Response of SiCf/Si3N4 Composites Under Static and Cyclic Loading—An Experimental and Statistical Analysis

Abstract

Monotonic tensile and fatigue response of continuous silicon carbide fiber reinforced silicon nitride (SiCf/Si3N4) composites has been investigated. The monotonic tensile tests have been performed at room and elevated temperatures. Fatigue tests have been conducted at room temperature (RT), at a stress ratio, R = 0.1 and a frequency of 5 Hz. It is observed during the monotonic tests that the composites retain only 30 percent of its room temperature strength at 1600°C suggesting a substantial chemical degradation of the matrix at that temperature. The softening of the matrix at elevated temperature also causes reduction in tensile modulus, and the total reduction in modulus is around 45 percent. Fatigue data have been generated at three load levels and the fatigue strength of the composite has been found to be considerably high; about 75 percent of its ultimate room temperature strength. Extensive statistical analysis has been performed to understand the degree of scatter in the fatigue as well as in the static test data. Weibull shape factors and characteristic values have been determined for each set of tests and their relationship with the response of the composites has been discussed. A statistical fatigue life prediction method developed from the Weibull distribution is also presented. Maximum Likelihood Estimator with censoring techniques and data pooling schemes has been employed to determine the distribution parameters for the statistical analysis. These parameters have been used to generate the S-N diagram with desired level of reliability. Details of the statistical analysis and the discussion of the static and fatigue behavior of the composites are presented in this paper.