Crack propagation in notched specimens of porous silicon carbide under cyclic loading

Notched specimens of porous silicon carbide with porosity 37% were fatigued under four-point bending at frequencies of 30 and 0.3 Hz. The fatigue life expressed in terms of time was rather insensitive to the test frequency, while that expressed in terms of cycles was much shorter for the case of 0.3 Hz than for 30 Hz. A time-dependent mechanism of stress corrosion cracking was mainly responsible for crack propagation, and stress cycling enhanced the crack-propagation mechanism. The crack length was estimated from the change in compliance of the specimen. The crack-propagation curve was divided into stages I and II. In stage I, the crack-propagation rate decreased even though the applied stress intensity factor became larger with crack extension, and then turned to increase in stage II. The transition from stage I to II took place at a crack extension of around 0.8 mm. This anomalous behaviour is caused by crack-tip shielding due to microcracking and asperity contact. Fractographic observations showed that the fracture path was along the binder phase between silicon carbide particles, or more precisely along the interface between particles and binders.