Effects of Plasticity on Patched and Unpatched Center Crack Tension Specimens

Composite patch repairs of aluminum structures are designed using linear elastic fracture mechanics, which compares crack tip predictions to a critical strain energy release rate or stress intensity. Analytical and numerical predictions are reasonable for linear-elastic behavior, but do not account for the elastic-plastic behavior observed at the crack tip of ductile materials below the ultimate load. This research used digital image correlation to study full field displacements and strains ahead of the crack tip for unpatched and patched center crack tension specimens loaded monotonically to failure. Both qualitative and quantitative comparisons show measureable effects of surface reinforcement on crack tip behavior. Free surface fracture response was an intrinsic property of the aluminum from the initial development of the plastic strain zone through crack blunting and ultimately through peak load with all stages occurring at crack opening displacements (COD) independent of the one-sided composite patch reinforcement. Patch reinforcement increased the ultimate tensile load 160% and the total achieved COD by 20% over the unpatched behavior. Increased capacity was related to an increase in accumulated large strain area and distributed plasticity measured on the free surface prior to the end of crack blunting. The direct correlation of fracture behavior indicates patched specimen failure can be predicted using unpatched specimen COD.