Ultrasonic Testing

Ultrasonic Testing

TWI – Training and Examination Services

Granta Park, Great Abington

Cambridge CB21 6AL

United Kingdom

Copyright © TWI Ltd

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 Contents

Contents

Physical Principles 5

Modes of Sound Energy 10

Generating Ultrasound 14

Pulse Length and Damping 19

The Sound Beam 20

Total Attenuation Loss 25

Acoustic Impedance 28

Snell’s Law 30

Probe Design 33

Test techniques 36

Ultrasonic Flaw Detector 41

Calibration and Sensitivity 45

Flaw Location 49

Flaw Sizing 51

Sensitivity Setting 58

Ultrasonic Equipment Checks 65

Practical Welding Inspection 73

Root Flaws 76

Face and Body Flaws 78

Plate Inspection 81

Inspection Procedure 82

Ultrasonic Thickness Measurement 91

References 94

Other relevant documents 95

Glossary 96

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 Physical Principles

Physical Principles

Sound is generated when something vibrates. You can twang a ruler on a table or flick a stretched
elastic band to verify this. The stretched surface of the rubber band or the ruler vibrates and sets
up a series of vibrations, sound waves, in the air. As the surface of the band or ruler pushes into the
air, the air molecules are forced together and a region of high-pressure forms; this process is called
compression. As the surface moves back, the air molecules move apart, forming a low-pressure area
(rarefaction). As the surface vibrates, alternate compressions and rarefactions are set up in the air
and travel out from the surface to form a sound wave. The air molecules don’t move with the wave
– they vibrate to and from in time with the vibrating surface.

If we plot the displacement of the particle against time, it will produce a sine wave as shown below.

Figure 1: Ultrasonic vibration.

The sound wave thus produced travels through the air at a speed of about 332 m/sec, at 0°C, at
sea level. We hear the sound when it hits a membrane in our ear and causes it to vibrate.

Sound will travel through any medium that has molecules to move but it travels faster in more elastic
materials because the vibrations are passed on more quickly. Sound travels faster in water or metal
than it does in air because liquids and solids are more elastic than air. The speed of sound in a
material increases with its stiffness (elasticity) and decreases with its density; more precisely, the
square root of the stiffness divided by the density gives the speed of sound.

A sound wave is generally described in terms of its frequency, velocity and wavelength.

Frequency

As sound is a series of vibrations, one way of measuring it is to count the number of vibrations per
second – the frequency. Frequency is measured in hertz (Hz). One vibration in one second is one
hertz. Two vibrations in one second is two hertz. Ten vibrations in one second is 10 hertz and 1000
vibrations in one second is 1000 hertz or one kilohertz (kHz). One million vibrations in a second is
one megahertz (MHz).

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 Physical Principles

The higher the frequency – the higher the note sounds – the higher the pitch. If you twang the ruler
or the rubber band hard, the noise is louder, it has greater amplitude, but the note remains the
same. If, however, you shorten the ruler or tighten the rubber band, they vibrate more quickly and
the note given out is higher, the frequency is greater. To raise the pitch of their instrument, guitar
players move their fingers down the frets, thus shortening the string and making it vibrate more
quickly.

We can only hear sounds between certain frequencies – more than 20 and less than 20,000 Hz. If
you were able to move your arm up and down 20 times a second, it would sound like a very low
hum. You cannot move your arm this fast, so you cannot hear the vibrations in the air caused by
your moving arm. A dog whistle vibrating at 25,000 Hz cannot be heard by humans, but it can be
heard by the sensitive ears of a dog.

Figure 2: The sound spectrum.

It rarely occurs to us that there is a whole world of sound that we cannot hear. Some other animals
can hear sounds at higher frequencies – bats can hear sound at 100,000 Hz – and some animals,
like snakes, have worse hearing than we have.

A sound with frequencies above the upper range of human hearing is called ultrasound. Sound below
about 16 Hz is called infrasound. Therefore, the definition of ultrasound is sound with a frequency
greater than 20 kHz.

There is an advantage to using lower frequencies: The lower the frequency, the more penetrating a
sound wave is – that is why foghorns give out very low notes and why the low throbbing notes from
your neighbour’s stereo set come through the wall rather than the high notes. Elephants and hippos
can communicate over distances of up to 30 kilometres using ultrasound, while whales can
communicate through water across an ocean!

Wavelength

A wave in the sea is a vibration of energy. As the wave passes a fixed point it produces a constant
rise and fall of energy. A complete vibration is a change in energy from maximum to minimum and
back to maximum. The distance over which one complete vibration of energy occurs is called a
wavelength.

A wavelength is the distance between the highest points of energy. It varies with the speed of sound
and with the frequency. Wavelength is represented by the Greek letter lambda (λ). We can work out

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 Physical Principles

the wavelength if we know the speed and frequency of a sound wave. Wavelength is the velocity in
metres per second divided by the frequency.

Figure 3: Wavelength, Velocity and Frequency relationships.

If we want to know the wavelength of a 200 Hz frequency sound wave travelling through air, we can
apply this formula, as we know that the speed of sound in air is 332 m/sec.

332
𝜆= = 1.66 𝑚𝑚
200

If we want to know the wavelength of a 2 MHz compression wave travelling through steel, we can
again use the formula, as we know the compressional speed of sound in steel, 5,920 m/s.

5,920
𝜆= = 2.96 𝑚𝑚
2,000,000

If we wanted to know the wavelength of a shear wave of 2 MHz in steel, we could use the formula
again but this time using the shear speed of sound in steel which is 3,250 m/s.

An easy way to remember how this formula works is to split it down within a triangle – with the
velocity, wavelength and frequency at the corners. The velocity must be placed at the top (note how
it forms a diamond shape) and the wavelength and frequency at either of the bottom two corners.

If we want to work out wavelength, we cover the wavelength symbol – this leaves 𝑣 over 𝑓 . If we
need to find the velocity, cover the V which gives us 𝜆 ∙ 𝑓. Covering the frequency (𝑓) will leave 𝑣
over λ.

𝑣 𝑣
𝜆= 𝑣 =𝜆∙𝑓 𝑓=
𝑓 𝜆

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 Physical Principles

So, the wavelength of ultrasonic waves is important because the shorter the wavelength, the smaller
the flaws that can be detected. Defects of a diameter of less than half a wavelength may not show
on the cathode ray tube (CRT). On the other hand, the shorter the wavelength the less the ultrasound
will penetrate the test material. Beam shape is also affected by wavelength. These factors will be
discussed later.

Resolution

Resolution is the ability of an equipment/probe combination to distinguish between two echoes from
reflectors that are close together. To have good resolution, a probe must present two signals on a
CRT screen from two separate reflectors: if it has poor resolution, the echoes from the two reflectors
appear as one signal on the screen.

In the early days of ultrasonic testing, we used 100 mm, 91 mm and 85 mm steps at the radius end
of the V1 block to test resolving power. However, today this is regarded as too crude a test and BS
4331 part 3 (now obsolete and superseded by BS EN 12668-3: Methods of assessing the
performance characteristics of ultrasonic flaw detection equipment Part 1: Overall performance on-
site methods) recommends that we should be able to recognise two discrete echoes less than two
wavelengths apart. Discrete echoes mean split by more than 6 dB (see Figure 4) or to more than
half the total height of the signals.

Figure 4: Echo resolution.

Signal amplitude

The amplitude of an ultrasonic signal is defined as the maximum displacement of the molecules from
their equilibrium position. The energy of an ultrasonic wave is in turn expressed as the square of the
amplitude.

The relative amplitude of ultrasonic signals is expressed using the decibel (dB), a logarithmic unit of
comparison. When we compare the height of two signals on the CRT screen, we are in fact comparing
the electric voltage that is being sent to the Y plates; electric voltage is proportional to the square
of the current. To compare two signals, we must use a formula that takes account of this fact:

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 Physical Principles

𝐻2 (1)
𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑖𝑛 𝑑𝐵 = 20 ∙ 𝑙𝑜𝑔10
𝐻1

For example, if we want to compare a signal of 50% FSH with one of 100% FSH on the display:

100
𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑖𝑛 𝑑𝐵 = 20 ∙ 𝑙𝑜𝑔10
50

Find the log10 of 2 in tables or a calculator

𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑖𝑛 𝑑𝐵 = 20 ∙ log10 2 = 20 ∗ 0.301 = 6.02 𝑑𝐵

So, the answer is 6 dB and this can be tested on a CRT screen by obtaining a signal from a backwall
echo on a test block and increasing or decreasing the gain until the signal touches the top of the
screen. Take out 6 dB with the gain control and the signal should drop to 50% full screen height
(FSH). If it does not, the vertical linearity of the UT set is out or inaccurate; the signal height is not
changing in accordance with energy from the probe.

Using the formula, we discover that:

12 dB difference means that one signal is 4 times bigger than another

10 dB difference means that one signal is 3 times bigger than another

20 dB difference means that one signal is 10 times bigger than another

Remember that decibels are only a means of comparing signals. All UT sets are different, so a defect
may be at FSH with a gain control reading of, say, 36 dB on one set and be at FSH on another set
with a gain control reading of only 28 dB on another set. The gain control allows us to set sensitivities
and forms the basis of ultrasonic sizing techniques.

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 Modes of Sound Energy

Modes of Sound Energy

Sound waves propagate due to the vibrations or oscillatory motions of particles within a material.
Within a freely vibrating medium each particle is subject to both inertial and elastic forces. These
forces cause particles to exhibit oscillatory motions comparable to the free vibration of a system of
masses and springs. The elastic restoring forces in a material can be described as microscopic spring
forces as shown below.

Figure 5: Representation of sound wave propagation using partial mass and microscopic restoring spring
forces.

This theory agrees with both Hooke’s Law and Newton’s second law. Hooke’s Law states that, within
the elastic limit of any body, the ratio of the stress to the strain produced is constant; therefore, the
more stress or force is placed on an object, the more it will strain or deform. Newton’s second law
of motion states that the force (F) equals the mass (m) times the acceleration (a).

𝐹 =𝑚∙𝑎 (2)

Where: 𝐹 = Force [N]

𝑚 = Mass [g]
𝑎 = Acceleration [ms-2]

The spring theory makes accurate predictions for the propagation of sound. The propagation of a
sound wave velocity is determined by the elastic properties and density of the material. The velocity
of a longitudinal wave is described by the following equation:

𝑣=𝑓∙𝜆 (3)

Where: 𝑣 = velocty [ms-1]

𝑓 = frequency [Hz]
𝜆 = wavelength [m]

Compressional waves

We cannot hear all sound; what we do hear is sound in a compressional mode, where molecules
vibrate backwards and forwards in the same direction as the energy of propagation – rather like
billiard balls in a line. A compressional wave of sound is also called a longitudinal wave: waves of
this type consist of alternate compression and dilation in the direction of propagation. As each
particle moves, it pushes or pulls the adjacent particle through elastic interconnection (see Figure
6). Gases, liquids and solids have elasticity, so compressional waves can travel in all of them.

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 Modes of Sound Energy

Figure 6: Compression wave propagation.

The velocity of a longitudinal wave is described by the following equation:

𝐸 ∙ (1 − 𝜇) (4)
𝑉𝐿 = √
𝜌 ∙ (1 + 𝜇) ∙ (1 − 2𝜇)

Where: 𝑉𝐿 = Velocity of longitudinal wave [ms-1]

𝐸 = Young's module of elasticity [Nm2]
𝜇 = Poisson ratio [-]
𝜌 = Material density [kgm3]

Sound travels through air in the compressional mode at 332 m/sec, through water at 1480 m/sec,
Perspex at 2730 m/sec, steel at 5920 m/sec and aluminium at 6320 m/sec.1

Shear waves

Sound can travel in solids in a shear mode as well as a compressional mode. In the shear mode,
molecules vibrate up and down, across the direction of propagation rather than to and from; for this
reason, the shear mode is also called the transverse mode, as particle vibration is transverse to the
direction of sound energy (see Figure 7).

In the shear or transverse mode, molecules of a solid move rather like beach balls floating on the
surface of the sea – they move up and down as a wave passes.

Figure 7: Shear wave propagation.

1
Sound can only travel through air and water in the compressional mode. Sound can travel through Perspex, steel and
aluminium in modes other than the compressional mode.

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 Modes of Sound Energy

This type of sound travel can only happen when the molecules through which it propagates are
joined together – as in a solid. A solid has rigidity as well as elasticity. Air and water, like other gases
and liquids, do not have rigidity. Shear or transverse waves cannot travel in gases of liquids for this
reason.

Shear (transverse) wave velocity can be written as:

𝐸 ∙ (1 − 𝜇) 𝐺 (5)
𝑉𝑆 = √ =√
2 ∙ 𝜌 ∙ (1 + 𝜇) 𝑃

Where: 𝑉𝑆 = velocity of longitudinal wave [ms-1]

𝐸 = Young's module of elasticity [Nm2]
𝜇 = Poisson ratio [-]
𝜌 = Material density [kgm3]

The speed of sound in the shear or transverse mode is less than it is in the compressional or
longitudinal mode. The shear speed of sound in steel is 3250 m/sec and in aluminium 3130 m/sec.
There is no shear or transverse speed for air or water, as shear waves cannot be supported in these
media.

Table 1: Comparison of compression wave and shear wave velocities in different materials

Material Compression velocity [m/sec] Shear velocity [m/sec]

Air 332 NA
Water 1480 NA
Steel 5920 3250
Aluminium 6320 3130
Perspex 2730 1430
Copper 4700 2260
Brass 4430 2120

Applying these values for one velocity of steel to the formula used previously for determining the
wavelength, it can be seen that for a given frequency that the wavelength of the shear wave is less
than that of the compression wave.

Table 2: Comparison of wavelength between compression and shear waves at different frequencies in steel.

Frequency [MHz] Compression wavelength [km] Shear wavelength [m]

0.5 11.8 6.5

1 5.9 3.2
2 2.95 1.6
4 1.48 0.8
6 0.98 0.54

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 Modes of Sound Energy

Rayleigh or surface waves

A third type of sound wave can travel along the surface of a solid: these are called Rayleigh or
surface waves.

Figure 8: Surface wave propagation.

The surface molecules vibrate in an elliptical motion, though only to a depth of one wavelength in
the carrier material. Surface waves are about 8% slower than shear waves and in steel they travel
at about 3000 m/sec.

Lamb waves

Another mode of sound travel is Lamb or plate waves which propagate in thin plate materials when
the plate thickness is about the same as the wavelength. Lamb or plate waves travel at velocities
which vary with the plate thickness and the wavelength. Particle motion is elliptical, as with surface
waves.

Figure 9: Lamb waves.

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 Generating Ultrasound

Generating Ultrasound

Sound is created when something vibrates. It is a stress wave of mechanical energy. The piezo-
electric effect changes mechanical energy into electrical energy. It is reversible, so electrical energy
– a voltage – can be changed into mechanical energy or sound, which is the reverse piezo-electric
effect. The first people to observe the piezo-electric effect were the Curie brothers who observed it
in quartz crystals.

Piezo-electric crystals

Jacques and Pierre Curie used quartz for their first experiments. Nowadays polarised ceramics are
used instead of quartz crystals.

Figure 10: Illustration of the piezo-electric effect showing the effect of an applied voltage on a crystal.

It was later discovered that by varying the thickness of crystals and subjecting them to a voltage,
they could be made to vibrate at different frequencies. (Figure 8: Surface wave propagation) The
frequency depends on the thickness of the piezo-electric crystal, according to the following formula:

𝑣
𝑡= (6)
2∙𝑓
Where: 𝑡 = Crystal thickness [m]
𝑣 = Velocity of sound in crystal [ms-1]
𝑓 = Frequency [Hz]

Quartz or silicon oxide (SiO2)

Found in granite as a natural crystal, quartz can produce compressional or shear waves according
to the way the crystal is cut. An X-cut crystal is cut in a direction that directly crosses the axis joining
two angles of the crystal. A Y-cut crystal is cut in a direction parallel to the axis joining two angles
of the crystal (see Figure 11: X-cut quartz crystal)

X-cut crystals produce a compressional wave.

Y-cut crystals produce a shear wave.

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 Generating Ultrasound

Figure 11: X-cut quartz crystal.

Quartz is not much used now. Several types of quartz crystal can be produced, each with its
advantages and disadvantages, some of which are listed below.

Advantages Disadvantages
Needs a lot of electrical energy to
Resistant to wear. produce a small amount of ultrasound,
which means it is inefficient.

Quartz crystals are susceptible to mode

Insoluble in water.
change.

High voltage is needed to give low

Resistant to ageing.
frequency sound.

Easy to cut to give the required

frequencie.

For these reasons quartz has been largely superseded by other piezo-electric materials.

Lithium sulphate (Li2SO4)

Crystals grow as a solution of lithium sulphate is evaporated.

Advantages Disadvantages
Most efficient receiver of ultrasound. Soluble in water.

Very low electrical impedance. Break easily.

Decompose at temperatures above

Operate well at low voltages.
130°C.

Do not age.

Very good resolution.

Crystals are easily damped to give short

pulse lengths (to give good resolution)

These disadvantages make lithium sulphate crystals unsuitable for industrial use, though they are
used for medical ultrasonics in the examination of pregnant women and patients suffering from
tumours. Polarised crystals were found to be most suitable for industrial use. Polarised crystals are

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 Generating Ultrasound

made by heating powders to high temperatures, pressing them into shape and allowing them to cool
in very strong electrical fields, which affect the atomic structure of the crystal lattice.

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 Generating Ultrasound

Barium titanate (BaTiO3)

Crystals are made by baking barium titanate at 1,250°C and then cooling it in a 2 kV/mm electrical
field.

Advantages Disadvantages
Its Curie temperature, at which the
Efficient generator of sound. crystal depolarises, is only about 120°C,
which makes it susceptible to heating.

Only needs a low voltage. Deteriorates over time.

Good sensitivity.

Lead metaniobate (PbNb2O6)

Crystals are made in a similar way to barium titanate.

Advantages Disadvantages
Much less sensitive than lead zirconate
Heavy internal damping.
titanate (PZT).

Gives out very narrow pulses of

ultrasound, which gives good
resolution.

