Study on the damage and fracture characteristics of coal rock based on the X-ray micro-CT scanning technology and statistical strength theory

Coal rock dynamic disasters such as rockburst and gas outbursts pose severe threats to the safe and efficient mining of coal mines in China; however, the mechanical mechanisms of those disasters are still unknown. Therefore, the evolution law and constitutive model of damage and fracture of coal rock was emphatically studied, starting from the rockburst-gas disaster that occurred at the coal mine roadway. Firstly, the X-ray high-precision micro-CT scanning experiment was performed, and the high-resolution 3D digital core of burst-prone coal was built utilizing 3D reconstruction technology. Furthermore, the box-counting dimension method and the 3D visual image processing software AVIZO were used to calculate the characteristic parameters of mineral bands and crack structures inside the coal sample. Secondly, a damage-fracture constitutive model considering three-parameter Weibull distribution, gas influence, and effective stress was proposed, based on CT scanning reconstruction and statistical strength theory. Thirdly, the triaxial compression experiments of gas-bearing coal were conducted, the model parameters were calculated using experimental results, the model was verified, and the damage evolution law of the progressive failure process of coal rock was revealed. Specifically, the experimental results of micro-CT scanning reconstruction and fractal calculation show that the surface area ratio and fractal dimension of the mineral bands are0.0035−0.0137and1.1214−1.2342respectively, while that of the fracture structures are0.0006−0.0040and1.0651−1.1454respectively. The surface area ratio and fractal dimension of mineral bands are generally larger than those of fracture structures, indicating that the number of discontinuous structures of burst-prone coal is significant, and its distribution is complex. Furthermore, the experimental results show that stress-strain curves are characterized by the class I curve; shear fracture usually occurs under triaxial compressive loads. The peak strength and peak strain increase with the rise in confining pressure. Moreover, the determination method of seven parameters within the damage fracture model is given, and those parameters and damage variable are solved based on experimental results. The calculation results show that this model can accurately reflect the progressive failure characteristics of coal rock, such as compressive deformation, quasi-linear elastic deformation, plastic deformation, peak strength, and post-peak softening. Additionally, in the first two stages, the damage variable of coal rock is less than 0.2. When it enters the plastic stage, especially in the post-peak step, the damage variable increases sharply until the failure occurs. Hence, the concept of damage threshold is proposed; here, the damage threshold of 0.2 is recommended as the early warning value of damage fracture of coal rock. The findings will further promote the development of damage mechanics and provide a theoretical basis for the prevention and control of coal and rock dynamic disasters.