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/SiC f ) 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°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‐III transition in terms of the stress relaxation of crack‐bridging fibers.