| The mechanisms that govern the strength of three Nicalon fiber-reinforced ceramic matrix composites (CMCs) have been elucidated. Specific regard was given to the volume-dependence of tensile strength, the fatigue resistance at ambient and elevated temperatures, and the notch-sensitivity of strength. Mechanism-based materials models were employed to describe these properties. The utility of the models was assessed through comparison with experiments.; The CMCs investigated in this study exhibit stochastic behavior, manifested as volume-dependent strength. The volume-dependence was demonstrated through a technique relating the strengths obtained in flexure and tension, coupled with the application of weakest-link statistics. The results indicate that the CMCs exhibit a high reliability, with Weibull moduli being in the range of {dollar}sim{dollar}20 to 40.; The reduction in strength under cyclic loading is due to degradation of the interface sliding stress, via a frictional wear process, and the associated reduction in the fiber bundle strength. This degradation was evaluated using hysteresis loop measurements, as well as fiber pullout and push-in techniques. The fatigue thresholds predicted using a model based on the degradation of the sliding stress showed reasonable agreement with the experimental measurements.; The moderately notch-insensitive behavior of the CMCs is due to a combination of stress redistribution, arising from inelastic straining, and volume-dependent strength. The variation in notch-sensitivity with notch size and shape was modeled using a non-linear constitutive law to describe the stress-strain response coupled with a fracture criterion that accounts for the size-dependence of the strength. Excellent correlations are obtained between experiment and theory.; Due to the poor chemical stability of Nicalon fibers, the strength-limiting mechanism at elevated temperatures is oxidation of the fibers, rather than degradation of the sliding stress. The oxidation mechanism was ascertained through a comparison of cyclic and static load tests, coupled with chemical analysis of the interfaces. The oxidation is expected to severely limit the use of Nicalon-based composites in high temperature applications. |
