Contact laws between solid particles

This paper presents a comprehensive study for the contact laws between solid particles taking into account the effects of plasticity, strain hardening and very large deformation. The study takes advantage of the development of a so-called material point method (MPM) which requires neither remeshing for large deformation problems, nor iterative schemes to satisfy the contact boundary conditions. The numerical results show that the contact law is sensitive to impact velocity and material properties. The contact laws currently used in the discrete element simulations often ignore these factors and are therefore over-simplistic. For spherical particles made of elastic perfectly plastic material, the study shows that the contact law can be fully determined by knowing the relative impact velocity and the ratio between the effective elastic modulus and yield stress. For particles with strain hardening, the study shows that it is difficult to develop an analytical contact law. The same difficulty exists when dealing with particles of irregular shapes or made of heterogeneous materials. The problem can be overcome by using numerical contact laws which can be easily obtained using the material point method.     

Meshless meso-modeling of masonry in the computational homogenization framework

In the present study a multi-scale computational strategy for the analysis of structures made-up of masonry material is presented. The structural macroscopic behavior is obtained making use of the Computational Homogenization (CH) technique based on the solution of the Boundary Value Problem (BVP) of a detailed Unit Cell (UC) chosen at the mesoscale and representative of the heterogeneous material. The attention is focused on those materials that can be regarded as an assembly of units interfaced by adhesive/cohesive joints. Therefore, the smallest UC is composed by the aggregate and the surrounding joints, the former assumed to behave elastically while the latter show an elastoplastic softening response. The governing equations at the macroscopic level are formulated in the framework of Finite Element Method (FEM) while the Meshless Method (MM) is adopted to solve the BVP at the mesoscopic level. The material tangent stiffness matrix is evaluated at both the mesoscale and macroscale levels for any quadrature point. Macroscopic localization of plastic bands is obtained performing a spectral analysis of the tangent stiffness matrix. Localized plastic bands are embedded into the quadrature points area of the macroscopic finite elements. In order to validate the proposed CH strategy, numerical examples relative to running bond masonry specimens are developed.     

A microstructure-based viscoplastic model for asphalt concrete

A microstructure-based viscoplastic continuum model is developed for the permanent deformation of asphalt concrete (AC). The model accounts for several phenomena that influence the permanent deformation of AC at high temperatures. These phenomena include strain rate dependency, confining pressure dependency, dilation, aggregate friction, anisotropy, and damage.The material anisotropy was included in the model by replacing the stress invariants in the yield function with invariants of both the stress and a microstructure tensor, which describes the aggregate orientation distribution. The components of the microstructure tensor were determined using image analysis techniques (IAT) conducted on digital images taken from two-dimensional cut sections of AC. Furthermore, damage was included in the model based on the effective stress theory to account for crack and air void growth that significantly reduces the load-carrying capacity of the material.Experimental data from triaxial compressive strength tests conducted at five strain rates and three confining pressures were used to develop a methodology to determine the material parameters that characterize AC permanent deformation. The model predictions were in a good agreement with the experimental measurements.     

Calculations of fracture process zones on meso-scale in notched concrete beams subjected to three-point bending

The paper describes investigations on fracture process zones (FPZ) at meso-scale in notched concrete beams subjected to quasi-static three-point bending. The simulations were carried out with the FEM using isotropic damage constitutive model enhanced by a characteristic length of micro-structure by means of a non-local theory. Concrete was modelled as a random heterogeneous three-phase material. The effect of the beam size, aggregate distribution, aggregate density, aggregate shape, aggregate size and characteristic length on the width and shape of FPZ and load-displacement curve was numerically investigated. The numerical results were compared with own test results using Digital Image Correlation method (Skar?yński et al., 2009a), the tests by Le Bellěgo et al., (2003) and the size effect law by Ba?ant (2004).     

