应力各向异性对散粒体三维强度影响的拓扑机理
TOPOLOGICAL MECHANISM OF THE STRESS ANISOTROPY INFLUENCE ON THE THREE-DIMENSIONAL STRENGTH OF GRANULAR MATERIALS
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摘要: 岩土类散体材料在实际中往往处于三维各向异性应力状态, 初始应力各向异性对散体材料后续加卸载过程的宏微观力学特性有一定影响. 采用离散单元法制得不同初始各向异性应力的试样, 在等平均主应力条件下进行不同加载方向的真三轴试验, 绘制了不同初始各向异性应力试样在\textπ平面上所得到的强度包络线, 并对峰值强度前不同初始应力试样向强度包络线上同一点加载过程中的微观结构变化, 以及峰值强度前、后相同初始各向异性应力试样的强接触网络的拓扑结构进行了分析. 研究发现, 峰值前不同初始各向异性应力对散体强度包络线影响不大, 峰值后试样的强度包络线形状与峰值前试样相似但对应的屈服应力更小; 在向强度包络线上某一点加载过程中, 峰值前不同试样内部微观结构在加载初期差异迅速减小且随着加载的进行逐渐趋于相同; 峰值前、后相同初始各向异性应力试样虽处于相同的应力状态, 但内部接触以及强接触网络的拓扑结构差异明显, 可能是影响颗粒材料三维强度的重要因素.Abstract: In practical engineering applications, granular materials in geotechnical systems are often subjected to complex three-dimensional anisotropic stress conditions. The initial anisotropy of the stress state can exert a significant influence on the mechanical behavior of granular materials during loading and unloading process at both macro and micro scales. In this study, a series of numerical specimens with varying degrees of initial anisotropic stress were generated using the Discrete Element Method (DEM). These specimens were then subjected to true triaxial tests under the condition of constant mean principal stress, with loading applied in different directions. Based on the simulation results, strength envelopes on the π plane were constructed for specimens with different initial stress anisotropies. In order to further investigate the internal mechanical evolution, the microscopic structural changes during loading towards a specific point on the strength envelope were compared across specimens with different initial stress states before reaching peak strength. Furthermore, the topology of strong contact networks was analyzed for specimens at both pre-peak and post-peak stages under the same initial anisotropic stress conditions. The results demonstrate that initial stress anisotropy has limited impact on the strength envelope of samples before reaching the peak strength. However, post-peak strength envelope exhibits a similar shape to pre-peak strength envelopes but with reduced yield strengths. As loading progresses towards a specific point on the strength envelope, differences in the internal microstructure caused by different initial stress conditions tend to diminish rapidly and converge gradually as loading continues. Despite being at the same macroscopic stress state, specimens with the same initial anisotropic stress display notable differences in their internal contact distribution and the topology of strong contact network before and after the peak strength. These differences in contact network evolution are likely critical factors governing the three-dimensional strength behavior and failure mechanisms of granular materials.