Investigation of surface acoustic wave propagation on a sphere using laser ultrasonics

Abstract

Using the noncontact laser ultrasonic technique, we investigate the propagation of surface acoustic waves on a sphere (SSAW). A finite thermoelastic line source gives rise to combined focusing and reversal effects. As for propagation on a cylinder surface, the reversal of the SSAW pulse is explained by the dispersion of the high frequency components of the laser-generated acoustic pulse. The focusing effect, due to the curvature of the surface, depends on the angular aperture of the source. From diffraction theory, optimal conditions for diffraction free propagation are derived, both in harmonic and pulse regimes. On a steel sphere of radius 25mm, Rayleigh waves excited by a 20ns YAG laser pulse focused onto a 3mm line propagate with a nearly constant amplitude.