Abstract: Defining the damage evolution characteristics of rocks under load is the key to analyze the damage mechanism of rocks. In this paper, a strain-related deformation intrinsic model of sandstone was developed by introducing a strain-related macro-modulus mathematical model. On the basis of the damage mechanics theory, the sandstone damage evolution law was analyzed by using the established intrinsic structure model. Moreover, uniaxial/triaxial compression tests were performed on rocks under different loading rates and surrounding pressure conditions using a rock triaxial loading system to systematically verify the rationality of the sandstone damage evolution model. The initial damage ω₀, loading damage ω_f and total damage ω_t of sandstone were obtained. Results demonstrate that loading rate has a small effect on the evolution of the three types of damage when loading rate is varied in the range of 0.0-0.5 mm/min. When surrounding pressure changes in the range of 0-10 MPa, the initial damage and total damage of the sandstone keep decreasing with the increase of surrounding pressure, while loading damage remains almost constant. The loading damage, as the rock damage threshold, is less influenced by loading rate and surrounding pressure and fluctuates within a certain range, which indicates that the crack expansion within the rock is the causative factor of damage. Finally, a method for estimating the in-situ modulus of rocks is proposed by combining the intrinsic model and damage theory presented in this paper, providing theoretical references for related projects.

Key words: initial damage, strain dependence, deformation constitutive model, damage evolution, uniaxial and triaxial compression test, rock in-situ model