Predicting Bond Strength

Methods are presented for predicting the load carrying capability of a bonded joint using relatively simple laboratory test samples. It is first shown that using an average stress criterion can lead to errors of an order of magnitude in predicted load carrying capability. A fracture mechanics approach is then shown to predict failure load accurately in a joint bonded with either a polyurethane or a relatively brittle epoxy when proper consideration is given to loading mode, temperature, and load rate. The principal contribution of this paper is in extending fracture mechanics theory to regions where classical singular points do not exist. Analyses are combined with test data to deduce an “inherent” flaw size.