Experimental Research on Rock Energy Evolution under Uniaxial Cyclic Loading and Unloading Compression

To reveal the characteristics of energy accumulation and energy dissipation during rock deformation and failure, cyclic uniaxial compression experiments under 36 different loading and unloading schemes were carried out on 180 red sandstone specimens using an MTS 815 rock mechanics test system (MTS Systems Corporation, Eden Prairie, MN). Based on the theory of thermodynamics and the analysis of the resultant stress-strain curves, a method of calculating the rock energy density from the characteristics of the loading and unloading curves was proposed. The energy densities of rock specimens under different loading and unloading rates were determined. The influence of loading and unloading rates on the rock energy evolution was discussed, and the evolution and distribution laws of energy accumulation and dissipation in the rock during the prepeak stage were revealed. The experimental results showed that rock energy density increases nonlinearly with an increase in the axial loading stress under different loading and unloading rates. The total absorbed energy density increases at the fastest rate, followed by the elastic energy density, and the dissipated energy density increases at the slowest rate. Elastic energy density shows a trend of "increasing first and then decreasing," while the dissipation energy density shows the opposite trend. In the static and quasi-static loading ranges, the loading and unloading rates have no clear effect on the elastic energy density but have a considerable effect on dissipation energy density. The smaller the loading or unloading rate, the larger the dissipation energy density.