This document discusses a study on using guided wave electromagnetic acoustic transducer (EMAT) techniques to inspect three-layered clad coin stock materials for delamination. The study involved: 1. Developing guided wave dispersion curves for the intact three-layer structure and for delaminated substructures. 2. Modeling guided wave propagation and interaction with laminations using finite element analysis. 3. Testing an EMAT inspection system installed on a coin stock production line, which successfully detected disbonds. 4. Verifying the guided wave modeling through destructive and offline nondestructive testing techniques.

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2. Guided
Wave
Clad Materials in Three-layered Coin Stock
In recent years (since the late 1960s), most worldwide coinage has transitioned from
solid precious metals (gold, silver, copper) to a multilayered clad material comprising
mostly a copper core and outer layers of alloys, which wear out less often and cost less
to produce.
The desire to produce only high quality coinage and the new implementation of clad
coin stock has been a difficult challenge. In the event of a clad disbond on coin stock, the
resultant minted coin might:
l have an open seam on the outer rim (best case);
l appear to have a “clamshell” rim that is widely opened (worse case);
l have no outer layer at all, that is, a headless or tail-less coin (worst case).
Clad metals are multilayered metals containing two or more layers that have been
bonded together. This bonding can be accomplished by cold or hot rolling, extrusion,
welding, diffusion bonding, casting, heavy chemical deposition, or heavy electroplating.
Clad metals offer the opportunity to combine desirable properties and characteristics
of individual metals and alloys into a material “system” that provides improved
characteristics over the individual metals, such as corrosion resistance, lower cost,
and high strength-to-weight ratio. In the event the bond quality is compromised,
these materials will not meet their original purpose; therefore, it is important to detect
disbonding between clad layers in a timely manner.
Guided Wave
Electromagnetic Acoustic
Transducer Technique for
Inspection of Clad Plates
by Syed Ali, Francisco Hernandez-Valle,
and Borja Lopez
feature • NDTMarketplace 25
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3. This paper introduces a new application of
guided wave electromagnetic acoustic transducers
(EMATs) for the detection of delamination in a
brass/copper/brass three-layered composite used
for coin stock. In addition to detection, using finite
element analysis and guided wave modal analysis,
it was possible to model and explain the results for
different discontinuity sizes and geometries.
The guided wave testing method is based on an
already proven technique previously implemented
in a steel mill pickle line to detect and identify
rejectable internal and surface flaws such as pencil
pipe, laminations, voids, and dissimilar material
inclusions. The detection of these flaws early in
the manufacturing process allowed the operator
the opportunity to “downgrade” the material
before value added processes were performed,
thus increasing the utilization of mill assets and
ensuring only quality material was processed further,
yielding acceptable quality at the end of additional
processes.
Experience with Multilayered Materials
A recent project with the supplier of coin stock
material for a national mint allowed the unique
opportunity to use advanced modeling and finite
element analysis tools developed to aid in the
efficient design of an application specific EMAT
sensor. The following research data provide
a fundamental investigation of guided wave
propagation in multilayered structures and the
interaction of guided waves with laminations. The
result of this research not only proves that guided
waves are applicable for lamination detection but
also provides a theoretical guideline for guided
wave sensor design and interpretation of guided
wave signals. Empirical testing was completed with
the installation of an EMAT inspection system in the
manufacturer’s facility. Actual inspection results are
presented in this paper.
Figure 1 shows a metallurgical photo of the
cross-section of the three-layered cladding product.
The normal thickness of the three-layered structure
is 1.63 mm (0.064 in.) in total and 0.41, 0.81, and
0.41 mm (0.016, 0.032, and 0.016 in.) for the brass,
copper, and brass layers, respectively.
In order to study the behavior of guided waves
in a delaminated composite plate, guided wave
dispersion curves of three material systems needed
to be obtained. The first system in this material
is the pristine structure with all three layers well
bonded. The second is a delaminated substructure
with the brass clad layer only. The third is the
remaining two-layered subsystem consisting of the
copper core and the other brass clad with good
bond.
Using the theory of guided wave propagation in
multilayered structures and a semi-analytical finite
element technique, the phase velocity and group
velocity dispersion curves were obtained and are
plotted in Figures 2 and 3 (Auld, 1990; Gao, 2007;
Gao, 2008; Hayashi et al., 2003; Matt et al., 2005;
Rose, 1999). These guided wave modes correspond
to a 2.03 mm (0.08 in.) wavelength, as is plotted
in the figures. These figures indicate a significant
difference in the guided wave dispersion curves for
the three systems.
When a guided wave mode encounters a
lamination, the original mode will be decomposed
into the possible wave modes in each subsystem.
When the lamination is short, these new modes
will meet at the end tip of the lamination and be
26 NDTMarketplace • feature
Figure 1. Metallurgical photo of the cross-section of the three-layered cladding product.
