Dispersive Properties of Leaky Interface Waves in Adhesive Layers

There are two types of guided modes we can speak of in connection with adhesive bonds. First, there are the guided modes of the adhesive joint as a sandwich-like, multi-layered structure which are basically Lamb-type plate modes, and second, there are the guided modes of the adhesive layer which are interface modes between two apparently semi-infinite adherend half-spaces (see Fig. 1). Guided modes of the first type have been used for ultrasonic inspection of adhesive bonds for almost fifteen years [1–5], but it became more and more clear that the sensitivity of this technique left much to be desired. It was recently established by the authors that guided modes of the second type offer superior sensitivity to both cohesive and adhesive properties of the adhesive bond [6]. This is not surprising at all since, although the adhesive layer is only a small percentage of the total joint thickness, all defects are expected in this crucial region or on its boundaries. Consequently, an acoustic wave which is effectively confined to this very region should be more sensitive than other modes trapped in the joint as a whole. The main purpose of this paper is to develop the analytical tools needed to study the dispersive properties of guided interface waves in adhesive layers and to compare the corresponding theoretical and experimental dispersion curve