不同围压下含预制裂隙煤体裂隙演化特征研究

Fracture evolution characteristics of prefabricated crack coal under different confininggpressures

为探究不同围压下含预制裂隙煤体裂隙演化特征,基于受载煤岩工业CT扫描系统开展不同围压下三轴压缩试验,通过受载煤体内部裂隙三维可视化分析,研究不同围压下煤体裂隙演化规律,借助PFC模拟软件,以裂纹数量为量化指标表征围压对裂纹发育程度的影响。此外,推导出考虑裂隙宽度的应力强度因子表达式,并以最大周向应力准则计算裂隙的理论起裂角度。结果表明:随着围压的增大,裂隙起裂角度减小,3,5,7 MPa围压下,翼裂纹在预制裂隙端部沿着最大主应力方向扩展,最终呈现为“X”或“Y”型拉-剪复合型破坏,9 MPa围压下,煤体翼裂纹发育受阻,次生裂纹充分扩展,呈现出次生裂纹沿共面扩展的剪切破坏形式;围压效应和剪切裂纹的互锁效应均对裂隙发育模式产生影响,但二者作用机制不同,前者为全局作用,后者必须在原生裂隙压密到一定程度时才发挥作用;矿物颗粒赋存状态影响裂隙的发育形态,由纯粹拉应力导致的翼裂纹较为完整,而次生裂纹则呈“残缺”状态,该残缺状态随围压增加而增加;峰值应力前,各裂纹数量呈先减少后增加的规律,拉伸裂纹占比不断减少;经典欧文Ⅰ型应力强度因子适用于张开型裂隙,采用考虑裂隙宽度的应力强度因子计算的起裂角度更接近实验室试验结果。

In order to explore the fracture evolution characteristics of coal with prefabricated cracks under different confining pressures,triaxial compression experiments under different confining pressures are carried out based on the loaded coal-rock industrial CT scanning system.Through the three-dimensional visualization analysis of the internal cracks of the loaded coal,the fracture evolution law of coal under different confining pressures is studied,and the influence of confining pressure on the degree of crack development is reflected by PFC simulation software with the number of cracks as a quantitative index.In addition,the expression of stress intensity factor considering crack width is deduced,and the theoretical crack initiation angle is calculated by the maximum circumferential stress criterion.The results show that the crack initiation angle decreases with the increase of confining pressure.Under the confining pressure of 3,5 and 7 MPa,the wing crack propagates along the direction of the maximum principal stress at the end of the prefabricated crack,and finally presents the"X"or"Y"type tensile-shear composite failure.The secondary crack is fully grown and presents the shear failure form along the coplanar expansion under the confining pressure of 9 MPa,which also inhibits the growth of the coal body's wing crack.Both the confining pressure effect and the interlocking effect of shear cracks have an impact on the fracture development model,but the mechanism of action is different.The former is a global effect,and the latter plays a role in the compaction of the original fracture to a certain extent.The occurrence state of mineral particles affects the development form of cracks.The wing cracks are relatively complete by pure tensile stress,while the secondary cracks are in an"incomplete"state,which increase with confining pressure.Prior to the peak stress,the proportion of tensile cracks continuously declines as the number of cracks first decreases and then increases.For open cracks,the traditional Owen type I stress intensity factor is appropriate.The crack initiation angle determined by the stress intensity factor considering crack width is much closer to the results of the laboratory tests.