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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 MPa√m, 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 compacttension 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.

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Silicon nitride exhibits fatigue based on cyclic crack propagation which is critical for components under repeated loading. Lifetime predictions for such components are usually based on power law formulations and are most sensitive to the crack growth exponent n. Various statistical procedures exist to determine the parameters from cyclic tests of un-cracked smooth samples. In this work, an analysis is presented for silicon nitride (SL200) lifetime data with a focus on the relation between load ratio and the exponent n. It is found that n increases with the load ratio which has also been observed for macroscopic cracks in silicon nitride earlier. A high degree of uncertainty is associated with the crack growth exponent due to the sparse lifetime database. A pooling strategy is presented which increases the sample size of the underlying lifetime distribution and successfully decreases the scatter in the crack propagation curves and the uncertainty in the crack propagation exponent and, thus, in the lifetime prediction.V. Sglavo-contributing editor Manuscript No. 28709.