer, and the test setup,
to validate that the desired level of precision and accuracy are achieved.
The term calibration standard is usually only used when an absolute value
is measured and in many cases, the standards are traceable back to
standards at the National Institute for Standards and Technology.
Display

In ultrasonic testing, there is also a need for reference standards.
Reference standards are used to establish a general level of consistency in
measurements and to help interpret and quantify the information contained
in the received signal. Reference standards are used to validate that the
equipment and the setup provide similar results from one day to the next
and that similar results are produced by different systems. Reference
standards also help the inspector to estimate the size of flaws. In a pulse-
echo type setup, signal strength depends on both the size of the flaw and
the distance between the flaw and the transducer. The inspector can use a
reference standard with an artificially induced flaw of known size and at
approximately the same distance away for the transducer to produce a
signal. By comparing the signal from the reference standard to that
received from the actual flaw, the inspector can estimate the flaw size.
Display

Calibration and reference standards for ultrasonic testing come in many
shapes and sizes. The type of standard used is dependent on the NDE
application and the form and shape of the object being evaluated. The
material of the reference standard should be the same as the material
being inspected and the artificially induced flaw should closely resemble
that of the actual flaw. This second requirement is a major limitation of most
standard reference samples. Most use drilled holes and notches that do not
closely represent real flaws. In most cases the artificially induced defects in
reference standards are better reflectors of sound energy (due to their
flatter and smoother surfaces) and produce indications that are larger than
those that a similar sized flaw would produce. Producing more "realistic"
defects is cost prohibitive in most cases and, therefore, the inspector can
only make an estimate of the flaw size. Computer programs that allow the
inspector to create computer simulated models of the part and flaw may
one day lessen this limitation.
Display

Test Range Calibration with normal
probe :
Before actual testing, the machine is
first set to a known distance range by
calibrating the CRT screen using back
wall echoes from test blocks
accurately machined to a standard
thickness.
The test material and the material of
the calibration block must be same.
Range - The maximum sound path
length that is displayed.
Display

The International Institute of Welding calibration block, IIW - V1 is the
standard block for setting up an ultrasonic flaw detector for testing
applications.
Display

I.I.W Calibration Standard. Calibration of Shear and compression wave
probes. Checking beam angle, beam exit point and resolution. Calibration
of time base and gain settings.These blocks are also produced with some
difference in design features.
Display

IIW – V1 block major dimensions : The plastic insert is used for checking
the sound generating power of the flaw detector.
Display

Normal probe placed on the face [ 25 mm thk ] of the IIW block for the
purpose of test range calibration. Echoes at multiple of 25 mm can be
obtained.
Display

Repetitive signal of the back reflection can be seen on the CRT screen.

Locations of echoes after 100 mm range calibration :
Test range 100 mm, 1 small scale division equals 2 mm.
Location of 1st back wall echo 25 / 2 = 12.5th division.
Location of 2nd back wall echo 50 / 2 = 25th division.

By positioning the echoes as shown in the picture, 125 mm range can
be calibrated :
1st echo 25 / 2.5 = 10th division
2nd echo 50 / 2.5 = 20th division
3rd echo 75 / 2.5 = 30th division

Echo Signals as displayed on the CRT screen after calibration.

Signal locations on a 100 mm calibrated screen, when the probe is placed
on a thickness of 25 mm :
Echoes set to 12.5, 25, 37.5 and 50 divisions.

A block of material, which is
accurately machined to a
standard thickness can be used
to calibrate different test ranges.
The block produces a series of
back wall signals at regular interval,
such as a block of 25 mm will
produce signals at 25, 50, 75, 100,
125 mm and so on.
To calibrate 100 mm full scale,
the setting of the calibrating echoes
will be
1st echo 25 / 2 = 12.5th division
2nd echo 50 / 2 = 25th division

A block of material, which is accurately
machined to a standard thickness can
be used to calibrate different test
ranges.
The block produces a series of back wall
signals at regular interval, such as a block of
25 mm will produce signals at 25, 50, 75,
100, 125 mm and so on.
To calibrate 100 mm full scale, the
setting of the calibrating echoes will be
1st echo 25 / 2 = 12.5th division
2nd echo 50 / 2 = 25th division 3rd echo
75 / 2 = 37.5th division.

Modern Digital flaw detectors allow easy set up of test parameters. Internal
data loggers can be used to record full waveform and setup information
associated with each test. These flaw detectors can display selected
information like echo amplitude, beam path, depth or distance readings.

Digital flaw detectors capture a
waveform digitally and then perform
various measurement and analysis
function on it. A clock or timer will
be used to synchronize transducer
pulses and provide distance
calibration. Signal processing may
be as simple as generation of a
waveform display that shows signal
amplitude versus time on a
calibrated scale, or as complex as
sophisticated digital processing
algorithms that incorporate distance
/ amplitude correction and
trigonometric calculations for
angled sound paths. Alarm gates
are often employed to monitor
signal levels at selected points in
the wave train to flag echoes from
flaws.

