Resistance to Fatigue at Intermediate Temperatures in Air for SiC Hi-Nicalon Fibers: Statistical Distributions of Lifetimes and Strength Degradation


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Silicon carbide-based fibers have been shown to be sensitive to delayed failure at temperatures <1200°C in air as a result of slow crack growth. The present article investigates the resistance to fatigue of Hi Nicalon SiC-based filaments in the intermediate temperature range where creep is ineffective. Statistical distributions of rupture times under constant and identical stresses were determined from static fatigue tests on single filaments at 500°C and 800°C. Closed form equations of statistical distribution of lifetimes as a function of stresses and specimen volume, of size effects on lifetime and of retained strength after fatigue were established. The slow crack growth constants were estimated from experimental statistical distributions of rupture times. Trends in the effects of applied stress and specimen size on lifetime, and in strength degradation in fatigue were predicted. Finally, the influence of preponderant factors was discussed.