Confined Tensile Testing of Porous Sandstone

The confined tension region of a failure surface, where both tensile and compressive normal stresses are present, is experimentally challenging to investigate and consequently not fully characterized. In particular, strength tests of rock at stress states associated with low mean stress are rare. To address this limitation, axisymmetric extension experiments on dog-bone specimens are used in combination with axisymmetric extension and compression experiments on right-circular cylinders of Dunnville sandstone to define the confined tension region and evaluate linear (Mohr–Coulomb and Paul-Mohr–Coulomb) with tension cut-offs and non-linear (Hoek–Brown and Fairhurst) failure criteria. The strength tests indicate that a failure criterion that includes a tension cut-off best captures failure in the low mean stress regime, − T/3 < P < C_o/3, where T = uniaxial tensile strength, P = mean stress, and C_o = uniaxial compressive strength. Fracture surfaces of the dog-bone specimens were examined for failure mode based on surface roughness and it was found that there is a transition of decreasing roughness from tensile (opening) failure to hybrid (opening and sliding) failure to shear (sliding) failure.