Lamb waves characterization of adhesion levels in aluminum/epoxy bi-layers with different cohesive and adhesive properties

This paper presents a study of the detectability of the level of adhesion at the Aluminum/Epoxy interface using the propagation of ultrasonic guided waves. The topic is relevant to the long-range ultrasonic inspection of structural adhesive bonding. The samples considered are epoxy-coated aluminum: samples were fabricated with partially or totally crosslinked epoxy groups and with different surface treatments of aluminum substrates. Experimentally measured dispersion curves and attenuation coefficients of a selected mode are used to quantitatively track changes at the epoxy-aluminum interface. The results show that modes could be very sensitive to the nature of the interface, that is to say to the level of adhesion, if properly selected. Numerical predictions via the Jones theological model and their comparison with experimental results highlight three adhesion levels: weak, medium and high one.