DEM simulations of sandstone under true triaxial compressive tests

Numerically simulated true triaxial compression tests (sigma(1) >= sigma(2) >= sigma(3)) are conducted in this study to elucidate the failure mechanism of sandstone using 3D discrete element method (DEM), in particular the effect of the intermediate principal stress (sigma(2)). Eight series of tests (sigma(3) = 0, 10, 20, 30, 40, 50, 70, and 100 MPa) are conducted. Within each series, sigma(2) is varied from sigma(2) = sigma(3) to sigma(2) = sigma(1) from test to test. For each test, sigma(1) is raised monotonically to failure while keeping sigma(2) and sigma(3) constant. The DEM simulations reveal the effect of sigma(2) on the variations of peak stress, Young's modulus, failure plane angles, the brittle-ductile transition, and the evolution of failure modes, the effect beyond the well-understood effect of sigma(3). The simulation is in qualitative agreement with the results obtained experimentally. Detailed analyses performed on the particle-scale responses further the understanding of the microscopic mechanisms. The distribution of contact force becomes more homogeneous with the increase of sigma(3), which leads to the resulting damage being more localized rather than diffused. The interaction between contact force distribution and coalescence of cracks determines the processes and patterns of fracturing in the sample scale. sigma(2) is found to affect the microscopic stress distribution as well as structure evolution, and this effect weakens with the increase of sigma(3).