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4. converted back to the wave modes of the three-
layered structure. When the lamination is long,
the group velocity of the wave modes in the two
subsystems may differ enough such that their wave
packages arrive at the end tip in a totally separated
time. It is important to mention that the final wave
mode is not necessarily the incident mode, but it can
be a combination of several modes.
A normal mode expansion technique was used
in this section to study the mode decomposition
behavior at the front tip of a lamination (Auld, 1990;
Gao, 2007). In the numerical calculation, all of the
possible incident wave modes were studied, and the
mode decomposition curves were generated. As an
example, the mode decomposition curves for incident
mode 4 are plotted in Figure 4.
feature • NDTMarketplace 27
20.3
17.8
15.2
12.7
10.2
7.6
5.1
2.5
0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
1
2
3
4
Frequency (MHz)
Phasevelocity(mm/µs)
Groupvelocity(mm/µs)
4.1
3.6
3.0
2.5
2.0
1.5
1.0
0.5
0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
Frequency (MHz)
4
Figure 2. Dispersion curves of the three-layered system: (a) phase velocity; and (b) group velocity.
(a) (b)
20.3
17.8
15.2
12.7
10.2
7.6
5.1
2.5
0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
Frequency (MHz)
Phasevelocity(mm/µs)
Phasevelocity(mm/µs)
2
1
3
4.1
3.6
3.0
2.5
2.0
1.5
1.0
0.5
0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
Frequency (MHz)
Figure 3. Dispersion curves of the one-layered subsystem: (a) phase velocity; and (b) group velocity.
(a) (b)
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5. 28 NDTMarketplace • feature
When mode 4 at 2045 kHz is used in the
incidence, mode 2 will be the dominant mode in
subsystem 1. However, in subsystem 2, there is a
combination of mode 4, mode 3, mode 2, and
mode 1, in which mode 4 has slightly greater
energy than the others.
At the end tip of the lamination, the modes in the
two subsystems will combine with each other and
form transmission wave modes in the three-layered
system. At the same time, reflected wave modes will
also be produced in the two subsystems. It is assumed
that the dominant decomposed mode in subsystem 1
and subsystem 2 are mode 1a and mode 2b. If the
lamination is short or the two modes have similar
group velocity, these two modes will interfere at the
end tip of the lamination and form transmitted modes
after the lamination. When the two modes arrive in
phase, wave energy will be converted back to the
incident mode; therefore, the amplitude drop will be
insignificant. On the other hand, if the two modes
arrive out of phase, significant mode conversion will
occur and produce a significant change in the receiving
signal. This will be measured with a significant
amplitude drop and time of flight change.
The three-layered brass/copper/brass clad products
are manufactured using the following process.
0.0351 mm
0.0254 mm 0.340 mm
0.0404 mm
3.175 mm
Figure 5. Sample metallurgical photo of corrugation of copper core in the composite strip.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
Frequency (MHz)
Modeamplitude
Modeamplitude
2
1
2
1
3
4
0.7
0.8
0.9
0.6
0.5
0.4
0.3
0.2
0.1
0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
Frequency (MHz)
Figure 4. Mode 4 incidence: (a) subsystem 1; and (b) subsystem 2.
(a) (b)
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6. l The copper core and brass surface clad
are annealed separately to reach a full
recrystallization.
l The copper core, 0.81 mm (0.032 in.), and two
layers of brass, each 0.41 mm (0.016 in.), are
roll bonded together. After the roll bonding, the
thickness of the strip is 1.63 mm (0.064 in.).
l After bonding, the composite strip is annealed,
the copper core is fully recrystallized, and the
alloy clad is only partially recrystallized.
l The composite strip is rolled to finish thickness,
1.63 mm (0.064 in.).
Although the overall thickness of the strip can be
well controlled, the core layer can be “wavy” after the
process. This makes the thickness of the two alloy
clad layers change in the rolling direction; this is also
called corrugation. The empirical estimation of the
period of the corrugation is between 7.62 and
10.2 mm (0.3 and 0.4 in.). A metallurgical photo
of the cross-section of the strip is shown in
Figure 5.
The influence of lamination width on guided
wave scattering was also studied. Modal analysis
expects a cyclic performance of a guided wave
mode with respect to the width of a discontinuity.
In this section, finite element modeling (FEM) is
used to reveal the details of the wave scattering
process using commercial software and to test
the hypothesis put forward with the guided wave
modal analysis. Using the same excitation mode,
a sequence of FEM simulations with changing
discontinuity width was performed, shown in
Figure 6.
Once all modeling was completed, an actual
laboratory system was assembled and installed in an
off-line re-rolling location (Figure 7). This system was
tested for nearly six months and data were studied
to continuously verify that these results matched the
developed models. The guided wave EMAT technique
of strip inspection requires the use of a separate
FEATURE • NDTMarketplace 29
Figure 7. Modeling verification test setup as part of an
offline rerolling location.
