Two-scale modeling of granular materials: A FEM-FEM approach

In the present paper, a homogenization-based two-scale FEM-FEM model is developed to simulate compactions of visco-plastic granular assemblies. The granular structure consisting of two-dimensional grains is modeled by the microscopic finite element method at the small-scale level, and the homogenized viscous assembly is analyzed by the macroscopic finite element method at large-scale level. The link between scales is made using a computational homogenization method. The two-scale FEM-FEM model is developed in which each particle is treated individually with the appropriate constitutive relations obtained from a representative volume element, kinematic conditions, contact constraints, and elimination of overlap satisfied for every particle. The method could be used in a variety of problems that can be represented using granular media.     

高原湖相泥炭质土蠕变特性及本构模型研究

高原湖相沉积泥炭质土的蠕变严重影响建构筑物的长期稳定和安全,有必要对其蠕变特性进行研究。对原状泥炭质土进行固结蠕变试验,采用分别加载方式获得其在不同荷载作用下应力应变随时间的变化曲线。研究结果表明:蠕变试验的等时及双对数曲线显示泥炭质土在各级荷载下具有明显的非线性特性;将泥炭质土的应变简化为线粘弹性ε₁与非线性粘塑性ε_n,通过线性Kelvin蠕变模型与经验模型,建立了半经验半理论固结蠕变模型,可分析其蠕变性状。对蠕变试样进行电镜扫描,分析在荷载作用下其微观结构变化机理,即泥炭质土低压时的变形主要来自于有机质和胶体的压缩及自由水排出,高压时的变形主要是土体颗粒间的错动。通过以上分析,可为预防和控制工后沉降提供参考。     

薄壁空心球节点承载能力的非线性数值分析

现行网架规程中的焊接空心球承载力设计公式是以试验为基础的经验公式 ,对以稳定性控制的球壳 ,难以求得较为精确的屈曲临界点。应用对称旋转薄壳屈曲的非线性有限元理论 ,对空心球节点进行大位移弹塑性屈曲分析 ,得出了可供设计参考的空心球承载力数据表 ,并回归出适用于大直径空心球计算的近似公式 ,以确定空心球节点的失效状态及临界荷载。     

考虑Cowper-Symonds黏塑性材料本构的向量式有限元三角形薄壳单元研究

薄壳结构的力学行为分析往往涉及显著的材料非线性效应,对爆炸、冲击等高速动力问题还需进一步考虑高应变率效应的影响。基于此,将可同时考虑应变硬化和应变率效应的Cowper-Symonds黏塑性材料本构模型（简称C-S模型）引入向量式有限元三角形薄壳单元,以实现金属薄壳结构的动力非线性分析。推导C-S模型的弹塑性增量分析步骤,并将其作为单独的计算分析模块引入薄壳结构的向量式有限元分析程序,适用于线弹性、理想弹塑性、双线性弹塑性和率相关C-S模型等4种材料模型条件下的薄壳结构分析。算例分析表明,所编制的向量式有限元程序可以有效实现非线性材料情况下薄壳结构的静、动力分析以及爆炸冲击作用下的非线性动力响应分析,验证了文中C-S模型理论推导和程序的正确、可靠。     

Long-term deflection of cracked composite beams with nonlinear partial shear interaction: I — Finite element modeling

In this paper, a uniaxial nonlinear finite element procedure for modeling the long-term behavior of composite beams at the serviceability limit state is presented. The finite element procedure follows a displacement-based approach. The nonlinear load-slip relationship of shear connectors as well as the creep, shrinkage, and cracking of concrete slab are accounted for in the proposed finite element procedure. The effects of creep and shrinkage of the concrete slab are considered only for uncracked concrete. The nonlinear iterative procedure adopted for tracking the nonlinear behavior of the composite beam implemented the total nodal deformations, not the incremental deformations, as the independent variables of any iteration. The results of the proposed finite element procedure were compared with the experimental results of four composite beams reported in the literature. The proposed finite element procedure was capable of predicting the deflections and stresses of the four beams with an acceptable degree of accuracy. A parametric study was conducted to study the effect of the nonlinearity of load-slip relationship of shear connectors and the cracking of the concrete deck on the long-term behavior of simply-supported composite beams.     