An extended micromechanics method for probing interphase properties in polymer nanocomposites

Inclusions comprised on filler particles and interphase regions commonly form complex morphologies in polymer nanocomposites. Addressing these morphologies as systems of overlapping simple shapes allows for the study of dilute particles, clustered particles, and interacting interphases all in one general modeling framework. To account for the material properties in these overlapping geometries, weighted-mean and additive overlapping conditions are introduced and the corresponding inclusion-wise integral equations are formulated. An extended micromechanics method based on these overlapping conditions for linear elastic and viscoelastic heterogeneous material is then developed. An important feature of the proposed approach is that the effect of both the geometric overlapping (clustered particles) and physical overlapping (interacting interphases) on the effective properties can be distinguished. We apply the extended micromechanics method to a viscoelastic polymer nanocomposite with interphase regions, and estimate the properties and thickness of the interphase region based on experimental data for carbon-black filled styrene butadiene rubbers.     

Modelling of compressive behaviour of concrete-like materials at high strain rate

Uniaxial compression tests are the most common tests for characterizing the strength of concrete-like materials. The dynamic compression strength of concrete-like material is typically obtained by Split Hopkinson Pressure Bar (SHPB) tests. The increase in material strength under dynamic loading is usually attributed to the strain rate effect and modelled with a dynamic increase factor (DIF). However, it was observed by some researchers that the radial inertial confinement caused apparent increase of dynamic strength of concrete-like specimen in SHPB tests. They attributed the material strength increase to this inertial effect, instead of the strain rate effect. In the present study, numerical analyses are performed to investigate the compressive behaviour of concrete-like material at high strain rates. A homogeneous macroscale model and a heterogeneous mesoscale model are developed in the study. In the macroscale model, the material is assumed to be homogeneous and isotropic. In the mesoscale model, the test sample is modelled as a three-phase composite consisting of aggregate, mortar matrix and interfacial transaction zone (ITZ) between the aggregate and the mortar matrix. The aggregate is assumed to be circular and the ITZ is modelled as a thin boundary around the aggregate. In the both models, the materials are assumed to be insensitive to the strain rate first. Therefore, the obtained strength enhancement is only due to the inertial confinement. Strain rate sensitive material properties are then used in the two models in the calculations. Numerical simulations of the concrete samples under compression at different strain rates are carried out. The relative contribution of the inertial effect and the strain rate effect on the compressive strength DIF is examined based on the numerical results. The failure process of concrete specimen is also studied.     

Preparation,characterization and combustion properties of Zr/ZrH_2 particles coated with α-FeOOH crystal grains

Zr/ZrH2 particles with irregular morphologies and broad size distribution were uniformly coated with acicular-FeOOH crystal grains via a facile route without using polymers or surfactants. The as-synthesized material was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), UV-vis diffusion reflection (UV-vis) and Raman spectrometry. Based on these characterizations, the synthesis mechanism was explained in terms of combined heterogeneous nucleation and...     

混凝土单轴压缩下细观损伤特性的CT研究

混凝土是一种非均质的材料,在细观层次上将混凝土看作由骨料、砂浆和两者之间的界面组成的三相复合材料.本文利用CT技术对混凝土的细观损伤过程进行实时扫描观测,获得了混凝土试件实时静力压缩CT图像,提取出图像上各点的CT数并根据分区理论定义了一个基于CT数的统计损伤变量.通过对图像和CT数以及损伤变量的分析表明混凝土试件在静力压缩条件下经历了压密阶段、扩容阶段,损伤急剧增大到破坏的细观损伤演化全过程,以此反映了混凝土细观损伤演化的特性.进而通过细观损伤对混凝土材料裂纹的扩展、贯通以及最后失稳破坏的破坏实质进行了有益的探索.     

单轴压缩条件下混凝土细观损伤演化机理的CT试验研究

混凝土是一种非均质的材料,在细观层次上将混凝土看作由骨料、砂浆和两者之间的界面组成的三相复合材料.本文在对混凝土进行单轴压缩试验的基础上,根据CT扫描图像反映的细观破损过程,分析了基于CT数平均值变化规律的特点;将混凝土材料的损伤过程进行了分段,提出了损伤变量的提取方法.随后结合试验得到的宏观应力应变曲线,经过拟合得到...     

