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. Copyright © 1990, Wiley Blackwell. All rights reserved