Lead zirconate titanate (PbZrO3, PbTiO3)

Lead zirconate titanate (PZT) crystals have the best all-round performance for industrial testing.

Advantages Disadvantages
A high Curie point, up to 350°C.

Good resolution.

Does not dissolve in water.

Tough and resistant to ageing.

Easily damped.

Because it has no major disadvantages, PZT is used in most probes.

Electromagnetic acoustic transducers

A feature of probes using piezo-electric crystals is that they require mechanical coupling to the solid
under inspection. This is achieved either by immersing them in a tank filled with a fluid (usually
water) or directly by the use of a thin (less than one quarter of the wavelength) fluid layer between
the two. When shear waves are to be transmitted, the fluid is also generally selected to have a
significant viscosity. The acoustic impedance of the couplant layer should also have a value
somewhere between that of the probe and that of the material being tested.

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 Generating Ultrasound

Electromagnetic acoustic transducers (EMATs) rely upon a totally different physical principle. When
a wire is placed near the surface of an electrically conducting object and a current of the required
ultrasonic frequency is applied, eddy currents will be induced in a near-surface region of the object.

EMAT probes are used for the detection of flaws and the determination of material properties such
as the precise velocity as well as attenuation measurements. They do not require the use of couplant
and as such can operate without contact at elevated temperatures and in remote locations.

EMAT probes are, however, inefficient and require strong magnetic fields and large currents to
produce ultrasound that is often weaker than that produced by piezo-electric transducers. Rare earth
materials such as samarium-cobalt and neodymium-iron-boron are often used to produce sufficiently
strong magnetic fields, which may also be generated by pulsed electromagnets.

EMAT probes generate ultrasonic waves due to the interaction between a static magnetic field of a
magnet and the high frequency magnetic field generated by a coil. The eddy currents produced in
the material due to the coil create a Lorentz force, causing the atomic lattice of the material to
oscillate and produce an ultrasonic wave. A magnetic structure component is also generated by the
EMAT and although not very efficient in terms of energy, the ultrasonic proportion can have useful
in-service applications (see Figure 12).

Figure 12: Method of ultrasonic wave generation from an EMAT probe.

Including a magnetic component in the structure of an EMAT probe can allow thickness measurement
of sealed tubes (e.g. ferromagnetic boiler tubes) at elevated temperatures without the necessity to
remove the oxide scale.

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 Pulse Length and Damping

Pulse Length and Damping

A pulse of ultrasound from a piezo-electric crystal has a length or width of several vibrations or
wavelengths. When you strike a bell, it continues to ring for several seconds as the metal continues
to vibrate. The vibrations get steadily weaker and the sound dies away. If you put your hand on the
bell you stop the vibrations and the sound dies away more quickly – you dampen the sound.

A piezo-electric crystal continues to vibrate after it is hit by an electrical charge. This affects the
sensitivity: the longer the pulse length, the worse the resolution. In most probes a slug of tungsten-
loaded Araldite is placed behind the crystal to cut down the ringing time and to shorten the pulse
length. Pulse length, duration and width are the same thing but we must not confuse them with
wavelength.

Pulse length (or width) is also sometimes called wave train length. It is defined in a number of ways
but not even the standards always agree. We choose the one in EN 1330 Part 4 NDT terminology –
Part 4: Terms used in ultrasonic testing, which defines it as the leading and trailing edges of a pulse
measured at a defined level below the peak amplitude.

Figure 13: Ultrasonic pulse.

A long pulse may be 15 wavelengths (cycles, vibrations) while a short pulse may have as little as
two cycles. The average pulse length is about five wavelengths. The longer the pulse length, the
more penetrating the ultrasound, as it contains more energy but the worse the sensitivity and
resolution; hence the need to compromise.

1 to 2 cycles

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 The Sound Beam

5 cycles

10 to 12 cycles

Figure 14: Pulse lengths.

The Sound Beam

The spread of sound waves from a piezo-electric crystal has been likened to the beam of a torch,
i.e. an elongated cone. Just as the intensity of light from a torch diminishes with distance, so sound
pulses get weaker the further they travel from the crystal. An acoustic sound wave has also
previously been described as being a single sinusoidal wave propagating through a material. These
analogies do not however present a totally true picture. The sound produced from an ultrasonic
crystal does not originate from a single point but rather it is derived from many points along the
surface of the piezo-electric crystal. This results in a sound field with many waves interacting or
interfering with each other (see Figure 15).

Figure 15: Interaction of the ultrasonic beam.

When waves interact. they overlay each other and the amplitude of the sound pressure or particle
displacement at any point of interaction is the sum of the amplitudes of the two individual waves.
When the waves are fully in phase, the result is additive or constructive and the intensity is doubled.
When completely out of phase, the result would be the amplitudes cancelling each other out. The
interaction can vary between these two extremes and the wave produced will equal the sum of the
amplitudes at all points with peaks of intensity referred to as nodes. In an ultrasonic probe the

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 The Sound Beam

situation is further complicated as sound originates from not just two but many points on the crystal
surface.

Figure 16: Additive nature of two sound waves interacting.

In an ultrasonic probe, one would expect the sound intensity to be highest at the probe face and to
fall away gradually as the distance from the probe increases. Due to interactions near the face of
the probe, however, the sound field is very uneven in this region with peaks and troughs in sound
intensity. This area of intensity variation is known as the near field or Fresnel zone. As one moves
farther away from the probe these variations are eliminated and the sound field behaviour becomes
more uniform. This region of the sound beam is referred to as the far field, or Fraunhofer zone. In
the far field, the intensity behaves as expected and is reduced exponentially with distance. The beam
spreads out as a circular wave front.

Near zone (or near field)

A piezo-electric crystal is made up of millions of molecules. Each of these vibrates when the crystal
is hit by an electric charge and they send out shock waves. The shock waves jostle each other.

Figure 17: Variations in sound intensity.

After a time, the shock waves or pulses even out to form a continuous front. The area between the
crystal and the point where the wave front evens out is what we call the near or Fresnel zone. Inside
the near zone, signals from a reflector bear no accurate relation to the size of the reflector, as the
sound vibrations are going in all directions. This affects the accuracy of flaw sizing of small reflectors
inside the near zone.

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 The Sound Beam

Figure 18: Regions of a sound beam.

The near zone of a crystal varies with the material being tested, but it can be worked out by a
formula:

𝐷2 (7)
𝑁𝑒𝑎𝑟 𝑍𝑜𝑛𝑒 =
4∙
𝐷2 ∙ 𝑓 (8)
𝑁𝑒𝑎𝑟 𝑍𝑜𝑛𝑒 =
4∙v
Where: 𝐷 = Crystal thickness [m]
𝑣 = Velocity of sound in crystal [ms-1]
𝑓 = Frequency [Hz]
𝜆 = Vavelength [m]

For example, the near zone of a 5 MHz compression probe with a 10 mm diameter crystal will be, in
steel

102 ∙ 5,000,000
= 21.1 𝑚𝑚
4 ∙ 5,920,000

The near zone of a 2.5 MHz probe with a 20 mm diameter crystal will be:

202 ∙ 2,500,000
= 42.2 𝑚𝑚
4 ∙ 5,920,000

We can deduce from the formula that:

The greater the diameter, the greater the near zone

The higher the frequency, the greater the near zone

In twin crystal or angle beam probes with a Perspex stand-off component in the probe body, some
or even all of the near zone is contained in the Perspex shoe. This must be taken into account when
the calculation for near zone is applied.

We will need to know the length of the beam path within the Perspex, the velocity of the Perspex
and the velocity of the test material, as well as the near zone within the test material.

The first step is to calculate the near zone in the test material; then the path length in the Perspex
is multiplied by the velocity in the Perspex before dividing by the velocity in the test material; this
is finally subtracted from the near zone length.

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 The Sound Beam

If a 2.5 MHz, 20 mm compression probe has a near zone of 42.2 mm and a Perspex shoe of 15 mm
in front of the transducer, then the length of the near zone within the steel is:

15 ∙ 2730 ÷ 5920 = 6.91 𝑚𝑚

42.2 − 6.91 = 35.31 𝑚𝑚 of the near zone is in the test material.

𝑇𝑤 (9)
𝑁𝑟𝑠 = 𝑁𝑠 ∙ (1 − ( ))
𝑁𝑤

Where: 𝑁𝑟𝑠 = Near zone path length remaining in the steel [mm]
𝑁𝑠 = Near zone in steel [mm]
𝑇𝑤 = Thickness travelled through wedge [mm]
𝑁𝑤 = Near zone in wedge [mm]
Figure 19: Near zone path distance in the steel test object and probe wedge.

Far zone

In the far zone the sound pulses follow the inverse square law, spreading out as they move away
from the crystal; the sound intensity decays exponentially.

If we double the distance, we quarter the intensity

If we halve the distance, we increase the intensity four-fold

1 (10)
𝐼=
𝑟2
Where: 𝐼 = Sound intensity [m-2]
𝑟 = Dystance from the crystal [m]

The higher the frequency of the crystal, the less the beams spread out. The angle of beam spread
can be found using the formula below:

Figure 20: Beam spread.

𝜃 𝐾∙ (11)
sin =
2 𝐷

𝜃 𝐾∙𝑉 (12)
sin =
2 𝐷∙𝑓

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 The Sound Beam

𝜃
Where: = Beam divergance angle [°]
2
𝐾 = Constant for the edge of the beam spread [-]
𝐷 = Diameter of crystal [m]
𝑓 = Crystal frequency [Hz]

Figure 21: K values for the beam edge.

Where the sound intensity drops by 6 dB (half the intensity), K is 0.56.

If we take the edge of the beam to be where the sound energy is 10% (20 dB) of the energy at the
beam centre, K is 1.08.

If we take the extreme edge of the sound beam to work out beam spread angles, then K is 1.22.

Example

The beam spread of a 10 mm and 5 MHz probe in steel is calculated as follows:

 5920
sin = 1.08 or 1.22 ∙
2 5000 ∙ 10

sin = 7.35° at the 20 dB point or 8.3° at the edge
2

So, the angle of beam spread is 14.7° if you take the edge of the beam to be where the energy is
10% of the main energy or 16.6° taking the extreme edge of the beam.

From the formula we can deduce that:

The higher the frequency, the smaller the beam spread.

The larger the crystal, the smaller the beam spread.

This is one of the reasons why low frequency probes have large diameter crystals.

NDT4-060420 24 Copyright © TWI Ltd

 Total Attenuation Loss

Total Attenuation Loss

EN 1330 defines attenuation as the decrease in sound pressure that occurs when a wave travels
through a material arising from absorption and scattering. The two components, absorption and
scatter, are defined as:

Absorption

Component of the attenuation resulting from transformation of ultrasonic energy into other types of
energy (e.g. thermal).

Absorption occurs as the sound pulse hits the molecules of the test material and makes them vibrate.
The energy lost in vibrating the molecules turns to heat. The rate of absorption varies from one
material to another and even from one type of steel to another. It is very high in Perspex, nylon and
lead and is low in aluminium.

Scatter

Randomly reflected energy caused by grain structure and/or by small discontinuities in the beam
path. Scatter occurs as sound energy is reflected from grains in the test material. The larger the
grains, the more scatter occurs. The grass at the bottom of the CRT screen is caused by reflections
from grain boundaries in the test material. More grass arises from cast iron or brass than from small
grained materials like refined steel or annealed aluminium.

The longer the wavelength of a sound pulse, the less energy is scattered. Where the wavelength is
smaller than the grain size, a sound pulse is scattered very quickly. It is for this reason that a low
frequency probe, with its longer wavelength, has greater penetration in a given material than a high
frequency probe.

Figure 22: Scatter of the ultrasonic beam at grain boundaries.

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 Total Attenuation Loss

Attenuation due to beam spread

The amount of energy reflected back depends on whether the reflector is bigger in area than the
sound beam at that distance. If the reflector is bigger in area than the sound beam, the signal on
the screen varies according to the law of large reflectors. If the reflector is smaller in area than the
sound beam at that distance, it obeys the law of small reflectors.

Both of these laws only apply beyond a distance of three times the length of the near zone.

6.1.1 Law of the large reflector

Large reflectors outside three near zones obey the inverse law.

A large reflector at 20 mm, if it is beyond three near zones, gives a signal at 80% FSH. If the dB
setting is not altered, a large reflector in the same material at 40 mm will give a signal at 40% FSH
(inverse law).

Figure 23: Law of the large reflector.

6.1.2 Law of the small reflector

Small reflectors outside three near zones obey the inverse square law.

A small reflector at 20 mm, if it is beyond three near zones, gives a signal at 80% FSH. If the dB
setting is not changed, a similar reflector in the same material at 40 mm will give a signal at 20%
FSH (inverse square law).

Figure 24: Law of the small reflector (i.e. smaller than the beam width).

Sound energy is lost in other ways:

Reflection inside the probe.

Scattering from a rough surface.

Non-metallic inclusions or laminations in test material.

Reflection from the surface of the test piece.

Mode change.

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 Total Attenuation Loss

Measurement of material attenuation

1. Place a compression probe on a piece of the test material and turn the backwall echo to
FSH2.
2. Obtain an echo from twice the distance of the BWE used and increase the signal height until
it is at FSH3.
3. To remove the effects of beam spread we subtract 6 dB (due to beam spread loss) from the
dB difference and divide the remainder by the distance the sound has travelled between the
two echoes. This is twice the distance shown on the CRT, as the sound has to travel to the
backwall and back to trigger the probe.
4. The answer will give the number of decibels lost per millimetre by attenuation. This can give
an assessment of plate quality and heat treatment.

If the difference between the first and second backwall echoes from a 75 mm thick block of steel
was 9 dB, what is the attenuation of the material?

9 dB due to beam spread and attenuation combined

9 𝑑𝐵 − 6 𝑑𝐵 = 3𝑑𝐵

This gives 3 dB due to attenuation only as the sound travels through the block.

The 75 mm block gives a sound path for the pulse echo of 150 mm; the sound has to travel to the
backwall, then back to the probe.

3
= 0.02
150

The attenuation within the block is 0.02 dB/mm

If we need the answer in dB/m, multiply by 1000 (1000 mm in a metre)

Attenuation is 20 dB/m

Attenuation checks have to be made when dealing with distance amplitude correction (DAC) and
distance gain size (DGS) systems; these will be discussed later.

2
If the back-wall echo (BWE) is within three near zones of the probe, use the first BWE outside the distance of three
near zones.

3
The dB difference.

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 Acoustic Impedance

Acoustic Impedance

When a sound pulse arrives at an interface between different materials at right angles, some sound
is reflected back into the material from whence it came. The rest of the sound, however, is
transmitted into the second material. This is due to the difference in acoustic impedance of the two
materials and is known as acoustic impedance mismatch or sometimes as interface behaviour.

We can calculate how much sound is transmitted and how much sound is reflected back if we know
the acoustic impedance of both materials.

Acoustic impedance is represented by the letter Z and is the velocity of sound in the material
multiplied by the material density:

𝑍 =𝜌∙𝑣 (13)

Where: 𝑍 = Acoustic impendance [kgs-1]

𝜌 = Density [kgm-1] (the Greek letter rho)
𝑣 = sound velocity [ms-1] (compressional or shear – depending on
the case)

Once we know the acoustic impedances of two materials, we can use a formula to work out how
much sound will be reflected back. The formula is:

𝑍1 − 𝑍2 2 (14)
( ) ∙ 100 = % 𝑟𝑒𝑓𝑒𝑙𝑒𝑐𝑡𝑒𝑑
𝑍1 + 𝑍2
Where: 𝑍1 = Acoustic impendance of the first material [kgs-1]
𝑍2 = Acoustic impendance of the second material [kgs-1]

Example

To calculate the amount of energy reflected back at a steel-water interface, we must find out the
acoustic impedances of steel and water. They are:

𝑍1 (steel) = 46.7 ∙ 106 kg/𝑚2 s 𝑍2 (water) = 1.48 ∙ 106 kg/𝑚2 s

So, applying the formula (14):

46.7 − 1.48 2 45.22 2

Refenergy = ( ) ∙ 100 = ( ) ∙ 100 = 0.938562 ∙ 100 = 0.8809 ∙ 100 →
46.7 + 1.48 48.18

Refenergy = 88.09%

The 88 % of the sound energy is reflected back at the interface. This means that 12 % of the energy
is transmitted at the interface. Using the same formula, the figures for other media can be worked
out. At a steel/oil interface, 91 % of sound energy is reflected back; at a glycerine/steel interface,
90 % of energy is reflected back.

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 Acoustic Impedance

These substances, water, oil and glycerine, are used as couplants in ultrasonics to transmit sound
energy from the probe into the test materials. So, in fact, only about 10% of the energy generated
by the probe crystal actually gets through the couplant into the test material.

When examining a piece of steel with a compression probe, we pass at most about 10% of sound
energy from the crystal into the steel. Even if all that energy is reflected back from the backwall or
a large flaw in the steel, only 10% of the returning energy will pass back through the interface into
the probe. Consequently, at most 1% of energy generated by a probe crystal will come back into a
probe, a very small amount indeed. A rule of thumb with UT is that whatever happens to sound
going in one direction, happens also in the reverse direction.

Table 3:Characteristic impedance of various materials.

Material Characteristic impedance ∙ 106 [kg/m2s]

Aluminium 17

Brass 36

Copper 41

Lead 27

Magnesium 93

Nickel 50

Steel 46.7

Glass 18

Polystyrene 29

Oil 1.3

Water 1.4

Air 0.0041

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 Snell’s Law

Snell’s Law

When sound waves pass obliquely (not at 90°) between materials having different acoustic velocities,
the direction of sound propagation is changed on passing through the interface and the sound wave
is said to have been refracted.

Light is also refracted when passing from one medium to another with a different velocity; this
means that objects seen across an interface appear to be shifted relative to where they really are.

Figure 25: Refraction.

Snell’s law states that the ratio between sound speeds in two materials is the same as the ratio
between the sine of the incident and refracted angles (to the vertical).

