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Coarse‐grained alumina is among those monolithic ceramics which can exhibit an increase in crack resistance with crack extension. This R ‐curve behavior is most pronounced for intergranular fracture and does not depend exclusively on material properties. Crack and specimen geometries also influence the shape of the R ‐curves. The magnitude of the effect increases with increasing crack surface roughness, which is microstructure‐dependent, and with crack‐opening displacement, which is geometry‐dependent. Based on experimental observations, a “dynamic” R ‐curve model is presented which relates the increasing resistance to an increasing crack tip shielding caused by crack surface bridging. Applying a J ‐integral approach, R ‐curves are calculated for two specimen geometries (short double cantilever beam and single‐edged notched beam) and different grain sizes. The good agreement between calculation and experiment indicates that the R ‐curve behavior of long cracks in alumina can be predicted by a simple wake model.
Steinbrech et al. (Sun,) studied this question.