N P Selvam ASNT NDT Level III / EN ISO 9712 NDT Level III / PED NDT Level III / SGNDT

Level III

ULTRASONIC TESTING

1.Introduction
• Ultrasonic Testing is a non-destructive method in which beams of high-frequency
sound waves are introduced into materials for the detection of surface and
subsurface flaws in the material.
• Discontinuities that produce reflective interfaces can be easily detected. The
major discontinuities which can be detectable are as follows,
▪ Cracks
▪ Laminations
▪ Shrinkage cavities
▪ Bursts
▪ Flakes
▪ Pores
▪ Disbonds
▪ Lamellar Tearing
▪ Solid Inclusions etc.,
• Most ultrasonic inspection instruments detect flaws by monitoring one or more
of the following:
▪ Reflection of sound from interfaces consisting of material
boundaries or discontinuities within the metal itself.
▪ Time of transit of a sound wave through the test piece from
the entrance point at the transducer to the exit point at the
transducer
▪ Attenuation of sound waves by absorption and scattering
within the test piece
▪ Features in the spectral response for either a transmitted or
a reflected signal
2. Principle: Acoustic Impedance mismatch
• Acoustic impedance is the measure of the opposition that a system presents to
the acoustic flow resulting of an acoustic pressure applied to the system
• Ultrasonic waves are mechanical vibrations. The amplitudes of vibrations in metal
parts being ultrasonically inspected impose stresses well below the elastic limit,
thus preventing permanent effects on the parts.
• Ultrasonic inspection is done at frequencies between 0.1 and 25 MHz, well above
the range of human hearing, which is about 20 Hz to 20 kHz.

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• The sound waves travel through the material with some loss of energy
(attenuation) and are reflected at interfaces. The reflected beam is displayed and
then analysed to define the presence and location of flaws or discontinuities.
• The degree of reflection depends largely on the physical state of the materials
forming the interface and to a lesser extent on the specific physical properties of
the material.
• Sound waves are almost completely reflected at metal/gas interfaces. Partial
reflection occurs at metal/liquid or metal/solid interfaces, with the specific
percentage of reflected energy depending mainly on the ratios of certain
properties of the material on opposing sides of the interface.
• It is also used to detect surface flaws, to define bond characteristics, to measure
the thickness and extent of corrosion, and (much less frequently) to determine
physical properties, structure, grain size, and elastic constants.

3. Application of Ultrasonic testing:

• Ultrasonic inspection is used to
▪ Detect and characterize of internal flaws
▪ Detect surface flaws
▪ Define bond characteristics
▪ Measure the thickness and extent of corrosion
▪ It was less frequently used to determine physical properties, structure,
grain size, and elastic constants.
• Ultrasonic inspection is used for quality control and materials inspection in all
major industries
▪ In-line techniques that have been developed for monitoring and
classifying material as acceptable, on-hold (salvageable), or scrap and
for process control.
▪ Inspecting Mill components: Rolls, shafts, drives, and press columns.
▪ Inspecting Power equipment: Turbine forgings, generator rotors,
pressure piping, weldments, pressure vessels.
▪ Inspecting nuclear fuel elements, and other reactor components.
▪ Inspecting Jet engine parts: Turbine and compressor forgings, and gear
blanks.
▪ Inspecting Aircraft components: Forging stock, frame sections.
▪ Machinery materials: Die blocks, tool steels, and drill pipe
▪ Railroad parts: Axles, wheels, and welded rail tracks
▪ Automotive parts: Forgings, ductile castings, and brazed and/or welded
components
▪ Ultrasonic inspection methods are particularly well suited to the
assessment of loss of thickness from corrosion inside closed systems,
such as chemical-processing equipment. Such measurements can often
be made without shutting down the process.

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4. Types of ultrasonic waves used for inspection

• On the basis of the mode of particle displacement, ultrasonic waves are
classified as longitudinal waves, transverse waves, surface waves, and Lamb
waves.
4.1. Longitudinal waves:
• It was also called as Compression waves and pressure waves, which are the type
of ultrasonic waves most widely used in the inspection of materials.
• These waves travel through materials as a series of alternate compression and
rarefaction in which the particles transmitting the wave vibrate back and forth in
the direction of travel of the waves.
• Longitudinal ultrasonic waves and the corresponding particle oscillation and
resultant rarefaction and compression are shown in Fig. 1.

• The distance between successive crest or successive trough (which equals the
distance for one complete cycle of rarefaction and compression) is the
wavelength (λ).
• The vertical axis could represent pressure instead of particle displacement. The
horizontal axis could represent time instead of travel distance because the speed
of sound is constant in a given material and because this relation is used in the
measurements made in ultrasonic inspection.
• Longitudinal ultrasonic waves are readily propagated in liquids and gases as well
as in elastic solids.

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• The mean free paths of the molecules of liquids and gases at a pressure of 1 atm
are so short that longitudinal waves can be propagated simply by the elastic
collision of one molecule with the next.
• The velocity of longitudinal ultrasonic waves is about 5920 m/s (19422.57 ft/s) in
steel, 1500 m/s (4921.26 ft/s) in water, and 330 m/s (1082.68 ft/s) in air.

4.2. Transverse waves:

• It was also called as shear waves which are also extensively used in the ultrasonic
inspection of materials. Transverse waves are visualized readily in terms of
vibrations of a rope that is shaken rhythmically, in which each particle, rather than
vibrating parallel to the direction of wave motion as in the longitudinal wave,
vibrates up and down in a plane perpendicular to the direction of propagation.
• A transverse wave is illustrated schematically in Figure 2, which shows particle
oscillation, wave front, direction of wave travel, and the wavelength, ,
corresponding to one cycle.

• Transverse waves cannot be supported by the elastic collision of adjacent molecular

or atomic particles. For the propagation of transverse waves, it is necessary that each
particle exhibit a strong force of attraction to its neighbours so that as a particle
moves back and forth it pulls its neighbour with it, thus causing the sound to move
through the material.
• The velocity of transverse wave is about 50% of the longitudinal wave velocity for the
same material.
• Air and water will not support the propagation of transverse waves.
• In gases, the forces of attraction between molecules are so small that shear waves
cannot be transmitted.

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4.3. Surface waves:

• It was also called as Rayleigh waves, which are another type of ultrasonic wave
used in the inspection of materials.
• These waves travel along the flat or curved surface of relatively thick solid parts.
• For the propagation of waves of this type, the waves must be travelling along an
interface bounded on one side by the strong elastic forces of a solid and on the
other side by the practically negligible elastic forces between gas molecules.
• Surface waves leak energy into liquid couplants and do not exist for any
significant distance along the surface of a solid immersed in a liquid, unless the
liquid covers the solid surface only as a very thin film. Surface waves are subject
to attenuation in a given material, as are longitudinal or transverse waves.
• Surface waves have a velocity approximately 90% of the transverse wave velocity
in the same material.
• The region within which these waves propagate with effective energy is not much
thicker than about one wavelength beneath the surface of the metal.
• At this depth, wave energy is about 4% of the wave energy at the surface, and the
amplitude of oscillation decreases sharply to a negligible value at greater depths.
• Surface waves follow contoured surfaces. For example, surface waves travelling
on the top surface of a metal block are reflected from a sharp edge, but if the
edge is rounded off, the waves continue down the side face and are reflected at
the lower edge, returning to the sending point. Surface waves will travel
completely around a cube if all edges of the cube are rounded off.
• Surface waves can be used to inspect parts that have complex contours.
• In surface waves, particle oscillation generally follows an elliptical orbit, as shown
schematically in Figure.
• The major axis of the ellipse is perpendicular to the surface along which the
waves are travelling. The minor axis is parallel to the direction of propagation.
• Surface waves can exist in complex forms that are variations of the simplified
wave form illustrated in Figure

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4.4. Lamb waves:

• It was also known as plate waves, are another type of ultrasonic wave used in the non-
destructive inspection of materials.
• Lamb waves are propagated in plates (made of composites or metals) only a few
wavelengths thick.
• A Lamb wave consists of a complex vibration that occurs throughout the thickness of the
material.
• The propagation characteristics of Lamb waves depend on the density, elastic properties,
and structure of the material as well as the thickness of the test piece and the frequency.
• Lamb wave behaviour in general resembles that observed in the transmission of
electromagnetic waves through waveguides.
• There are two basic forms of Lamb waves:

▪ Symmetrical, or dilatational
▪ Asymmetrical, or bending

• The form is determined by whether the particle motion is symmetrical or asymmetrical

with respect to the neutral axis of the test piece.
• Each form is further subdivided into several modes having different velocities, which can
be controlled by the angle at which the waves enter the test piece.
• Theoretically, there are an infinite number of specific velocities at which Lamb waves can
travel in a given material. Within a given plate, the specific velocities for Lamb waves are
complex functions of plate thickness and frequency.
• In symmetrical (dilatational) Lamb waves, there is a compressional (longitudinal) particle
displacement along the neutral axis of the plate and an elliptical particle displacement on
each surface.
• In asymmetrical (bending) Lamb waves, there is a shear (transverse) particle
displacement along the neutral axis of the plate and an elliptical particle displacement on
each surface.
• The ratio of the major to minor axes of the ellipse is a function of the material in which
the wave is being propagated.

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5. General Characteristics of Waves

• Ultrasonic waves are mechanical waves that consist of oscillations or
vibrations of the atomic or molecular particles of a substance about the
equilibrium positions of these particles.
• Ultrasonic waves behave essentially the same as audible sound waves. They
can propagate in an elastic medium, which can be solid, liquid, or gaseous, but
not in a vacuum.
• In many respects, a beam of ultrasound is similar to a beam of light; both are
waves and obey a general wave equation. Each travels at a characteristic
velocity in a given homogeneous medium. The velocity depends on the
properties of the medium, not on the properties of the wave.
• Ultrasonic beams are reflected from surfaces, refracted when they cross a
boundary between two substances that have different characteristic sound
velocities, and diffracted at edges or around obstacles.
• Scattering by rough surfaces or particles reduces the energy of an ultrasonic
beam.

5.1 Wave Propagation:

• Ultrasonic waves (and other sound waves) propagate to some extent in any
elastic material. When the atomic or molecular particles of an elastic material
are displaced from their equilibrium positions by any applied force, internal
stress acts to restore the particles to their original positions. Because of the
inter-atomic forces between adjacent particles of material, a displacement at
one point induces displacements at neighbouring points and so on, thus
propagating a stress-strain wave.
• The actual displacement of matter that occurs in ultrasonic waves is extremely
small.
• The amplitude, vibration mode, and velocity of the waves differ in solids,
liquids, and gases because of the large differences in the mean distance
between particles in these forms of matter.
• These differences influence the forces of attraction between particles and the
elastic behaviour of the materials. The relation of velocity to frequency and
wavelength was as follows,

λ=V/F
Where;

λ is wavelength (in meters per cycle).

V is velocity (in meters per second),

F is frequency (in hertz)

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5.2 Acoustic Impedance

• Sound travels through materials under the influence of sound pressure.
Because molecules or atoms of a solid are bound elastically to one another,
the excess pressure results in a wave propagating through the solid.
• The acoustic impedance (Z) of a material is defined as the product of its
density (ρ) and acoustic velocity (V).

Z = ρV
• Acoustic impedance is important in

1. The determination of acoustic transmission and reflection at the boundary

of two materials having different acoustic impedances.
2. The design of ultrasonic transducers.
3. Assessing absorption of sound in a medium.

• The units for acoustic impedance are kgm-2s-1 and values for the acoustic
impedance of some materials are given in the following table:

Material Density Speed of sound Acoustic impedance

(kgm-3) (ms-1) (kgm-2s-1x106)

Air 1.3 330 0.000429

Water 1000 1450 1.50
Perspex 1200 2680 3.22

• The greater the difference between the acoustic impedances of the two
materials at a boundary in the body the greater the amount of reflection –
two materials with the same acoustic impedance would give no reflection (or
refraction) while two with widely separated values would give much larger
reflections.

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6. Characteristics of ultrasonic wave:

6.1. Frequency (f):

• It is defined as the number of waves passing a point in a certain time.

• Normally it was calculated for one second,
• It was expressed in terms of hertz (Hz),
• One hertz is equal to one wave per second.
▪ 1 kHz = 1,000 Hz = 1 x 10 3 Hz
▪ 1 MHz = 1,000,000 Hz = 1 x 10 6 Hz
• Transducer fundamental frequency shall be calculated from the following
equation,

E f = V / (2 x T)

Where,
V= Velocity of Sound in the material. (V=VL for X-cut crystal and V=VS for Y cut Crystals)
T= Thickness of the Crystal.

6.2. Amplitude:

• It is defined as the distance from the centre line (or still position) to the top of a
crest or to the bottom of a trough.
• Amplitude is measured in percentage of the full screen height (%).
• Refer wave length figure, amplitude was shown as ‘a’

Amplitude calculations:

Case 1: For Larger reflectors (Larger than beam width)

Inverse Law,

A /A =D /D
1 2 2 1

Where,

A & A = Amplitude of first and second echo respectively.

