R-curve behaviour of the mixed-mode I/II delamination in carbon/epoxy laminates with unidirectional and multidirectional interfaces

To successfully predict the delamination behaviour of the laminated composite structures with fibre bridging, the R-curve has to be studied. This work experimentally investigates the complete R-curve behaviour of the unidirectional and multidirectional carbon/epoxy composite laminates. The delamination tests use the double cantilever beam (DCB), mixed-mode bending (MMB) and end-notched flexure (ENF) specimens for mode I, mixed-mode I/II and mode II loading, respectively. The test results show that the interfacial ply and mode mixity (phi-ratio) have significant influences on the initial fracture toughness, steady-state fracture toughness and fibre bridging length. The ratio between the steady-state fracture toughness and its initial value is approximately same for both interfaces, which indicates a similar enhanced effect of the fibre bridging on the fracture toughness. The Benzeggagh-Kenane (B-K) criterion is capable of representing the relation between the fracture toughness and the phi-ratio. Based on the DCB and MMB test results, the predicted values of the mode II fracture toughness via the B-K criterion are very close to the experimental ones, which illustrates the possibility of determining the mode II fracture toughness without executing the mode II delamination tests. Furthermore, a semi-empirical expression is proposed, which can well predict the mixed-mode I/II delamination behaviour.