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The article contains sections titled: 1.Introduction2.Physical Basis3.Generation and Detection3.1.Conventional Probes3.2.Piezocomposite Materials for Ultrasound Generation3.3.Phased Array Technique3.3.1.Phased Array Probes3.3.2.Applications4.Inspection Methods5.Display Methods6.Interpretation7.Inspection Standards
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2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Article No : o17_o02
Nondestructive Testing, 3. Ultrasonics
KANJI ONO,University of California, Department of Materials Science and
Engineering, Los Angeles, California, USA
ANTON ERHARD,Federal Institute for Materials Research and Testing, BAM, Berlin,
Germany
1. Introduction........................ 495
2. Physical Basis ...................... 495
3. Generation and Detection ............. 497
3.1. Conventional Probes ................. 497
3.2. Piezocomposite Materials for Ultrasound
Generation......................... 499
3.3. Phased Array Technique .............. 501
3.3.1. Phased Array Probes . . . ............... 504
3.3.2. Applications ........................ 505
4. Inspection Methods .................. 506
5. Display Methods .................... 507
6. Interpretation ...................... 509
7. Inspection Standards ................. 509
References ......................... 510
1. Introduction
The field of acoustics deals with sound propa-
gation in solids, liquids, and gases, whereby
the frequencies may lie above or below the
audio band, that is, infrasound (<16 Hz) and
ultrasound (>16 kHz), respectively. The speed
of sound depends on mechanical properties
of the material like elastic constants and
compressibility.
Interaction of imperfections such as grains,
cracks, pores, and inclusions with sound waves
results in reflection, refraction, scattering, and
attenuation due to discontinuities in elastic
moduli and density. By sending a pulse and
receiving its reflection, the presence and loca-
tion of defects can be determined (pulse–echo
technique).
Ultrasound was applied in nondestructive
testing (NDT) earlier than in medicine, but
visualization of the data in a unicolor-scale
image was first used in medical ultrasound
diagnostics (Fig. 1).
The principle of ultrasonic testing, for exam-
ple, the wave types that are suitable for different
applications, the frequency range employed,
reflectivity behavior, mode conversions, probe
types, and so on are described in various pub-
lications [1–4], and a newer ultrasonic method,
namely, the phased array technique, is treated in
[5].
2. Physical Basis
Reflection and Transmission of Ultrasonic
Waves. Since every material has boundaries
the interaction of ultrasonic waves with these
irregularities must be taken into consideration.
The most widely used wave types in ultrasonic
testing–longitudinal waves and shear waves–
have specific characteristics, and their interac-
tions with boundaries are also specific. Longitu-
dinal waves travel parallel to the direction of
compression and rarefaction, i.e., parallel to the
wave propagation (Fig. 2). In shear or transverse
waves, elastic displacement occurs perpendicular
to the direction of wave propagation (Fig. 3).
In both cases, the distance between two points
of the same phase is the wavelength l. In solids,
compressive waves and shear waves occur,
whereas in gases and liquids only longitudinal
waves exist. Equation (1) describes the sound
velocity c
L
of longitudinal waves as a function
of Young’s modulus E, Poisson’s ratio m, and
the density rof the material. The sound velocity
c
T
of shear waves is given by Equation (2), which
describes the relation with the shear modulus.
Article with Color Figures
DOI: 10.1002/14356007.o17_o02
Therefore, characterization of these two wave
types and their propagation conditions in solid
materials can be derived from the sound velocity
equations.
cL¼ffiffiffi
E
r
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1m
ð1þmÞð12mÞ
sð1Þ
cT¼ffiffiffi
E
r
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
2ð1þmÞ
sð2Þ
The wavelength lis defined by Equation (3)
l¼c
fð3Þ
where fis the ultrasonic frequency Table 1 lists
typical values of the sound velocity cfor various
materials.
Consider an incident longitudinal plane wave
traveling in perpendicular direction to a bound-
ary or interface between two materials, as shown
schematically in Figure 4 [2]. For a perpendicular
incident plane wave P
e
, only reflected wave P
r
and transmitted wave P
t
are possible. The ratio of
incident and reflected waves is the reflection
coefficient R(Eq. 4), and that of the incident and
color
fig
Figure 1. Sonogram of a fetus
Figure 2. Longitudinal wave
Figure 3. Shear wave
Table 1. Sound velocities for different materials
Material r,10
3
kg/m
3
Sound velocity,
km/s
Impedance
(W¼rc
L
),
10
6
kg m
2
s
1
c
L
c
T
Metals
Aluminum 2.7 6.32 3.13 17
Lead 11.4 2.16 0.70 25
Gold 19.3 3.24 1.20 63
Cast iron 6.9–7.3 3.5–5.8 2.2–3.2 25–42
Copper 8.9 4.70 2.26 42
Brass (MS 58) 8.4 4.40 2.20 37
Platinum 21.4 3.96 1.67 85
Silver 10.5 3.60 1.59 38
Carbon steel 7.7 5.92 3.23 45
Nonmetals
Epoxy resin 1.1–1.25 2.4–2.9 1.1 2.7–3.0
Alumina 3.6–3.95 9–11 5.5–6.5 32–43
Ice 0.9 3.98 1.99 3.6
Glass (flint) 3.6 4.26 2.56 15
Plexiglas 1.18 2.73 1.43 3.2
Porcelain 2.4 5.6–6.2 3.5–3.7 13
Quartz glass 2.6 5.57 3.52 14.5
Liquids
Glycerol 1.26 1.92 2.5
Diesel fuel 0.80 1.25 1.0
Engine oil 0.87 1.74 1.5
Water (20C) 1.0 1.483 1.5
496 Nondestructive Testing, 3. Ultrasonics Vol. 24
transmitted waves the transmission coefficient T.