Creep and time-dependent damage in argillaceous rocks

This paper presents the results of laboratory tests on the time-dependent behaviour of three rocks characterized by a high proportion of clay particles. The viscosity of these sedimentary rocks was studied under different loading conditions in uniaxial compression: static or cyclic creep tests and quasistatic tests (low-loading strain rate) were performed across various orientations of fabric planes.The quasistatic tests showed similarities in the mechanical response of these three argillaceous rocks: a late phase of dilation and a linear development of volumetric deformation before the beginning of unstable crack propagation. The development of secondary and tertiary creep phases during the creep tests highlighted the existence of a deviatoric stress threshold, below which only primary creep is observed. Long-term creep tests also showed that the volumetric variation is not constant during the development of viscoplastic deformations.A microstructural analysis of thin sections extracted from specimens after the tests, gave evidence of cataclastic and granular creep. Damage to the argillaceous matrix occurs and no cracks were observed in the quartz and carbonate grains. This evidence was also demonstrated for tests with loading at a high strain rate.Finally, this study highlights the significant viscoplasticity of argillaceous rocks. The mechanical properties deteriorate rapidly when crack propagation becomes unstable and the viscoplastic strains seem to be due to clay particle slips, known as granular creep.     

沥青混合料的拌和流动特性及和易性指数

为定量评价沥青混合料的拌和流动特性,深化认识混合料强度构成的演变机理,文章利用自行开发的一种变速拌和试验装置,在不同拌和速率（20~50r·min1）和拌和温度（130~180℃）下,对不同沥青种类、不同沥青用量和不同级配类型的沥青混合料进行拌和功率测试,分析拌和流动特性,建立拌和流变模型,定义拌和和易性指数。研究结果表明：拌和试验装置的功率测试误差小于5%,测试数据稳定可靠,可用来表征沥青混合料的拌和功率;拌和温度越高,拌和速率越小,所耗费的拌和功率越少,沥青混合料越容易拌和;沥青混合料的拌和流变模型服从线性的宾汉黏塑性模型,模型直线的斜率表征混合料的拌和黏度,截距反映内在塑限;拌和和易性指数可以定义为模型直线对拌和速率之面积的倒数,从理论上综合考虑黏塑性流动参数;沥青越黏稠,沥青用量越大,集料颗粒越粗,沥青混合料的拌和阻力越大,和易性越差;从强度构成的演变机理来看,集料的组成特性决定内在塑性极限,沥青砂浆的黏性决定黏性强度即牛顿黏性流强度。     

钢板件考虑损伤的循环弹塑性大变形分析

建立结构钢弹塑性各向异性损伤本构关系,采用U.L格式的壳体大挠度双重非线性有限元分析方法,对四边简支矩形薄钢板进行面内拉压循环荷载作用下的滞回性能分析,讨论残余应力、初始挠度、宽厚比对板件循环性能的影响.     

非饱和土的理论土-水特征曲线

基于热力学理论建立了规则等直径圆球土颗粒之间的弯液面方程,运用迭代法对这一高度非线性方程组进行了求解,进而确定了理论意义上的基质吸力。文中考虑了基质吸力的作用面积,提出了等效基质吸力和广义土-水特征曲线的概念。分析表明,饱和度为0时等效基质吸力为0,随着饱和度的增大,等效基质吸力逐渐增大,并在某一饱和度处达到最大,此时土的强度最高。针对等直径土颗粒松散排列方式进行了具体计算与讨论。计算表明,与基质吸力相比,等效基质吸力随饱和度的变化并不十分显著,而无论基质吸力还是等效基质吸力均随颗粒空间排列间隙的增大而迅速减小,由此所得到的两种土-水特征曲线强烈地依赖于接触角的变化。     

