B. Van De Steen et al., Numerical modelling of fracture initiation and propagation in biaxial tests on rock samples, INT J FRACT, 108(2), 2001, pp. 165-191
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.