含多裂隙煤体裂纹细观演化规律与相互作用机制

Microcrack evolution law and interaction mechanism of coal with multiple cracks

煤体中裂隙具有随机分布的特点,其位置分布及载荷作用下相互作用影响着煤体的稳定性.因此,为阐明煤体内多裂隙扩展演化规律及相互作用机制,文中开展了含随机三裂隙煤样的单轴压缩试验,借助数字图像相关法(Digital Image Correlation,DIC)监测试样变形过程,并采用数值模拟基于细观角度验证了裂纹的孕育、扩展及贯通的演化过程.另外,基于经典Kachanov法和应力场分析法推导并验证了单轴受压状态下有限板三裂隙尖端的应力强度因子表达式.研究结果表明:裂纹间相互作用对尖端应力强度因子影响可分划为增大、减小、无影响3种形式,根据推导所得有限板多裂隙尖端的应力强度因子表达式,可计算裂隙随机分布裂隙尖端应力水平,并且,可结合应变能密度因子准则准确预测裂隙尖端起裂位置;裂隙位置分布对初始应变场形成局部高应变区具有明显的导向作用,具体表现为:试样受载初期率先在各裂隙尖端产生损伤并逐步演化为局部高应变区,经扩展后形成高应变集中带,继而作为宏观裂纹相应的扩展路径;试样破坏形式为拉剪复合破坏,新生裂纹在加载初期以剪切形式出现,并在裂隙尖端形成翼型裂纹,随载荷继续施加,张拉裂纹数量快速增长,最终导致试样失稳.

The fractures in the coal body are characterized by random distribution,and their position distribution and interaction under load could affect the stability of the coal body. In this study,uniaxial compression experiments with random three-fracture coal samples were performed to elucidate the evolution law and interaction mechanism of multi-fracture propagation in coal. Beyond that,the digital image correlation method(DIC) was used to monitor the deformation process of the test sample,and the evolution process of crack gestation,propagation and penetration was verified by numerical simulation from meso-angle. Based on the classical Kachanov method and stress field analysis,the stress intensity factor expression of the three fracture tips of finite plates under uniaxial compression state was derived and verified. According to the results,the crack-to-crack interaction influences the tip stress intensity factor in three forms: “increase,decrease and no effect”. According to the derived stress intensity factor expression of the multi-fracture tip of the finite plate,the fracture tip stress level is calculated. Moreover,the fracture tip fracture position can be accurately predicted in combination with the strain energy density factor criterion;The fracture position distribution has an obvious guiding effect on the formation of a local high strain zone in the initial strain field,which is specifically manifested as follows: the specimen is first damaged at the tip of each fracture in the early stage of loading and gradually evolves into a local high strain zone. After expansion,a high strain concentration zone is formed and then serves as the corresponding expansion path of macroscopic cracks. The form of specimen failure is shear and tension failure. Apart from that,the new crack appears in the form of shear in the early stage of loading,and an airfoil crack is formed at the tip of the fracture. In addition,the number of tensile cracks grows rapidly with the continuous application of load,which eventually induces the instability of the sample.