Abstract: To explore the influence of true triaxial cyclic loading and unloading in principal stress directions on sandstone energy accumulation and dissipation, three cyclic loading and unloading tests in principal stress directions were carried out by using the self-developed true triaxial disturbed unloading rock testing system. Based on the stress-strain evolution law and the loading and unloading characteristics of true triaxial cyclic loading and unloading tests, the types of loading and unloading in the cyclic loading and unloading tests are classified. Through the comparative analysis of the surface cracks of rock mass after cyclic loading and unloading in three principal stress directions, it is found that the minimum principal stress cycle causes the severest damage to rock mass, followed by the intermediate principal stress, and the maximum principal stress causes the least damage. The elastic energy density, dissipated energy density, and input energy density of true triaxial cyclic loading and unloading are calculated by using the graph area integral method and the superposition method, respectively. The evolution laws of the above three densities with the increase of the number of principal stresses and the energy distribution during loading and unloading are analyzed. The rationality and accuracy of the proposed energy analysis method are verified by the true triaxial loading and unloading test, and the elastic energy released by unloading in three principal stress directions is analyzed. The damage caused by cyclic loading and unloading has little influence on the elastic energy stored in the rock mass. The influences of unloading independently in three principal stress directions on sandstone damage and energy dissipation are compared, and the advancing direction of the roadway is further discussed.

Key words: rock mechanics, true triaxial cyclic loading and unloading, energy accumulation, energy dissipation, loading and unloading classification, energy evolution