An elasto-plastic constitutive model for brittle-ductile transition in porous rocks

Abstract

A unified elasto-plastic constitutive model based on the observed behavior of porous rocks is proposed. The model utilizes the concept of critical state (CS), used for soils in the Cam clay models, to separate brittle dilatant deformations from the compactive ones. The CS represents ultimate (residual) state of stress at which large shear deformantions occurs with steady level of stresses and fixed porosity. The Cam-clay phenomenology is combined with the poroelasticity theory to develop a comprehensive elasto-plastic model for porous rocks. The yield surface is allowed to evolve with both plastic volumetric and deviatoric strain such that dilatant hardening/softening and compactive yielding can be predicted. The potential function is assumed to have the same form as the yield function, but the constants have different values to predict non-associative dilatancy. The model parameters are evaluated from the experimental data of sandstones and the model capabilities are demonstrated through the calculated mechanical response in triaxial tests under a wide range of confining pressures.