Steady-state granular flow in a 3D cylindrical hopper with flat bottom: macroscopic analysis

The information of a hopper flow at a particle scale, obtained from discrete particle simulation, is used to investigate the macroscopic dynamic behaviour of granular flow in a cylindrical hopper with flat bottom by means of an averaging technique. The macroscopic properties including velocity, mass density, stress and couple stress are quantified under the cylindrical coordinate framework, and an effort is made to link these variables to the microscopic variables considered. The velocity and density distributions are first illustrated to match qualitatively the experimental and numerical results, confirming the validity of the proposed averaging method. Four components of stress, T_zz, T_rr, T_rz and T_zr, and two dominant components of couple stress, M_{r θ} and M_{z θ}, are then investigated in detail. It is shown that large vertical normal stress is mainly observed in the region close to the bottom corner, large radial normal stress is observed within the particle bed as well as the bottom corner, and large shear stresses in the region adjacent to the vertical wall. The four stresses are relatively small in a region close to the orifice. Their magnitudes are mainly contributed by the interaction forces between particles and between particles and walls. However, the transport of particles also plays a significant role at the orifice, especially, in the vertical normal stress. The couple stress can be ignored except for the regions close to the vertical and bottom walls, where the most dominant components are M_{r θ} adjacent to the vertical wall and M_{z θ} close to the bottom wall. The magnitudes of these macroscopic variables depend on the geometric and physical parameters of the hopper and particles such as the orifice size and wall roughness of the hopper, and the friction and damping coefficients between particles although their spatial distributions are similar.     

Future continuum models for granular materials in penetration analyses

This paper presents an investigation on future continuum models for granular materials in penetration analyses. A two-dimensional discrete element method has been used to numerically simulate penetration tests on a granular ground. The stress paths of soil elements in the ground have been studied, and then used to highlight the main features of granular materials based on most-advanced knowledge in soil mechanics. The study shows that the penetration makes the soil near the penetrometer undergo a significant changes of stresses in both magnitude and direction. The soil of large deformation rate may arrive at a stress state slightly over the strength envelope obtained from conventional tests. As a result, shear dilatancy, rate-dependency, non-coaxiality and particle crushing are the four main features that future continuum models should capture for granular materials in penetration analyses.     

Role of porosity in controlling the mechanical and impact behaviours of cement-based materials

This work deals with the influence of porosity on the tensile, the compressive and the impact behaviours of two fine cementitious mortars—one with silica fume and one without. The addition of silica fume is shown to change the pore size distribution. The mix without silica fume is characterized by porosity at the scale of the grains of fine sand (approximately 100 μm), while silica fume addition results in a more porous matrix with pore sizes of millimetre-length size. The mortar with silica fume shows a higher quasi-static compressive and flexural strength whereas the mix without silica fume is observed to be less compressible (by irreversible reduction of volume) under heavy confinement pressure (quasi-oedometric tests) and shows better ballistic performance. A numerical simulation of the impact tests employing the Krieg, Swenson and Taylor model, which accounts for both deviatoric and volumetric inelastic behaviour of the material, was undertaken using the data from quasi-oedometric tests. These calculations follow the experimental results and confirm the influence of the macroscopic porosity on the impact performance of cement-based materials.     

芳纶纤维布约束混凝土轴心受压性能试验研究

在分析芳纶纤维布（KFF）对混凝土的增强作用机理的基础上,通过C20、C30、C35、C45、C50五个强度等级的普通混凝土φ150mm×300mm的圆柱体试件和用KFF约束混凝土试件的抗压强度对比试验,研究了KFF约束混凝土的轴心受压力学性能,定量地得到了KFF对混凝土试件受压承栽力的加强效果、强度比和混凝土自身强度、KFF粘贴层数的关系。研究结果表明：KFF对混凝土受压承载力的提高效果非常明显,在外粘1层KFF的情况下,KFF和普通混凝土轴心受压强度比在19．0～54．8％范围内,粘贴2、3层KFF和混凝土轴心受压强度比达51．9％、73．5％。KFF的加强效果与混凝土自身的强度、KFF粘贴层数具有良好的相关性,强度比随着混凝土自身强度等级的提高而减小,随着KFF粘贴层数的增加而增大,从而验证了KFF约束加固混凝土的有效性和可行性。     

Effect of omitting terms involving tube radii difference in shell models on buckling solutions of DWNTs

Continuum cylindrical shell models have been widely applied in the buckling analysis of carbon nanotubes. An explicit expression for the critical buckling strain of double-walled carbon nanotubes (DWNTs) may be obtained based on cylindrical shell models. The expression is usually simplified by neglecting the terms involving outer and inner tube radii difference. In this brief note, we present the critical buckling strains of DWNTs with the inclusion of these terms and investigate the quantitative effect of neglecting these terms on the critical strain. It was found that the omission of the terms related to outer and inner tube radii difference leads to an overprediction of the critical buckling strain as well as a possible change in the buckling mode shape. It is also observed that the effect of the terms is especially significant for DWNTs with small inner radius but is negligible when the inner radius is relatively large.     