Figure 26: Snell’s law.

sin 𝐼 𝑉1 (15)
=
sin 𝑅 𝑉2
Where: 𝐼 = Incident angle [°]
𝑅 = Refracted angle [°]
𝑉1 = Velocity in material 1 [ms-1]
𝑉2 = Velocity in material 2 [ms-1]

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 Snell’s Law

If we want to make a probe transmitting a shear wave at a certain angle, we have to transpose
the formula (15)

sin 𝑅 ∙ 𝑉1
sin 𝐼 =
𝑉2

For example, if we want a probe giving a 45° shear wave in steel, we must calculate the angle at
which to cut the Perspex wedge – the incident angle. The compression speed of sound in Perspex is
2730 m/s, the shear speed of sound in steel is 3250 m/s and the refracted angle we need is 45°.

sin 45° ∙ 2730 0.7071 ∙ 2730

sin 𝐼 = = → sin 𝐼 = 0.594
3250 3250

𝐼 = 36.44𝑜

However, when the incident angle in the Perspex shoe is less than 27°, both compression and shear
waves are generated in the steel. This makes interpretation very confusing. To get a shear wave on
its own, the angle of incidence must be more than 27.4°, called the first critical angle. This gives a
shear wave of 33° (the lowest standard angle probe manufactured is 35°).

Figure 27: Two sound modes.

If the incident angle is above 57.14°, the shear wave is replaced by a surface wave. This angle is
called the second critical angle.

Figure 28: First critical angle (left) and the second critical angle (Right).

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 Snell’s Law

The largest probe angle below a surface (90°) wave probe available from manufacturers without a
special order is 80°.

Shear waves on their own in steel are only possible when the incident angles are between 27.4 and
57.14°. This is worked out by the probe manufacturers and it must be borne in mind that a probe
which gives a refracted angle of 45° in steel will give a different refracted angle in other materials.

Critical angle calculation

Snell’s law can be used for working out critical angles in non-ferrous metals. For example, during
immersion scanning the incident material is water, so a whole new set of angles need to be worked
out. The first critical angle is the incident angle at which the compression wave in the test material
is generated at 90°. So, using Snell’s law:

sin 𝐼 2730 sin 90𝑜 =1 2730

𝑜
= → sin 𝐼 = → sin 𝐼 = 0.4580 → 𝐼 = 27.26𝑜
sin 90 5960 5960

The second critical angle is the incident angle at which a shear wave is generated in the material
at 90°. Use Snell’s law again (Formula (15)):

sin 𝐼 2730 sin 90𝑜 =1 2730

= → sin 𝐼 = → sin 𝐼 = 0.8246 → 𝐼 = 57.41𝑜
sin 90° 3240 3240

Mode conversion

When sound travels in a solid material, one form of wave energy can be transformed into another.
When a longitudinal wave strikes an interface at an angle, some of the energy can cause particle
movement in the transverse direction to start a shear (transverse) wave. This phenomenon is
referred to as mode conversion and will occur every time a wave encounters an interface between
materials of different acoustic impedance and the incident angle is not at 90° to the interface. Mode
conversion can, therefore, cause numerous spurious indications to arise during an inspection which
the inspector must eliminate.

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 Probe Design

Probe Design

In the US, a probe is usually called a search unit or formerly a transducer. However, we generally
now understand the transducer to be the crystal. There are a number of probe designs and
configurations. We shall deal with those most commonly used in weld, aerospace and general
ultrasonic testing.

Compression wave probes

Compression probes (see Figure 29) generate compressional or longitudinal waves in test materials
and are sometimes called normal degree probes. A typical compression probe is composed of a
crystal in a metal or plastic housing, with wires connected to it which carry the electrical pulse from
the flaw detector and cause the crystal to vibrate. The crystal is surrounded by a damping material
at the back to restrict vibration and a plastic disc in front to prevent crystal wear.

Figure 29: Compression wave probe: Schematic drawing (left) and cross-section (right).

Angle probes

An angle probe is a piezo-electric crystal mounted on a Perspex wedge at an angle calculated to

generate a shear (transverse) wave in the test material.

The wedge is made of Perspex because:

a. The compressional speed of sound in Perspex (2730 m/s) is lower than the shear velocity of
sound in steel (3250 m/s) so refracted angles are greater than incident angles.

b. Perspex is very absorptive and attenuates unwanted echoes from the compressional wave as
it hits the Perspex test material interface.

The piezo-electric crystal generates a compressional wave, which it transmits into the Perspex
wedge. When the compressional wave hits the bottom surface of the wedge, most of the energy is
reflected away from the interface and back into the Perspex. It is damped by tungsten powder in
epoxy resin on the Perspex wedge.

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 Probe Design

Figure 30: Angle probe: photograph (left) and schematic drawing (right).

If there is no couplant on the bottom surface of the Perspex wedge, all the energy is reflected back
into the probe. If there is couplant and if the probe is placed on a test material, sound energy passes
into the test material and generates a shear wave. Angle probes use compression probes mounted
on a wedge of Perspex. The wedge of such a probe is cut to a particular angle to enable the beam
to refract into the test material at a chosen angle.

Angle probes usually transmit a shear or sometimes a surface wave into test materials and are used
largely in weld testing, casting and forging inspection and in aerospace applications.

Twin crystal probes

A single crystal probe transmits and receives ultrasound with one crystal: the crystal transmits the
pulse and vibrates when the pulse returns from a backwall echo or a flow. However, when a single
crystal probe is used, a signal appears on the screen at the beginning of the time base. It is caused
by vibrations immediately adjacent to the crystal and is known by several names: initial pulse,
transmission signal, crystal strike or main bang.

For a single crystal probe, the length of the initial pulse is the dead zone and any signal from a
reflector at a shorter distance than this will be concealed in the initial pulse.

In twin crystal probes, the initial pulse is deliberately delayed beyond the left of the time base by
mounting the transducers of a twin (or double) crystal probe onto plastic wedges. This, in addition
to the focusing of the crystals, reduces the dead zone considerably and flaws can be assessed
anywhere except where the transmission and receptive beams do not overlap.

A twin or double crystal probe is designed to minimise the problem of the dead zone. A twin crystal
probe has two crystals mounted on Perspex shoes, angled slightly inward to focus at a set distance
in the test material. If the crystals were not angled, the pulse would be reflected straight back into
the transmitting crystal.

The Perspex shoes hold the crystals away from the test surface so that the initial pulse does not
appear on the CRT screen. The dead zone is greatly reduced to the region adjoining the test surface,
where the transmission and reception beams do not overlap.

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 Probe Design

Figure 31: Twin crystal probe: photograph (left) and schematic drawing (right).

Additional advantages of double

Disadvantages
crystal probes
Can be focused. Good contact is difficult with curved surfaces.

Difficult to size small defects accurately as the

Can measure thin plates. width of a double crystal probe is usually greater
than that of a single crystal probe.

Can detect near-surface flaws. The amplitude of a signal decreases the further a
reflector is situated from the focal distance – a
Good near-surface resolution. response curve can be made out.

Therefore, single and twin crystal probes are complementary.

Other probe types

Immersion probes are designed for use where the test part is immersed in water. They are
typically used inside a water tank or as part of a squirter or bubbler system in scanning applications.
Immersion transducers usually have an impedance matching layer that helps to get more sound
energy into the water and thus into the component being inspected. Immersion transducers can be
purchased in a flat, cylindrically or spherically focused lens. A focused transducer can improve
sensitivity and axial resolution by concentrating the sound energy to a smaller area.

Delay line probes, as the name implies, introduce a time delay between the generation of the
sound wave and the arrival of any reflected waves. This allows the crystal to complete its
transmission function before it begins to receive returning signals. Delay line transducers are
recommended for applications that require a contact transducer with good near-surface resolution
and are designed for use in applications such as high-precision thickness gauging of thin materials
and delamination checks in composite materials. They are also useful in high temperature
measurement applications since the delay line provides some heat insulation to the piezo-electric
element.

High frequency broadband probes with frequencies between 20 and 150 MHz are commercially
available and can dramatically improve flaw resolution and thickness measurement capabilities.

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 Test techniques

Test techniques

Ultrasonic inspections are largely performed by the pulse echo technique, in which a single probe is
used to both transmit and receive ultrasound. In addition to the fact that access is required from
one surface only, a further advantage of this technique is that it gives an indication of not only the
type of defect but also its size and exact location within the item being tested.

Pulse echo

The major disadvantage is that pulse echo inspection is reliant upon the defects having the correct
orientation relative to the beam in order to generate a returning signal to the probe and is therefore
not considered fail safe (see Figure 32). If the sound pulse hits the flaw at an angle other than 90°,
much of the energy will be reflected away and not return to the probe with the result that the flaw
will not show up on the screen.

Normal compression probe Shear wave angle probe

Figure 32: Schematic drawing of the pulse echo technique showing specular reflection from a discontinuity.

Through-transmission

Through-transmission was used in the early days of UT and is still used in plate and bar production.
A probe on one side of a component transmits an ultrasonic pulse to a receptor/receiver probe on
the other side. The absence of a pulse arriving at the receiver indicates a defect.

Figure 33: Through-transmission.

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 Test techniques

Advantages Disadvantages
Less attenuation of sound Defect cannot be located.
energy.

No probe ringing. Defect cannot be identified.

No dead zone on the screen. Component surfaces must be parallel.

Orientation of a defect does Vertical defects do not show.

not matter as much as on the
Requires access to both sides of the component.
pulse echo display.

Process must be automated.

Tandem scanning

Tandem scanning is used mainly to locate defects lying perpendicular to the surface. It involves the
use of two or more angle probes of the same angle of incidence and facing the same direction with
one probe acting as the transmitter and the others as receivers.

Figure 34: Tandem scanning (T=Transmitter, R=Receiver).

Contact scanning

Contact scanning is defined by BS EN 1330 as scanning by means of (an) ultrasonic probe(s) in

direct contact with the object under examination (with or without couplant). A thin film of couplant
between the probe and the test surface usually serves to transmit ultrasound, to lubricate the surface
and to reduce wear on the probe face. Ideally the acoustic impedance of the couplant should be
between that of the probe (Perspex) and that of the material under test.

Gap scanning

Gap scanning is a technique in which the probe is not in direct contact with the surface of the
specimen but rather coupled to it through a column of liquid no more than a few wavelengths thick.

Figure 35: Gap scanning.

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 Test techniques

Immersion testing

Immersion testing involves the test object being submerged in a liquid, usually water and the probe
being scanned at a fixed distance above the component. The water serves to provide constant
coupling conditions and amounts to a long fluid delay line. Although the probe itself requires a
compression wave, shear waves can be produced within the sample by angulation of the probe. This
technique frequently uses high frequency probes (25-50 MHz) and focused probes for automated
inspections and is suited to the inspection of complex components, see e.g. BS M36: Ultrasonic
Testing of Special Forgings by an Immersion Technique.

Wheel probes, squirters and bubblers are also considered to be immersion systems.

Figure 36: Immersion scanning.

Presentation

Figure 37: B, C and D scans.

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 Test techniques

Figure 38: P scan image.

Scan image consisting of (from top to bottom):

Top view C scan

Side view D scan

Echo view A scan (cumulative)

End view B scan

Projection view P scan

10.7.1 A scan

The flaw detector or UT set sends ultrasound energy into test materials. Some of this energy returns
to the set to be presented as information on a cathode ray tube (CRT) screen. This is an A scan
display with the amplitude of signals displayed as a function of time or distance.

10.7.2 B scan

This gives an end or cross-sectional view of the component being examined, with the position of the
probe displayed on one axis and the distance from the surface to the signal on the other (see Figure
37). The B scan is used in hospitals and on aircraft components. It is often used with specimens
immersed in water and with an automated scanning device.

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 Test techniques

10.7.3 C scan

The C scan gives a plan view of a defect (see Figure 37Figure 37: B, C and D scans.). It is often
used as an automated process to map out laminations in plate. It gives the area of a defect, so it is
good for plotting the extent of laminations in metal sheets.

10.7.4 D scan

The D scan gives a side view of the defect seen from a viewpoint normal to the B scan (Figure 37).
It is usually automated and shows the length, depth and through thickness of a defect. The D scan
should not be confused with the delta technique.

NDT4-060420 40 Copyright © TWI Ltd

 Ultrasonic Flaw Detector

Ultrasonic Flaw Detector

Principles

The ultrasonic flaw detector, which is part of the UT set, sends a voltage down a coaxial cable
(sometimes called the lead) to a probe. The piezo-electric crystal in the probe is hit by the voltage
and vibrates. The vibration creates an ultrasonic pulse which enters the test material. The pulse
travels through the material until it strikes a reflector and is reflected back to the probe.

It re-enters the probe, hits the crystal and vibrates it, causing it to generate a voltage. The voltage
causes a current which travels back to the flaw detector along the cable. The set displays the time
the pulse has taken through the test material and therefore the distance travelled back and the
strength amplitude of the pulse as a signal on the CRT screen.

In essence, a UT set transmits energy into a material via a probe and measures the time in
microseconds that the sound pulse takes to return to the probe. The controls on the UT set are
almost entirely concerned with presenting a display on the CRT screen for the operator to interpret.

Figure 39: Block diagram of an ultrasonic flaw detector instrument.

Cathode ray tube

The cathode ray tube (CRT) is a device for measuring very small periods of time. The CRT displays
electrical pulses on a screen in a linear time/distance relationship. This means that the longer the
distance on the on-screen time base, the longer the time that has been measured.

How the CRT works

A filament is heated in a vacuum tube. The heat causes the particles of the filament to vibrate and
electrons start ‘boiling’ out of the surface, a process known as thermionic emission.

A positive potential electric charge is positioned further down the vacuum tube and the negatively
charged electrons from the filament are attracted towards it.

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 Ultrasonic Flaw Detector

The electrons pass through a negatively charged focusing ring which pushes them towards the centre
of the tube, forcing them into a fine stream. This stream of electrons hits a phosphor-covered screen
at the end of the tube. The electron bombardment forces the phosphor to give out light and a green
dot appears on the screen.

The X and Y plates above, below and beside the electron stream carry potentials that move the
electron stream from side to side and up and down, moving the green dot on the screen.

The X plates control horizontal movement and the Y plates control vertical movement. By altering
the potential of the X and Y plates, the dot can be moved on the screen.

Pulse generation

The pulse generator in a UT set is a timer which gives out a number of electrical pulses every second.
This is called the pulse repetition rate or pulse repetition frequency (PRF) and must not be confused
with probe frequency. The PRF on most sets is about 1000 pulses/s, though this can be varied on
most sets from 50 for thick specimens to 1250 pulses/s for thinner specimens.

The pulse generator sends the pulse to the time base generator on the CRT and to the pulse
transmitter. The time base generator sends the green dot moving across the CRT screen by
generating a charge in the X plates in the tube.

Simultaneously the pulse transmitter sends an electric voltage down the coaxial cable to the piezo-
electric crystal in the probe. The crystal vibrates, transmitting the pulse of sound into the test
material. At the end of each pulse, the green dot on the CRT screen flies back to the left-hand side
of the screen to await the next pulse.

If the test material is thick, the dot must travel across the screen fairly slowly, as the pulse repetition
rate is lowered. Only one pulse may be in the test material at any one time or confusing echoes will
result. For this reason, the PRF is lowered when thicker specimens are examined.

Range control

The range control varies the speed of the green dot across the screen. It is divided into the coarse
range, which allows large changes in range (e.g. 10 to 100 to 500 mm), and the fine range which
allows small adjustments in distance between these. As mentioned above, the dot travels slowly for
thick specimens, while for thin specimens its speed is increased. Adjusting the speed of the dot in
relation to the time taken for the sound pulse to enter the test specimen and to be reflected back to
the probe is called setting a time base.

If the speed of the dot across the screen is not even, as a result of equipment failure, we say the
time base is not linear. Flaw detectors should be checked frequently to assess time base linearity.

Delay

The delay control makes the time base generator wait before sending the green dot moving across
the screen.

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 Ultrasonic Flaw Detector

Twin crystal and angle probes have Perspex blocks or wedges between the crystal and the test
material. This need not be shown on the CRT screen, so we adjust the delay to move it sideways off
the display, so the passage of the ultrasound through the Perspex in the probe does not appear on
the screen.

You can also use the delay control to wait until the sound has travelled part of the way through the
test piece itself before representation on the screen. For example, if you only want to look at the
bottom 25 mm of a 200 mm specimen, you can adjust the delay so that the green dot begins to
travel across the screen at 175 mm. During thickness checks, this can make for more accurate
readings for thicker specimens.

Calibrated gain/attenuator control

If the sound pulse sent into the test material is reflected back at the proper angle, it returns to the
probe and hits the receiver crystal. The crystal sends a current back to the UT set. For technical
reasons this current must be very small.

The current returning to the set goes to an amplifier which increases it and filters out irrelevant
signals. The returning current is alternating (AC) and must therefore pass through the rectifier before
going to the CRT.

The rectified current now goes to the attenuator, which uses a variable resistance to control the
current passed on to the CRT. The greater the resistance, the smaller the current. This attenuator
is controlled by the calibrated gain/attenuator control on the set.

From the attenuator, the current goes to the Y plates in the CRT. When the current hits the Y plates,
they pull the electron stream upwards and the green dot jumps from the bottom of the screen to
make a signal. The height of the signal is increased or decreased by turning the gain up or down.

This control is a method of controlling the amplitude of a signal. It is also a means of comparing the
height of one signal with the height of another. So, the UT set can tell us two things:

The position of a reflector below the probe

The comparative amount of energy reflected from that reflector

We can find the latter by comparing a signal from the reflector in the test piece with a signal from
an artificial reflector in a reference block.

Reject/suppression control

When measuring high attenuating material, there is often a corresponding high level of grass (US:
hash) on the time base. It is possible to reduce this to an acceptable level by means of the
reject/suppression control and, providing the calibration is verified, accurate thickness
measurements are possible. However, reject often makes the vertical axis non-linear so must NOT
be used if readings related to the decibel are made.

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 Ultrasonic Flaw Detector

The decibel

Gain is measured in decibel (dB) – tenths of a unit called a bel. When we compare the height of two
signals on the CRT screen, we are in fact comparing the electric voltage that is being sent to the Y
plates; electric voltage is proportional to the square of the current. To compare two signals, we must
use a formula that takes account of this fact (see Section 1.4).