1 2

D & D = Diameter of first and second echo respectively.

1 2

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Case 2: For Smaller reflectors (Smaller than beam width)

Inverse Square Law,

2 2
A / A = (D ) / (D )
1 2 2 1

Where,
A & A = Amplitude of first and second echo respectively.
1 2

D & D = Diameter of first and second echo respectively.

1 2

6.3. Wavelength:

• The wavelength (λ) of a wave is the distance from any point on one wave to the
same point on the next wave along.
• It was measured as a distance between successive crest or successive trough.
• Wavelength is measured in metres (m) - it is a length of the wave.

6.4. Velocity:

• It was defined as the distance travelled per unit time.

• It is measured in metres per second (m/s).
• It can be derived from the wavelength formula

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7. Piezo-electric effect:

• The conversion of electrical pulses to mechanical vibrations and the conversion of

returned mechanical vibrations back into electrical energy is the basis for
ultrasonic testing.
• The active element is the heart of the transducer as it converts the electrical
energy to acoustic energy, and vice versa. The active element is basically a piece
of polarized material (i.e. some parts of the molecule are positively charged,
while other parts of the molecule are negatively charged) with electrodes
attached to two of its opposite faces.
• When an electric field is applied across the material, the polarized molecules will
align themselves with the electric field, resulting in induced dipoles within the
molecular or crystal structure of the material.

• This alignment of molecules will cause the material to change dimensions. This
phenomenon is known as electrostriction. In addition, a permanently-polarized
material such as quartz (SiO2) or barium-titanate (BaTiO3) will produce an electric
field when the material changes dimensions as a result of an imposed mechanical
force. This phenomenon is known as the piezoelectric effect.

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7.1. Piezoelectric materials:

7.1.1. Quartz crystals:

• Initially the only piezoelectric element used in commercial ultrasonic
transducers was quartz.
• Properties of the transducers depended largely on the direction along which
the crystals were cut to make the active transducer elements.
• Principal advantages of quartz-crystal transducer elements are electrical and
thermal stability, insolubility in most liquids, high mechanical strength, wear
resistance, excellent uniformity, and resistance to aging.
• A limitation of quartz is its comparatively low electromechanical conversion
efficiency, which results in low loop gain for the system.

7.1.2. Lithium Sulphate

• The principal advantages of lithium sulphate transducer elements are ease of
obtaining optimum acoustic damping for best resolution, optimum receiving
characteristics, intermediate conversion efficiency, and negligible mode
interaction.
• The main disadvantages of lithium sulphate elements are fragility and a
maximum service temperature of about 75 °C (165 °F).
• It acts as a best receiver.

7.1.3. Polarized ceramics:

• Generally it has high electromechanical conversion efficiency, which results in
high loop gain and good search-unit sensitivity.
• Lead zirconate titanate is mechanically rugged, has a good tolerance to
moderately elevated temperature, and does not lose polarization with age.
• It does have a high piezoelectric response in the radial mode, which
sometimes limits its usefulness.

7.1.4. Barium titanate:

• It is also mechanically rugged and has a high radial-mode response. However,
its efficiency changes with temperature, and it tends to depolarize with age,
which makes barium titanate less suitable for some applications than lead
zirconate titanate.
• It acts as a best transmitter.

7.1.5. Lead metaniobate:

• It exhibits low mechanical damping and good tolerance to temperature. Its
principal limitation is a high dielectric constant, which results in a transducer
element with a high electrical capacitance.

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8. Properties of ultrasonic Wave :

8.1. Sensitivity or the ability of an ultrasonic inspection system to detect a

very small discontinuity is generally increased by using relatively high
frequencies (short wavelengths).

8.2. Resolution or the ability of the system to give simultaneous, separate

indications from discontinuities that are close together both in depth below
the front surface of the test piece and in lateral position, is directly
proportional to frequency band-width and inversely related to pulse length.
Resolution generally improves with an increase of frequency.

8.3. Penetration or the maximum depth (range) in a material from which

useful indications can be detected, is reduced by the use of high frequencies.
This effect is most pronounced in the inspection of metal that has coarse grain
structure or minute in-homogeneities, because of the resultant scattering of
the ultrasonic waves; it is of little consequence in the inspection of fine-grain,
homogeneous metal.

8.4. Beam spread or the divergence of an ultrasonic beam from the central
axis of the beam, is also affected by frequency. As frequency decreases, the
shape of an ultrasonic beam increasingly departs from the ideal of zero beam
spread. This characteristic is so pronounced as to be observed at almost all
frequencies used in inspection. Other factors, such as the transducer (search
unit) diameter and the use of focusing equipment, also affect beam spread.

Sensitivity, resolution, penetration, and beam spread are largely determined by the
selection of the transducer and are only slightly modified by changes in other test
variables.

The relation between Frequency, Wavelength, Resolution, and depth are as follows,
• Wavelength is inversely proportional to Frequency (F)
• Shorter wavelengths give a better resolution (R)
• High frequency will have shorter wavelength, shorter Penetration but good
resolution Low Frequency will have longer wavelength, Deep penetration, but
the resolution was poor.

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8.5. Beam divergence half angle:

• All ultrasonic beams diverge. In other words, all transducers have beam spread.
• The simplified view of a sound beam for a flat transducer was given in the
fallowing figure.
• The sound field of a transducer is divided into two zones; the near field and the
far field. The near field is the region directly in front of the transducer where the
echo amplitude goes through a series of maxima and minima and ends at the last
maximum, at distance N from the transducer

• In the near field

(Fresnel
zone), the beam has a complex shape that narrows. It can be calculated by using
the fallowing Equation,

N= (D2 – λ2)/ 4 λ

Where,

N = Nearfield Distance of the transducer

D = Element or Crystal Diameter
λ = Wavelength

• In the far field (Fraunhofer Zone) the beam diverges as shown in figure.
• For flat transducers as shown in Figure, the - 6 dB pulse-echo beam spread angle is
given by Equation :

Sin (α/2) = C x (λ / D)
Where,
α /2 = Half Angle Spread between -6 dB points
C = constant which varies according to the intensity of the beam as fallows
C = 1.22 for 0% intensity (outer surface or edge of the beam)
C = 1.08 for 10% intensity (-20 dB difference from centre axis intensity)
C = 0.56 for 50% intensity (-6dB difference from centre axis intensity)

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8.6. Incidence, Reflection ,Refraction and mode conversion:

• When an ultrasonic wave is incident at right angles on an interface between two

materials (normal incidence, i.e. angle of incidence = 0°) do transmission and
reflection occur at the interface without any change in beam direction.
• At any other angle of incidence, the phenomena of mode conversion (a change in
the nature of the wave motion) and refraction (a change in direction of wave
propagation) must be considered.
• These phenomena may affect the entire beam or only a portion of the beam and
the sum total of the changes that occur at the interface depend on the angle of
incidence and the velocity of the ultrasonic waves leaving the point of
impingement on the interface.
• All possible ultrasonic waves leaving this point are shown for an incident
longitudinal ultrasonic wave in Figure.
• Not all the waves shown in Figure will be produced in any specific instance of
oblique impingement of an ultrasonic wave on the interface between two
materials.
• The waves that propagate in a given instance depend on the ability of a waveform
to exist in a given material, the angle of incidence of the initial beam, and the
velocities of the waveforms in both materials.

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Snell's Law holds true for shear waves as well as longitudinal waves and can be
written as follows.

Where,
VL1 - longitudinal wave velocity in material 1.
VL2 - longitudinal wave velocity in material 2.
VS1 - Shear wave velocity in material 1.
VS2 - Shear wave velocity in material 2.

ϴ1 – Incident angle of longitudinal wave.

ϴ2 – Refracted angle of longitudinal wave.
ϴ3 – Reflected angle of shear wave.
ϴ4 – Refracted angle of shear wave.

8.7. Snell’s Law:

Snell's Law describes the relationship between the angles and the velocities of the
waves. Snell's law equates the ratio of material velocities V1 and V2 to the ratio of the
sine's of incident (Q1) and refracted (Q2) angles, as shown in the following equation.

Where:

VL1 is the longitudinal wave velocity in material 1.

VL2 is the longitudinal wave velocity in material 2.

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8.8. First critical angle:

• When a longitudinal wave moves from a slower to a faster material, there is

an incident angle that makes the angle of refraction for the wave 90 o. This is
known as the first critical angle.
• The first critical angle can be found from Snell's law by putting in an angle of
90° for the angle of the refracted wave or beam.
• At the critical angle of incidence, much of the acoustic energy is in the form of
an inhomogeneous compression wave, which travels along the interface and
decays exponentially with depth from the interface. This wave is sometimes
referred to as a "creep wave."
• Because of their inhomogeneous nature and the fact that they decay rapidly,
creep waves are not used as extensively as Rayleigh surface waves in NDT.
However, creep waves are sometimes more useful than Rayleigh waves
because they suffered less from surface irregularities and coarse material
microstructure due to their longer wavelengths.

The value of first critical angle shall be calculated from the following Equation,
Sin (ϴ1) = Sin 90° x ( VL1 / VL2)

Where,

ϴ1 – First critical angle , VL1 – Longitudinal wave velocity in First medium / material (M1) , VL2 - Longitudinal wave velocity in
Second medium / material (M2)

8.9. Second critical angle:

• Beyond the first critical angle, only the shear wave propagates into the
material. For this reason, most angle beam transducers use a shear wave so
that the signal is not complicated by having two waves present.
• In many cases there is also an incident angle that makes the angle of
refraction for the shear wave 90 degrees. This is known as the second critical
angle and at this point, all of the wave energy is reflected or refracted into a
surface following shear wave or shear creep wave. Slightly beyond the second
critical angle, surface waves will be generated.
• The value of Second critical angle shall be calculated from the fallowing
Equation,
Sin (ϴ2) = Sin 90° x ( VL1 / VS2)

Where,

ϴ2 – First critical angle , VL1 – Longitudinal wave velocity in First medium / material (M1)

VS2 - Shear wave velocity in Second medium / material (M2)

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8.10. Intensity of Reflection co-efficient (R) :

• Ultrasonic waves are reflected at boundaries where there is a difference in

acoustic impedances (Z) of the materials on each side of the boundary.
• This difference in Z is commonly referred to as the impedance mismatch.
• The greater the impedance mismatch, the greater the percentage of energy that
will be reflected at the interface or boundary between one medium and another
• The co-efficient of reflection (R) shall be calculated from the equation,

Where,

R = Reflection co-efficient

Z1 = Acoustic impedance of first medium.

Z = Acoustic impedance of second medium.

2

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Alternative form:

2 2
R = (r-1) / (r+1)

Where,

r = Impedance Ratio = (Z / Z )
2 1

R = Reflection co-efficient

8.11. Intensity of Transmission co-efficient (T) :

• Since the amount of reflected energy plus the transmitted energy must equal the
total amount of incident energy, the transmission coefficient is calculated by
simply subtracting the reflection coefficient from one (applicable only for normal
incidence i.e. incident angle perpendicular to the incidence surface)

R+T=1

Where, R = Reflection co-efficient T = Transmission co-efficient

• It can also be calculated as fallows,

2
T = 4r / (r+1)
Where,

r = Impedance Ratio = (Z / Z )
2 1

T = Transmission co-efficient

• For Oblique or inclined incidence

R = 1 – (L + S)
Where,
L = Transmission Co-efficient for Longitudinal wave
S = Transmission Co-efficient for Longitudinal wave

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8.12. Attenuation:

• The intensity of an ultrasonic beam that is sensed by a receiving transducer is

considerably less than the intensity of the initial transmission. The factors that
are responsible for the loss in beam intensity can be classified as

1. Transmission losses,
2. Interference effects
3. Beam spread.

• Transmission losses include absorption, scattering, and acoustic impedance

effects at interfaces.
▪ The absorption of ultrasonic energy occurs mainly by the conversion
of mechanical energy into heat. Elastic motion within a substance as a
sound wave propagates through it alternately heats the substance
during compression and cools it during rare-faction.

▪ Scattering of an ultrasonic wave occurs because most materials are not

truly homogeneous. Crystal discontinuities, such as grain boundaries,
twin boundaries, and minute nonmetallic inclusions, tend to deflect
small amounts of ultrasonic energy out of the main ultrasonic beam.

• Interference effects include diffraction and other effects that create wave
fringes, phase shift, or frequency shift.

▪ When the reflector is very small compared to the sound beam, as is

usual for a pore or an inclusion, wave bending (forward scattering)

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around the edges of the reflector produces an interference pattern in

a zone immediately behind the reflector because of phase differences
among different portions of the forward-scattered beam.
▪ The interference pattern consists of alternate regions of maximum
and minimum intensity that correspond to regions where interfering
scattered waves are respectively in phase and out of phase.

