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: The stress-strain curves of cyclic loading and unloading exhibit concave shapes, with closely aligned curves. As the confining pressure and upper limit stress increase, the hysteresis loop curves become progressively sparser, with their areas expanding. Low confining pressures and upper limit stresses inhibit the growth of circumferential strain. Compared to conventional triaxial tests, the peak strength, circumferential 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. Under cyclic loading, both the crack initiation stress and damage stress surpass those observed in 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. The findings elucidate the deformation and failure mechanisms of sandstone under graded equal amplitude cyclic loading and unloading, offering valuable insights for evaluating engineering stability.