Experimental investigation of the mechanical characteristics and energy dissipation of gas-containing coal under incremental tiered cyclic loading

The fluid–solid coupled with triaxial servo-controlled seepage experiments of coal containing gas was used to study coal samples from a coal mine in Sichuan Province, China. The mechanical characteristics, seepage laws, energy dissipation, and fracture evolution of raw coal under incremental tiered cyclic loading and different confining pressures were studied. The experimental results show that as the axial stress increases, the axial relative residual strain curve changes in an "N" shape, while the radial relative residual strain curve always increases. The change in the relative residual strain can be divided into three stages: compaction stage, elastic stage, and yield–failure stage. Except for in the last cycle, as the cycle number increases, the dissipated energy of coal increases linearly. With increasing cycle number, the difference in energy dissipation under different confining pressures gradually increases. During the yield–failure stage, some closed fractures inside the coal began to expand, and new fissures and cracks were generated, causing a large residual deformation and leading to a sharp increase in the dissipated energy and elastic modulus. Additionally, from the perspective of energy, a new method for calculating the elastic modulus (referred to as the equivalent elastic modulus) using the integral method was proposed.