A bond failure mechanism for adhesive on a metal substrate

Abstract

Butt-joint strength was measured on a series of epoxy resin-to-chrome plate adhesive bonds comparing the effect of curing the resin with different stoichiometric proportions of cross-linking agent. Adhesive bonds prepared with low stoichiometric proportions of curing agent consistently exhibited apparent adhesional failure. Adhesive bonds prepared with higher stoichiometric proportions of curing agent consistently yielded apparent cohesive failure. A point of direct correspondence was noted between trends of bulk elastic properties in the cured adhesive and joint strength with curing agent concentration. In the case of adhesive bond compositions which consistently yielded apparent adhesional failure an unexpected correlation was noted between butt-joint strength and the bulk tensile strength of the adhesive. In the more detailed evaluation of joint fracture surfaces by means of radiotracer techniques which followed, molecularly thin layers of cross-linked radioactive adhesive were anchored at the metal substrate surface prior to application of the non-radioactive bulk layer of adhesive and formation of the adhesive bond. Autoradiographs of the resulting radioactive joint fractures indicated large areas of the adherend surfaces which were completely free of adhesive in the case of apparent adhesional joint failures. Sufficiently thick pretreatments of the adherend bonding surface with cross-linked adhesive were found to prevent apparent adhesional failure, but produced no measurable increase in joint strength. It was found that all the preceding observations and correlations could be interpreted according to a proposed mechanism of joint failure in which it is assumed that the joint failure crack always originates in a bulk region of the adhesive.