Effects of fibre debonding and sliding on the fracture behaviour of fibre-reinforced composites

The effects of interfacial debonding and sliding on fracture characterisation of unidirectional fibre-reinforced composites are studied numerically under plane strain and small-scale bridging conditions. To account for the importance of fiber architecture, the fracture process zone (FPZ) characterised by discrete embedded reinforcement and cracking matrix, is of discontinuous bridging stresses. A bi-linear bridging law is incorporated in the finite element (FE) model to assess the effect of interfacial debonding and sliding. A linearly decreasing traction-separation law describes matrix cracking in brittle matrix. Transversely isotropic, orthotropic elastic constitutive relation is used for the effective properties of bulk brittle-matrix composites outside the FPZ. A boundary-layer formulation is imposed on the remote boundary based on the elastic solutions for mode I crack problems. Crack advance and fibre breakage are direct consequences of the constitutive modelling without any ad hoc crack growth criteria. The proposed FE model can eliminates unreal singular stresses ahead of the matrix crack. Fracture resistance (R) curves are obtained for a range of interfacial properties, as well as stress and bridging stress distributions. It is found that the toughness can be enhanced by modification of interfacial properties. Numerical results also show that fibre fracture results in a significant reduction of the steady-state toughness and relief of high stress concentration. However, strongly bonded interface implies a high interfacial strength, but a low toughness due to a low fibre strength.