Mixed-mode fracture of adhesively bonded CF/epoxy composite joints

Mixed-mode fracture behaviour of adhesively bonded CF/Epoxy composite joints was studied using a rubber-modified epoxy (Araldite® GY260) as the adhesive. Fixed-ratio mixed-mode specimens (G_I/G_II = 4/3) were applied in the experiments. Composite adherends were manufactured from carbon fibre/epoxy (5128 T300 BASF) prepreg. The global mixed-mode fracture energy, G_C of the joints with different bond thickness, t, was evaluated using compliance calibration obtained from the experiments, beam theory and finite element analysis. The crack path along the bond line in the adhesive joints was investigated using optical microscopy. It was found that G_C was dependent on both the bond thickness and the actual crack path in the joints. G_C increased mildly from thin bonds (t < 100 μm) to a plateau value for larger bond thickness (t > 100 μm). For poor bonding between the adhesive and the composite adherends, interfacial crack propagation was observed for very thin bonds with alternating crack jumping from one interface to another across the bond line. The crack propagated almost at one interface for large bonds with occasional kinking. However, for strong interfacial bonding, the crack was deviated from the mode-I precrack towards the composite adherend and propagated cohesively in the composite layer. Scanning electron microscopy on the fracture surfaces showed different morphology depending on the interfacial bonding between the adhesive and the composite adherend.