Surface response of a fluid-loaded solid to impulsive line and point forces: Application to scanning acoustic microscopy

Algorithms are presented for calculating the two- and three-dimensional time domain dynamic Green's functions of a pair of joined semi-infinite anisotropic elastic continua. They are used to calculate the normal surface displacement response of fluid-loaded solids to impulsive line and point forces. Particular attention is given to the resonant and singular features in the response associated with the Stoneley-Scholte interfacial wave, leaky; Rayleigh and pseudosurface acoustic waves, and lateral waves, i.e., surface skimming bulk waves of the solid and of the liquid. The various regimes are explored, in which the fluid sound speed and acoustic impedance range from small to large as compared to those of the solid. The effects of elastic anisotropy of the solid are illustrated with results for a carbon fiber composite and for the principal crystallographic cuts of a number of cubic crystals of anisotropy coefficient eta = 2 C-44 /(C-11 - C-12) greater and less than unity. Calculated images, representing the dependence of the normal displacement response on time and direction, are in good agreement with published acoustic microscopy images of a number of anisotropic solids that have been measured with a configuration of two,line focus or two point focus lenses. These images display prominent features due to leaky Rayleigh and pseudosurface waves, as well as sharper lateral wave structures. The mode of excitation and detection does not, however, couple into the water lateral wave and Sholte wave, which are absent from the measured images. This effect is simulated by-setting a finite cutoff, determined by the aperture angles of the lenses, to the spatial Fourier transform of the surface Green's function. [S0163-1829(98)06327-9].