Abstract
A zero-group velocity (ZGV) resonance method based on laser ultrasonic Lamb waves is proposed for measuring each layer thickness of a thin bilayer with uneven thickness at the interface. The relationship between the multiple ZGV frequency of Lamb waves and the thickness ratio of the bi-layer metal sheets was theoretically deduced, and the results showed that S1S2-ZGV frequency variation with the thickness ratio is non-monotonic, and S1S2-ZGV Lamb mode vanishes at some thickness ratios. Then a high-order S3S6-ZGV frequency is utilized to determine the desired thickness ratio. Experimentally, the two ZGV modes in a thin Al/Cu bilayer excited by laser and detected by laser Doppler vibrometer (LDV) were used to evaluate the thickness of each layer according to the theoretical curves.
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