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 Calibration and Sensitivity

Calibration and Sensitivity

Angle probes, initial checks and calibration

Before we can start to use an angle probe, we need to find out more about it. For instance:

Where is the sound coming out of the Perspex shoe?

Is it the angle that it is supposed to be?

Has the angle changed since it was last used?

So, we must check the probe before we can calibrate the time base for use. The following paragraphs
also describe a number of other performance checks which should be carried out at specified
intervals.

Finding the probe index

The point at which the centre of the beam leaves the probe and enters the test material is called the
probe index or emission point. It should be marked on each side of the probe and checked regularly.
As the probe surface wears down, the probe index can change. Stand-off measurements are taken
from the probe index and used to check the probe angle (another check that the UT technician must
perform regularly), so this is the master reference point or datum.

To find the probe index, place the probe on a Calibration Block No 1 (see BS EN ISO 2400 Ultrasonic
Testing Specification for Calibration Block No 1), also referred to as a V1 Block, and obtain an echo
from the 100 mm radius and establish it at more than 50% FSH using the gain control. Maximise
the echo by moving the probe backwards and forwards. Mark a line on each side of the probe directly
above the slots which indicate the centre of the 100 mm radius. This is the probe index, where the
axis of the beam leaves the Perspex shoe.

a) Calibration Block No 1 b) Schematic drawing of Calibration Block c) ‘A’ scan display

No 1 with angle probe at 100mm radius

Figure 40: Determination of probe index.

Checking the probe angle

For a 45° or 60° probe, place it on the Calibration Block No 1, approximately adjacent to where the
appropriate angle is inscribed, directed towards the plastic insert. Obtain a signal on the screen from
the plastic insert and maximise it. Find the position where the probe index coincides with the angle
indicated on the side of the No 1 Block and this will tell you the probe angle.

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 Calibration and Sensitivity

This procedure can be repeated for a 70° probe, but reflecting the energy from the plastic insert
radius is unreliable. Therefore we suggest you use the 1.5 mm hole as a target.

a) Angle probe in position b) Angle probe in position c) ‘A’ scan display

Figure 41: Determination of probe angle.

Calibration of shear waves for range

By range in angle probe testing we mean the distance a reflector is from the probe index.

It is possible on some flaw detectors to calibrate the time base to 100 mm range from the Calibration
Block No 1. However, this involves delaying the signal by 100 mm and not all equipment can do this
on the appropriate scale expansion setting, so we will confine ourselves to calibrating for 200 mm
full screen width.

12.3.1 Calibration with the Calibration Block No 1 (V1 block)

Place the probe on the Calibration Block No 1 and obtain a boundary echo from the 100 mm radius.
Establish this signal to more than 50% FSH using the gain control. Further maximise the echo by
moving the probe backwards and forwards, then keep the probe stationary.

Wind in or out on the scale expansion/range control to establish a second boundary echo at 200 mm
range.

Place the signal from 100 mm at 5 (half scale) on the time base and the one from 200 mm at 10
(full scale), using the delay and range controls. The time base is calibrated for 200 mm; longer
ranges can be catered for in multiples of 100 mm.

However, the Calibration Block No 1 is bulky and inconvenient for site work, so it is not always
possible to calibrate for 100 mm and we tend to use the Calibration Block No 2 (also referred to as
the V2 Block).

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 Calibration and Sensitivity

a) Schematic drawing of shear wave probe positioned on b) ‘A’ scan display

Block No 2

Figure 42: Calibration of the time base (range) of the flaw detector:

12.3.2 Calibration with the Calibration Block No 2 (V2 block)

The V2 Block (see BS EN ISO 7963 Non-Destructive Testing: Ultrasonic Testing; Specification for
Calibration Block No 2) is the most convenient calibration block to use with angle probes. It has two
arcs, at 25 and at 50 mm (see Figure 43).

12.3.3 Calibration for 100 mm

Place the probe on the block and point it at the 25 mm arc. Adjust the delay and range controls until
you have two signals on the screen; the first will represent 25 mm and the second will represent
100 mm. Maximise the signals by sliding the probe forward and backward. Adjust range and delay
until the first echo comes a quarter of the way across the screen at 2.5 and the second echo comes
at the extreme edge of the screen on the right-hand side at 10.

The time base now represents 100 mm. Check this by turning the probe around and pointing it at
the 50 mm arc. If you have calibrated correctly, the signal when maximised will come up exactly in
the middle of the screen at 5.

a) Schematic drawing of Block b) Block No 2 with 25mm and c) ‘A’ scan display for probe in
No 2 with angle probe, first 75mm arc (radius) position as pictured in a
reflection at 25mm and second
at 100mm

Figure 43: Calibrations using Block No 2.

12.3.4 Calibration for 200 mm

Point the probe at the 50 mm arc on the Block No 2 and obtain three echoes on the screen. These
represent 50, 125 and 200 mm. Maximise these signals by sliding the probe forward and backward.
Adjust the range and delay until the first signal comes a quarter of the way across the screen at 2.5
and the third echo comes at the extreme edge of the screen at 10.

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 Calibration and Sensitivity

a) Schematic drawing of Calibration Block No 2 with b) ‘A’ scan display with echoes at 2.5 (50 mm), 6.25
probe partitioned for the 50 mm arc (radius) (125 mm) and 10 (200 mm).

Figure 44 Block No 2 Calibration for 200 mm range.

Calibration for 250 mm

Point the probe at the 25 mm radius arc on a Calibration Block No 2 (V2 Block) and adjust the set
until you get four echoes. These represent 25, 100, 175 and 250 mm. Maximise these signals by
sliding the probe forward and backward. Adjust range and delay until the first echo comes one tenth
of the way across the screen at 1 and the fourth echo comes at the extreme edge of screen at 10.
Check on a Calibration Block No 1. On the 100 mm arc you should get one echo 4/10 across the
screen and the other 7/10 across the screen.

a) Schematic drawing of Calibration Block No 2 with b) ‘A’ scan display with echoes at 1 (25 mm), 4 (100
probe positioned for the 25 mm arc (radius) mm), 7 (175 mm) and 10 (250 mm).

Figure 45: Block No 2 Calibration for 250 mm range.

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 Flaw Location

Flaw Location

You can calculate the location of a flaw by using trigonometric formulas as shown below. You need
to know the angle of the probe and the stand-off measured from the centre of a weld.

Figure 46: Reflector depth and probe stand-off as a function of probe angle.

A general rule of thumb used to calculate the depth of an indication from the range on the screen
is:

45° probe – range is approximately 1.5 x depth

60° probe – range is exactly 2 x depth

70° probe – range is approximately 3 x depth

It is quicker and easier, however, to use a flaw location slide and a beam plot or even a piece of
clear plastic film with the probe angle drawn on it (see Figure 47). Use the slide as follows:

Draw a cross section of the weld on the transparent outer envelope of the slide

Draw a mirror image of the weld cross section immediately under it if the sound energy is
going to bounce off the backwall, ie using full skip

Use the printed datum line on the plastic envelope as the centre of the weld and measure
all stand-offs from it

Maximise the echo from a defect and mark where the index point falls on the parent metal
Measure its distance from the centre of the weld4

Move the weld datum line on the plastic envelope to the stand-off distance.

Look along the centre of the beam plot until you come to the range shown on the screen

Make a mark on the envelope; this represents the centre of the defect. It shows the position
of the defect in the weld body

4
The defect on a sketch as well as the stand-off and range of the centre of the defect.4

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 Flaw Location

a) Probe position for ‘full skip’ into the weld for b) Flaw location slide showing graduated range, stand-off
detection of side wall defect; and defect depth.

Figure 47: Determination of flaw location using the flaw location slide.

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 Flaw Sizing

Flaw Sizing
The 6 dB drop sizing method

This method is used for sizing large reflectors. If the probe is moved until the signal amplitude from
a reflector drops to half its original screen height, then it can be said that the sound beam is half on
and half off the reflector. So, by moving the probe until the signal from the end of a large reflector
halves in height, we can estimate that the edge of the reflector is immediately below the centre of
the probe.

This method is called the 6 dB drop sizing method because the amplitude of the signal drops by half,
which corresponds to 6 dB, when the probe is moved to the edge of a large reflector.5

a) Maximum signal position of probe

b) ‘A’ scan response at maximum signed height
c) Probe position for signal at 50% of maximum response
d) ‘A’ scan response at 50% of maximum response, i.e. 6 dB drop from maximum echo height.

Figure 48: 6 dB drop sizing.

5
The last peak on the screen before the probe goes off the end of the reflector is usually considered as the peak of
the reflector, rather than the maximum signal from the reflector.

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 Flaw Sizing

The 20 dB drop sizing method

We can use a beam plot to find the edge of a defect by using the edge of the sound beam.

If we know the width of a beam at a certain distance from the crystal, we can mark the distance
across a defect from where the extreme edges of the beam touch each end of the defect and then
subtract the beam width to get the defect size.

When the signal from the defect drops by 20 dB from its peak, we judge that the edge of the beam
is just touching the end through-thickness extremity of the defect. We can find the width of the
sound beam at that range by consulting the beam plot we have made.6

a) Top edge of the ultrasonic b) Maximum signal response from c) Bottom edge of the ultrasonic
beam detecting the bottom the defect; beam detecting the top edge of
edge of the defect; the defect.

Figure 49: Probe and ‘A’ scan displays.

Construction of a beam edge plot – 20 dB

Find the hole at a depth of 13 mm on an IOW block with a 0° probe and maximise the signal. Move
the probe until you get the highest signal you can from the hole, then turn the signal to FSH using
gain. Mark the position of the middle of the probe on the side of the block.

Figure 50: Construction of the 20 dB beam plot – maximum signal.

6
The last peak on the screen before the probe goes off the end of the defect is usually considered as the peak of the
defect, rather than the maximum signal from the defect.

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 Flaw Sizing

Move the probe to one side until the signal drops to 10% FSH (-20 dB) and mark the centre of the
probe on the side of the block.

Move the probe to the other side of the hole until the signal drops to 10% FSH
(-20 dB) and mark the centre of the probe on the block.

Use the distances between the marks on the block to plot the beam on a piece of graph paper.
Measure 13 mm depth on the paper then mark the distances of the probe centre at -20 dB from the
beam centre at 100% FSH on either side.

Now find the 25 mm hole and maximise the signal, turning it to 100% FSH. Move the probe to either
side of the hole, marking the centre of the probe on the side of the block where the signal drops by
20 dB.

Measure 25 mm on the paper and use the distances on the block to plot the beam dimensions at 25
mm.

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 Flaw Sizing

Repeat using the 32 mm hole. Join up the points marking the probe centre at 20 dB to obtain a
beam plot. 7

b-e Determination of the 20dB ultrasonic beam edge

Figure 51: Construction of the 20 dB beam plot.

Constructing an angle beam plot

An IOW reference block is convenient for constructing a beam plot. It has a number of 1.5 mm side-
drilled holes at different depths and is used mainly for setting sensitivity. Use a 20 dB beam edge
for 45° and 60° probes, but use a 10 dB beam edge for 70° probes. With the 70° probe, a 20 dB
beam spread is so wide and difficult to construct that it is effectively useless.

Figure 52: IOW reference block with 1.5 mm side-drilled holes at different depths.

We will start with a 60° probe. Using the probe, find the hole which is 13 mm below the top surface
and maximise the signal to 100% FSH. Mark where the index point comes on the block with a pencil
or crayon. Move the probe forward until the signal drops to one tenth screen height (20 dB drop).
Make a second mark on the block where the index point on the probe stands on the block.

7
We have only drawn the beam width in one plane, so the probe must be marked accordingly and used to measure
defects in this plane. We use knowledge of this beam spread to size defects, find their edges and hence their width,
length and sometimes orientation.

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 Flaw Sizing

Figure 53: Angle beam plot construction.

Move the probe backwards until the signal maximises and then drops down to 1/10 screen height.
Mark where the index point now stands and draw a vertical line on the block from the hole to the
upper edge. Measure the distances of the three index point marks from the top of the line and note
them down.

Now find an echo from the 19 mm deep hole and repeat the process, noting the distances and repeat
the process a third time using the 25 mm hole.

Take the slide out of a beam plotting chart and draw three faint lines across it at depths of 13, 19
and 25 mm.

Transfer the distances of the index points from the vertical lines to the relevant pencil lines on the
chart. Join the marks up. The centre line represents the main energy of the beam and the other two
marks represent the leading and trailing edges of the beam.

With a 45° probe, use the 19, 25 and 32 mm depth holes as the 13 mm hole may be in the probes
near zone.

Use a 10 dB drop with a 70° probe and instead of dropping the signal to 1/10 FSH for the leading
and trailing edges, use the 3/10 line on the screen.

Proving the beam plot

Use the six side-drilled holes in the IOW block.

Use the corner of the block as a reference point from which to measure stand-offs.

On the cover of the beam plotting chart, use the corner of the block to represent the centre line.

Calibrate the probe to 100 mm (200 mm for a 70° probe).

Obtain a signal from the top hole of the six, maximise it, then push the probe towards the block
corner until the signal drops to 1/10 FSH (3/10 for a 70° probe). Mark where the index point occurs
on the block and measure the stand-off. Note the range of the reflector on the screen.

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 Flaw Sizing

Use the stand-off and the range to plot the defect along the trailing edge of your beam spread. Mark
it on the slide cover.

Now obtain a signal from the bottom hole of the six, maximise it and turn it up to FSH on your
screen. Pull the probe back until the signal drops to 1/10 FSH (3/10 for a 70° probe). Plot the bottom
of the defect on your slide cover using the leading edge of your beam plot.

Lay the transparent slide over the IOW block; the top and bottom of the drilled holes should coincide
with the marks on the slide. If they do not, your beam plot is off or you are going wrong somewhere.
If they do, you have just sized a defect by the 20 dB drop method (10 dB drop for the 70° probe)

Figure 54: Example of bespoke reference block to check beam characteristics including beam edge
resolution and near-zone approximation.

Modified near zone angle probes

We must now consider the part of the beam which is in the near zone on an angle shear wave probe
because with a beam edge method of flaw sizing, we cannot assess small defects in the near zone.

However, the beam starts to travel in the plastic wedge and is then refracted and carries on in the
material being tested. We are only concerned with the part of the beam near zone registering later
than zero on the time base, ie in the test material. This is called the modified near zone.

Here is an example:

A 5 MHz shear probe has a 10 mm diameter crystal. The beam travels in Perspex for 10 mm. What
is the modified near zone?

𝐷2 𝑓 8 (16)
NZs =
4𝑉

102 ∙ 5 ∙ 1,000,000
𝑁𝑍𝑠 = 𝑚𝑚 = 38.46 𝑚𝑚
4 ∙ 3250 ∙ 1000

We must now subtract the Perspex wedge part of the beam which is 10 mm, multiplied by the ratio
of the Perspex and steel velocities which is

2730
𝑁𝑍𝑝 = 10 ∙ = 8.4 𝑚𝑚
3250

8
If totally developed in steel.

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 Flaw Sizing

Therefore: modified near zone is:

𝑁𝑍𝑚𝑜𝑑 = 38.46 – 8.4 = 30 𝑚𝑚

Horizontal beam plot

A number of methods can be used to find the -20 dB edge of a beam in the horizontal plane. Some
use the ends of the side-drilled holes in the IOW block to determine the edge. However, we prefer
to use the 1.5 mm through-drilled hole in the IOW calibration block.

Method:
Place the probe to pick up the 1.5 mm hole at ½ skip and maximise the signal from the
intersection of the hole and the opposite face. Mark the straight edge adjacent to the near
centre of the probe to indicate the beam centre.

Position a straight edge either in front of or behind the probe to hold the probe in the fixed
transverse position. Scan the probe laterally (sideways) until the hole signal drops by 20 dB.
Mark on the straight edge adjacent to the rear centre position of the probe. This registers
half a beam at the ½ skip range.

Scan the probe laterally the other way, through the maximum signal position, until the hole
signal again drops by 20 dB. Mark the straight edge as before.

You now have three marks on the straight edge to indicate the beam width at that range.
Transfer these to the beam plotting chart as appropriate.

Repeat steps a-d, but at full skip and 1½ skip for a 45° probe (only at full skip for a 60°
probe). Note that mode conversion reduces the 1½ skip signal on a 60° probe to too low a
level to be reliable.

Join up the three points on either side of the centre line to complete the beam. Only take
the lines back to the near zone because the edge is not reliable beyond that.

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 Sensitivity Setting

Sensitivity Setting

Setting a sensitivity level is essential to providing reproducible results when the same inspection is
carried out by different operators, using different probe set combinations and maybe working in
different locations. They must all see the same flaw giving the same signal height and therefore
have the same data on which to base their accept/reject decisions.

There are several systems for setting sensitivity. We have already encountered the method of setting
the first back wall echo (BWE) to FSH for lamination checks. However, when checking plates adjacent
to a weld, the second BWE should be set to FSH.

When setting the sensitivity, we must be sure that a signal from a defect will be visible on the CRT
screen and that we will be able to distinguish the defect signal from background noise or grass. All
UT sets differ slightly, so we cannot say, ‘Set the sensitivity to xdB’, as different probes and
equipment will give entirely different signals from the same reflector. The methods of setting
sensitivity have evolved to attain some uniformity.

Different methods are used in different places. At TWI, the IOW block is used as the recommended
method for PCN examinations. On North Sea contracts, either the distance amplitude correction
(DAC) curve or the American Society of Mechanical Engineering (ASME) curve is used. The DAC
method is recommended in BS EN ISO 17640 (Non-destructive testing of welds – ultrasonic testing
techniques, testing levels and assessment), while in Germany the distance gain size (DGS) system
is usually applied, especially when evaluating small reflectors.

The purpose of sensitivity setting is to find a gain level sufficient to find a flaw and depends on the:

Probe used, in particular its frequency.

Flaw detector.

Properties of the test material.

Ratio of noise to BWE or flaw echo.

The Institute of Welding (IOW) block

We met the Institute of Welding block when studying beam profiles. The block contains 1.5 mm
side-drilled holes at different depths and allows the holes to be detected from different angles with
angle probes. To use it is simple and straightforward.

