Fresnel array method for determining the dispersion curves in plates

Abstract

Experiments have been conducted on the thermoelastic generation of Lamb waves in plates by a Q‐switched ruby laser. Detection was performed both by interferometric laser probe and a miniature piezoelectric pinducer. The long‐term objective of this work is to determine various characteristics of the plate such as material properties, plate thickness, flaws, and inhomogeneities, or even bonding between plates by comparing experimentally determined points with the theoretical dispersion curves. The technique consists in forming on the surface of the specimen an array of confocal arc sources by passing the laser beam through a Fresnel lens. The array spacing produces a ‘‘forcing wavelength’’ for which only a few specific frequencies can propagate. Dispersion curves can be obtained by measuring the frequency content of the received signals for a range of wave numbers. Because of the narrow‐band nature of the technique, and because of the confocal geometry of the source distribution, this technique offers a relatively high signal‐to‐noise ratio. Good agreement is obtained between theoretical and experimental dispersion curves especially for the lower modes, thus showing that the proposed technique has potential for the above‐mentioned applications. [Work supported by the National Science Foundation.]