the complexity and cost of the systems. 1. In discontinuity reconstruction by
One application uses a steered, focused scanning with a beam width as narrow
ultrasonic beam for tests of tubes.43 An as possible. The results from all the
experimental array of 24 transducers was positions are superimposed to produce
developed in a ring that fit over the tube a cross sectional B-scan presentation of
and had four consecutive elements the discontinuity.
transmitting sequentially as a group,
similar to the sequential linear array. 2. In discontinuity classification by
scanning with a beam large compared
When using a sector scan for testing with discontinuity size. Figure 48
materials, the ultrasonic beam is steered shows how the maximum echoes from
over a wide sector from a fixed position. each transducer position can be
Compound scanning (scanning the same combined to form a compound scan
general area from different positions) amplitude locus curve.
offers several benefits: high redundancy,
exact determination of discontinuity Contact Modes Phased
location and high detectability when Arrays
using high intensity beams, especially for
crack tip detection.29 Phased arrays can operate in three basic
contact modes: manual, semiautomated
For discontinuity characterization, and automated. Like other types of
there are two approaches to take with ultrasonic testing, phased arrays can also
compound scanning. operate in immersion. The following
discussion describes the three contact
FIGURE 47. Linear array: (a) simple linear modes and how they are applied.87
scan; (b) sector scan (phased linear array).40
Manual Contact Phased Arrays
(a) Line 1
Normally, a portable phased array unit is
Line 2 used. These compact units have much of
the capability of larger units and typically
Line 3 have 128 channels. Manual phased arrays
operate in a similar manner to
conventional discontinuity detectors but
use a sector scan displayed on the screen.
This allows real time imaging with true
depth positioning.
Discontinuities can be imaged in
spectral color at true depth. The displayed
waveform corresponds to the angled line
(b) FIGURE 48. Generation of compound scan
amplitude locus curve: (a) maximum echo
Aperture amplitude; (b) beam profiles and transducer
locations.29
(a)
X1 X2 X3 X4 X5
(b)
Test object Discontinuity
240 Ultrasonic Testing
on the electronic scan and the gate and other applications where full data
corresponds to the two corresponding storage and display is required. With these
lines in the S-scan. Cursors are also inspections, the results can be displayed
available for discontinuity sizing. as corrosion maps, A-scans, B-scans,
C-scans, D-scans (or top, side and end
With manual phased array tests, the views) as shown in Fig. 50 or as required.
operator calibrates and couples in the FIGURE 50. Phased array test of weld,
usual contact manner but typically showing indications: (a) top view; (b) side
watches the electronic scan while view; (c) end view; (d) waveform.
scanning for early detection and (a)
characterizing of discontinuities. This
gives improved probability of detection (b)
and faster testing rates. For welds, a
transverse wave is used; the operator can (c)
increase scanning speed by using the
electronic scan. For other applications,
such as shaft inspections, a longitudinal
wave electronic scan may be more
appropriate.
Because no encoder is used, it is not
possible to generate a C-scan, However,
screen shots can be saved — a major
advantage of manual phased arrays.
Saving images reduces subjectivity, gives
better reporting and more repeatable
rescanning. Focused beams significantly
improve the ratio of signal to noise (and
hence the probability of detection).
Semiautomated Contact Phased
Arrays
Semiautomated tests are similar to manual
tests with one major difference; the scans
are encoded and all the data are stored.
The simplest semiautomated system is an
array with an encoder attached but
handheld scanners and belt scanners are
also used for some applications (Fig. 49).
The operator calibrates in the usual
manner; coupling is either with
commercial gels/oils or pumped water,
depending on the application,
requirements and test speeds. The
operator pushes the probe by hand,
whether using just an encoder or a
handheld scanner.
Semiautomated tests are typically used
for weld inspection, corrosion mapping
FIGURE 49. Typical semiautomated
handscanner for piping test.
(d)
Ultrasonic Pulse Echo Contact Techniques 241
Semiautomated scans permit a high Studies show that the best number of
degree of reliability and reproducibility, elements to have in an array aperture
with minimal cost and maintenance. between 20 and 30.46 Less than twenty
decreases the resolution and more than
Automated Contact Phased Arrays thirty has little effect on improving the
lateral and axial resolution. The same
Fully automated test systems are totally research also concludes that both the
mechanized; the operator controls the lateral and axial resolution are inversely
scanner from a computer, and all data are proportional to the aperture.
collected automatically. With phased
arrays, fully automated test systems are A linear array is typically constructed
normally single-axis systems because the by cutting through a single piezoelectric
second axis is performed by electronic ceramic to isolate the individual elements
scanning, discussed elsewhere in this (a technique called slotting) or by
volume. Typically, fully automated phased electroding the surface and leaving the
array systems are high end products with plate intact. The latter type of array is
significantly better capability than called monolithic.47-49 The lateral
semiautomated systems. For example, dimension of the array is always larger
scanning speed is typically at mechanical than its elevation dimension DA > LA) but
speeds of 100 mm·s–1 or faster. The the individual element is always smaller
ultrasonic test performed may be than its elevation dimension (LA > WA), so
complex, the data collection rate may be that each element can be approximated as
high and the arrays may be bigger. a line source.
Calibration is made to the code or Beam Focusing
specifications, which may be specifically
tailored to the application and As described above, a linear array can
equipment. The couplant is typically produce an ultrasonic wave front at any
pumped, and is probably water unless azimuth by timing the sequence of array
otherwise required. Some form of element transmissions so that the
coupling check is often required, for maximum acoustic intensity of all the
example, a through transmission for elements creates a line perpendicular to
welds or a normal beam signal. Generally, the direction of overall propagation. The
the large array wedges run smoothly and timing of the sequence dictates the
give less coupling problems than an direction of the wave front. For a steering
equivalent multiple-probe system. angle r, each of the array elements must
be delayed:44
At high scanning speeds, the plastic
wedges wear quickly, especially on carbon FIGURE 51. Design of multiple element linear
steel. Standard practice is to use wear pins array.44
(often tungsten carbide), positioned with
a specified maximum allowed gap to
ensure good water flow.
Linear Phased Arrays X Y
Linear phased arrays are an excellent DA
means of implementing a real time
ultrasonic imaging system. They allow θ
beam steering and focusing, signal
processing during image formation and a Z
potential for parallel processing to
increase data rates. LA
Beam Steering WA dA
Beam steering is based on the principles Legend
of geometric optics.44,45 Figure 51 shows a dA = element separation
representative linear array with eight DA = aperture depth
elements, where DA is the total lateral LA = aperture length
dimension or aperture, LA is the elevation WA = element width
dimension, dA is the interelement spacing
and WA is the size of the elements. X,Y,Z = dimensional axes
θ = steering angle
Steering can be performed in the lateral
dimension for both transmitting and
receiving at the azimuthal angle θ. The
lateral dimension DA can be between 13
and 25 mm (0.5 and 1.0 in.), with 16 to
64 elements.
242 Ultrasonic Testing
(12) tn = n dA sin θ + t0 lobes increases in amplitude. For good
c imaging, only the reflections from targets
in the path of the main lobe should be
where c is the velocity of ultrasound in received.
the test material, where n = 0, ±1, ±2 ...
with respect to the center element for the Grating lobes reduce the range for
individual elements from the center of the imaging and in some cases create multiple
array outward and where t0 is a constant images. The amplitudes of grating lobes
large enough to avoid negative delays. can be reduced by using the nyquist
sampling criterion, where the array
A spherical timing relationship can element spacing is half the wavelength or
produce focusing within a certain range of less.50 This small spacing translates into
the ultrasonic beam. Focal points are more transducers for the array and
limited by the transition range ZTR of the increases the angle between the main lobe
array and may be approximated:44 and the grating lobes. Also, using short
pulses reduces the relative amplitude of
(13) ZTR = dA2 the grating lobes.44
4λ0
Limitations of Phased Arrays
where λ0 is the wavelength corresponding
to the center frequency of the transmitted Linear phased arrays have a number of
limitations. For example, the response of
acoustic burst. For an ultrasonic beam transducer arrays is not ideal. This occurs
because of the coupling of the thickness
focused at range F with a direction of θ, mode and radial mode vibrations, which
leads to a decrease in the thickness mode
the delay times for the individual resonant frequency. It has been shown
elements are:44 that a width-to-thickness ratio of 0.65
provides the optimum transducer
F ⎡ ⎛ nd ⎞2 nd ⎤ sensitivity.51
c ⎢⎢1 ⎝⎜ F ⎟− F ⎥
(14) tn = ⎣ − 1 + ⎠ 2 sin θ ⎥ + t0 In addition, there are limitations to the
⎦ uniform angular response of the array
elements, due in part to coupling of
This focusing is restricted to the region energy to adjacent elements through the
around the focal point, a region known as transducer faceplate and interelement
the depth of field. The depth of field is material.52 Another problem is caused by
inversely proportional to the square of the the fact that the time delays that control
array aperture at a given focal distance. the sequencing of transmission and
reception are discreet, which results in a
The depth of field of a strongly focused quantized approximation of the smooth,
system can be improved during reception continuous curves needed for ideal
using dynamic focusing. The receive focus focusing and steering. The jagged curve
of the array is rapidly changed to track produces error grating lobes in the image,
the range of reflections synchronously. especially at high steering angles. The best
Multiple foci are possible by focusing the way to reduce this effect is to decrease the
receiver on a point close to the time delay increments.53-56
transducer. After those reflections are
received, the focus is moved by increasing Phased Linear Array Design
the delay times to receive more
reflections. The design of a phased linear array
considers many critical factors, including
Linear Array Field Patterns (1) the number of elements (especially
with regard to focusing), (2) whether the
A real time scan is composed of a transducer is in contact with test material
complete transmission and reception in or immersed in a liquid, (3) whether the
multiple azimuthal directions. A phased electrical excitation is burst or pulsed and
array could be steered in any azimuthal (4) control of signal transmission and
direction but practical considerations reception.
limit steering to ±0.8 rad (±45 deg) from
the center element, depending on the The electronics of the phased linear
element size and frequency.44,45 A far field array are crucial to its operation. The
pattern for a phased array transmitting problem with focusing the received
straight ahead has a main lobe with some reflections is to provide signal delays so
small grating lobes, each lobe having a that the echoes arriving at different times
width of λ·(DA)–1. The grating lobes occur are made coincident when they are
at angular spacings of λ·(dA)–1. summed.56-59 One approach to this is
multiplexing each array element onto a
As the far field pattern of a linear fast analog-to-digital converter and
phased array beam is steered to greater storing the results in memory. The data
angles, the amplitude of the main beam are taken out of memory with the
decreases and one of the adjacent grating
Ultrasonic Pulse Echo Contact Techniques 243
appropriate delay, then summed. The lobe surrounded on both sides by regions
delays are related to the geometry of the of high angular damping.
targets: (1) the reflections from the
different ranges arrive at increasingly Before 1990, almost all phased array
higher rates after transmission and (2) the imaging systems had been developed for
curvature of the received wave front varies medical applications. A monolithic
inversely with the time delay after phased array (Fig. 52) has used both
transmission. longitudinal and transverse waves
appropriate for nondestructive testing.47-49
Also, targets produce a linear time
delay in their echoes at the receiver A sequential linear array with
aperture. In one representative system, 32 contacting transducers has been
charged coupled devices are used as developed for tests of aluminum.60 The
analog delay lines, each array element array uses longitudinal waves with a
sending its output signal through two center frequency of 3 MHz. A second
delay lines controlled by independent array was designed to use transverse waves
clocks. Clock 1 is linear across the array at half the wavelength of the longitudinal
and produces the steered beam; clock 2 wave. Another technique, called interlaced
has a parabolic curvature. The variation of scanning, uses a linear array for the helical
the frequency of clock 2 has a parabolic scanning of cylindrical objects for
curvature. This variation of clock 2 discontinuities, especially longitudinal
controls the summation of the different cracks.61 Because of the geometry of the
curved wave fronts produced by the testing problem, a phased linear array is
different ranges, so reception can be not needed, just a linear array with each
focused. Combining the received transducer transmitting and receiving.
reflections can also be implemented in
software if all of the signals are stored. An Phased Planar Arrays
alternative technique uses a pipelined
sampled delay focusing technique that A phased planar (or matrix) array is
eliminates memory addressing and some basically an extension of the
hardware.58 two-dimensional phased linear array into
the third dimension. This extension
The elements of the linear array do not produces the planar array’s major
have to be equidistant. For example, in advantage — the ability to steer the
one phased linear array the elements are ultrasonic beam — but this comes at a
spaced according to a sine high cost: a considerable increase in the
distribution:29,38 complexity of electronics to control the
sequencing of individual transducers. For
(15) xi = DA ⎡ − ⎛ π ⎞⎤ this reason, phased planar arrays have had
⎢i p⎜ ⎟⎥ limited use in nondestructive testing. The
n ⎣ ⎝ ni ⎠⎦ theory for controlling the phased planar
array is similar to that of phased linear
where i is the number of that element in arrays and is based on the timing of the
relation to the array center, n is the total array element’s firing.
number of elements and p is a positive
number smaller than n·(2π)–1. This Charge coupled devices have been used
distribution concentrates elements in the as delay lines in a square N × N transducer
center and thins them out toward the array that requires the use of N + 1 delay
ends — the directivity diagram has a main line modules.59 Each row is thought of (in
the X direction) as a linear array
FIGURE 52. Monolithic phased linear array connected to a delay line that controls the
for nondestructive testing.49 Y direction steering. The phase delays for
any single element add linearly, therefore
WA the X and Y direction delay lines produce
independent phase effects. Phased planar
dA arrays are described mathematically
LA elsewhere.62-68
During reception, the focus is shifted
to each zone by using switchable delay
elements, creating an expanding
aperture.63 Design approaches include the
application of different spatial sampling
patterns for sparse array transducer
design62 and both experiments and
simulations for designing phased array
transducers.63-66 Newer techniques in
transducer design also include the
miniaturization of the transducer arrays,
which not only improves the
244 Ultrasonic Testing
characteristics of the arrays but also between the pulses. Then the reflected
expands the range of applications.67,68 pulses were combined.
