Thermal shock fracture in unidirectional fibre-reinforced ceramic-matrix composites

The onset of multiple matrix cracking due to thermal shock in unidirectional fibre-reinforced ceramic-matrix composites is investigated in this study. A simple, semi-empirical formula is developed that allows prediction of the critical quenching temperature differential, ΔTc, that initiates fracture, as a function of the processing temperature of the composite, the temperature of the quenching medium, and material properties measured at room temperature. The approach considers the anisotropic stress field generated during the shock. Application of the formula to the cases of four different CMCs reinforced with Nicalon fibres revealed significant discrepancies with experimental results, which were attributed to a reduction in the effective value of the interfacial shear stress during the shock due to the bi-axial nature of the applied stress field. The phenomenon was modelled successfully using a modified Coulomb-type friction law and estimates were obtained for the value of the stress reduction factor, A, that characterises the heat transfer conditions. A methodology based on this analysis is then proposed that enables reasonable estimates of ΔTc for the class of CMCs under consideration.