Acoustic microscopy of elastic discontinuities

Abstract

In the reflection-scanning acoustic microscope, interference fringes may be obtained from cracks and grain boundaries running at an angle to the surface. The periodicity of these fringes suggests that while sometimes they are caused by reflection of longitudinal waves within the specimen, when the feature is approximately normal to the surface, Rayleigh waves are responsible, and this confirms the dominant role played by Rayleigh waves in the contrast in acoustic microscopy. The variation of contrast with defocus may be expressed as a Fourier transform of the reflectance function of a specimen; oscillations in V(z) then arise as the transform of the change in phase around the Rayleigh angle. When Rayleigh waves strike a surface-breaking discontinuity, they may be strongly reflected even though the discontinuity is much less than a wavelength thick. This enables fine cracks and other features which would not be resolved according to conventional criteria to be revealed very clearly in acoustic micrographs.