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The fracture toughness ( K Ic ) of sintered alumina was measured using notched beam (NB) and indentation/strength‐in‐bending (ISB) techniques. K Ic (NB) decreases with increasing grain size. For fine‐grained materials (<5 μ m ), NB results overestimate K Ic , and exhibit a substantial notch‐radius sensitivity. A stress‐intensity‐derived model is used to explain this notch sensitivity. The ISB results are very similar to those obtained using the double‐cantilever‐beam (DCB) method and show an increasing fracture toughness with increasing grain size. The differences between the NB and ISB (DCB) results for coarser‐grained materials are thought to be related to R ‐curve behavior.
Mussler et al. (Mon,) studied this question.