Energy evolution and damage analysis of true triaxial cyclic loading and unloading of sandstone at different rates

To investigate the effects of different rates on the energy and damage evolution of sandstone during true triaxial cyclic loading and unloading damage, the true triaxial compression test, true triaxial primary loading and unloading test, and true triaxial cyclic loading and unloading test were conducted with the aid of cubic sandstone specimens by using the self-developed true triaxial disturbance unloading rock test system. The tests show that the peak strength of sandstone under true triaxial conditions increases with the cyclic loading and unloading rate. The evolution of the elastic energy density in the σ₁ direction does not vary with the cyclic loading and unloading rate, while the dissipative energy density decreases with the cyclic loading and unloading rate, both of which show a nonlinear increase with the number of cycles. With the increase of the cyclic loading and unloading rate, the ratio of the dissipative energy density to the total input energy density in the σ₁ direction decreases and the ratio of the elastic energy density to the total input energy increases. With the increase of the number of cycles, the growth rate of elastic energy density in the σ₂ and σ₃ directions gradually increases with the increase of cycle number, while the growth rate of dissipative energy density changes less in the first five cycles and increases significantly at the end of the cycle. The elastic energy density in the σ₂ and σ₃ directions increases with the increase of cyclic loading and unloading rate, while the dissipative energy density increases in the opposite direction. The elastic energy density in the σ₂ direction increases with the cycle number as a percentage of the input energy density, and an inflection point appears in the sixth cycle. The elastic energy density to input energy density in the σ₃ direction decreases and then increases with the increase of cycle number, showing a “V”-shaped trend. It is found that the elastic energy density and dissipative energy density are linearly related to the input energy density, and the elastic energy density-input energy density and dissipative energy density-input energy density are linearly correlated. The damage of sandstone under the action of true triaxial cyclic loading and unloading is divided into three stages: initial damage, slow development of damage, and rapid development of damage, and the damage variables show nonlinear growth with the increase of cycle number.