Measurement of stresses by line-focus acoustic microscopy

In this paper, line-focus acoustic microscopy has been used to determine local near-surface stresses in isotropic materials. Two surface wave modes, namely a leaky Rayleigh wave and a leaky surface-skimming longitudinal wave, have been excited by the acoustic microscope. It has been observed that the changes of the wave velocities are linearly proportional to the applied stresses, as predicted by acoustoelastic theory. The non-uniform stress field in a loaded specimen has been determined from wave velocity measurements by the use of acoustoelastic constants obtained from a calibration test. The measured stresses are in good agreement with the results calculated by a finite element method. A self-calibrating method, which determines the stress profile directly from velocity measurements without a calibration test, is proposed and the results are compared with experimental data.