• Beam spreading involves mainly a transition from plane waves to either

spherical or cylindrical waves, depending on the shape of the transducer
element face.

• Co-Efficient of Attenuation (α in dB/mm) shall be calculated from the

fallowing equation,

α = (Δ dB) / (2 x T)

Where,

α- Attenuation co-efficient

T – Thickness of the material

ΔdB - Difference in gain

• ΔdB shall be calculated by the fallowing equation,

ΔdB = 20 log 10 (X2 /X1)

Where,

ΔdB - Difference in gain.

X1 – Amplitude of first echo.

X2 – Amplitude of second echo.

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Major Variables in Ultrasonic Inspection

The major variables that must be considered in ultrasonic inspection include both the
characteristics of the ultrasonic waves used and the characteristics of the parts being
inspected. Equipment type and capability interact with these variables.
The major variables are
• Velocity
• Acoustic impedance
• Angle of incidence
• Critical angles
• Beam intensity

9. Ultrasonic Inspection Methods:

• The major methods of ultrasonic inspection are as follows,

▪ The pulse-echo method.

▪ Through transmission method.
▪ Frequency modulation method.
▪ Spectral Analysis method.
• The primary difference between the first two methods is that the transmission
method involves only the measurement of signal attenuation, while the pulse-
echo method can be used to measure both transit time and signal attenuation.

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9.1. The pulse-echo method:

• It is the most widely used ultrasonic method, involves the detection of echoes
produced when an ultrasonic pulse is reflected from a discontinuity or an
interface of a test piece.
• This method is used in flaw location and thickness measurements. Flaw depth is
determined from the time-of-flight between the initial pulse and the echo
produced by a flaw.
• Flaw depth might also be determined by the relative transit time between the
echo produced by a flaw and the echo from the back surface.
• Flaw sizes are estimated by comparing the signal amplitudes of reflected sound
from an interface (either within the test piece or at the back surface) with the
amplitude of sound reflected from a reference reflector of known size or from the
back surface of a test piece having no flaws.

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9.2. Through transmission method:

• It may include either reflection or through transmission, involves only the

measurement of signal attenuation.
• This method is also used in flaw detection. In the pulse-echo method, it is
necessary that an internal flaw reflect at least part of the sound energy onto a
receiving transducer.
• However, echoes from flaws are not essential to their detection. Merely the fact
that the amplitude of the back reflection from a test piece is lower than that from
an identical work piece known to be free of flaws implies that the test piece
contains one or more flaws.
• The technique of detecting the presence of flaws by sound attenuation is used in
transmission methods as well as in the pulse-echo method. The main
disadvantage of attenuation methods is that flaw depth cannot be measured.
• Various types of testing’s performed by transmission mode are as follows,
▪ Pitch catch testing (separate transmitter and receiver probes)
▪ Continuous beam testing
▪ Lamb wave testing.

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9.3. The frequency modulation (FM) method:

• It was the precursor of the pulse-echo method, is another flaw detection

technique. In the FM method, the ultrasonic pulses are transmitted in wave
packets whose frequency varies linearly with time.
• The frequency variation is repeated in successive wave packets so that a plot of
frequency versus time has a saw tooth pattern. There is a time delay between
successive packets. Returning echoes are displayed on the readout device only if
they have certain characteristics as determined by the electronic circuitry in the
instrument.
• Although not as widely used as the pulse-echo method, the FM method has a
lower signal-to-noise ratio and therefore somewhat greater resolving power.

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9.4. Spectral analysis:

• It can be used in the through transmission or pulse-echo methods, involves

determination of the frequency spectrum of an ultrasonic wave after it has
propagated through a test piece.
• The frequency spectrum can be determined either by transmitting a pulse and
using a fast Fourier transform to obtain the frequency spectrum of the received
signal or by sweeping the transmission frequency in real time and acquiring the
response at each frequency.
• The increasing use of the pulse method is attributed to improvements in the
speed of digital fast Fourier transform devices.
• Spectral analysis is used in transducer evaluations and may be useful in defect
characterization.
• Spectral analysis can also be used to measure the thickness of thin-wall
specimens. A short pulse of ultrasound is a form of coherent radiation, in a thin-
wall specimen that produces front and back wall echoes, the two reflected pulses
show phase differences and can interfere coherently.
• If the pulse contains a wide band of frequencies, interference maxima and
minima can occur at particular frequencies, and these can be related to the
specimen thickness.

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10.Basic Equipment

Most ultrasonic inspection systems include the following basic equipment

configurations,
• An electronic signal generator or pulser
• A transducer
• A couplant
• An electronic device to amplify
• A display or indicating device
▪ The display device may be a CRT (Oscilloscope).
▪ The chart or strip recorder.
▪ The marker, indicator, or alarm device.
▪ The computer printout.
• An electronic clock,
Although the electronic equipment used for ultrasonic inspection can vary greatly in
detail among equipment manufacturers, all general-purpose units consist of a power
supply, a pulser circuit, a search unit, a receiver-amplifier circuit, an oscilloscope, and
an electronic clock. Many systems also include electronic equipment for signal
conditioning, gating, automatic interpretation, and integration with a mechanical or
electronic scanning system. Moreover, advances in microprocessor technology have
extended the data acquisition and signal-processing capabilities of ultrasonic
inspection systems.

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10.1. Power Supply:

▪ Circuits that supply current for all functions of the instrument constitute
the power supply, which is usually energized by conventional 115-V or 230-
V alternating current.
▪ There are many types and sizes of portable instruments for which the
power is supplied by batteries contained in the unit.

10.2. Pulser Circuit:

▪ When electronically triggered, the pulser circuit generates a burst of

alternating voltage.
▪ The principal frequency of this burst, its duration, the profile of the
envelope of the burst, and the burst repetition rate may be either fixed or
adjustable, depending on the flexibility of the unit.

10.3. Search Units:

▪ The transducer is the basic part of any search unit. A sending transducer is
one to which the voltage burst is applied, and it mechanically vibrates in
response to the applied voltage.
▪ When appropriately coupled to an elastic medium, the transducer thus
serves to launch ultrasonic waves into the material being inspected.
▪ A receiving transducer converts the ultrasonic waves that impinge on it
into a corresponding alternating voltage. In the pitch-catch mode, the
transmitting and receiving transducers are separate units; in the pulse-
echo mode, a single transducer alternately serves both functions. The
various types of search units are as listed below,
1) Piezo-electric transducer
2) EMA transducer
3) Magnetostriction transducer

10.4. Receiver-amplifier circuits:

▪ Electronically amplify return signals from the receiving transducer and

often demodulate or otherwise modify the signals into a form
suitable for display.
▪ The output from the receiver-amplifier circuit is a signal directly
related to the intensity of the ultrasonic wave impinging on the
receiving transducer.
▪ This output is fed into an oscilloscope or other display device.

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10.5. CRT- Oscilloscope:

▪ Data received are usually displayed on an oscilloscope in either video mode
or radio frequency mode. In video mode display, only peak intensities are
visible on the trace; in the RF mode, it is possible to observe the waveform
of signal voltages.
▪ A CRT (oscilloscope) is an electronic tube designed to display electrical
data.
▪ The basic CRT consists of four major components.
1. Electron Gun
2. Focussing & Accelerating Anodes
3. Horizontal & Vertical Deflection Plates
4. Evacuated Glass Envelope

▪ Heater element is energized by alternating current to obtain high emission

of electron from cathode. Control grid is biased negative with respect to
cathode it controls the density of electron beam to focus the electron
beam on the screen focusing anode is used. the focusing anode operate at
a potential of twelve hundred (1200 V) and accelerating anode at 2000 V to
accelerate the electron beam.
▪ Two pairs of deflection plates provided in the CRT these deflection plates
are mounted at right angle to each other to provide electron beam
deflection along vertical and horizontal axis of the screen.
▪ The screen consists of a glass which is coated by some florescent material
like zinc silicate, which is semitransparent phosphor substance.
▪ When high velocity electron beam hits the phosphorescent screen the light
emits from it. The property of phosphor to emit light when its atoms are
excited is called fluorescence.

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10.6. Clock:

▪ The electronic clock, or timer, serves as a source of logic pulses, reference

voltage, and reference waveform.
▪ The clock coordinates operation of the entire electronic system.

11.Pulse Echo Method

11.1. Operation of a pulse echo Ultrasonic system:

• At regular intervals, the electronic clock triggers the signal generator, which
imposes a short interval of high-frequency alternating voltage or a unipolar
(negative) spike on the transducer.
• Simultaneously, the clock activates a time-measuring circuit connected to the
display device.
• The operator can preselect a constant interval between pulses by means of a
pulse-repetition rate control on the instrument. Pulses are usually repeated 60 to
2000 times per second. In most commercially available flaw detectors, the pulse-
repetition rate is controlled automatically except for some larger systems. Also,
most systems are broadband when they transmit, but may be tuned or filtered
for reception.
• The operator can also preselect the output frequency of the signal generator. For
best results, the frequency (and sometimes the pulse-repetition rate) should be
tuned to achieve the maximum response of the transducer (resonance in the
vibrating element) and maximum signal-to-noise ratio (lowest amount of
electronic noise) in the electronic equipment.
• The transducer then converts the pulse of voltage into a pulse of mechanical
vibration having essentially the same frequency as the imposed alternating
voltage.
• The mechanical vibration (ultrasound) is introduced into a test piece through a
couplant and travels by wave motion through the test piece at the velocity of
sound, which depends on the material.
• When the pulse of ultrasound encounters a reflecting surface that is
perpendicular to the direction of travel, ultrasonic energy is reflected and returns
to the transducer.
• The returning pulse travels along the same path and at the same speed as the
transmitted pulse, but in the opposite direction. Upon reaching the transducer
through the couplant, the returning pulse causes the transducer element to
vibrate, which induces an alternating electrical voltage across the transducer.
• The induced voltage is instantaneously amplified (and sometimes demodulated),
then fed into the display device. This process of alternately sending and receiving

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pulses of ultrasonic energy is repeated for each successive pulse, with the display
device recording any echoes each time.
• Theoretically, the maximum depth of inspection is controlled by the pulse-
repetition rate. For example, if a 10 MHz pulse is transmitted at a pulse-repetition
rate of 500 pulses per second, a longitudinal wave pulse can travel almost 12 m
(40 ft) in steel or aluminum before the next pulse is triggered. This means one
pulse can travel to a depth of 6 m (20 ft) and return before the next pulse is
initiated.
• Practically, inspection can be performed only to a depth that is considerably less
than the theoretical maximum. Sound attenuation in a test piece can limit the
path length. The practical limit varies with the type and condition of the test
material, test frequency, and system sensitivity.
• It is highly desirable for all ultrasonic vibrations (including successively re-
reflected echoes of the first reflected pulse) to die out in the test piece before the
next initial pulse is introduced. As a rule, the pulse-repetition rate should be set
so that one pulse can traverse the test piece enough times to dissipate the sonic
energy to a non-displayable level before the next pulse is triggered.
• Both sound attenuation and pulse reverberation are of little consequence except
when inspecting large parts (for example, in the axial inspection of long shafts).
• Pulse-echo inspection can be accomplished with longitudinal, shear, surface, or
Lamb waves.
• Straight-beam or angle beam techniques can be used, depending on test piece
shape and inspection objectives.
• Data can be analyzed in terms of type, size, location, and orientation of flaws, or
any combination of these factors. It should be noted, however, that some forms
of data presentation are inherently unable to pin-point the location of flaws
unless the flaws are favorably oriented with respect to the transmitted sonic
beam.
• Discontinuity or flaw type, location, and orientation often influence the
procedures and techniques used to estimate size. Sometimes it is advantageous
to use separate sending and receiving transducers for pulse-echo inspection.
(Separate transducers are always used for through transmission inspection.)
• These separate transducers can be housed in a single search unit or in two
separate search units. The term pitch-catch is often used in connection with
separate sending and receiving transducers, regardless of whether reflection
methods or transmission methods are involved.

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11.2. Presentation of Pulse-Echo Data

Information from pulse-echo inspection can be displayed in different forms. The

basic data formats include:

11.2.1. A-scan presentation :

• This format provides a quantitative display of signal amplitudes and time-

of-flight data obtained at a single point on the surface of the test piece.
• The A-scan display, which is the most widely used format, can be used to
analyze the type, size, and location (chiefly depth) of flaws

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11.2.2. B-scan presentation:

• This format provides a quantitative display of time-of-flight data obtained

along a line of the test piece.
• The B-scan display shows the relative depth of reflectors and is used
mainly to determine size (length in one direction), location (both position
and depth), and to a certain degree the shape and orientation of large
flaws

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11.2.3. C-scan presentation:

• This format provides a semi quantitative or quantitative display of signal
amplitudes obtained over an area of the test piece surface. This
information can be used to map out the position of flaws on a plan view of
the test piece.
• A C-scan format also records time-of-flight data, which can be converted
and displayed by image-processing equipment to provide an indication of
flaw depth.
• Gating- An electronic depth gate is another essential element in C-scan
systems. A depth gate is an electronic circuit that allows only those echo
signals that are received within a limited range of delay times following the
initial pulse or interface echo to be admitted to the receiver-amplifier
circuit.

