Dynamic evolution and damage fracture characteristics of coal mass during confining pressure unloading

To study the dynamic fracture evolution and damage characteristics of coal during confining pressure unloading, the mechanical experiment was carried out on coal samples using an in-situ loading and unloading CT scanning system. The stress-strain curves of coal samples during the process of confining pressure loading were obtained, and the dynamic evolution of geometric characteristics, such as ratio, volume, fractal dimension, quantity, length and angle of cracks in two-dimensional plane and three-dimensional space was statistically analyzed. The correlation among three-dimensional crack volume, fractal dimension, and unloading amount was analyzed, and the relationship between crack volume and damage degree of coal sample was established. The dynamic evolution characteristics of shear and tensile fracture damage during confining pressure unloading and its influence mechanism on final failure of coal samples were explored. The results show that the axial strain of coal increases continuously with the increase of unloading stress, and the fracture rate increases with the increase of unloading amount, showing a sudden increase trend. In addition, the three-dimensional crack volume, fractal dimension and overall damage variable show a nonlinear exponential growth trend with the increase of unloading amount. It can be concluded that the confining pressure unloading can simultaneously induce shear and tensile fractures of coal samples and the damage degree of the two significantly influences the final failure mode of the coal.