Adhesive Nonlinearity and the Prediction of Failure in Bonded Composite Lap Joints

A theoretical model is derived that predicts failure in adhesively bonded lap joints loaded by in-plane shear. This model is based on shear lag assumptions and accounts for a nonlinear adhesive shear stress-strain relationship so that the development of plastic strain is tracked. Failure of the joint is determined when the plastic strain reaches its ultimate value. By accounting for adhesive plasticity, this model permits the design of joints with higher load-carrying efficiency than designs based on simple elastic-to-failure adhesive constitutive behavior. Example calculations presented in this paper show that the theoretical model predicts failure load accurately for a carbon/epoxy adherend joint (within 6%), in comparison with finite element analysis (FEA) predictions, for joints with adhesive bondlines of 0.33 mm or less. For the thicker bondlines studied, a severe strain localization effect was observed in the FEA models to occur at the joint interface corners, and therefore the theoretical model over-predicted failure load by up to 22% for 2.08 mm bondline carbon/epoxy adherends.