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28 January 1999 Impact of damage on propagation of Lamb waves in plates
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A consorted effort is ongoing to utilize spectral analysis of Lamb Waves to rapidly characterize and to detect damage in plates. To optimize test set-up, to understand the limitations of the methodology, and to verify the experimental results, an effort is under way to simulate conditions normally encountered in actual cases. The impact of propagation of Lamb waves in the presence or absence of damage has been simulated using a finite element algorithm, 2D Fourier transform can be used to identify individual modes, and to measure the amplitude and propagation velocity of each mode in a thin plate. Mode conversion due to the presence of a crack can be readily identified with this method. The damage was modeled as notches with different widths and depths. Three different materials were modeled for this purpose: Aluminum 6061-T6, Steel A36, and Graphite- Epoxy; the latter being analyzed in directions parallel nd perpendicular to the fibers. The focus of the work was on low-frequency range where the fundamental symmetric and anti-symmetric modes are dominant. The ratio of notch depth to plate thickness and the absolute notch depth were considered as possible controlling parameters for the sensitivity in the damage detection. The effect of the width of the flaw on the transmissivity of the Lamb waves was also considered and found to be minimal.
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Cesar Tirado and Soheil Nazarian "Impact of damage on propagation of Lamb waves in plates", Proc. SPIE 3586, Nondestructive Evaluation of Aging Aircraft, Airports, and Aerospace Hardware III, (28 January 1999);

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