Notch sensitivity of fatigue life in a Sylramic TM/SiC composite at elevated temperature

Abstract

The effects of holes and notches on the fatigue life of an advanced Sylramic™/SiC composite at 815°C have been examined. At this temperature, fracture occurs by an oxidative embrittlement mechanism, common to most SiC-based composites. In unnotched specimens, embrittlement is manifested at stresses above the matrix cracking limit, σ mc, leading to fracture following relatively short exposure times (∼100 h). As a consequence, a fatigue threshold is obtained at a stress, σ th≈ σ mc. This threshold is due to the absence of an easy path for oxygen ingress when matrix cracks are not present. In center-hole specimens, an analogous threshold is obtained, at a stress, σ th ≈ σ mc k e , where k e is the elastic stress concentration factor (= 2.5). That is, once the cracking limit is exceeded at the hole edge, embrittlement and fracture ensue. The threshold stress for center-notch specimens with stress concentration k e=7 is numerically similar to that of the center-hole specimens with k e=2.5, indicating some tolerance to local stress levels above the global matrix cracking limit in sharply notched geometries. Non-linear finite-element calculations of the stresses in the center-hole and center-notch specimens are used to infer the local conditions associated with the threshold. A key result is that the damage tolerance and notch insensitivity normally associated with inelastic straining cannot be exploited at temperatures at which the embrittlement mechanism operates. The implication is that composite structures with holes and notches must be designed extremely conservatively to ensure long lifetimes (> 100 h).