Pulse-echo method :
This method uses short pulses of sound that travel through the part to either
locate a flaw or the back side of the part. It’s suitable for flaw detection or
thickness testing. The time it takes for the sound to travel through the part
and bounce back is calculated using the simple equation, d = v t / 2 where d
is the distance from the surface to the discontinuity, v is the velocity of
sound waves and t is the round-trip transmit time. The operator moves a
transducer over the surface of the part, and the tester will display the
echoes.
Pulse echo method - An inspection method in which the echo amplitude and
time indicate the presence and position of a reflector.

There are three ways to display
information collected from an
ultrasonic tester. They’re known as A-
scan, B-scan and C-scan.
The A-scan presentation displays the
relative amount of energy received
on the vertical axis and elapsed time
along the horizontal axis.
The B-scan display is a cross-
sectional view with travel time
displayed along the vertical axis and
linear position of the transducer
displayed along the horizontal axis.
C-scan presentations are used with
automated data acquisition systems.
The C-scan displays information
along a plane of the image parallel to
the scan pattern of the transducer.
Gaps in the scan pattern represent
defects within the material.

The most commonly used ultrasonic testing technique is pulse echo A scan,
wherein sound is introduced into a test object and reflections (echoes) from
internal imperfections or from the part's geometrical surfaces are returned to
a receiver to produce a display where the distance of the reflector can be
read on a calibrated scale.

Pulse-Echo - One Transducer
• Ultrasound reflected from the sample is used.
• Can determine which interface is delaminated.
• Requires scanning from both sides to inspect
all interfaces.
• Provides images with high degree of spatial
detail.
• Peak Amplitude, Time of Flight (TOF) and
Phase Inversion measurement
Through Transmission - Two Transducers
• Ultrasound transmitted through the
sample is used.
• One Scan reveals delamination at all
interfaces.
• No way to determine which interface is
delaminated.
• Less spatial resolution than pulse-echo.
• Commonly used to verify pulse-echo
results.
Pulse-Echo Through Transmission
Transmit
&
Receive
Transmit
Receive
Inspection Modes

Through Transmission
Receive
Transmit
3
3
2
2
1
1

A-SCAN
Initial Pulse
Front surface
Interface of
interest
Back surface
Transducer
Sample

A-SCAN
A-Scans provide the following information:
1. Amplitude / % of full screen height (FSH)
2. Phase / positive or negative peak
3. Time / Depth
Amplitude %FSH 0%
100%
-100%
_
+ Phase
Phase
Time / Depth

B-Scan
Front surface
Back surface
Front surface Signal from
indication
Back surface
The blue line (B-scan gate) represents the depth
of information recorded.
Signal from
indication

IP
Front surface
C-SCAN
Area of interest
Back surface
The red box (data gate) indicates
the depth of information.

Immersion testing setup
C-scan - An ultrasonic data presentation which provides a plain view of
the test object, and discontinuities therein.

Immersion testing machine
Immersion testing - An ultrasonic
examination method in which the
search unit and the test part
are submerged (at least locally) in
a fluid, usually water.

Immersion testing machine with 3 axis probe manipulator

Immersion testing machine This is another name for a top (or plan) view
image. C-Scans can be obtained from immersion testing systems (where a
0° compression wave probe is scanned across an area through a water
path, i.e. non-contact scanning) or from direct 0° contact scans. Depending
on the mode of operation selected, the color coding levels on the image may
represent signal amplitude or range. The latter case is used for automated
corrosion mapping where on-screen cursors can be used to show the
thickness at any point and sectional thickness plots.

Probe focussing
Focused beam - Converging energy of the sound beam at a specified
distance.
B-scan presentation – A means of ultrasonic data presentation which
displays a cross section of the specimen indicating the approximate length
(as detected per scan) of reflectors and their relative positions.

Type of test part which requires c scan recording.

Not all of the inclusions and back wall reflections are
discernable when evaluating through thickness conditions of
plate, pipe wall and aboveground storage tank floors. The
application of B-Scan technology has improved the ability to
make an assessment of pitting, inclusions and wall loss due to
corrosion and erosion.
B-Scan presentation of ultrasonic evaluation of an inspected
area of plate or pipe essentially, converts all of the A-Scan
recordings and compiles them into a single path through
thickness profile of the areas of interest. This side view or
profile gives the inspector the capability of defining pitting and
corrosive conditions more accurately.
Geometry MINI IIW:
1" X 2" X 6". Contains 1" diameter hole, 2" radius 1/4"
deep cutout test side, 3 side drilled holes, and a 3/4" sq.
x .100" deep cutout.

Delay Line Transducers
Delay line transducers are single element
longitudinal wave transducers used in
conjunction with a replaceable delay line.
One of the reasons for choosing a delay
line transducer is that near surface
resolution can be improved. The delay
allows the element to stop vibrating before
a return signal from the reflector can be
received. When using a delay line
transducer, there will be multiple echoes
from end of the delay line and it is
important to take these into account.
Another use of delay line transducers is in
applications in which the test material is at
an elevated temperature. The high
temperature delay line options are not
intended for continuous contact, they are
meant for intermittent contact only.

