Crack growth in cracked-lap-shear specimens with multidirectional composite adherends under quasi-static loading

Quasi-static crack growth behaviour involving crack migration into composite adherends was investigated using cracked-lap-shear specimens. The baseline specimen configuration consisted of quasi-isotropic carbon/epoxy laminates providing 45° ply at the adhesive/adherend interface. Extensive use of ex-situ X-ray CT imaging together with supplemental FE analysis enabled description of three-dimensional discrete cracking mechanisms simultaneously growing at multiple sites including the bondline, the 45° ply of the strap adherend next to the adhesive, and the interfaces between the ply and its adjacent 0°　ply. Specimens with different interface plies were tested to study their effects on crack migration, which highlighted extensive evolution of cracks and delaminations in multiple layers of the strap adherend for the specimens with 90° ply at the interface. Further, the influence of a fabric carrier in the adhesive and testing environments was separately examined for the baseline specimen configuration. While crack types observed were basically the same, the timing for their formation, the significance as well as the ease of their extension were seen to be largely varied among the test conditions, affecting the resistance to unstable growth of the delamination at the 45°/0° ply interface of the strap. The detailed three-dimensional crack migration behaviour of the bonded composite joint and its dependence on adhesive material, layup and environments revealed by the present work provides insights towards more robust joint design or material improvement which can prevent unpredicted/unstable crack growth.