Strength of rocks in the confined tension region, where the minimum principal stress is tensile, has only infrequently been measured and is not well understood. Quasi-static confined Brazilian tests under a range of confining stresses (2.76 to 27.58 MPa) where used to determine the strength of sandstone in the confined tension region. The test results indicate that the strength in the confined tension region was a strong function of the intermediate principal stress: increasing the intermediate principal stress significantly increased the strength of the sandstone. The strength data were well fit by the Mogi–Coulomb criterion, which accounts for the intermediate principal stress. Unconfined Brazilian strength data were not well fit to the Mogi–Coulomb criterion derived from the confined Brazilian test data, consistent with a transition from tensile to shear processes dominating failure with increasing confining pressure. Observations of post-failure fracture surfaces reveal more indication of shear processes with increasing confining pressure. Numerical simulations from combined finite-discrete element method are compared to the experimental results and reflect similar conditions for failure compared to the experimental tests in the confined tension region.

We present results of an investigation into the mechanisms of damage in granular microstructures conducted through direct numerical simulation with the combined Finite-Discrete Element Method (FDEM). Scanning Electron Microscope (SEM) images of a pressed crystalline powder are directly meshed, resolving grain-grain interfaces. Semi-ductile microfracture is simulated by prescribing a combination of inter-granular brittle fracture and intra-granular grain plasticity. Pristine (undamaged) and damaged microstructures are simulated in uniaxial compression tests and compared to experimental uniaxial compression measurements from literature. The simulation results show that the observed microscale mechanisms of damage (microfracture predominantly around and sometimes through grains and crack associated pore-growth) can well explain degradation of strength observed in the laboratory measurements. A method of tracing grain boundaries from SEM images is described and applied to meshing of a microstructure damaged through cyclic thermal loading. By calibrating the simulations to the damaged and undamaged experimental measurements, micro-mechanical/structural insight is gained into the mechanisms of damage for the material. The results show that the SEM-based micro-characterization of damage can explain the degradation in effective strength observed in the testing and can be accurately modeled using the presented methods.

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