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• A-scan and B-scan data are usually presented on an oscilloscope screen; C-

scan data are recorded by an x-y plotter or displayed on a computer monitor.
With computerized data acquisition and image processing, the display formats
can be combined or processed into more complex displays.
• A-scan display is basically a plot of amplitude versus time, in which a
horizontal baseline on an oscilloscope screen indicates elapsed time while the
vertical deflections (called indications or signals) represent echoes. Flaw size
can be estimated by comparing the amplitude of a discontinuity signal with
that of a signal from a discontinuity of known size and shape; the discontinuity
signal also must be corrected for distance losses.

11.3. Interpretation of Pulse-Echo Data:

• The interpretation of pulse-echo data is relatively straightforward for B-scan
and C-scan presentations.
• The B-scan always records the front reflection, while internal echoes or loss of
back reflection, or both, are interpreted as flaw indications.
• Flaw depth is measured as the distance from the front reflection to a flaw
echo, with the latter representing the front surface of the flaw.
• The length of a flaw can be measured as a proportion of the scan length or can
be estimated visually in relation to total scan length or to the size of a known
feature of the test piece.
• The position of a flaw can be determined by measuring its position along the
scan with respect to either a predetermined reference point or a known
feature of the test piece.
• C-scan presentations are interpreted mainly by comparing the x and y
coordinates of any flaw indication with the x and y coordinates of either a
predetermined reference point or a known feature of the test piece.
• The size of a flaw is estimated as a percentage of the scanned area. If a known
feature is the size or position reference for the interpretation of either B-scan
or C-scan data, it is presumed that this feature produces an appropriate echo
image on the display.
• In contrast to normal B-scan and C-scan displays, A-scan displays are
sometimes quite complex. They may contain electronic noise, spurious
echoes, or extra echoes resulting from scattering or mode conversion of the
transmitted or interrogating pulse, all of which must be disregarded in order
to focus attention on any flaw echoes that may be present.

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• Furthermore, flaw echoes may exhibit widely varying shapes and amplitudes.
Accurate interpretation of an A-scan display depends on the ability of the
operator to:

▪ Recognize the type of flaw based on echo shape or echo-

intensity effects.
▪ Determine flaw location by accurately measuring echo position
on the time trace.
▪ Estimate flaw size, mainly from echo amplitudes with or without
simultaneously manipulating the search unit.
▪ Assess the quality of the test piece by evaluating the A-scan data
in terms of appropriate specifications or reference standards.

11.4. Signal-conditioning and gating:

▪ Circuits are included in many commercial ultrasonic instruments. One

common example of a signal-conditioning feature is a circuit that
electronically compensates for the signal-amplitude loss caused by
attenuation of the ultrasonic pulse in the test piece.
▪ Electronic gates, which monitor returning signals for pulses of selected
amplitudes that occur within selected time-delay ranges, provide
automatic interpretation.
▪ The set point of a gate corresponds to a flaw of a certain size that is located
within a prescribed depth range. Gates are often used to trigger alarms or
to operate automatic systems that sort test pieces or identify rejectable
pieces.

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12. Piezoelectric Transducers:

Contact-Type Units:

▪ Although contact-type search units can sometimes be adapted to automatic

scanning, they are usually hand held and manually scanned in direct contact
with the surface of a test piece. A thin layer of an appropriate couplant is
almost always required for obtaining transmission of sound energy across the
interface between the search unit and the entry surface.
▪ Piezoelectricity is pressure-induced electricity, this property is characteristic of
certain naturally occurring crystalline compounds and some man-made
materials.
▪ As the name piezoelectric implies, an electrical charge is developed by the
crystal when pressure is applied to it. Conversely, when an electrical field is
applied, the crystal mechanically deforms (changes shape). Piezoelectric
crystals exhibit various deformation modes; thickness expansion is the
principal mode used in transducers for ultrasonic inspection.
▪ The most common types of piezoelectric materials used for ultrasonic search
units are quartz, lithium sulfate, and polarized ceramics such as barium
titanate, lead zirconate titanate, and lead metaniobate.

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12.1. Straight-Beam Units:

• This type of search unit is hand held and manually scanned in direct contact
with the surface of the test piece. The contact face is subject to abrasion and
in most cases is required to couple efficiently to metals having high acoustic
impedances.
• This type of search unit projects a beam of ultrasonic vibrations perpendicular
to the entry surface. It can be used for either the reflection (echo) method or
the through transmission method.
• Inspection can be performed with either one or two search units. With the
technique using a single search unit, the search unit acts alternately as
transmitter and receiver.
• The backing is usually a highly attenuative, high density material that is used
to control the vibration of the transducer by absorbing the energy radiating
from the back face of the active element.
• When the acoustic impedance of the backing matches the acoustic impedance
of the active element, the result will be a heavily damped transducer that
displays good range resolution but may be lower in signal amplitude.
• If there is a mismatch in acoustic impedance between the element and the
backing, more sound energy will be reflected forward into the test material.
The end result is a transducer that is lower in resolution due to longer
waveform duration, but may be higher in signal amplitude or greater in
sensitivity.

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12.2. Dual Element Transducers:

• Dual element transducers utilize separate transmitting and receiving

elements, mounted on delay lines that are usually cut at an angle.
• This configuration improves near surface resolution by eliminating main bang
recovery problems. In addition, the crossed beam design provides a pseudo
focus that makes duals more sensitive to echoes from irregular reflectors such
as corrosion and pitting.
• One consequence of the dual element design is a sharply defined distance/
amplitude curve.
• In general, a decrease in the roof angle or an increase in the transducer
element size will result in a longer pseudo-focal distance and an increase in
useful range.

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12.3. Angle-Beam Units:

• A plastic wedge between the piezoelectric element and the contact surface
establishes a fixed angle of incidence for the search unit.
• The plastic wedge must be designed to reduce or eliminate internal reflections
within the wedge that could result in undesired false echoes.
• Angle-beam search units are used for the inspection of sheet or plate, pipe
welds or tubing, and test pieces having shapes that prevent access for straight
beam.
• Angle-beam search units can be used to produce shear waves or combined
shear and longitudinal waves, depending on the wedge angle and test piece
material.
• There is a single value of wedge angle that will produce the desired beam
direction and wave type in any given test piece.
• A search unit having the appropriate wedge angle is selected for each specific
application.
• The surface wave search unit is an angle-beam unit insofar as it uses a wedge
to position the crystal at an angle to the surface of the test piece. It generates
surface waves by mode conversion.
• The wedge angle is chosen so that the shear wave refraction angle is 90° and
the wave resulting from mode conversion travels along the surface.
• The nominal refraction angles are 45°, 60°, 70° which are commonly used for
weld inspection.

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12.4. Delay-Tip Units:

• Its primary application is in thickness measurement and for thin materials that
require a high degree of resolution.
• The delay shoe allows the indication from the front surface of the test piece to
be delayed by the transmission time through the delay shoe.
• This separates the front-surface signal from the large excitation pulse, thus
eliminating much of the dead zone encountered in contact inspection with a
search unit that does not have a delay shoe.
• Reducing the extent of the dead zone allows the piezoelectric crystal to
respond to front and back reflections that occur close together in time.
• This provides improved accuracy in the thickness measurement of thin plate
and sheet.
• The major application of delay tip transducers is in which the test materials
are in elevated temperatures.

11.5. Array Transducers:

• In recent years, there has been a growing need to increase the speed of
ultrasonic inspections. The fastest means of scanning is the use of an array of
transducers that are scanned electronically by triggering each of the
transducers sequentially.
• Such transducers consist of several crystals placed in a certain pattern and
triggered one at a time, either manually or by a multiplexer.
• Array transducers can either transmit normal to their axis or can have an angle
beam. To perform beam steering, sound is generated from the various crystals
with a predetermined phase difference. The degree of difference determines
the beam angle.

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11.6. Paintbrush Transducers:

• The inspection of large areas with small single-element transducers is a long
and tedious process. To overcome this and to increase the rate of inspection,
the paintbrush transducer was developed.
• It is so named because it has a wide beam pattern that, when scanned, covers
a relatively wide swath in the manner of a paintbrush.
• Paintbrush transducers are usually constructed of a mosaic or series of
matched crystal elements. The primary requirement of a paintbrush
transducer is that the intensity of the beam pattern not varies greatly over the
entire length of the transducer.
• Paintbrush transducers are designed to be survey devices; their primary
function is to reduce inspection time while still giving full coverage. Standard
search units are usually employed to pinpoint the location and size of a flaw
that has been detected by a paintbrush transducer.
13. Electromagnetic Acoustic Transducers (EMAT) :

• Electromagnetic-acoustic (EMA) phenomena can be used to generate

ultrasonic waves directly into the surface of an electrically conductive
specimen without the need for an external vibrating transducer and coupling.
Similar probes can also be used for detection, so that a complete non-contact
transducer can be constructed.
• The method is therefore particularly suitable for use on high-temperature
specimens, rough surfaces, and moving specimens. The received ultrasonic
signal strength in EMA systems is 40 to 50 dB lower than a conventional
barium titanate probe, but input powers can be increased.
• The principle of EMA transducers is illustrated in Figure. A permanent magnet
or an electromagnet produces a steady magnetic field, while a coil of wire
carries an RF current. The radio frequency induces eddy currents in the

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surface of the specimen, which interact with the magnetic field to produce
Lorentz forces that cause the specimen surface to vibrate in sympathy with
the applied radio frequency.
• When receiving ultrasonic energy, the vibrating specimen can be regarded as
a moving conductor or a magnetic field, which generates currents in the coil.
• The clearance between the transducer and the metal surface affects the
magnetic field strength and the strength of the eddy currents generated, and
the ultrasonic intensity falls off rapidly with increasing gap.
• Working at 2 MHz, a gap of 1.0 to 1.5 mm (0.04 to 0.06 in.) has been found to
be practicable provided it is kept reasonably constant.

14. Magnetostriction Transducers :

• Although Magnetostriction transducers are seldom used in the ultrasonic

inspection of metals, Magnetostriction has advantages in the Lamb wave
testing of wire specimens.
• Magnetostrictive materials change their form under the influence of a
magnetic field, and the most useful Magnetostrictive material in practice are
iron, nickel and cobalt as well as many alloys of these three elements to create
ultrasonic waves in a liquid.
• A stack of plates of Magnetostrictive material with a coil through them can
produce an ultrasonic beam at right angles to the plate stack, and the
frequency.

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• A magnetostrictive ultrasonic transducer consists, essentially, of series of

laminations of a magnetostrictive (or ferromagnetically) active material
attached directly to a vessel or tank which holds the liquid to be ultrasonically
activated. A coil of wire is positioned either around or in proximity to the
laminations of magnetostrictive material to provide the oscillating magnetic
field required to cause them to, in turn, vibrate. The basic construction is
shown below.

15. Immersion Ultrasonic testing :

• In conventional immersion inspection, both the search unit and the test piece are
immersed in water. The sound beam is directed into the test piece using either a
straight-beam (longitudinal wave) technique or one of the various angle-beam
techniques, such as shear, combined longitudinal and shear, or Lamb wave.
• Immersion-type search units are basically straight-beam units and can be used for
either straight-beam or angle-beam inspection through control and direction of
the sound beam (Figure [a to d] ).
• In straight-beam immersion inspection, the water path (distance from the face of
the search unit to the front surface of the test piece) is generally adjusted to
require a longer transit time than the depth of scan (front surface to back
surface) so that the first multiple of the front reflection will appear farther along
the oscilloscope trace than the first back reflection.

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• This is done to clear the displayed trace of signals that may be misinterpreted.
Water path adjustment is particularly important when gates are used for
automatic signaling and recording. Longitudinal wave velocity in water is
approximately one-fourth the velocity in aluminum or steel; therefore, on the
time base of the oscilloscope, 25 mm (1 in.) of water path is approximately equal
to 100 mm (4 in.) of steel or aluminum. Therefore, a rule of thumb is to make the
water path equal to one-fourth the test piece thickness plus 6 mm ( in.).

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16.1. Immersion transducer Units:

• The advantages of immersion inspection include speed of inspection, ability to
control and direct sound beams, and adaptability for automated scanning.
• Angulation is used in immersion inspection to identify more accurately the
orientation of flaws below the surface of the test piece.
• If the direction of the sound beam, its point of entry, and its angle of incidence
are known, the direction and angle of refraction within the test piece can be
calculated from the following equation,

Sin α / Sin β = V1 / V2

Where,

α= Angle of incidence
β= Angle of refraction
V1=Velocity in first medium
V2= Velocity in second medium

• Only a single search unit is required for conventional immersion inspection,

regardless of incident angle. The construction of a conventional immersion-
type search unit is shown in Figure

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• The change in the focal length can be predicted by the following equation.
For example, given a particular focal length and material path, this
equation can be used to determine the appropriate water path to
compensate for the focusing effect in the test material.

