Strengthening–Weakening Coupled Rheological Model for Sandstone Based on Cyclic Loading–Unloading Creep Experiments

Creep modeling and parameter identification have long been critical issues in rock mechanics. In this study, cyclic loading–unloading creep tests were conducted on a porous sandstone. The evolution characteristics of instantaneous, viscoelastic, and viscoplastic strains under different stress states were revealed through strain decomposition, where strengthening and weakening effects in rock deformation were observed. When the axial stress was lower than the long‐term strength, both the elastic and viscoelastic stiffness of sandstone were strengthened, while the viscoplastic deformation developed convergently. As the stress increased, the rock underwent a stiffness strengthening–weakening transition. Based on these findings, we proposed a rheological model containing the strengthening–weakening effects and conducted parameter optimization according to the decomposed creep strains. Using the integrated rheological model and the optimized parameters, the overall creep strain curves of sandstone were reproduced. The theoretical curves matched well with the experimental data, thus proving the rationality of the results.