Thermoelastic Attenuation of Rayleigh Waves.

Thermoelastic relaxation is an intrinsic mechanism of Rayleigh wave attenuation in solids. Since the strain varies with depth, an irreversible flow of the heat from one part of the solid to another generates entropy. This entropy converts the energy associated with the Rayleigh waves into heat causing the waves to attenuate. This effect is enhanced if the thermal expansivity varies with depth. Two cases are considered. The first case is when the variation in expansivity is confined to the surface of the solid. The Grueneisen parameter is expressed as a sum of decaying exponential terms. This case approximates a thin single layer placed on a semi-infinite slab. In the second case, the expansivity varies periodically with depth. The Grueneisen parameter is now expressed as a Fourier series. This represents a multilayered material which is assumed to approximate a composite. In the thin single layer case, several configurations were examined. These include, aluminum on quartz, aluminum on lithium niobate, silver on sapphire, gold on gallium arsenide, and glass on lithium niobate. The maximum attenuation occurs when the thickness of the thin layer is comparable to the wavelength of the Rayleigh wave. A similar conclusion results for the multilayered case. A graphite-aluminum composite was considered and it was found that the thermoelastic effect dominates over scattering at frequencies lower than 0.5 MHz. The maximum attenuation occurs when the layer spacing of the composite is of the dimension of the Rayleigh wave. Thermoelastic attenuation is compared to other attenuation mechanisms. These are the Akhieser mechanism, scattering, airloading, and piezoelectricity. The frequencies where thermoelastic attenuation equals the other mechanisms and where thermoelasticity is dominant is determined for each case. The role of the thermoelastic effect in ion implantation is estimated. It is found that thermoelasticity plays a significant role in Rayleigh wave attenuation in ion implanted materials. The possibility of using thermoelastic attenuation in nondestructive evaluation is discussed.