FRACTURE AND FATIGUE OF A NICALON/CAS CONTINUOUS FIBER-REINFORCED GLASS CERAMIC MATRIX COMPOSITE

The fracture and fatigue behaviour of Nicalon/CAS continuous fibre-reinforced glass-ceramic matrix composite was studied at temperatures of up to 1000-degrees-C in both air and vacuum. Using chevron-notched test-pieces in bending, high nominal toughness values are measured at ambient temperature and at 1000-degrees-C in vacuum. In contrast, toughness values obtained in air decrease progressively with test temperature increase from 600 to 1000-degrees-C, and at 1000-degrees-C they are reduced by a factor of three from their values at ambient temperature. Marked changes in micromechanisms of crack growth under cyclic loading are also observed in air as the test temperature is increased: multiple cracking occurs at ambient temperature, while dominant Mode I cracks can be produced at 1000-degrees-C. A further study has been carried out in air on plane-sided test-pieces at a temperature at 1000-degrees-C, under both monotonic and cyclic loading. At ambient temperature, effects of cyclic loading have been deduced, while crack growth at 1000-degrees-C in air appears to be dominated by static loading with little effect of cyclic loading. These subcritical crack growth and toughness observations are consistent with changes that occur in the fibre/matrix interfaces at elevated temperatures in air reported in the literature.