Environmental Effects on the Isothermal and Thermomechanical Fatigue of SCS-6/TIMETAL 21S Unidirectional Composites

Abstract

The effect of environment on the fatigue behavior of SCS-6/TIMETAL 21S [0]{sub 4} composites is examined through a comparison of fatigue lives and damage progression for tests performed in air and high-purity helium. Isothermal and thermomechanical tests were conducted at 650 C and 150 to 650 C, respectively. Out-of-phase thermomechanical fatigue (TMF) lives of specimens tested in the inert environment show a {times}2 increase in life. In-phase TMF lives in inert and air environments, which are governed primarily by fiber bundle strength and matrix stress relaxation, are comparable. Isothermal fatigue tests in the inert environment performed at 1.0 Hz show that at high stresses the life is not affected by the environment but at lower stresses a {times}3.5 increase in life is observed. Reducing the fatigue frequency to 0.01 Hz causes no change in life at low stresses in the inert condition as compared to the low-frequency air condition. At high stresses, the behavior is governed primarily by the statistics of fiber bundle strength. Life fraction models that consider time-dependent and cycle-dependent behavior as well as fiber- and matrix-dominated failure modes are used to correlate observed behavior in these experiments.