In this special case the reflection and transmis-
sion coefficients only depend on the acoustic
impedance W¼rc
L
.
R¼W2W1
W1þW2ð4Þ
T¼2W2
W1þW2ð5Þ
Here indexes 1 and 2 indicate the two media. For
oblique incidence the angle between the wave
propagation directions and the normal at the
surface must be taken into consideration. In such
cases, both longitudinal and shear waves are
reflected or refracted regardless of the nature of
the incident wave (Fig. 5). This is known as mode
conversion, and the angles of reflection and
refraction are defined by Snell’s law (Eq. 6)
sin b
c1L ¼sin d
c1S ¼sin a
c2L ¼sin g
c2S ð6Þ
where the indexes 1 and 2 refer to the two media,
and L to the longitudinal and S to the shear wave.
3. Generation and Detection
3.1. Conventional Probes
Ultrasonic waves are generated and detected by
using a transducer element that converts electri-
cal signals to mechanical ones and vice versa [6].
A thin disk of piezoelectric crystal or polarized
ceramic is used. Quartz, lithium sulfate, and
lithium niobate are used as single-crystal
elements, whereas lead zirconate–titanate is the
most common ceramic element. The transducer
element is enclosed in a protective casing with a
thin ceramic or plastic faceplate and an attenua-
tive backing such as a tungsten–epoxy mixture
(see Fig. 6 A). The backing is needed to dampen
the resonance of the transducer element and to
broaden the frequency response. The casing also
Figure 4. Normal incidence of a longitudinal plane wave
Figure 5. Reflection and transmission of longitudinal and
shear waves
Figure 6. Typical ultrasonic search units
A) Straight-beam unit; B) Angle-beam unit
a) Case; b) Epoxy potting; c) Tungsten-loaded backing;
d) Electrical connections; e) Cable connector; f) Transducer
element; g) Faceplate; h) Plastic wedge
Vol. 24 Nondestructive Testing, 3. Ultrasonics 497
acts as an electrical shield. A thicker faceplate
(a delay tip) may be used to separate the front-
surface signal from the large excitation pulse.
The frequency characteristics of the packaged
transducer or search unit are dictated primarily
by the thickness resonance of the transducer
element, which occurs when the thickness equals
one-half of the wavelength, causing constructive
interference of waves reflected at the surface.
To generate ultrasonic pulses with a search
unit, an electronic pulse generator is used to
produce a train of short pulses or tone bursts (up
to ten oscillations). These are repeated 50 to 2000
times per second. A short pulse contains a wide
spectrum of frequencies, while the frequency
spectrum of a tone burst is centered at its base
frequency. When the search unit is excited with a
pulse (or tone burst), the frequency spectrum of
the ultrasonic wave output is the product of the
frequency spectra of the input pulse (or tone
burst) and the search unit.
The waveform of the output can be obtained
from the frequency spectrum by inverse Fourier
transform. This waveform W
0
can also be ex-
pressed as the convolution of the input waveform
W
i
and the input response W
s
:
W0ðtÞ¼ZWiðttÞWsðtÞdt
where W
s
is the characteristic waveform for an
infinitely short pulse (delta-function shape). Typ-
ical waveforms and the frequency spectra of
narrow-band and broad-band search units are
shown in Figure 7. High peak voltages (300–
500 V) are needed for a short pulse because only
part of the pulse energy is used. This often leads
to the failure of thin, high-frequency transducer
elements. With a tone burst, the base frequency
must be matched to the nominal resonance fre-
quency of the search unit. Since the tone burst is
limited in bandwidth, the peak voltage can be
much lower to obtain the same ultrasonic energy
output. Alternatively, a much stronger wave can
be generated with a tone burst than with a short
pulse for a given applied voltage.
The sound field immediately in front of the
faceplate is basically a plane wave, but the
intensity varies depending on the position
and distance from the search unit because
of diffraction effects. This region is known as
the near field, which extends to a distance N
x
given by
Nx¼D2
t=4lu
where D
t
is the diameter of the transducer
element. For flaw detection, the near field is
difficult to use because of the intensity variation
(near-field effect). Near-field effects in the mate-
rial under inspection can be eliminated byplacing
a delay tip between it and the search unit, whereby
the near-field effect is confined to the delay tip.