Find a hole on the block that approximately coincides with the thickness of the material you are
testing. Double the thickness if you are examining at full skip, i.e. bouncing your sound beam off
the backwall.

Obtain a signal from the hole and turn the gain control until the signal is at FSH.

Work out the transfer correction.

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 Sensitivity Setting

You have now set the sensitivity and can be assured that flaws having the equivalent reflectivity of
1.5 mm side-drilled holes will appear on the screen.

This method has several advantages:

Simple to use.

Provides a uniform system of reference.

A fairly large and visible echo is assured from small flaws.

Side-drilled hole reflectors are independent of angle.

But also, some disadvantages:

Block is heavy and expensive.

Only applies to 1.5 mm side-drilled holes.

Not a reliable method for sizing defects.

Sensitivity will be higher for ranges shorter than the SDH used.

Distance amplitude correction (DAC) curves

BS EN ISO 17640 and all US specifications recommend this method. A special reference block of the
same material as the test object is usually necessary, though the curves can be constructed from
an IOW block.

The type of block recommended by EN ISO 17640, which is an ASME block, is shown below.

Figure 55: Distance amplitude correction (DAC) Figure 56: The IOW reference block containing a
reference block. series of 1.5 mm SDH at different depths. A 60°
angle probe is shown in position on the block.

The procedure described in EN ISO 17640 for constructing a DAC curve is:

Calibrate the time base for the maximum sound path length to be used

Adjust the gain so that the amplitude from the series of reflectors falls between 20 and 80%
FSH

Record the gain setting used for plotting the DAC curve and reference this to some other
reflector, such as the radius in a V calibration block. This action enables the gain to be reset
without the reference block.9

9
Should the difference in height between the largest and the smallest echoes exceed the range of 20-80% FSH, the
line shall be split and separate curves plotted at different gain settings. The difference in gain between the two
curves shall be noted.

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 Sensitivity Setting

Without altering the gain setting, maximise the amplitude of each reflector in turn and mark
the tip of the signal, either on the screen or on a transparent overlay

Figure 57: DAC construction from a series of reflector SDHs to cover the thickness range of
the test component.

Examine the test material as instructed in the specifications, comparing the signals from
discontinuities to the curves on your screen. Any signal above the curve shows a reflector larger
than the reference hole. Accept or reject discontinuities as instructed in the specification you are
working to.

Advantages
A quick way of accepting or rejecting discontinuities without too much time consumed in
sizing reflectors.

Some idea can be gained of the size of the discontinuities in relation to the reference holes.

Uniformity provided by all technicians constructing their curves from the same test block.

Disadvantages
Curves must be constructed for each probe in conjunction with each UT set.

Transfer correction must be worked out.

Flat-bottomed holes (FBH)

Blocks are drilled with flat-bottomed holes to precise diameters at set distances from the top of the
block. These diameters and distances are stamped on the side of the block.

When setting sensitivities, the specification or technique will specify the block to be used and the
amplitude of the signal to be obtained from the FBH. Blocks are cut for use with 0° probes or angle
probes in different materials. This method is mostly used in aerospace applications.

Advantages
Easy to use.

Uniformity assured when different technicians use the same blocks.

Blocks can be made from different materials.

Disadvantages
Fairly rigid system for specific applications.

Large number of blocks needed for different settings.

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 Sensitivity Setting

Transfer correction usually needed.

Blocks for angle probes are rarely cut exactly normal to the beam.

You may hear these blocks called HITT or ALCOA blocks, after the originators.

Using noise

Work out the maximum range at which you will be examining the test material. Place the probe on
the material with couplant applied. Turn up the gain until you have 2 mm grass on the screen at the
maximum range. You will now have the assurance of knowing that any discontinuity larger than the
grain size will show up on the screen.

Advantages
Quick and easy.

No reference block is needed.

Any defect larger than the material grains will show up.

No transfer correction needed.

Disadvantages
No accurate sizing of the defect.

Discontinuities near the surface of the test material may be hidden in the grass.

Transfer correction

Reference blocks usually have smooth machined surfaces, while test objects frequently have
rougher, more uneven surfaces. Also the attenuation of sound in the reference block might be
different to that in the test material. Usually, the attenuation in the reference block is smaller than
that of test material, but not always. This means that allowances must be made for the differences
in sound energy transfer between probe and test material and probe and reference block. More
energy can be passed into a reference block than into a rougher surfaced component.

Therefore, the artificial defects in a reference block may give higher amplitude signals (anything up
to 6 dB or even more) than signals from similarly sized discontinuities in the test materials.

Allowances have to be made for this and corrections made for different surfaces. This allowance is
named transfer correction or transfer loss.

There are several methods of determining the transfer correction, some requiring the construction
of separate DAC curves and some requiring calculation according to formulae. Two simple methods
are explained below.

Compression probe method

Place the probe on the reference block and turn the BWE up to FSH. Note the gain settings. Now
place the probe on the test material and at a similar range bring the BWE to FSH. Again note the
gain setting. The difference between them is the transfer correction.

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 Sensitivity Setting

Angle probe method

As you cannot get a BWE with angle probes from a plate or pipe wall, you have to use two probes
with the same angle.

Place the two probes opposite each other on the reference block with one probe transmitting and
the other receiving, so that the sound energy is bounced off the backwall and caught by the receiving
probe (pitch and catch).

Figure 58: Schematic drawing of the transfer correction method for angle probes.

Maximise the signal and adjust the gain until it is at FSH.

Place the two probes on a piece of test material of the same thickness as the reference block and
repeat the process.

Note the difference between the two gain settings. This is the transfer correction needed.

Other methods of transfer correction are described in EN 17640 and in literature concerning the
distance gain size (DGS) system.

The distance gain size (DGS) method

The DGS system relies on the laws of large and small reflectors in the far zone and was developed
to relate the amplitude of a signal to various sizes of perfect disc reflectors (flat-bottomed holes),
so it does not actually size flaws but relates them to an equivalent reflector. The relative heights of
signals from different sizes of flat-bottomed holes at different distances were plotted as curves.

Reflector sizes are expressed in terms of the probe diameter and distances from the probe are
expressed as multiples of the near zone.

If you have a signal from a flaw at a certain depth, you can compare the signal size to what the
signal of a BWE should be at that depth and estimate the size of a flat-bottomed hole that would
give such a signal at that depth. The defect can then be sized according to its flat-bottomed hole
equivalent.

The attenuation factor for the test material must be taken into consideration when using the DGS
system.

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 Sensitivity Setting

Example

You are using a 5 MHz 10 mm diameter compression probe on a 100 mm steel plate and you find a
defect at 60 mm depth which gives a signal at FSH with a 30 dB gain setting. What is its flat-
bottomed hole equivalent?

First, work out the probe near zone. It is 21 mm, so the defect is at a distance of three near zones.

Now get a BWE and find what the dB reading is. Say it is 20 dB when the BWE is at FSH. 100 mm is
five near zones. What will it be at 60 mm – three near zones? Refer to the DGS curves. If the BWE
is at FSH with 20 dB at 100 mm, by the law of large reflectors and according to the BWE line on the
DGS curves, a BWE at 60 mm should reach FSH at 16 dB – 4 dB less than at 100 mm.

The signal height from the flaw is 30 dB, which is 14 dB more than the BWE. Look down the scale
14 dB at three near zones from the BWE and you find that the nearest line is at 0.5 of the probe
diameter. The probe diameter is 10 mm so the nearest equivalent flat bottomed hole to the flaw had
a diameter of 5 mm.

By a similar process, a sensitivity setting can be worked out for a flat-bottomed hole of a certain
diameter at a given range to a given screen height and the flaw detector gain set accordingly.

Advantages
Can choose a gain level for sizing.

Tells you the smallest defect you can find at a given range.

Provides the basis for an accept/reject system.

Gives a rough equivalent to the size of a flaw.

Uniformity between results from different technicians.

Disadvantages
Operators must keep referring to a chart and making calculations.

Attenuation must be taken into account.

No account is taken of the flaw orientation.

Most effective on small defects.

An equivalence system, not a sizing system.

Flaw surfaces and shapes are not ideal reflectors; therefore, the signal amplitudes are not
the same as those of a comparable flat-bottomed hole.

For angle probes, plastic slides have been manufactured by Krautkramer to fit over the CRT screen.
The set is calibrated and gain setting is performed by bringing the BWE or the echo from the 1.5
mm hole on the V2 block up to marks on the slide. Flat-bottomed hole equivalents for flaws can then
be read straight off the slide. The DGS system is widely used in Germany.

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 Sensitivity Setting

Signal-to-noise ratio (SNR)

It has been mentioned elsewhere that frequency and wavelength have a major influence on flaw
detection. However, the detection of a defect is also influenced by many other factors. The amount
of sound that reflects from a defect is, for example, dependent on the acoustic impedance mismatch
between the flaw and the surrounding material. A gas-filled defect such as a lack of fusion is
generally a better reflector than a metallic inclusion because the difference in acoustic impedance is
greater between air and metal than between a metal and another metal.

The nature of the surrounding material also greatly affects the detection of defects with coarse-grain
structures, reducing defect detectability. A measure of detectability of a flaw and the effect of the
many factors involved is its signal-to-noise ratio (SNR). The SNR is a measure of how the signal
from the defect compares to other background reflections (categorised as noise). An SNR of 3-1 is
often required as a minimum.

The absolute noise level and the absolute strength of an echo from a small defect depend on a
number of factors:

Probe size and focal properties.

Probe frequency, bandwidth and efficiency.

Inspection path and distance (water and/or solid).

Interface (surface curvature and roughness).

Flaw location with respect to the incident beam.

Inherent noisiness of the metal microstructure.

Inherent reflectivity of the flaw, which is dependent on its acoustic impedance, size, shape
and orientation.

Cracks and volumetric defects can reflect ultrasonic waves quite differently. Many cracks are
invisible from one direction and strong reflectors from another.

Multi-faceted flaws will tend to scatter sound away from the transducer.

General factors to consider with respect to SNR and therefore defect detection:

Increases with increasing flaw size (scattering amplitude). The detectability of a defect is
directly proportional to its size.

Increases with a more focused beam. In other words, flaw detectability is inversely
proportional to the transducer beam width.

Increases with decreasing pulse width. In other words, flaw detectability is inversely
proportional to the duration of the pulse produced by an ultrasonic transducer. The shorter
the pulse (often higher frequency), the better the detection of the defect. Shorter pulses
correspond to broader bandwidth frequency response.

Decreases in materials with high density and/or a high ultrasonic velocity. The SNR is
inversely proportional to material density and acoustic velocity.

Generally increases with frequency.

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 Ultrasonic Equipment Checks

Ultrasonic Equipment Checks

Linearity of time base

General

This check may be carried out using a standard calibration block, e.g. Block 1 (see BS EN ISO 2400),
and a compression wave probe. The linearity should be checked over a range at least equal to that
which is to be used in subsequent testing.

Method
Place the probe on the 25 mm thickness of Calibration Block 1 and adjust the controls to
display ten BWEs.

Adjust the controls so that the first and last BWEs coincide with the scale marks at 1 and 10.

Increase the gain to bring successive backwall echoes to 80% FSH. The leading edge of each
echo should line up with the appropriate graticule line.

Record any deviations at approximately half screen height. Deviations should be expressed
as a percentage of the range between the first and last echoes displayed (ie 225 mm).

Tolerance

Unless otherwise specified by the testing standard, a tolerance of ±2% is considered acceptable.

Frequency of checking

This check shall be carried out at least once per week.

Figure 59: A’ scan flaw detector showing signal amplitude vs distance (transit time) for linearity of time base
checks.

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 Ultrasonic Equipment Checks

Figure 60: A’ scan (echo amplitude at approximately 80% vs depth distance) presentations.

Linearity of equipment gain

General

This is a check on both the linearity of the amplifier within the set and the calibrated gain control. It
can be carried out on any calibration block containing a side-drilled hole; the probe should be the
same that is used in subsequent testing. Reject/suppression controls shall be switched off.

Method
Position the probe on a calibration block to obtain a reflected signal from a small reflector,
e.g. the 1.5 mm hole in Calibration Block No 1.

Adjust the gain to set this signal to 80% FSH and note the gain setting (dB).

- Increase the gain by 2 dB and record the amplitude of the signal.

Remove the 2 dB again and return the signal to 80% FSH.

Reduce the gain by 6 dB and record the signal amplitude.

Reduce the gain by a further 12 dB (18 in total) and record the signal amplitude.

Reduce the gain by a further 6 dB (24 in total) and record the signal amplitude.

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 Ultrasonic Equipment Checks

Tolerance
Table 4 Expected screen height results and limits for equipment gain check.

Expected screen height

Gain, dB Recorded amplitude Limits
(%)
+2 101 No less than 95%
0 80 Reference line
-6 40 37-43%
-12 20 17-23%
-18 10 8-12%
-24 5 Visible, below 8%

Frequency of checking

The check shall be carried out at least once per week.

Figure 61 ‘A’ scan presentations showing respective echoes set at 80% FSH.

Probe index and beam alignment

16.3.1 Index point

General

The probe index only needs to be checked in shear wave angle probes, but for them it should be the
first probe characteristic to be checked. The standard Calibration Block No 1 may be used for this
purpose.

Method
Position the probe on the appropriate side of the block to obtain a reflection from the
quadrant.

Move the probe backwards and forwards to maximise the amplitude of the reflected signal.

When the signal is at maximum, the probe index will correspond to the engraved line on the
block. Mark this position on the side of the probe.

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 Ultrasonic Equipment Checks

Tolerance

The tolerance depends on the application, but for plotting of defects it is recommended that the
probe index be accurate to within 1 mm.

Frequency of checking

When a probe is in continuous use, it is recommended that the check be carried out every few hours;
otherwise, a daily check is recommended.

16.3.2 Beam alignment (squint)

With the probe still in position, a check of the beam alignment can be performed. If the probe beam
is correctly aligned, the edge of the probe will be parallel to the edge of the block. If this is not the
case, measure the squint angle between the two edges.

The tolerance depends upon the accuracy of defect plotting required.

This check should be carried out once per week.

Figure 62: Amplifier and gain control checks at 0 db to 24 db.

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 Ultrasonic Equipment Checks

Beam angle

General

The beam angle can be checked on several calibration blocks, e.g. Calibration Block No 1 (BS EN
ISO 2400) or Beam Calibration Block (BCB, A5 Block). The beam angle check shall preferably be
made on a probe in conjunction with the flaw detector to be used in subsequent testing.

Method
Place the probe in such a position as to receive a reflected signal from the selected transverse
hole in the calibration block (eg the 19 mm deep hole in the BCB)

Maximise the signal from the hole and mark the index point of the probe on the block

Measure the distance from the marked point on the block to the edge of the block. Knowing
the position of the drilled hole will allow the beam angle to be calculated10

Tolerance

The accuracy achieved by the described method is 1.5°.

The accuracy in this case can only be assumed to be 3°.

Frequency of checking

When a probe is in continuous use, it is recommended that the check be carried out at least every
few hours; otherwise, a daily check is recommended.

Sensitivity and signal-to-noise ratio

General

The main objective of this check is to provide the operator with a simple method which will identify
the deterioration in sensitivity of the combination of probe and flaw detector.

Method (Figure 63: Sensitivity and signal-to-noise ratio (SNR) check.)

Place the probe on Calibration Block No 1 (also referred to as the V1 Block) and adjust its
position to maximise the signal from the 1.5 mm diameter hole

Adjust the gain control to set this signal to 20% FSH and note the dB setting

Increase the gain until the overall system noise (electronic noise and grain structure grass)
at the same range as the target hole reaches 20% FSH and note the new dB setting

10
If only a rapid check is required, maximise the signal from the 50 mm hole in Calibration Block No 1. The angle can
then be assessed by visual interpolation between the reference markings on the block

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 Ultrasonic Equipment Checks

The first gain measurement noted provides a check on the sensitivity of the probe and flaw
detector and the difference between the first and second measurements (dB) gives the
SNR1112

Tolerance

The tolerance depends on the application. Any deterioration in the sensitivity value indicates a
problem with the probe or flaw detector. A low SNR would be typical of a coarse-grained material.

Frequency of checking

Unless otherwise agreed, the check shall be carried out once per probe per day.

Pulse duration

General

This check on the combination of probe and flaw detector measures the effect on the displayed signal
of probe damping, amplifier bandwidth, built-in suppression and smoothing circuits. The standard
No 1 Calibration Block may be used for this check.

Method
Calibrate the time base in millimetres to a range that is to be used in subsequent testing.

Maximise the signal from the 1.5 mm side-drilled hole for shear wave probes or a BWE for
compression wave probes and set its peak to 100% screen height.

Measure the width of the signal in millimetres at the 10% screen height position.

If desired, the measurement in millimetres can be converted into microseconds by dividing

it by the relevant sound velocity.

Tolerance

The tolerance depends upon the application. A long pulse duration will limit range resolution and
indicate the need for a resolution check13, while a short pulse duration may indicate that the flaw
detector has built-in suppression that could prevent the observation of small signals.

Frequency of checking

Unless otherwise agreed, the check should be carried out daily.

11
A demonstration of the sensitivity of the probe and the flaw detector on a calibration block does not guarantee that
the same size of reflector could be detected in the work piece.

12
If it is desired to check the sensitivity as a function of range, the use of the standard Beam calibration block
‘also referred to as the A5 block) is recommended for longer ranges.

13
A resolution check is described in Section 16.7.

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 Ultrasonic Equipment Checks

Figure 63: Sensitivity and signal-to-noise ratio (SNR) check.

Resolving power (resolution)

General

This check determines the ability of an ultrasonic flaw detection system to give separate indications
of discontinuities which are situated close together and simultaneously hit by the sound beam.

Method
Calibrate the time base to a range of 0-100 mm for either the compression or the shear
wave probe

Place the probe so that the axis of the beam impinges upon the 2 mm step in the A7
‘resolution’ calibration block for shear wave probes, or the 3 mm step for compression wave
probes.

Adjust the position of the probe so that the echoes from the two targets are of the same
height (approximately half the full graticule height).

The steps are said to be resolved when their echoes are clearly separated at half maximum
echo height or lower14.