Phased Annular Arrays Early designs of phased annular arrays
had flat faces and focusing only with the
Annular arrays cannot be steered but their receive mode. The transmit mode was
focusing abilities along the central axis typically only a weakly insonifying pulse.
can be considerably enhanced. Focusing is To determine the beam pattern
enhanced by time delays in the path of characteristics of the annular array, the
the reflections detected by the array beam profiles of the transducers are
elements and then summing those results. measured during transmission. According
As described earlier, dynamic focusing to the reciprocity principle, the sensitivity
during reception of the ultrasonic of a transducer acting as a receiver is
reflections is achieved by varying the time identical to its beam pattern distribution
delays. In transmission mode, the when transmitting.71
ultrasonic beam may not be dynamically
focused because it originates from only A seven-element phased annular array
one annulus. However, the focal length has been designed with the capability of
along the axial direction can be varied by dynamic focusing using separate
changing the delays between the transmitters and receivers in each
sequential excitation of the annuli, the element.72 The lack of dynamic control
outermost to the center. during the transmit phase of scanning has
been circumvented by different
Annular Array Design researchers by transmitting with the
center element only, by conical or line
The primary factors for the design of an focusing of the entire aperture or by using
annular array are the size and number of switched zone focusing (described
array elements and the frequency of below).73 Otherwise, the image is limited
transmission. The primary factors to a predetermined depth of focus.72
influencing the use of an annular array
are the calculation of the focal depth and A flat faced 12-element array (one
the time delays. Much of the theory for element per annulus) has been developed
the design and use of annular arrays is with an expanding aperture. In this
based on the theories of geometric optics. system, four central annuli are always
The characteristics of sound propagation, active. The outer eight are inactive for
however, do not closely follow those of short focal distances and active for longer
light. Specifically, parallel light rays do distances, to effect dynamic focusing.
not focus whereas parallel sound beams
emanating from a flat source focus at FIGURE 53. Transmitting and receiving
D2A·(4λ)–1. segments for first and second pulses of
segmented transducer array.70
If this difference is ignored, then the
sound beam’s focal length is shorter than (a)
expected. In the discussion of annular
array theory that follows, this φ
consideration should be kept in mind and
strict experimentation should be (b)
performed to verify any results predicted
by the theory. Legend
transmitting
Phased annular arrays were first receiving
discussed in the mid 1950s as a means of
extending the depth of field of strongly
focused apertures while maintaining
adequate lateral resolution.69 However, it
was not until the 1970s that the
technique was perfected for practical
applications.
In one developmental system, a device
was designed with a single annulus that
was not a phased annular array but a
segmented configuration that allowed
different subelements to transmit and
receive (see Fig. 53).59,70 All elements were
capable of transmitting and receiving but
the reception could not be dynamically
focused. To reduce side lobe effects, two
pulses were used, with the transmitting
segments shifted one segment over
Ultrasonic Pulse Echo Contact Techniques 245
The f number is a characterization of dynamic focusing because of the
the focusing ability of a transducer and is improved lateral resolution, lower side
the ratio of the focal length to the lobe effects and simpler electronics.73 A
aperture diameter. The beam width and separate ultrasonic pulse is transmitted for
therefore the lateral resolution in the focal each zone, so it takes several pulses for
plane have a linear relationship with the f each A-scan. A means has been developed
number when the f number is greater for choosing zones according to the depth
than 2.74 The design allows for larger of field. To determine the depth of field,
elements, lower time delays and lower the axial pressure distribution about the Z
refocusing rates. axis is based on H.T. O’Neil’s theory of
focusing radiators:73,76
A time delay is needed for an annulus
of radius r located away from the central ⎛ Δφ(z) ⎞
element of the array:74 ⎜ sin ⎟
⎜ 2 ⎟
r2 + z2 − z (20) p(z) = ρv0 f0 πa2 Δφ(z)
c
(16) t = z ⎜ 2 ⎟
⎜⎝ ⎟⎠
where c is the acoustic velocity through where a is radius of beam at point z
the material (millimeter per microsecond) (millimeter), f0 is the center frequency
and z is the axial distance of the reflection (hertz), v0 is the amplitude of the particle
source (millimeter). velocity at the source, Δφ(z) is the phase
shift along the Z axis and ρ is the density
The largest time delay is at the of the medium (gram per cubic
minimum focal distance for any active millimeter).
element. The key to dynamic focusing is
refocusing the array on receive as the Figure 54 shows a typical pulse echo
phase shifts occur across the elements of distribution about the area of focus.
the array. For a frequency f0 with a phase
shift of 0.5 π, the maximum refocusing Depth of field also may be defined as
rate tS must be designed for:74 the depth over which the axial response
multiplied by z2 remains over 50 percent
(17) tS = f0 r2 of its value at the focus:73
z2
( )2
(21) Df = 7.1 λ Nf
This design assumes that the plane of where Df is the depth of field (meter) and
Nf is the f number.
observation is in the far field and the
This definition can be used to define
annular elements are treated as infinitely
the transition distances between the
thin.
FIGURE 54. Typical pulse echo distribution for focused array,
It has been shown that superior showing depth of field.73
focusing, lower side lobe levels and Relative amplitude
1.0
simplified electronics can be achieved by
using concave annuli.73,75 To focus a
concave annular array with a radius of
curvature Rc at a depth of field df, the time
delay tj between the jth element of radius
aj and the central element is
approximated:75
⎛ 1 − 1 ⎞
⎜ ⎟
a2j ⎝ R df ⎠
(18) tj ≈ 2c
This expression is a good 6 dB width 0.5 Axis of
approximation for all depths of field 20 dB width transducer
where the outer annulus radius aN satisfies
[aN·(df)–1]2 << 1. Rc is chosen to minimize 0.1
the maximum time delays:75
(19) 1 = 1 ⎛ 1 − 1 ⎞
R ⎜ ⎟
2 ⎝ df1 df2 ⎠
Position off axis Depth of field
where df1 is the shortest focal depth used
and df2 is the longest focal depth used.
Switched zone focusing has been
proposed as an alternative to the faster
246 Ultrasonic Testing
different focal zones. With different focal A concave design with the highest
zones, both the transmit and receive frequency possible and the largest
modes can be strongly focused. To choose aperture was chosen for a handheld
the number of focal zones Nz for an array, transducer design.77 To choose the
Eq. 22 may be used:73 number of elements for the array, an
acceptable level of beam degradation was
(22) Nz = zmax − zmin sought by diagramming the lateral point
Df av response for transducers with different
numbers of elements using a simulation
where Df av is the depth of field with the program.75 The number of elements has
average f number, and:73 little effect on the –6 dB resolution of the
transducer but significant influence on
(23) Nf av = zmin + zmax the side lobe levels.
4a
The gaps between elements should be
where a is the radius of the beam at the minimized to achieve maximum
focal point of interest. Despite the success sensitivity and to lower the sensitivity of
of this design, dynamic focusing remains side lobes. To determine the width of each
the primary technique for implementing a annulus, it is necessary to test the phase
phased annular array. shift. The phase shift across a focused
aperture, with respect to a given focal
The design of a phased annular array point, is determined from the difference
requires several difficult compromises. between the longest and shortest
Elements of equal width proportional to distances from the aperture to that point.
the f number of the system have been The phase shift for each focal zone must
used for expanding the aperture system.74 not exceed a certain value, say 0.5 π. The
Each element of the array should have the phase shift affects the maximum width of
same area so that they have similar an element with respect to any point in
electrical impedance properties and the focal zone. The maximum phase shift
identical phase shifts across each element occurs at the minimum focal distance:74
for any axial position. However, elements
of equal area cause lateral modes of (24) w ≈ λ zmin
vibration leading to pulse degradation.77 4r
The frequency is usually chosen This is the common geometry for a flat
according to the attenuation annular array. The phase shift across a
characteristics of the test material.78 concave array is related to the difference
However, a higher frequency is desired between these two distances from the
because it optimizes the lateral resolution aperture to the point of interest. For a
and the depth of field is proportional to depth of field df, the phase shift is given:75
the frequency.73 According to mode
coupling theory, the ratio of width to (25) w = πa2 ⎛ 1 − 1⎞
thickness should be at least 2. This λ ⎝⎜ df R ⎠⎟
provides a sufficient separation between
the thickness and lateral modes of Annular Array Applications
vibration of the piezoelectric crystals.79
Applications for annular arrays include
The equivalent source resistance of an medical imaging, pulsed doppler volume
array element in receive mode, and flow meters and tests of turbine disks.80,81
therefore its maximum attainable An annular array was chosen to reduce
signal-to-noise ratio, is inversely the number of individual transducers
proportional to the square root of the needed to test the turbine disk. The array
element area. Element area should be is segmented into quarters with randomly
maximized but must be weighed against spaced but equal area elements (Fig. 55).
overall transducer size and the number of Each segment has its own channel and
elements. Studies with concave annular the elements are randomly spaced to
arrays have tried to approximate a focused reduce the effects of grating lobes.
solid-to-solid aperture. This
approximation is affected by the number, Another annular array has been
the width and the spacing of the annuli designed for nondestructive testing with
and affects how closely an array matches the capability of three-dimensional
the theory.74 steering by heavily segmenting the
rings.82 The divided ring array has 48
The annular array’s ultrasonic beam segments on two rings (Fig. 56). It can be
can be shaped but, to achieve uniform steered in the range of ±0.8 rad (±45 deg)
wide and narrow beams, many elements by longitudinal wave excitation and ±0.5
are required, increasing the complexity to ±1.2 rad (±30 to ±70 deg) by transverse
and cost of the electronic control
hardware and initiating the need for
compromise.78
Ultrasonic Pulse Echo Contact Techniques 247
wave excitation. The system performs wave focused at 50 mm (2 in.), and the
better than a phased linear array for instrument will calculate the time delays
discontinuity reconstruction. and focus.
Applications of Phased Pressure Vessel Inspections
Array Systems
The ultrasonic test patterns can be
Applications of phased array systems for complex, as can the geometry.88 For
inspection and health monitoring take example, pressure vessels normally require
advantage of improvements in transducer tests in compliance with the ASME Boiler
design and signal processing capabilities.83 Code, having two separate transverse wave
Examples include monitoring for cracks in angles.89 Good practice would also
industrial plant facilities at high include time-of-flight diffraction, all of
temperatures84,85 and the development of which can be performed in a single linear
piezoelectric wafer active sensors which scan with phased arrays. Figure 57 shows
can be embedded into thin walled a typical pressure vessel test using a
structures and scan the structure for delivery system that follows a magnetic
cracks using lamb waves.86,87 strip.
Commercial phased array systems have Results are often displayed as merged
user friendly software that performs all data displays, with multiple views such as
calculations. For example, the operator top, side and end views plus time-of-flight
can program a 0.8 rad (45 deg) transverse diffraction (Fig. 58). The software often
has special features for data analysis,
including linked the cursors,
FIGURE 55. Diagram of segmented annular FIGURE 57. Magnetic wheel tracker performing contact
phased array weld test.
array with random spacing and 1.5 rad
(90 deg) segmentation.80
FIGURE 56. Optimized arrangement of
divided ring array.82
13
8
Y axis (arbitrary unit) 5
1
3
7
–11
–15 13
–15 –11 –7 –3 1 5 9
X axis (arbitrary unit)
248 Ultrasonic Testing
three-dimensional cursors, overlays and configuration of several oblique
full data storage. discontinuity detection setups by simply
downloading focal laws.91 No mechanical
Tailored Weld Inspections adjustments are needed, and better
coverage is obtained. Figure 59 shows a
Phased arrays are well suited for tests photograph of a full-body inspection
customized for a specific component or system, which uses contact phased arrays
weld profile and for the expected to inspect for longitudinal, transversal
discontinuities. A standard for customized and up to six different oblique
weld tests is for automated ultrasonic discontinuities, measuring ±0.21, ±0.38,
testing of pipelines, which uses the zone ±0.79, ±1.17 and ±1.36 rad (±12, ±22, ±45,
discrimination technique.89,90 Here, the ±67 and ±78 deg). Different water wedges
weld is divided into zones, and each zone hold each array group; each water wedge
is inspected using a well focused, correctly is usually optimized for detection of one
angled beam. This type of test is easily or two discontinuity types.
performed using phased arrays and the
setups can be automated. FIGURE 59. Phased array technique for full-body test of
rotating pipe.
