Nondestructive evaluation of adhesive joints in layered structures using nonlinear measurements of second harmonics of ultrasonic Lamb waves

Based on the nonlinear measurements of the second harmonics of ultrasonic Lamb waves, this work develops a promising method for NDE of the properties of adhesive joints in layered planar structures. When ultrasonic Lamb waves have a strong nonlinearity, the measurements of amplitude-frequency curves for the second harmonics of Lamb waves at the surfaces of the given layered planar structures (aluminum sheet-epoxy-aluminum sheet) have been carried out. It is verified that there is no overlapping of multi-modes of the double frequency Lamb waves in the received signals. The experiment results show that both the stress wave factors of the nonlinear Lamb waves and the frequency values corresponding to the peaks of the amplitude-frequency curves of the second-harmonic signals can effectively characterize the properties of the adhesive joints.