DISPERSION CURVE ANALYSIS OF LASER-GENERATED LAMB WAVES

Experiments have been conducted with a Q-switched ruby laser to study thermoelastic generation of Lamb waves in plates. The objective of thi s study was to resolve experimentally the various modes that propagate in a thick plate after absorption of a short laser pulse at its upper surface. The methodology consists of acquiring several laser-generate d waveforms at various points on the plate. These waveforms are proces sed with a two-dimensional fast Fourier transform, in space and time, to obtain the dispersion curves of the plate, in wave-number and frequ ency. The technique can be used to determine which modes propagate in the structure as well as their relative amplitudes. This method, which is commonly used in structural acoustics, has not previously received much attention in laser ultrasonics even though it is particularly we ll adapted for measuring dispersion curves, because optical methods al leviate the problem of irreproducible coupling that exists with conven tional piezoelectric ultrasonic transducers.