Chapter 6
Ultrasonic Testing
AN INTRODUCTION TO THE WORLD OF SOUND
Physicists and acoustic engineers tend to discuss sound pressure levels (SPLs)
in terms of frequencies, partly because this is how our ears interpret sound.
What we experience as “higher pitched” or “lower pitched” sounds are pressure vibrations having a higher or lower number of cycles per second. In a
common technique of acoustic measurement, acoustic signals are sampled in
time and then presented in more meaningful forms such as octave bands or
time frequency plots. Both of these popular methods are used to analyze sound
and better understand acoustic phenomena.
The fundamental principles being the same, there are more advanced developments in the field, and basic principles are dissected and used to address
more specific needs of the ever-growing industry. Some of these specialized
“subsections” or new developments in ultrasonic testing are listed below. In
this chapter, some of these will be addressed, but for details, readers are
directed to the specialized industry expert companies because some of these
are proprietary techniques.
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UT: shear wave ultrasonic testing
AUT: automated ultrasonic testing
PAUT: phased array ultrasonic testing
TOFD: time of flight diffraction ultrasonic testing
GWUT: guided wave ultrasonic testing
LGWUT: long-range guided wave testing
SWUT: surface wave ultrasonic testing technology
THEORY OF SOUND WAVE AND PROPAGATION
In fluids such as air and water, sound waves propagate as disturbances in the
ambient pressure level. Although this disturbance is usually small, it is still
audible to the human ear. The smallest sound that a person can hear, known
as the threshold of hearing, is nine orders of magnitude smaller than the
ambient pressure. The loudness of these disturbances is called the SPL, and
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it is measured on a logarithmic scale in decibels. Mathematically, SPL is
defined as:
SPL ¼ 20 log10 ðP=Pref Þ
where Pref is the threshold of hearing and P is the change in pressure from
the ambient pressure.
Table 3-6-1 gives a few examples of sounds and their strengths in decibels
and Pascals.
The entire sound spectrum can be divided into three sections: audio,
ultrasonic, and infrasonic. The audio range falls between 20 and 20,000 Hz.
This range is important because its frequencies can be detected by the human
ear. This range has a number of applications, including speech communication
and music.
The ultrasonic range refers to the very high frequencies: 20,000 Hz
and higher. This range has shorter wavelengths that allow better resolution in imaging technologies. Industrial and medical applications such
as ultrasonography and elastography rely on the ultrasonic frequency
range.
On the other end of the spectrum, the lowest frequencies are known as the
infrasonic range. These frequencies can be used to study geological phenomenon such as earthquakes.
As stated in the introduction, the term ultrasonic is the name given to the
study and application of sound waves having frequencies higher than those
that can be heard by human ears.
Ultrasonic nondestructive testing is the use of ultrasonic sound spectrum to
examine or test materials or measure thickness without destroying the material. The testing frequencies range from 100,000 cycles per second (100 kHz)
to 25,000,000 cycles per second (25 MHz).
Ultrasonic testing does not give direct information about the exact nature of
the discontinuity. This is deduced from a variety of information, such as
materials property and its construction.
TABLE 3-6-1 Pressure Amplitude and Decibel Level
Example of Common Sound
Pressure Amplitude
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Pa
Decibel Level
Threshold of hearing
20*10
0 dB
Normal talking at 1 m
0.002 to 0.02 Pa
40 to 60 dB
Power lawnmower at 1 m
2 Pa
100 dB
Threshold of pain
200 Pa
134 dB
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THEORY OF SOUND
Sound is the mechanical vibrations of particles in a medium. When a sound
wave is introduced in a material, the particles in the material vibrate about a
fixed point at the same frequency as the sound wave. The particles do not
travel with the wave but react to the energy of the wave. It is the energy of the
wave that moves through the material.
The length of a particular sound wave is measured from through to trough,
or from crest to crest. The distance is always the same. This distance is known
as the wavelength (l). The time taken for the wave to travel a distance of one
complete wavelength (l) is the same amount of time it takes for the source to
execute one complete vibration. The velocity of sound (V) is given by the
following equation:
V¼lF
where, l is the wavelength of the wave and F is the frequency of the wave.
