基于能量耗散机制的砂土微观力学特性研究
发布时间：2018-09-07 16:10
【摘要】：砂土作为一种典型的散体颗粒集材料,其颗粒的几何、物理性质以及微观力学行为组成了砂土颗粒集在宏观尺度上的力学特性,如应力应变行为、剪胀效应、剪切局部化现象以及诱发向性异性等。借助于二维离散元分析软件PFC2D模拟了一系列砂土颗粒集样本在平面应变下的准静态剪切实验,本文试图通过砂土颗粒在剪切过程中的微观力学行为的研究,包括平动、转动、接触本构、破碎以及自组织结构等,全面地了解砂土材料的多尺度力学特性,并构建其微观力学行为与宏观力学行为之间的联系。 在微观尺度上,颗粒形状、表面纹理与破碎行为构成了砂土颗粒主要的几何、物理特征。本文分别提出了采用颗粒重叠成簇的方法模拟不规则形状砂土颗粒,并采用自定义的接触本构模型模拟颗粒表面纹理的抗滚动效应,以及采用改进型聚粒算法生成可破碎性砂土颗粒,全面地讨论了不同的颗粒几何、物理特征对砂土宏-微观力学行为的影响,并根据颗粒集系统的能量分配机制对这种影响的内在机理进行了阐述。此处,从宏观尺度上看,砂土作为一种类连续介质材料,为了描述其力学行为,本文提出采用自由网格法计算砂土颗粒集的局部应变张量,通过剪切过程中局部剪切应变空间分布发展来反映砂土剪切带的形成演化过程。 颗粒集系统能量分配机制作为砂土微观力学行为与宏观力学行为的纽带贯穿于全文中,其在微观尺度上以颗粒的力学和运动参量为基础,并通过统计分析方法可以深入地解释了砂土宏观力学行为的内在物理机制。通过对剪切过程中砂土的宏-微观力学行为以及能量分配机制的研究,得到如下结论： 由形状和表面抗滚动纹理所引发的颗粒抗转动效应都能显著地提高砂土的抗剪切强度。然而,这两种颗粒抗转动效应的微观作用机制却完全不同：不规则形状颗粒间的内锁力可以提高接触点滑动摩擦力的发挥程度以及减缓剪切带的发展,从而贡献于抗剪强度;表面抗转动纹理则大大地提高了颗粒接触点的弹性转动势能储存以消耗外界功,贡献于抗剪强度。由于其微观作用机制的不同,不同的颗粒抗转动效应在宏观上表现出完全不同的力学行为,特别是在应力峰值下,抗滚动模型定义的圆形颗粒集与非规则形状颗粒集相比,其剪切诱发的局部化现象以及各向异性特征更为明显。 砂土颗粒的破碎伴随着整个剪切过程,但破碎增长速率则不断下降,达到临界状态后开始趋于平缓；在临界状态下,由于剪切带的形成,颗粒破碎主要集中在剪切带中,且不断地反复破碎生成更多、更小的碎片颗粒。由于颗粒的反复破碎有效地增加了剪切带中颗粒摩擦的比表面积,从而强化了系统的弹性应变能储存,表现为砂土在临界状态下的应变强化和体积剪缩过程。另一方面,通过剪切过程中颗粒级配演化过程,发现颗粒破碎过程实际上是一个颗粒级配朝着砂土终极级配优化的过程,终极级配下颗粒粒径服从良好的分形特征。在考虑颗粒破碎准则的前提下,得到砂土颗粒粒径的分形维数约为1.3。 最后,基于接触力网络结构的拓扑识别,发现了砂土颗粒在剪切过程中的自组织结构行为。本文提出细观尺度下的颗粒子域结构单元构建了颗粒集的结构体系以抵抗外界荷载与变形作用。其中,3-cycle单元是系统中最稳定的结构单元,且对力链传递存在双重支撑作用；随着n-cycle单元阶次的提高,其结构稳定性逐渐降低。根据颗粒集体系结构稳定性的演化过程,砂土的剪切带发展可以理解为稳定的低阶结构单元向不稳定的高阶结构单元转化的局部化发展过程,也正是高阶结构单元在剪切带中的聚集引起了砂土在临界状态中的剪胀效应。更为重要的是,根据不同类型结构单元物理力学性能的研究,结构单元间的协调作用机制使得颗粒集系统从微观的离散化力学状态向宏观的均匀化力学状态进行了重要地过渡。从而,更加全面和深入地了解了砂土的多尺度力学行为。 总体来讲,根据砂土颗粒的微观力学行为并通过能量分析手段建立其与砂土宏观力学行为之间的联系,旨在研究砂土在剪切过程中变形破坏的内在机理,为砂土的屈服准则以及本构模型研究提供一定的理论支持,并最终服务于岩土工程实践与设计中。
[Abstract]:Sand is a typical granular aggregate material. Its geometry, physical properties and micro-mechanical behavior constitute the mechanical properties of granular aggregates at macro-scale, such as stress-strain behavior, dilatancy effect, shear localization and induced anisotropy. A series of quasi-static shear tests of sand particles under plane strain are carried out in this paper. The micro-mechanical behavior of sand particles in shear process is studied, including translation, rotation, contact constitutive, breakage and self-organization structure. The multi-scale mechanical properties of sand materials are comprehensively understood and their micro-mechanical behavior and microstructure are constructed. The relationship between macroscopic mechanical behavior.
