Study on the mechanical properties of unloading damaged sandstone under cyclic loading and unloading.

To reveal the mechanical properties of rocks under stress disturbance and unloading confining pressure, conventional triaxial compression tests, triaxial compression tests on unloading damaged sandstone, and cyclic loading and unloading tests on unloading damaged sandstone were conducted. Then, the evolutionary characteristics of dissipated energy in sandstone under cyclic loading and unloading were explored, and damage variables were proposed. The crack development characteristics were analyzed from a microscopic perspective. The study results reveal that: (1) the sandstone exhibits obvious brittle failure under different stress paths, and the macroscopic failure mode is dominated by shear failure. As the number of cycles increases, the load-bearing capacity, elastic modulus, and deformation modulus of the sandstone will be significantly reduced if it suffers greater unloading damage. (2) The cyclic action in the early stage inhibits the development of the internal fracture. However, the inhibitory effect is significantly reduced for specimens with larger unloading quantities. The damage variable in the cyclic loading and unloading is about 50.00% of that in the unloading, indicating that unloading confining pressure is the dominant factor for specimen failure. (3) The extension of microcracks within the sandstone is dominated by intergranular cracks, and the number of cracks increases with the increase of unloading quantity. After cyclic loading and unloading, the structure becomes looser. The test results deepen the understanding of rock mechanical behavior and fracture evolution under cyclic loading and can provide a basis for structural stability improvement under stress disturbance and unloading confining pressure.