A number of sound waves travel through the solid matter; some of them are
listed below.
In a longitudinal wave, also called a compression wave, the particles
vibrate back and forth in same direction as the motion of the sound. The ultrasonic vibrations in liquids and gases propagate in longitudinal waves only.
This is because liquids and gases have no shear rigidity.
In a shear wave, also called transverse wave, the particles vibrate back and
forth in a direction that is at right angle to the motion of sound.
It is also possible in some specific limits, to produce shear waves that travel
along the free boundary or surface of a solid. These surface or Rayleigh
waves penetrate the material to a depth of only a few particles.
In solids, all the three modes of sound waves can propagate.
The shortest ultrasonic wavelengths are of the order of the magnitude of
the wavelength of visible light. Because of this, ultrasonic wave vibrations
possess properties very similar to the light waves that are they can be reflected,
focused, and refracted.
High-frequency particle vibrations of sound waves are propagated in
homogeneous solids in the same manner as directed light beams. Sound beams
are reflected either partially or totally at any surface acting as a boundary
between the object and the gas, liquid, or other type of solid. The ultrasonic
pulses reflect from discontinuities, thereby enabling detection of their presence
and location. Some of the terminology specially associated with ultrasonic
testing methods is described next.
PIEZOELECTRICITY
Piezoelectricity refers to the electricity produced by a vibrating crystal and its
reversion back to vibrations of the crystal.
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When electric current is applied into the crystal, the crystal transforms the
electrical energy to mechanical vibrations and transmits the vibrations through
a coupling medium into the test material. These pulsed vibrations propagate
through the object with a velocity that depends on the density and elasticity of
the material.
SOUND BEAM REFLECTION
High-frequency sound wave act in a similar way as light waves. If the wave is
interrupted by an object, most of the sound beam is reflected. Crystals or transducers then pick up these sound beams and present them as vertical deflections of a
horizontal trace or a line base on a cathode ray tube (CRT) or an oscilloscope. This
type of presentation is called A-scan, other presentations being the B-scan, which
presents a cross-sectional image of the discontinuity and the material being
inspected. The C-scan presentation displays the discontinuity in plan-view.
SOUND BEAM FREQUENCIES
Most ultrasonic testing is available within 400 kHz to 25 MHz. These vibrations
are beyond the audible range and propagate in the test material as waves of
particle vibrations. Sound beams of all frequencies can penetrate fine-grained
material without difficulty. When using high frequencies in coarse-grained
material, interpretation becomes difficult because interference in the form of
scattering is noted. Depth of penetration is better achieved by lower frequencies.
The selection of specific frequency for testing is mainly dependent on the
material property and goal of the testing.
Frequencies up to 1 MHz are generally a good choice, and they have better
penetration and have less attenuation, and they scatter less by coarse grains
and rough surfaces. The disadvantage of low frequencies is that they have
large angle of divergence, so they cannot resolve small flaws.
On the other side, the high-frequency transducers emit more concentrated
beams with better resolving power, but they are more scattered by coarse
grains and rough surfaces.
Frequencies above 10 MHz are normally not used in contact testing
because the higher frequency transducers are thinner and fragile. As the frequency of sound vibrations increases, the wavelength correspondingly
decreases and approaches the dimension of the molecular or atomic structure.
In immersion testing, however, all frequencies can be used because there is no
physical contact between the transducer and the material being tested.
SOUND BEAM VELOCITIES
Ultrasonic waves travel through solids and liquids at relatively high speed,
but they are relatively rapidly attenuated or die down. The velocity of a
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TABLE 3-6-2 Sound Velocities in Various Mediums
Longitudinal Velocity
3
cm/mSec
Material
Density g/cm
Air
0.001
0.033
738
Water
1.00
0.149
3,333
Plastic (Acrylic)
1.18
0.267
5,972
Aluminum
2.8
0.625
13,981
Steel
0.56
Cast Iron
0.35 to 0.56
Mercury
13.00
0.142
3,176
Beryllium
1.82
1.28
28,633
specific mode of sound is a constant through a given homogeneous material.
