  We used computer simulations to study spontaneous strain localization in
granular materials, as a result of symmetry breaking non-homogeneous
deformations. Axisymmetric triaxial shear tests were simulated by means of
standard three-dimensional Distinct Element Method (DEM) with spherical grains.
Carefully prepared dense specimens were compressed between two platens and, in
order to mimic the experimental conditions, stress controlled, (initially)
axisymmetric boundary conditions were constructed. Strain localization gave
rise to visible shear bands, previously found experimentally under similar
conditions by several groups, and different morphologies could be reproduced.
We examined the stress-strain relation during the process and found good
agreement with experiments. Formation mechanism of shear bands is discussed.
