In metallurgy, cladding refers to the bonding together of dissimilar metals, normally achieved by extruding two or more metals through a die or pressing sheets together under high pressure. Timely detection of delamination that occurs occasionally during the cladding processes is very important for the industry. This paper presents an EMAT system based on ultrasonic guided wave techniques. The analysis of a three-layer, brass/copper/brass product is also presented including dispersion curves, and interaction of ultrasonic guided wave with delamination defects. The authors observed a cyclic behavior of guided wave propagation with the increase of defect size. An explanation is introduced and proved with finite element analysis. The results presented in this paper will have a very significant impact on understanding of delamination detection in multilayered composite structures including adhesive bonded structures.

1. Delamination Detection in Composite Clad Products Using Ultrasonic
Guided Wave EMATs
H. Gao, S. M. Ali, J. Monks, and B. Lopez
Innerspec Technologies, Inc., Lynchburg, VA 24501
ABSTRACT. In metallurgy, cladding refers to the bonding together of dissimilar metals, normally
achieved by extruding two or more metals through a die or pressing sheets together under high
pressure. Timely detection of delamination that occurs occasionally during the cladding processes is
very important for the industry. This paper presents an EMAT system based on ultrasonic guided
wave techniques. The analysis of a three-layer, brass/copper/brass product is also presented including
dispersion curves, and interaction of ultrasonic guided wave with delamination defects. The authors
observed a cyclic behavior of guided wave propagation with the increase of defect size. An
explanation is introduced and proved with finite element analysis. The results presented in this paper
will have a very significant impact on understanding of delamination detection in multilayered
composite structures including adhesive bonded structures.
Keywords: Ultrasonic, Guided Wave, EMAT, Delamination, Composites
PACS: 43.20.Mv 43.35.Zc 43.20.Gp
INTRODUCTION
Multi-layered composites are widely used to enhance the structural capabilities of
different materials. In composites created by cladding or adhesive bonding, delamination
is an ever-present risk that can compromise the quality of the final product. Timely
detection of potential delamination is very important to save production cost and prevent
consequent failure.
Nondestructive evaluation using guided waves has attracted a lot of attention in
recent years because of its ability to cover large spans of product at a distance. Innerspec
Technologies has developed many guided wave systems for in-line and in-service
nondestructive testing using electromagnetic acoustic transducer (EMAT) technolgy.
EMAT is very effective way of exciting and receiving specific guided wave modes, so it is
a natural choice for these applications.

2. In this paper, we introduce a new application of guided wave EMATs for detection
of delamination in a brass/copper/brass three-layered composite. In addition to detection,
using finite element analysis and guided wave modal analysis we were also able to model
and explain the results from different defect sizes/geometries.
EXPERIMENTAL SYSTEM AND TEST RESULTS
Figure 1 (a) shows an EMAT lab testing system. A temate®
PowerBox 2 is used
for excitation signal generation and data acquisition. A lap top computer and temate®
PowerUT software is used for signal processing and display. Figure 1(b) shows a three
layered brass/copper/brass test sample and the through transmission sensor arrangement. A
unidirectional EMAT transmitter is used to excite guided waves at the left side, and a
receiver is at the right side. In this scheme, defects between the transmitter and receiver
pair will be detected from the changes in the through transmission signal. During in-line
inspection, composite strips move under the sensor and gets inspected at a speed of around
1 m/s or more.
(a) (b)
FIGURE 1. A EMAT lab testing system includes an instrument, sensor assembly, and a sample.
(a) (b)
(c) (d)
FIGURE 2. Example though transmission signals from four test samples. (a) good sample, (b) with 0.2
inch wide defect (c) with 0.3 inch wide defect (d) with 0.625 inch wide defect.
Figure 2 shows test results on a good sample and three defective samples. The
defects are intentional made during the manufacturing process. The widths of the
delaminations are about 0.2 inch, 0.3 inch, and 0.625 inch respectively. The fourth order
guided wave mode at 2045 kHz is used for the test. All the defects can be detected from