The region beyond N
x
is known as the far field and
a spherical wave propagates as shown in Figure 8.
No diffraction fringe occurs, and the wave inten-
sity decreases with the square of the distance. The
spread jof the far-field sound beam is dictated by
l
u
/D
t
and is given by
j¼2sin
1ð1:22lu=DtÞ
For example, j¼44in steel at 1 MHz with
D
t
¼19 mm. For the same D
t
at 5 MHz, j
decreases to 8.7. A smaller value of D
t
allows
flaws to be more accurately located at short
distances, but this advantage is lost as the beam
spreads with increased penetration. A higher
Figure 7. Ultrasonic waveforms and their frequency spectra
A) Narrow band; B) Broad band
Figure 8. Near- and far-field regions
498 Nondestructive Testing, 3. Ultrasonics Vol. 24
frequency beam spreads less for a given value of
D
t
or allows the use of a smaller D
t
.
For special laboratory studies, ultrasonic
pulses can be generated by means of the ther-
moelastic effect with a strong laser beam. Elec-
tromagnetic transducers allow noncontact
generation and detection. They are suited for
automated inspection of pipes and plates at
elevated temperature or with rough surfaces,
for which noncontact generation is desirable for
ease of use. Commercially available units can
be used up to 3 MHz. A strong magnetic field
is required, and a special powering unit must be
used. Nonmagnetic test pieces lower the
efficiency. Limits in the frequency range of
operation and lower signal-to-noise ratios are
some of the disadvantages.
By using the principle of refraction, ultrasonic
search units can be designed to generate angle
beams. A plastic wedge is housed with a trans-
ducer element as shown in Figure 6 B. The
direction of the beam is valid for a specified
material. Surface wave search units are also made
with the same design concept.
In the pulse-echo mode, the same search unit
is used for generation and detection of ultrasonic
waves. Dual-element search units are also used
which house separate transducer elements for
detection and generation. Two separate search
units are employed for the through-transmission
mode, in which one is dedicated for receiving the
ultrasonic waves. Received ultrasonic waves
produce low-level electrical signals. Electronic
amplifiers with low noise levels (even for a wide
bandwidth of 0.1–50 MHz) are used to provide
good signal-to-noise ratios. When a single search
unit is used for transmission and receiving, an
input protection circuit is required and the input
amplifier must be capable of rapid recovery from
an overload due to the excitation pulse.
The sound beam can be focused by means of
an acoustic lens attached to the front surface
of the transducer. This requires a water path
between the lens and the test piece. Transducers
can be spherically focused to a spot or cylindri-
cally to a line. For the latter, a rectangular
transducer element is used. Spherical focusing
achieves the same for pipe and tubing inspection.
Both can increase near-surface resolution with-
out increasing the transducer frequency. Focus-
ing also reduces the effects of surface roughness
and contour.
Focal distance is given in water path length.
Path length in a solid is converted to an equiva-
lent path length in water by multiplying by the
ratio of sound velocities. Focal distance should
then be equal to (water path) þ(solid path)
(sound velocity in solid)(sound velocity in
water). Search units of various focal distances
(typically, 1–25-cm water path) are available.
3.2. Piezocomposite Materials for
Ultrasound Generation
In most ultrasonic applications the piezoelectric
effect is the basis for ultrasound generation.
Sound field generation and reception are per-
formed by ultrasonic transducers and ultrasonic
probes. The active sound generation tool is a
special ceramic with piezoelectric properties. In
NDT, ceramic materials like barium titanate
BaTiO
3
, lead zirconate titanate Pb(Zr,Ti)O
3
(PZT), lead titanate PbTiO
3
(PT), and lead
metaniobate PbNb
2
O
6
(PN), as well as the semi-
crystalline polymer poly(vinylidene fluoride)
(PVDF) in the form of ‘‘monolithic’’ disks are
commonly used for ultrasound generation.
Meanwhile, other crystal configurations have
also been used as the basis for ultrasound gener-
ation. The use of 1-3 composite materials has the
advantage of higher coupling coefficient and
more effective ultrasound generation in compar-
ison with monolithic ceramics and polymers.
The Pennsylvania State University’s Materials
Research Laboratory developed a large variety of
piezoelectric composite materials that consist of
a ceramic and a polymer phase with different
connectivities [7, 8]. The connectivity in one,
two, or three dimensions in a composite is desig-
nated as 1, 2, or 3. Therefore, a piezocomposite
consisting of piezoelectric ceramic rods aligned
in parallel and embedded in a polymeric resin
matrix is called a 1-3 composite. The ceramic
rods connected in only one direction, i.e., the
poled direction of the material, have a connec-
tivity of 1. On the other hand, the polymer phase
connects in all three dimensions, and has a
connectivity of 3. Figure 9 shows schematically
the 1–3 piezocomposite arrangement [9–11].
