Cyclic fatigue of ceramics. A fracture mechanics approach to subcritical crack growth and life prediction

Cyclic fatigue, and specifically fatigue-crack propagation, in ceramic materials is reviewed both for monolithic and composites systems. In particular, stress/life (S/N) and crack-propagation data are presented for a range of ceramics, including zirconia, alumina, silicon nitride, SiC-whisker-reinforced alumina and a pyrolytic-carbon/graphite laminate. S/N data derived from unnotched specimens often indicate markedly lower lives under tension-compression compared to tension-tension loading; similar to metals, 108-cycle 'fatigue limits' generally approach -50% of the tensile strength. Crack-growth results, based on studies on 'long' (3mm) cracks, show fatigue-crack propagation rates to be markedly power-law dependent on the applied stress-intensity range, ΔK, with a threshold, ΔKTH, of the order of approximately 50% of Kc. Conversely, for 'small' (μm) surface cracks, fatigue-crack growth is seen to occur at ΔK levels some 2 to 3 times smaller than ΔKTH, and to show a negative dependency on applied stress intensity. At ambient temperatures, lifetimes are shortened and crack-growth rates are significantly accelerated by cyclic, compared to quasi-static loading, although limited data suggest the reverse to be true at very high temperatures in the creep regime. Such results are discussed in terms of the primary crack-tip shielding (toughening) mechanisms and potential mechanisms of cyclic crack advance. Finally, implications are discussed of long and small crack cyclic fatigue data to life prediction and safety-critical design of ceramic components.