It is now well known that the principal source of toughening during resistance-curve (R-curve) behavior in non-transforming monolithic ceramics derives from crack-wake phenomena, in particular from a zone of bridging grains behind the crack tip which screen it from the applied loads (crack-tip shielding) (Evans, 1990). Specifically, where fracture is intergranular, the elastic and frictional tractions generated via the contact of opposing crack faces act to reduce the local driving force in the immediate vicinity of the crack tip. Indeed, various attempts have been made to quantify the magnitude and distribution of bridging tractions acting across the faces of cracks growing under monotonic loads in order to understand the mechanics of R-curve toughening and how it relates to local microstructure.

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