Although it seems easy to produce such
ceramic materials with the knowledge of wafer
technology, there are some difficulties in the
manufacturing processes. These difficulties have
Vol. 24 Nondestructive Testing, 3. Ultrasonics 499
limited, to a degree, their applicability and have
increased the cost of manufacturing piezocom-
posite probes. Research and development activi-
ties in the past try to improve these processes and
to introduce piezocomposite transducers for
NDT. Further, the improvements in processes
and materials have broadened the applicability
and enhanced the reliability of this material. The
most common fabrication method for piezocom-
posite material for ultrasonic probes is known as
‘dice and fill’’ (Fig. 10) [12]. In the dicing
process, a matrix of ceramic rods is produced.
These ceramic rods can be smaller than 0.05 mm
in width, with cuts separating them by less than
0.025 mm. After dicing, the gaps between the
rods are filled with resin, and subsequently a
grinding process reduces the thickness to give
the required frequency. The plating process (e.g.,
with gold) and polarization procedure are the
final steps of manufacturing piezocomposite for
ultrasonic transducers.
With the introduction of phased array probes,
piezocomposite manufacturing processes have
become more critical, because of the large num-
ber of small piezoelectric elements in a typical
array probe [13]. Individual phased array ele-
ments are often created by dicing a larger, single
piece of piezocomposite material, as shown in
Figure 11. In some cases, array elements can be
less than 0.25 mm in width. Figure 11 shows
clearly the matrix of the composite material with
the small rods and the filler between.
These types of crystals are widely employed
for the design and fabrication of ultrasonic probes,
independent of the probe type, i.e., straight-beam
probes, angle-beam probes, twin-crystal probes,
and phased array probes. The higher sensitivity
of piezocomposite is advantageous independent
of the component or construction for which ultra-
sonic inspection is required.
A further advantage of such piezoelectric
material is its flexibility; the production of focus-
ing probes and adaptation to curved surfaces are
easer than with monolithic crystals. The technol-
ogies for the production of piezocomposites,
especially the chip-dice technology, are funda-
mental to phased array probe design. Therefore,
the growth of the whole phased array technique –
probe including electronic devices and computer
technology – is closely related to chip technology.
With increasing use of electronic chips in nearly
all technical products like household machines,
cars, etc. the market price drops and becomes of
interest for the small global market of ultrasonic
phased array equipment.
color
fig
Figure 9. 1-3 piezocomposite arrangement
a) Resin; b) Piezoelectric ceramic rods
color
fig
Figure 10. Dice-and-fill method for fabricating piezocom-
posite material [12]
Figure 11. Phased array elements diced into 1-3 piezocom-
posite material
a) Rods and filler of the composite material; b) Phased array
elements
500 Nondestructive Testing, 3. Ultrasonics Vol. 24
3.3. Phased Array Technique
When inspection methods with different angles
of incidence are required, a multiprobe arrange-
ment or phased array technique is helpful. In the
phased array technique destructive and con-
structive interference are used for beam forming
procedures. Prerequisite for these is that the
sound is generated by a crystal of size smaller
than or equal to the ultrasonic wavelength,
which ensures that Huygens’ principle is appli-
cable, as shown schematically in Figure 12.
Figure 12A shows the situation for destructive
interference. The thus-generated sound field has
a straight propagation direction, known from
monolithic crystal. For sound field forming,
constructive interference as illustrated in Figure
12B is necessary. Typical industrial ultrasound
applications utilize short pulse lengths. The
advantage of phased array techniques are as
follows:
.Steering the angle of incidence
.Steering sound field parameters such as open-
ing angle by changing the number of elements
or kneeling of the side loops due to pulse form
changing between the elements
.Focusing the sound field at a certain distance
Figure 13 shows the above-mentioned possi-
bilities of sound field steering by means of
changing delay time. The pulse configuration
and the delay time distribution at the symbolic
printed elements are qualitative.
Angle beam steering is accomplished by ap-
plying a linear time delay function to the indi-
vidual elements. For an unfocused sound field,
the angle of incidence is directly proportional to
the applied time delay; with increasing applied
time delay, the angle and delay time decrease.
The delay time tis calculated using Equation (7),
where ais the desired angle of incidence, n¼0,
Figure 12. Principle of sound field steering by means of delay time
Figure 13. Principle of sound field steering by delay time
distribution
A) Steering the angle of incidence; B) Focusing; C) Side loop
kneeling
Vol. 24 Nondestructive Testing, 3. Ultrasonics 501
1, 2, ...the number of array elements, cthe
sound velocity in the examined material, and d
the distance between the centers of two side-by-
side elements.
tn¼nd
csin að7Þ
This equation can be derived with the help of
Figure 14 for an element spacing of d¼lx.
