Determination of each layer thickness of thin bilayer using laser-based multiple zero-group velocity Lamb waves.

• Determination of the thickness of each layer of a thin metal bilayer by applying multiple zero-group velocity Lamb modes. • Deducing the relationship between the zero-group velocity frequency of Lamb waves and the thickness ratio of the bi-layer metal sheets. • Thickness measured by zero-group velocity Lamb wave method agreed well with the metallographic microscope results. 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 S 1 S 2 -ZGV frequency variation with the thickness ratio is non-monotonic, and S 1 S 2 -ZGV Lamb mode vanishes at some thickness ratios. Then a high-order S 3 S 6 -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. [ABSTRACT FROM AUTHOR]