Prediction of plastic strain accumulation in continuous fiber reinforced laminates by a constitutive ply model

The present paper is concerned with the prediction of plastic ply strains which accumulate in continuous fiber reinforced laminates with polymeric matrix materials. The study is based on a constitutive model which is implemented within the Finite Element Method. Plastic strains are expected to evolve when the ply is subjected to a pronounced shear load and/or to pronounced transverse compression. Two plasticity mechanisms are modeled at ply level under plane stress assumption (i.e. for thin shells and plates). They either concern the evolution of plastic shear strains or the evolution of plastic normal strains. The proposed plasticity model is combined with an existing ply level continuum damage model. The capabilities of the proposed model are assessed by comparing its predictions to experimental data from literature. Emphasis is placed on loading conditions which drive the evolution of plastic strains. Excellent correlation with experimental results is shown for proportional as well as for non-proportional ply loadings.     

DEM simulation and experimental validation for mechanical response of ellipsoidal particles under confined compression

The representation of non-spherical particles in discrete element method (DEM) has not been addressed adequately. Although the multiple sphere method (MSM) is the most popular approach to describe non-spherical particle shape, the validity of the MSM has not been established yet. The purpose of this study is to examine the validity and adequacy of the MSM. A uni-axial confined compression test was designed and set up to study the mechanical behaviour of an ellipsoidal granular assembly under vertical loading and the load transfer to the contacting boundary. Four levels of multi-sphere approximation for an axi-symmetric ellipsoidal particle were employed in DEM simulation to investigate the adequacy of multi-sphere approximation. A comparison on compression characteristics between the numerical and experimental results was made and discussed in this paper. Most of the compared physical properties showed reasonable agreement, indicating that capturing the key linear dimensions of a non-spherical particle may be sufficient to predict reasonable results. A small number of sub-spheres (say, N?≥?5) for representing an axi-symmetric ellipsoidal particle can give plausible results. However, the DEM simulations also produced a certain extent of discrepancy in loading stiffness with experiments. Plausible explanations are provided and require further investigation.     

基于有限元法的果蔬保鲜包装箱通风孔设计

通过建立不同通风孔设计的果蔬保鲜包装箱有限元模型,模拟分析了其应力分布云图,探讨了果蔬保鲜包装箱通风孔设计的主要参数。分析结果表明,长圆形、靠近纸箱中部且对称分布的偶数个通风孔设计,能有效减少应力集中,避免纸箱压缩强度的大幅下降。为实际应用中果蔬保鲜包装箱通风孔结构设计提供参考。     

单桩在压与拔荷载下桩侧摩阻力的有限元计算研究

通过有限元分析,对垂直受力桩在桩顶压、桩顶拔及桩底托等加载方式下的桩侧摩阻力进行了探讨研究。重点研究在不同加载条件下,桩侧摩阻力影响因素。研究结果表明：桩周土应力路径、主应力方向的旋转、桩身本身的压缩和伸长、桩径的膨胀和收缩是不同加载方式下桩侧摩阻力发展和分布有所差异的主要影响因素。     

Effect of punch and specimen dimensions on the confined compression behavior of S-2 glass/epoxy composites

The behavior of composite materials under transverse loading is often characterized using quasi-static experimental methods. A complete characterization requires experimentation for a range of specimen and impactor dimensions, necessitating a sizeable test matrix. However, if dimensional effects on specimen damage behavior can be understood, the number of tests required can be reduced. The present study considers confined compression testing of an S-2 glass/SC15 resin composite under quasi-static loading to understand damage initiation and propagation during penetration. A test matrix of 29 punch–specimen combinations was tested to determine the fracture behavior and applied load at failure. A finite element (FE) model of the experiments was also created to study the internal stress distributions. Characteristic fracture angles and applied stresses at failure were observed to be independent of punch–specimen dimensions. However, varying the dimensions of the punch and specimen was found to change the behavior of the internal stresses. Various failure criteria were evaluated using the numerical stress distributions to predict damage initiation. A Mohr–Coulomb compression–shear interaction on the fracture plane was found to best explain the observed experimental behavior.     

