Non contact thin film local thickness measurements through a thick plate using zero group velocity lamb modes.

Abstract

It has been shown that a homogeneous isotropic plate can support non‐propagating Lamb modes having a zero group velocity (ZGV) for nonzero values of their wave number. Optical means such as a pulsed laser source and a heterodyne interferometer can be used for non‐contact generation and detection of these elastic waves. With this experimental setup, local and accurate measurements of material parameters such as bulk waves velocities and very small plate thinning detection were obtained from the resonance spectrum of ZGV Lamb modes. A thin layer deposited on the plate induces a significant down shift of the set of ZGV resonance frequencies. Experimentally, thin gold layers down to 100 nm were detected through a 1.5‐mm thick Duralumin plate. Optical measurements were performed at the same point as the generation on the non‐covered face of the thick plate. The frequency shift, which is typically 1 kHz for the S1‐Lamb mode at 1.9 MHz, can be approximated by a formula, which allows us to calculate the layer thi...