Frequency of checking

This check shall be carried out monthly, or when too long a pulse duration is suspected.

14
The 3 mm steps between the 9 mm and 3 mm drilled holes in the A6 calibration block may also be used when checking
compression probes.

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 Ultrasonic Equipment Checks

Figure 64: Flaw detector and probe system resolving power (resolution) check to give separate indications of closely
situated reflections (note how discontinuities a and b above give clear echo resolution).

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 Practical Welding Inspection

Practical Welding Inspection

Equipment (instrumental and probe) sensitivity is set in accordance with one of the methods
described in Section 15, with calibration covered in Section 12 and flaw location and sizing covered
in Sections 13 and 14, respectively.

Identifying flaws in butt welds

It is not always easy to identify a defect, but by noting its position in the weld and moving the probe
around the defect and watching the changing signal on the screen you can come to a reasonably
accurate conclusion. Knowledge of the welding process is essential, as is knowledge of the weld
preparation, weld dimensions, size of the gap and other factors. Slag is unlikely in a TIG weld and
lack of sidewall fusion is not likely in the middle of the weld metal. Cracks are more likely in thicker
welds than in thinner welds and fusion defects are more likely to result from automatic than manual
welding processes.

The shape, amplitude and time spread of a reflector, as represented on the screen, can also give
clues as to the identity of a flaw. Specular reflectors are those with a mirror-like face, where all the
sound is reflected back to the source of energy, providing that the probe and flaw are correctly
orientated. A sidewall fusion flaw is nearest to this ideal.

However, at the other extreme, porosity can be considered as a large number of small spherical
reflectors which cause the energy to reflect everywhere, rather like the light reflecting from a disco
ball hanging from the ceiling. Porosity is a diffuse reflector.

By combining these movements and watching the movement of the signal on the screen, conclusions
can be drawn. The characteristics of different defects are shown in the accompanying diagrams, with
explanations adjacent.

Guidance in the classification of ultrasonic indications can be found in:

EN ISO 23279 Ultrasonic testing Characterisation of indications in welds

EN ISO 16827 Ultrasonic testing 80 Characterisation and sizing of discontinuities

EN ISO 23279 contains a flowchart to be followed in order to determine the exact nature of any
indications. The stages involved are:

Echo amplitude.

Directional reflectivity.

Echostatic pattern (A scan).

Echodynamic pattern.

The first stage of assessing the echo amplitude involves comparing the amplitude of an indication to
DAC level and classifying it into one of the four categories shown in the table below.

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 Practical Welding Inspection

Table 5 Echo amplitude vs DAC level

S1 S2 S3 S4

DAC –10 dB DAC +6 dB DAC –6 dB 9/15 dB

Indications falling into the S1 category would be immediately discounted. All other indications would
then proceed to be assessed for directional reflectivity, which is defined as the variation in echo
amplitude from a discontinuity in relation to the angle at which the ultrasonic beam is incident upon
it. A spherical indication would show the same echo amplitude over a wide range of incident angles,
e.g. 45o, 60o and 70o, and is said to have low directional reflectivity. A large smooth planar reflector
would show a great variation in echo amplitude and would therefore be said to have high directional
reflectivity.

The next two stages of the process analyse firstly the shape of the signal as displayed on the A scan
equipment and finally the behaviour of the signal when the probe is scanned at 90 o to the
discontinuity (traversing). Echostatic patterns are categorised as:

Single and smooth.

Single and jagged.

Multiple.

a) Single and smooth reflector b) Single and jagged reflector c) Multiple facet reflector.

Figure 65: Echostatic patterns.

With respect to echodynamic patterns, indications fall into one of five categories dependent upon
the changes observed in the signal on the A scan in response to probe movement. To aid the
identification of defects, there are four basic probe movements:

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 Practical Welding Inspection

Figure 66: Probe movements to determine echodynamic patterns. (a) Lateral probe movement (b)
Traversing probe movement (c) Orbital probe movement and (d) Rotational movement.

Figure 67: The five echo dynamic patterns.

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 Root Flaws

Root Flaws
The signal from a root flaw will appear on the time base while you are scanning laterally along a
straight edge, at a fixed position from the root. Once the signal is maximised by getting the best
reflection from the flaw, it can be assumed that the centre of the beam is hitting the bottom of the
flaw. Fine adjustment of the straight edge will perhaps be necessary.

Excess penetration
Echo amplitude between 10-90%, dependent on depth and probe angle.

Multi-range signal – echo falls rapidly when traversed with 70° probe; also the range
increases.

Probe movement – echo falls rapidly when angle probe traverses forward.

Measurement – it is not possible to measure the depth with an angle probe. Length
measurement is difficult (usually 6 dB).

Figure 68: Excess penetration.

Root concavity
Echo sharp and large, with reduced range. Often mode conversion with 60° probe.

Probe movement – traversing backwards, the echo falls more rapidly than the lack of
penetration.

Measurement – use centre of beam and 20 dB drop (trailing edge) for height. Not always
possible to measure height.

Figure 69: Root concavity.

Root crack
Usually high amplitude response with fir tree appearance.

Probe movement – orbit, echo held over large angle.

Lateral, echo held with multi-range signals and variations on time base.

Measurement – 6 dB for length. Traverse forward with 20 dB for height.

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 Root Flaws

Figure 70: Root crack.

Lack of root fusion

Similar to corner reflector with large, narrow echo from both sides.

Probe movement – confirm with 70° probe, when traversed.

Large movement for 20 dB drop. Orbit: echo falls rapidly.

Measurement – lateral use 6 or 20 dB drop. Traverse use 20 or 10 dB for 70° probe.

Figure 71: Lack of root fusion.

Misalignment
Large single echo from one side. No echo from opposite side.

Probe movement – traverse back echo falls rapidly.

Measurement – lateral for 6 dB drop.

Figure 72: Mismatch.

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 Face and Body Flaws

Face and Body Flaws

A degree of additional flexibility can be applied when flaws are located at the face or in the body of
a weld. The diagrams below illustrate the characteristic shape of the screen presentation, but they
are ideal rather an actual.

Lack of fusion

Echo large, single, narrow at time base when on the sidewall. Poor echo from opposite side. Confirm
by skip scan.

Probe movement:

Rotate or orbit – echo falls rapidly.

Lateral or traversing – echo height held.

Measurement: For depth use 20 dB. For length use 6 or 20 dB.

Figure 73: Lack of fusion

Crack
Multiple peak reflector: usually high amplitude, but dependent on type of crack and size;
echo with fir tree appearance.

Probe movement:

Orbit – echo over larger angle than with fusion defects.

Lateral – signal held with varying height.

Measurement: For length use 6 or 20 dB. For depth use 20 dB.

Figure 78: Crack

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 Face and Body Flaws

Gas pore
Spherical even reflector: single peak echo, narrow profile, similar to drilled hole or radius of
calibration block intensity (approximately 50% CRT)

Probe movement:

Rotate, lateral and traversing – echo falls rapidly.

Orbit – echo height remains.

Measurement: Impractical to measure height and length. Report as isolated reflector. Equate
reflectivity against disc area or DGS.

Figure 74: Gas pore.

Porosity
Multiple peak echo: Low intensity (20% CRT), broad at time base due to numerous ranges.

Probe movement: Orbit – echoes held with amplitude variations.

Measurement: Indicate area by pinpointing last maximum signal from traversing and lateral
scans.

Figure 75: Porosity.

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 Face and Body Flaws

Linear inclusion (slag)

Echo may be wide at time base and will be multi-faceted, due to having more than a single range.
Height will vary between 20-90%.

Probe movement:

Orbit (traversing is similar) – echo held with various maxima and minima.

Rotational – echo will drop quickly.

Lateral – will produce large variations in height, perhaps with total loss of signal for distances
shorter than the beam width.

Figure 76: Linear inclusion (slag).

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 Plate Inspection

Plate Inspection

EN 10160 describes methods for inspecting uncoated steel plates between 6 and 200 mm in
thickness for internal discontinuities. It details:

Three quality classes.

Equipment to be used.

Calibration requirements (two BWEs).

Coupling (normally with water but oil and paste are also acceptable).

Scanning plan – 200 or 150 mm grids, dependent upon the quality class.

Sensitivity setting.

Sizing techniques for defects (6 dB).

Acceptance criteria.

Reporting requirements.

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 Inspection Procedure

Inspection Procedure
EN ISO 17640 (NDT of welds – Ultrasonic testing – Techniques, testing, levels and assessment)
describes the procedure for examining welds and details beam paths to be used for welds of different
configurations, e.g. plates, pipes, nozzles and nodes. To standardise methods of examination, BS
EN ISO 17640 and BS EN ISO 16811 (NDT – Ultrasonic testing – Sensitivity and range setting)
recommend the use of DACs and different sensitivity settings to match the criticality of different
examinations. The sensitivity is set higher for examining, say, a high-pressure steam pipe in a
chemical plant than it is for a comparatively low-pressure line in a refinery.

Different procedures are followed in different projects. Usually, the test procedure is formulated
before the job starts so the technicians know exactly what is expected of them.

The following is the procedure laid down in EN ISO 17640, recommended for the inspection of
examination test pieces:

Compression scan to check the parent metal on either side of the weld for laminations and
to check through-thickness dimensions.

Root scan to check the root for longitudinal defects such as lack of penetration, lack of root
fusion, cracking or mismatch.

Weld body scan with shear probes to check the sides of the weld and the weld body for
longitudinal defects like lack of fusion, cracks, slag and porosity.

Transverse weld scan to check the weld for transverse and chevron cracking.

Reporting.

Information required prior to testing

BS EN ISO 17640 specifies that the following information should be specified before commencing an
inspection:

Method for setting the reference level.

Method to be used for evaluation of indications.

Acceptance levels.

Testing levels.

Manufacturing and operating stage at which inspection is to be performed.

Qualification of personnel.

Extent of testing for transverse indications.

Requirements for tandem testing.

Parent metal testing prior to and/or after welding.

Whether or not a written test procedure is required.

Requirements for written testing procedure.

BS EN 17640 also states that it is essential that the operator performing an inspection on a welded
joint shall have access to the following:

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 Inspection Procedure

Written test procedure.

Types of parent material and product form.

Manufacturing and operating stage at which inspection is to be performed.

Time and extent of post-weld heat treatment.

Joint preparation and dimensions.

Requirements for surface conditions.

Welding procedure or relevant information on the welding process.

Reporting requirements.

Acceptance requirements.

Extent of testing including requirements for transverse defects if relevant.

Personnel qualification level.

Procedures for corrective action when unacceptable indications are revealed.

BS EN 17640 itself would, in many cases, satisfy the requirement for a written test procedure

Compression scan

Make a visual examination of the weld. Note if there is any spatter, rust or inaccessible areas. Look
for surface defects, lack of fill, undercut and gross misalignment, cracks or surface porosity.

Find the centre of the weld. The stand-off must be measured from there. If the root is detectable,
find the root from both sides with a 60° probe. Mark where the index point falls on both sides of the
weld when the root signal is maximised; the weld centre is midway between these two marks. Note
the thickness of the parent metal on either side of the weld and ensure there is no counter boring.

Figure 77: Weld root detection.

Examine the parent metal on either side of the weld with a 0° compression probe with sensitivity
set at either 2nd BWE to FSH or the sensitivity given in the procedure. Look for changes in thickness
and lamination. You should cover the parent metal on either side of the weld to full skip distance for
the highest angle probe, that is for a 60° or 70° probe. Mark any discontinuities found in the parent
metal.

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 Inspection Procedure

Figure 78: Parent material comparison probe scan.

Draw a cross section of the weld on the slide of your beam plotting chart once you are sure of the
weld dimensions.

If the cap of the weld has been dressed ground flush with the parent metal, examine the body of
the weld with the 0° probe. If a backing bar has been used, check bonding at the root if possible.
Check the root on a double-sided weld if the cap has been ground flush. Lack of penetration at the
root will sometimes show with a 0° probe.

If you are examining a single V weld with a dressed cap, a 0° probe scan of the weld can reveal lack
of inter-run fusion and large pockets of slag.

Root scan

Use a 70° probe for the root scan if possible. Work out the half skip distance for a 70° probe to put
the beam centre exactly through the centre of the root.

Measure out the half skip distance on the parent metal on either side of the weld, measuring from
the centre line of the weld. Draw lines on either side of the weld at this distance. Note the range of
the root centre along the main beam.

Figure 79: Root Scan using an angle probe at a fixed ‘stand-off’.

Set the gain from an IOW block so that a hole at the depth of the root will give an echo to FSH. (Or
set the gain according to the set procedure.)

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 Inspection Procedure

Place the probe index point on the line and put a magnetic ruler or strip behind it as a guide. Move
the probe laterally along the weld. Signals from a good root should be small, while defects will give
large signals. Signals from a fully penetrated root will usually appear just beyond the range of the
root centre.

Figure 80: Fixed ‘stand-off’ scan using a straight edge.

Mark any defects on the parent metal or the magnetic guide behind your probe. Find the height of
the defect using a 20 dB drop on the trailing edge of your beam. Record the defect on a sketch and
note the stand-off distance and range.

Weld scan

Many standards and procedures demand that the first weld scan be done with a probe beam which
meets the joint face at as near to 90° as possible. This has almost always meant a 60° probe,
although probe angles have become steeper lately and 70° probes are increasingly used.

If a 60° probe is used first, follow up with a 70° probe if the plate thickness is less than 25 mm and
a 45° probe if it is over 20 mm. There is a bit of an overlap.

Use your beam plot and flaw location slide to work out the half skip distance to the root centre and
full skip distance plus half the weld width to the top of the weld. Mark these stand-off distances on
the parent metal on either side of the weld. Note the range of the root can cap.

Figure 81: Angle probe raster scanning from the weld cap edge to the full skip position.

Set the gain relative to DAC according to the sensitivity setting given in the procedure.

Move the 60° probe backwards and forwards along the weld on either side so that the beam covers
the sidewalls and centre body of the weld (see Figure 81). Defects should maximise between the

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 Inspection Procedure

root and cap signals. If any signal occurs near the cap, use a couplant-covered finger to damp any
signal that may be coming from the cap, thus identifying its position.

Make a second scan of the weld body with a 45° probe for materials with more than 20mm thickness
or a 70° probe for less than 25 mm thickness.

Check the area just under the cap with a 45° probe if you are examining thicker materials. You may
find porosity that the 60° probe has not revealed. Check any defect already noted and see if it is
longer than shown with the 60° probe.

On thinner materials, a 70° probe will confirm defects already noted and may help you distinguish
between signals from a defect and signals from, say, the weld cap or mode change.

Transverse scan

Place a 60° or 70° probe beside the weld cap angled slightly inwards (see Figure 82) and move the
probe along the weld to find transverse cracking. Turn the probe round and check in the opposite
direction. Examine the weld along the other side in both directions.

Figure 82: Shear wave angle probe scan to detect transverse defects.

If the weld has been dressed, push the probe along the centreline of the weld in both directions and
then push it along both edges of the weld in both directions.

Double V welds

If a weld had been welded from both sides, it can be examined from both sides. The method for
examining a double V weld is not much different from that used for a single V weld.

Examine the root with a 70° probe straight into the root at 1/4 skip (see Figure 83). Run the probe
laterally along the weld with sensitivity set from a hole in the IOW block at a suitable range; this
should show any lack of penetration at the root.

Figure 83: Double V weld root examination from ¼ skip.

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 Inspection Procedure

Make body scans with 60° and 45° probes from both sides of the weld if the plate is thick (over 25
mm) using ¼ to ½ skip stand-off.

Figure 84: Weld body scan between a ¼ and ½ skip stand-off using 45° and 60° probes.

On thinner plates, use a 60° and a 70° probe from one side of the weld only between ¼ and full skip
and ½ weld thickness stand-off positions. Take great care in locating the defects and reporting, as
you will have to use a mirror image on your plotting chart and this can lead to confusion. You may
find a defect and place it on the wrong side of the weld.

Pipes

Circumferential pipe welds can be examined in the same way as butt welds in plates are examined,
that is:

Compression scan to check the parent metal on either side of the weld for laminations and
to check through-thickness dimensions.

Root scan using a flexible strip at the back of the probe will help to ensure that the centre
of intensity of the beam goes into the centre of the root.

Weld body scan with shear probes to check the sides of the weld and the weld body for
longitudinal defects like lack of fusion, cracks, slag and porosity.

Transverse weld scan – difficulty may arise here as you must be sure that the beam
reaches the bore of the pipe.

This flexible guide is convenient for marking defects. All defects must be measured from a datum
point. The button at the top of the weld is a convenient datum point. When examining a longitudinal
weld on a pipe or when doing a transverse check on a circumferential weld, you must choose a probe
angle that will reach the bore of the pipe (see Figure 84). The formula below provides this angle.

Figure 85: Ultrasonic beam.

So, a 12 cm OD pipe with a 1 cm wall will need what probe angle?

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 Inspection Procedure

10
= 0.8333 which is the sine of 56
12

A 60° probe will not reach the bore of this pipe, so a 45° probe is advised.

Reporting

A detailed test report will normally be produced for each item of test and will cover all of the salient
parameters that affect the quality and integrity of the test as laid out in the test procedure, which
must be made available to the test technician/operator prior to starting the test along with written
instructions detailing the components to be tested, the specific test procedure,
specifications/standards and acceptance criteria to be applied along with any special instructions
that might apply (eg PPE to be used, use of photographs etc.). The test report will normally include
an assessment of the condition of the component against the specified acceptance criteria.

Specific details that may be included in the test procedure are as follows:

Title of the test procedure.

Description (including sketch/drawing if relevant) of components including materials and

surface condition.

Scope detailing general requirements of the test (eg type of ultrasonic instrument and
settings together with probe and scanning details).

Reference documents (eg codes, standards, client requirements, personnel qualifications).

Definitions and abbreviations used.

Responsibilities (personnel involved in the test sequence including identification of test

component, carrying out the test and making the area of testing safe).

Personnel qualifications (technician undertaking the test, evaluating the results/indications

and procedure preparation).

Technique procedure, equipment and settings (if applicable), initial cleaning, surface
preparation and access requirements.

Examination details, diagnostic area and scan overlap .