Damage initiation and growth in metals. Comparison between modelling and tomography experiments

Damage in heterogeneous model materials was measured using high-resolution X-ray absorption tomography. The material consisted of an aluminium matrix containing 1% and 4% of spherical ceramic particles acting as nucleation sites for an interface decohesion mechanism of damage. The damage initiation stage was quantified using the global population of particles in the 4% material. A strain path change experiment was then applied to the 1% material. The sample was first deformed in tension in order to create elongated cavities and then compressed at 45^° to rotate and close these cavities. The results of a model based on the Rice and Tracey approach accounting for the presence of particles inside the cavities and calculating their rotation with assuming a linear hardening plastic behaviour of the matrix were compared with the observations. The model was modified to account for the damage initiation phase. It was shown to give a good global prediction of the void volume fraction provided that the physical, mechanical and morphological information are corresponding in the experimental and the model cases. The cavity rotation experiment was also shown to compare well with the calculation although only one cavity was sufficiently opened after compression to allow the comparison.     

Discrete fracture modeling of asphalt concrete

A heterogeneous fracture approach is presented for modeling asphalt concrete that is composed of solid inclusions and a viscous matrix, and is subjected to mode-I loading in the fracture test configuration. A heterogeneous fracture model, based on the discrete element method (DEM), is developed to investigate various fracture toughening mechanisms of asphalt materials using a high-resolution image processing technique. An energy-based bilinear cohesive zone model is used to model the crack initiation and propagation of materials, and is implemented as a user-defined model within the discrete element method. Experimental fracture tests are performed to investigate various fracture behavior of asphalt concrete and obtain material input parameters for numerical models. Also, bulk material properties are necessary for each material phase for heterogeneous numerical models; these properties are determined by uniaxial complex modulus tests and indirect tensile strength tests. The main objective of this study is to integrate the experimental tests and numerical models in order to better understand the fracture mechanisms of asphaltic heterogeneous materials. Experimental results and numerical simulations are compared at different test conditions with excellent agreement. The heterogeneous DEM fracture modeling approach has the potential capability to understand various crack mechanisms of quasi-brittle materials.     

DRAG ON SUBMICRON NANOPARTICLE AGGREGATES

A new procedure was developed for estimating the effective collision diameter of an aggregate composed of primary particles of any size. The coagulation coefficient of two oppositely charged particles was measured experimentally and compared with classic Fuchs theory, including a new method to account for particle non-sphericity. A second set of experiments were performed on well-defined nanoparticle aggregates at different stages of sintering, i.e. from the aggregate to the fully sintered stage. Here, electrical mobility was used to characterize the particle drag. The aggregates are being built from two different size-fractionated nanoparticle aerosols, the non-aggregated particles are discarded by an electrofilter and then they are passed through a furnace at concentrations low enough not to induce coagulation.     

MORPHOLOGY OF BLACK CARBON AEROSOLS AND UBIQUITY OF 50-NANOMETER BLACK CARBON AEROSOLS IN THE ATMOSPHERE

Different-sized aerosols were collected by an Andersen air sampler to observe the detailed morphology of the black carbon （BC） aerosols which were separated chemically from the other accompanying aerosols, using a Scanning Electron Microscope equipped with an Energy Dispersive X-ray Spectrometer （SEM-EDX）. The results indicate that most BC aerosols are spherical particles of about 50 nm in diameter and with a homogeneous surface. Results also show that these particles aggregate with other aerosols or with themselves to form larger agglomerates in the micrometer range. The shape of these 50-nm BC spherical particles was found to be very similar to that of BC particles released from petroleum-powered vehicular internal combustion engines. These spherical BC particles were shown to be different from the previously reported fullerenes found using Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight Mass Spectrometry （MALDI-TOF-MS）.     