At the micro-scale, particle shape, surface texture and crushing behavior constitute the main geometric and physical characteristics of sand particles. In this paper, a method of overlapping particles into clusters is proposed to simulate irregular sand particles, and a custom contact constitutive model is used to simulate the anti-rolling effect of grain surface texture, and an improved method is adopted. The influence of different particle geometry and physical characteristics on the macro-micro mechanical behavior of sands is discussed comprehensively, and the intrinsic mechanism of the influence is explained according to the energy distribution mechanism of the particle collection system. In order to describe its mechanical behavior, a free-mesh method is proposed to calculate the local strain tensor of sand particle set, which reflects the formation and evolution of sand shear band by the spatial distribution of local shear strain during shear process.
The energy distribution mechanism of granular aggregation system is the link between the micro-mechanical behavior and the macro-mechanical behavior of sand. It is based on the mechanical and kinematic parameters of particles at the micro-scale. The internal physical mechanism of the macro-mechanical behavior of sand can be explained by statistical analysis. The following conclusions are drawn from the study of macro micro mechanical behavior and energy distribution mechanism of sand.
The shear strength of sands can be significantly improved by the anti-rotation effect of particles caused by the shape and surface anti-rolling texture. However, the micro-mechanism of the anti-rotation effect of the two kinds of particles is completely different: the internal locking force between irregularly shaped particles can increase the exertion of sliding friction at the contact point and slow down the shear band. The development contributes to the shear strength; the surface anti-rotation texture greatly improves the elastic rotational potential energy stored at the particle contact point to consume external energy and contribute to the shear strength. The shear-induced localization and anisotropy of the circular particle set defined by the anti-rolling model are more obvious than that of the irregular particle set.
The breakage of sand particles is accompanied by the whole shear process, but the growth rate of breakage decreases continuously and becomes gentle after reaching the critical state. In the critical state, due to the formation of the shear band, the breakage of sand particles mainly concentrates in the shear band, and breaks repeatedly to produce more and smaller debris particles. The specific surface area of particle friction in the shear band is effectively increased, thus the elastic strain energy storage of the system is strengthened, which is represented by the strain strengthening and volume shearing process of sand in critical state. The fractal dimension of sand particle size is about 1.3 considering the criterion of particle breakage.
Finally, based on the topological identification of the contact force network structure, the self-organizing behavior of sand particles in shear process is discovered. The particle subdomain structure element is proposed to construct a particle-set structure system to resist external load and deformation. The shear band development of sandy soil can be understood as the local development process of the transformation from stable low-order structural elements to unstable high-order structural elements according to the evolution process of structural stability of granular system. More importantly, according to the study of physical and mechanical properties of different types of structural elements, the mechanism of coordination between structural elements makes the granular aggregation system proceed from microscopic discrete mechanical state to macroscopic homogeneous mechanical state. An important transition has been made, so that the multi-scale mechanical behavior of sand can be understood more comprehensively and thoroughly.
