Influence of porosity on the hydrostatic constraint factor for evaluating toughness from Vickers indentation cracks in brittle materials

Discrepancies have been noted between the predictions of classical mechanics for dense bodies and the Vickers indentation cracking behaviour of porous YBa2Cu3O7-x ceramics, and in particular a shift in the 'universal' indentation diagram of Niihara et al. The hydrostatic constraint factor is measured as the ratio of hardness to yield stress on porous sintered steels. Its introduction into the universal indentation diagram of porous ceramics allows retrieval of the behaviour predicted for dense materials. The question of whether the evolution of the constraint factor with porosity could be common to all porous materials densifying under constrained compression is addressed.