土体应变局部化现象中非共轴机制的宏细观分析

Strain localization is one of the main reasons for the loss of stability and the progressively failure of geostructures. In which the non-coaxiality of the soils plays a non-negligible role in the prediction of the bifurcation and the description for the variation of the shear bands. The anisotropy of the soil is the essential reason of the non-coaxiality. Hence, in order to study the strain localization behavior in soils, the macro-meso method will be adopted, and an anisotropic non-coaxial constitutive model based on the micro-polar theory will be developed. On the mesoscopic, the relationship between the soil fabric and anisotropy will be obtained; On the macroscopic, an anisotropic constructive model considering the micro fabric will be established, the non-coaxial flow rule and the non-coaxial stress-dilatancy relation will be introduced. At the same time, the transformation of macro-meso parameters will be built through the bridge of RVE. By all these means, the typical simple shear and plane stain compression tests will be simulated. In order to verify and improve the proposed constitutive model, the corresponding laboratory experiments incorporating DIC method will also be carried out. Finally, The interaction mechanism of the anisotropy and the non-coaxiality of the soils and the resulting influence on the bifurcation and the development of shear bands will be illuminated. And the research results will offer some theoretic supports for the reveal of the mechanism of the progressive failure of the geostructures due to the strain localization phenomenon.

应变局部化现象是导致岩土工程失稳渐进破坏的主要原因之一。其中，土体非共轴特性对土体局部化分岔的预测及剪切带的发展都会产生不可忽视的影响，而土体各向异性是引起非共轴行为的本质原因。为此，本项目拟采用宏细观结合的方法建立基于微极理论的各向异性非共轴本构模型，对土体应变局部化行为开展系统深入的研究。细观上，通过离散元方法获取土体组构对各向异性的表征；宏观上，基于微极理论框架，建立与细观组构相关的各向异性屈服准则，引入非共轴流动法则及非共轴应力-剪胀关系。同时，以代表性体积元（RVE）为桥梁建立宏细观参数的传递，对典型的简单剪切实验及平面应变压缩实验进行数值模拟。并结合数字相关方法（DIC）开展相应的室内试验，验证和完善所发展的本构模型。最终为阐明土体各向异性和非共轴特性的相互作用机制及其对土体分岔和剪切带发展演化的影响，揭示由应变局部化导致的岩土工程结构渐进破坏机理提供科学认知与理论支撑。

本项目针对土体非共轴特性对应变局部化行为的影响这一问题开展了研究。首先，引入了基于角点屈服面的非共轴流动法则，建立了基于微极理论的宏观弹塑性非共轴本构模型，并采用自带误差控制的修正欧拉算法，编写了基于ABAQUS的UEL子程序。通过简单剪切实验，分析了非共轴塑性模量对应力应变关系和非共轴角的影响，非共轴塑性模量的值越小，加载初期产生的非共轴角越大，且材料表现出“更软”的力学响应。通过平面应变压缩实验，探究了不同边界条件对分岔和剪切带倾角、宽度的影响。其次，通过引入颗粒细观组构张量与应力张量联合不变量，建立了一个各向异性本构模型，各向异性程度越强材料表现出了更强的非共轴行为。通过边坡及浅基础等典型工程问题证实了该模型模拟剪切带的适用性。最后，以RVE为中间媒介，由颗粒半径、法向切向接触刚度等计算得到宏观变形参数的值，由颗粒间接触力、接触方向、颗粒位置等细观信息计算得到RVE的应力、应变，并进一步得到剪胀系数和摩擦系数随加载的演化曲线，实现了宏细观参数的有效传递，建立了基于微极理论的宏细观非共轴本构模型。得到了非共轴应力剪胀关系曲线，发现非共轴越强材料的剪缩行为越显著。通过与试验对比，发现非共轴模型的模拟结果与试验吻合更好。本项目建立的基于微极理论的宏细观结合的弹塑性非共轴本构模型，既反映了颗粒细观属性的影响，又避免了离散元颗粒数量巨大导致的计算效率低下的问题，同时对于边值问题，还有效克服了传统本构模型网格依赖性的缺点，对分析实际工程中由应变局部化现象所引起的渐进失效破坏等问题有广阔的应用前景。