Evolution mechanism and quantitative characterization of initial micro-cracks in marble under triaxial compression

Abstract

The initial micro-cracks affect the evolution characteristics of macroscopic deformation and failure of rock but are often ignored in theoretical calculation, numerical simulation, and mechanical experiments. In this study, we propose a quantitative analysis model to investigate the effects of initial micro-cracks on the evolution of marble deformation and failure. The relationship between the micro-crack propagation and the marble failure characteristics was comprehensively studied by combining theoretical analysis with a micro-computed tomography (micro-CT) scanning technique. We found that with the increase of confining pressure, the matrix elastic modulus of the marble first increased and then tended to be stable, while the micro-cracks increased exponentially. The sensitivity ranges of the marble sample matrix elastic modulus and micro-cracks to confining pressure were 0–30 MPa and 30–50 MPa, respectively. The porosity and Poisson’s ratio decreased exponentially. The increasing proportion of internal micro-cracks led to an increase in the sample non-uniformity. The samples presented mainly shear failure under triaxial compression, and the failure angle decreased linearly with the increase of confining pressure. The convergence direction of cracks decreased gradually. This quantitative analysis model could accurately portray the relationship between the overall macroscopic deformation and the deviatoric stress of the samples at the compaction and the linear elastic stages, thus deepening the understanding of the stress–strain behavior of rocks. 目的 岩石宏观变形破坏演化特征受其内部初始微裂纹的影响,但在理论计算、数值模拟和力学实验中,这部分影响往往被忽略。本文旨在提出一个定量分析模型来研究初始微裂纹对岩石变形破坏演化过程的影响。 创新点 1. 建立初始微裂纹占比定量分析的理论模型;2. 揭示围压对大理岩模型参数演化的影响。 方法 1. 通过理论推导,建立一种能进行岩石初始微裂纹占比定量分析的理论模型,并基于三轴压缩试样的应力分解改进该模型的表达式(公式(13)和(14));2. 通过三轴压缩试验,确定岩石初始裂隙精确分析的拟合区间,并分析围压对大理岩试样模型参数演化的影响(图5~7);3. 结合微CT扫描技术,对受载岩样的裂隙演化特征进行讨论与分析(图10和11)。 结论 1. 所建立的岩石初始微裂纹占比定量分析模型参数物理意义明确、确定方便;2. 随着围压的增加,试样孔隙度和泊松比均以指数函数的形式递减,基质部分弹性模量先增大后趋于稳定,而微裂纹弹性模量呈指数增长;3. 试样破坏是试样内部微裂纹扩展的结果,且宏观破坏角随着围压的增大而线性减小。