Rapid Imaging of Microstructure using Spatially Resolved Acoustic Spectroscopy

Microstructure can have profound effects on the bulk mechanical properties of a material, such as its strength and susceptibility to failure under stress. Several well known methods for imaging microstructure exist, including essentially destructive techniques such as etching and electron back-scattered diffraction, and contact techniques such as the scanning acoustic microscope. SRAS – spatially resolved acoustic spectroscopy – is a relatively new laser ultrasound technique that uses surface acoustic wave velocity as its contrast mechanism. Unlike the techniques above it is non-contact, non-destructive, can be used to image large samples, and provides quantitative velocity information for acoustic waves propagating in one or more directions. The technique uses the relative efficiency with which waves of either a fixed frequency or k-vector are excited by a grating to obtain the velocity, unlike most laser ultrasound techniques which rely on direct phase or time of flight measurements. This makes SRAS very tolerant to acoustic aberrations and poor signal to noise ratios, and hence is a robust technique. Lateral resolutions of the order of 25μm, and velocity resolutions better than 1m/s are illustrated by striking images of microstructure from industrially relevant materials.