Microcrack-dependent fracture of damaged rock

This paper is concerned with theoretical and experimental estimations of fracture-load degradation due to an increase of damage within a rock. The physical interpretation of this increase will involve a change in length of the process zone and a reduction in modulus. A linear cohesive-zone model of the type introduced by Dugdale and Barenblatt is formulated for large-scale yielding in a geometric sense to demonstrate the effect of microcracking on the critical load of a stationary crack. Experiments with a limestone in the chevron-notched, round beam geometry are conducted at elevated temperatures up to 150°C to evaluate the fracture load as a function of damage. When a material such as rock is heated slowly and uniformly, intergranular thermal stresses develop independent from a temperature gradient because of the local nonhomogeneity or anisotropy of the matrix. With the assumption of brittle behavior, thermal microcracking is shown to contribute to the degradation of the fracture load, as measured by the toughness, mainly through a reduction in modulus. Preliminary results indicate, however, that the energy dissipated in fracturing the damaged rock actually increases.