Evaluation of thermal material properties based on SAW dispersion in the laser-induced dynamic subsurface temperature gradient

Dispersion of surface acoustic waves (SAWs) caused by a dynamic surface heating-cooling cycle has been applied for evaluation of thermal properties. A laser-generated broad-band SAW pulse is propagated through the material whose elastic properties and density were modified by the dynamic inhomogeneous temperature field produced by laser heating and subsequent cooling. A sample with depth-dependent material properties introduces a phase velocity dispersion in SAW propagation. Dispersion was measured by a differential technique, which helped to isolate the thermally induced dispersion and eliminate the contributions of other possible sources of dispersion. The dispersion curves were measured at several time instants, which allowed evaluation of the following parameters: thermal diffusivity, temperature dependence of the shear modulus, and the temperature magnitude. The inverse problem was solved by fitting the experimental dispersion curves into the theoretical ones by means of differential evolution techniques.