Rheological and fractal characteristics of granular sludge in an upflow anaerobic reactor

The rheological and fractal characteristics of the granular sludge in an upflow anaerobic sludge blanket (UASB) reactor were investigated in this study. The influences of sludge concentration and temperature on the rheological properties of the granular sludge were evaluated, and the Bingham model was adopted to describe its rheology. In addition, image analysis was used to determine the sludge fractal dimension. The results indicate that the UASB granular sludge showed a shear-thinning behavior. The relationships between the limiting viscosity and the sludge concentration, as well as the limiting viscosity and temperature could be respectively modeled using an exponential equation and Arrhenius equation well. The Bingham model was able to adequately describe the rheology of the granular sludge. The fractal dimension of the granular sludge, 2.79+/-0.03, was larger than that of some other aggregates, suggesting that the granular sludge were more compact and denser. Furthermore, the relationship between rheological and fractal properties of the granular sludge could be properly described with the model proposed by Shih et al. [1990. Scaling behavior of the elastic properties of colloidal. Phys. Rev. A 42, 4772-4779].     

A force-based model for composite steel-concrete beams with partial interaction

This paper presents a new force-based beam-column element for the nonlinear analysis of composite steel-concrete beams with partial interaction. The element is made up of three components: (a) a fiber beam-column element that models the behavior of the steel girder, (b) a fiber beam-column element that models the behavior of the concrete deck, and (c) a bond element that models the transfer of forces between the steel and concrete elements through shear connectors. The model neglects uplift and frictional effects. The fiber beam-columns are force-based elements that depend on force interpolation functions. A linear bending moment and a constant axial force serve as the interpolation functions. An important factor that favors the use of force-based elements in modeling composite structures is their ability to treat any type of distributed element loads. Distributed element loads are applied internally in a continuous manner by force superposition at the control sections. The state determination of these elements is based on an iterative solution that determines the element resisting forces and stiffness matrix. The bond element is a spring-type element that assumes a linear bond stress variation along the length. The nonlinear behavior of the composite element derives entirely from the constitutive laws of the steel, the concrete and the shear connectors. The paper concludes with a correlation study to investigate the validity of the model. Good agreement between analysis and experimental results was observed.     

非连续变形计算力学模型弹塑性数值分析方法及其应用

在笔者所建立的非连续变形计算力学模型弹性分析方法基础上 ,进一步考虑材料的弹塑性 ,基于分区参变量最小势能变分原理 ,建立了非连续变形计算力学模型弹塑性分析的数学列式和求解方法 ,分析了不同材料特性的基础梁和岩基相互作用问题。计算结果表明 :这种非连续变形计算力学模型弹塑性分析方法将材料的非线性与多体相互作用系统的接触非线性或界面非连续性相耦合 ,能够模拟多体间的非线性相互作用特性 ,是一种处理岩土工程中多体非连续变形体系的新颖而有力的数值计算方法     

交错桁架体系的抗震性能动力分析

对交错桁架体系的抗震性能进行了分析 ,建立了单向水平地震作用下交错桁架体系协同分析的力学模型。在动力时程分析的过程中考虑了构件的剪切变形 ,并在单元水平上考虑了几何非线性。通过交错桁架体系设计实例的弹性和弹塑性时程分析 ,对这种体系的抗震性能做出了评价     

单层网壳结构在地震作用下的弹塑性动力响应研究

采用U L有限元列式 ,基于各向同性强化模式 ,Von Mises屈服准则和Prandtl Renss增量理论 ,针对强烈地震作用 ,考虑结构的几何和材料非线性 ,研究单层网壳在多维地震作用下的弹塑性动力响应计算。提出了一种有效的适用于网壳结构非线性动力响应的增量 /迭代有限元计算方法 ,考虑了转角大位移的精确计算 ,并对一个短程线形单层球面网壳结构进行了地震响应的双重非线性时程分析研究 ,探讨了结构的延性抗震性能     

A viscoplastic constitutive model for hot mix asphalt in compression at high confining pressure

