Rayleigh Wave Velocity Mapping Using Scanning Acoustic Microscope

Abstract

In a Scanning Acoustic Microscope (SAM) amplitude of focused acoustic beam reflected by a sample is utilized to produce acoustic images and to measure local elastic property for effective nondestructive characterization of materials. The most important acoustic rays involved in both imaging and quantitative measurements in an acoustic lens are shown in Fig.1. The extra contribution to the reflected signal from the Rayleigh waves generated at the interface between water and the sample surface enhances the contrast in acoustic images. Amplitude acoustic images produced at a defocus are very effective in revealing the microstructure structure, surface and near surface defects, flaws, micro cracks etc. On the other hand an interference between the direct reflected ray (PO) and the Rayleigh ray (AB-BC-DE-EF) [see Fig.1] produces a V(z) curve which displays periodic minima as the distance between the lens and the sample is varied. The periodicity in the V(z) curve is directly related to the Rayleigh wave velocity. This makes an acoustic microscope a quantitative tool for measurement of local elastic property. Several methodologies have been developed to analyze the V(z) curve to obtain high accuracy in the measurement of Rayleigh wave velocity. A computationally intensive procedure with additional experimental data on a sample that doesn’pt support Rayleigh waves has been velocity with an accuracy of 1 part in 104 m/s. Although this tedious and time consuming procedure is very useful for high accuracy single location measurements, time necessary to produce an image of the variation Rayleigh wave velocity over an area becomes forbiddingly too large.