Interpretation of results and evaluation of indications against the acceptance criteria with a
sketch showing the positions of indications if required.

What follows is a sample NDT report. The headings within it are taken from BS EN ISO 17640,
(Ultrasonic Testing – Techniques, testing levels and assessment) which specifies these as the
minimum content.

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 Inspection Procedure

Table 6 Ultrasonic Test Report conforming to EN ISO 17640

Maker Type Serial number

Ultrasonic Test
Report

Name of the
Date of report
inspection

Reference standard

Item Inspected

Material

Product form

Dimensions

Location of weld

Specification

Operator Certification

Configuration

Stage of manufacture

Surface condition

Date of test

Equipment

Flaw detector

Ultrasonic Probes

Maker Type Frequency Serial number

Couplant

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 Inspection Procedure

Technique

Testing
levels

Extent of test

Location of
scanning
areas

Reference
point

Identification
of Probe
position

Time base
range

Sensitivity
level

Reference
level

Parent metal

Acceptance
level

Deviations
from
standard

Results

Co-ordinates Maximum Type Length Accept/Reject

amplitude

Figure 86: Ultrasonic Test Report conforming to EN ISO 17640

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 Ultrasonic Thickness Measurement

Ultrasonic Thickness Measurement

Standards to be used for reference include:

BS EN 14127. Non-destructive testing – Ultrasonic thickness measurement.

BS EN ISO 16811. Non-destructive testing – Ultrasonic testing. Sensitivity and Range

setting.

BS EN 1330-4. Non-destructive testing – Terminology. Part 4, Terms used in ultrasonic

testing.

Measurement modes

The precise thickness of a component part can be determined by accurately measuring the transit
time for a short duration ultrasonic pulse generated by a transducer to travel through the material
thickness once, twice or several times.

The material thickness can be calculated by multiplying the sound velocity of the material with the
transit time and dividing the result by the number of times that the pulse has transited the material
thickness.

Four ultrasonic measurement modes are given below and illustrated in Figure 87:

Mode 1: Measure the transit time from an initial excitation pulse to a first returning echo,
minus a zero correction to account for the thickness of the transducer wear surface and the
couplant layer (single echo mode).

Mode 2: Measure the transit time from the end of a delay line to the first back wall echo
(single echo delay line mode).

Mode 3: Measure the transit time between back wall echoes (multiple echoes).

Mode 4: Measure the transit time for a pulse travelling from the transmitter to a receiver in
contact with the back wall (through transmission mode).

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 Ultrasonic Thickness Measurement

Figure 87: Ultrasonic Measurement Modes.

22.1.1 Requirements test object, instruments, probes and reference blocks

Test Object

The object to be tested shall enable ultrasonic propagation/transmission and have access to apply
the probe to the test surface that will be free of all dirt, grease, scale or any material that could
interfere with the examination.

If the surface to be tested is coated, the coating will have tight adhesion to the base parent material,
otherwise it must be removed.

When measuring through a coating, the coating thickness and sound velocity need to be known
unless Mode 3 above is being used.

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 Ultrasonic Thickness Measurement

Ultrasonic Instruments

The following Ultrasonic measurement instruments can be used:

Dedicated ultrasonic thickness measurement instrument with a numerical display that shows the
measured thickness.

Dedicated ultrasonic thickness measurement instrument with a numerical display that shows the
measured thickness and the ‘A’-scan waveform presentation.

‘A’-scan display instruments that are designed primarily for flaw detection and may also include a
numerical display of measured thickness.

Probes

Generally, compression (longitudinal) wave probes are used based on either of the following
configurations:

Dual element probes (see Probe A3 in Figure 87Figure 87: Ultrasonic Measurement Modes.)

Single transducer probes (see Probe A in Figure 87)

Reference Blocks

The ultrasonic measurement system (i.e. instrument and probe) must be calibrated on a sample(s)
or reference block that closely represent the component whose thickness is to be measured in terms
of dimensions, material and structure and the reference sample thickness range should cover the
range of component thicknesses to be measured. Either the thickness or the sound velocity of the
reference sample block shall be known.

Thickness calibration reference blocks are commercially available for steel and aluminium in the form
of ladder step wedges and curved step wedges.

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 References

References

EN ISO and other Standards

[1] BS EN ISO 11666. Founding – Non-destructive testing of welds – Ultrasonic testing –

Acceptance levels, 2018.

[2] BS EN 1330-1. Founding – Non-destructive testing – Terminology – Part 1: List of general

terms, 2011.

[3] BS EN 1330-2 Founding – Nondestructive testing – Terminology – Part 2: Terms common

to NDT methods, 2017.

[4] BS EN 1330-4, Founding – Non-destructive testing – Terminology – Part 4: Terms used in

ultrasonic testing, 2017. 137p.

[5] BS EN ISO 23279, Founding – Non-destructive testing of welds – Ultrasonic testing –

Characterization of indications in welds, 2017. 112p.

[6] BS EN ISO 17640.Founding – Non-destructive testing of welds – Ultrasonic testing –

techniques, testing levels and assessment, 2017. 92p.

[7] BS EN ISO-16810, Founding – Non-destructive testing – Ultrasonic Testing.General

principles, 2017.

[8] BS EN ISO 16811, Founding – Non-destructive testing – Ultrasonic Testing.Sensitivity and

range setting, 2017. 125p.

[9] BS EN ISO 16823. Founding – Non-destructive testing – Ultrasonic Testing.Transmission

technique, 2011.

[10] BS EN ISO 16826., Founding – Non-destructive testing – Ultrasonic Testing. For

discontinuities perpendicular to the surface, 2011.

[11] BS EN ISO 16827, Founding – Non-destructive testing – Ultrasonic Testing.Characterization

and sizing of discontinuities, 2017.

[12] BS EN 10160, Founding – UT of steel flat product of thickness equal to or greater than 6mm
(reflection method), 2017. 123p.

[13] BS EN 10228-3, Non-destructive testing of steel forgings. Part 3: Ultrasonic testing of

ferritic or martensitic steel forgings, 2017.

[14] BS EN 10228-4 Non-destructive testing of steel forgings. Part 4: Ultrasonic testing

of austenitic and austenitic-ferritic stainless-steel forgings,2016

[15] BS EN ISO 17635, Non-destructive examination of welds – General rules for metallic
materials,2016

[16] BS EN ISO 2400 Non-destructive testing – Ultrasonic Testing Specification for

calibration block No.1, 2012, 76p.

[17] BS EN 12668-1 Non-destructive testing – Characterization and verification of ultrasonic

examination equipment. Part 1: Instruments, 2010

[18] BS EN 12668-2 Non-destructive testing – Characterization and verification of

ultrasonic examination equipment. Part 2: Probes, 2010.

[19] BS EN 12668-3 Non-destructive testing – Characterization and verification of

ultrasonic examination equipment. Part 3: Combined equipment, 2013, 22p.

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 References

[20] BS EN 12680-1 Founding – Ultrasonic examination. Part 1: Steel castings for general
purposes. 2003.

[21] BS EN ISO 7963 Non-destructive testing – Ultrasonic testing – Specification for calibration
block No. 2. 2010, 78p.

[22] M 36 Ultrasonic testing of special forgings by an immersion technique using flat bottomed
holes as a reference standard, 1970, 59p.

[23] M 38 Guide to compilation of instructions and reports for the in-service non-destructive
testing of aerospace products, 1971.

[24] EN 473 Superseded by BS EN ISO 9712, 2008.

[25] EN ISO 9712 Non-destructive testing. Qualification and certification of personnel, 2012

[26] EN 4179 Aerospace series. Qualification and approval of personnel for non-destructive
testing. 2017

[27] ISO 18175 Non-destructive testing. Evaluating performance characteristics of ultrasonic

pulse-echo testing systems without the use of electronic measurement instruments – First
Edition, 2004.

[28] BS EN 14127 Non-destructive testing. Ultrasonic thickness measurement, 2011, 136 p.

[29] EN ISO 6520-1 Welding and allied processes — Classification of geometric imperfections in
metallic materials — Part 1: Fusion welding, 2007

Other relevant documents

Associated Reading

NDT Ed.org – Introduction to ultrasonic testing

http://www.ndted.org/EducationResources/CommunityCollege/Ultrasonics/cc_ut_inde
x.htm

Procedures and 'Recommendations for Ultrasonic Testing of Butt Welds' 2nd edition. The Welding
Institute

'Ultrasonic Flaw Detection for Technicians' by J C Drury. Obtainable from the British Institute of Non-
Destructive Testing

Mathematics and Formulae in NDT. Edited by Dr. R Halmshaw. Obtainable from the British Institute
of Non-Destructive Testing

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 Glossary

Glossary

The following is a compilation of the more common terms used in connection with radiographic
testing. Other lists can be found in EN 1330-3 and ASME V, Appendix A.

A
Acoustic emission Method of flaw detection which uses an array of accurately positioned transducers
(AE) to listen to the structure under stress. The high-frequency sound given out by a
growing crack would be detected by the transducer array.

Acoustic impedance Coupling of two media together to provide optimum transfer of acoustical energy
matching between them.

Acoustical shadow Effect produced in a body by its geometry or by a discontinuity in it, whereby
ultrasonic energy when travelling in a particular direction is prevented from
reaching a certain region within the body.

Angle of incidence Angle which the axis of an ultrasonic beam makes with the normal to a tangent
plane of a surface at its point of impingement as it travels towards that surface

Angle of reflection Angle which the axis of an ultrasonic beam makes with the normal to the tangent
plane of a reflecting interface, at the point of impingement of the incident wave,
as it travels away from that interface in the same medium.

Angle of refraction Angle which an ultrasonic beam makes with the normal to a tangent plane of an
interface as it travels away from that interface into the second material.15

Angle of squint Angle between the side edge of the probe and the projection of the beam axis on
the plane of the probe face.16

Angle probe Contact probe from which the main lobe of waves propagates at any angle other
than 0° or 90° to the normal to the tangent plane of the surface at the place
where the probe is positioned.

A-scan Ultrasonic flaw detector display in which the pulse amplitude is represented as a
displacement along one axis (usually the Y axis) and the travel time of the
ultrasonic pulse is represented as a displacement along the other axis (usually the
X axis).

ASME Cross-drilled holes with diameter and position as required by the ASME pressure
vessel code.17.

Attenuation Diminution in the level of acoustic energy as it propagates through material

Attenuation Factor determined by the diminution in the amplitude of a wave per unit distance
coefficient travelled.18

Attenuator Electrical device by which the amplitude of an ultrasonic signal can be adjusted by
calibrated increments.

Automatic scanning Systematic relative displacement of the ultrasonic beam and the material under
test by other than manual means

AVG/DGS diagram. Series of curves which show the relationship between distance along the beam to
gain in dB compared with a particular back wall echo and the size of a particular
flat-bottom hole reflector

15
A synonymous term is beam angle

16
For angle probes this normally relates to deviation in the lateral direction

17
ASME = American Society of Mechanical Engineers

18
The attenuation coefficient is composed of two parts, one proportional to frequency (termed absorption), the
other dependent on the ratio of grain size to wavelength and arising from scatter

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 Glossary

Back wall echo Pulse of ultrasonic energy reflected from the boundary of a body directly opposite
(BWE) to the surface on which the probe(s) is/are positioned and returned to that surface
by the shortest possible path.19

Beam angle See angle of refraction.

Locus of points of maximum intensity in the far field in a beam of ultrasonic

Beam axis. waves, and its geometrical prolongation into the near field.

Point on the surface of a body through which the beam axis passes (cf. probe
Beam index index).

Divergence of the main lobe of an ultrasonic beam in the far field.20

Beam spread.

Bottom echo (first) See back wall echo (first

B-scan display Two-dimensional graphical plot showing the apparent size and position of
reflectors in the test piece on a cross-sectional plane which is normal to the test
surface and contains the beam axis of the probe during a single line scan

Calibration block Piece of material of specified composition, heat treatment, geometric form and
surface texture, by means of which the performance of ultrasonic flaw detection
equipment can be assessed and calibrated for the examination of material of the
same general composition.21

Calibration reflector Reflector of ultrasonic waves, such as a drilled hole, machined slot or the end face
of a specimen representative of the material under test, which can be used to
calibrate or set up equipment for inspection purposes

Characteristic Complex ratio of sound pressure to particle velocity at a point in the path of a
impedance purely progressive sound wave. For a non-dissipative material it is equal to the
product of density and velocity.

Combined double See double crystal probe

probe.

Constructive When two positive or negative sound waves from two sources (Huygens's
interference Principle) meet at a point at the same instant, the wave is reinforced and assists
the sound to propagate into the material under test. This is constructive
interference and takes place in the near field. See also destructive interference.

Compressional wave Form of wave motion in which the particle displacement at each point in a material
is parallel to the direction of propagation.22

Contact scan Scanning carried out by means of ultrasonic probe(s) in contact with the body
under examination

Convergence point Point of intersection of the axes of the transmitting and receiving sound fields in a
double crystal probe

Corner effect Reflection of ultrasonic energy back to a point coincident with or very close to its
point of origin, after impinging successfully on two or three orthogonal surfaces.

19
The term is generally restricted to normal compressional waves and is sometimes referred to as bottom echo
(first)

20
The beam spread is proportional to the ratio of the wavelength to the diameter of the ultrasonic crystal

21
For specification and use of calibration blocks, see EN ISO 2400 and EN ISO 7963, respectively

22
A synonymous term is longitudinal wave; also sometimes referred to as dilatational wave or irrotational wave.

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 Glossary

Couplant Liquid or pliable medium interposed between two solids to assist the passage of
ultrasonic waves between them.23

Coupling film See couplant

Coupling losses Reduction in amplitude of ultrasonic waves as a result of their passage through
the couplant

Coupling medium See couplant

Coupling monitor Probe operating in the receive mode and positioned such that it detects ultrasonic
energy originating from a second probe or multiple probes, thereby monitoring
that successful coupling is taking place between the second probe and the body.

Critical angle Angle of incidence of a beam of ultrasound on an interface at which one of the
refracted wave modes has an angle of refraction of 90 o

Cross-drilled holes Cylindrical holes drilled parallel to the test surface and at right angles to the
vertical plane of the probe, the cylindrical surfaces of which form the ultrasonic
reflectors.

Cross talk Acoustical or electrical signal leakage across an intended barrier.24

Crystal (ultrasonic) Part of a single crystal or polycrystalline plate having piezo-electric properties,
used for the generation and/or detection of ultrasonic waves

Crystal array Single housing containing an orderly assembly of crystals which may be energised
together in groups, with or without time delay, to give directional effects, focused
beams or variable angle beams

Crystal backing Material attached to the rear surface of a crystal to increase damping

Crystal loading Mechanical power per unit surface area delivered by a crystal to a medium
acoustically coupled to it.

Crystal mosaic Regular assembly of ultrasonic crystals, in which each crystal is of identical
material and cut and mounted so that the assembly of crystals tends to behave as
though it were a single crystal

C-scan display Two-dimensional graphical projection of the test surface showing in plan view the
apparent size and position of reflectors in the volume inspected by scanning an
area of test surface

Curie point Temperature above which a ferromagnetic material loses its polarisation

Curved crystal Non-planar crystal generally used to improve coupling or focusing.

Cylindrical reflector Reflecting surface in the form of a circular cylinder

D
Damped train . Wave train in which the amplitudes of successive waves diminish

Dead zone Region in a material adjoining the surface of entry from which no direct echoes
from discontinuities can be detected due to the characteristics of the ultrasonic
equipment in association with the material under test and its surface condition

Decay technique Technique of using ultrasonic waves to assess the quality of a material or a bond
by studying the amplitudes of successive echoes.

Decibel (dB) Unit used to express the magnitude of change in the amplitude of an ultrasonic
signal, defined by the equation: dB = 20 log10 (A1/A0), where A0 is a reference
amplitude.

Defect detection A particular level of sensitivity setting of an ultrasonic flaw detector for revealing
sensitivity the presence of defects in a given application

23 In the majority of cases, the couplant is a liquid interposed between the probe and the body under examination.
Synonymous terms are coupling film and coupling medium

24 Sometimes referred to as cross noise

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 Glossary

Delayed time-base Cathode ray tube display in which the initial part of the time scale is not shown.
sweep

Depth scan Manipulation of an ultrasonic shear wave probe over the surface of a body so as to
cause an oblique beam to traverse a particular plane section of the body.

Destructive When a positive sound wave from one point source (Huygens's Principle) passes a
interference negative sound wave from a second point source at the same instant, the pressure
sound wave is nullified at that point. This is destructive interference, which results
in areas of maximum and minimum pressure giving spurious indications from any
reflectors in this area. This area of interference is the near field. See also
constructive interference

Diffuse reflection Reflection of an ultrasonic wave from a rough surface so that the reflected energy
is detectable over a range of angles on either side of the theoretical angle of
specular reflection, ie reflection in a non-specular manner.

Dilatational wave See compressional wave

Directional Relationship between the angle made with the normal to the surface of a reflector
sensitivity by a beam of ultrasonic waves and the amplitude to the resultant echo.

Direct scan See single traverse technique and indirect scan.

Dispersive medium Material in which the phase velocity of an ultrasonic wave varies with frequency.

Display Form in which ultrasonic data is presented for interpretation, generally on a

cathode ray tube. See A-scan, B-scan, C-scan and D-scan

Distance amplitude Change in amplification of ultrasonic signals to provide equal amplitude from equal
correction (DAC) reflectors at different distances in the same material.

Distance amplitude Curve constructed from the peak amplitude responses from reflectors of equal
curve area at different distances in the same material.

Distortional wave See shear wave

Double bounce See triple traverse technique

technique

Double crystal probe Probe comprising two separate crystals in a single housing, one acting as a
transmitter and the other as a receiver. Note: Synonymous terms are twin crystal
probe and combined double probe

Double probe . Ultrasonic testing technique using one probe for transmission and the other for
technique reception.

Double skip Ultrasonic testing technique where the distance between the point where the
technique waves enter the body and the region under examination is twice the skip distance.

