Time- and frequency-domain characteristics of laser-generated ultrasonic surface waves

Abstract

The generation of elastic waves by illumination of a free metal surface with a laser pulse was studied to establish a quantitative basis for laser ultrasonics in nondestructive evaluation of surfaces. Experiments were carried out using a Q-switched Nd:YAG laser for generation and a wideband piezoelectric sensor for detection. A theoretical model for laser generation was developed for the thermoelastic energy regime. This model integrates over point source Green’s functions, suitably spread throughout the illuminated region. Good agreement was found between experiment and theory for characteristics in both time and frequency domains, for surface waves excited in the thermoelastic regime. For a given laser pulse energy, the highest Rayleigh wave peak frequency and bandwidth occurred when the Gaussian laser-beam half-width a was reduced just enough to begin surface damage. Once such damage commenced, further spot size reduction slightly lowered the peak frequency. Quantitative agreement with theory was found for the observed dependence, on both source–receiver distance and a, of the amplitudes for both the Rayleigh wave and the surface wave traveling at the bulk pressure wave speed (sP). In the nearfield, the sP wave overlapped the Rayleigh wave and made a greater contribution to the observed waveform than in the farfield.