Welding bands are used for delivery
systems. Scanning speed is high,
100 mm·s–1 (240 in.·min–1), so a 0.9 m
(36 in.) pipe is scanned in under 60 s.
Depending on the configuration, up to
20 MB of data are collected each minute
and are saved to two separate storage
locations. The data are displayed in real
time so the operator can make rapid
accept/reject decisions.
As with pressure vessels, the coupling is
pumped water (or methanol water mix in
cold countries). Wedges are mounted with
wear pins, and coupling checks are
performed. This approach has been used
for millions of welds.
Pipe Mills
Pipe mills are extensive users of contact
ultrasonic testing, usually continuously
with many different pipe diameters and
wall thicknesses. Phased arrays have
significant setup advantages over
conventional systems, allowing fast
FIGURE 58. Phased array test of weld: (a) top view and side
view; (b) end view and waveform.
(a)
(b)
Ultrasonic Pulse Echo Contact Techniques 249
PART 7. Moving Transducers11
Ultrasonic Tomography flight of the reflected signal. The gross
shape of the discontinuity or material
Computed tomographic imaging is the interface can be estimated by successive
reconstruction by computer of a scans around the boundary of the
tomographic plane or slice of a test object. discontinuity.
Such imaging is achieved using several
different types of energy, including However, because most ultrasonic
ultrasound, electrons, alpha particles, energy is scattered in the forward
lasers and radar. By definition, a direction, the receiving transducer must
tomograph of an object is a have high sensitivity and electronics are
two-dimensional visualization of a very required to measure backscattered signals
thin cross section through the object. The at high signal-to-noise ratios. A technique
Greek word τομοσ, tomos, means “slice.” A based on time of flight has been
cross section of an object can be at any developed for the reconstruction of the
location and orientation. In ultrasonic image from backscattered ultrasound
transmission tomography, the image of when a fan beam is used.
the cross section is a two-dimensional
reconstruction of many one-dimensional Ultrasonic tomography requires more
A-scans taken from many directions. computer hardware and software than
conventional ultrasonic techniques. The
This cross sectional technique hardware has three basic components:
eliminates the superposition of features (1) data acquisition, (2) data storage and
that occurs when a three-dimensional processing and (3) image display of the
object is displayed in a two-dimensional processed data. A data acquisition system
imaging format. The superposition, consists of the transducer (single or
sometimes called structural noise, makes phased array) in an immersion tank and
discontinuity detection and can be either normal to the material
characterization more difficult because surface or at an oblique angle.
reflective objects from outside the plane
of interest are included. Tomographic During scanning the transducer is
imaging is much more highly detailed. In moved across the plane of the test object.
addition, computers that reconstruct the This movement can be with about
image also provide access to image 0.15 rad (several degrees) of freedom to
enhancement algorithms. follow the contour of irregularly shaped
components. The storage and processing
Tomography can be divided into two system stores the raw scanned data and
types with different applications: then performs calculations on the data to
reflective and transmission tomography. produce the cross sectional image. The
Reflective ultrasonic tomography is used image may be a two-dimensional plot
to locate and size discontinuities, erosion constructed by comparing many adjacent
and corrosion of metals and can be used cross sections. Software is required to
to characterize voids and inclusions. perform this processing. Because of the
Transmission ultrasonic tomography can number of scans required, a tomographic
be used for determining differentiations in image takes longer to construct than a
material density, composition or residual conventional ultrasonic image.
stress. Both sides of the test object must
be accessible and the lateral resolution is Synthetic Aperture
limited by the lateral resolution of the Focusing
transmitting and receiving transducers.
Ultrasonic synthetic aperture focusing is a
Reflection Tomography computer enhanced imaging technique
for the detection and characterization of
Ultrasonic reflection tomography is an discontinuities. It takes advantage of the
outgrowth of the transmission technique nonlinear phase shift of a reflection as a
and is designed for providing quantitative discontinuity is linearly scanned.
images displaying a specific acoustic Improved lateral resolution and a higher
parameter of the test material. The size signal-to-noise ratio are achieved by using
and location of the detected discontinuity this phase shift, mathematically
or material interface can be closely simulating the focusing of an ultrasonic
estimated by the amplitude and time of
250 Ultrasonic Testing
lens that is focused on every point in a the test material and the coupling
test object. medium. This collection of A-scans then
must be processed to create a single
Synthetic aperture focusing requires a image.
computer. Processing many signals for a
single final image has a great advantage: Processing of Synthetic Aperture
the system assists in signal interpretation Data
and displays a visually understandable
image. The synthetic aperture focusing The processing of the data begins by
technique can produce unambiguous choosing A-scans processed as a unit to
images of discontinuities, especially those construct the synthetic aperture (the
with irregular refractive surfaces, and can synthetic aperture can be considered a
eliminate the blurring caused by complex synthetic transducer, the S transducer).
angular scattering at discontinuities. The number of A-scans is determined by
These effects are averaged out by the the spacing between scans. The beam
processing of the technique. width at the depth of field, usually
composed of an odd number of scans, is
In conventional ultrasonic testing, the used to facilitate the processing described
resolution depends on the size of the below. The increase in resolution and
aperture, the area over which data can be signal-to-noise ratio in synthetic aperture
collected from a single point and is focusing is degraded if scans are included
limited by the physical size of the that do not contain the target
transducer. Synthetic aperture focusing discontinuity.
simulates an aperture larger than those
that can be realistically used with a small The basic process includes giving each
aperture transducer. The focus of the aperture element signal a time shift that is
transducer is assumed to be a point of the inverse of the time shifts resulting
constant phase at the interface of from the geometry, the ultrasonic velocity
couplant to material, where all the sound in the test material and in the immersion
waves pass before diverging in a cone. couplant. The signals then are summed
point by point along the length of the
The angle of the cone is determined by center aperture element and divided by
the diameter of the transducer and the the number of signals summed.
focal length. The width of the cone at a
given depth corresponds to the aperture For synthetic aperture focusing to
that can be synthesized. The path length work, the target discontinuity must be in
and travel time to any reflector located the near field of the S transducer. To
beneath the focal point is calculated from achieve one-wavelength lateral resolution,
the corresponding ray path. The path the transducer must be focused at a depth
length corresponds to the phase shift seen about equal to its diameter. However, as
in the signal for the transducer position. the target discontinuity is scanned by
The signals from the adjacent positions in aperture elements further away from the
the synthesized aperture are shifted by the discontinuity, the ultrasonic beam
phase and then added to the signal. The intensity decreases. When the S
final images are usually three-dimensional transducer is created during processing,
maps of the discontinuities and the area this phenomenon effectively apodizes the
surrounding the discontinuities. S transducer aperture, smoothing the near
field fluctuations and improving the
Because the synthetic aperture focusing image.
technique is based primarily on the
processing of stored data, different The data processing can be either
algorithms can be applied to the data. A analog or digital but the digital has
conventional, large aperture transducer advantages, such as ease of manipulation.
has a limited depth of field, a drawback More than one sample may be taken at
that can be circumvented by each aperture element and these signals
simultaneous, multiple-depth focusing can be averaged. If the electronic
algorithms. Multiple-depth focusing lets amplifiers in the system have zero mean
the scanning be much more flexible gaussian noise, then this signal averaging
because the final image shows increases the signal-to-noise ratio by the
discontinuities at various depths for a square root of the number of sums.
component with a relatively complex
shape. The averaging increases the amplitude
accuracy by giving a better estimate of the
The raw data are A-scans stored as the exact level of the signal relative to the
broadband transducer is scanned over the quantization steps in the analog-to-digital
surface of the test object. A curve is made converter. One concern to note is the
of the peak amplitudes caused by the accuracy of the time shift that must be
reflection from the discontinuity for each incorporated during processing.
aperture element. The size of this curved Quantization errors in calculating this
path is determined by the width of the time shift cause phase errors in the image.
ultrasonic beam. The curvature and apex The suggested limit on this phase shift is
of the curve depend on the depth of the 0.44 rad (26 deg).
discontinuity, the ultrasonic velocity in
Ultrasonic Pulse Echo Contact Techniques 251
The final image can be displayed as a of such techniques and their adaptations
two-dimensional line isometric plot, an for ultrasonic testing of materials.
isometric projection of contour plots, a
two-dimensional gray scale image or a A third factor that has influenced the
color coded isometric projection. development of ultrasonic testing is the
proliferation of high speed computers.
Linear Synthetic Aperture Their availability has enabled the practical
Focusing use of calculation intensive procedures
such as synthetic aperture focusing and
The synthetic aperture focusing technique ultrasonic tomography.
can be performed in two-dimensions,
producing a linear scan. This so-called
linear synthetic aperture focusing technique
produces a discontinuity distribution in a
plane perpendicular to the surface (a
B-scan) that shows the discontinuity size
and position.
Another variation uses linear synthetic
aperture focusing and ultrasonic
holography. The two-dimensional
holography results in a C-scan image of
the discontinuities and the linear
synthetic aperture focusing results in a
B-scan. Combining these two scans gives a
top view and a cross sectional view of the
discontinuity distribution. This decreases
the computation time compared to that
required for synthetic aperture focusing.
Errors can be introduced in processing
if there are deviations in the test surface.
Researchers have worked to compensate
for scanning on deviating surfaces by
using enhanced processing algorithms. A
ray is traced from the focal point of the
transducer to the desired range of the
target discontinuity.
If the lateral position of the ray is close
enough to the desired position, then the
ultrasonic path and the time delay to the
target discontinuity is calculated.
Otherwise, the ray angle is changed and
traced again. Some A-scans do not contain
the discontinuity because of the sloping
of the surface.
Conclusion
There are many techniques for using more
than one ultrasonic transducer. These
techniques have generally arisen in
response to specific testing problems that
could not be solved with the use of one
transducer. This process has led to the
development of through-transmission,
various pitch catch and other techniques
whose applications are controlled by the
geometry and acoustic characteristics of
the test material.
Another area that has influenced the
application of sophisticated ultrasonic
tests is medical imaging. Through
extensive medical research, techniques
requiring large investments in electronics
and other equipment have been
successfully developed (various array
configurations and tomography are
examples). This development allows use
252 Ultrasonic Testing
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Ultrasonic Pulse Echo Contact Techniques 259
7
CHAPTER
Ultrasonic Scanning
Yoseph Bar-Cohen, Jet Propulsion Laboratory,
Pasadena, California (Parts 1 to 9)
Byron B. Brenden, Battelle Pacific Northwest
Laboratories, Richland, Washington (Part 10)
Govinder P. Singh, Karta Technology, Incorporated,
San Antonio, Texas (Part 2)
PART 1. Ultrasonic Coupling1
An ultrasonic wave, propagating from a signal. In addition, a separation is
transducer to a test object, crosses several maintained between the test object’s
interfaces with different acoustic internal reflections and the repetitive
impedances (the product of density and reflections in the water path. This
acoustic velocity) before being detected by adjustment is necessary to avoid
the receiving transducer. If the receiving interference between the various
transducer is the same as the source, the reflections and to simplify the evaluation
configuration is called pulse echo, of the response.
whereas if the receiving transducer is a
different transducer, then the Couplant path length is related to test
configuration is known as through object thickness for immersion coupling:
transmission. In either case, the mismatch
of the acoustic impedances causes an (2) dc > Vc Ndtm
energy loss from reflections, which can be Vtm
very high if air is present in the wave
path. where dc is couplant path length
(millimeter), dtm is test material thickness
To improve the energy transfer from (millimeter), N is desirable number of
the transducer to the test object, two
coupling techniques are used: contact and repetitive reflections in the test material,
immersion. The difference between these
two techniques is related primarily to the Vc is acoustic velocity of the couplant
length, or depth, of the couplant medium (meter per second) and Vtm is acoustic
between the transducer and the test velocity of the test material (meter per
object.
second).
Contact Coupling
Limitations of Immersion
With contact coupling, the transducer is Coupling
pressed onto the test object surface to
reduce the couplant thickness to a As a rule in immersion coupling, the
minimum. This thickness needs to be velocities ratio (couplant to test media) is
such that its resonance frequency is much about 1:6 for ceramics, 1:4 for metals and
higher than the transducer spectral 1:2 for composites and plastics. A
response range. constraint over the path length of the
coupling medium as expressed in Eq. 2
(1) d << V limits the practical usage of the
2 fmax immersion method to relatively thin
sections. Because most aerospace
where d is the couplant thickness structures meet this requirement, the
(millimeter), fmax is the transducer’s immersion technique is very popular for
maximum frequency measured at 6 dB testing aircraft components.
below peak frequency (hertz) and V is the
couplant’s acoustic velocity (millimeters There are other limitations of the
per second). The thickness d should be immersion technique.
less than 0.25 λ (an antiresonance) or
d ≤ V·(4f )–1. 1. Because of weight, immersion systems
lack portability and are impractical in
Immersion Coupling the field.
Immersion coupling uses a long fluid 2. The hardware is complex and
delay line. The distance between the relatively expensive.
transducer and the test object is long
enough to separate in the time domain 3. The technique is not recommended
the reflections from the test object front for test objects susceptible to
surface and the transducer’s excitation corrosion.