Direct numerical simulations of gas–solid–liquid interactions in dilute fluids

The dynamic interactions between gas bubbles, rigid particles and liquid can lead to profound nonlinearities in the aggregate behavior of a multiphase fluid. Predicting the nonlinear dynamics of the multiphase mixture hence requires understanding how the phases interact at the scale of individual interfaces, but these interactions are notoriously difficult to resolve in models. The goal of this paper is to develop and validate a computational method capable of capturing the complex flow interactions between gas bubbles and rigid particles immersed in a Newtonian liquid. We focus on multiphase systems that are dilute enough for the solid and gas components to move through and be moved by the ambient liquid. We use level sets with a topology-preserving advection scheme to track the gas interfaces. To include the motion of the rigid particles, we couple distributed Lagrange multipliers to an immersed-boundary method. The high viscosity contrast between the liquid and the gas requires both time splitting and approximate factorization to efficiently solve the governing equations consisting of the conservation of mass, momentum and energy. To resolve interactions between interfaces that vary drastically in size, we refine our mesh adaptively in the vicinity of the boundary.     

混凝土多尺度应力响应方程及其数值模拟

针对混凝土的多相多尺度材料组成特征及其复杂力学响应问题, 首先, 根据混凝土中各组成材料的几何特征, 将C-S-H凝胶、硬化水泥浆体、砂浆及混凝土细观组成分别视为纳观、微观、亚细观和细观尺度上的复合材料, 并利用颗粒空间堆积方法, 重构了混凝土各尺度复合材料的简化几何模型; 其次, 基于重构的几何模型和等效夹杂理论, 通过等效刚度的升阶计算和应力响应的降阶计算, 建立各尺度复合材料应力响应之间的过渡关系, 推导混凝土多尺度应力响应方程, 并编制相应的计算程序; 最后, 以单轴压缩载荷作用为例, 数值计算载荷作用下混凝土各尺度复合材料中的应力响应, 分析骨料空间位置和相互作用以及水化产物刚度、几何形状和空间取向对其应力响应的影响规律. 结果表明, 单轴压缩载荷作用下, 混凝土细观组成中的应力分布并不均匀; 骨料颗粒之间的距离影响到混凝土中的应力分布, 其有效影响范围约为骨料粒径的6倍; 水泥水化产物的刚度、几何形状和空间取向是影响其应力分布的重要因素, 刚度越大, 所受应力越大, 与载荷作用方向的夹角越小, 长椭球形水化产物沿载荷作用方向的应力越大, 扁椭球形水化产物与之相反.      

骨料堆积注浆再生混凝土的抗压强度及其影响因素

为提高再生粗骨料利用率及降低再生混凝土质量控制难度,本文采用骨料堆积注浆法制备早强型再生混凝土。利用Griffith断裂力学理论及"实际水料比与名义水料比"的差异,分析了骨料性能对混凝土强度的影响,并通过扫描电子显微镜(SEM)分析了再生混凝土界面过渡区(interfacial transition zone,ITZ)的特点。结果表明,骨料强度及吸水率均对再生混凝土强度具有影响,且骨料吸水作用对混凝土强度提高具有促进作用,再生混凝土的界面结合强度较高。     

Field fluctuations in multicomponent mixtures

Fluctuations of electrostatic and elastostatic fields in a random phase mixture may be characterized by mean values and square means of the fields in each component. Exact relations between the square means and the analytical properties of the effective moduli are established for isotropic mixtures. Moreover, a modified effective medium procedure for calculating the field fluctuations in mixtures with aggregate topology is proposed. Explicit results are given for mixtures of isotropic components and spherical grain shapes. Particularly strong fluctuations occur in strongly heterogeneous media near the percolation threshold.     

混凝土细观随机骨料结构与有限元网格剖分

在细观层次上,混凝土被认为是一种由粗骨料、水泥砂浆及二者间的粘结带所组成三相非均质复合材料。本文首先基于蒙特卡罗随机抽样原理,用“取和放”方法在计算机上产生形状、尺寸和骨料颗粒分布与真实混凝土相似的随机骨料结构,再使用有限元分析软件ANSYS对骨料区域及砂浆区域分别划分网格,并编程在骨料和砂浆之间生成三角形三结点可控制厚度粘结单元,从而使三相网格缝合为一个整体,为混凝土非线性有限元分析提供可靠的细观计算模型。最后利用建立的模型进行混凝土轴心受拉和轴心受压的仿真模拟,在细观层次研究的基础上揭示出混凝土的宏观力学性能。