WP = F - MP (Ctm – Cw)

Where,

WP = Water Path

MP = Material Path / Depth

F = Focal Length in Water

Ctm = Sound Velocity in the Test Material

Cw = Sound Velocity in Water

• There are three broadly classified scanning methods that utilize immersion-
type search units:

1. Conventional immersion methods in which both the search unit and

the test piece are immersed in liquid.
2. Squirter and bubbler methods in which the sound is transmitted in a
column of flowing water.
3. Scanning with a wheel-type search unit which is generally classified as
an immersion method because the transducer itself is immersed

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15.2 Focused immersion transducer Units:

• Sound can be focused by acoustic lenses in a manner similar to that in which

light is focused by optic lenses. Most acoustic lenses are designed to
concentrate sound energy, which increases beam intensity in the zone
between the lens and the focal point.
• When an acoustic lens is placed in front of the search unit, the effect
resembles that of a magnifying glass; that is, a smaller area is viewed but
details in that area appear larger. The combination of a search unit and an
acoustic lens is known as a focused search unit or focused transducer;
• For optimum sound transmission, the lens of a focused search unit is usually
bonded to the transducer face. Focused search units can be immersion or
contact types.
• Acoustic lenses are designed similarly to optic lenses. Acoustic lenses can be
made of various materials; several of the more common lens materials are
methyl methacrylate, polystyrene, epoxy resin, aluminum, and magnesium.

• The important properties of materials for acoustic lenses are:

▪ Large index of refraction in water
▪ Acoustic impedance close to that of water or the piezoelectric element
▪ Low internal sound attenuation

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▪ Ease of fabrication
• Immersion transducers are available in three different configurations as listed
below,
▪ unfocused (“flat”),
▪ spherically (“spot”) focused, and
▪ Cylindrically (“line”) focused.
• Focusing is accomplished by either the addition of a lens or by curving the
element itself. The addition of a lens is the most common way to focus a
transducer.

• The advantages of focused search units are listed below; these advantages
apply mainly to the useful thickness range of 0.25 to 250 mm (0.010 to 10 in.)
below the front surface:
▪ High sensitivity to small flaws
▪ High resolving power
▪ Low effects of surface roughness
▪ Low effects of front-surface contour
▪ Low metal noise (background)

16. Inspection standard:

The standardization of ultrasonic inspection allows the same test procedure to be
conducted at various times and locations, and by both customer and supplier,
with reasonable assurance that consistent results will be obtained.
Standardization also provides a basis for estimating the sizes of any flaws that are
found.

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The inspection or reference standards for pulse-echo testing include test blocks
containing natural flaws, test blocks containing artificial flaws, and the technique
of evaluating the percentage of back reflection. Inspection standards for
thickness testing can be plates of various known thicknesses or can be stepped or
tapered wedges.

16.1. Test blocks containing natural flaws:

▪ These are metal sections similar to those parts being inspected. Sections known to
contain natural flaws can be selected for test blocks. Test blocks containing natural
flaws have only limited use as standards, for two principal reasons:
▪ It is difficult to obtain several test blocks that give identical responses. Natural flaws
vary in shape, surface characteristics, and orientation, and echoes from natural flaws
vary accordingly.
▪ It is often impossible to determine the exact nature of a natural flaw existing in the
test block without destructive sectioning.

16.2. Test blocks containing artificial flaws:

▪ It consists of metal sections containing notches, slots, or drilled holes. These test
blocks are more widely accepted as standards than are test blocks that contain
natural flaws.
▪ Test blocks containing drilled holes are widely used for longitudinal wave, straight-
beam inspection. The hole in the block can be positioned so that ultrasonic energy
from the search unit is reflected either from the side of the hole or from the bottom
of the hole.
▪ The flat-bottom hole is used most because the flat bottom of the hole offers an
optimum reflecting surface that is reproducible.

17. Standard Reference Blocks:

• Many of the standards and specifications for ultrasonic inspection require the
use of standard reference blocks, which can be prepared from various alloys,
may contain holes, slots, or notches of several sizes, and may be of different
sizes or shapes.
• The characteristics of an ultrasonic beam in a test piece are affected by the
following variables, which should be considered when selecting standard
reference blocks:
▪ Nature of the test piece
▪ Alloy type
▪ Grain size
▪ Effects of thermal or mechanical processing
▪ Distance-amplitude effects (attenuation)
▪ Flaw size

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▪ Direction of the ultrasonic beam

19.1. Area-amplitude blocks:

▪ It provides artificial flaws of different sizes at the same depth. Eight blocks
made from the same 50 mm (2 in.) diameter round stock, each 95 mm (3 ¾
in.) high, constitute a set of area-amplitude blocks.
▪ The block material must have the same acoustic properties as the test piece
material.
▪ Each block has a 20 mm (3/4 in.) deep flat-bottom hole drilled in the center of
the bottom surface. The hole diameters vary from 0.4 to 3.2 mm
▪ The blocks are numbered to correspond with the diameter of the holes.
▪ Block No. 1 has a 0.4 mm (1/64 in.) diameter hole, No. 2 has a 0.8 mm (
2/64in.) diameter hole, and so on, up to No. 8, which has a 3.2 mm ( in.)
diameter hole. Similar area amplitude blocks made from 49 mm (1 in.)
square stock are sometimes known as Series-A blocks.

Block No. FBH Diameter Block Diameter Metal Distance

mm(in) mm (in) mm(in)
1 0.396 (1/64) 50 (2) 95 (3 ¾)

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2 0.793 (2/64) 50 (2) 95 (3 ¾)

3 1.190 (3/64) 50 (2) 95 (3 ¾)
4 1.587 (4/64) 50 (2) 95 (3 ¾)
5 1.984 (5/64) 50 (2) 95 (3 ¾)
6 2.381 (6/64) 50 (2) 95 (3 ¾)
7 2.778 (7/64) 50 (2) 95 (3 ¾)
8 3.175 (8/64) 50 (2) 95 (3 ¾)

19.2. Distance-amplitude blocks:

• It provides artificial flaws of a given size at various depths. From ultrasonic

wave theory, it is known that the decrease in echo amplitude from a flat-
bottom hole using a circular search unit is inversely proportional to the square
of the distance to the hole bottom.
• Distance-amplitude blocks (also known as Series-B blocks) can be used to
check actual variations of amplitude with distance for straight-beam
inspection in a given material.
• They also serve as a reference for setting or standardizing the sensitivity (gain)
of the inspection system so that readable indications will be displayed on the
oscilloscope screen for flaws of a given size and larger, but the screen will not
be flooded with indications of smaller discontinuities that are of no interest.
• On instruments so equipped, these blocks are used to set the sensitivity-time
control or distance-amplitude correction so that a flaw of a given size will
produce an indication on the oscilloscope screen that is of a predetermined
height regardless of distance from the entry surface.

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• There are 19 blocks in a Series-B set. All are 50 mm (2 in.) dia blocks of the
same material as that being inspected, and all have a 20 mm (3/4in.) deep
flat-bottom hole drilled in the center of the bottom surface.
• The hole diameter is the same in all the blocks of a set; sets can be made with
hole diameters of 1.2, 2.0, and 3.2 mm (3/64, 5/64, and 8/64 in.).
• The blocks vary in length to provide metal distances of 1.6 to 145 mm (1/16to
5 3/4 in.) from the top (entry) surface to the hole bottom. Metal distances are:
▪ 1.6 mm ( 1/16 in.)
▪ 3.2 mm through 25 mm (1/8 in. through 1 in.) in increments of 3.2
mm (1/8 in.)
▪ 32 mm through 150 mm (1 1/4 in. through 5 ¾ in.) in increments of
13 mm ( ½ in.)

FBH Metal Path

Sl.no. Block No. Diameter Distance
mm mm(in)
1 3,5,8 -0006 1.2 / 2.0 / 3.2 1.6 (0.0625)

2 3,5,8 -0012 1.2 / 2.0 / 3.2 3.2 (0.125)

3 3,5,8 -0025 1.2 / 2.0 / 3.2 6.4(0.250)

4 3,5,8 -0037 1.2 / 2.0 / 3.2 9.6(0.375)

5 3,5,8 -0050 1.2 / 2.0 / 3.2 12.8(0.500)

6 3,5,8 -0062 1.2 / 2.0 / 3.2 16.0(0.625)

7 3,5,8 -0075 1.2 / 2.0 / 3.2 19.2(0.750)

8 3,5,8 -0087 1.2 / 2.0 / 3.2 22.4(0.875)

9 3,5,8 -0100 1.2 / 2.0 / 3.2 25.4(1.000)

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10 3,5,8 -0125 1.2 / 2.0 / 3.2 32(1.250)

11 3,5,8 -0175 1.2 / 2.0 / 3.2 45(1.750)

12 3,5,8 -0225 1.2 / 2.0 / 3.2 58(2.250)

13 3,5,8 -0275 1.2 / 2.0 / 3.2 71(2.750)

14 3,5,8 -0325 1.2 / 2.0 / 3.2 83(3.250)

15 3,5,8 -0375 1.2 / 2.0 / 3.2 97(3.750)

16 3,5,8 -0425 1.2 / 2.0 / 3.2 110(4.250)

17 3,5,8 -0475 1.2 / 2.0 / 3.2 123(4.750)

18 3,5,8 -0525 1.2 / 2.0 / 3.2 136(5.250)

19 3,5,8 -0575 1.2 / 2.0 / 3.2 149(5.750)

• Each Series-B block is identified by a code number consisting of a digit, a dash,

and four more digits. The first digit is the diameter of the hole in one sixty-
fourths of an inch. The four other digits are the metal distance from the top
(entry) surface to the hole bottom in one hundredths (00.00) of an inch.
• For example, a block marked 3-0075 has a 1.2 mm ( 3x(1/64)in.) dia hole and
20 mm (00.75 in.) metal distance.

19.3. ASTM blocks:

• It can be combined into various sets of area-amplitude and distance-

amplitude blocks.
• The ASTM basic set consists of ten 50 mm (2 in.) dia blocks, each with a 20
mm ( ¾ in.) deep flat-bottom hole drilled in the center of the bottom surface.
• One block has a 1.2 mm ( 3/64 in.) dia hole at a 75 mm (3 in.) metal distance.

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• Seven blocks have 20 mm ( 5/64 in.) dia holes at metal distances of 3.2, 6.4,
13, 20, 40, 75, and 150 mm ( 1/8, 1/4, 1/2 , 3/4 , 1 1/2 , 3 and 6 in.).
• The remaining blocks have 3.2 mm ( 8/64 in.) dia holes at 75 and 150 mm (3
and 6 in.) metal distances.
• The three blocks with a 75 mm (3 in.) metal distance and hole diameter of 1.2,
2.0, and 3.2 mm (3/64 , 5/64 , and 8/64 in.) form an area-amplitude set, and
the set with the 2.0 mm ( 5/64 in.) dia holes provides a distance-amplitude
set.
• In addition to the basic set, ASTM lists five more area-amplitude standard
reference blocks and 80 more distance-amplitude blocks.

19.4. IIW blocks:

These blocks are mainly used to calibrate instruments prior to contact inspection
using an angle-beam search unit; these blocks are also useful for:
• Checking the performance of both angle-beam and straight-beam search
units
• Determining the sound beam exit point of an angle-beam search unit

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• Determining refracted angle produced

• Calibrating sound path distance
• Evaluating instrument performance

There are two types of IIW blocks as listed below,

• IIW-Type -1 (IIW-V1) Block
• IIW-Type -2 (IIW-V2 or miniature) Block

The miniature angle-beam block shown in Figure, is used in a somewhat similar

manner as the larger IIW block. Both the 25 and 50 mm (1 and 2 in.) radius surfaces
provide ways for checking the location of the index mark of a search unit and for
calibrating the time base of the instrument in terms of metal distance. The small hole
provides a reflector for checking beam angle and for setting the instrument gain.

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18.Couplant:
• A couplant 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 air
and solids (i.e. such as the test specimen) is large.

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• The couplant displaces the air (poor transmitter of sound waves) and makes it
possible to get more sound energy into the test specimen so that a usable
ultrasonic signal can be obtained. In contact ultrasonic testing a thin film of
oil, glycerine or water is generally used between the transducer and the test
surface.
• In immersion ultrasonic testing both the transducer and the part are
immersed in the couplant, which is typically water.