4. Tests can be performed only on objects
with a thickness below the limit of
Eq. 2.
262 Ultrasonic Testing
Advantages of Immersion 6. Total immersion in a water bath helps
Coupling suppress surface waves that
inordinately increase signals from
Immersion testing has significant minor outside surface discontinuities.
flexibility. It is possible, for instance, to
use immersion techniques in most contact 7. The water column provides a delay
coupling applications. The immersion line that allows the very strong initial
technique also provides significant signal to pass through the amplifier
coupling uniformity and the simplicity of before the weaker signals return to the
changing the insonification angle without instrument. This is particularly
changing the transducer. In addition, the advantageous when testing small tube
transducer is not exposed to wear during sizes and thin plates.
use and immersion techniques are ideal
for automated testing. The immersion
technique provides good acoustic
impedance matching to composite
materials.
Water is the most common fluid used
for immersion coupling, because of its
availability and low cost. To inhibit
chemical or biological aqueous reactions,
various inhibitors can be added to water.
To prevent air bubbles from forming
and accumulating on the surface of test
objects, detergent additives are commonly
mixed with the water. Such additives
reduce the surface tension between the
water and test object, making air bubbles
less likely.
When the tank is filled or the water is
moved (in scanning or filtering, for
example), air tends to dissolve in the
water and increase the attenuation. This
attenuation increase can be as high as
6 dB and can be avoided by keeping water
movement slow or by allowing the air to
be outgassed before testing. The
outgassing process can last from 15 to 60
min after filling the water tank. Generally,
water is filtered before and during use to
ensure that it does not contain particles
that create false test indications.
Immersion scanning has seven primary
advantages.
1. No special transducer adapters or
shoes are required when changing the
size or shape of the test object.
2. Simple continuous adjustment of the
incidence angle of the sound beam is
permitted. This capability is essential
for contour following of complex
shaped structures or when developing
a test procedure.
3. The coupling liquid is continuously
available.
4. Because intimate contact is not
required, testing speed is significantly
faster.
5. The immersion technique is not
influenced so greatly as the contact
technique by loss of coupling due to
ovality of tubing, surface conditions or
dimensional variations.
Ultrasonic Scanning 263
PART 2. Ultrasonic Test Techniques1,2
Ultrasonic test systems can be used with discontinuities can be estimated from the
three forms of ultrasonic scanning: amplitude of the reflected signal. The type
A-scan, B-scan and C-scan (Fig. 1). Each of discontinuity can be determined by
kind of scan provides a different set of analysis of the amplitude and phase
information. Computer based test systems information. The A-scan technique is the
can display the results of all three most widely used and can be displayed on
scanning techniques. most standard ultrasonic instruments.
A-Scan Technique B-Scan Technique
The ultrasonic A-scan presents With the ultrasonic B-scan, the test object
one-dimensional data showing the is scanned along one axis to produce a
response along the beam path at a specific presentation of its cross section. The
location of the test object. Such scans can location along the scanning path is
produce detailed information about shown on the X axis and time of flight
discontinuities in the scanned material. values are shown along the Y axis.
Because a cross section is produced, the
The depth of discontinuities is B-scan is less practical for large volumes of
indicated by the time of flight as material.
measured from the time base of the
cathode ray tube. The size of The B-scan is popular for medical
diagnosis where cross sections are useful.
FIGURE 1. Comparison of scanning In medical applications, the angular
techniques: (a) lamination in plate; manipulation of the transducer is
(b) A-scan of discontinuity; (c) B-scan of monitored to prevent image distortion
discontinuity; (d) plan view of C-scan (entire and the display is adjusted to account for
surface of plate must be scanned to produce changes in the beam angle along the cross
plan view). section of the examined area.
(a) C-Scan Technique
(b) The ultrasonic C-scan is applied to the
test object in a raster pattern and presents
Front surface a view of the discontinuity’s area as seen
Lamination from above. Discontinuity location and
Back surface size data are available from changes in
amplitude as a function of position.
(c) Top of plate
Modern C-scan systems use computers
Bottom of plate to control the transducer position and to
acquire, display, document and store the
(d) test results. The computer synchronously
acquires the digitized position of the
transducer and the associated value of a
specific ultrasonic parameter. The position
can be obtained by various means,
including optical encoders or sonic
digitizers.
In most cases, the parameters obtained
include position, time of flight or the
amplitude of reflection or transmission at
a certain time range. These parameters are
digitized with the aid of an
analog-to-digital converter and stored in
the memory of the computer for further
processing.
C-scan systems can typically scan at
speeds up to 500 mm·s–1 (20 in.·s–1) or
264 Ultrasonic Testing
higher. Speeds must be kept at a level that Transducer Alignment
does not induce water turbulence, which
introduces noise and degrades the Because of the directivity pattern of the
reliability of the test. transducers, the two transducers must be
exactly oriented so that the receiving
Multiple transducers can be used in transducer receives the maximum amount
C-scan tests for through-transmission tests of sound energy. Otherwise, an incorrect
or phased array configurations. As an measurement of velocity or signal
alternative to mechanical scanning, these amplitude is taken. The impedance and
phased array transducers perform frequency characteristics of the two
synchronously and their scanning transducers must be exactly the same —
location is indicated by the cursor the transducers must be calibrated to
position on the computer display. minimize signal modulation problems
caused by instrumentation or the test
Ultrasonic C-scan systems are large in setup. The angle of the ultrasonic beam
size and most are limited to on-site testing can be normal to the test object surface
conditions. With the increasing (Fig. 3)4 or at an oblique angle (Fig. 4)5
availability of inexpensive computing but the receiver must be precisely aligned
capability, scanners for field applications with the transmitter. Table 1 is a list of
have also been designed. constraints for a normal incidence
transducer and a typical test object, along
Through-Transmission with the ambiguities eliminated when
Tests these constraints are followed.
The through-transmission technique uses The through-transmission technique
two ultrasonic transducers located on can be used for discontinuity detection,
opposite sides of a test object. One
transducer acts as an ultrasound FIGURE 3. Diagram for sending and receiving signals in
transmitter and the other transducer is through-transmission configuration.3
passively receives the ultrasound. The
transducers can be in contact with the Digitizing
material, or the test object can be oscilloscope Interface bus
immersed in liquid couplant. If this is not
possible, water column transducers or Preamplifier
squirters can be used to transmit the
sound to the material surface (Fig. 2).3 Pulser/ Computer
receiver Synchronized to time base
FIGURE 2. Through-transmission transducer Test object Receiving In immersion
In immersion tank transducer tank
configuration: (a) in immersion tank;
(b) using water columns.2 Tank
(a) Sound beam Water
Sending Receiving Sending
transducer transducer Transducer
Through-transmission Pulse echo
Test object Discontinuity FIGURE 4. Through-transmission
configuration with transducers at oblique
angle.4
(b) Discontinuity Couplant supply Sending
transducer
Water
Sound Test object
beam
Sending transducer Receiving transducer
Test object Receiving
transducer
Ultrasonic Scanning 265
for material characterization and for
thickness measurement, except in
materials that are thick and highly
attenuative. The usual test setup is to
align the two transducers and move them
simultaneously over the component.
When a discontinuity or a change in a
material’s composition is encountered, the
signal amplitude changes because the
ultrasonic beam is reflected, scattered or
obscured by the discontinuity.
However, the through-transmission
technique cannot distinguish between
voids and delaminations because the
attenuation characteristics of the two
discontinuities are identical. Also, a
discontinuity’s location in the thickness
of the material cannot be determined
using through-transmission tests.6 If the
thickness is known, the ultrasonic signal’s
time of arrival can be used to calculate the
velocity, and its velocity can then be used
to characterize the material’s acoustic
impedance. Changes in the acoustic
impedance represent changes in the
composition of the test material.
Through-transmission can be used for
thickness measurements by recording the
time of flight for the ultrasonic beam in a
material whose acoustic impedance is
known.
TABLE 1. Ultrasonic wave attenuation in water as function
of temperature.
_T__e_m_p__e_r_a_tu__re__ _____________A__tt_e_n__u_a_t_io_n______________
°C (°F) 10–15 Np·m–1·Hz–2 10–12 dB·m–1·Hz–2
0 (32) 56.9 0.494
5 (40) 44.1 0.383
10 (50) 36.1 0.313
15 (60) 29.6 0.257
20 (68) 25.3 0.219
30 (86) 19.1 0.165
40 (104) 14.6 0.127
50 (122) 12.0 0.104
60 (140) 10.2 0.089
266 Ultrasonic Testing
PART 3. Immersion Coupling Devices1
The key to immersion coupling is the is raised above the transducer and the
presence of a continuous fluid medium in ultrasonic test is performed.
the path between the transducer and the
test object. This condition can be Bubbler Devices
maintained by the various devices
detailed below. While immersion of the The device known as a bubbler contains a
transducer and the test object in a water transducer and a captured water column.
tank is the most widely used form of The test object is positioned below the
immersion coupling, other forms are also water column opposite the transducer.
finding widespread usage, particularly the The bubbler maintains a constant flow of
water jet. water through the gap between the
bubbler adapter (Fig. 6) and the test
The cost of an ultrasonic test increases object. The transducer mounting unit is
with the complexity of the coupling designed to provide the desired angle of
device. Therefore, choosing the device is a incidence for the beam.
budgetary decision that must be weighed
against the intention of the test For continuous tests, it is preferable to
procedure. Any of the coupling devices couple the bubbler to the test object’s
below can be used for automated testing. bottom. With a weak water flow, this
arrangement makes it easier to ensure that
Immersion Tanks the area between the transducer and the
test object is always filled with water.
The technique of coupling a transducer to When the test object is placed below the
a test object by submerging both in a bubbler, a strong water flow is required to
water tank has been in use for ultrasonic expel air from the system. As shown in
testing since the early 1940s. In the 1980s, Fig. 6, water is fed to the bubbler cavity
the use of immersion tanks increased through a pipe nipple. If this is done at
substantially with the development of sufficient pressure, a water cushion is
automated scanning systems. formed and the bubbler can slide over the
test object without touching it.
In a typical configuration, scanning
systems are assembled on the immersion FIGURE 5. Short immersion tank for
tanks and the transducer is moved scanning test objects with constant cross
sequentially in at least two normal section.
directions, either manually or
automatically, following a programmed Manipulator
scanning plan. A manipulator permits
adjustment of the beam angles and Rubber seal
remote control of the distance between
the transducer and test object. Test tube
An immersion tank can be used to test Rubber seal
many shapes, including plates, wires and
contours. Computer controlled systems
can follow complex shapes by changing
the insonification angles to maintain a
constant angle of incidence.
The immersion testing of stiff materials
with a constant cross section (such as
pipes and rods) can be simplified to avoid
the high cost of large tanks.4 These
materials may be tested by passing them
through a short tank with two windows
that match the test object cross section
(Fig. 5). To prevent water leakage, the
windows have rubber seals in the gap
between the test object and the window.
After the test object is inserted through
the windows, the water level in the tank
Ultrasonic Scanning 267
The bubbler is used in a variety of field water jet is smooth. These reflections can
applications. As an example, one has been sometimes make the squirter coupling
installed on a manual scanner to conduct unsatisfactory for the pulse echo
a normal beam test of glass epoxy tubing.5 technique. Squirters are more commonly
The bottom section of the bubbler was used with the through-transmission
machined to fit the outside diameter of technique, where water jets are applied on
the tube, and a fixture was designed to both sides of the test object.
hold the bubbler and its angle steady
while scanning. When using squirters with the pulse
echo technique, the response is very
Water Jet Devices sensitive to the angle of incidence. To
ensure a sufficient signal-to-noise ratio,
If enough pressure is available, a water jet the water jet should be within 0.04 rad
can provide noncontact coupling of the (2 deg) of normal to the surface. The
transducer to a test object over a distance surface needs to be smooth and free of
of 120 mm (4.75 in.) or more. Special scratches or wrinkles, a difficulty when
hydrodynamic considerations are used in testing composite laminates.
the design of the squirter to ensure a
minimum of bubbles or turbulence. This FIGURE 7. Water jet for ultrasonic tests:
capability is important for rapid (a) schematic diagram; (b) automated
automated testing where the manipulator system capable of contour following.
might collide with an uneven surface or
with a projection from the test object. (a)
Water jet coupling has an advantage over
immersion when testing large structures. To ultrasonic
No large volume of water or tank is instrument
required: coupling is maintained by
pressurizing a water column and draining Water couplant
the water after it strikes the test object.
Examples of a water jet are shown in Transducer
Fig. 7). Figure 7b is an automated
manipulation system that can follow a (b)
contour.
Water jets are widely used in the
aerospace industry, where many
assemblies have a large volume of air in
their internal structures. This air causes
flotation capability and immersion is not
practical. Furthermore, water can
penetrate into such structures and induce
corrosion.
A squirter produces many disturbing
reflections at the contact point behind the
front surface reflection, even when the
FIGURE 6. Cross section of bubbler device for immersion
tests.
