SIZE EFFECTS AND A MEAN-STRENGTH CRITERION FOR CERAMICS

On the basis of a large amount of research on the fracture behaviour of ceramics, a new failure criterion is advanced. It is pointed out that the rupture of structural ceramics is governed by the mean stress in a specific zone, rather than by the peak stress value. The size of the process zone was derived as a material constant, and reflects the relation between the toughness and the strength of the material. The results show that the critical maximum stress varies with the stress gradient: the steeper the stress gradient, the higher the critical peak stress. Different experiments concerning the size effect have been carried out and the reason for size effects has been explained by a mean strength theory.