Acousto-ultrasonics for nondestructive evaluation of epoxy adhesive joint integrity in complex structures

In industry, achieving a perfect adhesive joint is a quite complicated matter. It requires following rigorous procedures for surface preparation to minimize the risks of surface contamination and to increase glue impregnation on both the surfaces to be bonded. The bonding and curing must also be carried out in controlled atmosphere: temperature and humidity must be continually monitored to ensure an optimal curing and to guarantee maximal bonding strength. Once the adhesive joint is cured, few nondestructive testing methods are available to evaluate bonding quality. This article presents three acousto-ultrasonic methods used to detect and localize defects in epoxy adhesive joints: wave attenuation, Lamb wave generated by Hertzian contact, and nonlinear Time Reversal (TR) based Nonlinear Elastic Wave Spectroscpoty TR-NEWS methods associated to chirp-coded excitations. Results showing the detectability of defects are presented. An immediate test of its capability to detect scatterers will be practically implemented using approach recently proposed and leading to in-situ epoxy adhesive joint pseudo-tomography.