Non-destructive characterisation of adhesive bond strength by Lamb waves

Abstract

The non-destructive evaluation of adhesive structures is a challenging problem. Up until now, there does not appear to be a practical technique to evaluate the bond strength of adhesive joints. It has been already confirmed that acousto-ultrasonic (AU) signals are sensitive to material property variations. AU signals are mainly composed of multiple Lamb modes when the thin plate-shape structures are tested by the AU technique. Because of the complication of Lamb waves, the conventional AU technique evades further analysis for signal components; generally, stress wave factors are constructed to measure the energy dissipation of the AU signals in the materials and thereby to determine the whole material properties. The construction of stress wave factors and other AU parameters, and the selection of signals of interest are subjective to a certain extent, and generally depend on 'trial and error'. In this paper, the adhesively bonded lap-shear aluminium joints with different bond strength are tested by the AU technique. Using time-frequency analysis, instead of the stress wave factor method, the multiple Lamb modes in AU signals were separated in time-frequency space. The propagation features of the main Lamb modes, such as attenuation, dispersion and mode conversion, were extracted from time-frequency distribution of AU signals and were correlated with the bond strength of adhesive joints. Good correlation was obtained.