Quantitative evaluation of surface damage in brittle materials by acoustic microscopy

We describe theoretical methods to assess the effects of distributions of surface breaking cracks on the velocity of surface acoustic waves (Rayleigh waves). The cracks are considered either as linear, one-dimensional cracks, or as semi-elliptical two-dimensional cracks. We find reductions in surface acoustic wave velocity of up to similar to 10% can be produced by high crack densities. We compare the theoretical results with experimental data; models produce a good fit to the experimentally determined variations in surface acoustic wave velocity with frequency on damaged surfaces. We also consider the influence of surface residual stresses on the effects of surface cracks on surface acoustic wave velocity. Compressive stresses partially close surface-breaking flaws and reduce their effect on surface acoustic wave velocity considerably. We also report experimental studies on the use of acoustic microscopy to characterize experimental studies on the polishing quality in alumina.