基于细观拓扑结构演化的颗粒材料剪胀性分析

剪胀性是包括岩土材料在内的摩擦性颗粒材料的重要特征之一, 其形成机制与颗粒体系内部拓扑结构的演化有关. 基于颗粒体系细观数据, 可对颗粒体系内部的拓扑结构特征及演化进行分析, 进而建立拓扑演化与宏观剪胀变形之间的联系. 采用离散单元法, 根据密实、中密和松散摩擦性颗粒材料双轴试验的宏微观数据, 从拓扑参量演化及接触网络拓扑变化所引起的3类细观结构几何特征等方面研究了颗粒材料宏观剪胀变形的拓扑机制. 研究表明: 密实和中密颗粒体系双轴试验过程存在应变软化和剪胀现象, 与细观结构的拓扑演化与几何各向异性相关, 而松散颗粒体系剪胀效应不明显; 将接触网络剖分成多边形力环结构并根据拓扑演化定义新生、消失和不变3类细观结构, 在加载过程中不同拓扑属性细观结构所占比例及各向异性存在较大差异, 且较大尺寸的力环可承担较大的几何各向异性; 新生力环结构在双轴加载过程中能够表现出更加明显的剪胀特性, 颗粒体系整体的剪胀变形受到新生结构拓扑变化及恒定结构几何变化的综合影响. 

DILATANCY ANALYSIS OF GRANULAR MATERIALS BASED ON MESOSCOPIC TOPOLOGICAL EVOLUTIONS

Dilatancy is one of the most important characteristics for frictional granular materials, especially for geo materials. It is widely accepted that the mechanism of dilatancy could be related to the evolution of the internal topological structure within the granular system. Based on meso-structural data of granular assemblies, features of the internal topological structure evolution in the granular system can be captured, which could further help to correlate the mesoscopic topological evolution and the macroscopic deformation properties including dilatancy. In this paper, the discrete element method (DEM) was used to conduct biaxial tests on dense, medium-dense and loose frictional granular materials, respectively. According to those DEM data from macroscopic to microscopic levels, the topological mechanism for dilatancy of granular materials are investigated in terms of network parameters (e.g., coordination number and clustering coefficient) and deformation features of 3 types of mesoscopic structures induced by topological exchanges. The results show that the significant strain softening and dilatancy occur for dense granular samples under biaxial loading, which is related to the topological and geometric changes of mesoscopic structures. The medium dense sample also exhibits dilatancy features but the degree is less evident, and the loose sample only shows contractancy and strain hardening during the shearing process. The contact network could be tessellated to force loop structures with the polygon shapes, and further classified into new, lost and constant categories by considering the topological exchanges. The anisotropy and composition evolutions of three groups of force loop structures are different, and loops with larger size could exhibit higher geometrical anisotropy. Under deviatoric loads, the new loop structures are easily related to higher dilatancy, and the dilatancy mechanism of the overall granular system could be influenced by the comprehensive effects of the topological evolutions of new meso structures and geometrical evolutions of constant meso structures.