Sound field [ intensity distribution ] of a probe : The near field is an area
of space in which the sound waves are not uniform. The ultrasonic beam
is more uniform in the far field, where the beam is spread out in a pattern
originating from the center of the transducer. The variations that occur in
the near field eventually change to a smooth and declining amplitude, at
which point the far field begins.

Sound field, Near and Far zone of a normal probe : The near field is an area
of space in which the sound waves are not uniform. The ultrasonic beam is
more uniform in the far field, where the beam is spread out in a pattern
originating from the center of the transducer. The variations that occur in the
near field eventually change to a smooth and declining amplitude, at which
point the far field begins.
Beam spread - A divergence of the ultrasonic beam as the sound travels
through a medium.

Sound field of a normal beam probe
Ultrasound spreads out from a true
parallel beam and the intensity per
unit area reduces with distance
from the source.
Near field - The region of the
ultrasonic beam adjacent to the
transducer and having complex
beam profiles. Also known as the
Fresnel zone.

Sound field of a probe :
The sound field of a probe is
divided into two zones.
Near zone : Intensity in this zone
vary because of interference effect.
Signal from a constant reflector
vary from place to place. This zone
is not suitable for flaw
measurements.
Near zone length D2 / 4 λ, where D
is element diameter and λ is
effective wavelength.
Far zone : is after near zone, and
intensity is inversely proportional to
square of distance. This zone is
suitable for flaw measurements.
Half Beam spread γ = 1.22 λ / D

Sound field of a probe :
Near field length for a 5 MHz,
10 mm dia circular crystal in
steel [ V = 5.9 X 106 mm ]
NZ = D2 / 4 λ or D2 f / 4 V
or 10 X 10 X 5 X 106 / 4 X
5.9 X 106
or 21.2 mm
Half Beam spread
γ = 1.22 λ / D

Because of variation of sound intensity at different distances, the signal from
a constant reflector vary with distance.
DAC [distance amplitude correction] - Electronic change of amplification to
provide equal amplitude from equal reflectors at different depths.

Signal amplitude from a 2 mm dia FBH for a 2 MHz, 24 mm dia probe. Distance
amplitude response curve - A curve showing the relationship between the
different distances and the amplitudes of ultrasonic response from targets of
equal size in an ultrasonic response from targets of equal size in an ultrasonic
transmitting medium.
Display

Signal amplitude comparison for a back wall and from a 2 mm dia FBH for
2 MHz angle probes.
Echo amplitude from a large reflector such as a back wall is inversely
proportional to the distance. Echo amplitude from a small reflector such as a
flaw is inversely proportional to the square of distance, i. e. signal of a small
reflector becomes one-fourth if its distance is doubled.
Display

Calibration Blocks :Reference block - A block that is used both as a
measurement scale and as a means of providing
an ultrasonic reflection of known characteristics. Sensitivity – A measure of
the smallest ultrasonic signal which will produce a discernible
indication on the display of an ultrasonic system. Reference block - A block
that is used both as a measurement scale and as a means of providing
an ultrasonic reflection of known characteristics.

Flat Bottom Holes which can be used for setting test sensitivity with normal
probes and comparing disc equivalent reflectors in wrought products.

Flat Bottom Holes which
can be used for setting test
sensitivity with normal
probes and comparing disc
equivalent reflectors in
wrought products.

ASTM set of 10 Flat Bottom Hole blocks which can be used for
checking dead zone, resolution and drawing Distance Amplitude
Correction [ DAC ] curves for normal beam testing. Wave
Interference
-------

Calibration Methods
 Calibration refers to the act of evaluating and adjusting the
precision and accuracy of measurement equipment. In
ultrasonic testing, several forms of calibration must occur.
First, the electronics of the equipment must be calibrated to
assure that they are performing as designed. This operation
is usually performed by the equipment manufacturer and will
not be discussed further in this material. It is also usually
necessary for the operator to perform a "user calibration" of
the equipment. This user calibration is necessary because
most ultrasonic equipment can be reconfigured for use in a
large variety of applications. The user must "calibrate" the
system, which includes the equipment settings, the
transducer, and the test setup, to validate that the desired
level of precision and accuracy are achieved. The term
calibration standard is usually only used when an absolute
value is measured and in many cases, the standards are
traceable back to standards at the National Institute for
Standards and Technology.

