A strain softening/hardening constitutive model for rock based on newly modified GZZ criterion

The generalized Zhang-Zhu (GZZ) criterion successfully extends the widely used Hoek-Brown criterion to a three-dimensional (3D) version by considering the intermediate principal stress. Based on the GZZ criterion, a constitutive model within the elastic-perfectly plastic framework has already been developed. In this paper, a general strain-softening/hardening constitutive model is proposed aiming to enhance the performance with respect to post-failure behavior prediction. The plastic deviatoric strain is taken as the fundamental variable in a series of exponential functions to control the softening/hardening behavior of rock. The same type of exponential function but with different parameters is used to describe the evolution of the plastic potential function. The constitutive model has been implemented in a 3D finite-difference code and used to analyze two types of rock tested under a true triaxial condition. The results indicate that the calculated stress versus strain and lateral strain versus axial strain relations based on the constitutive model are in good agreement with those from the experiments. A systematic parametric study is also performed to explore the effect of the strain-softening/hardening parameters on the post-failure behavior of rock.