松砂不稳定行为的数值模拟研究

针对松散砂土的不稳定行为和静态液化现象，开展了一系列的双轴剪切试验离散元数值模拟，从宏细观的角度研究了松砂不稳定行为的各种影响因素及其发生机理。通过数值模拟发现，砂土的不稳定行为不仅与初始孔隙比有关，而且与土体结构的各向异性和围压有关。当加载方向与由颗粒接触法向所表征的结构各向异性主方向不共轴时，不稳定行为的发生会同时伴随着各向异性主方向的旋转。两个方向的角度差愈大，不稳定行为愈容易发生，颗粒的细观运动和细观结构重组现象也愈剧烈。不稳定状态点处的剪切荷载主要由颗粒间法向接触力承担，切向接触力仅起次要作用。     

砂土直剪力学性状的非圆颗粒模拟与宏细观机理研究

基于PFC2D非圆颗粒单元的二次开发,对砂土直剪力学过程进行了非圆颗粒仿真模拟,分析了数值试样的应力–剪胀关系并与实际砂土进行对比,探讨了颗粒位移与颗粒旋转特征及其与剪切带演化的内在关联,研究了主应力与主应变增量的非共轴效应,揭示了细观组构各向异性的演化规律及其与宏观剪切强度之间的宏细观关联。研究结果表明,数值试验能够较好的模拟实际砂土的应力–剪胀关系和剪切过程主应力与主应变增量的非共轴效应;剪切带的演化与颗粒位移和颗粒旋转密切相关,颗粒形状影响剪切带的厚度;试样宏观的剪切强度主要受控于粒间法向接触力的分布及其各向异性演化;整个加荷过程中,剪切带内大主应力的偏转方向与法向接触力各向异性的主方向保持了良好的一致性。     

基于随机散粒体模型的堆石体真三轴数值试验研究

基于三维变形体离散单元法,模拟堆石体的真三轴试验,研究堆石体在三向不等应力状态下的强度和变形特性。真三轴数值试验装置采用六刚性板加荷方式,等中主应力比路径加载。数值试验结果表明：真三轴数值试验能较好反映堆石体在三向不等应力状态下的应力和变形规律,数值模拟得到宏观应力变形特性与试验规律相似;等中主应力比参数的大小对堆石在三向应力状态下应力和强度特性有显著的影响,堆石体在 3 个加载方向的变形也随该参数的变化而变化,应力比参数从 0 到 1 变化过程中中主应变方向先压缩后膨胀,小主应变方向一直处于压缩状态;堆石体的内摩擦角随着值的增加而增大,基本符合 Lade-Duncan 破坏准则;在细观层面上,围压越高,值越大,颗粒配位数越大; 加载过程中,颗粒接触法向和法向接触力各向异性程度加强,各向异性主方向角由水平向转向大主应变方向,试样各向异性系数的演化规律和试样的宏观应力变形曲线相对应,试样的宏观力学特性与细观组构存在内在的关联。     

考虑颗粒破碎的堆石体三维随机多面体细观数值模拟

 基于随机颗粒不连续变形模型,在颗粒的细观单元之间插入界面单元,采用黏聚力裂缝模型模拟界面单元的起裂、扩展和失效,研究颗粒破碎对堆石体强度和变形的影响。从宏观、细观2个层面分析细观数值模拟结果,结果表明：本文提出的方法能直接模拟三维颗粒破碎现象,颗粒破碎降低堆石体的强度,其机制在于颗粒破碎弱化堆石体的剪胀性能,使得由于剪胀引起的内摩擦角显著降低,颗粒破碎的影响随围压的增加而增大;颗粒破碎时伴随着声发射,高围压下声发射数多于低围压下,表明高围压下颗粒破碎更明显,与试验结果一致;在剪切过程中,颗粒间接触法向逐渐由水平方向向竖直方向倾斜,法向接触力的主方向与加载方向相同,颗粒间平均法向接触力逐渐增大。     

基于数字图像技术的砂土液化可视化动三轴试验研究

常规动三轴试验不能实现试样细观结构的观测,对原有CKC型振动三轴仪实施可视化改进,开发用于试验过程试样细观结构观测的试验系统。在砂土振动液化试验中,利用显微数码摄录技术全程动态摄录整个试验过程砂土细观结构的变化,从摄录录像中提取特征时刻的数字图像照片,导入自主开发的数字图像细观结构分析软件DeoDIP,对比分析液化前后砂土细观结构(颗粒定向性、接触法向、接触数)的演化规律,追踪标志砂颗粒的运动轨迹。结果表明:液化时,颗粒长轴没有明显的优选方向,制样引起的初始各向异性被消除;接触法向呈现各向异性特征,其主方向在整个试验过程中一直偏于竖直方向;平均接触数发生了损失,液化排水后得到恢复,说明试样振动密实。液化前,各标志颗粒具有基本相同的运动轨迹,颗粒运动以平动为主,旋转作用很小,颗粒运动与试样变形保持较好的整体性;颗粒之间相对运动加剧,颗粒之间不断接触分离,部分颗粒出现悬浮现象,且颗粒旋转明显。可视化三轴试验结果不仅有助于研究砂土液化的细观力学机理,并能为液化细观数值模拟提供试验基础。     

