The effect of elastic anisotropy on contrast in the scanning acoustic microscope

The contrast obtained with polycrystalline specimens in the scanning acoustic microscope is computed by taking account of the anisotropic stiffness tensor. The resulting variation of signal with defocus is fundamentally different from that which could be obtained with an isotropic material regardless of the values of the elastic properties. In many cases this results largely from the excitation of pseudo-surface waves which effect the response of the microscope in a similar manner to ‘leaky’ Rayleigh waves. The reasons why some materials fail to give good acoustic images of grain structure are discussed.