R-Curve Behavior of Long Cracks in Alumina

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.