RESPONSE OF SICF SI3N4 COMPOSITES UNDER STATIC AND CYCLIC LOADING - AN EXPERIMENTAL AND STATISTICAL-ANALYSIS/

Monotonic tensile and fatigue response of continuous silicon carbide f iber reinforced silicon nitride (SiCf/Si3N4) composites has been inves tigated The monotonic tensile tests have been performed at room and el evated temperatures. Fatigue tests have been conducted at room tempera ture (RT), at a stress ratio, R = 0.1 and a frequency of 5 Hz. It is o bserved during the monotonic tests that the composites retain only 30 percent of its room temperature strength at 1600 degrees C suggesting a substantial chemical degradation of the matrix at that temperature. The softening of the matrix at elevated temperature also causes reduct ion in tensile modulus, and the total reduction in modulus is around 4 5 percent. Fatigue data have been generated at three load levels and t he fatigue strength of the composite has been found to be considerably high; about 75 percent of its ultimate room temperature strength. Ext ensive statistical analysis has been performed to understand the degre e of scatter in the fatigue as well as in the static test data. Weibul l shape factors and characteristic values have been determined for eac h set of tests and their relationship with the response of the composi tes has been discussed. A statistical fatigue life prediction method d eveloped from the Weibull distribution is also presented. Maximum Like lihood Estimator with censoring techniques and data pooling schemes ha s been employed to determine the distribution parameters for the stati stical analysis. These parameters have been used to generate the S-N d iagram with desired level of reliability Details of the statistical an alysis and the discussion of the static and fatigue behavior of the co mposites are presented in this paper.