Low-cycle fatigue of Nicalon™-fiber-reinforced ceramic composites

Abstract

An experimental investigation of damage and fracture in fiber-reinforced ceramic composites under low-cycle fatigue has been performed. Several different composites have been studied, each reinforced with ceramic-grade Nicalon™ fibers, but with varying fiber architectures and matrix materials. Correlations have been established between the presence of matrix cracks within the longitudinal tows or laminae, the hysteresis in the stress/strain response, and the propensity for fatigue fracture. Fatigue was found to be most prevalent when both matrix cracks are present and the stress/strain response exhibits hysteresis. In such cases, a fatigue threshold was obtained, typically at a stress of 65–80% of the ultimate strength of the pristine composite. Furthermore, the interface sliding stress diminished with cycling, reaching a saturation level of ≈1/3–1/2 of the initial value. This reduction is predicted to cause a fatigue-strength reduction to ≈80% of the ultimate strength, broadly consistent with the measured thresholds. The inference is that the degradation in fatigue strength is attributable in a large part to the degradation in the sliding stress, with the balance being associated with a degradation of the fibers themselves. The latter conclusion has been corroborated through measurements of the fracture-mirror radii on the fibers in both pristine and fatigued specimens.