Generally speaking, according to the micro-mechanical behavior of sand particles and by means of energy analysis, the relationship between sand particles and macro-mechanical behavior of sand is established. The purpose is to study the internal mechanism of deformation and failure of sand in shear process, and to provide certain theoretical support for the study of yield criterion and constitutive model of sand, and ultimately serve geotechnical engineering. Process practice and design.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：TU521
本文编号：2228752
[Abstract]:Sand is a typical granular aggregate material. Its geometry, physical properties and micro-mechanical behavior constitute the mechanical properties of granular aggregates at macro-scale, such as stress-strain behavior, dilatancy effect, shear localization and induced anisotropy. A series of quasi-static shear tests of sand particles under plane strain are carried out in this paper. The micro-mechanical behavior of sand particles in shear process is studied, including translation, rotation, contact constitutive, breakage and self-organization structure. The multi-scale mechanical properties of sand materials are comprehensively understood and their micro-mechanical behavior and microstructure are constructed. The relationship between macroscopic mechanical behavior.
At the micro-scale, particle shape, surface texture and crushing behavior constitute the main geometric and physical characteristics of sand particles. In this paper, a method of overlapping particles into clusters is proposed to simulate irregular sand particles, and a custom contact constitutive model is used to simulate the anti-rolling effect of grain surface texture, and an improved method is adopted. The influence of different particle geometry and physical characteristics on the macro-micro mechanical behavior of sands is discussed comprehensively, and the intrinsic mechanism of the influence is explained according to the energy distribution mechanism of the particle collection system. In order to describe its mechanical behavior, a free-mesh method is proposed to calculate the local strain tensor of sand particle set, which reflects the formation and evolution of sand shear band by the spatial distribution of local shear strain during shear process.
The energy distribution mechanism of granular aggregation system is the link between the micro-mechanical behavior and the macro-mechanical behavior of sand. It is based on the mechanical and kinematic parameters of particles at the micro-scale. The internal physical mechanism of the macro-mechanical behavior of sand can be explained by statistical analysis. The following conclusions are drawn from the study of macro micro mechanical behavior and energy distribution mechanism of sand.
The shear strength of sands can be significantly improved by the anti-rotation effect of particles caused by the shape and surface anti-rolling texture. However, the micro-mechanism of the anti-rotation effect of the two kinds of particles is completely different: the internal locking force between irregularly shaped particles can increase the exertion of sliding friction at the contact point and slow down the shear band. The development contributes to the shear strength; the surface anti-rotation texture greatly improves the elastic rotational potential energy stored at the particle contact point to consume external energy and contribute to the shear strength. The shear-induced localization and anisotropy of the circular particle set defined by the anti-rolling model are more obvious than that of the irregular particle set.
The breakage of sand particles is accompanied by the whole shear process, but the growth rate of breakage decreases continuously and becomes gentle after reaching the critical state. In the critical state, due to the formation of the shear band, the breakage of sand particles mainly concentrates in the shear band, and breaks repeatedly to produce more and smaller debris particles. The specific surface area of particle friction in the shear band is effectively increased, thus the elastic strain energy storage of the system is strengthened, which is represented by the strain strengthening and volume shearing process of sand in critical state. The fractal dimension of sand particle size is about 1.3 considering the criterion of particle breakage.
Finally, based on the topological identification of the contact force network structure, the self-organizing behavior of sand particles in shear process is discovered. The particle subdomain structure element is proposed to construct a particle-set structure system to resist external load and deformation. The shear band development of sandy soil can be understood as the local development process of the transformation from stable low-order structural elements to unstable high-order structural elements according to the evolution process of structural stability of granular system. More importantly, according to the study of physical and mechanical properties of different types of structural elements, the mechanism of coordination between structural elements makes the granular aggregation system proceed from microscopic discrete mechanical state to macroscopic homogeneous mechanical state. An important transition has been made, so that the multi-scale mechanical behavior of sand can be understood more comprehensively and thoroughly.
Generally speaking, according to the micro-mechanical behavior of sand particles and by means of energy analysis, the relationship between sand particles and macro-mechanical behavior of sand is established. The purpose is to study the internal mechanism of deformation and failure of sand in shear process, and to provide certain theoretical support for the study of yield criterion and constitutive model of sand, and ultimately serve geotechnical engineering. Process practice and design.
【学位授予单位】：华中科技大学
【学位级别】：博士
【学位授予年份】：2013
【分类号】：TU521
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