Clad metals are composite metal containing two or more layers that have been bonded together. The bonding may have been accomplished by rolling, extrusion, welding, diffusion bonding, casting, heavy chemical deposition, or heavy electroplating. Clad metals offer the opportunity to combine desirable properties and/or characteristics of individual metals and alloys into a material "system" that provides improved characteristics over the individual metals. In the event the bond quality is compromised, these materials will not meet their original purpose. Disbond in clad layers is very similar to an internal void in single layer materials such as steel strip material.

1. Guided Wave EMAT Technique for Composite Plate Inspection
Syed Ali1
, Huidong Gao2
, Jeff Monks3
, Borja Lopez4
1,2,3,4
Innerspec Technologies
4004 Murray Place, Lynchburg VA 24501
(434)847-2023; Fax (434)948-1313; email sali@innerspec.com, hgao@innerspec.com, jmonks@innerspec.com,
blopez@innerspec.com ,
INTRODUCTION
Clad metals are composite metal containing two or more layers that have been bonded together. The bonding may
have been accomplished by rolling, extrusion, welding, diffusion bonding, casting, heavy chemical deposition, or
heavy electroplating. Clad metals offer the opportunity to combine desirable properties and/or characteristics of
individual metals and alloys into a material "system" that provides improved characteristics over the individual
metals. In the event the bond quality is compromised, these materials will not meet their original purpose. Disbond
in clad layers is very similar to an internal void in single layer materials such as steel strip material.
CLAD METALS IN COIN STOCK
A recent application of strip inspection was for the inspection of clad coin stock material. In recent years (late 1960’s)
most worldwide coinage transitioned from solid precious metals (gold, silver, copper) to a multi-layered clad material
comprised mostly of a copper core and outer layers of alloy to appear silver or gold but wear longer and obviously cost
far less to produce.
The desire to produce only high quality coinage and the new implementation of clad coin stock has been a difficult
struggle. In the event of a clad disbond on coin stock, the resultant coin minted may:
• Best Case: Have an open seam on the outer rim
• Worse Case: May appear to have a “clam-shell” rim which is widely opened
• Worst Case: Have no outer layer at all (a head-less or tail-less coin).
In this paper, we introduce our new application of guided wave EMATs for 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, we were also able to model and explain the results for different defect sizes and
geometries.
Our guided wave method is based on an already proven technique previously implemented in a steel mill pickle line to
detect internal and surface flaws such as pencil pipe, laminations, voids and dissimilar material inclusions. The
detection of these flaws early in the manufacturing process allow the operator the opportunity to “downgrade” the
material before value added processes are performed thus increasing the utilization of mill assets and assuring only
quality material was processed further, yielding acceptable quality at the end of additional processes.
EXPERIENCE WITH MULTI-LAYERED MATERIALS
Our 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 recently developed in-house at Innerspec Technologies to aid in
the efficient design of application specific EMAT sensor. The following research data provides 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.
Some 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 0.064 inch in total and 0.016’’, 0.032’’, and 0.016’’ for the brass, copper,

2. and brass layers respectively.
Figure 1: Metallurgical photo of the cross section of the three layered cladding product.
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 the copper core and the other brass clad with good bond.
Using the theory of guided wave propagation in multilayered structures [1-2] and a semi-analytical finite element
technique [3-5], the phase velocity and group velocity dispersion curves are plotted in Figures 2 and 3. These guided
wave modes correspond to 0.08’’ wavelength as is plotted in the figures. These figures indicate that there is a
significant difference in the guided wave dispersion curves for the three systems.
Figure 2: Dispersion curves of the three layer system (a) phase velocity (b) group velocity
Figure 3: Dispersion curves of the one layer subsystem (a) phase velocity (b) group velocity
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
1
2 3
4
4
2
2
1
3

3. and be 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.
Normal mode expansion method [2, 5] is used in this section to study the mode decomposition behavior at the front
tip of a lamination. In the numerical calculation, we studied all the possible incident wave modes, and generated the
mode decomposition curves. As an example, the mode decomposition curves for incident mode 4 are plotted in
Figure 11.
When mode 4 at 2045 kHz is used in the incidence, mode 2 will be the dominant mode in the subsystem I. However,
in the subsystem II, there is a combination of mode 4, mode 3, mode2, and mode 1, in which mode 4 has slightly
larger energy then others.
Figure 4: Mode 4 incidence (a) subsystem I (b) subsystem II
At the end tip of the lamination, the modes in 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. Assume the dominant de-composed mode in subsystem I and subsystem II are mode Ia, and mode IIb.
If the lamination is short or the two modes have similar group velocity, these two modes are going to 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 in the following process.
• The copper core and the brass surface clad are annealed separately to reach a full re-crystallization.
• The copper core, 0.032’’, and two layers of brass each 0.016’’, are roll bonded together. After the roll
bonding, the thickness of the strip is 0.064’’.
• After bonding, the composite strip is annealed, the copper core is fully re-crystallized, and the alloy clad is
only partially re-crystallized.
• The composite strip is rolled to finish thickness, 0.064’’.
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 layer changes in the rolling direction, this is also called corrugation.
The empirical estimation of the period of the corrugation is between 0.3’’ and 0.4’’. A metallurgical photo of the
cross section of the strip is shown in Figure 5.
1
2
1
3
2
4

4. Figure 5: Sample metallurgical photo of corrugation of copper core in the composite strip.
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 defect. In this section, finite element modeling is
used to reveal the details of the wave scattering process using “Abaqus” and to test the hypothesis put forward with
the guided wave modal analysis. With the same excitation mode and a sequence of FEM simulations with changing
defect width was performed.
Figure 6: Amplitude and time of flight measurement of the receiving signal.
Once all modeling was completed, an actual Laboratory system was assembled and installed in an off line re-rolling
location. This system was tested for nearly 6 months and data was studied to continuously verify that these results
matched the models developed. The guided wave EMAT method of strip inspection requires the use of a separate
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 defects in strip materials can easily be detected.
Figure 7: Modeling verification and Signal variance at disbond

5. CONCLUSION
An ultrasonic guided wave EMAT system is introduced in this paper for delamination detection in multilayered
composite products. Our system has successfully detected both intentional and natural delamination in a
brass/copper/brass clad composite. Ultimately a production system was designed and installed to operate in-line 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 temate®
Pi-GW system has now been verified in operation on both single layer and multi-layer clad materials.
With the successful implementation of proprietary guided wave modeling and finite element analysis, the
implementation of a clad multi-layered 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 methods.
EMAT generated guided waves are key to an in-line inspection of such materials as the EMAT method is the only
way to efficiently generate guided lamb waves into production inspection environments..
Figure 8: Result from Installed on-line System for Natural defect about 3 inch long and 0.3’’ wide
REFERENCES
1. J.L. Rose, Ultrasonic Waves in Solid Media, Cambridge University Press, (1999).
2. B.A. Auld, Acoustic Fields and Waves in Solids. Krieger Publishing Company (1990).
3. T. Hayashi, W.J. Song, and J.L.Rose, Ultrasonics 41, pp: 175-183, 2003.
4. H. Matt, I. Bartoli, and F. Lanza di Scalea, Journal of Acoust, Soc. Am 118(4): pp: 2240-2252, 2005.
5. H. Gao, "Ultrasonic Guided Wave Mechanics for Composite Structural Health Monitoring", Ph.D. Thesis,
Penn State University (2007).
6. H. Gao, S. M. Ali, J. Monks, and B. Lopez, Review of Progress in Quantitative Nondestructive
Evaluation Vol. 28B, pp: 1121-1126.
temate®
Pi-GW system as installed at Clad Coin Stock Factory