Ultrasonic Attenuation Caused by Thermoelastic Heat Flow

Abstract

Expressions for the attenuation of sound waves in a standard linear viscoelastic body have been derived and have been applied to the attenuation due to the thermoelastic effect. The thermoelastic attenuation for sound waves has been computed for single crystals and polycrystalline materials under application of the high-frequency approximation of sound propagation. For single crystals the attenuation in the main propagation directions have been computed explicitly for cubic and hexagonal crystals taking into account the anisotropy of the elastic properties and thermal expansion. Comparison with experiments in the megacycle region shows that the attenuation caused by heat flow between the grains of a polycrystalline specimen is usually overpowered by scattering at the grain structure. Also the computed attenuation caused by heat flow between the regions of compressions and rarefaction of the wave is for many materials much smaller than the measured values. For single crystals of certain metals, however, the computed and the measured values have the same order of magnitude. It can be shown that in these cases the attenuation is composed of a thermoelastic component and of a component resulting from dislocations. In zinc the two components can be separated experimentally and the agreement between the measured and the computed thermoelastic damping is very good.