Oxidation under stress of SiC-based fibers at intermediate temperature and ambient air

Abstract

If the temperature and environment effects on the oxidation of SiC fibers have been extensively documented, the impact of external stresses (stress-oxidation coupling effect) remains questioned. Therefore, interrupted static fatigue tests were conducted on filaments. Surviving specimens (>60%) were subsequently tensile tested and fracture surfaces recovered for analysis. The external stress was found to have no impact on the oxidation regime (linear or parabolic) nor on its rate, which is comparable to literature data. As long as the slow crack growth (SCG) incubates (negligible growth rate), the oxidation was shown to govern the embrittlement. Fracture surface analysis revealed tensile residual stresses, attributed to a wedge effect when specimens were unloaded. Rare tests (<2%) were interrupted while the crack had significantly grown highlighting the successive crack fronts. Sources of residual stresses and differences between fiber types (flaw position inducing a bias on lifetime prediction) are discussed. In these circumstances, a coupling effect between the oxidation embrittlement and the SCG could be assumed for moderate stress levels, which is pertinent for most of ceramic matrix composite applications.