颗粒形状及粒间摩擦角对堆石体宏观力学行为的影响

基于三维变形体离散单元法对堆石体进行细观研究,采用随机模拟技术生成堆石体三维数值试样,模拟其常规三轴剪切试验,研究堆石体颗粒形状及粒间摩擦系数对宏观力学性能的影响,揭示其细观组构和细观力学参数与堆石体宏观特性的关联性,建立细观力学参数与宏观力学参数的相关关系。数值模拟结果表明：颗粒形状对堆石体宏观特性影响明显,颗粒的长短径比越大,数值试样的峰值强度和残余强度均明显增大,峰值强度对应的轴向应变也逐渐增大,而数值试样的初始模量有所减小;在细观层面上,颗粒的长短径比越大,颗粒的接触法向向加载方向倾斜越明显,接触法向的各向异性程度越大;粒间摩擦角对堆石体宏观特性的影响显著,随着粒间摩擦角的增大,峰值强度明显增大和初始模量均明显增大,数值试样在剪切时的体积收缩量也有所增大,反映了堆石颗粒间的相互作用对颗粒体骨架的宏观力学行为的影响。     

砂土单调剪切特性的非圆颗粒模拟

针对纯圆颗粒模拟的缺陷,利用颗粒流"团颗粒"方法开发了形状近似椭圆的"椭圆团颗粒",研究了颗粒形状变化对数值试样宏观剪切特性的影响及其细观机理;分析了加荷过程中平均接触数的变化规律以及颗粒形状对临界接触数的影响;对比了纯圆颗粒和椭圆团颗粒的不同颗粒旋转特性,并初步探讨其机理;分析了剪切过程中椭圆团颗粒试样的组构各向异性演化规律。结果表明,颗粒形状对试样的强度和变形特性具有重要影响,其在细观机理上与平均接触数有关;剪切荷载作用下椭圆团颗粒试样呈现明显的组构各向异性,颗粒长轴逐渐趋于水平定向,接触法向逐渐偏于加荷的大主应力方向。     

各向异性砂土宏微观特性三维离散元分析

各向异性对砂土强度和变形特性有显著的影响,为了研究各向异性砂土的宏微观特性,基于三维离散元法,对7个不同沉积角的试样进行了一系列的三轴模拟试验。利用"Clump"命令生成近似椭球形状颗粒,并且采用三维抗转动模型来模拟颗粒间的抗转动能力。离散元模拟结果与已知室内试验结果吻合很好。结果表明:随着沉积角的增大,偏应力和轴向应变的关系逐渐由应变软化向应变硬化发展。沉积角较小的试样剪胀性更强并且容易到达临界状态,颗粒组构–应力联合不变量(表征颗粒长轴组构张量和应力张量的相对角度)的值接近于-1,且颗粒长轴组构各向异性先增大后减小;然而对于沉积角较大的试样,在轴向应变50%处,仍不能达到临界状态,并且联合不变量的值大于-1,颗粒长轴组构各向异性先减小后不断增大。对于法向接触组构,组构主轴方向迅速向应力主轴方向偏转,组构各向异性的演化规律与偏应力随轴向应变的演化规律相似。     

循环荷载下砂土变形的细观数值模拟I:松砂试验结果

采用颗粒流方法模拟不同排水条件下砂土的双轴试验,研究了循环荷载作用下松砂渐进破坏过程中配位数、接触方向、粒间接触力的演化规律,应用表征上述量的组构参数研究了砂土的诱发各向异性,探讨了饱和砂土液化、状态转换面产生的微细观机理。研究表明:宏观的液化对应于细观组构上配位数的连续累积丧失和粒间接触力的不断减小,其根本原因在于循环荷载往返过程中,组构各向异性与应力各向异性发展的不匹配。研究成果对于揭示砂土变形的细观机理以及建立砂土的细观力学模型都具有意义。     

循环荷载下砂土变形的细观数值模拟Ⅰ：松砂试验结果

采用颗粒流方法模拟不同排水条件下砂土的双轴试验，研究了循环荷载作用下松砂渐进破坏过程中配位数、接触方向、粒间接触力的演化规律，应用表征上述量的组构参数研究了砂土的诱发各向异性，探讨了饱和砂土液化、状态转换面产生的微细观机理。研究表明：宏观的液化对应于细观组构上配位数的连续累积丧失和粒间接触力的不断减小，其根本原因在于循环荷载往返过程中，组构各向异性与应力各向异性发展的不匹配。研究成果对于揭示砂土变形的细观机理以及建立砂土的细观力学模型都具有意义。     

