Fatigue behaviour of silicon nitride studied by indentation flexure method

Fatigue crack growth has been studied in self-reinforced gas pressure sintered silicon nitride (GPS Si3N4) using four-point bending of indented samples (indentation flexure fatigue - IFF) at room temperature under static and cyclic conditions. The method is simple, cost-effective, material and time saving, and produces stable crack growth. However, the analysis requires an estimation of the residual stress intensity factors produced by the indentations and assumptions about the shape of the indentation flaws to calculate the applied stress intensity factors. Crack growth parameters were calculated assuming a Paris's power law dependence. It was found that crack growth resistance was significantly reduced under cyclic loading due to the wear of the bridging ligaments in the crack wake. In the case of static fatigue, the resulting Paris's exponent was compared with the value obtained from the dynamic fatigue constant stress rate testing in 4-point bending using different loading rates. The crack growth mechanisms were identified using scanning electron microscope and have been correlated to the results of the fatigue tests.