Mechanisms and Models for Delamination in the Presence of Through-thickness Reinforcement

Prior detailed microscopic observations have revealed the essential mechanisms of damage in through-thickness reinforcing tows when a delamination crack passes. As expected, the damage sequence depends quite strongly on whether the crack is loaded in Mode I or Mode II and, if loading is mixed mode, the order of loading. Here, micromechanical models are presented that show how geometry and constituent material properties affect the deformation and displacement of a bridging tow. From these models, the effective bridging law for a bridged crack model of the delamination can be derived. An analytical model is presented that predicts the relationship between the crack displacement vector (mode I and Mode II displacements) and the bridging traction vector that acts on the fracture surfaces. Criteria for rupture or pullout of the bridging tow are incorporated leading to predictions of the ultimate strength of the bridging ligaments in mixed mode conditions. Given the traction law, which may be a material property, the fracture behaviour of a part can be predicted.