Fatigue Crack Growth in Ceramics and Ceramic Composites at High Temperatures

Abstract

The susceptibility of ceramics and ceramic composites to subcriticai fracture under the influence of cyclic stresses has been a topic of considerable research during the past decade. Experiments conducted on both unreinforced and reinforced ceramics at room and elevated temperatures have shown¹ that true cyclic fatigue effects may occur under certain conditions as a consequence of one of the following mechanisms. (1) The generation of tensile residual stresses ahead of stress concentrations in ceramics subjected to far-field cyclic compression can induce the nucleation of stable mode I fatigue cracks at room and elevated temperatures.² This crack formation and growth process is aided by the existence of a zone of permanent deformation ahead of the stress concentration upon unloading from the far-field compressive stress.³ (2) Intrinsic differences between the crack-tip deformation characteristics induced by static and cyclic loads at elevated tempertures can lead to cyclic fatigue effects in some monolithic and reinforced ceramics.⁴ These effects generally arise from such factors as viscous flow of glassy films formed at grain boundaries and interfaces, grain boundary sliding, frictional sliding of a discontinuous reinforcement within the crack-tip region, or the breakage of the reinforcement.^{5, 6} (3) Degradation of grain bridging or reinforcement bridging of the crack faces in response to repeated cyclic loading can amplify the effective driving force at the crack tip such that the rates of crack growth under cyclic loads are significantly more pronounced than under static loads.^{7, 8}