 In ultrasonic testing, there is also a need for reference standards.
Reference standards are used to establish a general level of
consistency in measurements and to help interpret and quantify
the information contained in the received signal. Reference
standards are used to validate that the equipment and the setup
provide similar results from one day to the next and that similar
results are produced by different systems. Reference standards
also help the inspector to estimate the size of flaws. In a pulse-
echo type setup, signal strength depends on both the size of the
flaw and the distance between the flaw and the transducer. The
inspector can use d a reference standard with an artificially
induced flaw of known size anat approximately the same distance
away for the transducer to produce a signal. By comparing the
signal from the reference standard to that received from the actual
flaw, the inspector can estimate the flaw size.
 This section will discuss some of the more common calibration and
reference specimen that are used in ultrasonic inspection. Some of
these specimens are shown in the figure above. Be aware that are
other standards available and that specially designed standards
may be required for many applications. The information provided
here is intended to serve a general introduction to the standards
and not to be instruction on the proper use of the standards.

Introduction to the Common Standards
 Calibration and reference standards for ultrasonic testing come
in many shapes and sizes. The type of standard used is
dependent on the NDE application and the form and shape of
the object being evaluated. The material of the reference
standard should be the same as the material being inspected
and the artificially induced flaw should closely resemble that of
the actual flaw. This second requirement is a major limitation of
most standard reference samples. Most use drilled holes and
notches that do not closely represent real flaws. In most cases
the artificially induced defects in reference standards are better
reflectors of sound energy (due to their flatter and smoother
surfaces) and produce indications that are larger than those
that a similar sized flaw would produce. Producing more
"realistic" defects is cost prohibitive in most cases and,
therefore, the inspector can only make an estimate of the flaw
size. Computer programs that allow the inspector to create
computer simulated models of the part and flaw may one day
lessen this limitation.

The IIW Type Calibration Block
 The standard shown in the above figure is commonly known in the
US as an IIW type reference block. IIW is an acronym for the
International Institute of Welding. It is referred to as an IIW "type"
reference block because it was patterned after the "true" IIW block
but does not conform to IIW requirements in IIS/IIW-23-59. "True" IIW
blocks are only made out of steel (to be precise, killed, open hearth
or electric furnace, low-carbon steel in the normalized condition with
a grain size of McQuaid-Ehn #8) where IIW "type" blocks can be
commercially obtained in a selection of materials. The dimensions of
"true" IIW blocks are in metric units while IIW "type" blocks usually
have English units. IIW "type" blocks may also include additional
calibration and references features such as notches, circular groves,
and scales that are not specified by IIW. There are two full-sized and
a mini versions of the IIW type blocks. The Mini version is about one-
half the size of the full-sized block and weighs only about one-fourth
as much. The IIW type US-1 block was derived the basic "true" IIW
block and is shown below in the figure on the left. The IIW type US-2
block was developed for US Air Force application and is shown below
n the center. The Mini version is shown on the right.

IIW type blocks are used to calibrate instruments
for both angle beam and normal incident
inspections. Some of their uses include setting
metal-distance and sensitivity settings,
determining the sound exit point and refracted
angle of angle beam transducers, and
evaluating depth resolution of normal beam
inspection setups. Instructions on using the IIW
type blocks can be found in the annex of
American Society for Testing and Materials
Standard E164, Standard Practice for
Ultrasonic Contact Examination of Weldments.

The Miniature Angle-Beam or ROMPAS
Calibration Block
 The miniature angle-beam is a calibration block
that was designed for the US Air Force for use in
the field for instrument calibration. The block is
much smaller and lighter than the IIW block but
performs many of the same functions. The
miniature angle-beam block can be used to check
the beam angle and exit point of the transducer.
The block can also be used to make metal-
distance and sensitivity calibrations for both angle
and normal-beam inspection setups.

AWS Shearwave Distance/Sensitivity Calibration
(DSC) Block
 A block that closely resembles the miniature angle-
beam block and is used in a similar way is the DSC
AWS Block. This block is used to determine the
beam exit point and refracted angle of angle-beam
transducers and to calibrate distance and set the
sensitivity for both normal and angle beam
inspection setups. Instructions on using the DSC
block can be found in the annex of American
Society for Testing and Materials Standard E164,
Standard Practice for Ultrasonic Contact
Examination of Weldments.

 AWS Shearwave Distance Calibration (DC) Block
 The DC AWS Block is a metal path distance and beam exit
point calibration standard that conforms to the requirements
of the American Welding Society (AWS) and the American
Association of State Highway and Transportation Officials
(AASHTO). Instructions on using the DC block can be found
in the annex of American Society for Testing and Materials
Standard E164, Standard Practice for Ultrasonic Contact
Examination of Weldments.
 AWS Resolution Calibration (RC) Block
 The RC Block is used to determine the resolution of angle
beam transducers per the requirements of AWS and
AASHTO. Engraved Index markers are provided for 45, 60,
and 70 degree refracted angle beams.

