Experimental investigation and micromechanical modeling of the brittle-ductile transition behaviors in low-porosity sandstone

This paper presents a unified experimental and numerical investigation on mechanical behaviors of a sandstone with brittle-ductile transition. A series of triaxial compression tests are performed at room temperature under a wide range of confining pressure, from which some critical states of damage are originally identified. It is shown that shear-induced dilation occurs in brittle faulting while macroscopic dilatant cataclastic flow takes place in the rock under elevated pressures without occurrence of shear bands by strain localization. There is a clear trend for peak strength to increase nonlinearly with confining pressure and that microcracking-related local friction is the dominant mechanism of inelastic deformation. In order to describe the nonlinear mechanical behaviors of the sandstone, a micromechanics-based isotropic plasticity-damage coupling model is formulated, in which two critical states of damage at rock failure are taken into account to establish the relations between the model's parameters and experimental data. Theoretical predictions of the strength envelope and nonlinear mechanical responses of the sandstone show a quantitative agreement with the test results including the main features of stress-strain curves with different confining pressures.