• This method of coupling makes it easier to maintain consistent coupling while

moving and manipulating the transducer and/or the part.
• The following should be considered in selecting a couplant:
▪ Surface finish of test piece
▪ Temperature of test surface
▪ Possibility of chemical reactions between test surface and couplant
▪ Cleaning requirements
• Some of the essential characteristics of couplant are as listed,
▪ Couplant used on nickel based alloys shall not contain more than
250ppm of sulphur
▪ Couplant used in contact inspection should be applied as a uniform,
thin coating to obtain uniform and consistent inspection results.
▪ Heavy oil, grease, or wallpaper paste may not be good choices when
water will suffice, because these substances are more difficult to
remove.
▪ It is sometimes appropriate to add glycerin to increase viscosity;
however, glycerin tends to induce corrosion in aluminum and
therefore is not recommended in aerospace applications.
19. Calibration of ultrasonic equipment:

19.1. Time-Base Linearity or Horizontal Linearity:

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• Horizontal Limit is the maximum readable length of horizontal deflection that

is determined either by an electrical or a physical limit in the A-scan
presentation of an ultrasonic testing instrument.
• Horizontal limit is expressed as the maximum observed deflection in
inches or in mm from the left side, or the start, of the horizontal line
representing the time base.
• The horizontal limit is full scale.
• Horizontal Linearity shall be calibrated by Using IIW block and a straight beam
search unit
• Place the search unit on the IIW block and adjust the gain, sweep, and sweep
delay to obtain six back reflections on the CRT.
• The first back reflection should be located at the left side of the base line (the
initial pulse will be off screen), and the 6th back reflection should be located
at the right side of the base line.

19.2. Amplitude linearity or vertical Linearity:

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• Calibration in terms of metal distance or reflector depth assumes a linear

oscilloscope sweep for the instrument, which can be checked using a straight-
beam search unit.
• The search unit is placed on either surface C or D to obtain multiple echoes
from the 25 mm (1 in.) thickness.
• Reduce or add dB (usually 6db for easiest calculations), and notice the change
in echo amplitudes corresponding to the decibels.
• These echo indications will be aligned with evenly spaced grid lines or scale
marks if the time base is linear.
• Linearity within ±4% (or less) of the full-scale value of thickness is usually
obtainable.
• The same shall be followed for both angle beam shear wave as well as Normal
and TR probes correspondingly.

19.3. Probe Index Point:

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• Calibration of an angle-beam pulse-echo ultrasonic test system for flaw

detection can be accomplished using an IIW standard calibration block such as
the one shown in Fig. (a).
• The large (100 mm, or 4 in., radius) curved surface at one end of the block is
used to determine the index point of angle-beam search units (the point
where the beam leaves the unit).
• The mark labeled "beam exit point" in Fig. (a) Is at the center of this 100 mm
(4 in.) radius. Regardless of the angle of the search unit, a maximum echo is
received from the curved surface when the index point of the search unit is at
the "beam exit point."
• To determine the index point of an angle-beam search unit, the search unit is
placed on surface A in the position shown in Fig. (a) and is moved along the
surface of the block until the echo is at maximum amplitude.
• The point on the search unit that is directly over the beam exit point of the
block can then be marked as the index point of the search unit.

19.4. Probe index angle:

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• Once the index point of the search unit is marked, the 50 mm (2 in.) dia hole is
used to determine the angle of the beam in the low-carbon steel from which
the block is prepared.
• The search unit is placed on surface A or surface B and is aimed toward the 50
mm (2 in.) dia hole. Then, the search unit is moved along the surface until a
maximum-amplitude echo is received.
• At this position, the index point on the search unit indicates the beam angle,
which is read from one of the degree scales marked along the sides of the
block at the edges of surfaces A and B refer Fig. (b).
• By knowing the beam path value of the artificial defect at known depth from
IIW-V1 Block or IIW-V2 block, the true indexing angle (refracted angle) of the
angle beam probe shall be calculated as follows,

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19.5. Analytical method for determining probe index and probe angle
simultaneously:
• Analytically the probe index point and probe true beam angle shall be
predicted by the following method
• Plotting a cure with depth of the side drilled hole on ‘X’ axis and position of
the probe face end in ‘Y’ axis. As shown.
• The maximum indications were captured from the side drilled holes (SDH) at
various depths (minimum three side drilled holes) and noted the location of
the probe from the SDH and the corresponding hole depth was noted and
plotted on the graph as shown in the figure.
• The line joining these plotted points will intersect the ‘X’ axis at a distance of X
from the origin.
• The distance X from the origin will give the exact value of probe index.
• The included angle (θ) between the line intersecting on ‘X’ axis will give the
probe beam angle.

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19.6. Delay correction for the Shear wave probe :

• A delay correction for the plastic wedge is required to correctly set the
scale to 100 mm with each shear wave probe.
• Scan the 100 mm radius with the shear wave probe for which the range
is to be calibrated and maximize the signal obtained from the radius.
• Observe the location of this indication on the scale and bring it to the
actual full scale location using the zero and delay control.

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19.7. Sensitivity :

• The relative sensitivity of an angle-beam search unit in combination with a

given instrument can be defined by placing the unit on either surface A or B
and reflecting the beam from the side of the 1.5 mm (0.060 in.) dia hole Fig.
(a).
• The position of the search unit is adjusted until the echo from the hole is
maximum, then the gain of the instrument is adjusted to give the desired
indication height.
• When no back reflection is expected, the sensitivity of a straight-beam system
is defined by placing the search unit on either surface B or F in line with the
1.5 mm (0.060 in.) dia hole.
• The position of the search unit is adjusted until the echo from the side of the
hole is maximum, then the gain of the instrument is adjusted to give the
desired indication height.
• When a back reflection is expected, a plastic insert can be used in the 50 mm
(2 in.) dia hole to gage the sensitivity of a straight-beam system.
• The plastic material and insert thickness are specified to have the absorption
characteristics of 50 mm (2 in.) of steel.
• With this calibration, the search unit is placed on the side of the insert facing
surface C, and the number of echoes and the height of the last echo indication
are noted.
• The gain was increased to a minimum of additional 6dB from the reference
level to improve the detection sensitivity during Inspection.

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19.8. Resolution:
• A straight-beam search unit, as well as the instrument, can be checked for
back-surface resolution by placing the search unit on surface A and reflecting
the beam from the bottom of the 2 mm (0.080 in.) wide notch and from
surfaces B and E on either side of it, as shown in Fig.(d).
• With proper resolution, the indications from these three surfaces should be
clearly separated and not overlapped so as to appear as one broad, jagged
indication. Because resolution is affected by test conditions and by
characteristics of the search unit and instrument amplifier, this degree of
resolution sometimes may not be obtainable.

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19.9. Beam spread:

• It can be determined by moving the search unit in one direction (toward

either higher or lower beam angles) from the point of maximum-amplitude
echo until the echo disappears and noting the beam angle at the index point.
• The search unit is then moved in the opposite direction, past the point of
maximum-amplitude echo to the point where the echo again disappears.
• The beam spread is the difference between the angles indicated by the index
point at these two extreme positions.

19.10. The dead zone:

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• It is the depth below the entry surface that cannot be inspected because the
initial pulse interferes with echo signals. An indication of the length of the
dead zone of a straight-beam search unit can be obtained by placing the
search unit on surface A or F in line with the 50 mm (2 in.) dia hole Fig. (a).
• When the search unit is placed on surface A, a discernible echo from the 50
mm (2 in.) dia hole indicates a dead zone of less than 5 mm (0.2 in.). Similarly,
when the search unit is placed on surface F, a discernible echo from the 50
mm (2 in.) dia hole indicates a dead zone of less than 10 mm (0.4 in.).
• Alternatively, the length of the dead zone can be measured by calibrating the
time base of the instrument, then measuring the width of the initial-pulse
indication at the base of the signal, as illustrated schematically in Figure.

19.11. Range Setting:

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• The range for a search-unit and instrument system for straight-beam

inspection can be set for various distances by use of the IIW block.
• From surface F to the 2.0 mm (0.080 in.) wide notch is 200 mm (8 in.), from
surface A to surface B is 100 mm (4 in.), from surface E to surface A is 91.4 mm
(3.60 in.), and from surface C to surface D is 25 mm (1 in.).
• For weld scanning the range shall be set such that the overall weld shall be
covered in half and full skip distances (scanning distances).
• For Normal beam scanning screen range was set to get at least first three back
walls echoes on the screen.

Weld scanning with angle beam probe, Range = 0 – (1.25 times the full beam path)
Normal beam or Dual element (TR) Probe, Range = 4 X Thickness of the job.

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19.11. Distance amplitude correction:

• Acoustic signals from the same reflecting surface will have different
amplitudes at different distances from the transducer.
• DAC – Distance amplitude Correction curve is a method of compensating for
the fact that the pulse-echo response of a reflector will decrease as the
distance of the reflector from the ultrasonic probe increases.

• This occurs because the transmitted ultrasonic beam spreads out as it travels
from the probe to the reflector and so the further the reflector is from the
probe the lower the energy of the sound that actually hits the reflector.
• Similarly, the further the reflected pulse has to travel the lower the energy
that is received back at the probe.
• The material from which DAC block was made shall be of the same product
form and material specification or equivalent P- number grouping as the
material being inspect.

• The DAC is generated by plotting the amplitude of a known calibration

reflector (Generally Side drilled holes and notches are used for non-tubular
and tubular inspections correspondingly) at different distances from the
probe.
• The gain of the flaw detector is set so that the amplitude of the nearest
reference reflector (the one at the shallowest depth) is at 80% FSH.
• The amplitudes of the same calibration reflector at further depths are then
plotted on the flaw detector screen whilst the gain setting is maintained
constant.
• The DAC is generated by plotting a curve through these amplitudes.

• During the inspection, the amplitude of any echo which appears on the
screen can easily be compared to the signal amplitude of the reference
reflector at the exactly the same range.

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19.12. Transfer correction:

• A correction of instrument sensitivity is required to compensate for
differences between the reference standard surface roughness, contact area,
and acoustical attenuation characteristics and those of the part being
examined.
• Amplitude transfer correction should be performed at initial examination of a
group of similar welds and / or materials and whenever significant changes in
surface roughness, condition profile, or coating are observed.
• If the transfer loss was measured more than +6dB, Weldments conditioning
should be performed before doing inspection.
• Transfer correction was done by employing two identical angle beam probes,
one acting as transmitter and the other as receiver.
• The probes are directed each other on the reference block at one skip
distance and the signal is adjusted to 80% of full screen height.
• The probe was then placed at two skip distance and the echo signal amplitude
was noted on the graphical plot representing the CRT grid system.
• The two points are connected by a straight line on the graphical plotter.
• Without altering equipment setup, reflections are obtained at one and two
skip distances on the member to be examined.
• These two points are entered on the graphical plot and connected by a
straight line.The difference in amplitude between these two straight lines
gives the transfer correction value in dB.
• The transfer correction value was added with the reference gain during
inspection and evaluation.

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20. WELD SCANNING:

• Welded joints can be ultrasonically inspected using either the straight-beam
or the angle-beam technique.
• Angle-beam inspection is used most often, one reason being that the
transducer does not have to be placed on the weld surface, but is typically
placed on the relatively smooth base metal surface adjacent to the weld.
• In angle-beam inspection, the wedge angle is usually selected to produce
shear waves in the part being inspected at an optimum angle to detect serious
flaws.

• The types of flaws usually encountered in welds are

▪ Porosity,
▪ Entrapped slag,
▪ Incomplete penetration,
▪ Incomplete fusion,
▪ Cracks,
▪ Lamellar tearing,
▪ Lamination,
▪ Undercut.
• Serious flaws, such as cracks and incomplete fusion, usually extend
longitudinally along the weld and give especially clear signals when the sound
beam strikes them at right angles.
• Spherical porosity will produce a small amplitude echo, even when the sound
beam strikes at an angle to the joint.
• Slag inclusions may produce stepped indications, which are maximum at right
angles to the orientation of the slag.
• A large inclusion may produce multiple signals as different portions of the
inclusion are scanned.

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20.1. Probe Movement or Scanning Pattern:

• Weld shall be scanned using a back and forth movement of a probe

accomplished by a slight angular (Rotational) movement.
• The length of the transverse movement shall be sufficient to ensure that the
center of the Beam crosses the weld profile in two directions, a full Vee-Path
and one forth times a skip distance.

