Tailoring Substrate Stiffness in Bi-Material Adhesive Joints

Abstract

Joining composites to metals is a common challenge in the automotive and aerospace industries because of delamination and fiber breakage caused by drilling holes in the composite to accommodate mechanical fasteners. Adhesive bonding offers a compelling alternative to mechanical fastening, with additional advantages of having fewer parts, lesser weight, and a more uniform stress distribution when compared to mechanical fasteners. However, the relatively low strength of such joints is a limiting factor preventing widespread adoption in high-strength structural applications. One way to improve bi-material adhesive joint strength is to drill holes in the metallic substrate to allow the adhesive to ooze through and form a mechanical interlock, with potential to significantly increase the joint strength. Such ‘perforated’ joints have been explored in recent literature [1–5], and experimentally found to increase the strength of adhesive joints under tensile loads. However, all such studies have used an equally-spaced, rectangular array of holes, and examined only double lap joint configurations (DLJ). Such techniques cannot be directly translated to the more common single lap joint configurations (SLJ) due to the presence of bending-induced peel stress concentrations towards the axial ends of the bond area under tensile loads. The presence of perforation holes in a stress concentration zone would be detrimental to the joint performance. In this work, perforated SLJs with a rectangular-array hole pattern and a ‘tailored’ hole pattern were tested in tension-shear, and the peak loads were compared against baseline joints without perforations. Three different adhesives were used to induce different failure types and identify the specific conditions in which perforations provide an advantage in SLJs.