The Mechanical Behaviors of Sandstones in Response to True Triaxial Graded Perturbations of the Intermediate Principal Stress

In this study, true triaxial graded cyclic loading and unloading intermediate principal stress tests were performed on sandstones. The effects of graded cyclic increments (GCI) (1, 3, and 5 MPa) and minimum principal stresses (4, 5, 6, and 7 MPa) on the mechanical responses of sandstones during graded cyclic loading were examined. The experimental results reveal that during the cyclic tests, plastic hysteresis loops are clearly observed along all three principal stress directions, indicating memory characteristics. As the graded cycles proceed, the mechanical responses (elastic modulus, volumetric strain) exhibit a nonlinear attenuation trend; the strains are larger in the unloading than those in the loading. Both GCI and the minimum principal stress level are of great significance to rock behaviors under graded cyclic loading conditions. Overall, lower minimum principal stress levels result in stronger deformation responses and shorter fatigue life. Final failure morphology indicates more pronounced spalling under higher cyclic stress increments and minimum principal stresses. The dissipated energy of the rock mass increases exponentially with the graded cyclic amplitude, and the evolution of the damage variable follows a “slow‐fast‐stable” trend. On the above, sandstone is more prone to yield under lower minimum principal stresses and higher GCIs.