A thermodynamics-based model for brittle to ductile behaviour and localised failure of porous rocks

This study presents the development of a thermodynamics-based constitutive model for describing the macro-mechanical behaviour of porous rocks. The model formulation is developed within the well-established framework of generalised thermodynamics, or thermodynamics with internal variables (TIV), in order to guarantee the thermodynamics admissibility of the results. A new way for coupling different mechanisms of energy dissipation is proposed within the framework of TIV, so that the essential aspects of macro-mechanical behaviour of porous rocks ranging from brittle to ductile, dilation and compaction, as well as various modes of localised failure, under shearing at a wide range of confining pressures are correctly captured. The pressure sensitive behaviour of porous rocks is captured by introducing a shift stress in the formulation of the yield function, through the concept of 'frozen elastic energy' or non-dissipative part of the plastic work, which is stored within the material during loading. The proposed model allows the investigation of several important aspects of macro-mechanical behaviour of porous rocks at both material and specimen levels.