Energy accumulation and dissipation test and analysis method of height-diameter ratio sandstone

To explore the influence of aspect ratio on rock energy accumulation and dissipation, the uniaxial compression test, uniaxial cyclic loading-unloading test, and uniaxial stage loading and unloading test were carried out on the sandstone specimens with the same diameter and different lengths using RMT-150 B rock mechanics test system. Comparing the uniaxial compression and the uniaxial cyclic loading-unloading curves of sandstone, it is found that the uniaxial compression curve is approximately the outer section of the uniaxial graded loading and unloading curve, and the strength of uniaxial graded loading and unloading is slightly higher than that of uniaxial compression due to hardening. The analysis on mechanical characteristics of the sandstone under uniaxial cyclic unloading demonstrates that as the cycle number increases, the residual deformation decreases gradually until the cycle generates residual deformation of sandstone showing the nonlinear pseudo elastomer characteristics. If the loading continues, the nonlinear pseudo elastomer features disappears, the residual deformation appears again. Therefore, the residual deformation evolution can be summarized as a large residual deformation-the gradual decrease of residual deformation-no residual deformation-residual deformation again. Based on the evolution analysis of the elastic energy of sandstone, the number of cycles has little effect on the elastic energy of sandstone, meaning that fatigue damage has little effect on the elastic energy of sandstone. Therefore, the elastic energy of sandstone is equal to that of the uniaxial compression test when the load at each unloading point is equal to that of the uniaxial compression test. Comparison between the elastic energy of each unloading point and the first loading-unloading in the uniaxial cyclic loading-unloading tests was made to verify the correctness of the hypothesis. It is found that the elastic energy evolution of sandstone obeys the law of linear energy storage by the energy analysis of the uniaxial compression test of sandstone with different aspect ratios. The energy evolution analysis of the uniaxial compression test with the same size is carried out based on the uniaxial loading and unloading test of sandstone and the linear energy storage law, which provides a new method for the energy analysis of the uniaxial compression. The results show that the elastic energy is directly proportional to the height-diameter ratio and inversely proportional to the storage limit when the load is equal. The new method proposed in this paper is compared with the traditional energy analysis method, which calculates the elastic energy by formulas. It is found that the method proposed in this paper is highly accurate. Based on the evolution law of dissipated energy, the damage of sandstone with different aspect ratios is further analyzed. It is found that the damage curve of sandstone exhibits a nonlinear evolution, and the damage degree of sandstone with a larger aspect ratio is higher when the load is equal. The elastic energy accumulated in sandstone during fracture is converted into the surface energy of the fracture surface and the kinetic energy of fragments. The energy storage limit is inversely proportional to the height-diameter ratio. Therefore, when L/D≥2.0,the shear failure mainly occurs in one direction. When L/D<2.0,the rock is mainly a relatively complex double shear or conical failure, and the pressure relief of borehole is further discussed from the perspective of energy.