Figure 6. Receiving signal measurements: (a) amplitude; (b) and time of flight.
(a) (b)
0.05
0
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.6
2.5 5.1 7.6 10.2 12.7 15.2 17.8 20.3 23 25.4
Width of discontinuity (mm)
Signalamplitude
0 2.5 5.1 7.6 10.2 12.7 15.2 17.8 20.3 23 25.4
Width of discontinuity (mm)
Timeofflight(µs)
105
95
90
100
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7. 30 NDTMarketplace • FEATURE
transmitter and receiver held at a fixed mechanical distance
from each other. In a generally homogenous material, the
time of flight of the transmitted signal to the receiver and the
amplitude of the received signal is near constant. By closely
monitoring the time of reception of the EMAT generated
ultrasonic energy and the amplitude of this signal, the
presence of rejectable flaws in strip materials can easily be
detected, shown in Figure 8.
Conclusion
An ultrasonic guided wave EMAT system is introduced
in this paper for delamination detection in multilayered
composite products. The system successfully detected
both intentional and natural delamination in a brass/
copper/brass clad composite. Ultimately, a production
system was designed and installed to operate inline at line
speeds. Since its installation, disbonded coin stock has
been eliminated. New guided wave dispersion curves have
been developed for other types of coin stock clad alloy
inspection. The operation is fully automated and the system
includes an automated paint marking system to identify
disbonded areas on the strip’s edge while processing.
The guided wave EMAT system has now been verified
in operation on both single-layered and multilayered clad
materials. With the successful implementation of proprietary
guided wave modeling and finite element analysis, the
implementation of a clad multilayered inspection system
was successfully fielded in a coin stock supplier’s factory for
100% line speed inspection. The results have been verified
via destructive and off-line nondestructive techniques. EMAT
generated guided waves are key to an inline inspection of
such materials, as the technique is the only way to efficiently
generate guided lamb waves into production inspection
environments.
Figure 8. Signal variance: (a) indicating a disbond; and (b) indicating a natural discontinuity approximately 7.62 cm (3 in.) long and
0.76 cm (0.3 in.) wide.
100
80
60
40
20
0
22.86 21.74 20.62 19.51 18.39 17.27
Position (cm)
100.0
97.5
95.0
92.5
90.0
87.5
85.0
82.5
80.0
Timeofflight(µs)
Amplitude(%)
(b)
100
80
60
40
20
0
15.32 20.39 25.47 30.54 35.62 40.69
100.0
97.5
95.0
92.5
90.0
87.5
85.0
82.5
80.0
Amplitude(%)
Position (cm)
Timeofflight(µs)
(a)
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8. FEATURE • NDTMarketplace 31
Authors
Syed Ali: M.S., ASNT NDT Level III, Innerspec Technologies, 4004
Murray Pl., Lynchburg, Virginia 24501; (434) 948-1301; fax (434)
948-1313; e-mail sali@innerspec.com.
Francisco Hernandez-Valle: Ph.D., Innerspec Technologies, 4004
Murray Pl., Lynchburg, Virginia 24501; (434) 948-1301; fax (434)
948-1313; e-mail fhernandez@innerspec.com.
Borja Lopez: Innerspec Technologies, 4004 Murray Pl., Lynchburg,
Virginia 24501; (434) 948-1301; fax (434) 948-1313; e-mail
blopez@innerspec.com.
References
Auld, B.A., Acoustic Fields and Waves in Solids, Krieger Publishing
Co., Malabar, Florida, 1990.
Gao, H., “Ultrasonic Guided Wave Mechanics for Composite
Structural Health Monitoring,” Ph.D. thesis, Pennsylvania State
University, 2007.
Gao, H., S.M. Ali, J. Monks, and B. Lopez, “Delamination Detection
in Composite Clad Products using Ultrasonic Guided Wave EMATs,”
Review of Progress in Quantitative Nondestructive Evaluation,
Vol. 28B, 2008, pp. 1121–1126.
Hayashi, T., W.J. Song, and J.L. Rose, “Guided Wave Dispersion
Curves for a Bar with an Arbitrary Cross-section, a Rod and Rail
Example,” Ultrasonics, Vol. 41, 2003, pp. 175–183.
Matt, H., I. Bartoli, and F. Lanza di Scalea, “Ultrasonic Guided
Wave Monitoring of Composite Wing Skin-to-spar Bonded Joints in
Aerospace Structures,” Journal of the Acoustical Society of America,
Vol. 118, No. 4, 2005, pp. 2240–2252.
Rose, J.L., Ultrasonic Waves in Solid Media, Cambridge University
Press, Cambridge, United Kingdom, 1999.
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