Movement A – Lateral scanning at constant skip distance (best to detect root discontinuities)

Movement B – Swivel scanning (best for sizing discontinuities)

Movement C – Direct scan or Traverse scanning (best to detect sub surface discontinuities)

Movement D – Zigzag Scanning (Vee path scanning)

Movement E – Orbital scanning (best for sizing volumetric discontinuities)

Movement F – Skew angle scanning (best to detect transverse discontinuities)

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Weld Scanning Techniques:

• Shear wave from an angle-beam transducer progresses through a flat test

piece by reflecting from the surfaces at points called nodes.
• The linear distance between two successive nodes on the same surface is
called the skip distance and is important in defining the path over which the
transducer should be moved for reliable and efficient scanning of a weld.
• The skip distance can be easily measured by using a separate receiving
transducer to detect the nodes or by using an angle-beam test block or it can
be calculated.
• Once the skip distance is known, the region over which the transducer should
be moved to scan the weld can be determined.
• This region should extend the entire length of the weld at a distance from the
weld line of approximately to 1 skip distance, as shown in Fig.
• A zigzag scanning path is used, either with sharp changes in direction (Fig.) or
with squared changes

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20.2. Angle beam scanning:

• A straight beam transducer, producing a longitudinal wave at normal
incidence into the test piece, is first used to locate any laminations in or near
the heat-affected zone. This is important because an angle beam transducer
may not be able to provide a return signal from a laminar flaw.
• The second step in the inspection involves using an angle beam transducer to
inspect the actual weld. Angle beam transducers use the principles of
refraction and mode conversion to produce refracted shear or longitudinal
waves in the test material.
• This inspection may include the root, sidewall, crown, and heat-affected zones
of a weld.
• The process involves scanning the surface of the material around the
weldments with the transducer.
• This refracted sound wave will bounce off a reflector (discontinuity) in the
path of the sound beam.
• With proper angle beam techniques, echoes returned from the weld zone may
allow the operator to determine the location and type of discontinuity.
• To determine the proper scanning area for the weld, the inspector must first
calculate the location of the sound beam in the test material. Using the
refracted angle, beam index point and material thickness, the Beam path
Distance and skip distance of the sound beam is found.
• By using the calculations the transducer locations on the surface of the
material corresponding to the crown, sidewall, and root of the weld shall be
clearly identified.

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20.3. Weld scanning Calculations:

• The Pythagorean Theorem states that, in a right triangle, the square of

hypotenuse is equal to the sum of squares of opposite side and adjacent side.
• In a right triangle with sides a, b and hypotenuse c, trigonometry determines
the sine and cosine of the angle θ between side ‘a’ and the hypotenuse as
shown in the diagram.

• By which the below formula can be derived as fallows,

▪ Half Beam path = (T / Cos ϴ )

▪ Full Beam Path = 2 X (T / Cos ϴ )

▪ Half Skip Distance = (T X tan ϴ )

▪ Full Skip Distance = 2 X (T X tan ϴ )

▪ Depth of a defect ( in half Beam path) = (Beam path X Cos ϴ)

▪ Depth of a defect ( in Full Beam path) = [ (2X T) - (Beam path X Cos ϴ) ]

▪ Surface Distance or location of the Defect = (Beam path x Sin ϴ)

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Weld scanning -Angle beam scanning -formulae Derivations:

By Pythagoras Theorem
Cos ϴ = Adjacent side / Hypotenuse
Sin ϴ = opposite side / Hypotenuse
From the Weld scanning configuration,
Adjacent side = Thickness,
Opposite side = Skip distance,
Hypotenuse = 1/2 Beam path.
Applying these terms in trigonometric functions,
Sin ϴ = ½ skip distance / ½ Beam path
Cos ϴ = Thickness / ½ Beam path
By Re-arranging,

½ Beam path = T / Cos ϴ

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By applying the value of ½ Beam path value,

Sin ϴ = ½ skip distance / (T/ Cos ϴ)
Re-arranging,

½ Skip Distance = T x (sin ϴ / Cos ϴ ) = T x tan ϴ

These two Equations shall be rewrite for the multiples of ½ beam path and ½ skip distances
by multiplying by the constant n, where n gives number of
½ beam path and ½ skip distance values.

Depth of a defect and Location:

While determining the depth of the defect and location, the beam path for
corresponding evaluation was well know from the CRT, it can be predictable whether
the discontinuity indication was at ½ Beam paths or full Beam path or one and half
beam path etc.

If the defect was between 0 to ½ Beam path Distance,

Depth of the defect = Beam path x Cos ϴ

If the defect was between ½ to full Beam path Distance,

Depth of the defect = 2T – (Beam path x Cos ϴ)

Location of the defect or projected Distance,

Location of the Defect = (Beam path x Sin ϴ)

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20.4. Scanning Pattern for Single V Butt joint

(Full skip) (Full skip)

Figure 1: Scanning for parent metal

½ skip ½ skip
Guide strip Guide strip

Figure 2: Scanning for critical root

Full skip Full skip

½ skip ½ skip

Figure 3: Scanning for weld body

Figure 4: Scanning for transverse

defect

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20.5. Geometrical analysis of indications in weld scanning:

• A graphical card as shown in figure will be used to determine and analysis the
sound beam path in the member and it will provides all the required details
for the triangulation solution of discontinuity location.

• The weld geometry shall be drafted in the plotting cards for evaluation of
indications as shown in the figure.

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• From which the location of the defect, depth, size of the defect shall
be clearly extracted.

21. Inspection of Flat-Rolled Products:

• Rolled strip, sheet, and plate can be ultrasonically inspected by using either
straight-beam or angle-beam pulse-echo techniques for contact or immersion
inspection
• Contact inspection is more widely used, with straight-beam inspection from
the top surface of a test piece, planar discontinuities to which flat-rolled
products are susceptible are readily detected, and their locations and limits
are easily and accurately determined.
• Laminations are particularly detrimental when pieces cut from the plate are to
be subsequently welded to form large structural assemblies.
• Laminations usually occur centered in the thickness of the plate and are
usually centered in the as-rolled width of the plate.
• Laminations do not extend to the surface except at sheared or flame-cut
edges and may be difficult to detect visually unless the lamination is gross.
• Ultrasonic inspection is the only reliable way to inspect a plate for
laminations.

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• The loss of back reflection technique is the technique most often used for the
inspection of plate for planar discontinuities, especially laminations.
• This technique is similar to the percentage of back reflection technique except
that rejection is based on complete or partial loss of the back reflection
indication.
• The system is calibrated by placing the search unit over a sound (lamination-
free) region of the plate to be inspected.
• The pattern on the oscilloscope screen is then a series of multiple back
reflection indications from the bottom surface of the plate, as shown for the
top inspection of 28.4 mm (1.12 in.) thick steel plate in Figure (a).
• The instrument controls are adjusted to produce a first back reflection height
of about 75 to 100% of full screen height. General scanning of the plate is
done at this sensitivity level.
• Minor sensitivity adjustments can be made to accommodate surface
roughness, provided no laminations or other flaws are encountered.
• As the search unit passes over the edge of a lamination during scanning,
additional indications appear on the oscilloscope screen approximately
midway between the initial pulse and the first back reflection and between
the multiple back reflection indications.
• As the search unit is moved farther over the laminated region, indications of
the laminations become stronger until finally the original pattern is replaced
by a series of multiple lamination indications spaced at approximately half the
thickness of the plate.
• The size of a lamination can be determined by moving the search unit back
and forth across the flawed region and, when lamination indications drop to
40% of full screen height, marking the plate at a location corresponding to the
center of the search unit.

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22. Inspection of Extrusions and Rolled Shapes:

• Flaws in extrusions and in rolled shapes are usually longitudinally oriented,

that is, parallel to the direction of working.
• Both contact and immersion inspection are used to inspect for these flaws,
usually employing their longitudinal-beam or angle-beam techniques. In some
cases, surface waves are used to detect surface cracks or similar flaws.
• Steel bars and pipe are usually contact inspected for internal flaws, using
either the straight-beam or the angle-beam technique, or both techniques
simultaneously.
• One method of inspecting round bar is to scan along the bar with a straight
beam search unit having a curved plastic shoe to achieve good contact with
the bar. Because the surface of the bar is curved, the sound beam enters the
bar at various angles.
• This may produce both longitudinal and shear waves in the bar and provides
for inspection of the entire cross section of the bar. Another method of
inspecting round bar can also be used on pipe. In this method, the bar or pipe
is rotated to provide inspection of the entire cross section.
• Steel rails for railroads can be contact inspected in service with special
equipment that detects cracks--either transverse cracks emanating from the
edges of the railhead or from hydrogen flakes in the center of the railhead, or
cracks emanating from fishplate bolt holes.
• Larger rail-testing equipment employs the pulse-echo method. Often this
equipment is mounted on special railroad cars. Most inspection with this
equipment is done by angle-beam techniques, but straight beam inspection is
also used. With some equipment, coupling is through an oil film; in other
equipment, wheel-type search units are used.
• Seamless tubes and pipes can be contact tested on the mechanized
equipment, but immersion inspection is used more often. Usually, the tube or
pipe is rotated and driven past the immersed search units, which are
positioned to produce an angle beam in the wall of the test piece.
• The immersion tank has glands on either side through which the part passes
during inspection.

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23. Inspection of Forgings:

• Forgings can be ultrasonically inspected for internal flaws such as pipe,

internal ruptures, flakes, and nonmetallic inclusions.
• Inspection is usually done with frequencies of 1 to 5 MHz, with the beam
normal to both the surface of the forging and the direction of maximum
working; this orientation is best for detecting most flaws in forgings.
• Angle-beam inspection employing shear waves is sometimes used for rings or
hollow forgings.
• Contact inspection is performed on most forgings that have fairly uniform
dimensions. Because of the difficulty in complete contact inspection of
irregularly shaped forgings, immersion inspection may be preferred.
Alternatively, contact inspection can be performed at an earlier stage of
production before the shape becomes too irregular; however a rough
machining of the forging surface is recommended.
• Contact inspection is also used for inspecting forgings in service

24. Inspection of Castings:

• Both contact inspection and immersion inspection are used to detect in

castings such flaws as porosity, tears and cracks, shrinkage, voids, and
inclusions. Figure illustrates typical ultrasonic indications from four types of
flaws found in castings.
• Although immersion inspection is preferred for castings having rough and
irregular surfaces, any one of the inspection techniques previously described
can be used.
• The choice of technique in a specific instance will depend mainly on casting
size and shape. Ultrasonic indications from four types of flaws found in
castings because many castings are coarse grained, low-frequency ultrasound
may have to be used.
• Castings of some materials are so coarse grained that extensive scattering
makes ultrasonic inspection impractical. These materials include some
brasses, stainless steels, titanium alloys, and cast irons.
• Echo dynamics for casting inspection was as shown in the figure

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25. Echo Dynamics pattern of reflectors in weld scanning:

25.1. Point reflectors response:

• At any position the A scan response from the point reflector will shows a
sharp indication echo
• As the probe is moved these sharp echo will attain its maximum peak
amplitude before falling smoothly to the noise level (reverberation or grass
echo level i.e., below 5 % amplitude).
• Porosity was the best example of point reflectors.

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25.2. Extended Smooth reflecting surfaced reflectors:

• At any position the A scan response from the point reflector will shows a
sharp indication echo
• When the probe is moved to get the sizing of the discontinuity the echo will
rise smoothly (with a little change in amplitude or no change) and maintained
until the beam moved off from the reflectors edge.
• Lack of side wall fusion is the best example of smooth reflecting surface
reflectors.

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25.3. Extended Rough reflecting surfaced reflectors:

• The angle of incidence of the probe plays a vital role in getting indications
from the rough reflecting surfaced reflectors.

a) Normal Incidence:

• At normal incidence (i.e. perpendicular to the reflecting surface) A-Scan

will show a single ragged echo.
• As the probe is moved further the amplitude response will undergo a large
fluctuations in amplitude (more than 6 dB variations)
• These fluctuations are caused because the reflection will be from the
different facets of the reflector surface and by random interference of
waves which are scattered from facets.

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b) Oblique incidence:

• At an oblique incidence of the beam angle to the rough reflecting surface

reflectors, the travelling echo pattern with the envelope of a train of signals
will be detected.
• As the probe is moved each subsidiary peak echo will travels through the
pulse envelope and then the amplitude will fall.
• The overall signal may show large fluctuations in amplitude (+6 dB)
• It’s very complicated to determine the individual peak signal, since it will
fluctuates with slight probe movement.

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25.4. Multiple reflector Response:

• At any position the A scan response from the multiple reflector will shows a
cluster of signals which may or may not be well resolved in range.
• As the probe is moved the signal raises and falls randomly.
• The signal from each separate reflector element will show a separate peak
indication as shown in figure.

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26. Welding defects:

26.1. Crack :
• Cracks are rupture of metal caused by severe heat and stress.
• Crack can occur anywhere in the weld metal, heat affected zone and in
the base metal.
• Longitudinal cracks propagate along the weld length.
• Transverse cracks are perpendicular to the weld seam.
• Crater cracks form at the weld surface and radiate out in many
directions [ visible at surface ].
• Base metal cracks may be in any orientation to the weld.
• Tight crack produces small indication or no indication at all.
• Opened cracks usually produce recognizable high amplitude broad based
indication with wide signal envelope.

