Mechanisms of laser-generated ultrasound in plates

A theoretical study on the thermoelastic generation mechanisms of the characteristic elastic waveforms excited axisymmetrically by a pulsed laser in plates is presented by employing the method of eigenfunction expansion. The method gives a systematic treatment and allows one to investigate not only the thermoelastic generation of longitudinal, transverse and surface acoustic waves in thick samples, but also the excitation of the Rayleigh-Lamb wave modes in thin materials. The three-dimensional theoretical modelling takes account of both the sub-surface thermoelastic source (due to thermal diffusion and optical penetration into the solid) and the surface source resulting from the thermoelastic mode conversion at the illuminated surface. The excitation of the transient Lamb waves is also studied thoroughly in thin isotropic materials, and the results indicate that the Lamb waves characterized by a spike (termed 'sheet waves') and the frequency-modulated oscillation signal in a very thin sample will approach the Rayleigh wave in nature as the thickness of sample increases. The numerical technique provides a quantitative tool for experimental determination of material properties.