Abstract
Fatigue properties in the noncyclic loading of ceramics with R-curves are studied. Particular attention is directed to the potential role of R-curves in the enhancement of fatigue limits. A numerical algorithm for solving the appropriate differential equations of rate-dependent failure is developed. Our formalism specifically incorporates a crack-size dependent toughness function, based on grain-localized interfacial bridging, and a hyperbolic-sine velocity function, representative of a fundamental activation process. In a case study, dynamic fatigue (constant stressing rate) and static fatigue (constant applied stress) data for a coarse-grained alumina with a pronounced R-curve are analysed. With foreknowledge of the toughness parameters, the intrinsic crack-tip velocity function is deconvoluted. This intrinsic function is distinguished from the usual “apparent”, or “shielded”, (and demonstrably nonunique) function determined directly from the external load. It is confirmed that the R-curve, by virtue of its stabilizing influence on the crack growth, significantly enhances the fatigue limit, and confers the quality of “flaw tolerance” on fatigue lifetimes.
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