三点弯曲条件下不同层理面强度对裂纹扩展过程的影响

Effects of different bedding plane strength on crack propagation process under three points bending

  • 摘要: 煤矿、页岩气开采等工程均涉及到层状岩体力学问题,层状岩体的裂纹破裂模式直接决定了工程施工稳定性及能源高效开发利用。为了揭示层理面强度性质对裂纹扩展过程的影响,利用ABAQUS模拟软件,通过全局嵌入0厚度cohesive单元的方法,创建了与最大主应力方向垂直的层理面,并且基于页岩XRD矿物成份分析结果,利用Python语言对软件进行二次开发,实现了页岩的层理各向异性与矿物颗粒非均质性。通过控制层理面的强度参数,研究了在三点弯曲条件下,层理面强度性质对裂纹扩展过程的影响。通过MATLAB编程处理模拟结果,实现了加载过程中的声发射(AE)模拟,根据声发射定位图和声发射能量数据,进一步分析了裂纹破裂类型及动态扩展过程。结果表明:层理面抑制了裂纹沿着最大主应力方向扩展,层理面强度越弱抑制效果越强;层理面强度较弱时,裂纹先从层理界面处起裂,产生顺层横向裂纹,阻隔应力波的传递,导致预制裂纹尖端应力场波及范围变小应力集中程度变强;层理面的存在导致裂纹扩展路径复杂,穿层破裂与顺层破裂交替出现,裂纹表现为断续位错式的破裂特征,层理面强度与最大主应力共同决定了裂纹路径;穿层破裂主要沿着最大主应力方向产生,表现为拉张破裂类型,顺层水平迁移裂纹表现为剪切破裂类型;层理面强度越弱,克服顺层破裂所需能量越大,峰值载荷越大;层理面强度与声发射事件数呈负相关性,层理面越弱声发射事件数越多,拉张、剪切破裂越多,顺层裂纹长度越长。

     

    Abstract: The issue of layered rock mechanics is critical for many geological engineering projects,such as coal min- ing,development of shale gas by hydraulic fracturing and so on. The layered rock failure model directly determines the stability of rock mass and the efficiency of energy development. In order to reveal the influence of bedding plane strength on the fracture propagation process,a bedding plane perpendicular to the direction of the maximum principal stress was created based on the method of globally embedding a cohesive unit with 0 thickness using ABAQUS simula- tion software. Combined with the results of X-ray diffraction mineral composition analysis,the software was redeveloped in Python language to realize the bedding anisotropy and mineral particle heterogeneity of shale. The effect of the strength properties of the bedding plane on the fracture growth process under the three-point bending condition was studied by adjusting the strength parameters of the bedding plane. In addition,the acoustic emission (AE) simulation during the loading process was implemented by MATLAB programming. The crack type and dynamic propagation process are further analyzed based on AE location map and AE energy data. The results show that the bedding plane inhibits the propagation of cracks along the direction of maximum principal stress,and the inhibiting effect will be en- hanced if the strength of the bedding plane is weak. Therefore,the crack initiation will occur at the bedding interface when the bedding plane strength is weak,which generates transverse cracks along the bedding. It prevents the trans- mission of stress wave,resulting in a smaller range of the stress field and the enhancement of stress concentration at the crack tip. The crack propagation path becomes complicated due to the existence of the bedding plane,which is mani- fested by the alternating occurrence of the penetrating fracture and the bedding fracture. Meanwhile,the crack presents the characteristics of discontinuous dislocation type. Therefore,the crack propagation path is determined by the bed- ding plane strength and the maximum principal stress. Moreover,the penetrating fracture belongs to the tensile type and mainly occurs along the direction of maximum principal stress,while the bedding fracture belongs to the shear type. If the bedding plane strength is weak,more energy will be required to penetrate the bedding fracture,and the peak load will also become higher. The weaker the bedding plane strength,the more the number of AE events,so there is a negative correlation between them. Similarly,the more tension and shear rupture events occur,the longer the crack length along the layer is.

     

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