The converging-surface-acoustic-wave technique: anaylsis and applications to nondestructive evaluation

Abstract

Converging surface-acoustic waves (SAW) are generated by irradiating the inspected material with an annular-shaped pulsed laser beam. The converging-SAW pulse arrival is detected by a laser interferometer focused on the center of the annulus, where the converging effect produces a strong amplification of the ultrasonic pulse. This technique can be applied either to the detection of defects or to the characterization of the material by measuring the SAW velocity or attenuation. In this paper we present an analysis of the converging-wave propagation in order to explain some features of the detected signal, such as its shape and amplitude for different positions of the probing beam. A comparison with the signal intensities expected for a diverging as well as a collimated SAW is also presented. Applications of this technique to the characterization of anisotropic materials as well as to the detection of subsurface planar defects are presented and discussed.