Double transceiver Ultrasonic testing technique involving the use of two probes, each used
technique simultaneously as a transmitter and a receiver

Double traverse Testing technique in which a beam of ultrasonic waves is directed into a region of
technique a body under examination after having been reflected by a surface of the body.
Note: A synonymous term is single bounce technique

D-scan Two-dimensional graphical projection onto a plane normal to the test surface and
to the projection of the beam direction on the test surface, showing the apparent
size and position of reflectors in the volume inspected by scanning an area of test
surface.

Dynamic range Range of signal amplitude that can be handled by electronic or ultrasonic
equipment without overloading or excessive distortion and without being too small
for detection. Note: Usually expressed in decibels (dB).

Echo. Distinct pulse of ultrasonic energy reflected from any surface or discontinuity

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Electro-magnetic Transducer in which eddy currents are produced at a conducting surface adjacent
acoustic transducer to the transducer in the presence of a static magnetic field, the interaction of the
(EMAT) two fields producing a mechanical deformation of the surface thereby generating
ultrasonic vibrations and vice versa. Note: Often referred to as an EMA transducer.

Electronic noise. Unwanted random signals that vary rapidly with time, caused by electronic pick-up
and thermal noise in the amplifier of the flaw detector

Expanded time-base Increased speed of time-base spot sweep which enables echoes from a selected
sweep. region within the thickness or length of a body to be displayed in greater detail on
the screen of the ultrasonic flaw detector

Far field Region in an ultrasonic beam where the intensity is inversely proportional to the
square of the distance. Note: Sometimes referred to as the Fraunhofer region

First critical angle Angle of incidence of a longitudinal wave in one medium such that the refracted
longitudinal wave is 90° in the second medium, ie along the surface, only the
transverse wave being transmitted into the second medium.

Flat-bottom hole Cylindrical blind hole with a flat bottom, the flat bottom being used as the
(FBH) ultrasonic reflector

Flat-bottom hole Size of flat-bottomed hole which gives an ultrasonic indication equal to that from
equivalent the discontinuity, at the same range.

Flaw location scale Specially graduated device that can be attached to a shear wave probe which, in
conjunction with the position of the flaw echo on the screen of the cathode ray
tube, gives a direct reading of the location of the discontinuity within the body.

Focused probe Probe incorporating an acoustic lens or a suitably curved crystal, so as to produce
focusing of the ultrasonic beam.

Frequency (f) Number of cycles or complete particle oscillations in one second, expressed in
hertz (Hz).

Fresnel region See near field.

Full-skip technique Ultrasonic testing technique whereby the inspection of a surface region of a body
is accomplished by using shear waves entering the same surface at a point one
skip distance away.

Gap scanning Form of scanning in which the probe carrier follows the contour of the material
under examination but the probe, whilst not in direct contact with its surface, is
coupled to it through a layer or jet of liquid which is maintained between the
surfaces of the probe and the material

Gate . Electronic means of monitoring a selected region of the cathode ray tube display
of an ultrasonic flaw detector

Ghost echo Indication arising from an incorrectly matched combination of pulse repetition
frequency and time base frequency

Ghost images See ghost echo

Grass Spatially random signals arising from the reflection of ultrasonic waves from grain
boundaries and/or microscopic reflectors in a material.

Half skip technique Ultrasonic testing technique in which the inspection of a surface region of a body is
accomplished by using a shear wave beam entering from the opposite surface at a
point corresponding to the half skip-distance.

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Hard face probe Probe in which the contact surface is of a hard material, such as steel or ceramic,
to minimise wear

Head wave Shear wave generated by mode transformation when a compressed wave travels
at a grazing angle on a free solid surface. Note: In steel, the head wave is at 33°

Holography Ultrasonic image from two transducers, the beams of which are positioned to
(ultrasonic) produce an interface pattern, usually on a liquid surface, which when illuminated
by laser light produces a visible indication of ultrasonic wave intensity distribution

Huygens’s principle States that any finite source of sound is considered to be constructed of an infinite
number of point sources, with sound radiating out from each.

Immersion probe Compressional wave probe designed to be used while immersed in a liquid

Immersion testing Ultrasonic testing technique in which the material under test and the probe(s) are
immersed in a tank of water or other liquid.

Indexing Automatic measuring of probe position, usually electrically, to generate probe

position data that can be recorded

Indirect scan Use of surface(s) of a body to direct an ultrasonic beam into a particular region of
the body.

Interface Transition region between two materials of different characteristic impedance in

acoustical contact

Interface signal Displayed ultrasonic signal arising from the part reflection of an incident pulse at
an interface

Interface trigger Interface signal used as the initiating point from which other timing sequences (eg
gate position) are referenced

Internal echoes Unwanted signals generated within an ultrasonic probe

Irrotational wave See compressional wave.

Lamb wave Term applied to those modes of vibration which propagate in a plate. Note 1: A
synonymous term is plate wave. Note 2: In general, both compressional and shear
elasticity are involved, together with plate thickness and frequency; also, the
propagation shows dispersion.

Lateral resolution Ability of an ultrasonic flaw detection system to give separate indications from two
reflectors having the same range within the sound beam. See also range
resolution

Logarithmic amplifier Amplifier where the output is related logarithmically to the amplitude of the input
signal

Logarithmic For a damped train, the natural logarithm of the ratio of the peak values of the
decrement amplitudes of two successive cycles.

Longitudinal wave See compressional wave.

Love wave Acoustical wave which propagates along a stratum bounded on both sides by
layers of material which differ from the stratum in their elastic properties. Note:
The particle displacement of the wave is parallel to the wave front and to the
stratum.

Magnetostrictive Transducer in which the application of a magnetic field on the active element of
effect/transducer the transducer produces mechanical deformation of the active element, thereby
generating ultrasonic vibrations and vice versa.

Maximum working Total distance over which a probe will transmit sufficient energy to find the
range smallest reflector to be detected.

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 Glossary

Mode conversion See mode transformation

Mode transformation Process by which a wave of a given mode of propagation is caused to generate
waves of other modes by reflection or refraction at a surface boundary

Multiple echo Repeated reflection of an ultrasonic pulse between two or more surfaces or
discontinuities in a body.

Multiplexer Device for electrically connecting probes to channels in sequence.

N-distance Distance from the probe to the N-point (see M36).

N-point Position in an ultrasonic beam where the intensity of sound on the beam axis
reaches a final maximum before beginning a uniform reduction with distance (ie
far zone; see M36)

Near field Region in an ultrasonic beam subject to variations in intensity due to diffraction
effects, extending from the source of radiation to the last axial maximum in
intensity. Note: Synonymous terms are near zone and Fresnel region.

Near zone See near field

Normal probe Probe from which waves propagate at 90° to its contact surface.

Parasitic echo See spurious echo.

Piezo-electric Transducer in which the application of an electric field across the active element
effect/transducer produces mechanical deformation of the active element thereby generating
ultrasonic vibrations and vice versa

Pitch and catch Ultrasonic testing technique involving the use of two separate probes, one being
technique used to transmit the ultrasonic energy into the body and the other positioned so
as to receive the reflected energy from a discontinuity.25

Plane wave Wave in which points of the same phase lie on parallel plane surfaces

Plate waves See Lamb waves.

Poisson’s ratio Ratio of transverse strain to tensile strain.

Primary scan axis Major direction of probe scanning movement.

Probe Electromechanical device, usually incorporating one or more ultrasonic crystals,

and functioning as a generator and/or receiver of ultrasonic waves.

Probe array Array of probes which may comprise: (1) Probes in a mechanical holder which
scan together and are used sequentially, individually and/or in pairs. (2) A single
unit comprising probes used as in (1).

Probe face Part of a probe through which ultrasonic waves are transmitted and received.

Probe index Point on a shear wave or surface wave probe through which the emergent beam
axis passes.26

Probe shoe delay Time taken for the transmitted ultrasonic wave to traverse the probe shoe and to
be reflected back to the ultrasonic crystal.

Proportional output Output signal from ultrasonic or electronic equipment which is proportional to the
peak amplitude of an input ultrasonic pulse, such as a defect echo

Pulse Short electrical or acoustical wave train.

Pulse amplitude Pulse height of a signal, usually base to peak, when displayed in an A-scan

Pulse Duration See pulse length.

25
In variations of the technique, more than two probes may be used.

26
The index can vary slightly depending on the method of measurement

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Pulse echo technique Technique in which the presence of a discontinuity in a material is indicated by a
reflection of pulses from it

Pulse energy Total energy associated with a single pulse.

Pulse envelope Outline of a pulse indication.

Pulse length Time interval between the leading and trailing edges of a pulse, usually measured
at the half-amplitude value. Note: Synonymous terms are pulse duration and
pulse width.

Pulse repetition Number of pulses transmitted per second.

frequency (PRF)

Pulse width See pulse length.

Quadruple traverse Technique in which a beam of ultrasonic waves is directed into a region of a body
technique under examination, after having been reflected successfully three times by
surfaces of the body. Note: A synonymous term is triple bounce technique.

Range resolution Ability of an ultrasonic flaw detection system to give separate indications from two
reflectors at similar range within the sound beam. See also lateral resolution

Rayleigh waves Particular type of surface wave which propagates on the surface of a body with
effective penetration of less than a wavelength

Reference piece An aid to interpretation in the form of a test piece of the same nominal
composition, significant dimensions and shape as a particular object under
examination. Note: Such pieces may or may not contain natural or artificial
imperfections.

Reference standard Artificially produced imperfection of predetermined dimensions, usually a notch or

hole, used for the sole purpose of establishing the test sensitivity of the ultrasonic
equipment

Reflecting surface Interface at which the ultrasonic beam encounters a change in characteristic
impedance

Reflection factor See reflection coefficient.

Reflection technique Technique in which the presence of discontinuities in a material is indicated by

receiving the reflected energy from them.

Refracting prism A prism, usually of plastic material, which when placed in acoustical contact
between an ultrasonic crystal and a body will cause ultrasonic waves to be
refracted at a known angle into that body

Refractive index Ratio of the velocity of an incident wave in one material to the velocity of a
refracted wave in a second material in acoustical contact with the first material.

Reject (rejection) Reduction of grass by the elimination of all signals below a predetermined
amplitude. Note: A synonymous term is suppression

Resolution Ability of an ultrasonic flaw detection system to give separate indications of

discontinuities having nearly the same range and/or lateral position with respect
to the beam axis.

Resonance technique Examination technique which involves varying the frequency of ultrasonic waves to
excite a maximum amplitude of vibration in a body, or part of a body, generally
for the purpose of determining thickness from one side only.

Reverberation time Time required for the intensity of an unsustained vibration in a closed system to
decrease to one millionth of its initial value, e.g. by 60dB.

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Ringing time Time during which the mechanical vibrations of a crystal continue after the
electrical pulse has stopped.

Rotational wave See shear wave.

Scale expansion See expanded time-base sweep

Scan pitch Pitch or distance between lines of scan during passage of the probe(s) over the
scan area

Scanning Systematic relative displacement of the ultrasonic beam and the material under
test.

Scatter (ultrasonic) Energy reflected in a random way by small reflectors in the path of a beam of
ultrasonic waves (eg grain boundaries).

Schlieren system Optical system used to display an ultrasonic beam visually, by passing it through a
transparent medium.

Screen marker Small electronically generated pulses following one another at a preset time
interval, presented on a time-base sweep to provide a calibration less dependent
on the linearity of the time base.

Second critical angle Angle of incidence of a longitudinal wave in one medium such that the refracted
transverse wave is at 90o, ie along the surface of the second medium.

Sensitivity Ability of an ultrasonic system to identify a small reflector in the far distance.

Sequence number In an automatic testing system, the order of connection of channels and probes
required to perform defined scans.

Shadow technique Technique in which a discontinuity in a material is revealed by the acoustical

shadow it produces.

Shadow zone Region in a body which cannot be reached by ultrasonic energy travelling in a
given direction, because of the shape of the body or a discontinuity in it.

Shear wave Form of wave motion in which the particle displacement at each point in a material
is at right angles to the direction of propagation. Note: Synonymous terms are
distortional wave, rotational wave and transverse wave.

Shear wave probe Probe for generating or detecting shear waves.

Short pulse . Pulse which has few (usually less than 1.5) cycles in the time interval over which
its amplitude exceeds half of its maximum amplitude

Side lobe Peak or pronounced shoulder in an ultrasonic beam lying to one side of the main
beam.

Signal-to-noise ratio Ratio of the amplitude of an ultrasonic echo arising from a discontinuity in a
(SNR) material to the amplitude of the average background noise.

Single bounce See double traverse technique.

technique.

Single probe Technique involving the use of a single crystal probe for both transmitting and
technique detecting ultrasonic waves.

Single traverse Examination technique in which a beam of ultrasonic waves is directed into a
technique region of a body under examination without intermediate reflection. Note: A
synonymous term is direct scan

Sizing technique Technique which enables an estimate of the size of a discontinuity to be made
from its ultrasonic responses. Note: Examples of sizing techniques are 6dB drop
(half maximum) technique, 20dB drop technique and maximum amplitude
technique.

Skip distance For a beam of shear waves entering a body, the distance measured over the
surface of the body between the probe index and the point where the beam axis
impinges on the surface after a single reflection from the opposite surface.

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Snell’s law States that the angle of refraction is a function of the angle of the incident beam
and the change in relative velocity between the two materials.

Soft-faced probe Probe in which the contact surface is a flexible membrane and a space between
the crystal and membrane is filled with a liquid couplant.

Soft-tipped probe Probe in which a thick flexible medium forms the coupling between its crystal and
the surface of the material under test.

Sound attenuation Reduction in the level of sound intensity due to distortion, scatter and beam
spread.

Specific acoustic Property of a medium which determines the amount of reflection that occurs at an
impedance interface with another medium. Note: It is defined mathematically as the product
of the density of the medium and the velocity of the wave travelling through it.

Specular reflection A mirror-like reflection of an ultrasonic beam such that the angle of incidence is
equal to the angle of reflection

Spherical reflector Surface of spherical or near spherical form, separating two media of different
characteristic impedance.

Spherical wave Wave in which points of the same phase lie on the surfaces of concentric spheres.

Spurious echo Term used for any indication not obviously associated with a discontinuity or
boundary. Note: A synonymous term is parasitic echo.

Spurious indication See spurious echo.

Stand-off Block, usually of plastic material, which serves to separate the ultrasonic crystal(s)
from the surface of the test piece. Note: The use of such blocks is generally
confined to compression wave probes.

Stationary wave Effect produced by the superposition of wave trains moving in opposite directions
with the formation of stationary nodes and antinodes

Subsidiary maxima Irregular fluctuations in the response of a small reflector as an ultrasonic beam is
scanned over it.

Suppression See reject.

Surface noise Unwanted signals at very short range, produced by ultrasonic waves being
reflected within the coupling film and from irregularities of the surface

Surface preparation Processing of a surface necessary to render it suitable for providing good
acoustical coupling for ultrasonic testing

Surface wave Ultrasonic wave which propagates on the surface of a body.

Surface wave probe Probe for generating and/or detecting surface waves.

Swivel scan Shear wave technique used to provide information about the form of a previously
located discontinuity, the probe(s) being positioned at a constant distance from
and directed at the discontinuity and rotated by an angle of up to 360 o.

Tandem probe Technique involving the use of two probes, one transmitting the ultrasonic energy
technique into the body and the other positioned to pick up any energy reflected from a
discontinuity. Note: The probes are usually scanned together at a fixed separation
and the technique is mostly used for the detection of vertically oriented, through-
wall, planar defects.

Test block Piece of material capable of propagating ultrasonic waves and suitable for
assessing particular features of ultrasonic equipment performance.

Test surface Surface of a piece of material through which ultrasonic waves pass.

Threshold Minimum signal amplitude that is regarded as significant in a particular ultrasonic

examination.

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Time base Trace on the screen of a cathode ray tube which is generated in such a way that
distance along it is proportional to time.

Time base range Maximum ultrasonic path length that can be displayed on a particular time base.

Time corrected gain Facility of flaw detectors to represent flaws of equal reflective size with the same
screen amplitude, irrespective of their depth in the material.

Time marker See screen marker.

Toe-in-semi-angle Half the angle between the normals to the crystal faces in a twin crystal probe.

Total attenuation Diminution of intensity of a particular mode, with travel range, of an ultrasonic
beam of any form arising from the combined effects of absorption, scatter and
geometric beam spread.

Total internal Reflection which occurs when the angle of incidence is greater than the critical
reflection angle and the reflection coefficient is unity.

Transceiver Probe used to generate and detect ultrasonic energy.

Transducer Electroacoustical device for converting electrical energy into acoustical energy and
vice versa.

Transmission Ratio of ultrasonic wave intensity transmitted across an interface to the total wave
coefficient energy incident upon the interface

Transmission point Point on the time base which corresponds to the instant at which ultrasonic energy
enters the material under examination.

Transmission Technique in which the quality of a material is assessed by the intensity of the
technique. ultrasonic radiation incident on the receiving probe after the waves have travelled
through the material.

Transverse wave. See shear wave.

Trigger/alarm Condition where the equipment indicates that a piece of material is suspect.
condition.

Trigger/alarm level. Level at which the ultrasonic equipment is required to differentiate between
acceptable and suspect material.

Triple bounce See quadruple traverse technique.

technique.

Triple traverse Technique in which a beam of ultrasonic waves is directed into a region of a body
technique. under examination after having been reflected successively by two surfaces of the
body. Note: A synonymous term is double bounce technique.

Twin crystal probe. See double crystal probe.

Ultrasonic frequency. Any frequency of vibration greater than the range of audibility of the human ear,
generally taken as greater than 20kHz.

Ultrasonic mode Device which causes vibrations of a particular mode (eg compressional) in one
changer. body to produce vibrations of another mode (eg shear) in another body.

Wavelength (λ). Perpendicular distance between two wave fronts with a phase difference of one
complete period.

Ultrasonic wave. Disturbance which travels through a material at ultrasonic frequency by virtue of
the elastic properties of that material.

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Wedge (ultrasonic). Device placed between the probe and the test surface for the purpose of causing
ultrasonic waves to pass between the two at a particular angle.

Wetting Agent. Substance added to a coupling liquid to decrease its surface tension.

Young’s modulus of In an elastic material, the ratio of tensile stress to tensile strain.
elasticity.

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