The ultrasonic wave velocities through the various materials are given in
Table 3-6-2. The difference in sound velocity is due to the difference in the
density and elasticity of each material. Yet it may be noted that density alone
is not able to account for the variations because it may be noted that beryllium
has high sound velocity, although it is less dense than aluminum, and the
acoustic velocity of water and mercury is nearly the same even though the
density of mercury is 13 times greater than that of water.
While discussing properties of sound, we introduced that the sound beams
are refracted and subjected to mode conversion, resulting in a combination of
shear and longitudinal waves. What principles govern the of mode
transformation?
When a longitudinal ultrasonic wave is directed from one medium into
another of different acoustic properties at an angle other than normal to the
interface between the two media, a wave mode transformation occurs. The
resultant transformation depends on the incident angle in the first medium and
on the velocity of sound in the first and second media. In each transformation,
there is an equal angle of reflection back into the first medium. Snell’s law is
used to calculate angle transformations based on the sound path angles and the
sound velocities of the two media (Figure 3-6-1).
SNELL’S LAW OF REFLECTION AND REFRACTION
Figure 3-6-1 shows an illustration of Snell’s law of reflection and
refraction. We note that the sine of the incident angle a is to the sine of
d (longitudinal) or e (shear) refracted angle as sound velocity of the
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(a)
b
a
c
Second medium
Air
Snell’s Law of Reflection
(b)
b
a
c
First medium
Second medium
d
e
Legend:
Longitudinal wave
Shear wave
Snell’s Law of Refraction
FIGURE 3-6-1 (a) and (b), Snell’s law of reflection and refraction.
incident medium 1 is to the sound velocity of the refracted medium 2.
The same relationship exists when mode conversion occurs within the
same medium, using the sound velocities of different waves in the
equation. This can be used to calculate refracted and reflected angles. For
part A of the sketch, these equations can be written as:
sin a=sin d ¼ Longitudinal velocity in medium 1=Longitudinal
velocity in medium 2
sin a=sin e ¼ Longitudinal velocity in medium 1=Shear velocity
in medium 2
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or
sin a ðlongÞ=sin c ðshearÞ ¼ Longitudinal velocity in medium 1=Shear
velocity in medium 1
In B part of Figure 3-6-1, the equation will be:
sin a ðshearÞ=sin c ðlongÞ ¼ Shear velocity in medium 2=Longitudinal
velocity in medium 2
We note that as the incident angle is increased from normal, this results
in only the longitudinal wave. This longitudinal angle d also increases until
it reaches 90 degrees. At this point, no more longitudinal wave is entering
the second medium. This angle of the medium is called the first critical
angle.
As the incident angle is further increased, the shear angle e also increases
until it becomes 90 degrees. At this point, the entire shear wave in the second
medium is transformed into the surface wave. This is called second critical
angle.
These calculations use simple centerline of the beam as input. The actual
application, however, is more complex because the sound beam has width and
divergence. The amplitude of the sound is also higher at its center line, and it
gradually dies down on the outer edges.
UNDERSTANDING THE VARIABLES ASSOCIATED WITH
ULTRASONIC TESTING
The sound velocity through a given material is the distance that sound energy
will propagate in that material in a given time and is a function of material
density, the material’s acoustic impedance, and the material’s temperature.
Because sound velocities are relatively high, the most expression is in
meters or feet per second. The sound velocity of a shear wave in a given
material is usually one half of a longitudinal and about 1.1 times that of a
surface wave. Specific velocities are tabulated in handbooks and are used for
calculations to determine the angle transformations.
The effect of temperature on sound velocity is normally not very significant
in most metals but must be considered when calculating angles in plastics if
they are used as wedges for shear wave search units.