离散元中破碎自组织对颗粒破碎影响研究

从数学领域的阿波罗填充法入手,建立了4种破碎自组织,并借助线性膨胀法保证破碎前后质量守恒。在此基础上,开展了不同破碎自组织的数值试验,研究了破碎自组织对级配演化以及材料的宏细观力学特性等的影响。结果表明:颗粒级配曲线的分形维数和颗粒间的平均应力随破碎自组织中颗粒数目增多而下降,而相对破碎率B_r和材料的压缩性随自组织中颗粒数目增多而增大。加载过程中的法向接触和接触力玫瑰图表明,自组织中颗粒数目愈多,材料的各向异性程度愈低,颗粒法向接触数目愈多,而法向接触力愈小。另外,配位数及接触力的概率密度也与破碎自组织存在密切联系。     

Experimental study on compaction-induced anisotropic mechanical property of rockfill material

The anisotropy of rockfill materials has a significant influence on the performance of engineering structures. However, relevant research data are very limited, because of the difficulty with preparing specimens with different inclination angles using traditional methods. Furthermore, the anisotropy test of rockfill materials is complex and complicated, especially for triaxial tests, in which the major principal stress plane intersects with the compaction plane at different angles. In this study, the geometric characteristics of a typical particle fraction consisting of a specific rockfill material were statistically investigated, and the distribution characteristics of particle orientation in specimens prepared via different compaction methods were examined. For high-density rockfill materials, a set of specimen preparation devices for inclined compaction planes was developed, and a series of conventional triaxial compression tests with different principal stress direction angles were conducted. The results reveal that the principal stress direction angle has a significant effect on the modulus, shear strength, and dilatancy of the compacted rockfill materials. Analysis of the relationship between the principal stress direction angles, change in the stress state, and change in the corresponding dominant shear plane shows that the angle between the compacted surface and dominant shear plane is closely related to interlocking resistance associated with the particle orientation. In addition, different principal stress direction angles can change the extent of the particle interlocking effect, causing the specimen to exhibit different degrees of anisotropy.     

Numerical investigation into the stiffness anisotropy of asphalt concrete from a microstructural perspective

A micromechanical model was built in this paper to investigate the stiffness anisotropy of asphalt concrete (AC) using the discrete element method. Four three-dimensional cubic AC digital samples with different aggregate particle orientations were built using discrete element software PFC3D. The aggregate gradation and shape, air voids and mastic included in the digital samples were modeled using different contact models, with due consideration of the volumetric fractions of the different phases. Laboratory uniaxial complex modulus test and indirect tensile strength test were conducted to obtain material input parameters for numerical modeling. Simulation of the uniaxial cyclic compressive tests was performed on the four cubic samples loaded in three different directions. Dynamic stiffness in different directions was calculated from the compression stress–strain responses. Results show that the AC stiffness is significantly dependent on preferential orientation of aggregate particles. The AC stiffness in the long-axis direction of aggregate particles is shown to be up to 43% higher than the stiffness in the particle short-axis direction. The stiffness anisotropy of AC decreases as the mixture temperature drops.     

Visualization of force networks in 2D dense granular materials

Dense granular matter is a conglomeration of discrete solid and closely packed particles. As subjected to external loadings, the stress is largely transmitted by heavily stressed chains of particles forming a sparse network of larger contact forces. To understand the structure and evolution of force chains, a photoelastic technique was improved for determining stresses and strains in the assemblies of photoelastic granular disks in this paper. A two-dimensional vertical slab was designed. It contains 7200 polydispersed photoelastic disks and is subjected to a localized probe penetrating at the top of the slab to mimic the cone penetration test. The interparticle contact force distribution was found a peak around the mean value, a roughly exponential tail for greater force and a dip toward zero for smaller force. The force chain network around the probe tip was depicted, and the contact angle distribution of particles in force chains was found to be well aligned in the directions of major principal stress.     

Visualization of force networks in 2D dense granular materials

Dense granular matter is a conglomeration of discrete solid and closely packed particles. As subjected to external loadings, the stress is largely transmitted by heavily stressed chains of particles forming a sparse network of larger contact forces. To understand the structure and evolution of force chains, a photoelastic technique was improved for determining stresses and strains in the assemblies of photoelastic granular disks in this paper. A two-dimensional vertical slab was designed. It contains 7200 polydispersed photoelastic disks and is subjected to a localized probe penetrating at the top of the slab to mimic the cone penetration test. The interparticle contact force distribution was found a peak around the mean value, a roughly exponential tail for greater force and a dip toward zero for smaller force. The force chain network around the probe tip was depicted, and the contact angle distribution of particles in force chains was found to be well aligned in the directions of major principal stress.