Effects of cyclic loading and unloading rates on the energy evolution of rocks with different lithology

The deformation-to-failure process of rocks is accompanied by energy dissipation and release, indicating that it is an energy conversion process. To investigate the influence of lithology as well as the loading and unloading rates on the evolution and distribution of rock energy, the MTS 816 rock mechanics testing system was used for performing uniaxial compression and uniaxial cyclic loading–unloading tests on the 165 samples of four rock types. The total energy density, elastic energy density, and dissipated energy density absorbed by rocks of different lithology were obtained, and the evolution and distribution laws of lithology and loading and unloading rates on the accumulation and dissipation of rock energy were investigated. The results revealed that the energy density of all rock samples increased nonlinearly with the increase in axial stress, and the elastic energy density increased gradually first and subsequently rapidly with the increase in the axial stress. The evolution curves were not affected by the loading and unloading rates. The dissipated energy density increased gradually with the increase in axial stress, and the discrepancy of the evolution curve was large. The proportion of the elastic energy density varied nonlinearly with the increase in axial stress or the cycle index, exhibiting the evolutionary process of increasing, then stabilizing, and finally decreasing. However, the proportion of the dissipated energy density exhibited an opposite trend. The elastic energy accumulated in the rock sample at the pre-peak stage was considerably higher than the dissipated energy, and the proportion of the elastic energy density was large. With the increase in loading and unloading rates, the proportion of elastic energy exhibited an approximate growth trend, and the proportion of dissipated energy exhibited an approximate decreasing trend.