ANALYSIS OF LOCALIZATION OF DEFORMATION AND COMPLETE STRESS-STRAIN RELATION FOR MESOSCOPIC HETEROGENOUS BRITTLE ROCK MATERIALS WHEN AXIAL STRESS IS HELD CONSTANT WHILE LATERAL CONFINEMENT IS REDUCED
Abstract：Stress redistribution induced by excavation of underground engineering and slope engineering results in the unloading zone in parts of surrounding rock masses. The mechanical behaviors of crack-weakened rock masses under unloading are different from those of crack-weakened rock masses under loading. The pre-existing microcracks will be closed under loading，but they will be open under unloading. The deformation of rock material under unloading is more larger than that under loading. The frictional sliding activation on pre-existing cracks will occur under loading，but the frictionless sliding activation on pre-existing cracks may occur under unloading. Under same condition，the axial stress at which some pre-existing cracks begin to propagate in a stable fashion under unloading is less than that under loading；and the axial stress at which some pre-existing cracks begin to propagate in an unstable fashion under unloading is less than that under loading. The strength of rock material under unloading is less than that under loading. A micromechanics-based model has been proposed for brittle rock material undergoing irreversible changes of microscopic structures due to microcrack growth when axial stress is held constant while lateral confinement is reduced. The basic idea of the present model is to classify the constitution relation of rock material into four stages including some of the stages of linear elasticity，pre-peak nonlinear hardening，rapid stress drop，and strain softening，and to investigate their corresponding micromechanical damage mechanisms individually. Special attention is paid to the transition of structure from rearrangements on microscale to the macroscopic inelastic strain，to the transition from distribution damage to localization of damage and the transition from homogeneous deformation to localization of deformation. The closed-form explicit expression for the complete stress-strain relation of rock materials containing cracks under unloading is obtained. The results show that the complete stress-strain relation and the strength of rock materials under unloading depend on the crack spacing，the fracture toughness of rock materials and orientation of the cracks，the crack half-length and the crack density parameter.