26.2. Lack of penetration :

• Incomplete filling and not bridging the gap of the weld root opening.
Weld metal does not extend entirely through the joint thickness.
• For welding from one side, this discontinuity is open to the surface.
Produces high amplitude sharp indication from the corner of the root
face, detectable from both the sides of the weld.
• Plotting does not cross over the root centerline. Cross over indicates
probability of root lack of fusion or root undercut.

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26.3. Lack of fusion / Cold lap :

• Failure of the weld metal to fuse along the edges of the base metal [
bevel ] or lack of bond between adjacent weld passes [ an area of the
parent metal or already solidified weld metal does not get melted to
fuse with the weld metal ].
• Usually caused by improper heat or poorly prepared weld surfaces.
• Lack of fusion may have slag associated with it.
• Produces high amplitude sharp indication, only when intercepted at 90 0,
usually detectable from one side of the weld only.
• When slag is present, the defect is detectable from both the sides of
the weld.

26.4. Slag / Inclusions :

• Entrapment of foreign material in the weld metal.

• Slag may be small irregular fragments or elongated.
• Slag lines follow the direction of welding and will be located along the
weld groove edge or between passes following a valley left by weld
passes.
• Produces forked, broad based, lower height indication, Usually detectable
from both the sides of the weld.

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26.5. Porosity :

• Porosity occurs when gasses in the molten weld metal fails to escape
before solidification of the weld material.
• Occurs anywhere within the weld. Isolated pore is a single spherical
gas pocket.
• Scattered porosity is random distribution of single pores. Cluster
porosity is a group of pores.
• Wormhole is an elongated [ tunneling ] pore. Hollow bead is an
elongated gas pocket that tunnels down the root pass in the
direction of welding.
• Single pore produces a narrow base sharp small amplitude indication.
• Single and dispersed / scattered pores are difficult to detect.
• Cluster will produce broad base indication with multiple peaks.

26.6. Root concavity :

• The root of the weld is fused but the center of the root weld pass is
below the surface of the adjacent base material.
• This defect occurs on joints that are welded from one side only, where
excessive melting of the underside occurs.
• This discontinuity is open to the surface. Detectable as low amplitude
signal from both the sides of the weld.
• Plotting short of plate thickness with no crossover.
• Difficult to detect if wide and shallow. [ a 450 probe is preferred ]

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26.7. Undercut :

• Undercut is a groove cut along the edge of the weld, caused by

excessive melting of base material and left unfilled by the weld
metal.
• Undercut forms on any or all the four edges of the weld.
• It is open to the surface.
• Produces sharp indication at half or full skip beam path, amplitude
depends on depth of cut [ 45 0 probe preferred ].
• May produce twin peaked signal due to beam spread hitting both
the defect and the root bead.
• Plots short of plate thickness with no cross over.

26.8. Excess Penetration :

• Excess metal accumulation at root, occurs mostly after root repair. Low
amplitude signal from the root bead, detectable from both the sides,
distinguishing feature is ringing of the falling edge of the signal.
• Beam path just longer than half skip beam path.
• Plots deeper than plate thickness level with cross over.

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26.9. Misalignment with / without penetration :

• Misalignment of the members is visible at the surface.

• Produces high amplitude root signal detectable from the member which
is lower in height.
• Scanning from the other member does not produce any indication.

26.10. Backing Bar :

• The inherent gap between the backing bar and the base material and
the edges of a narrow backing strip produce indications.

26.11. Other defects :

▪ Irregular weld surface,

▪ Excessive capping,
▪ Under fill,
▪ Irregular root penetration,
▪ Burn through,
▪ Tungsten inclusion [ GTAW ],

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27. Advantages and Disadvantages:

The principal advantages of ultrasonic inspection as compared to other methods

for nondestructive inspection of metal parts are:

• Superior penetrating power, which allows the detection of flaws deep in the
part
• Ultrasonic inspection is done routinely to thicknesses of a few meters on
many types of parts and to thicknesses of about 6 m (20 ft) in the axial
inspection of parts such as long steel shafts or rotor forgings
• High sensitivity, permitting the detection of extremely small flaws
• Greater accuracy than other nondestructive methods in determining the
position of internal flaws, estimating their size, and characterizing their
orientation, shape, and nature
• Only one surface needs to be accessible
• Operation is electronic, which provides almost instantaneous indications of
flaws. This makes the method suitable for immediate interpretation,
automation, rapid scanning, in-line production monitoring, and process
control.
• With most systems, a permanent record of inspection results can be made for
future reference
• Volumetric scanning ability, enabling the inspection of a volume of metal
extending from front surface to back surface of a part
• Nonhazardous to operations or to nearby personnel and has no effect on
equipment and materials in the vicinity
• Portability
• Provides an output that can be processed digitally by a computer to
characterize defects and to determine material properties

The disadvantages of ultrasonic inspection include the following:

• Manual operation requires careful attention by experienced technicians

• Extensive technical knowledge is required for the development of inspection
procedures
• Parts that are rough, irregular in shape, very small or thin, or not
homogeneous are difficult to inspect

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• Discontinuities that are present in a shallow layer immediately beneath the

surface may not be detectable
• Couplants are needed to provide effective transfer of ultrasonic wave energy
between transducers and parts being inspected
• Reference standards are needed, both for calibrating the equipment and for
characterizing flaws.
28. Appendix
Table 1- Sound Velocity, Density, Acoustic impedance
Velocity m/s Density Acoustic
Material
Longitudinal Transverse g/cm3 Impedance

Aluminum 6320 3130 2.7 17.0

Bismuth 2180 1100 9.8 21.4

Brass 4430 2120 8.1 35.9

Cadmium 2780 1500 8.6 23.9

Cast iron 2200 3500 6.9 24.2

Concrete 4600 - 2.0 9.2

Copper 4700 2260 8.9 41.8

Glass 4260 2560 3.6 15.3

Glycerin 1920 - 1.3 2.5

Gold 3240 1200 19.3 62.5

Lead 2160 700 11.4 24.6

Magnesium 5770 3050 1.7 9.8

Nickel 5630 2960 8.8 49.5

Perspex 2740 1520 1.18 3.2

Platinum 3960 1670 21.4 84.7

Polyethylene 2340 925 0.94 2.2

Polyvinyl chloride
2395 1060 1.4 3.353
(PVC)
Porcelain 5600 3500 2.4 13.4

Quartz Glass 5570 3515 2.6 14.5

Silver 3600 1590 10.5 37.8

Steel (low alloy) 5920 3250 7.85 46.6

Tin 3320 1670 7.3 24.2

Titanium 6230 3180 4.54 28.3

Tungsten 5460 2680 19.1 104.3

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Uranium 3200 - 18.7 59.8

Water 20⁰C 1480 - 1.0 1.48

Zinc 4170 2410 7.1 29.6

ULTRASONIC TESTING FORMULAE

1. Acoustic impedance calculation:

Where:

Z = Acoustic Impedance in kg/m2s

 = Density
V = Velocity

2. Wavelength calculation:

Where:

 = Wavelength
V = Velocity
F = Frequency

3. Snell’s Law (Refraction):

Where:

Angle of the Incident

 =
Wave
Angle of the Reflected
R =
Wave
Velocity of Incident
V1 =
Wave
Velocity of Reflected
V2 =
Wave

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4. Reflection Co-efficient (R) :

Where:

R = Reflection Coefficient
Z1 = Acoustic Impedance of Medium 1

Z2 = Acoustic Impedance of Medium

5. Transmission co-efficient (T) :

T = (1-R) = 4Z1Z2 / (Z1+Z2)2

Where:

R = Reflection Coefficient
Z1 = Acoustic Impedance of Medium 1

Z2 = Acoustic Impedance of Medium

6. Near Field (N) or Fresnel Zone calculation:

Where:

N = Near Field
D = Transducer Diameter
 = Wavelength
V = Velocity

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7. Beam Spread Half Angle:

Where:

 = Wavelength
D = Transducer Diameter
V = Velocity
F = Frequency

8. Deci Bell (dB) Gain or Loss calculation:

Where:

dB = Decibel

P1 = Pressure Amplitude 1

P2 = Pressure Amplitude 2

9. Weld scanning formulae:

• Half Beam path = (T / Cos ϴ )

• Full Beam Path = 2 x (T / Cos ϴ )

• Half Skip Distance = (T x tan ϴ )

• Full Skip Distance = 2 x (T x tan ϴ )

• Depth of a defect ( in half Beam path) = (Beam path x Cos ϴ)

• Depth of a defect ( in Full Beam path) =[ (2x T) - (Beam path x Cos ϴ) ]

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• Surface Distance or location of the Defect = (Beam path x Sin ϴ)

Solved Examples
Example 1

Calculate the end of the near field when using a 5 MHz, 0.375 inch diameter transducer to
inspect a component made of brass. The sound velocity in brass is 0.1685x106 inch/second

Near field formula

Where: N = Near Field Length or Transition from Near Field to Far Field
D = Diameter of the Transducer
F = Frequency of the Transducer
 = Wavelength (cycles/second) Note: MHz is used in the Java applet for ease of use.
V = Velocity of Sound in the Material

Substitute the values into the formula.

Complete the square and cancel terms where possible.

The near field will extend into the material 1.04 inch from the transducer face. Within this
near field area, it is hard to predict the signal amplitude from a reflector.

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Example 2

What is the incident angle that will produce a 70 degree refracted shear wave in steel using a
Lucite wedge.

First establish the values.

• 1= the value to be determined

• 2= 70 degrees
• V1= 0.106 in/s (sound velocity of a longitudinal wave in Lucite)
• V2= 0.128 in/s (sound velocity of a shear wave in steel)

Plug the known values into the equation.

Multiply both sides of the equation by 0.940 to solve for Sin .

Finally, take the inverse sine of 0.778 to determine the angle whose sine equal 0.778.

Θ1 = 51.1 degrees

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Example 3

If the incident angle is 24 degrees when setting up an immersion inspection, what is the
refracted shear wave angle in aluminum?

First establish the values.

• 1= 24 degrees
• 2= the value to be determined
• V1= 0.148 cm/s (sound velocity of a longitudinal wave in water)
• V2= 0.313 cm/s (sound velocity of a shear wave in aluminum)

Determine the sine of 24 degrees with a calculator or lookup table.

Sin 24 = 0.407
Plug in this value and cross multiply and divide.

Finally, take the inverse sine of 0.861 to determine the angle whose sine equal 0.861.

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Example 4

Two sound pressure measurements are made using an ultrasonic transducer. The output
voltage from the transducer is 600 mv for the first measurement and 100 mv for the second
measurement. Calculate the difference in the sound intensity, in dB, between the two
measurements?

Substitute in the voltage values:

Divide to get a decimal value for the ratio:

Take the log of 0.1667:

Multiply:

The sound intensity changed by -15.56dB. In other words, the sound intensity decreased by
15.56 dB

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Example 5

If the intensity between two ultrasonic measurements increases by 6 dB, and the first
measurement produces a transducer output voltage of 30 mv, what was the transducer output
voltage for the second measurement?

Substitute the know information in to the equation:

Divide both sides of the equation by 20

Simplify:

Clear the log:

Simplify:

The voltage output for P2 is 60mv. Notice that a 6dB increase in sound intensity doubled the
voltage output.

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Example 6

Consider the sound pressure difference between the threshold of human hearing, 0 dB, and
the level of sound often produce at a rock concert, 120 dB. (Note: prolonged sound levels
above 85 dB are considered harmful, while levels above 120 dB are unsafe.)

where: P1is the sound pressure of the reference level, and P2 is the sound pressure
experienced at the rock concert.

Divide both sides by 20:

Clear the log:

So the sound pressure at a rock concert is 106 or one million times greater than that of the
threshold of human hearing.

Example 7

Calculate the thickness of the normal beam transducer crystal size for the transducer firing on steel
at 4 MHz. (Beam Longitudinal velocity =5920 m/s)

W.K.T,

f = V / (2t)
Where,

F= frequency of the probe

V= velocity of the beam in a material

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t = Thickness of the crystal.

On Substituting,

4 X 10 6 = (5920 X 10 3 ) / (2t)
2t = 5920000/4000000
2t = 1.48
T = 0.74 mm
Crystal thickness = 0.74mm
Example 8

Calculate the attenuation co-efficient ( α ) for the following, the thickness of the specimen was
25mm.

W.K.T

α = (Δ dB) / (2 x T)

on substitution,

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α = 2 / (2 X 25)
α = 0.04 dB/mm

Example 9

Calculate the wave length of ultrasonic energy in lead (VL = 2.1X105 cm/s) at 1 MHz.

W.K.T

Wavelength λ=V/F
On substitution,
λ = 210000 / 1000000
=0.21 cm
The wavelength = 0.21cm or 2.1mm

Example 10
Calculate the dB required to raise the second back wall echo amplitude to that of first.

W.K.T ΔdB = 20 log 10 (X2 /X1)

= 20 log 10 (80/60)
= 2.498 dB.

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Example 11

Calculate the true angle for 4MHz 60 ⁰ shear wave probe in aluminium.

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