Test object Adapter
Holder
Angulation element Transducer
Pipe nipple
Water
268 Ultrasonic Testing
Wheel Transducers A rubber cup is attached to the
transducer assembly and the cup is filled
Wheel transducers (Fig. 8) consist of a with fluid (Fig. 9). Either a flat or a
plastic tire filled with coupling fluid under focused transducer can be used with the
pressure. During an ultrasonic test, the boot attachment and the angle of
tire rolls over the test object and incidence can be controlled by the
maintains a continuous coupling between manipulator on which the transducer is
it and the transducer. The transducer is mounted.
attached solidly to the wheel’s shaft and is
positioned a few millimeters from the FIGURE 9. Boot attachment for water
surface of the tire. The transducer can be column capture.
manipulated to transmit at an angle that
excites transverse waves in the test object. Transducer
The angle between the plane of incidence
and the rolling direction of the transducer Fluid couplant Boot attachment
can be adjusted to any rotation angle Couplant
between 0 and 1.57 rad (0 and 90 deg).
Test object
Testing with the transducer wheel is
performed by rolling the transducer with
light pressure while scanning the test
object manually or automatically. The tire
creates reflections that need to be
discriminated from the significant
reflections of the test object.
Boot Attachment
The boot attachment uses a rubber or
plastic enclosure to maintain a water path
between the transducer and the test object
(as in the wheel transducer).
FIGURE 8. Wheel transducers: (a) straight
ultrasonic beams; (b) angle beams.
(a)
Liquid filled tire
Stationary axle
Piezoelectric
crystal
(b)
α
Ultrasonic Scanning 269
PART 4. Water Couplant Characteristics1
Frequency Downshift Wave Velocity in Water
Water as a coupling medium distorts The velocity of sound in water varies as a
transmitted signals at high frequencies, function of temperature. The equation
where the ultrasonic wave is significantly relating the ultrasonic velocity of fresh
attenuated and the peak frequency of a water to its temperature is:
broad band signal is downshifted.
Neglecting losses due to diffraction, (5) Vfw = 1410 + 4.21 T − 0.037 T 2
attenuation can be expressed as follows:
where T is the water temperature (celsius)
( )(3) A = A0 exp −αf nX and Vfw is fresh water ultrasonic velocity
(meter per second).
where A is attenuated amplitude, A0 is
unattenuated amplitude, f is frequency Additional factors such as pressure and
salinity contribute to an increase in
(hertz), n is an exponent of the frequency ultrasonic velocity. These conditions play
an important role when tests are in sea
dependence, X is propagation distance water. Under such conditions, the
(meter) and α is a frequency dependent following correction factor needs to be
added.
amplitude attenuation coefficient of the
medium (Np·m–1·Hzn). (6) Vsw = Vfw + 1.1 S − 1.8 × 10−5 d
Note that the attenuation (decibels per where d is the depth below the water
surface (meter), S is salinity of water (parts
meter) unites at a specific frequency: per thousand) and Vsw is salt water
dB·m–1 = +8.6859 α fn. The value of α for ultrasonic velocity (meter per second).
water is 2.4 × 10–14 Np·m–1·Hz2 and varies
It is common to use the term standard
with water temperature and purity. water velocity, to refer to 1.5 km·s–1. This
value corresponds to sea water near the
The following expression has been surface at a temperature of 15 °C (60 °F)
and a salinity of 32 parts per thousand at
reported for the downshift of the a depth of 1 m (40 in.).7
frequency when n = 2:5
Attenuation in Water
(4) f peak = f0
2α X∇2 + 1 Ultrasonic attenuation in water changes
as a function of temperature. Typical
where f0 is unattenuated peak frequency experimental values of attenuation versus
(hertz) and ∇ is f0 × percent temperature are listed in Table 1.8
bandwidth ÷ 236. Attenuation in water also changes with
pressure. Typical values of attenuation as
The percent bandwidth (–6 dB) is from a function of pressure are given in Table 2.
the unattenuated spectrum. The
propagation distance X in Eq. 4 is for the TABLE 2. Ultrasonic wave attenuation in water as function
signal’s trip from the transducer to the of pressure at 30 °C (86 °F).
target and back. The bandwidth refers to
the width of the pulse echo spectrum ____P_r_e_s_s_u_r_e____ _____________A_t_t_e_n_u_a_t_i_o_n_____________
under an impulse excitation. As an MPa (atm) 10–15 Np·m–1·Hz–2 10–12 dB·m–1·Hz–2
example, 5 percent downshift can be
observed when a 25 MHz signal is 0 (0) 18.5 0.161
reflected from a target 12.7 mm (0.5 in.) 50 (500) 15.4 0.134
away from the transducer with a 100 (1000) 12.7 0.110
bandwidth of 50 percent. 150 (1500) 11.1 0.097
200 (2000) 0.086
When the downshift becomes too 9.9
high, it is recommended to replace the
water as a coupling medium with a high
velocity, low attenuation solid medium.
This medium serves as a bond between
the transducer and the test object.
270 Ultrasonic Testing
PART 5. Pulse Echo Immersion Test Parameters1
Parameter Analysis more useful for surface wave devices.
Various signal processing functions
Pulse echo immersion test systems can use (filtering, windowing, transforming) can
four parameters to detect and characterize be used with such systems.
discontinuities: (1) back surface reflection
amplitude, (2) amplitude of extraneous Computer based systems use image
reflections, (3) time-of-flight enhancement techniques to improve the
measurements and (4) spectral response. detectability of discontinuities. Such
systems can also characterize
A schematic view of a typical pulse discontinuities in automatic systems by
echo response is shown in Fig. 10. Time evaluating the received parameters. One
gates, superimposed on an A-scan display, suggestion is to combine C-scan imaging
are used to examine the first three and discontinuity identification by using
parameters. Windows are used in unique ultrasonic features that
combination with a fast fourier transform characterize the discontinuities.9 The
to analyze the frequency domain technique is called feature mapping and
response. uses an array processor to examine signals
in real time.
The analysis of a parameter can be
done with analog hardware or a computer Generally, test results are presented on
based digital system. Analog systems have the computer monitor in colors (or in
high speed performance but also have shades of gray scale) to provide high
predefined options that are relatively resolution. This technology is supported
limited. If signals are digitized and by developments in image processing and
analyzed by a microprocessor, a large enhancement techniques from related
variety of options become available. For fields such as X-ray tomography.
real-time performance, hard coded
firmware is used with a program that can Back Surface Reflection
acquire specific parameters. This approach Amplitude
provides high speed data acquisition but
the test parameters are predefined and Back surface reflection amplitude serves as
cannot be changed. Generally, such a measure of the material attenuation and
systems are similar in performance speed as a detector for anomalies that affect the
to the analog systems. energy of a traveling acoustic wave. This
parameter is a fast indicator of
For high versatility, the desired discontinuities and is widely used to
parameters can be acquired and processed detect delaminations, porosity and
digitally. Digital instruments can digitize a microcracks.
full, single-shot A-scan signal at rates
above 500 MHz. For frequencies up to the Back surface reflection amplitude is an
gigahertz range, sampling techniques are indirect indicator and can be sensitive to
used. Such high frequency systems are irrelevant sources. For example, geometry,
surface roughness and variations in front
FIGURE 10. Schematic view of typical pulse echo test and back surface conditions can cause
parameters. changes in back surface reflection
amplitude. Rejection of the test object on
Front surface the basis of a change in this parameter
can be done only after careful
Reflection Gate 1 Back consideration because of the uncertainty
amplitude reflection in determining the source of the change.
amplitude
Gate 2 The loss or absence of back surface
reflection is evidence that the transmitted
Time of flight sound is being absorbed, refracted or
Gate 3 reflected so that the energy does not
return to the transducer. Loss of back
surface reflection can result from many
causes and does not serve as a
quantitative measure of material
properties. With the gain levels used in
ultrasonic tests, at least several back
Ultrasonic Scanning 271
surface reflections are obtained in Large Discontinuities
acceptable materials, particularly in
metals. Individual discontinuities larger than the
effective beam diameter are also indicated
Loss of back surface reflection may be by reflections between the front and back
determined by measuring the ratio of the surfaces. However, the discontinuity size
number of back surface reflections in a cannot be determined by comparing it
reference material of thickness equivalent with a reference block. The extent of the
to the number of back surface reflections discontinuity can be obtained by moving
in the test material. The loss may also be the transducer along the surface of the
evaluated by reducing the gain setting to test object and finding the points at
give slightly less than a saturated which the discontinuity indication is still
(maximum undistorted) signal from the maintained.
first back surface reflection in a reference
material. The amplitude of this signal is The boundaries of the discontinuity are
then compared with the amplitude of the determined by marking the points where
back surface reflection in the test object. the reflection drops 6 dB below the
amplitude of the reflection at the center
Amplitude of Extraneous of the discontinuity. As an alternative
Reflections approach, a C-scan system can be used
with its receiver gain or attenuation
Discontinuities can be detected directly by calibrated so that, when a reference
examining their reflections from the bulk standard is scanned, the exact size of
of the material. These reflections appear discontinuities is displayed.
between the reflections from the front
and the back surfaces (Fig. 10). Time gates Time of Flight
are set in this region, and signals above a
preset threshold indicate discontinuities. Time-of-flight measurements typically are
The reflection pattern can indicate the made between the test object’s front
discontinuity. The reflection amplitude surface and the next significant reflector.
provides a measure of the discontinuity When this reflector is the object’s back
size. surface, there are no discontinuities. Pulse
echo measurements of time of flight
Small Discontinuities provide very good resolution. Test surface
resolution of 0.5 mm (0.02 in.) and far
Individual discontinuities that are small surface resolution of 0.25 mm (0.01 in.)
compared to the effective beam diameter can be reliably obtained with a 5 MHz,
are evaluated by comparing their highly damped transducer.
amplitude of reflection with the
amplitude of reflection from a standard Time-of-flight measurements are made
hole in a reference block. Extensive with the aid of a time gate that defines
experience in the aerospace industry has the boundaries of the time domain area of
shown this technique to have acceptable interest. To reduce the effect of ultrasonic
reliability. noise, only signals above a preset
threshold are measured. To avoid the
Efforts have been made to develop pulse emitted by the transducer (called
quantitative techniques for determining the initial pulse or main bang signal) and to
the size of small discontinuities. synchronize the time gate with a specific
Algorithms using born approximation10 material depth, it is common to trigger
were developed but showed limited the system on the first significant signal
success because they require prior that arrives at the receiver after the initial
knowledge of several material and pulse.
discontinuity parameters.
When testing complex structures, the
When using reference blocks to transducer’s angle of incidence and its
evaluate small discontinuities, the distance from the test object can change
estimated discontinuity size is generally as a function of location. As a result, the
smaller than the actual discontinuity size. reliability of the test is hampered. The
The surface of a test object and the surface capability of triggering the system with
of a discontinuity in a test object are the first reflection overcomes the effect of
usually not so flat and smooth as the water path length changes. This function
surface of the reference block and the flat is very useful for flat structures such as
bottom hole in the reference block. In steps or slightly curved surfaces.
addition, the attenuation in the test
object is commonly different than the To maintain a constant angle of
one in the reference standard. incidence, some computer based systems
have the ability to follow contours. This is
performed by software that contains
information about the contour or by
sensing devices that maintain
perpendicularity of the beam to the test
272 Ultrasonic Testing
object front surface. The programmed
technique is preferable for high scanning
speeds, but it is time consuming to
manually enter the test object’s
configuration into the computer before
testing. Contour sensing devices are
practical for tests of many objects with
the same configuration.
Time-of-flight (depth) data as a
function of position can be used to
produce a three-dimensional display of
discontinuities. Figure 11 shows a C-scan
three-dimensional view of impact damage
in a composite laminate. The distribution
of delaminations as a function of depth is
visible in the figure.
Frequency Domain
Analysis
A transformation of broad band signal
reflections to the frequency domain can
reveal information that is difficult to
identify in the time domain.
Commercially available systems can
produce fast fourier transforms at speeds
very close to real time.
The spectral response allows
visualization of features that are
associated with discontinuity
characteristics. Spectral analysis has been
reported as a potential nondestructive
testing tool for bonded structures11 and
composite materials.12
FIGURE 11. Computerized three-dimensional
C-scan image of impact damage in
composite laminate.
Ultrasonic Scanning 273
PART 6. Interpretation of Immersion Ultrasonic
Test Indications1
Indications from Reference Indications from Large
Standards Discontinuities
If a discontinuity’s size is smaller than the Discontinuities that are large compared
ultrasonic beam diameter, then the size with the ultrasonic beam size are
can be related to the response from generally easy to detect if their surfaces
known discontinuities in the A-scan. are reasonably smooth and parallel to the
Figure 12a illustrates ultrasonic test surface. A large discontinuity in a die
indications from reference blocks having forging is illustrated in Fig. 13.
