Near field inversion method to measure the material properties of a layer

This study develops an in situ methodology that uses ultrasonic waves to measure the material properties of a layer. These material properties include the longitudinal and shear wave speeds, as well as the thickness of the layer. The unknown properties are determined by comparing measured experimental data with values obtained from a theoretical model. Emphasis is placed on the effectiveness of measuring both the in-plane, and the out-of-plane surface displacement components with a laser Doppler vibrometer. An inversion scheme compares (in the frequency domain) the experimentally measured data with data predicted using the theoretical model, and an error-function quantifies the difference between these values. Finally, a downhill-simplex algorithm is used to minimize this error-function and thus determine an optimum set of material properties.