Experimental study on the mechanical properties and deformation failure characteristics of sandstone under graded equal amplitude cyclic loading and unloading.

To investigate the effects of graded equal amplitude cyclic loading on rock deformation and failure, this study examines the influence of confining pressure and upper limit cyclic stress on the deformation parameters, stress-strain hysteresis curves, and macroscopic failure characteristics of sandstone during cyclic loading and unloading processes. Through a series of three-stage cyclic loading and unloading tests conducted under varying confining pressures, the findings indicate the following. With the increase of confining pressure and upper limit stress, the axial hysteretic loop of cyclic loading and unloading shows a tendency of expansion, and the hysteresis loop gradually changes from dense to sparse. However, the change of the hoop hysteresis loop is only related to the confining pressure, and the ability to resist the hoop deformation is the best under the confining pressure of 10 MPa. Compared to conventional triaxial tests, the peak strength, axial peak strain, and elastic modulus of sandstone decrease under cyclic loading and unloading conditions, whereas the peak axial strain increases. The Poisson's ratio initially decreases and then increases with increasing confining pressure. Cyclic loading and unloading deteriorates the strength of rock mass, and the damage stress is greater than that of conventional triaxial specimens. With the increase of confining pressure, the growth effect on the crack initiation stress gradually weakens compared with conventional triaxial tests. In both test conditions, the rock samples exhibit distinct brittle failure characteristics, with macroscopic failure modes predominantly manifesting as shear failure. However, cyclic loading and unloading inhibit microcrack formation. These findings elucidate the deformation and failure mechanisms of sandstone under graded constant-amplitude cyclic loading, providing valuable insights for stability assessment and hazard mitigation in open-pit mine slopes.