Microscopic mechanism of loading rate of saturated coal sample mechanics and damage characteristics

In order to explore whether the advance rate of the working face will have a secondary impact on the safe and efficient production of coal mine after coal seam water injection prevents impact pressure and relieves the danger, uniaxial compression tests of dry and saturated coal samples under different loading rates were carried out, and the loading rate effects of peak intensity, acoustic emission energy, RA value and AF value, fracture morphology, fractal dimension and impact tendency characteristics of saturated coal samples are explored, and the microscopic mechanism of loading rate of saturated coal sample damage failure characteristics was revealed. The results show that with the increase of loading rate, the peak intensity of dry and saturated coal samples first decreases and then increases, and the loading rate of 0.01mm/s is the critical loading rate leading to the intensity transition. The macroscopic failure mode of saturated coal samples under different loading rates is a tensile-shear composite failure dominated by shear failure, and the maximum acoustic emission energy value first decreases and then increases, and reaches the minimum value at the critical loading rate. The proportion of microscopic shear fractures in saturated coal samples first decreases and then increases, and reaches a minimum value at the critical loading rate. The fracture morphology of saturated coal samples transitions from long trough-like fractures to completely irregular fractures, and the critical loading rate was the turning point at which a large number of irregular fractures begin to appear. With the increase of loading rate, the proportion of small-sized coal chips crushed by saturated coal samples decreased, and the proportion of large-sized coal chips increased. The fractal dimension of dry and saturated coal samples gradually decreased, the fitting curve satisfies the power function law, and the fractal dimension of saturated coal samples increases compared with that of dry coal samples. With the increase of loading rate, the K_E of dry and saturated coal samples decreased first and then increased, reaching a minimum at the critical loading rate. The inhibition effect of coal seam water injection on the impact pressure of the working face is higher than the inducing effect of the loading rate on the working face. Before the critical loading rate, with the increase of the loading rate, the pore water pressure inside the microfractures of the saturated coal sample increased less, the contribution stiffness was small, and the competitiveness was weak, while the crack growth rate increased faster and the competitiveness was stronger. After that, the pore water pressure increased greatly, the contribution stiffness increased, and the competitiveness was strong, while the crack growth rate increased relatively slowly and the competitiveness was weak. Under the competition of two factors, pore water pressure and crack growth rate, the mechanical and damage characteristics of saturated coal samples with different loading rates showed nonlinear characteristics.