Microscopic mechanism of loading rate of saturated coal sample mechanics and damage characteristics
YANG Ke, ZHANG Zhainan, HUA Xinzhu, LIU Wenjie, CHI Xiaolou, LYU Xin, WANG Yu
为探究煤层注水防冲解危后，工作面推进速率是否会对煤矿井下安全、高效生产造成二次影响，开展了不同加载速率下干燥及饱和煤样的单轴压缩试验，探究了饱和煤样的峰值强度、声发射能量及声发射的RA 值与AF 值、断口形貌、分形维数以及冲击倾向性特征的加载速率效应，揭示了饱和煤样损伤破坏特征的加载速率微观作用机制。研究表明：随加载速率增大，干燥及饱和煤样峰值强度先减小后增大，加载速率0.01 mm/s 为导致强度转折的临界加载速率。不同加载速率下饱和煤样宏观破坏模式均为以剪切破坏为主的拉-剪复合破坏，最大声发射能量值先减小后增大，在临界加载速率时达到最小值。饱和煤样微观剪切裂隙占比先减小后增大，在临界加载速率达到极小值。饱和煤样破裂断口形貌由长槽状裂隙向完全不规则裂隙过渡，临界加载速率是大量不规则裂隙开始出现的转折点。随加载速率增大，饱和煤样破碎的小粒径煤屑质量占比减小，大粒径煤屑质量占比增大；干燥及饱和煤样分形维数均逐渐减小，拟合曲线满足幂函数规律，且饱和煤样较干燥煤样分形维数增大。随加载速率增大，干燥及饱和煤样的KE 均存在先减小后增大的规律，在临界加载速率达到极小值。煤层注水对工作面冲击地压的抑制作用要高于加载速率对工作面的诱冲作用。在临界加载速率之前，随加载速率增大，饱水煤样微裂隙内部孔隙水压力增幅较小，贡献刚度小，竞争力较弱，而裂纹扩展速率增幅较快，竞争力较强；之后孔隙水压力增幅较大，贡献刚度增大，竞争力较强，而裂纹扩展速率相对增幅较慢，竞争力较弱。不同加载速率饱和煤样在孔隙水压力和裂纹扩展速率2 个因素的相互竞争下，导致其力学及损伤特征规律呈现非线性特征。
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 KE 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.
rock mechanics； loading rate； saturated coal sample； acoustic emission energy； fracture morphology； fractal dimension； impact tendency； microscopic mechanisms