The effect of stress state factor on fracture of sandstones under true triaxial loading

Experimental results on rock deformation and fracture under true triaxial compression have revealed a misfit between strain state and stress state, strain state varying from generalized compression to generalized shear at σ3 = 0. This misfit can lead to data misinterpretation during the stress field reconstruction after unloading. Fracture of rock specimens under true triaxial compression occurs by a combined longitudinal/transverse shear and produces the highest dilatancy. An increase in the hydrostatic pressure level diminishes limiting values of shear strains and suppresses the dilatancy effect. A maximum of dilatancy coincides with a maximum of fresh surface area formed during the fracture of the rock. The generalized cleavage of rocks becomes energetically disadvantageous in a true triaxial compressive stress field. Some sandstone becomes more brittle under true triaxial compression (σ2 = 0) at low values of the minimal stress component (σ3) due to high initial porosity and dilatancy.