Research on improved nonlinear viscoelastic-plastic damage model of rock and parameter identification

How to accurately characterize the nonlinear characteristics of rock mass creep has long been a research hotspot. The traditional Kelvin model cannot accurately characterize the nonlinear variation process in the decay creep stage and the dual characteristics of power function and exponential function in the accelerated creep stage, so it needs to be modified. Based on nonlinear rheological theory and damage theory, this paper establishes a damage creep model that simultaneously considers instantaneous elastic strain, nonlinear viscoelastic strain, viscous strain, and nonlinear viscoplastic strain. The differential damage constitutive equations under one-dimensional and three-dimensional stress states are derived, the method for determining model parameters is presented, and the model is verified based on the triaxial compression creep test results of sandstone. Meanwhile, a sensitivity analysis is performed on the parameters of the modified model. The results show that the modified model can accurately reflect the entire creep process of sandstone under different confining pressures. In uniaxial compression creep tests, the peak strain is most sensitive to changes in the parameters η₂ and η₄. Under high confining pressure, the peak strain is most sensitive to changes in the parameters β, n, and η₄.