The frequency of ultrasound used for testing is usually between 1 and 6
MHz. The most common frequency for weld inspection is 2.25 MHz. The
transducer element, when excited, resonates at its natural frequency. The
resulting frequency is not a single frequency but a relatively narrow band of
frequencies. Of these, one or more respond with highest amplitude. The frequency is related to the thickness of transducer element. Frequency is
decreased as the thickness of the element is increased.
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The piezoelectric property of the transducer is affected by the natural
frequency of the element and must be considered to obtain maximum sound
amplitude. It may be noted that broadband pulse generation is usually effective
and used with portable equipment.
As explained earlier in discussion of the theory of sound, the wavelength
(l) is the function of velocity (V) and frequency (F):
V¼lF
The expected minimum size of reflector (flaw) detectable with ultrasonic
sound is about one half wavelength (l/2) as measured in a direction perpendicular to the direction of sound propagation.
SELECTION OF TEST EQUIPMENT
A variety of ultrasonic test applications have prompted industry to develop
specialized test equipment. The enormous development of electronics has allowed
manufacturers to add more sophisticated functional features to their equipment,
increasing portability, leading to better results, and taking away several calculations from the hands of the operators and adding them to the machine as features.
The equipment with miniaturization in size and higher speed is very common
features of this new equipment. Other developments include:
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Typically, real-time 320 240 pixels (QVGA)
A-trace (40-Hz update) display
Distance amplitude flaw gating (DAG)
Weld trig
Freeze and insta-freeze for spot weld applications
Precision thickness measurement capability, combined flaw & thickness
mode,
2 MB of memory for substantial storage capability and USB connectivity
High-speed scrolling and encoded B-scan
IP and IF gating
Adjustable damping
Multi-color liquid crystal display (LCD) screens
Other features such as SplitView, SplitScan, AutoTrack, and Quarter VGA
resolution
But in its very basic form, the ultrasonic principles are the same, and the
equipment available is based on those principles. Longitudinal wave ultrasound is generally limited in use to detecting inclusions and lamellar types of
discontinuities in the base material. Shear waves are most valuable in detection
of weld discontinuities because of their ability to furnish three-dimensional
coordinates for discontinuities. As stated earlier in this section, the sensitivity of shear wave is about twice that of longitudinal wave, the frequency and
search unit size being constant.
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0.080 Hole 0.080
0.6
0.6 0.4
15
3.6
55
100
1.5
2.2
4
Type 1
R=1
0.125
6.6
Type 2
0.92
1
30
50
165
35
4
Type 2
300
60° 70°
R=4
6
Type 1
R = 25
3
23
25
2
60° 70°
2
9
91
12
0.080
15
40° 50° 60°
40° 50° 60°
1.2 1.42
2
8
80°
8
4
US CUSTOMARY DIMENSIONS
5
10
R = 100
15
20
80°
200
100
SI DIMENSIONS (MM)
Notes:
1. The dimensional tolerance between all surfaces involved in retaining or calibrating shall be within +0.005 inch (0.13 millimeter)
of detailed dimension.
2. The surface finish of all surfaces to which sound is applied or reflected from shall have a maximum of 125 µin. r.m.s.
3. All materials shall be ASTM A36 or acoustically equivalent.
4. All holes shall have a smooth internal finish and shall be drilled 90 degrees to the material surface.
5. Degree lines and identification marking shall be indented into the material surface so that permanent orientation can be maintained.
6. Other approved reference blocks with slightly different dimensions or distance calibration slots are permissable.
FIGURE 3-6-2 Calibration block.
Shear wave angles are measured in the test material from a line perpendicular to the test surface. The search unit’s angle selection is based on the
expected flaw orientation. Usually, it is a good practice to use more than one
search angle to ensure proper detection of flaws. Three of the most common
angles used for shear wave testing are 70-, 60-, and 45-degree probes
(Figure 3-6-2).