For sound field focusing, delay time distribu-
tions with the behavior of lenses are necessary.
Focusing is accomplished by applying a delay
function; various functions such as parabolic
functions can be used for this purpose. The focal
depth is inversely proportional to the applied
time delay; therefore, with decreasing maximum
applied time delay, the focal depth increases. The
formula governing this behavior is shown in
Equation (8) for a focused ultrasonic beam prop-
agating at a¼0 into a material, where fis the
required focal depth. The other variables are
defined above.
tn¼f
c1ffiffiffiffiffiffiffiffiffiffiffiffi
1þnd
f
s
"# ð8Þ
The focal depth fis calculated by means of a well-
known optics equation. Equation (9) describes
the relationship between optical and acoustical
focal depth [14].
f0¼N
1f0ðf00:82f02þ0:43f03Þð9Þ
where f0¼f
a
/N,f
o
is the optical focus depth, f0
the focus factor, f
a
the acoustical focus depth, and
Nthe near-field length.
With the help of Figure 15 the interpretation of
Equation (9) is as follows: Geometrical focusing
is only possible if the aperture size (in the present
case the crystal size D) is much larger than the
wavelength, but this condition, mostly true in the
optical field, is not the case in the acoustic field.
That is the reason why in ultrasonics the near field
length, i.e., ‘‘natural sound field focusing’’, must
be taken into consideration, as illustrated in
Figure 15. Thus, sound field focusing in the
particular case of NDT with ultrasound is only
possible within the near-field length. The result
of this approach is a fixed focus depth. Another
application is the dynamic focusing procedure
shown in Figure 16. The delay times of the
received signals are dynamically adapted with
increasing or decreasing time of flight. There-
fore, the actual focal depth is equal to the actual
time of flight. The transmitted sound field must
reflect these circumstances, i.e., a sound field
with a suitable amplitude must exist in the total
focusing depth region [15].
Another sound field focusing method is self-
focusing [16]. The principle is as follows: Trans-
mission of a divergent sound field to a reflector in
Figure 14. Calculation of the delay time
Figure 15. Basis for sound field focusing
502 Nondestructive Testing, 3. Ultrasonics Vol. 24
a volume of a material similar to the examined
material. The reflected sound field is measured
with all elements of an array. With a mathemati-
cal algorithm all received signal are added in
such a way that the sum over all signals has a
maximum.
Both steering the angle of incidence and sound
field focusing can be carried out simultaneously.
Equation (10) describes this behavior.
tn¼f
c1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1þnd
f

2
2nd
fsin a
s
2
43
5ð10Þ
The above-mentioned procedure for sound field
steering is more or less the historical version.
Nevertheless, most commercial phased array
equipment uses the delay time principle. In the
meantime also other sound field steering tech-
niques have become available, for example,
the ‘so-called sampling phased array (SPA)
approach [17, 18]. The SPA (Fig. 17) measures
elementary waves generated by individual
elements of the sensor array to reconstruct the
composite phased array signal for any arbitrary
angle or focal depth. Currently (ca. 2010), few
such systems are commercially available.
While a conventional phased array requires the
use of a delay law to create a particular beam in
the test object, SPA synthetically generates
beams as a post-processing operation. In SPA
the data is collected by firing in turn all or
selected elements and, for each firing, receiv-
ing across the entire array. Because of their
small size (less than or equal to the wave-
length), each element generates a nearly
Figure 16. Dynamic focusing
color
fig
Figure 17. Principle of sampling phased array
Vol. 24 Nondestructive Testing, 3. Ultrasonics 503
point-source sound field that sonifies the entire
test volume. The returning ultrasonic echo
signals from a single shot by one probe element
are captured by all of the probe elements, as
showninFigure17.
The benefits of SPA are threefold. Firstly,
the matrix can be reconstructed for the widest
possible range of sonification angles for a
given array, which improves detection of
defects with different orientations. Secondly,
the complete volume of the material can be
imaged through the use of appropriate recon-
struction algorithms such as the total focusing
method (TFM), which can achieve a focus at
every point within the image, without the need
to generate individual plots of amplitude
versus time (A-scans). Thirdly, once the data
are collected, they can be stored for auditing
purposes and processed to produce any image
type, either immediately or at a later date. For
example, the received signals can then be used
to reconstruct at one or more arbitrary angles
and/or focal depths.
3.3.1. Phased Array Probes
At the beginning of the phased array technique,
some scientists had the hope that with an array
coupled directly on the surface all ultrasonic
inspection tasks could be fulfilled. The mistake
of this idea was that with increasing angle of
incidence the effective aperture size rapidly de-
creases (see Fig. 12). Thus, the opening angle
also increases with increasing angle of incidence,
and therefore determination of the defect coor-
dinates becomes ambiguous. Two methods are
feasible; for the generation of longitudinal
waves, sound field steering around the straight
angle of incidence, i.e., coupling the array direct-
ly to the surface, while for the generation of shear
waves the array is on a wedge with a fixed angle
of incidence. Variation around this angle delivers
an angle scan. With the knowledge of Snell’s law,
choice of longitudinal or shear waves is possible,
too. The relative orientation of the elements on
the wedge offers other possibilities of sound field
variations [19].
