Fatigue threshold R-curves for predicting reliability of ceramics under cyclic loading

For monolithic, grain-bridging ceramics, the crack-size dependence of the fatigue threshold during bridging zone development presents a difficulty in its application in design. This paper demonstrates how a fatigue threshold R-curve may express this crack-size dependence, analogous to the traditional fracture toughness R-curve, and may be used to predict the endurance limit and retained strength under cyclic loading conditions. Furthermore, the fatigue threshold R-curve may be deduced from the behavior of millimeter-scale, through-thickness, fatigue cracks via an accurately measured crack bridging stress profile. Both an alumina ceramic with large steady-state bridging zones (∼2 mm), where the predicted and experimentally measured fatigue threshold R-curves agree well over a range of crack sizes from 0.06 to 7 mm, and a (Y 2O 3-MgO)-doped Si 3N 4 ceramic, where bridging zones are much shorter (∼100 μm), are investigated. The approach provides a useful alternative to performing difficult short-crack fatigue experiments, particularly for materials with small bridging zones.