Energy dissipation and dilation processes of rock mass under incremental cyclic loading and unloading

The deterioration of the mechanical properties of rock mass in underground engineering due to energy dissipation and microfracture accumulation under cyclic loading and unloading (CLU) has become a hot research topic in recent years. In order to elucidate the relationship between energy dissipation and the dilation processes, an incremental CLU mode is proposed and employed in triaxial tests in this paper. First, the methods and the procedures of the test were proposed to enhance the success rate, and the triaxial tests under incremental CLU were performed at confining pressures ranging from 0 MPa to 50 MPa. Second, the damage variables were defined and calculated from the perspective of energy dissipation, and the relationships between the damage variables and the energy parameters, as well as principal strains (plastic strain and plastic shear strain) were analyzed in detail. Third, a two-parameter shear dilation angle model was established under different confining pressures. The results show that the failure mode is tensile failure in conventional uniaxial compression, and is X-type shear failure in the uniaxial test with incremental CLU, and is shear dilatation failure mode in the triaxial test with incremental CLU, due to volume dilatation and damage accumulation after significant energy dissipation. The energy dissipation rates for single loading and unloading showed a "U" shaped trend as the number of the CLU increased. The reason for this observation is that the energy dissipation rate is greater due to the particle compaction in the initial compaction stage and structural damage of rock material in failure stage. The dilation angle first experiences a nonlinear rapid increase to a certain peak value and then gradually decays, as the rock shear dilation and failure occur gradually with the accumulation and development of cracks.