Abstract
An experimental technique is proposed to analyze and control laser-generated Lamb waves propagating in aluminum plates. The technique consists in forming on the surface of the specimen an array of concentric arc sources by passing the laser beam through a Fresnel lens. The spacing between the illuminated arcs produces a “forcing wavelength” for which only a few specific frequencies (those which satisfy the dispersion relation) can propagate in the sample. Dispersion curves can be obtained by measuring the frequency content of the received signals for a range of wavenumbers. The technique offers a relatively high signal-to-noise ratio because of the narrow-band nature of the method and because of the confocal geometry of the source distribution. Reasonably good agreement is obtained between theoretical and experimental dispersion curves especially for the lower modes, thus showing that the proposed technique may have some potential for some specific applications in laser ultrasonic nondestructive evaluation.
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