Deducing the Fatigue Crack Growth Rates of Natural Flaws in Silicon Nitride Ceramics: Role of R-Curves

摘要

Fatigue failure is a concern when high-strength, high-toughness silicon nitride ceramics are used in mechanical components and the growth of natural flaws will determine the usable upper bound strength. In this study a fracture resistance curve (R-curve) model is incorporated into an established method for deducing natural flaw growth rates from a combination of strength and fatigue life data for smooth specimens. Experimental data for a commercial silicon nitride, SL200, were examined. When compared with results deduced using a constant fracture toughness model, the new method gives more physically realistic growth rate results. Specifically, by incorporating the R-curve the deduced fatigue threshold is equal to the reported intrinsic toughness for crack propagation of 2.2 Mpam~(1/2), whereas the constant fracture toughness model gives a physically unrealistic threshold value. Furthermore, much better agreement is achieved with the growth rates measured using macroscopic compact-tension specimens. Overall, it is concluded that the R-curve effect should not be ignored when deducing the fatigue crack growth rates of natural flaws in high-toughness silicon nitride ceramics.