STUDY ON FATIGUE RHEOLOGICAL DAMAGE MODEL OF SILTSTONE UNDER CYCLIC LOADING

The fatigue test of siltstone under cyclic loading is carried out, aiming at the problem that the deformation law and the damage evolution characteristics of rock under cyclic loading cannot be described rigorously by the existing rheological models. According to the analysis of the test results, a damage variable established by Kachanov's creep damage theory is introduced into a viscoplastic element with strain trigger and stress threshold, then a new rheological constitutive model under cyclic loading is constructed through series with Burgers model and the viscoplastic element. In the meanwhile, the sine wave cyclic load stress function is used to replace the constant stress in the rheological differential constitutive equation. The one-dimensional and three-dimensional differential damage constitutive equation of rock is inferred by the integral method, and then the viscoelastic plastic rheological damage equation is inferred by a superposition principle. The parameters of the new rheological model are derived by fitted fatigue deformation curves of siltstone. The applicability verification shows that: the new rheological model can not only accurately reflect the attenuation and steady-state rheological deformation stage of siltstone, but also effectively describe the accelerated rheological stage when the upper limit of cyclic load is higher than the fatigue strength. Finally, the critical damage threshold and instability criterion of accelerated rheology are proposed, and the prediction methods of the start time, duration and fatigue life of accelerated rheology are developed. The model has certain theoretical significance for the long-term stability prediction of rock engineering.