INFLUENCE OF STRESS RATIO ON THE ELEVATED-TEMPERATURE FATIGUE OF A SILICON-CARBIDE FIBER-REINFORCED SILICON-NITRIDE COMPOSITE

The influence of stress ratio on the tensile fatigue behavior of a unidirectional SiC-fiber/Si3N4-matrix composite was investigated at 1200-degrees-C. Tensile stress ratios of 0.1, 0.3, and 0.5 were examined. Fatigue testing was conducted in air, at a sinusoidal loading frequency of 10 Hz. For peak fatigue stresses below the proportional limit of the composite (approximately 195 MPa at 1200-degrees-C) specimens survived 5 x 10(6) cycles, independent of stress ratio. At peak stresses above the proportional limit, fatigue failures were observed; fatigue life decreased significantly as the stress ratio was lowered from 0.5 to 0.1. Creep appears to be the predominant damage mechanism which occurs during fatigue below the proportional limit. Both mechanical cycle-by-cycle fatigue damage and creep contribute to specimen failure at peak stresses above the proportional limit.