Variation of the angle of incidence is shown
schematically in Figure 18A, and variation of the
skewing angle in Figure 18B, in which the ele-
ments are oriented perpendicular relative to Fig-
ure 18A. Both sound field variations are possible
with one arrangement by using a matrix array
(Fig. 18C). In the meantime the number of dif-
ferent types of phased array probes for different
applications is increasing and becoming com-
mercially available. An example of a twin-crystal
phased array probe is shown in Figure 19, with
the advantage that through scanning of, e.g., 16
elements the sensitivity area moves along the
depth direction. In this particular case, applica-
tion of the phased array technique performs the
scanning procedure by changing the active
element group of the crystal along the total
crystal size, whereas the angle of incidence is
nearly constant. This example demonstrates the
variety of the phased array approach. Such an
arrangement of crystal elements can be employed
with advantage in the inspection of austenitic
Figure 18. Variation of the angle of incidence and skewing angle
A) Variation of the angle of incidence; B) Variation of the skewing angle; C) Matrix array
504 Nondestructive Testing, 3. Ultrasonics Vol. 24
welded components for longitudinal and trans-
verse cracklike defects.
3.3.2. Applications
In the past phased array probes were employed
for the inspection of components with complex
geometries like the spherical dome or bottom of a
nuclear pressure vessel, but nowadays also weld
inspection is done on pipes and plates, that is, on
all components for which ultrasonic inspection is
required. It is impossible in the framework of this
section, to describe all application areas using
phased array techniques as an inspection tool.
Some examples in which the basic ideas of the
phased array technique become clear are given in
the following.
The inspection of turbine shafts with dia-
meters up to 2000 mm was carried out in the
past by using straight-beam probes adapted with
wedges for the generation of longitudinal waves
with inclinations of 0, 7, 14, 21, and 28, as well
as in the opposite direction [20]. This manual
inspection with wedge-modified straight-beam
probes is time-consuming. In addition to this
inspection procedure, also angle-beam probes
with 45 and 70are required by standards. A
phased array arrangement with varying angle of
incidence in the range between 28 and –28can
save time and increase the reproducibility and
reliability of the inspection result due to a mech-
anized procedure with computer data storage.
As shown in Figure 20, the phased array
probe is coupled at a radial position on the
specimen surface. All received echoes are
stored in the memory in a virtual pixel net during
shaft rotation. This has the result of merging
signals from many different incidence angles
and probe positions on the turbine shaft into a
single image representing all the data. If a flaw
in the shaft is present, it will normally provide
signals from different orientations. Echotomo-
graphy [360merged sector scan (S-scan)] is in
general suitable for all rotationally symmetric
objects.
Figure 19. Twin-crystal phased array probe
a) Skewing angle; b) Number of active elements (e.g., 16);
color
fig
Figure 20. Turbine shaft inspection by the phased array technique
a) Phased array probe; b) B-scan; c) Shaft rotation; d) Pixel system; e) Rectified A-scan
Vol. 24 Nondestructive Testing, 3. Ultrasonics 505
Figure 21 shows another example: inspection
of a nozzle corner [21]. The inspection of this
component requires modification of both the
angles of incidence and the skewing angles dur-
ing probe movement around the nozzle. For data
visualization and evaluation, the echotomogra-
phy approach is also a successful tool. Indeed,
application of the phased array technique for this
special problem was the breakthrough of the
technique in the nuclear field. Since then the
phased array method was also optimized for
other inspection problems, e.g., weld inspection
[22, 23].
The probe configuration shown in Figure 22
can reduce the inspection time and increase the
reliability. In both phased array probes, a group
of active elements (e.g., 10) move along the
inclined wedge surface, and the sound moves
along the coupling surface. Variation of the angle
of incidence in addition to the scanning proce-
dure covers the whole examination range in the
plane of incidence. Total weld examination takes
place due to probe movement parallel to the weld.
The above examples give an idea about the
possibilities and the variation range of phased
array applications. The phased array technique
is used meanwhile in many technical fields: rail
wheel and track inspection [24–26], weld
inspection in general [27–30], inspection of
aircraft components [31–33], inspection of pres-
surized components [27, 34], and so on. The
phased array technique realizes an old dream of
ultrasonic inspection to have a widespread
sound field for defect detection independent of
defect orientation, and a focused sound field for
the estimation of the defect coordinates and
evaluation of defect size.
