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Copyright © 2010 by ASME

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Proceedings of the ASME 2010 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS2010

September 28 - October 1, 2010 Philadelphia, PA, USA

SMASIS 2010-3780

INVESTIGATION OF INCIDENT LAMB WAVE PARAMETERS ON DETECTION OF COMPOSITE DELAMINATION

Ratneshwar Jha Inho Kim Dulip Widana-Gamage Department of Mechanical and Aeronautical Engineering

Clarkson University, Potsdam, New York 13699-5725 ABSTRACT

A preliminary investigation of excitation signal parameters

on the detection of delamination in composite plates is

presented. Composite plates typical of aerospace

applications are used and excitation is provided through

integrated PZT actuator. A scanning laser vibrometer is

used for recording structural responses. Experimental

results at three frequencies (15, 18, and 20 kHz) and two

pulse widths (3.5 and 5.5 cycles) show the significant

effects of incident Lamb waves on damage signature.

Numerical studies using a commercial finite element code

(ABAQUS) show good correlation with experimental

results with appropriate level of structural damping.

INTRODUCTION

The use of composite materials has many benefits for

engineering structures; however, the damage mechanisms

of composites are very different from those of metals. One

particular damage mechanism, namely delamination cracks,

can be initiated by loading, impact, or manufacturing

defects. The crack lengths can reach critical value before

visual inspection. Lamb waves (fundamental symmetric and

asymmetric modes) have shown particular promise for

damage diagnosis of composite structures1-7

. The

fundamental idea behind Lamb wave propagation based

diagnostics is that different types of damages interact

differently with waves. Therefore, based on the measured

time history of the propagated wave, the traveling time,

speed reduction, and wave attenuation parameters are

extracted and used as the damage identification variables.

Further processing of the measured signals (e.g., using

wavelet transform, Hilbert-Huang transform, etc.) helps

damage recognition.

The incident Lamb waves depend on the excitation signal

parameters (pulse shape, amplitude, frequency and number

of cycles to be sent during each pulse period) and may have

significant impact on damage detection. Published papers

discuss the effects of actuation pulse parameters in a

general way, but specifics are seldom reported. Kessler1

considered the actuation pulse parameters, but presented

results for 15 kHz 3.5 cycles only. Several authors suggest

higher frequencies, but Diamanti and Soutis7 consider

frequencies below 50 kHz to be particularly sensitive for

composite diagnostics. The fundamental anti-symmetric

Lamb mode A0 is generated at frequencies below 50 kHz

which has much lower phase velocity than the symmetric S0

mode and thus a smaller wavelength making it more

sensitive to damage detection. Delamination sizes well

below the wavelength of the propagating mode were

successfully detected by Diamanti and Soutis7.

This paper presents a preliminary investigation of excitation

signal parameters on the detection of delamination in

composite plates. We use composite plates typical of

aerospace applications and provide excitation using

integrated PZT actuator. A scanning laser vibrometer is

used for recording structural responses. Experimental

results at three frequencies (15, 18, and 20 kHz) and two

pulse widths (3.5 and 5.5 cycles) show significant effect on

damage signature. Numerical studies using a commercial

finite element code (ABAQUS) show good correlation with

experimental results with appropriate level of structural

damping.

LAMB WAVE DISPERSION CURVES

Lamb waves result from the superposition of guided

longitudinal and transverse (shear) waves. Lamb waves

travel in thin plates with unconstrained boundaries and have

the capability of traveling long distances with little

attenuation. Due to their propagation characteristics, Lamb

waves can be used as a means to detect both superficial and


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internal flaws in a structure. Local stiffness degradation,

crack or delamination (for composite structures) causes

reflection, dispersion, attenuation and mode shape change.

These changes in wave characteristics can be used to

diagnose structural defects.

Lamb waves are highly dispersive, exhibiting dependency

of phase/group velocity on wave excitation frequency.

Development of dispersion curves, which map modal

phase/group velocities with respect to propagation

frequency, provide an important tool in examining the

propagation characteristics of Lamb waves. Due to the

dependency of longitudinal and transverse wave velocity on

the properties of the medium, dispersive characteristics vary

with material properties and geometry. The dispersive

characteristics of the cross-ply composite plates used in our

experiments (carbon-epoxy, AS4/3501-6, [0/90]2s) are

shown in Figure 1.