 30 FBH Resolution Reference Block
 The 30 FBH resolution reference block is used to evaluate the
near-surface resolution and flaw size/depth sensitivity of a
normal-beam setup. The block contains number 3 (3/64"), 5
(5/64"), and 8 (8/64") ASTM flat bottom holes at ten metal-
distances ranging from 0.050 inch (1.27 mm) to 1.250 inch
(31.75 mm).
 Miniature Resolution Block
 The miniature resolution block is used to evaluate the near-
surface resolution and sensitivity of a normal-beam setup It can
be used to calibrate high-resolution thickness gages over the
range of 0.015 inches (0.381 mm) to 0.125 inches (3.175 mm).
 Step and Tapered Calibration Wedges
 Step and tapered calibration wedges come in a large variety of
sizes and configurations. Step wedges are typically
manufactured with four or five steps but custom wedge can be
obtained with any number of steps. Tapered wedges have a
constant taper over the desired thickness range.

Distance/Sensitivity (DS) Block
The DS test block is a calibration
standard used to check the horizontal
linearity and the dB accuracy per
requirements of AWS and AASHTO.

Distance/Area-Amplitude Blocks
 Distance/area amplitude correction blocks typically are
purchased as a ten-block set, as shown above. Aluminum sets
are manufactured per the requirements of ASTM E127 and steel
sets per ASTM E428. Sets can also be purchased in titanium.
Each block contains a single flat-bottomed, plugged hole. The
hole sizes and metal path distances are as follows:
 3/64" at 3"
 5/64" at 1/8", 1/4", 1/2", 3/4", 11/2", 3", and 6"
 8/64" at 3" and 6"
 Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6
Aluminum, and Type 304 Corrosion Resistant Steel. Aluminum
blocks are fabricated per the requirements of ASTM E127,
Standard Practice for Fabricating and Checking Aluminum Alloy
Ultrasonic Standard Reference Blocks. Steel blocks are
fabricated per the requirements of ASTM E428, Standard
Practice for Fabrication and Control of Steel Reference Blocks
Used in Ultrasonic Inspection.

Flat Bottom Hole blocks Distance Amplitude
Correction [ DAC ] curves for normal beam
testing.

The 30 FBH resolution reference
block is used to evaluate the near-
surface resolution and flaw
size/depth sensitivity of a normal-
beam setup. The block contains
number 3 (3/64"), 5 (5/64"), and 8
(8/64") ASTM flat bottom holes at
ten metal-distances ranging from
0.050 inch (1.27 mm) to 1.250
inch (31.75 mm).

The miniature resolution block is used to evaluate the near-surface
resolution and sensitivity of a normal-beam setup It can be used to
calibrate high-resolution thickness gages over the range of 0.015
inches (0.381 mm) to 0.125 inches (3.175 mm).

Nineteen block sets with flat-bottom holes of a single size and
varying metal path distances are also commercially available.
Sets have either a #3 (3/64") FBH, a #5 (5/64") FBH, or a #8
(8/64") FBH. The metal path distances are 1/16", 1/8", 1/4", 3/8",
1/2", 5/8", 3/4", 7/8", 1", 1-1/4", 1-3/4", 2-1/4", 2-3/4", 3-14", 3-
3/4", 4-1/4", 4-3/4", 5-1/4", and 5-3/4". The relationship between
the metal path distance and the signal amplitude is determined
by comparing signals from same size flaws at different depth.
Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6
Aluminum, and Type 304 Corrosion Resistant Steel. Aluminum
blocks are fabricated per the requirements of ASTM E127,
Standard Practice for Fabricating and Checking Aluminum Alloy
Ultrasonic Standard Reference Blocks. Steel blocks are
fabricated per the requirements of ASTM E428, Standard
Practice for Fabrication and Control of Steel Reference Blocks
Used in Ultrasonic Inspection.

Nineteen block sets with flat-
bottom holes of a single size
and varying metal path
distances are also commercially
available. Sets have either a #3
(3/64") FBH, a #5 (5/64") FBH,
or a #8 (8/64") FBH. The metal
path distances are 1/16", 1/8",
1/4", 3/8", 1/2", 5/8", 3/4", 7/8",
1", 1-1/4", 1-3/4", 2-1/4", 2-3/4",
3-14", 3-3/4", 4-1/4", 4-3/4", 5-
1/4", and 5-3/4". The
relationship between the metal
path distance and the signal
amplitude is determined by
comparing signals from same
size flaws at different depth.
The sets are commonly sold in
4340 Vacuum melt Steel, 7075-
T6 Aluminum, and Type 304
Corrosion Resistant Steel.

Area amplitude blocks are an
eight-block set and look very
similar to Distance/Area-
Amplitude Blocks. However,
area-amplitude blocks have a
constant 3-inch metal path
distance and the hole sizes are
varied from 1/64" to 8/64" in
1/64" steps. The blocks are
used to determine the
relationship between flaw size
and signal amplitude by
comparing signal responses for
the different sized holes. Sets
are commonly sold in 4340
Vacuum melt Steel, 7075-T6
Aluminum, and Type 304
Corrosion Resistant Steel.

Probe placed on a FBH block, the larger echo is from the back of the block
and the smaller one is from the hole bottom.

Signal amplitude increases as the hole diameter / area increase.
Assuming no other signal loss, echo height and reflection areas are related
as,
Sig ampl 1 / Area 1 = Sig ampl 2 / Area 2
Amplitude - The vertical pulse height of a signal, usually base to peak, when
indicated by an A- Scan presentation.