3. the amplitude reduction in the signal. However, an interesting phenomenon is observed
that the signal amplitude does not change monotonically with the increase of defect width.
NUMERICAL SIMULATION OF DEFECT SCATTERING
Finite element simulation is used to verify the phenomenon observed in the
experiment through a parametric study using ABAQUS. Figure 3 illustrates the three
layered structure with a delamination of width d, which varies from 0 to 1 inch with 0.04
inch step. The simulation result shown in Figure 4 indicates that the amplitude of the
signal varies cyclically with the increase of defect width. The amplitude reduction can be
very significant at the valleys and may also be very slight at the peaks.
FIGURE 3. Illustration of parametric study of delamination in a three layered composite structure
FIGURE 4. Simulation result of through transmission signal amplitude as a function of delamination width
MODAL ANALYSIS AND EXPALANATION
In order to explain the interesting cyclic behavior of wave interaction with
delamination, a detailed guided wave modal analysis is performed and presented in this
section. 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 of the three layered structure is shown in Figure 5.
d

4. FIGURE 5. Phase velocity and group velocity dispersion curve of the three layered composite structure
FIGURE 6. Phase velocity dispersion curves of (a) the one layered subsystem (b) the two layered subsystem.
FIGURE 7. Mode decomposition dispersion curves from the incidence of fourth order mode in the three
layered structure (a) the one layered subsystem (b) the two layered subsystem.
At the delaminated region, the incident guided wave mode decomposes into wave
modes in two subsystems, one with only the brass layer, and the other with brass and
copper. The dispersion curves of the two subsystems are plotted in Figure 6. Normal mode
expansion technique [2, 5] is used to study the mode decomposition at the front tip of the
delamination. Figure 7 shows the amplitude spectrum of decomposed wave modes when
the incident wave is mode 4. For the case of incident mode at 2045kHz, the dominant
wave mode in the two subsystems are mode 2 and mode 4 respectively.
At the end tip of delamination, the waves in the two subsystem converts back to the
wave modes in three layered structure. Since the EMAT receiver has an effective mode
1
2
2
1
3
4
2
1
3
4

5. selection, the amplitude of the receiving signal is strongly affected by the amount of
energy converted back to the incident mode. After a detailed modal analysis and finite
element simulation, our explanation to the cyclic behavior of amplitude as a function of
defect width is as follows.
• When the two dominant modes in the subsystems arrive at the end tip of the
defect in phase, the resultant mode will be dominated by the original
incident mode. The amplitude of the transmission signal will be large.
• When the two dominant modes arrive out of phase, the amplitude of the
transmission signal will be small.
• For the situations in between, the amplitude of the receiving signal will vary
graduation between the two extreme situations.
The numeric formula of period of the cyclic behavior is shown in Equation 1.
12
211
pp
pp
CC
CC
f
d
−
= (1)
Cyclic periods calculated from Equation 1 and periods obtained from FEM simulation for
several different incident wave modes are listed in Table 1. The excellent match between
expectation and numerical simulation proves our explanation to the cyclic phenomenon.
TABLE 1. Comparison of the period from modal analysis and FEM simulation
Frequency
(kHz)
Incident
mode
d (inch)
Modal Analysis
d (inch)
FEM Simulation
2045 4 0.454 0.46
1870 4 0.388 0.38
1729 4 0.281 0.28
959 1 0.231 0.23
CONCLUSIONS
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 delaminations in a brass/copper/brass clad composite.
In the experiment, the authors observed a very important cyclic behavior of the
signal amplitude as a function of defect width. This phenomenon is then confirmed with
finite element analysis. After detailed guided wave modal analysis and mode
decomposition and recombination study, the authors provided an explanation to this cyclic
phenomenon. A formula is given to estimate the periodicity of the cyclic change, which is
also verified with finite element simulation.
The observation and explanation of the cyclic behavior improved our
understanding of guided wave interaction with lamination defects. In a final sensor design
for the EMAT system, the cyclic behavior is considered and controlled to ensure confident
detection of delaminations of difficult size and also minimized false indication. The result
of this study will have a broad impact on the understanding of delamination detection in
other composite structures such as adhesively jointed composites and lap joints.

6. 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).