(a)

(b)

Figure 1. Analytical dispersion curves for AS4/3501-6 [0/90]2s: (a) Phase velocity and

(b) Group velocity

NUMERICAL SIMULATIONS

A finite element model to simulate Lamb wave propagation

in carbon-fiber epoxy laminate using finite element code

ABAQUS has been developed. The model is used to

simulate transient dynamic response for healthy and

delaminated cases. The AS4/3501-6 carbon-epoxy

composite laminate model has dimensions of 250 x 126 x

1.25 mm (10 x 5 x 0.05 in) and consists of 8 cross-ply

[0/90]2s layers similar to the experimental plate. The plate

is meshed with 15,750 C3D8I solid elements to ensure at

least 10 nodes per wavelength1. It is restricted in all degrees

of freedom at nodes along the two ends as shown in Figure

2. In order to simulate the piezoelectric excitation similar to

the experimental studies, actuating piezoelectric transducer

(PZT-5H) has been modeled using 164 piezoelectric

elements (C3D8E in ABAQUS). Perfect bonding between

plate and PZT is assumed and an input of 30V is applied to

PZT.

(a)

(b)

Figure 2. (a) Finite element model of composite plate

with PZT; (b) isoparametric view of the PZT bonded to the plate

A representative delamination between the 4th

and the 5th

layers of the laminate is introduced by disconnecting nodes

in the region. The delamination area is10 mm wide (shown

red in Figure 2) located 80 mm from the center of PZT. The

through-width delamination covers 4% of the plate area.

The electrical excitation given to the PZT has the shape of a

tone burst obtained using a Hanning window. The dynamic

analysis is performed using ABAQUS/ Standard solver.


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Solutions are obtained for 3.5 cycles of the input tone burst

at carrier frequency of 15 kHz. Out of plane velocity

responses are recorded at several sensor points. A series of

out of plane velocity fields for healthy plate are shown in

Figure 3 for illustrating wave propagation.

100 µs

220 µs

Figure 3. Lamb wave propagation and scattering at different time instants

Response of the plate for the input of 15 kHz 3.5 cycles for

healthy and delamination cases are shown in Figure 4. The

responses for the two cases show clear difference in the

wave amplitude near delamination area with delaminated

plate showing significantly larger amplitude.

Responses at sensor location just before the delamination

(Figure 5a) show significant differences in the wave packets

for healthy and delaminated cases. The amplitude of the

first wave packet is increased due to reflections from

delamination. Further observation at this sensor location

reveals formation of an extra wave packet right after the

incident wave. For the sensor location on top of the

delamination (Figure 5b) the increase in wave amplitude for

the first wave packet is smaller and the second wave packet

has much reduced amplitude. Similar trends were observed

15 kHz, 5.5 cycles input as well.

(a)

(b)

Figure 4. Response at time instance 480 µs for (a) healthy plate (b) plate with delamination


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(a)

(b)

Figure 5. Responses recorded at sensor location (a) Just before delamination (b) On top of delamination

Structural damping has a significant effect on wave

propagation, including initial amplitude and attenuation.

Rayleigh linear damping model available in ABAQUS/

Standard is used to introduce damping to the plate. In

Rayleigh damping, it is assumed that damping matrix is a

linear combination of mass and stiffness matrices.

�� (1)

where � and � are user defined constants. For a given

mode the fraction of critical damping, �� , can be

expressed in terms of the damping factors � and � as:

��

��

��

� (2)

where � is the natural frequency of a mode. This implies

the mass proportional damping parameter, � , damps the

lower frequencies and the stiffness proportional damping ,

�, damps the higher frequencies. In our studies, the most

appropriate values for � and � are found to be 0 and 4x10-7

through comparisons with experimental data. Figure 6

shows a comparison of FE results with experimental data at

a point with x=150 mm, y=64 mm.

(b)

Figure 6. FE vs. experimental response of healthy case for 15 kHz 3.5 cycles input

The FE responses match well with experimental results for

the first two wave packets. Subsequent responses show

some deviations possibly due to the differenc

and experimental conditions such as boundary, damping,

bonding between plate and PZT (assumed to be perfect

FE), etc. Since the first few wave packets contain most

important information, the FE analysis with PZT modeling

and damping values will be used in future for simulations

with other input excitations.