Determination of inelastic heat fraction of OFHC copper through dynamic compression

An approach to determine the inelastic heat fraction (IHF) value of metal by high-speed compression is established by combining dynamic deformation, infrared (IR) photography and finite element simulation. OFHC copper specimens are dynamically compressed and infrared thermographs captured at a rate of 1000 images/s. FEM simulation of the deformation is undertaken and the initial IHF value input adjusted until the computed average surface temperature matches the experimental data. It is found that for the IHF value identified, the predicted surface temperature distribution also exhibits good correlation with experimental results. For final strains in the range of 44–60%, a consistent IHF value of 0.68 is obtained. Using this value, the surface temperature of a sample deformed to a different final strain and at a different strain rate is predicted by FEM simulation and the numerical results show good agreement with test data in terms of average surface temperature and surface temperature distribution. The temperature field for the entire specimen is also predicted. Results indicate that high-speed compression at a strain rate of 1000/s to a final engineering strain of 70% may result in initiation of dynamic recovery in OFHC copper.     

Simulation of microstructural evolution in materials with misfitting precipitates

To predict the behavior of directional coarsening and the temporal evolution of the shape of coherent precipitates in two-phase materials, a dislocation-free model is proposed, based on a combination of statistical mechanics and linear elasticity. This model takes elastic anisotropy and isotropic interfacial energy into account. Based on an example of isolated precipitates under plane strain condition, the influence of particle size, inhomogeneity, direction and sign of external loads on the equilibrium shape will be discussed in terms of a generalized thermodynamic force acting on the interface. To simulate the morphological diffusion process of typical microstructures with several random distributed misfitting inclusions, a computational technique in form of a finite element Monte Carlo simulation is presented. Within this numerical technique, no restrictions on the particle shape or the elastic anisotropy of both phases are made.     

结构在浅水中的振动和声辐射特性研究

建立了浅水域声学边界元方程和相应的有限元／边界元流固耦合振动方程,探讨了水深对结构振动固有频率、振动响应和声辐射的影响。研究结果表明：水深变化对结构固有频率有影响,随着水深减小,结构固有频率降低,特别是当水深与结构浸深接近时,各阶固有频率大大降低,且振型顺序也可能发生变化;水深变化对结构振动响应和声辐射均有影响,其中对声辐射特性的影响较为复杂,即使水深超过结构浸深的10倍,水深变化也会影响到结构的声辐射功率级。     

MODE JUMPING AND ATTRACTIVE PATHS IN MULTIMODE ELASTIC BUCKLING

This paper summarizes a part of the first author's Ph.D. Thesis completely devoted to multimode elastic buckling within an FEM strategy. The theoretical arguments unfold among critical points on radial paths (the unique post-critical paths variationally defined), algebraic characterizations, proposition demonstrations and so on, by aiming to prove that the complexity of the phenomenon of multimode buckling (secondary bifurcations, post-critical attractive paths) can be theoretically explained. © 1997 by John Wiley & Sons, Ltd.     

高性能纤维束纤维尺度横向压缩数值模拟

为深入研究高性能纤维束纤维尺度横向压缩过程及变形机制,基于ANSYS有限元数值分析平台,采用APDL编程语言,在纤维尺度建立了高性能纤维束具有统计意义的三维数值生长模型。该模型考虑了纤维束内部纤维的随机分布与偏转及纤维之间在三维空间不产生相互贯穿的物理特性。利用数值模拟方法研究了纤维束在横向压缩时的变形情况,不仅获得了与纤维束压缩实验一致的非线性变形过程,而且得到了压力与纤维体积含量的定量关系,与经典的理论预测进行对比,获得了较好的支持。     

数值模拟人内耳半规管膜迷路的机械压痕实验

该实验数值模拟机械压痕实验,研究机械压痕激励下人半规管膜迷路壶腹嵴顶的位移响应,探求半规管结构和平衡机理的关系。基于文献实验结果数据,建立人半规管膜迷路的三维弹性流体动力学模型,采用液固耦合方法数值模拟机械压痕实验过程。研究结果表明：低频正弦压痕载荷时(<10/Hz),半规管整合激励产生的嵴顶位移响应与压痕峰值线性相关,相位相同。低频范围内的正弦压痕载荷,频率越低,水平半规管嵴顶位移峰值衰减速度越快,衰减比例大,趋于稳定时间越长。该实验建立了有效的半规管膜迷路液固耦合模型,描述了机械压痕激励与旋转激励的定量关系,为深入研究前庭力学及理解平衡机理奠定一定的基础。