Vertical linearity :
. Use three ASTM blocks, all with 3-inch metal travel distances and one
each with a 3/64, 5/64, and 8/64 inch diameter flat-bottom hole (FBH).
b. Move the search unit over the surface of the block with the 5/64 inch
FBH until maximum response is
obtained from the FBH. Make sure that the reject control and filters
are in the "off" or minimum positions. Adjust the instrument gain control
until the FBH signal is 35% of saturation on the CRT.
c. Leave the gain fixed as adjusted above in d. Maximize the FBH
signal on the 3/64 and 8/64 FBH blocks. Record the FBH signal
amplitudes. d. If the instrument is linear, the signals from the 3/64 and
8/64 FBHs will be 13% ± 3% and 90% ± 5% of
saturation respectively. Thus, a 3/64 FBH signal between 10% and 1
6% of saturation is considered linear; an 8/64 FBH signal between 85%
and 95% of saturation is considered linear. e. Instruments not linear within
the above limits SHALL be repaired or replaced. Area
amplitude response curve - A curve showing the changes in amplitude at
normal incidence from planar reflectors of different areas located at equal
distances from the search unit in an ultrasonic-conducting medium.

Side drilled hole block may also be used for setting up test sensitivity..
Display

Side drilled hole block may
also be used for setting up
test sensitivity..
Display

Drawing DAC curve with Side drilled hole block. These method is normally
used for weld testing.
Display

Drawing DAC with Flat Bottom Hole blocks.
Display

Drawing DAC with Side drilled hole block.
Display

Drawing DAC with Side drilled hole block. Straight beam - A vibrating pulse
wave train traveling normal to the test surface.
Display

Digital flaw detectors can draw DAC and the screen display can be saved
for future use.
Display

DAC curve is used for signal comparison.
Display

Thickness Measurements :
Thickness measurements are performed using a conventional flaw detector
and a compression wave probe, which sends longitudinal waves into the
component at normal incidence to the surface. Signals are displayed on the
flaw detector screen in the form of an A-scan, in which the horizontal axis
represents distance and the vertical axis represents signal amplitude.
Since a 0° compression probe is being used, the horizontal axis is
equivalent to depth from the scanning surface. When the probe is placed
on the surface of the component, a reflection appears at a range
corresponding to the thickness of the component at that point. The use of
an A-scan display allows the operator to distinguish more easily between
signals originating from embedded plate flaws and the nominal back wall
response. Also, the dynamics of the back wall echo can be observed on the
A-scan display to detect the presence of pitting. Conventional twin-crystal
0° compression probes are generally used to detect hidden corrosion.
However, where pitted surfaces are being assessed for remaining
thickness, pencil probes are used. These have a pointed tip which is
designed to fit into the pits, so that the remaining thickness can be
measured where the external pitting is at its most severe.
Display

Display
High-frequency transducer. Transducers use frequencies from 0.5 MHz all
the way up to 25 MHz--and sometimes up to 50 MHz. The higher the
frequency, the more sensitivity.
Normal incidence shear wave transducer. This type of transducer emits
shear waves directly into the material without having to use an angle-beam
wedge.

Flaw Detection :
Straight beam testing is used for examining bar stock for internal flaws.
Display

Angle beam testing is used for examining welds for internal flaws.
Display

Display
The shell of the mill roles are regularly monitored by ultrasonic testing.

Display
In a mill roll hard shell [ about 3 inches thick ] is bonded to a softer core.
The bonding can be tested by straight beam examination.
Cracking in shell material can be examined with angle beam probes.
Depth of surface breaking cracks can be estimated using Surface wave
probes.

Display
Fractured roll surface.

Display
Normal probes are used for testing Ingots. Large ingots are forged to
Blooms or Billets. Small ingots are rolled into bars.
Efficiency of testing depends on the surface roughness and the grain size
of the ingot.
Ingots are tested for Center line piping, large Inclusions and Voids.

Display
Normal probes are used for testing billets. Billets are produced by pressing
ingots or in a continuous caster.
Forged billets are tested for crack, piping etc,

Display
Slabs are produced by
continuous casting and are
rolled in to plates. Thin slab
castings are rolled into strips
and sheets. Slabs, produced by
continuous casting may be
rolled directly in to plates with
out ultrasonic examination.
Cast slabs may be tested with
normal beam probes before
they are rolled into plates.
Common defects are,
Segregation, Laminations,
Shearing separation of edges.

Display
Plate testing is one of the major applications of normal beam probes.
Plates are produced by rolling.
Plates are tested for lamination, cracks and large inclusions which also
produce laminar discontinuities. Surface breaking cracks are tested with a
450 angle beam probe.

