Anomalous cyclic fatigue-crack propagation behavior of small cracks in monolithic, grain-bridging ceramics

Cyclic fatigue properties of two monolithic, high-toughness SiC ceramics were examined, with emphasis placed on differences between long- (> 3 mm) and small-crack (< 250 μm) behavior. Studies were performed on two microstructures in which sintering additives were systematically varied. Small cracks were initiated with Vickers indents placed on the tensile surfaces of beams, and crack extension was monitored optically under an applied cyclic load. For small cracks, high growth rates which exhibited a negative dependence on the far-field driving force were observed. Such behavior was rationalized both in terms of indent-induced residual stresses and the relative size of cracks compared to bridging zone lengths.