4. Inspection Methods
When a search unit is placed directly over the
surface of a part under test in contact inspection,
a thin layer of liquid couplant is used to provide a
low loss path for wave transmission. Water, oils,
glycerol, greases, and resins are used as cou-
plants. Certain soft rubbers have been specially
color
fig
Figure 21. Nozzle corner inspection with a phased array probe coupled to a reactor pressure vessel (RPV)
a) Probe; b) Skewing angle; c) Examination area; d) Indication generated due to interference in the cladding (fishtail-shaped);
e) Crack indications
color
fig
Figure 22. Weld inspection
506 Nondestructive Testing, 3. Ultrasonics Vol. 24
formulated to provide dry coupling of a search
unit. In immersion inspection, the part under
inspection and the search unit are placed in a
tank of water. Here, a water path exists in place of
a couplant layer. A search unit is usually fitted
with a waterproof connector and attached to a
manipulator. It is usually set manually but is
increasingly used under computer control.
In conventional immersion inspection, the
search unit and the test piece are placed in a
water-filled tank. The search unit is connected via
a manipulator and an extension tube to an elec-
tromechanical scanning device (or a probe ma-
nipulator bridge). The basic scanning device
provides X–Y movements of prearranged scan-
ning patterns. Advanced units add computer
control that adapts to the shape of a test piece.
Turntables and roller drives allow efficient scan-
ning of round disks and cylindrical test pieces. A
rotating reflector inside a tube can be used to
reflect the ultrasonic beam of an immersion
search unit toward the tube wall, allowing the
inspection of thickness changes or flaws by
means of normal or shear beams (Fig. 23)
[35]. Scanning is synchronized with ultrasonic
data acquisition, and flaw indications are dis-
played at corresponding locations. An X–Y
recorder has been used for such a display, but
recent equipment can store the scan position and
ultrasonic data from 10
7
–10
8
locations in digital
form. Real-time or post-test analysis of the stored
data provides detailed color displays.
Wheel-type search units consist of a station-
ary transducer element, a liquid path inside a
rubber tire, and a rolling wheel to provide
continuous contact with a test piece. The trans-
ducer element can be placed for straight-beam or
angle-beam inspection. The wheel unit can be
stationary and a test piece moved past it, or it may
be moved over a fixed test piece.
A search unit can be fitted with a squirter, a
nozzle through which water streams out under
pressure, which provides a column of water for a
sound beam to reach the test piece. The squirter
technique can be combined with robotics. This
combination eliminates the size limitation of
immersion testing, can be adapted to complex
contours, and is suited for automatic operation.
Another method, known as the bubbler method,
uses a small tank with overflowing water. A
search unit is placed in this tank, and a test piece
is moved over it.
5. Display Methods
Results of ultrasonic testing are commonly pre-
sented in A-scan, B-scan, and C-scan and, more
recently, with reconstructed images. The A-scan
presentation is a plot of amplitude versus time, as
shown in Figure 24. Front reflection, back reflec-
tion, and a flaw echo are indicated on the time
axis, which corresponds to depth. The height of
the flaw echo is related to the size of the flaw. In
the absence of a flaw, an A-scan over a longer
period also shows the ultrasonic attenuation of
the test piece. Multiple back reflections, whose
heights decrease exponentially, are detected.
The B-scan presentation is used to display the
depth and length of a flaw. The transducer is
moved along the front surface, as shown in
Figure 25. When a flaw echo exceeds a prede-
Figure 23. Ultrasonic inspection of pipes
Figure 24. A-scan display for pulse-echo technique
a) Front reflection; b) Flaw echo; c) Back reflection
Vol. 24 Nondestructive Testing, 3. Ultrasonics 507
termined threshold, an indication is recorded on
the display along with indications of the front-
and back-surface signals. Convenient visual ob-
servation is possible with manual scanning and a
storage oscilloscope. Mechanized scanning al-
lows more accurate determination of the position
of a flaw.
TheC-scanpresentationismostusefulin
identifying defective areas in a plane view of a
test piece. The transducer is moved back and
forth over the front surface (Fig. 26). When a
flaw echo above a threshold is present, it is
recorded on an X–Y recorder, which is also
scanned synchronously. The use of several
electronic timing gates, each sensing the flaw
echo over a specified time interval, enables
scanning for flaws at predetermined depth
ranges. Newer inspection systems are usually
equipped with a computer that controls scan-
ning, ultrasonic data acquisition (in digitized
form), and display. These allow data storage,
display of C-scan records at different depths,
and a variety of image processing. For exam-
ple, flaws at different depths can be displayed
in different colors.
In specialized inspection systems, A-scan
records are accumulated along with location data
during a more complex mechanical scanning of
various shapes. For tubular products, helical
scanning is used, either internally or externally.
For pressure vessels, remote-controlled crawlers
are used. Industrial robots are used on contoured
surfaces. Three-dimensional images of flawed
regions can be reconstructed from these data,
and different perspectives of flaw images can be
obtained. A reconstructed view of flaws in a weld
is shown in Figure 27. In both reconstructed
imaging and C-scan displays, the use of color
enhances visual perception.