83 mm (3.25 in.) from the discontinuities
to the surface and hole sizes from 0.04 to When using normal gain settings, the
3 mm (0.015 to 0.125 in.) in increments discontinuity indication saturates the
of 0.4 mm (0.015 in.). The same display and the amplitude of the
instrument setting was used for all test indication has little quantitative meaning.
blocks. Typically, a strong loss of back surface
reflection is observed because the
Figure 12a shows that the 0.4 mm discontinuity reflects nearly all the
(0.015 in.) hole is barely discernible while ultrasonic energy. The dimensions of the
the reflections from others are clearly discontinuity may be determined from
indicated (increases with the size). the distance the transducer can be moved
Figure 12b shows B-scan presentations while maintaining an indication.
obtained with a constant gain setting for
all eight reference blocks. Indications from
Three-Dimensional
Indications from Small Discontinuities
Discontinuities
Discontinuities in materials that have
The simplest type of discontinuity to been forged, rolled or extruded are
detect has three basic characteristics: essentially two-dimensional. Occasionally
(1) relatively smooth surfaces, a three-dimensional discontinuity is
(2) effectively two dimensions but several encountered. Figure 14 shows a section
wavelengths in width and (3) major from 56 × 75 mm (2.25 × 3 in.) rolled
dimensions parallel to the test surface so steel bar. In Fig. 15a, the indication is
that the ultrasonic beam intersects the from a 2.0 mm (0.015 in.) flat bottom
major dimension with normal incidence. hole whose surface distance is equal to
that of the discontinuity. The display on
Small discontinuities form a significant Fig. 15b is from the discontinuity in the
part of the discontinuities encountered in test object. The first indication is from the
ultrasonic tests of airframe components, front surface and the second indication is
particularly in wrought aluminum from the discontinuity (note that it
products. Foreign materials or porosity in saturates the display). No reflection is
the cast ingot are rolled, forged or obtained from the back surface because
extruded into wafer thin discontinuities sound is not transmitted through the
during fabrication. Fabrication tends to discontinuity.
orient the maximum dimensions of the
discontinuity parallel to the surface. Testing from one surface does not
indicate whether the discontinuity is a
The display in radio frequency form void as in Fig. 15c or whether it is a thin,
shows the front surface reflection, the laminar discontinuity with only two
echo from the discontinuity and the back major dimensions. This can be ascertained
surface reflection. The indication has the by testing from two opposite surfaces. A
same amplitude as that obtained from a test from the top surface of the object
reference block with a 2.0 mm (0.075 in.) (Fig. 15b) indicates that the discontinuity
flat bottom hole. is below the object surface about
60 percent of the thickness. Testing from
the two opposite surfaces (not shown)
indicates that the discontinuity has a
274 Ultrasonic Testing
FIGURE 12. Ultrasonic indications from thickness on the order of 20 percent of
reference blocks with flat bottom holes; 0.4 the thickness. The possibility of two
to 3.2 mm (0.015 to 0.125 in.) in diameter, discontinuities is eliminated by testing
all reflecting surfaces at same distance: from all four sides. However, the
(a) A-scans; (b) B-scans. likelihood of such an occurrence is
considerably more remote than that of a
(a) (b) single discontinuity of the type shown in
Fig. 15.
25 Percent 0.4 mm Loss of Back Surface
(0.015 in.) Reflection
0.8 mm Loss of back surface reflection is an
(0.03 in.) important parameter that can be
examined with many ultrasonic
1.2 mm discontinuity detectors. Special care is
(0.045 in.) required when interpreting or measuring
loss of back surface reflection to ensure
1.6 mm that geometrical considerations (such as
(0.06 in.) roughness or taper) are not responsible for
the reflection loss.
2.0 mm
(0.075 in.) The process is simple to implement for
test objects with relatively smooth
surfaces and with nearly parallel front and
back surfaces. Both large or small
discontinuities can cause this loss of
reflection since part or all of the energy is
reflected from the discontinuity. The loss
of back surface reflection is a very
important parameter when there is no
significant individual discontinuity.
Among the causes for such a condition
FIGURE 13. Ultrasonic indication of
discontinuity in aluminum alloy die forging:
(a) A-scan indication; (b) cross section
through discontinuity.
(a)
2.4 mm (b)
(0.1 in.)
2.8 mm
(0.11 in.)
3.2 mm
(0.125 in.)
Ultrasonic Scanning 275
are (1) discontinuities with rough surfaces noise between the front and back
or orientations at steep angles to the reflections, indicating porous material.
surface, (2) large grain size, (3) a number The photomicrograph of Fig. 16 reveals
of very small discontinuities (such as the size of this porosity.
porosity) and (4) fine precipitate particles.
FIGURE 15. Ultrasonic A-scan indications
Figure 15) illustrates ultrasonic from aluminum plate: (a) multiple back
indications from an aluminum plate. The reflections from solid aluminum plate;
A-scan shown in Fig. 15a is from a 75 mm (b) cross section of solid plate; (c) multiple
(3 in.) aluminum plate (cross sectioned in back reflections from porous plate with
Fig. 15b). The A-scan in Fig. 15c is from a same gain setting as in Fig. 15a; (d) cross
porous plate of the same material and section of porous plate; (e) indication from
thickness (Fig. 15d). The solid plate shows porous plate with gain higher than Fig. 15a
five repetitive reflections and the porous or 15c and expanded time base.
plate shows four reflections. Figure 15c (a)
shows no indication of discontinuities
between the front and back reflections. (b)
Figure 15e is the same as Fig. 15c with a
higher gain setting and an expanded time 75 mm (3 in.)
base scale. The high peaks in Fig. 15e are
reflections from the front and back of the (c)
material.
(d)
Figures 15c and 15e show indications
of excessive porosity or coarse grain — a 75 mm (3 in.)
loss of back surface reflection occurs at
low gain with no occurrence of (e)
reflections. However, at high gain there is
FIGURE 14. Ultrasonic scan of
three-dimensional discontinuity: (a) A-scan
indication from 2 mm (0.075 in.) reference
block; (b) A-scan indication from
discontinuity shown in Fig. 14c, using the
same gain setting as in Fig. 14a;
(c) discontinuity detected in rolled steel bar.
(a)
(b)
FIGURE 16. Photomicrograph of aluminum
alloy containing porosity (94×).
(c)
75 mm (3 in.)
276 Ultrasonic Testing
Metallurgical Factors in although there are variations to this
Indication Formation generalization (Fig. 17).
Consideration of the metallurgical and Discontinuities in extrusions are nearly
fabrication history of materials is always elongated in the direction of
extremely valuable for interpreting of extrusion (along the long axis of the
ultrasonic test indications. extrusion). In the case of plate and
extrusions, it is very important to note
In general, discontinuities in wrought recurring discontinuity indications when
products tend to be oriented in the scanning parallel to the direction of grain
direction of grain flow. The maximum flow.
dimension of the discontinuity is in the
direction of maximum metal flow during Such a situation can be seen in a cross
fabrication. This generalization is not true section of an extrusion. Ultrasonic tests
for discontinuities that result from from the surface can indicate
processes subsequent to forging, extrusion discontinuities equivalent to an
or rolling. indication from a 2.0 mm (0.08 in.) flat
bottom hole in a reference block. The
Plate and Extrusions indications may occur along a 1.2 m (4 ft)
length of the extrusion even if no
Grain direction (the direction the metal continuous discontinuity is evident. Loss
flows during working) is relatively simple of back surface reflection occurs with the
to determine in a plate. Discontinuities individual indications. An important clue
are generally parallel to the plate surface that the discontinuity is large and
and elongated in the direction that continuous is if the discontinuities appear
received the maximum amount of rolling, to be about the same distance below the
surface and in a line coinciding with the
FIGURE 17. Discontinuities oriented in grain. Extrusions are usually visible on the
transverse direction in Unified Numbering butt end of the extrusion. In this case, the
System A97075, heat treatable, temper 6, discontinuity is extended across the
wrought aluminum alloy rolled plate: complete length of the extrusion but was
(a) A-scan indication; (b) cross section of not evident on the ends, even after they
discontinuity. were caustically etched.
(a) Back reflection Die Forging
Discontinuity Grain flow is a complex process in die
forgings. Discontinuities are not
necessarily oriented parallel to the surface
or elongated in the long dimension of the
test object. With the typically complicated
geometry of forgings, these factors make
detection and evaluation of
discontinuities difficult.
In some instances where large,
complex die forgings are being tested, it is
possible to section a sample forging to
determine grain flow in various parts of
the test object. Results of such destructive
procedures help determine the most likely
orientation of discontinuities for
subsequent ultrasonic test setups.
(b) Test Indications Requiring 64 mm (2.5 in.)
Special Consideration
Contoured Surfaces
Reflections from fillets and concave
surfaces may produce test indications
between the front and back reflections
and these can be confused with
indications from discontinuities. Such
spurious indications result from sound
reflected back to the transducer at a time
equivalent to the time of flight from a
discontinuity at a given distance below
the surface. It is sometimes difficult to
Ultrasonic Scanning 277
distinguish between discontinuities and indications remain relatively uniform in
false indications from curved surfaces. shape and magnitude when the
transducer is moved around the edge. The
Frequently, if a false indication results false indication is caused by surface waves
from a contoured surface, the amplitude reflected from a nearby edge on the
of the indication is related to the extremely smooth surface.
amplitude of the reflection from the front
surface. In this case, the amplitude of the Such false indications can be
front surface echo diminishes as the false eliminated by slightly disturbing the entry
indication increases. surface — coating with wax crayon or a
thin film of petroleum jelly. One of the
A false indication tends to be distinguishing characteristics of this false
consistent as the transducer is moved indication is its consistency. It is good
along the contoured surface. A reflection practice to be suspicious of any indication
from a discontinuity tends to be strongly that is unusually consistent in amplitude
localized. False indications from and appearance when the transducer is
contoured surfaces are more likely to passing over the test object.
result in a broad based indication.
Discontinuity indications are typically Location of Discontinuities
sharp spikes.
Because of the near zone effect and
If false indications result from equipment recovery time, discontinuities
reflections around a contoured surface, it very close to the test surface cannot
is sometimes possible to distinguish them always be detected at angles normal to the
by interrupting the ultrasonic beam surface. However, indications of
between the transducer and the surface of discontinuities are sometimes evident at a
the test object with a foreign object (a distance slightly less than that at which a
piece of sheet metal, for example). If the definite individual peak is observed. The
indication is a reflection from a curved sound wave reflected from the
surface, shielding a portion of the curved discontinuity near the surface interferes
area may eliminate the false indication with sound waves reflected from the front
and allow the major portion of the beam surface but the ultrasonic equipment is
to enter the test object. unable to resolve or separate the energy
into two distinct signals.
Edge Effect
A slight variation in the appearance of
Irrelevant indications are sometimes the front reflection does not necessarily
produced near the edges of rectangular indicate a front-surface discontinuity: a
shapes. This type of indication is observed variation in the flatness or roughness of
when the transducer is placed close to the the test surface can also produce a
test object’s edge. This effect is the result variation in the indication. Roughness or
of reflections from the edges, even though flatness variations sufficient for causing
the ultrasound enters the top of the object fluctuations in a front surface indication
and is not refracted by the corner. can usually be detected by touch. When
fluctuations of the front reflection cannot
One distinguishing characteristic of the be attributed to surface condition, the
edge effect is its consistency. There can be possibility of a discontinuity near the
some variation in the distance below the surface should be investigated by testing
surface (typically a fourth to a half of the from the opposite surface. Front surface
test object thickness) but the location and discontinuities may also cause a loss of
characteristics of edge effect indications back surface reflection. To improve the
are consistent. As the transducer travels detection of front surface discontinuities,
parallel to the edge of the test object, the double transducer techniques can be used.
indication remains relatively uniform in
appearance and amplitude. In contrast to Another technique to verify the
this, an indication from a discontinuity presence of front surface discontinuities is
generally shows variation in amplitude the use of thin tungsten foil as a reflector.
because of roughness in the Because tungsten has a very high acoustic
discontinuity’s surface. In addition, impedance (more than 108 kg·m–2·s–1), the
discontinuities that give a continuous reflection coefficient is very high. Such
indication over several millimeters of foil can be placed over a suspect area to
transducer travel are generally of obtain a reference reflection near the
sufficient size to reduce back reflections. front surface reflection.
