Energy Evolution and Fractal Characteristics of Sandstones Under True Triaxial Cyclic Loading and Unloading

Abstract

To study the fractal characteristics and energy evolution of sandstones under true three-dimensional stress states, a true triaxial compression test and a cyclic loading and unloading test of sandstone specimens under different loads were carried out using a self-developed true triaxial disturbance testing system. Based on the evolution law of true triaxial cyclic loading and unloading stress–strain, the types of loading and unloading in the cyclic loading and unloading test were delineated, and the reasons for the change in peak maximum principal stress intensity under different paths were analyzed. By analyzing the crushing characteristics of rock samples under different paths, it was found that the staged cyclic loading and unloading caused the greatest damage to the rock mass, while the equal-amplitude and unequal-lower-limit staged loading and unloading caused the least damage to the rock mass. Based on fractal theory, it was found that the rock samples under path V had the highest fractal dimension D. The elastic energy density, dissipated energy density, and input energy density of true triaxial cyclic loading and unloading under different paths were calculated by graphical area integration and superposition methods, respectively, to analyze the evolution of the three with the increase in the loading and unloading cycles and the energy distribution during the loading and unloading process. True triaxial cyclic loading and unloading tests revealed a linear relationship between the elastic energy density and total input energy density of the rock mass, and the energy storage coefficient exceeded 0.5, regardless of the loading path.