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煤张开型裂隙三维宏细观演化特征及扰动因素探究

Study on three-dimensional mesoscopic evolution characteristics and disturbance factors of coal open fractures

  • 摘要: 裂隙演化方式受控于诸如矿物特征及围压条件等内外环境,为探究含裂隙煤体裂隙宏细观演化特征及影响因素的围压效应,基于工业CT扫描系统及其搭载的三轴加载系统对含裂隙煤体开展三轴静载试验,以多角度联合表征,对原生裂隙、矿物及围压的内外条件相互作用机制做出合理解释。结果表明:①围压会改变煤体初始损伤显著区位置,使其随围压升高由裂隙尖端过渡至煤体上、下端,且微孔隙和大尺寸裂隙之间比微孔隙和微孔隙之间更易相互贯通,并产生新的宏观裂纹。②围压升高使得三维动态分形维数由缓慢增加、快速增加和平稳增加转变为平稳增加、快速增加和缓慢增加的发展阶段,可表征裂隙的时间演化规律。③含裂隙煤体在单轴或低围压下呈纵向拉伸破坏,高围压会使其破坏方式趋于剪切,并通过2种途径提升煤体强度。④起裂角理论值偏离试验值程度随围压增加而增加,与煤体由矿物分布引起的离散度数值关系一致。⑤根据裂隙的受力成分及矿物分布特征将裂隙扩展行为分为直驱、绕核和错核3种类型,该扩展行为受围压对裂隙的作用力成分影响,由相对纯粹拉应力、拉伸–剪切复合应力和相对纯粹剪应力作用的裂纹分别对应以上3种扩展行为,即对裂隙的扩展影响形式表现为以围压为主,矿物赋存形态为辅。

     

    Abstract: The evolution of fractures is affected by internal and external environments such as the distribution characteristics of mineral particles and confining pressure conditions. In order to explore the macro-meso evolution characteristics and influencing factors of fractures in fractured coal under different confining pressures, based on the industrial CT scanning system and its equipped triaxial loading system, the triaxial static load test of fractured coal was carried out. The interaction mechanism of internal and external conditions of primary fractures, mineral particles and confining pressure is reasonably explained by multi-angle joint characterization. the results showed: ① The confining pressure will shift the significant area of the initial damage of the coal body, so that it transits from the fracture tip to the upper and lower ends of the coal body with the increase of confining pressure, and it is easier to connect between micropores and large-sized fractures than between micropores and micropores, and new macroscopic cracks are generated. ② The increase of confining pressure will lead to the change of three-dimensional dynamic fractal dimension from slow increase, rapid increase and steady increase to the development stage of steady increase, rapid increase and slow increase, which can be used to characterize the time evolution law of fractures. ③ The fractured coal body exhibits longitudinal tensile failure under uniaxial or low confining pressure. The high confining pressure will change its failure mode and tend to shear failure, and enhance the strength of the coal body through two ways. ④ The deviation between the theoretical value and the experimental value of the cracking angle increases with the increase of confining pressure, which is consistent with the discrete numerical relationship caused by the distribution of mineral particles in coal. ⑤ According to the stress composition of the fracture and the distribution characteristics of mineral particles, the fracture propagation behavior is divided into three types: direct drive, bypassing mineral particles and staggering mineral particles. The above propagation behavior depends on the confining pressure to change the force component of the crack. The cracks under the action of relative pure tensile stress, tensile-shear composite stress and relative pure shear stress correspond to the above three propagation behaviors, that is, the influence form of crack propagation is mainly confining pressure, supplemented by the occurrence form of mineral particles.

     

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