SiC Fiber/Borosilicate Glass Composite (Part 3)

The influence of thermal stresses on the mechanical properties of thermal-shocked Nicalon SiC fibre/Pyrex composites has been analysed by considering the effect of thermal-mismatch stress on the mechanical properties of composite constituents. The thermal-mismatch stresses developed between devitrified cristobalite, and fibre and glass matrix were calculated using the values of material parameters obtained from experiment. This shows that the high thermal mismatch stresses originating from larger coefficient of thermal expansion of devitrified cristobalite extended the preexisting matrix crack, and caused fibre debonding. Thus, the reductions in the modulus and the interfacial stress of thermal-shocked composites in comparison with those of as-fabricated composites resulted from the matrix devitrification. The lower proportional limit of thermal-shocked composite was attributed to the lower interfacial stress and larger pre-existing cracks compared with those of as-fabricated composite. The reduction in ultimate strength of thermalshocked composite was correlated with the decreases of fiber hardness and interfacial stress.