Crack‐Interface Grain Bridging as a Fracture Resistance I, Mechanism in Ceramics: I, Experimental Study on Alumina

TLDR

The study observes crack growth in tapered cantilever beam and indented flexure specimens of coarse‑grained alumina. Microscopic evidence shows that grain‑localized bridging across the crack interface in coarse‑grained alumina produces a strongly increasing resistance characteristic, with stable but discontinuous fractures, bridging segments spaced 2–5 grain diameters that persist over ~100 grain diameters behind the crack tip, and secondary microfracture and frictional tractions driving bridge separation, while alternative toughening mechanisms were not observed, indicating that crack‑interface bridging may be a general phenomenon in nontransforming ceramics.

Abstract

Direct microscopic evidence is presented in support of an explanation of R ‐curve behavior in monophase ceramics by grain‐localized bridging across the newly formed crack interface. In situ observations are made of crack growth in tapered cantilever beam and indented flexure specimens of a coarsegrained alumina. The fractures are observed to be highly stable, typical of a material with a strongly increasing resistance characteristic, but are discontinuous at the microstructural level. Associated with this discontinuity is the appearance of overlapping segments in the surface fracture trace around bridging grains; the mean spacing of such “activity sites” along the trace is about 2 to 5 grain diameters. These segments link up with the primary crack beneath the specimen surface, and continue to evolve toward rupture of the bridge as fracture proceeds. The bridges remain active at large distances, of order 100 grain diameters or more, behind the crack tip. Scanning electron microscopy of some of the bridging sites demonstrates that secondary (interface‐adjacent) microfracture and frictional tractions are important elements in the bridge separation process. Evidence is sought, but none found, for some of the more popular alternative models of toughening, notably frontal‐zone microcracking and cracktip/internal‐stress interaction. It is suggested that the crackinterface bridging mechanism may be a general phenomenon in nontransforming ceramics.