Display
Plate scanner :
For scanning a large number of
plates, a plate tester which uses a
paint brush or an array of probes
are used to speed up testing. Plates
are usually tested for laminar
defects.
Laminations are a type of
discontinuity with separation or
weakness aligned parallel to the
rolled surface of a metal. They are
generally the result of defects
internal to the material in bulk,
flattened and elongated by rolling.
Piping, large blowholes and large
inclusions produce laminar
discontinuities.

Plates : A laminar defect which causes complete loss of back reflection and
can not be contained within a 3 inches diameter circle is rejected. Any
lamination which is within one inches from the welding edge must be cut off
before welding.

Display
Normal probes are used for testing drop Forgings.
Forgings are tested for Laps, Flash line cracks, forging bursts, crack,
flakes, piping, blowholes, inclusions, segregations and coarse grain
structure.

Display
Forgings are normally tested by back reflection method. tesalso processed
by pressing. processed by pressing.

Display
Rough forged bars limits the efficiency of testing. Forged bars are
generally rough machined before ultrasonic testing. Internal flaws are
detected by normal probing and surface breaking cracks are to be
detected by angle probing.

Display
Rough forged blanks, the surface condition may limit the sensitivity of
testing. Blanks are tested for bursts.

Display
Rolled rings are ultrasonic tested for laminar defects.

Display
Flanges are generally machined before ultrasonic testing.

Display
Bars and shapes are also tested by straight beam probes. For circular
testing shapes, a matching curved plastic shoe is usually fitted to front of the
probe, which improves sound transmission, produces a delay path and
significantly reduces the dead zone. Bars are tested for piping, seams, laps,
cracks, lamination, chevrons and stringers and coarse grained condition.
Surface breaking cracks are best detected by magnetic particle testing.
.

Display
Casting are also tested by straight beam
probes. Sensitivity may be limited due to the
material type, surface roughness and
complicated shapes.
Defects include shrinkage, cracks,
Inclusions, voids, porosity, cold shuts,
coarse grain etc.
in extrusions, Inclusions, Pipes, Seams,
Laps, Die drag etc.

Display
Valve body casting are also tested by
straight beam probes. Efficiency may be
limited due to the material type, surface
roughness and complicated shapes.
Although the ultrasonic method of
inspection has not been in common use
for as long as radiographic methods, it
nevertheless is a valuable tool for
examining heavy wall castings for
internal discontinuities. The first ASTM
specification for ultrasonic inspection of
steel castings was issued in 1970 and is
for longitudinal-beam ultrasonic
inspection of heat treated carbon and
low alloy steel castings. This inspection
method is in general not useful for
austenitic steel castings due to large
grain size of these castings. It is well
recognized that ultrasonic inspection
and radiography are not directly
comparable. However, the technique is
invaluable in detecting discontinuities in

Display
Large Casting are also
tested by straight beam
probes. Efficiency may
be limited due to the
material type, surface
roughness, material
thickness, and
complicated shapes.

Display
Bend parts are tested by a combination of normal and angle probing.
Major defect, cracks at the outer surface.

Display
Seamless tubes are tested using angle beam probes. Important defects
are cracks, seams and inclusions.

Display
Seamless tubes are tested using angle beam
probes..

Display
Notches for pipe testing.

Display
If the sensitivity calibration block is different, correction for transfer loss is
required.

Dead zone :
A single element
normal probe has
a dead zone
starting
immediately after
the entry surface
where flaws
cannot be
detected. The
width of the initial
pulse shows the
dead zone during
testing.

Curved objects reduces probe
contact area with significant
loss of sound transmission.

Delay Line Transducers
Delay line transducers
incorporate a short plastic wave
guide or delay line between the
active element and the test piece.
They are used to improve near
surface resolution and also in
high temperature testing, where
the delay line protects the active
element from thermal damage.
The delay line can be contoured
to match the curvature of round
objects.

Dead zone of a single element probe can be eliminated by dual element
arrangement. This transducer uses a pitch-and-catch effect. It uses two
elements. One element transmits the signal, while the other one receives it.
The probe generates longitudinal waves into a delay line. The angled
arrangement of the elements produce a pseudo focus where the detection
sensitivity is maximum.
Dual element probe :

Dead zone - The distance in the material from the surface of the test object
to the depth at which a reflector can first be resolved under specified
conditions. It is determined by the characteristics of the search unit, the
ultrasonic test instrumentation, and the test object.

Dual Element normal probes :
Dual element transducers utilize separate transmitter and receiver elements
in a single assembly. They are often used in applications involving rough
surfaces, coarse grained materials, detection of pitting or porosity, and they
offer good high temperature tolerance as well.
These probes are available in different element sizes and operating
frequencies. Different probes are used for thickness measurements and flaw
detection.

Dual Element Angle probes :
are available in different element sizes and
operating frequencies. Dual element angle
probes are used for thin materials and coarse
grained welds. To further improve signal to
noise ratio, it is possible to use dual element,
transverse wave probes or lens focused
transverse wave probes. As a consequence,
there is a restricted range of maximum
sensitivity.
Dual search unit - A search unit containing two
elements, one a transmitter, the other a
receiver.

