VISCOELASTICITY OF CERAMICS AT HIGH-TEMPERATURES

The dependence of the fracture toughness, K(IC), on the loading rate h as been calculated. On the basis of linear elastic fracture mechanics (LEFM) a strong dependence of the fracture toughness on the loading ra te is obtained if subcritical crack growth is taken into account. If t he subcritical crack growth parameters n and B are sufficiently small, which correspond to a high velocity of crack extension, the fracture toughness should decrease at lower loading rates. This behaviour is si milar to the well-known decrease of bending strength. The experimental results for alumina containing glassy phase as a model material, howe ver, show a maximum in a certain regime of loading rates. A model is e stablished, which combines LEFM and the viscoelasticity, and leads to a maximum of K(IC) at a certain loading rate dependent on the viscosit y of the glassy phase.