Nonlinear Viscoelastoplastic Fatigue Model for Natural Gypsum Rock Subjected to Various Cyclic Loading Conditions

To investigate its fatigue damage characteristics, natural gypsum rock was tested under multiple cyclic loading conditions. According to the fatigue test, a nonlinear viscoelastoplastic fatigue model was established by combining several fatigue elements to describe the fatigue process of gypsum rock based on the equivalent stress assumption. The results indicate that the fatigue damage behavior of gypsum rock is substantially different from that of other types of rocks because of its particular mode of damage involving crystal and tabular crystal microstructures. The fatigue characteristics of gypsum rock, such as the fatigue life and fatigue deformation evolution, were found to be sensitive to multiple cyclic loading conditions such as the stress level, cyclic frequency, and stress amplitude. The proposed nonlinear viscoelastoplastic fatigue model could predict the fatigue deformation evolution of gypsum rock well, and the fatigue model parameters varied depending on the cyclic loading conditions. When the upper limit stress was less than the fatigue damage threshold of gypsum rock, the proposed fatigue model was similar to the modified Kelvin model, which could only describe the decelerating and stationary stages of the deformation evolution. When the upper limit stress was higher, the proposed model could describe the entire fatigue deformation process of gypsum rock well, especially the acceleration stage.