Dual element probe uses a
long plastic delay line which
eliminates the initial echo
from the screen. A cross talk
barrier [ cork ] separates the
transmitter and receiver
delay lines and does not
allow detection of entry
surface signal.The return
signals from the transmitter
delay line are not detected
because the transmitting
probe has no receiving
function.
Cross talk - The signal
leakage [acoustic or electric]
across an intended acoustic
or electric barrier.

Dual element probe uses a long plastic delay line which eliminates the
initial echo from the screen. A cross talk barrier [ cork ] separates the
transmitter and receiver delay lines and does not allow detection of entry
surface signal.The return signals from the transmitter delay line are not
detected because the transmitting probe has no receiving function.

Sensitivity curve for twin probes :
The arrangements of the elements produces a pseudo focus where the
sensitivity of the probe is maximum and after this distance the sensitivity
drops rapidly. For detecting small flaws the usable test range is around
50 mm.

A twin probe being used with an ultrasonic flaw detector. -

Sheet metals : can be tested by ;
Dual probes when thickness permits.
Through transmission through water column using two probes known as
squirter technique.
Through transmission using a reflector plate on the other side of the sheet in
immersion method using a single probe and control echo.
By use of lamb waves.
Control echo - Reference signal from a constant reflecting surface, such as
a back reflection.

Examination of bonding is an important application of a twin probe.
White metal lining on carbon steel are checked for bonding integrity.
Titanium and stainless steel bonding to carbon steel plates are checked
frequently.

Bond testing : When there is a
difference in acoustic impedance, an
echo is obtained from the bonding
interface which is set to a predetermined
height. When lack of bonding is present
the interface echo increases in height
significantly and multiple reflections may
be produced.

One of the important applications of Twin probe is in a Digital Thickness
Gauge :
The gauge has a built in thickness reference for initial calibration.
Thickness is digitally displayed in mm or inches. Nominal accuracy 0.1 mm.
The gauge can measure different materials with suitable calibration or
correction.

Ultrasonic thickness gauges use
back wall reflection to measure
wall thickness. Sound travels at
different speeds through different
materials.For example the acoustic
velocity of steel is 5.95 mm per
microsecond and that of aluminum
is 6.32 mm per microsecond. The
gauge can be set to any acoustic
velocity depending on the material
being measured. The instruction
book includes a table of acoustic
velocities for common materials.
Acoustic velocity of unknown
material can be measured if a
sample of known thickness is
available.
Initial calibration is performed on
the 5 mm steel block supplied with
the thickness gauge.

Digital gauging is extensively used for measuring the remaining thickness
of corroded plates in ships. Internal corrosion pitting and general erosion in
most metals. A-Scan thickness surveys are also used for the inspection of
parent material for inclusions and laminations. Generally used for thickness
surveys on pressure vessels, pipelines, storage tanks and ship hulls.

The digital gauge is also used
for measuring the remaining
thickness of corroded pipes in
chemical plants. Thickness
measurements are performed
using a conventional flaw
detector and a compression
wave probe, which sends
longitudinal waves into the
component at normal
incidence to the surface.
Signals are displayed on the
flaw detector screen in the
form of an A-scan, in which
the horizontal axis represents
distance and the vertical axis
represents signal amplitude.
Since a 0° compression probe
is being used, the horizontal
axis is equivalent to depth
from the scanning surface.

When the probe is placed on the
surface of the component, a reflection
appears at a range corresponding to the
thickness of the component at that
point. Conventional twin-crystal 0°
compression probes are generally used
to detect hidden corrosion. However,
where pitted surfaces are being
assessed for remaining thickness,
pencil probes are used. These have a
pointed tip which is designed to fit into
the pits, so that the remaining thickness
can be measured where the external
pitting is at its most severe.

Some Digital gauge can measure thickness of a part through the paint
coating.
They work on echo to echo measurement principle.

The use of an A-scan display allows
the operator to distinguish more
easily between signals originating
from embedded plate flaws and the
nominal back wall response. Also,
the dynamics of the back wall echo
can be observed on the A-scan
display to detect the presence of
pitting.

A through paint test gauge in use.

A step block is recommended for
dual probe range calibration
when an ultrasonic flaw detector
is to be used. This block is also
used for checking accuracy of
measurements.

Ladder step wedge for thickness gauge calibration with thickness
measuring dual probes or checking the accuracy of digital gauges. The
block has 8 steps from 1mm to 8mm.

DSR calibration block. Designed to determine near surface resolution of
dual thickness gauging probes and verify sensitivity of dual thickness
gauges.

The minimum required accuracy is + / - 0.1
mm.
A 10 mm step may read between
9.9 to 10.1 mm.
Digital gauges may have display resolution of
.001 mm or better but it is not the accuracy of
the equipment.
The gauge may incorporate a built-in data
logger for recording many thickness readings.
The stored data can be downloaded to a
serial printer or a personal computer. A
Windows compatible data transfer program is
included.

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