Crack strain and energy evolution analysis of coal damage under cyclic loading and unloading conditions

Triaxial cyclic loading and unloading (TCLU) compression tests are conducted on coal samples with different initial confining pressures. Crack strain and energy evolution laws are analyzed, revealing the mechanical characteristics and behaviors under different confining pressures, such as damage evolution, energy conversion, unloading deformation and failure characteristics. The peak strength, expansion stress, and peak strain have larger growth than the increased amplitude of residual strength with increasing initial confining pressure. They have a good linear relationship between the characteristic stresses and initial confining pressures. Calculation and analysis of crack volumetric strain, crack axial strain and radial strain present the characteristics of each stage on crack development. The crack evolution law during coal failure is explored from the microscopic perspective. The deformation and failure of the coal is a complex energy conversion process. The damage variables in the TCLU process are defined by the energy index. Each test cycle produces irrecoverable damage to the coal, and the damage evolution curves increases in a stepwise nonlinear accumulation. The damage evolution curves demonstrate the damage of the coal under the TCLU path and [describe](javascript:;) the initial damage characteristics of the coal, it has [practical](javascript:;) significance to underground mining of mimic disturbance conditions.