In this study, the experimentally observed rate-dependent viscoplastic hardening–softening of hot mix asphalt (HMA) is modeled using Perzyna’s flow rule and a rate-dependent yield stress function. The yield stress function, which is adopted from the convolution integral in linear viscoelasticity, is applied with a strain hardening rule in order to address both the sudden and gradual changes in rate dependence of the viscoplastic media that are due to repetitive creep and recovery loading. The experimental study is performed to characterize the behavior of HMA and verify the developed model at 55 °C with 500 kPa confining pressure. The developed model shows good agreement with the measured viscoplastic responses of HMA under various load levels and pulse times of repetitive creep and recovery loading histories.     

粘性土的弹粘塑性本构方程及其应用

从常规试验成果出发，用弹粘塑性应力－应变－时间本构方程来描述粘性土的剪胀特性和时间效应。并采用Ｐｅｒｚｙｎａ的粘塑性方程，结合关联和不相关联流动法则，建立应力－应变－时间关系，对粘性土的有效应力路径特别是塑性区的剪胀特性和应力－应变关系的时间依存性进行了数值模拟计算，得出了与试验相近的结果。通过计算结果和试验结果的对比分析，探讨了该计算模型的合理性和可行性。     

澳凼第三大桥斜拉桥的非线性地震反应及TMD初步控制研究

本文结合一座澳门澳第三大桥斜拉桥的工程设计实例 ,采用大型有限元分析软件ANSYS ,选取空间BEAM188和BEAM4单元建立动力计算模型。为了分析斜拉桥的材料非线性地震反应 ,采用转动弹簧COMBIN40单元来模拟桥墩延性塑性铰的非线性性能。用编好的选波程序合理地选取三条地震波 ,在大震情况下沿桥的纵、横向分别输入 ,并把桥的非线性地震反应与线性地震反应作了分析比较。另选取MASS2 1和COMBIN14单元对斜拉桥进行了TMD初步控制研究 ,研究结果表明减震效果明显。     

天然裂隙土坡渐进变形解析

首先介绍了一个结构性土的各向异性粘弹塑性模型,通过分析速度场与粘塑性应变率场的对应关系,建议了一个考虑天然土体结构破损及吸力影响的渐进变形解析模型。该模型能够考虑结构性、各向异性、粘滞性以及结构破损程度、裂隙发育程度(用吸力大小反映)等对边坡变形的影响。基于结构渐进破损变形原理,应用该模型分析天然裂隙土体的渐进变形破坏过程,并结合某土坡实测数据进行了对比分析。     

非定常西原黏弹塑性流变模型的应用研究

 在西原模型的基础上,引入非线性Bingham黏塑性元件与Kelvin元件,建立能完整反映蠕变全过程的非定常西原黏弹塑性流变模型。当岩石处于高应力状态时,其黏塑性流变参数随时间呈非线性变化。根据ABAQUS的UMAT格式要求以及非定常西原黏弹塑性流变模型的平面应变(应力)有限元理论,编制相应的黏弹塑性流变模型程序。数值试验结果表明,所建立的非线性黏弹塑性流变模型可以统一描述不同应力状态下岩石的蠕变过程,具有广泛的适用能力。采用该模型对岩石边坡进行流变数值分析,在较大堆载下,边坡顶点进入加速蠕变阶段,达到破坏状态;中点和底点处于低应力状态,为黏弹性阶段,位移趋于稳定。结果表明该模型是正确和可靠的,具有良好的应用前景。     

岩质边坡弹黏塑性计算参数位移反分析研究

 在引入复变量求导法的前提下,依据弹黏塑性有限元基本原理,采用牛顿–拉普拉斯变换迭代优化法来反算计算参数,运用Fortran语言编制弹黏塑性位移反分析程序,通过2个实例的验证,依据关键测点位移,既可反算强度参数又可反算流变参数,搜索效率高、计算准确,充分说明该方法在岩质边坡的参数反算中的可行性。