A-Scan Equipment
In A-scan systems, the data are presented as returned signal from the material
under test. The data are presented on an oscilloscope. The horizontal base line
on the oscilloscope screen indicates from left to right the elapsed time, and the
vertical deflection shows signal amplitude. For a given velocity in the specimen, sweep can be calibrated directly across the screen in terms of distance or
depth of penetration into the sample. Conversely, when the dimension
(thickness) of the specimen is known, the sweep time may be used to
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determine ultrasonic velocities. The height of the indications or “pips” represents the intensities of the reflected sound beams. These are used to determine the size of the discontinuity, the depth, or distance to discontinuity from
any given surface the sound beam is either entering or reflecting back. The
main advantage of this type of presentation is that it gives the amplitude that
can be used to determine the size and position of the discontinuity.
B-Scan Equipment
B-scan is especially useful when the distribution and shape of large discontinuities within a sample cross section is of interest. In addition to the basic
components of the A-scan equipment, B-scan provides the following
functions.
1. Retains the image on the oscilloscope screen by use of a long persistence
phosphor coating
2. Deflection of the image-tracing spot on the oscilloscope screen is synchronized with the motion of the transducer along the sample.
3. Image-tracing spot intensity modulation or brightness is in proportion to
the amplitude of the signals received.
C-Scan Equipment
C-scan equipment is intended to provide a permanent record of the test when
high-speed automatic scanning is used in ultrasonic testing. C-scan equipment
displays the discontinuities in a plan view. It does not give the depth or
orientation of the discontinuity.
TESTING PROCEDURE
Most of the testing is carried out as per the written procedures for specific
work. These are based on the applicable code of construction. Hence, the
discussion here is of general application, and specifics must be developed
meeting the work requirements.
Before testing with shear wave angle units, it is good practice and some
codes mandate that the material is scanned with a longitudinal unit to ensure
that the base material is free from such discontinuities that would interfere
with shear wave evaluation of flaws.
Some of the basic rules of testing are:
1. The sound path distance is basically limited to a specified limit generally
up to 10 inches.
2. Three basic search unit angles used are 70, 60, and 45 degrees as measured
from a line normal to the test surface of the material.
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3. It is assumed that any flaw will be normal to the test material surface and
parallel to the weld axis. The flaw orientation would be the most serious
direction for flaws in most welds.
4. The 70-degree search unit would provide the highest amplitude response
from the type of flaw described earlier followed by the 60- and 45-degree
search units. Hence, the order of preference in use shall be the same.
5. The relative amplitude response from the flaw is in direct proportion to its
effect on the integrity of the material and weld. The generally accepted
diminishing order of flaw severity in welds and material is:
a. Cracks
b. Incomplete fusion
c. Incomplete penetration
d. Inclusions (e.g., slag)
e. Porosity
6. Ultrasonic indications are evaluated on a decibel amplitude basis. Each
indication to be evaluated is adjusted with the calibrated decibel gain or
attenuation control to produce a reference level height on the CRT, and the
decibel setting number is recorded as indication level a.
7. The reference level b is attained from a reflector in an approved calibration block. The reflector indication is maximized with search unit
movement and then adjusted with the gain or attenuation control to produce a reference-level indication. This decibel reading is the reference
level.
8. The decibel attenuation factor c, used for weldments testing, is at the rate of
two decibels per inch of sound path after the first inch. Example: A 6-inch
sound path would produce an attenuation factor of (6 1) 2 ¼ 10.
9. Decibel rating d for flaw evaluation is in accordance with the construction
code requirement, with gain control this is attained by applying the
equations a b c ¼ d. However, for equipment with attenuation control,
b a c ¼ d.
Case studies associated with actual calibration testing of welds are
included.
Role of Coupling in Testing
The couplant material is used to maintain full contact of the transducer with
the material surface. This allows the transfer of sound wave. It also helps full
coverage of the test surface during testing.
Coupling material should be hydraulic in nature and have good wetting
properties to cover the material surface. The couplant materials used include
water, oil, grease, glycerin, and cellulose gum powder mixed with water.
The cellulose gum powder is most common material used for testing. It has
significant advantage over others because it is low cost; its viscosity can be
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changed with the addition of more water; it is not a slipping hazard; it does not
form a contaminant film on the material surface; and its residue is easily
removable, so it is not an obstacle for any further work such as repair involving
welding.
