Examining the influence of the loading path on the cracking characteristics of a pre-fractured rock specimen with discrete element method simulation

Abstract

Damage in a rock mass is heavily dependent on the existence and growth of joints, which are also influenced by the complex stress states induced by human activities (e.g., tunneling and excavation). A proper representation of the loading path is essential for understanding the mechanical behaviors of rock masses. Based on the discrete element method (DEM), the influence of the loading path on the cracking process of a rock specimen containing an open flaw is examined. The effectiveness of the model is confirmed by comparing the simulation results under a uniaxial compression test to existing research findings, where wing crack initiates first and secondary cracks contribute to the failure of the specimen. Simulation results confirm that the cracking process is dependent upon both the confining pressure and the loading path. Under the axial loading test, a higher confining pressure suppresses the development of tensile wing cracks and forces the formation of secondary cracks in the form of shear bands perpendicular to the flaw. Increase of confining pressure also decreases the influence of the loading path on the cracking process. Reduction of confining pressure during an unloading test amplifies the concentration of tensile stress and ultimately promotes the appearance of a tensile splitting fracture at meso-scale. Confining pressure at the failure stage is well predicted by the Hoek-Brown failure criterion under quasi-static conditions.

Abstract

目的

结构面是岩体的重要组成部分,其对岩体工程结构的变形和失稳有着重要的影响。本文旨在探究加载路径和围压对预制裂缝试样裂纹扩展和强度的影响,揭示不同加载路径下含预制裂缝岩石试样的微观破坏机理。

创新点

1. 设计可以模拟各种工程场景(如隧道、边坡开挖)下应力状态变化的加载路径,包括轴向加载试验、围压卸载试验和围压卸载伴随的轴向加载试验;2. 研究试样裂纹扩展路径及其与加载路径和围压的关系;3. 通过对局部应力集中和裂纹类型的分析,探讨影响断裂过程的微观机制。

方法

1. 基于试验数据,开展离散元模拟参数校准,实现对岩石试样宏观力学参数的再现(表3和图4);2. 基于校准获得微观参数,并开展三种加载路径下预制裂缝岩石试样数值模拟试验,获得加载路径与围压对试样裂纹扩展的影响(图5~12);3. 通过分析试样加载过程的应力分布以及微裂纹类型的占比,获得预制裂缝试样 裂纹扩展的微观机理(图13~16)。

结论

1. 预制裂缝岩石试样加载过程中,裂纹扩展路径与轨迹受围压与加载路径的共同作用,围压的增大降低了加载路径对裂纹扩展的影响。2. 加载过程中,不同类型裂纹的产生主要与不同局部应力的集中有关;在轴向加载条件下,会出现剪切应力的局部集中,抑制了张拉裂纹沿主应力方向扩展,但也会促进垂直于预制裂缝方向的剪切带的产生;卸围压的加载条件下会出现张拉应力的局部应力集中,进而促进细观张拉裂纹的产生。3. 准静态条件下,预制裂缝岩石试样的失效阶段可以通过Hoek-Brown准则进行预测。