Surface Conditions Discontinuities Oriented at Angle
to Surface
Occasionally, test objects with smooth,
shiny surfaces produce irrelevant or false Discontinuities oriented at an angle to the
indications. When testing plates with front surface may be difficult to detect
smooth finish surfaces, for example, and evaluate if care is not exercised.
consistent indications may exist beyond
the front surface reflection. The
278 Ultrasonic Testing
Generally, it is desirable to scan first at a Ultrasonic testing was performed on a
comparatively high gain level to detect similar sample again heat treated to attain
discontinuities oriented at an angle to the grain refinement. The sample showed an
test surface. It has been shown that a absence of noise on the reference line,
2.0 mm (0.08 in.) diameter flat bottom indicating that with proper heat
hole oriented at 0.44 rad (25 deg) to the treatment a finer grain size was obtained
front surface is not discernible on the (Fig. 18c). Microscopic examination
display if the transducer is parallel to the revealed a refined grain size of ASTM
surface.13 This test was conducted using a standards 6 to 8.
gain level giving a peak height 50 percent
of the screen from a 2.0 mm (0.08 in.) flat In another test performed on forgings
bottom hole at normal incidence. of a nickel based alloy, a frequency of
5 MHz was used. One forging produced
Ultrasonic waves obey Snell’s law in a seven back reflections whereas another
fashion similar to light. Therefore, it is showed no back reflections with the same
necessary to manipulate the transducer
when evaluating discontinuities oriented FIGURE 18. Effect of grain size on ultrasonic
at an angle to the surface so that the indications from Unified Numbering System
sound beam strikes the plane of the G43400 nickel chromium molybdenum alloy
discontinuity at right angles. Even though steel (both A-scans obtained with the same
manipulation is accomplished, gain): (a) photomicrograph of large grain
discontinuities oriented at angles to the material; (b) ultrasonic A-scan indication for
surface result in indications with Fig. 18a; (c) photomicrograph of fine grain
magnitudes slightly lower than those for material; (d) ultrasonic A-scan indication for
discontinuities parallel to the surface. This Fig. 18c.
difference is not large. Indication (a)
amplitude is a function of angle between
the discontinuity and the front surface. (b)
The transducer can be manipulated to
obtain maximum indication height. (c)
In some instances, not only is the
general plane of the discontinuity
oriented at an angle to the surface but the
surface of the discontinuity may also be
irregular. The amplitude of the indication
may not indicate a large discontinuity
because most of the sound may be
reflected to the transducer. Discontinuities
of this type are generally large compared
with the transducer size and evidence of
the indication may persist as the
transducer is moved over the test object.
Occasionally, discontinuities large
compared to the transducer have a
relatively smooth, flat surface but lie at an
angle to the surface. Bursts in large
forgings fit this category and tend to lie at
0.79 rad (45 deg) to the surface. Such
discontinuities present a nearly
continuous test indication. Because of the
change in distance that the sound must
travel, the indication moves along the
base line of the display instrument as the
transducer is moved.
Grain Size Discontinuities (d)
In an ultrasonic test of Unified
Numbering System G43400 nickel chrome
molybdenum alloy steel at 5 MHz, an
unusually high noise level was detected.14
A study of this material showed very large
grain size compared with ASTM grain size
standards of 1 to 4 (Fig. 18a). The large
grains found in the as-received condition
resulted from (1) high temperature during
hot working and (2) subsequent improper
annealing.
Ultrasonic Scanning 279
gain level, transducer and test frequency. greater than θin, or about 0.35 rad (20 deg)
Microscopic examination was made of to normal. A schematic description of the
these and other forgings to determine if A-scan at this point is shown in Fig. 19b.
any internal discontinuities were The initial pulse (emitted by the
responsible for the ultrasonic pattern and transducer) is shown at the left and the
to compare the grain size. The forgings reflection from the front surface is shown
with the unusually large grain size showed next. As the transducer moves in an axial
a loss of back reflections even though no direction, the refracted beam (position 1)
internal discontinuity was present. is just cutting across the corner of the
Further investigations revealed that rabbit groove and producing an
prolonged or improper forging insignificant indication. The reflection
temperature could cause the abnormally from a far surface is shown at the right
large grain size. side.
Interpretation of Crack Indications
Indications from Rotor
Wheels In Fig. 20a, the transducer has been
moved farther right in an axial direction
The tests discussed below use as an and the sound beam (position 2) is
example the straight beam immersion test reflected from a crack. This produces an
of an aircraft component. Figure 19a indication on the A-scan (Fig. 20b). The
illustrates the area tested ultrasonically in rabbit groove indication has disappeared
an aluminum compressor rotor wheel. because the ultrasonic beam has been
The ultrasonic beam from the transducer repositioned. Because the ultrasonic pulse
is directed at an incidence angle of requires about 8 µs to make a round trip
0.09 rad (5 deg) at the surface periphery. through 25 mm (1 in.) of aluminum, the
time interval between the indication from
According to Snell’s law, the angle of the front surface and that from the crack
the refracted beam θ1 is about four times is 2 ms. At the position shown in Fig. 20,
the crack starts at about 6.5 mm (0.25 in.)
FIGURE 19. Ultrasonic testing of aluminum below the surface.
compressor rotor wheel at beam position 1:
(a) beam hits corner of rabbit groove; As the transducer is moved axially, the
(b) A-scan from position 1, showing refracted ultrasonic beam is reflected from
indication from corner of rabbit groove. the face of the crack at a depth of about
(a) Transducer FIGURE 20. Ultrasonic testing of aluminum
compressor rotor wheel at beam position 2:
θ (a) beam misses rabbit groove and strikes
N crack; (b) A-scan indication from position 2
shows crack reflection.
(a) Transducer
θ1
(b) Front surface Rear surface
Initial
pulse
(b) Front Rear
surface Crack surface
Initial pulse
Corner of
rabbit groove
Legend
N = normal incidence
θ = angle of incidence
θ1 = angle of propagation
280 Ultrasonic Testing
3 mm (0.125 in.) below the surface. The orientation. Figure 21a shows the
crack indication moves toward the front direction of the ultrasonic waves and the
surface indication and the time interval position of the discontinuity in the cross
between the two indications becomes section of the wheel.
about 1 µs. As the transducer moves, the
crack indication seems to move toward During testing, the wheel is rotated on
the front surface reflection. This indicates a turntable immersed in water. The A-scan
that the crack is not parallel to the surface display is shown in Fig. 21a for the
but instead is approaching the surface condition in which the transducer is
(the sound beam’s angle of incidence has oriented over a solid metal path. Both the
not changed with the change in the A-scan and B-scan presentations are
transducer’s axial position). shown (in Fig. 21) with the transducer
over an area containing a crack. The
Indications of Weld Cracks discontinuity is large enough in this case
to cause a reflection of high amplitude
A cross section of a welded turbine rotor and a complete loss of back surface
is shown in Fig. 21. In this wheel, a rim of reflection (Fig. 21d).
forged stainless steel is welded to a hub of
forged ferritic material. Despite advanced The distance between front surface
welding techniques, cracks occasionally reflection and the reflection from the
develop in the rim’s heat affected zone. crack indicates the crack’s depth below
These occur often enough to require the surface. Care must be exercised when
100 percent testing. angulating the sound beam close to the
interior interface of weld and rim material
The wheel cracks lie in a plane parallel — the beam can be reflected off these
to the face of the wheel and extend in a faces and is rapidly attenuated.
radial direction. Ultrasonic testing is the Consequently, this area is not always
only means to detect cracks in this effectively tested by the ultrasonic
method. Radiographic tests are also used
FIGURE 21. Ultrasonic testing of welded for the weld area.
turbine rotor: (a) beam position for crack in
heat affected zone; (b) A-scan indication Indications of Metallic Inclusions
over sound material; (c) A-scan indication and Segregations
over crack, also showing loss of back
reflection; (d) B-scan indication over crack. Metallic inclusions in the same plane as
cracks give similar indications in the heat
(a) Transducer affected zone. They are found most
frequently in the rim and farther away
(b) Front from the weld area.
surface
Amplitude Rear surface Ultrasonic testing is also used for solid
(percent of scale) forged turbine rotor wheels of certain
100 stainless steel alloys fabricated without
welding. The technique is used chiefly to
50 detect sharp cracks from forging.
0 Forgings may also contain segregates
that reflect ultrasonic beams. These
(c) Front indications cannot be distinguished from
those caused by cracks. Metallurgists have
surface determined that the mechanical
Crack properties of materials containing
segregates meet normal service
(d) Front surface requirements for room temperature tensile
and elevated temperature stress rupture
Crack properties. These segregates can cause
indications with amplitudes of 15 to
Rear surface 90 percent of the front surface reflection
(Fig. 21c). The more highly concentrated
ones cause higher amplitude indications.
These peaks can appear between the first
and second multiples of the rear surface
reflection.
Discontinuities in this stainless steel
occur in either a loose or tightly bound
condition. Fortunately, these
discontinuities do not occur frequently.
Although they do not adversely affect the
mechanical properties, they cannot be
accepted. If they should occur in a critical
area like the serrations of the rotor wheel,
Ultrasonic Scanning 281
they might propagate and cause specifications do not permit any
catastrophic failure. ultrasonic indications between front and
rear surface reflections.
Indications of Forging Bursts
Indication from Surface Contour
Figure 22 illustrates the cross section of a Blending
stainless steel rim of a turbine wheel
containing forging bursts. These are Surface conditions may cause false
irregularly shaped cavities caused by indications during ultrasonic tests. The
rupture of the material during forging. safest way to avoid false indications is to
Forging bursts are rejectable, are likely to have the surface prepared to completely
be clustered in groups and produce many avoid ultrasonic wave scattering. A
A-scan indications of various amplitudes. common practice is to require that the
The reflections from inclusions may also surface of test objects have at least a 2 µm
be of varying amplitude but are more (8 × 10–5 in.) root mean square finish.
likely to be widely scattered. By
comparison, the indication from a crack is Surface treatment such as blending or
sometimes continuous for as much as one grinding can produce misleading
fourth the circumference of the rim, with ultrasonic test results. Figure 23 shows the
complete loss of reflection from the far cross section of a turbine rotor where a
surface. slight depression has been made by a
blending operation in the otherwise
Indications of Small Inclusions smooth surface. The depression has been
exaggerated to illustrate the condition
The rotor wheel rims are also more clearly. In actual cases these
ultrasonically tested for nonmetallic depressions may be so slight that they
inclusions too small to be detected cannot be seen under water and their
radiographically. These inclusions do not presence can be verified only by touch.
seriously affect the mechanical properties
of the materials as do cracks and forging Under certain conditions, such
bursts. depressions can produce false indications
as shown in the A-scan in Fig. 23b. If the
The inclusions are often randomly sound waves enter the surface at the edge
located. If they are exposed on a of the blended area, they can be scattered
machined surface such as a serration used in such a way that part of the beam
for the insertion of a bucket, a costly travels across the surface of the blend,
rejection is the result. Rim acceptance reflects from the opposite edge and
appears as a discontinuity indication.
FIGURE 22. Ultrasonic testing of stainless Because sound travels much slower in
steel rotor wheel with forging bursts: water than in metals, the amplitudes and
(a) beam position on fillet area; (b) A-scan position of false indications with respect
indications from forging bursts between to front and back surface reflections
front surface and rear surface.
FIGURE 23. False indication from surface
(a) Forging bursts blending: (a) path of ultrasonic beam
producing the false indication (dashed);
Transducer (b) A-scan showing false indication between
front surface and rear surface indications.
(a) Transducer
(b) Front (b) Front surface
surface
Initial pulse
Forging bursts
Rear surface
Rear surface
False indication
282 Ultrasonic Testing
depend largely on the size, depth and more clearly. The indications from this
contour of the blended area and the path surface condition are depicted on the
length in the metallic test object. A-scan in Fig. 24 and are generally
characterized by an increased number of
A good practice is to machine the scattered signals next to the particular
as-forged rotor wheel to a 2 µm surface reflection.
(8 × 10–5 in.) root mean square finish with
opposite faces parallel wherever possible Slight surface etching can remove the
so that the effects of varying shape and cause of these reflections. They are a
contour can be minimized. This is more concern because they can mask
necessary when testing materials that may indications from an actual discontinuity.
contain inclusions, such as carbide bands
whose reflections can be confused with
those from serious discontinuities.
In these cases, the forging vendor (who
also tests the wheels ultrasonically) is
hired to prepare the test objects by
machining. Enough stock is provided in
the forging and after machining, before
ultrasonic testing, so that the machine
finish contours and tolerances can be
maintained. This procedure is costly but
pays for itself in greater reliability of
ultrasonic testing for critical components.
After preliminary machining and
ultrasonic testing, the rotor wheels are
heat treated before shipment from the
forging vendor’s plant.
Indications from Heat Treating
Scale
Heat treating can produce a thin
imperceptible scale or film on the surface
of rotor wheels and this can in turn
produce confusing ultrasonic test
indications. Figure 24a shows the cross
section of a machined turbine rotor wheel
as obtained from a forging vendor.
The transducer is directed onto the
surface of the rotor in an area containing
a thin scale. The size of this scale has been
exaggerated to illustrate the condition
FIGURE 24. False indications from heat
treating scale: (a) path of ultrasonic beam
entering scaled surface; (b) A-scan
indications showing false indications
between front surface and back surface
indications
(a) Transducer
(b) Front surface Rear surface
False indications
Ultrasonic Scanning 283
PART 7. Immersion Testing of Composite
Materials1
Discontinuities in release paper on which impregnated
Composite Laminates (uncured) composite plies are delivered.
Composites are used in applications Another foreign material that can be
requiring materials with high ratios of left in the composite is peel ply. Peel plies
stiffness to weight or of strength to are used to prevent bonding of the
weight. Composites possess a complex laminate to the mold during cure and
failure mechanism which causes sometimes pieces of peel plies are
difficulties in establishing design criteria introduced into the laminate bulk.
and nondestructive test techniques. Generally, the presence of such inclusions
inhibit bonding between plies.
