Anisotropy Imaging of Materials

Abstract

We describe in this paper an imaging technique in which a pulsed laser, operating as an ultrasonic point-source, is scanned across the surface of a specimen and the generated elastic waves are detected by a point-receiver. The signals detected along a scan line, comprise a scan image which contains the spatial- and temporal-dependence of the elastic wave field in a specimen that can be processed to recover all the elastic constants of the material and to detect microstructural features, including damage. By monitoring the amplitude of a particular wave arrival over a number of scan lines, an anisotropy image is generated that is analogous to a ballistic-phonon image but differs from it in that the measurements are made at MHz frequencies, at ambient temperatures and in metals and non-metals alike. We show the results of measurements made on single crystal specimens of Si, zinc and in a number of polymeric composite materials. Anisotropy-induced focusing effects are observed in particular directions in each of these materials. An interpretation of these images has been obtained using Monte Carlo simulation.