HIGH-TEMPERATURE PLASTICITY EFFECTS IN BRIDGED CRACKS AND SUBCRITICAL CRACK-GROWTH IN CERAMIC COMPOSITES

The time-dependent plasticity of the crack-wake bridging zone at high temperatures is found to control the rate of subcritical crack growth in continuous-fiber-reinforced ceramic composites. Subcritical crack growth measurements of ceramic matrix composites were conducted on materials consisting of chemical-vapor infiltrated SiC matrix reinforced with Nicalon fibers (SiC/SiC(f)) having C and BN fiber-matrix interfaces. Crack velocities were determined as a function of stress intensity in pure Ar and in Ar + 2000 ppm O2 at 1100-degrees-C. A stage II regime, where the crack velocity is weakly dependent on the applied stress intensity, characterized the V-K data over a range of stress intensities corresponding to the R curve of the material. This stage II behavior was followed by a stage III, or power-law, regime at higher stress intensities. Oxygen increased the crack velocity in the stage II regime and shifted the stage II to stage III transition to lower stress intensities. A two-dimensional micromechanics approach modeled the time-dependence of crack bridging by allowing fiber creep and was able to rationalize the observed velocity measurements and the stage II to stage III transition.