Numerical modelling of fracture initiation and propagation in biaxial tests on rock samples

A two-dimensional boundary element code, based on the displacement disconti nuity method is used to simulate a confined compression test. The method ta kes account of the granular nature of the rock and of the presence of pre-e xisting defects. Fracture propagation is thought to depend, amongst other f actors, on the crack orientation, the residual friction angle, the dilation angle, and the: confining pressure. To obtain a more precise understanding of the influence of these properties on the crack growth process, their in fluence on the normal stress and the excess shear stress on potential fract ure planes ahead of the crack tip is investigated for a single crack config uration. The orientation of the potential fracture planes proves to be the most important parameter determining fracture growth. A series of numerical experiments is carried out to determine the influence of the tessellation pattern used to represent the granular nature of the rock. Both the influen ce of the type of tessellation and the tessellation density are evaluated, and reasons tor the differences in behaviour are presented. The results of the simulations with the Delaunay and a Voronoi tessellation with internal fracture paths compare well with the fracture pattern obtained in laborator y tests. The pre-peak non-linearity in the stress-strain response obtained with the Voronoi tessellation and the post-peak strain softening obtained w ith the Delaunay tessellation are combined in one model. For that purpose, a Voronoi tessellation with internal fracture paths is used, whereby the pr operties elf the elements of the polygons and of the internal fracture path s are assigned different values. The role that is played by shear failure a nd the influence of dilation on the localisation process is determined by m eans of some further numerical experiments. It is shown that at the scale. at which the material is modelled, shear failure is required for a shear ba nd to develop.