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The modified static loading technique for estimating static fatigue limits was used to study the effects of oxidation and temperature on the static fatigue limit, K,,, for crack growth in sintered silicon carbide. For as-machined, unoxidized sintered silicon carbide with a static load time of 4 h, KIo ~2 . 2 5 MPa * m'" at 1200" and ~1 . 7 5 at 1400°C. On oxidation for 10 h at 12OO0C, Kro drops to ~1 . 7 5MPam"' at 1200" and -1.25 at 1400°C when tested in a nonoxidizing ambient. Similar results were obtained at 1200°C for tests performed in air. A tendency for strengthening below the static fatigue limit appears to result from plastic relaxation of stress in the crack-tip region by viscous deformation involving an oxide grain-boundary phase for oxidized material and, possibly, diffusive creep deformation in the case of unoxidized material.

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