(a)

Figure 7. AS4/3501-6 pre-preg cross ply composite plate (a) schematic diagram with delamination area (b) healthy

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experimental response of healthy case for 15 kHz 3.5 cycles input

The FE responses match well with experimental results for

Subsequent responses show

the differences between FE

and experimental conditions such as boundary, damping,

assumed to be perfect in

Since the first few wave packets contain most

important information, the FE analysis with PZT modeling

will be used in future for simulations

EXPERIMENTAL INVESTIGATIONS

The composite plate used in this study

AS4/3501-6 pre-preg using vacuum bagging and oven

curing technique. (Dimensions

numerical simulations.) Hi-temp mold release wax from

PARTALL was applied between 4

the delaminated plate. A piezoelectric (PZT) actuator

(diameter 13.5 mm and thickness

onto the composite plate using epoxy as shown in Figure

(b)

cross ply composite plate (a) schematic diagram with delamination area (b) healthy

composite plate with PZT actuator


experimental response of healthy case for 15 kHz 3.5 cycles input

EXPERIMENTAL INVESTIGATIONS

in this study was fabricated from

preg using vacuum bagging and oven

. (Dimensions and layup are given under

temp mold release wax from

PARTALL was applied between 4th

and 5th

layers to make

A piezoelectric (PZT) actuator

diameter 13.5 mm and thickness 0.22 mm) was affixed

site plate using epoxy as shown in Figure 7.

(b)

cross ply composite plate (a) schematic diagram with delamination area (b) healthy


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A National Instruments PXI 6339 and a BNC-2110 board

were used to generate signals and a QuickPack® power

amplifier was used to amplify the actuation signal. Tone

burst excitations at 3 different frequencies (15, 18 and 20

kHz) and 2 different cycle numbers (3.5 and 5.5 cycles)

were used to generate A0 Lamb waves. A Scanning Laser

Doppler Vibrometer (SLV) was employed to acquire the

signals at designated locations. The experimental set up is

shown in Figure 8.

(a)

(b)

Figure 8. (a) Experimental setup (b) Schematic diagram

Table 1 shows time of flight (TOF) for each signal at a

location 60 mm away from the center of PZT and the phase

velocity calculated using TOF. Similar values for TOF and

phase velocity are obtained for 5.5 cycles. With these TOF,

the reflected wave packets from boundaries can be

identified. Based on Lamb wave dispersion curve,

calculated phase velocities indicate that the signals are

Lamb wave A0 mode.

Table 1. TOF and Phase Velocity measured at 60 mm from center of PZT

15kHz,

3.5Cycle

18kHz,

3.5Cycle

20kHz,

3.5Cycle

TOF(ms) 0.0977 0.0928 0.0910

Phase

velocity(km/s) 0.614 0.646 0.659

(a)

(b)

Figure 9. Acquired signal at 80 mm away from PZT (a) Healthy plate (b) Delaminated plate

0 0.5 1 1.5 2 2.5-1

-0.5

0

0.5

1

Time(ms)

Norm

aliz

ed a

mplit

ude

0 0.5 1 1.5 2 2.5-1

-0.5

0

0.5

1

Time(ms)

Norm

aliz

ed a

mplit

ude


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Figures 9(a) and 9(b) show the 15 kHz 3.5 cycle acquired

signals at 80 mm away from PZT (which is 8 mm away

from the beginning of delamination area). For convenient

visualization, obtained signals are normalized with the

maximum value of the first wave packet at 20 mm from the

PZT actuator. For the healthy plate, two boundaries are

constrained (upper and bottom of plate) and left and right

sides are free. For delaminated plate, there is an additional

boundary due to the delamination area. The first wave

packet for healthy plate is the incident wave and other wave

packets are boundary reflections. With TOF of 0.278 ms

(Figure 4a), the second packet travels a distance of 170.9

mm. This distance indicates that the second wave packet is

the reflected wave from bottom of the plate since this

sensor location is 85 mm away from bottom boundary. For

the delaminated plate (Figure 4b), there are combined

packets between first and second waves. These combined

packets indicate delamination as they include reflected

waves from the delamination area (based on TOF analysis).

