Inverse calculation of material parameters for a thin-layer system using transient elastic waves.

The inverse calculation of material parameters of a thin-layer system is investigated using transient elastic waves. The inverse problem is formulated as an optimization problem in which the norm of the discrepancies between the calculated and measured normal surface displacements is minimized through the simplex algorithm. The theoretical result is first solved using the Laplace transform and the transient response is then implemented analytically by Cagniard's method. In the experiment, the source time function is generated by the brittle fracture of a pencil lead on the surface of the thin-layer system, and a National Bureau of Standards (NBS) conical transducer is used to record the surface responses. To obtain reliable inverse results for material parameters, a two-step inverse calculation procedure is proposed. The recovered material parameters of the specimens agree well with the theoretical values and experimental results.