Mechanical properties and energy of sandstone under cyclic loading in evolutionary pattern experimental studies

In order to explore the mining failure law of deep coal seam floor and clarify the mechanical behavior and energy change in the floor strata during mining, the mechanical properties and energy evolution law of sandstone under cyclic loading with different confining pressures (20, 30, 40 MPa) were studied using the Rock Top multi-field coupling tester. The results are as follows: (1) the hysteresis phenomenon of a rock stress-strain curve under cyclic loading is evident. Moreover, the hysteresis loop migrates to the direction of strain increase, and the fatigue damage caused by cyclic loading has a certain weakening effect on the peak strength of rock; (2) both the number of cycles and the axial strain show a nonlinear change characteristic that satisfies the quadratic function relationship. Among them, the stress level of the rock is the main factor affecting the fitting effect; (3) under the same confining pressure, with an increase in cycle level, the macroscopic deformation of the rock increases, the accumulation of fatigue damage in the sample increases, and the irreversible deformation of the rock increases, which leads to an increase in energy input and dissipation; (4) in terms of elastic energy and dissipation energy, elastic energy plays a dominant role. In the initial cycle, the rock is destroyed, and the rock energy loss is great. After the second cycle, the input energy is mainly stored in the rock in the form of elastic energy, and only a small part of the input energy is released in the form of dissipation energy; (5) the confining pressure can improve the efficiency of rock absorption and energy storage, enhance the energy storage limit of rock, and limit the dissipation and release of partial energy of rock. The greater the confining pressure, the more evident the limiting effect, and the more significant the dominant position of elastic energy; and (6) the change in the energy dissipation ratio can be divided into three stages: rapid decline stage, stable development stage and rapid rise stage. The greater the increase in dissipation energy, the greater the degree of rock damage. The evolution process of the energy dissipation ratio can reflect the internal damage accumulation process of rock well, which can be used as the criterion of rock instability.