(a) (b)

(c)

Figure 10. Acquired signal from healthy and delaminate plates at 80 mm away from PZT:

(a) 15 kHz, 3.5 Cycle (b) 18 kHz, 3.5 Cycle (c) 20 kHz, 3.5 Cycle

0 0.5 1 1.5 2 2.5-1

-0.5

0

0.5

1

Time(ms)

Nom

aliz

ed a

mplit

ude

Healthy plate

Delaminated plate

0 0.5 1 1.5 2 2.5-1

-0.5

0

0.5

1

Time(ms)

Nom

aliz

ed a

mplit

ude

Healthy plate

Delaminated plate

0 0.5 1 1.5 2 2.5-1

-0.5

0

0.5

1

Time(ms)

Nom

aliz

ed a

mplit

ude

Healthy plate

Delaminated plate


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The effect of incident (excitation) wave frequency is shown

in Figure 10. Signals from both healthy and delaminated

plates are presented for 3.5 cycle case with 15, 18, and 20

kHz frequencies. The shape of the wave packets change as

expected due to different velocities for the three

frequencies. All three cases show that delaminated plate has

larger wave amplitude due to reflected wave packet from

delamination area. Among these frequencies, 18 kHz seems

to indicate delamination most clearly. Wavelet transforms

of the signals at 18 kHz show appearance of additional

frequency components after 1.4 ms for the delaminated

plate (Figure 11). Number of cycles of the incident wave is

another important parameter. Comparison between 3.5 and

5.5 cycle signals show that 5.5 cycle case gives a better

indication of delamination (Figure 12). Further studies will

include other sensor locations, frequencies, and

quantification of energy of the reflected wave.

(a)

(b)

Figure 11. Wavelet transform of signals at 80 mm away from PZT: (a) Healthy plate (b) Delaminated plate

Time (µs)

Frequency (kHz)

Time (µs)

Frequency (kHz)


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Figure 12. Acquired signal from healthy and delaminate plate at 80 mm away from PZT: (a) 18 kHz, 3.5 Cycle (b) 18 kHz, 5.5 Cycle

CONCLUSIONS

This study has presented a presents a preliminary

investigation of excitation signal parameters on the

detection of delamination in composite plates. We used

composite plates typical of aerospace applications and

provided excitation using integrated PZT actuator. A

scanning laser vibrometer was used for recording structural

responses. Experimental results at three frequencies (15, 18,

and 20 kHz) and two pulse widths (3.5 and 5.5 cycles)

showed significant effect on damage signature. Numerical

studies using a commercial finite element code (ABAQUS)

showed good correlation with experimental results with

appropriate level of structural damping. Further studies will

include other sensor locations, frequencies, and

quantification of energy of the reflected wave.

REFERENCES

1. S. S. Kessler. 2002. Piezoelectric-Based In-Situ Damage

Detection of Composite Materials for Structural Health

Monitoring Systems, Ph.D. Thesis, MIT, Cambridge, MA.

2. P. S. Tua, S. T. Quek and Q. Wang. 2004. Detection of

Cracks in Plates using Piezo-actuated Lamb Waves, Smart

Materials and Structures, 13:643-660.

3. W. Lestari and P. Qiao. 2005. Damage Identification for

Carbon/Epoxy Laminated Composite Structures Based on

Wave Propagation Analysis, 46th AIAA/ASME/ASCE/

AHS/ASC Structures, Structural Dynamics & Materials

Conference, Austin, TX.

4. S. Banerjee, F. Ricci, E. Monaco, L. Lecce and A. Mal.

2007. Autonomous Impact Damage Monitoring in a

Stiffened Composite Panel, Journal of Intelligent Material

Systems and Structures, 18(6):623-633.

5. Y.-H. Kim, D.-H. Kim, J.-H. Han and C.-G. Kim. 2007.

Damage Assessment in Layered Composites using Spectral

Analysis and Lamb Wave, Composites Part B: Engineering,

38: 800-809.

6. A. Raghavan and C. E. S. Cesnik. 2007. Review of

Guided-wave Structural Health Monitoring, The Shock and

Vibration Digest, 39(2):91-114.

7. Diamanti K, Soutis C. Structural health monitoring

techniques for aircraft composite structures. Prog

Aerospace Sci (2010), doi:10.1016/j.paerosci.2010.05.001.

0 0.5 1 1.5 2 2.5-1

-0.5

0

0.5

1

Time(ms)

Nom

aliz

ed a

mplit

ude

Healthy plate

Delaminated plate

0 0.5 1 1.5 2 2.5-1

-0.5

0

0.5

1

Time(ms)

Nom

aliz

ed a

mplit

ude

Healthy case

Delaminated case

(a) (b)

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