Constitutive model of Viscoelastic-plastic strain characteristics and damage of South China red sandstone under chemical environment

In this paper, a graded creep cyclic loading and unloading test was carried out to explore the viscoelastic-plastic rheological characteristics of red sandstone in South China under acidic environment. The mechanical parameters and viscoelastic-plastic strain characteristics of the acidic solution corrosive specimen were obtained by analyzing the rock deformation behavior by the improved viscoelastic-plastic strain separation method, and the instantaneous strain was separated into instantaneous elastic strain and instantaneous plastic strain, and the creep-strain strain was separated into viscoelastic strain and viscoplastic strain. Based on the nonlinear rheological theory, the constitutive equations of creep damage of rock in one-dimensional and three-dimensional stress states are derived. The results show that acid corrosion has little effect on the resistance to elastic deformation of red sandstone, the elastic deformation modulus remains basically the same at the same stress level, and the instantaneous elastic strain and viscoelastic strain are close to linear relationship with the deviator stress. The increase of acidity of the solution increases the plastic deformation of rock samples, in which the instantaneous plastic modulus increases gradually with the increase of the number of loading and unloading, and the instantaneous plastic strain increment decreases. The viscoplastic modulus gradually decreases with the increase of stress level, and the increase of viscoplastic strain increases. The creep flexibility and creep rate of viscoplastic have the same trend with the change, and increase with the increase of solution concentration and stress level. By introducing the chemical damage variable considering the plastic deformation of the material, the creep damage constitutive equation considering the viscoelastic-plastic strain separation is established, which can describe the variation of strain with time during loading and unloading, and verifies the rationality and accuracy of the proposed model by comparing it with the traditional Nishihara model.