Most composites are made of layers
containing many fibers bonded with a During layup, ply gap can occur in a
matrix of different or equal composition. laminate if the various impregnated
The layers are stacked at various fiber composite tapes are not properly
orientations, depending on design positioned and a gap is left between them.
requirements. The resulting This gap is filled with a pocket of resin
heterogeneous, anisotropic, layered and causes a thickness reduction at its
characteristics hamper some center.
nondestructive test techniques well
established for homogeneous, isotropic Discontinuities such as delaminations,
materials. inclusions, porosity and ply gap can lead
to property degradation not accounted for
In aerospace, three composite systems in design and ultimately can shorten the
are commonly used: graphite epoxy (for composite’s service life.
critical structures), glass epoxy and plastic
epoxy. The diameter of the fibers varies Results of Composite
from 1 to 10 µm (4 × 10–5 to 4 × 10–4 in.) Discontinuities
for graphite and 5 to 15 µm (0.0002 to
0.0006 in.) for glass. Several types of At less than 2 percent of volume, porosity
discontinuities are commonly induced provides improved fracture toughness.
during the manufacture and service life of However, porosity also reduces
a composite structure. compressive and interlaminar shear
strength and compromises the fatigue life
Causes of Composite of the material. Porosity can produce an
Discontinuities increase in the moisture equilibrium level
and aggravates the thermal spike
Composite systems have a tendency to phenomenon. Both of these conditions
nucleate porosity if the volatile lead to deterioration in the material’s
components in the resin are not properly elastic properties.
removed during cure. At curing, trapped
air is pushed out along the fibers, Delaminations are a more severe
typically along the fibers and between discontinuity because they do not transfer
composite layers because of the high resin interlaminar shear stresses. Under
content in these regions. Once the curing compressive loading, delamination can
composite passes the gel stage, it begins to cause rapid and catastrophic buckling
harden and air is trapped in porosity or failure. The presence of a peel ply inside
voids. In addition to porosity, improper the laminate is harmful because of the
cure can lead to a partial delamination of low interlaminar shear it provides.
the composite plies.
The effect of ply gaps depends on the
Generally, delaminations result from stacking order and the discontinuity
hole drilling and impact damage. location. As an example, for [0, +45, 90,
Delaminations can also be the result of –45]2S laminate, a 2.5 µm (0.0001 in.) gap
stresses at the free edges of the composite, in the 0 rad (0 deg) layers reduces the
when the transverse tensile or shear tensile strength by 8 percent. The same
strength is exceeded. During layup, size gap in the 1.57 rad (90 deg) layer
foreign materials tend to be introduced, reduces the strength by 17 percent.15
particularly the plastic carrier film and the
284 Ultrasonic Testing
Ultrasonic Testing of discontinuities. Typical discontinuities
Composite Laminates that affect attenuation are porosity, voids,
impact damage and deviations in volume
Composites are usually tested for ratio of resin to fiber.
delamination using the straight beam
immersion technique. Time of flight is Many factors not related to material
measured to map the depth distribution quality may also affect attenuation, so
of discontinuities. Loss of back surface that attenuation changes serve as a
reflection provides a profile of severity. discontinuity indicator only when a
Time of flight can be used to identify severe change is observed. Porosity, for
delaminations of less than 1 mm (0.04 in.) example, is typically detected by its effect
diameter in graphite epoxy laminates with on attenuation. However, at low volume
±0.2 mm (±0.008 in.) accuracy.16 (below 3 percent), porosity can have
about the same effect as surface roughness
Figure 25 shows A-scan images or geometrical variations.
obtained from a 16-layer graphite epoxy
composite bonded to an aluminum Reflector Plate
honeycomb through a protective layer of
glass epoxy and adhesive bond. The In some thin structures, the ultrasonic
ultrasonic test uses short duration pulses beam attenuation may not be high
in the range of 100 ns. enough for testing. When this occurs, it is
common to use a reflector plate to detect
Figure 26 shows time-of-flight C-scan small discontinuities. The reflector plate
imaging of delaminations that resulted may be highly polished metal designed
from impact damage. Using the computer for the purpose. The bottom of the
to conduct a three-dimensional rotation, submersion tank may also be used.
the depth distribution of the
delaminations is clearly identified as The plate is used with the straight
shown above (Fig. 11). beam immersion technique and the test
object is placed between the transducer
The examination of back surface and the plate. Tests are conducted by
reflection amplitude provides a measure monitoring the changes in reflection
of the attenuation and can reveal the amplitude from the front surface of the
presence of material changes and plate after passing twice through the test
object.
FIGURE 25. Delamination detection inAmplitude Tests of Composite Tubing
graphite epoxy (depth of discontinuities is(relative scale)
indicated by time of flight): (a) trace Composite tubing is commonly
without delamination; (b) trace with manufactured by a filament winding
delamination. process rather than by stacking layers to
build a laminate. During the manufacture
(a) of composite tubing, several types of
discontinuities can be induced and some
1 of these may cause deterioration in
performance. For such tubes, straight
2
FIGURE 26. Computerized C-scan of impact
3 damage in graphite-to-epoxy laminate.
Travel time
(relative scale)
(b)
1
Amplitude
(relative scale)
4
Travel time
(relative scale)
Legend
1. front surface reflection.
2. reflection from fiber/glass layer.
3. rear surface reflection from adhesive layer.
4. reflection from 1 mm (0.04 in.) delamination.
Ultrasonic Scanning 285
beam ultrasonic tests can be performed 2. Knot appears as a local increase in
with water jet or bubbler instruments.4 tube thickness. As the discontinuity is
approached, an additional reflection
Characteristic Responses of splits off from the front reflection. The
Tubing Discontinuities additional reflection advances
gradually with respect to the front
The discontinuities below can be induced reflection and its amplitude grows
in a filament winding process. The steadily. In parallel, there is a decrease
characteristic responses when tested with in the amplitudes of the other echoes.
straight beam ultrasonics are also detailed When the center of the discontinuity
below. Each of these discontinuities has is reached, the reflection pattern
distinct characteristics that can be used acquires the appearance of a standard
with computer software to identify the echo train, advanced in time and
discontinuity automatically (Table 3). strongly attenuated. This behavior is
shown in Fig. 28.
1. Concealed cut, or ply gap, is a
discontinuity that produces a resin 3. Lack of rovings appears in a helical
pocket. The mismatch of acoustic configuration around the tube as a
properties in the discontinuity area, as local decrease in tube thickness. When
well as the presence of increased resin, the transducer is near the
cause a loss of back surface reflection discontinuity, a decrease of the echo
as shown in Fig. 27. train amplitude is observed and
simultaneously the whole train is
FIGURE 27. Ultrasonic A-scan indication of concealed cut: displaced from the main radio
(a) 3 mm (0.1 in.) away from discontinuity center; (b) at frequency pulse because of the
discontinuity center. increase in the time delay. Lack of
rovings also involves shorter time of
(a) flight but with no significant effect on
ultrasonic velocity. Whenever the
Amplitude severity of this discontinuity increases,
(relative scale) the ultrasonic changes are more
distinct. Reflection patterns from lack
of rovings are shown in Fig. 29.
Travel time or depth FIGURE 28. Ultrasonic A-scan indications of knot (dashed line
(relative scale) indicates location of front surface reflection): (a) 10 mm
(0.4 in.) from discontinuity center; (b) 4 mm (0.15 in.) from
(b) discontinuity center; (c) 2 mm (0.08 in.) from discontinuity
center; (d) at discontinuity center.
Amplitude
(relative scale) (a)
Travel time or depth (b) Advancing toward
(relative scale) (c) discontinuity location
(d)
Travel time or depth
(relative scale)
286 Ultrasonic Testing
4. Impact damage involves the FIGURE 30. Ultrasonic A-scan indications Advancing toward
appearance of local cracks and from impact damage: (a) 10 mm (0.4 in.) discontinuity location
delaminations. When the transducer is from discontinuity center; (b) 4 mm
near the discontinuity, additional (0.15 in.) from discontinuity center;
reflections appear and the reflection (c) 2 mm (0.08 in.) from discontinuity
from the inner diameter surface is center; (d) at discontinuity center.
decreased. This decrease reaches a
maximum at the center of the damage (a)
and small reflections may also appear
in the interval between the first and (b)
second reflections. The amplitude of
the reflection from the outer tube (c)
surface is not changed at the
discontinuity location and increased (d)
attenuation is measured. The
reflection pattern of impact damage is
shown in Fig. 30.
5. A resin starved layer has a
nonlocalized nature, extending
through a tube over a large area. This
discontinuity causes the appearance of
additional reflections originating at
the surface of the starved layer. The
time of flight between the
discontinuity reflection and the front
reflection depends on the depth of the
resin starved layer. This discontinuity
has some of the characteristics of
delaminations in laminates (Fig. 31).
FIGURE 29. Ultrasonic A-scan indications of Travel time or depth
lack of rovings (dashed line indicates (relative scale)
location of front surface reflection):
(a) 10 mm (0.4 in.) from discontinuity FIGURE 31. Ultrasonic A-scan indications of
center; (b) 4 mm (0.15 in.) from resin starved layer: (a) reference pattern;
discontinuity center; (c) 2 mm (0.08 in.) (b) discontinuity characteristics.
from discontinuity center; (a)
(d) at discontinuity center.
Advancing toward
(a) discontinuity location
(b)
Advancing toward
(c) discontinuity location Travel time or depth
(relative scale)
(d)
(b)
Travel time or depth
(relative scale) Advancing toward
discontinuity location
Travel time or depth
(relative scale)
Ultrasonic Scanning 287
TABLE 3. Characteristic parameters of various discontinuities in filament wound tubes.
Discontinuities Additional Increase of Shift of First Change of Velocity Changes near
Reflection Attenuation Reflection Time of Flighta Change Discontinuityb
Concealed cut — yes — — — —
Knot yes yes yes yes — yes
Lack of rovings — yes yes yes — yes
Impact damage yes yes — — — yes
Resin starved layer yes — — yes — —
Flexible resin — yes — yes yes —
Low modulus fibers — — — yes yes —
a. Change in time of flight can indicate thickness variation without corresponding change in ultrasonic velocity.
b. Several discontinuities in one location can be detected ultrasonically before positioning of transducer at discontinuity
location.
6. Flexible resin is a nonlocalized
discontinuity caused by the wrong
resin, an incorrect amount of hardener
or unsatisfactory curing. Ultrasonic
pulses are attenuated in flexible resin
much more than in a properly
hardened resin (17 dB in glass epoxy,
for example). In addition, flexible
resin is associated with some decrease
in the ultrasonic velocity. The
identification of this discontinuity
requires that reference velocity and
attenuation values be compared with
those of the tubing being tested.
7. Low modulus fibers constitute another
nonlocal discontinuity. It is caused by
use of the wrong fibers in the
composite. This discontinuity exhibits
a decrease in ultrasonic velocity by as
low as 10 percent of the original
velocity. No significant attenuation
change is observed.
These discontinuity types are listed in
Table 3.
288 Ultrasonic Testing
PART 8. Angle Beam Immersion Techniques1
Beam refraction occurs when the Scanning Systems
ultrasonic wave is not perpendicular to
the test object surface. Immersion For disks, cylinders, tubes and various
coupling provides the versatility of other shapes with axial symmetry, a
choosing any desired incidence angle. turntable may be used in combination
There are several reasons for performing with the rectilinear bridge of C-scan
angle beam tests: (1) to induce transverse ultrasonic systems. Turntable systems are
waves, (2) to insonify at an angle that provided with self-centering chucks or
provides maximum reflection from other handling devices to hold test
discontinuities, (3) to avoid specular objects in a properly aligned and secured
reflection from the test object’s front position. Even though the rotation speeds
surface, (4) to induce surface waves and can reach thirty revolutions per minute,
(5) to induce lamb waves. the tendency to produce turbulence at
such speeds enforces slower speeds.
Principles of Angle Beam
Tests When testing cylinders with a
turntable, C-scan images can be produced
When an ultrasonic beam impinges on a on paper wrapped around a drum placed
surface at an incidence angle other than on a recorder. The drum height is at least
normal, the transmitted wave is refracted equal to the test object’s height and its
according to Snell’s law. This
phenomenon is used in the immersion FIGURE 32. Propagation of sound in tube
ultrasonic test of tubular shapes. Because wall: (a) sawtooth pattern of distorted
the angles of the sound beam are adjusted longitudinal wave; (b) sound flooding
from 0.18 to 0.52 rad (10 to 30 deg), the around thin walled tube.
refraction sends the beam around the
tubular annulus. (a) Sound beam
Mode of Beam Propagation Tubing
The sound is propagated around a pipe (b) Sound beam
wall in a saw tooth or zigzag pattern, and
the mode of propagation is either Tubing
transverse or longitudinal vibration. The
saw tooth pattern is fairly sharp and a
discontinuity can be detected only when
its position coincides with the beam path.
Empirical data indicate that the sound
is propagated around the thin wall of the
small diameter tubing as a severely
distorted longitudinal wave. As the
boundaries of the material approach each
other, they offer increased interference
and the longitudinal velocity is
appreciably reduced. This behavior
indicates that the wave mode is modified.
With progressively thinner tube walls,
the reflection nodes become less sharp.
When the wall is very thin, the sound
floods around the very narrow metal wall.
These two concepts of the travel pattern
are depicted in Fig. 32. Observations of
thin walled tubing tests indicate that the
sound wave phenomenon may be
explained by comparison with the
formation of lamb waves.
Ultrasonic Scanning 289
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