散体颗粒介质变形局部化宏–细观机制研究

散体颗粒是自然界中广泛存在的一类特殊的介质,受内部组构影响,散体颗粒介质在外荷载作用下发生明显的变形局部化效应,这也是岩土力学领域一直研究的热点问题之一。然而目前对于其机制研究方面大都局限于细观统计参数的分析,忽视了宏观现象与细观机制的内在联系。本文以砂土颗粒介质为例,运用离散元法对其在直剪试验过程中的宏–细观力学特性及变形破坏机制进行系统分析,并取得一些有意义的认识。根据数值试验中试样在不同法向应力下剪应力比的发展,从颗粒运动角度探讨试样宏观抗摩擦特性变化趋势的细观机制;发现描述细观组构的各向异性参数及主方向与宏观上剪应力比的发展具有同步性,反映试样宏细观演化的统一性;通过对于颗粒旋转的统计分析,揭示颗粒间摩擦作用是维持细观力学结构相对稳定的重要因素;通过对力链网络的形态演化分析,发现试样大主应力方向与各向异性主方向一致,力链密集程度随剪切过程下降,试样孔隙度随之上升;提出不同法向应力下的2种主要力链结构的力学模型,通过稳定性分析与能量累积释放理论解释宏观力学参数波动情况的原因,揭示散体颗粒介质变形局部化和体应变剪胀的细观机制。

MACRO-AND MESO-MECHANISM STUDY OF STRAIN LOCALIZATION IN GRANULAR MATERIAL

The strain localization in granular material due to its internal fabric under external load is a popular topic in the field of geotechnical engineering. However，current studies have focused on the analysis of statistical parameters，while the intrinsic link between the macroscopic phenomena and the mesoscopic mechanism has been ignored. The macroscopic and mesoscopic mechanical characteristics and the mechanism of deformation failure in the direct shear tests were systematically analyzed with the discrete element method(DEM). According to the development of shear stress ratio under the different normal stresses during the shearing process，anti-friction properties of granular material were discussed from the perspective of particle movement. A synchronic development between the anisotropic parameters of fabric and the macro-scale stress ratio was found. Through the statistical analysis of particle rotation，the friction between particles was revealed to be an important factor to maintain the relative stability of the mesoscopic structure. The morphological analysis of the networks of force chains revealed an agreement between the direction of principle stress and the principle direction of anisotropy. The decrease of the force chain intensity and the increase of porosity during the shearing tests were discovered. Two main mechanical models of force chain structures under different normal stresses were proposed. The fluctuation of macro-scale mechanical parameters was explained with the stability of system and the cumulative release of energy，showing the microscopic mechanism of deformation localization and dilatancy.