Figure 25. A) Pulse-echo technique; B) B-scan display
Figure 26. A) Pulse-echo technique; B) C-scan display
Figure 27. Reconstructed image of weld flaws
508 Nondestructive Testing, 3. Ultrasonics Vol. 24
6. Interpretation
Echo intensity in an A-scan display depends on
the size, shape, and distance of the flaw. For disks
of various sizes in water, the intensity of the echo
varies as shown in Figure 28 [1]. The reduction in
echo intensity results from attenuation and
spreading of the beam. Compensation for this
reduction is provided by a distance–amplitude
correction (DAC) circuit in most ultrasonic in-
struments. By comparing the echo intensities of
flaws with those of flat-bottom holes in reference
blocks, flaw sizes can be estimated. Another
approach is to use the ratio of echo intensity to
the back-surface reflection as an indicator of an
unacceptable flaw. An echo intensity equal to
50% of the back reflection is one such rejection
criterion.
The shape of the echo depends on the shape,
orientation, and sound-reflecting characteristics
of an interface. Smooth metal–air interfaces that
are normal to the beam produce sharp echoes.
Curved or rough interfaces (pores, cracks, lami-
nations) produce broadened echoes. Changing
the beam orientation and the position of the
search unit often provides clues to flaw shape
and orientation. Loss of back reflection is another
factor in flaw size estimation. This is due to the
reflection of a sound beam by a flaw, which may
not produce a flaw echo if the reflected beam is
directed away from the search unit. In this case,
the loss of back reflection indicates the presence
of a flaw. Scattering from slags, inclusions, and
large grain structures also results in the loss of
back reflection.
The large grain structures of anisotropic
materials (e.g., stainless steel or brass) produce
beam scattering, which is indicated as random
echoes between the front and back reflections.
These are called grass and make the discrimina-
tion of small flaw echoes difficult. Spurious
reflections are also produced by fillets. A choice
of suitable inspection position is important for
complex geometries.
7. Inspection Standards
Ultrasonic inspection is performed under speci-
fied procedures established by national bodies,
such as the ASTM, DIN, and JIS. They require
the use of standard reference blocks, which may
be of different sizes prepared from different
alloys, containing holes, slots, or notches. Two
types of standard blocks are widely used. One
type is the area–amplitude or distance–amplitude
block, including ASTM E 127 blocks. Each of
the ASTM blocks is 51 mm in diameter and has a
19-mm-deep flat-bottom hole. The size of the
holes varies from 0.40 to 6.4 mm in diameter,
and the distance from the surface to the bottom of
the hole varies from 3.2 to 152 mm. By changing
the hole size at a given distance, these blocks can
be used to relate the amplitude of the flaw signal
to the area of a flaw, and to check the linearity
and sensitivity of a pulse-echo inspection system.
For a given hole size at various depths, distance–
amplitude blocks can evaluate variations of
echo ‘amplitude with distance for straight-beam
inspection in a given material. These blocks must
be made from the same material as the test piece
in order to estimate flaw sizes at various depths.
However, a set of blocks can be used as reference
blocks for performance calibration of a test
system.
Another reference block is known as the IIW
type. It is a nearly rectangular steel plate (25
100360 mm) with a curved edge of radius
100 mm at one end. The block has a notch, a
slot, and large and small holes (50 and 1.5 mm
diameter), which are used together with the
curved edge to determine the sensitivity, propa-
gation angle and beam spread of angle-beam
search units. The IIW block can also be used
for simple sensitivity evaluation of straight-
beam units. A miniature version based on the
same concept is also used.
Figure 28. Distance–echo amplitude–flaw size diagram
Vol. 24 Nondestructive Testing, 3. Ultrasonics 509
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Testing, Hilger, Bristol 1991.
L. Cartz: Nondestructive Testing, ASM International,
Materials Park, OH 1999.
C.-H. Chen (ed.): Ultrasonic and Advanced Methods for
Nondestructive Testing and Material Characterization,
World Scientific, Singapore 2007.
R. Halmshaw: Industrial Radiology. Theory and Practice,
2. ed., Chapman & Hall, London 1995.
C. J. Hellier: Handbook of Nondestructive Evaluation,
McGraw-Hill, New York, NY 2001.
S.-S. Lee, I.-k. Park, S.-j. Song (eds.): Advanced Nondestruc-
tive Evaluation, World Scientific, Singapore 2008.
P. E. Mix: Introduction to Nondestructive Testing, 2. ed.,
Wiley, Hoboken, NJ 2005.
L. Mordfin (ed.): Handbook of Reference Data for Nonde-
structive Testing, DS 68, ASTM, West Conshohocken,
PA 2002.
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tion for Manufacturing and Construction, Hemisphere,
New York 1990.
L. W. Schmerr, S.-J. Song: Ultrasonic Nondestructive
Evaluation Systems, Springer, Boston, MA 2007.
Vol. 24 Nondestructive Testing, 3. Ultrasonics 511
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