Energy Dissipation and Damage Evolution Characteristics of Shale under Triaxial Cyclic Loading and Unloading

Abstract

Rock engineering is highly susceptible to cyclic loads resulting from shale gas exploitation; it is very important to study the failure mechanism of underground rock mass under cyclic load. To investigate the energy dissipation and damage evolution characteristics of shale under triaxial cyclic loading and unloading conditions, a series of triaxial incrementally cyclic loading and unloading tests under different confining pressures (10 MPa, 15 MPa, 20 MPa, and 30 MPa) were carried out. The variation of plastic strain of shale under four confining pressures was analyzed, and the evolution characteristics of dissipated energy and energy dissipation ratio were discussed. The results show that: the peak strength and peak strain of shale increase with the increase in confining pressures. Meanwhile, the plastic strain of rock under cyclic loading increases rapidly first, then develops steadily, and finally increases sharply with the increase in axial strain. The energy dissipation ratio-strain curve presents a spoon-shaped evolution feature, and can be divided into three stages: a linear decline stage, a steady development stage, and a rapid increase stage, respectively. However, the trend of elastic modulus of shale was opposite to that of the energy dissipation ratio. Accordingly, the energy dissipation ratio can be regarded as the damage factor to describe the degradation of shale. Based on the evolution of dissipated energy, a theoretical equation of shale stress-strain evolution was established. By substituting the test data into the formula, it is found that the calculated results are basically consistent with the test data, and the peak strains and peak stress calculated by the equation are in good agreement with the test data. The findings could provide important theoretical support for the energy dissipation and damage evolution analysis in the failure process of shale under cyclic stress conditions.

Experimental Study on Mechanical and Damage Evolution Characteristics of Coal during True Triaxial Cyclic Loading and Unloading

“…Ding et al [ 8 ] implemented cyclic loading and unloading tests of coal samples under different stress levels, studied the deformation and energy evolution characteristics of coal samples during cyclic loading and unloading, and obtained the damage evolution characteristics of coal based on energy dissipation. Li et al [ 9 ] studied the damage evolution characteristics of shale during triaxial cyclic loading and unloading, and established the plastic strain damage model of confining pressure, damage accumulation coefficient and average damage factor. Meng et al [ 10 ] used the MTS815 rock mechanics test system to carry out triaxial cyclic loading and unloading tests on limestone with different surrounding rocks, and studied the strength, dilatancy deformation and damage mechanical properties of rock during cyclic loading and unloading.…”