Influence of microcracking on crack-tip toughness of alumina

Al2O3-samples with different grain sizes were produced and the crack-tip toughness KI0, also called intrinsic fracture toughness, was determined using two different measurement techniques. It appeared that KI0 depends strongly on specimen grain size with the mechanistic link provided by the increase of microcrack density with grain size. Results obtained using measurements of crack opening displacements (COD) lie considerably lower than values based on a method using bending tests of pre-notched bend bars. It is suggested that the latter method relies on the use of a notch with a microcrack, which in fact is different than the behavior of a long crack alone.