SUBCRITICAL CRACK-GROWTH IN CVI SILICON-CARBIDE REINFORCED WITH NICALON FIBERS - EXPERIMENT AND MODEL

Subcritical crack growth measurements of ceramic-matrix composites have been conducted on materials consisting of CVI SiC matrix reinforced with Nicalon fibers (SiC/SiCf) having C and BN fiber-matrix interfaces. Velocities of effective elastic cracks were determined as a function of applied stress intensity in pure Ar and in Ar plus 2000 ppm O-2 at 1100 degrees C. A stage-II regime, where the crack velocity depends only weakly on the applied stress intensity, was observed in the V-K diagrams over a range of applied stress intensities that correspond to the R-curve of the materials. This stage-II behavior was followed by a stage-III, or power-law, regime at higher stress intensity values. Oxygen was observed to increase the crack velocity in the stage-II regime and to shift the stage-II-to-stage-III transition to lower stress intensity values. A 2D micromechanics approach was developed to model the time dependence of observed crack-bridging events and is able to rationalize the measured effective crack velocities, the time dependence of the crack velocity, and the stage-II-to-stage-II transition in terms of the stress relaxation of crack-bridging fibers.