Nondestructive Testing for Environmental Degradation of Adhesive Joints

Abstract

The work described here is an ultrasonics based experimental study which aims to address the lack of a reliable technique for detecting strength loss in adhesive joints after exposure to hot wet environments. This is manifested as a change in the failure mode of an adhesive system from a cohesive failure in the as-made condition, that is failure through the adhesive, to an adhesive failure, failure between the adhesive and adherend, after exposure to a hot, wet environment. This work has been concerned with the bonding of aluminum using two part epoxy adhesive. The reason for the change in failure mode is thought to lie in changes in the oxide layer which is present between the aluminum and the epoxy. The oxide layer generally has a porous structure into which epoxy can penetrate, forming a micro-composite layer, referred to as the interlayer. It is the detection of changes in this interlayer which present the biggest problem to current N.D.T. techniques for adhesive joints [1]. This is largely a problem of size, the interlayer being typically no larger than a few microns thick, sandwiched between several hundred microns of epoxy and several millimetres of aluminum. It is the need to detect changes in such a thin layer through such a thick layer which presents the biggest problem.