Before testing, the equipment is checked for linearity and calibrated to
cover the scope of work. The calibration blocks used for calibrating ultrasonic
equipment before testing can be standard or very specific to the task at hand. A
typical IIW block is shown in Figure 3-6-3. Both U.S. customary unit and
metric unit calibration blocks are shown. Note that the block has the beam exit
angle for correcting the angle of sound beam exit from the transducer. This is
an important step in the accuracy of the test. Most standards and code mandate
that before beginning calibration and testing, the beam exit angle is reestablished, and all future calculations are based on the correct angle of the beam
path. This is one of the basic calibration steps; further detailed calibration
steps are also involved in testing.
ATOMIZATION OF ULTRASONIC TESTING SYSTEMS
At the beginning of chapter, we introduced some terms such as AUT, TOFD,
pulse echo, and so on. We will discuss a few of these as an introduction
because more and more ultrasonic inspections are now being done by automated systems. Most of the AUT systems are applied to and associated with
girth welds, but that is not to assume that the technique is not used for longitudinal butt weds.
Techniques used are to be based on zonal discrimination of the weld cross
section, whereby the weld is divided into approximately equal vertical
inspection sections (zones), each being assessed by a pair of ultrasonic
transducers. These inspection zones are typically 2 to 3 mm (0.08e0.12 in) in
height. For most applications, this requires the use of contact-focused transducers. This is essential to avoid excessive overlap with adjacent signals and
interference with signals originating from off-axis geometric reflectors. The
system is able to provide an adequate number of inspection channels to ensure
the complete volumetric examination of the weld through thickness in one
circumferential scan. The instrument is capable of providing a linear “A” scan
presentation for each selected channel. The AUT inspection channels allow the
volume of the weld scanned to be assessed in accordance with the inspection
zones.
It is important to have calibrated Instrument linearity within 5% of the
ideal acceptable linearity for both linear and logarithmic amplifiers. Each
inspection channel is suitable for selecting pulse-echo or through-transmission
mode, gate position and length for a minimum of two gates, and gain.
Recording thresholds is set so that the system can select display signals
between 0% and 100% of full screen height for simple amplitude and transit
time recording and from 0% to 100% for B-scan or “mapping” type recording
Ultrasonic Testing Chapter j 6
355
of data. B-scan mapping is described later. Two recordable signal outputs per
gate are available, in either analog or digital form, and they are representative
of signal height and time of flight. These systems are made suitable for
recording on a multichannel recorder or computer data acquisition software
display.
THE RECORDING SYSTEM
A marker circuit suitable for connection to the recorder or acquisition system
is used. Its accuracy is very important for full and proper coverage of the weld,
especially the circumferential welds. This acquisition is important as a means
of electronically determining circumferential weld distance to an accuracy of
at least 10 mm (0.4 in); often an optical encoder is used as a distance marker.
The programmed scan lengths ensure that all probes cover the maximum
circumferential distance required for a given circumference of the weld. The
suitable correction factor is used for equipment that has the encoders traveling
on a track or welding guide band to ensure that the circumferential distance
recorded on the chart corresponds to the transducer position on the circumferential weld outer surface. Each transducer for weld discontinuities records
for confirmation of the acoustic coupling arranged on the chart or display.
MAPPING (TIME OF FLIGHT DIFFRACTION)
B-scan or “mapping” displays are used for volumetric flaw detection and
characterizations, and TOFD techniques are often added to improve characterization and sizing. TOFD techniques are often used to augment pulse-echo
techniques; however, a TOFD technique is not intended to replace the pulseecho techniques.
When TOFD technique is used for mapping, the recording system should
be capable of a 256-level greyscale display and be capable of recording full
R-F waveforms for the TOFD transducer pairs.
The above is only a brief a typical AUT system. The details of AUT inspection of welds should be assessed. Often an engineering critical assessment
is carried out to assess the level of critical flaws that should be evaluated and
accepted or rejected. The selection of specific equipment and its limits and
advantages should be evaluated in terms of the specific quality level required
for the specific weld.