Finite strain rock plasticity: stress triaxiality, pressure, and temperature effects

An elasto-plasticity theory is used to model the deformation of geological materials under various confining pressures and moderate temperatures. The effects of material hardening (or softening due to volumetric strains) are included, and the corresponding elasto-plastic rate constitutive relations are developed. To study the influence of pressure and temperature on the constitutive parameters, we use some published data of laboratory experiments on certain rocks. It is shown that over a wide range of pressures and low to moderate temperatures, when the rate effect can be ignored, the model can be used to describe the behaviour of geological materials. Based on this theory, dilatancy (i.e., inelastic volumetric expansion) of an intact granite is studied under conventional triaxial stress states. The effect of pressure and temperature on the magnitude of dilation and on the stress (measured relative to the peak stress) at the onset of dilatancy is investigated. It is found that, consistent with experimental data, the theory predicts this stress to be about 50% of the peak stress, but its specific value depends on pressure and temperature. As an illustration, stress-strain curves for intact granite at relatively shallow crustal depths are then predicted for possible application to the study of crustal deformation and for the prediction of fault behaviour.