Tough-to-brittle transitions in ceramic-matrix composites with increasing interfacial shear stress Original Research Article

The possibility of decreasing ultimate tensile strength associated with increasing fiber/matrix interfacial sliding is investigated in ceramic-matrix composites. An axisymmetric finite-element model is used to calculate axial fiber stresses versus radial position within the slipping region around an impinging matrix crack as a function of applied stress and interfacial sliding stress τ. The stress fields, showing an enhancement at the fiber surface, are then utilized as an effective applied field acting on annular flaws at the fiber surface, and a mode I stress intensity is calculated as a function of applied stress, interface τ and flaw size. The total probability of failure due to a pre-existing spectrum of flaws in the fibers is then determined and utilized within the Global Load Sharing model to predict fiber damage evolution and ultimate failure. For small fiber Weibull moduli (m≈4), the local stress enhancements are insufficient to preferentially drive failure near the matrix crack. Hence, the composite tensile strength is weakly affected and follows the shear-lag model predictions, which show a monotonically increasing strength with increasing τ. For larger Weibull moduli (m≈20), the composite is found to weaken beyond about τ=50 MPa and exhibit reduced fiber pullout, both leading to an apparent embrittlement and showing substantial differences compared with the shear-lag model. Literature experimental data on an SiC fiber/glass matrix system are compared with the predictions.