基于随机多尺度力学模型的混凝土力学特性研究

提出一种混凝土材料宏观力学特性分析的新方法--随机多尺度力学模型分析方法.从描述混凝土材料的细观尺度入手,首先采用Monte Carlo法生成由骨料颗粒及砂浆基质组成的混凝土试件的随机骨料模型;然后采用材料特征单元尺度法来剖分有限元网格并投影到建立的随机骨料模型上,各单元网格的力学特性则采用复合材料等效化方法来确定.通...     

全级配混凝土三维细观模型的建模方法研究

根据混凝土材料细观结构特点,采用混合同余算法生成随机数,模拟混凝土骨料在形状和空间分布方面具有的随机性特点。按照混凝土级配理论,分析了不同级配混凝土细观组成特点,研究了不同粒径骨料的含量和分布规律,提出了一种高效的三维骨料生长和凸性判定算法,研究了骨料生长过程中的形状控制技术,避免了尖角、薄片状骨料的出现;在骨料投放过程中采用综合投放算法,大幅度提高了骨料一次性投放成功率和骨料体积含量,缩短了计算时间。在三维细观有限元网格剖分方面,采用向量判别法,优化了材料属性判定,提高了计算效率。算例表明,该文提出的细观模型,在模拟混凝土静力特性和冲击破坏特性方面具有较高的可靠性。     

粗骨料种类对刚性弹贯穿混凝土靶剩余速度的影响

针对弹体贯穿混凝土靶问题,混凝土细观尺度模型能够反映混凝土中各介质对混凝土力学行为的影响。本研究编写了三维随机球形粗骨料的生成和投放程序,并采用一种基于背景网格的材料识别技术建立了含有水泥砂浆和粗骨料两种网格的三维混凝土细观有限元模型,同时将界面过渡区简化为这两种网格之间的一种可失效的接触。基于这一模型,运用连续有限元软件对刚性弹贯穿混凝土过程进行了数值仿真,并分析了三个不同种类的粗骨料对剩余速度的影响。结果表明,粗骨料的强度和密度分别在较低和较高的着靶速度下对剩余速度有一定影响。     

闭孔泡沫金属三维细观模型建模方法

该文在深入分析闭孔泡沫金属CT扫描图像的基础上,根据其细观结构特点,提出了泡沫金属三维细观模型建模方法。首先,根据闭孔泡沫金属胞孔形状和尺寸分布特性,提出了采用随机椭球体模拟胞孔的建模方法,通过采用随机投放算法,建立了三维胞孔随机投放模型;其次,提出了有限元网格剖分算法,通过引入材料属性识别算法,建立了泡沫金属三维有限元细观模型。在此基础上,研究了冲击荷载下泡沫金属的力学性能,分析了细观损伤破坏机理和能量吸收特性。结果表明,该文建立的三维细观模型,能够较好地反映泡沫金属材料的力学性能和细观损伤破坏机理。     

基于刚体─弹簧模型的混凝土微裂纹行为模拟

本文在细观上将混凝土看成是由粗骨料颗粒与硬化水泥浆体组成的两相非均质复合材料,以随机颗粒模型代表混凝土的细观结构,研究了一种对随机颗粒模型结构进行全自动网格剖分的方法,采用适宜处理微裂纹行为的刚体一弹簧模型（RigidBodySpringModel）,探讨了预先存在的微裂纹对混凝土性能的影响,建立起混凝土的细观结构与宏观性能之间的关系,同时也对混凝土在拉伸荷载下的裂纹传播行为进行了研究。     

三维德洛内三角剖分算法探析

三维德洛内三角剖分算法是一种新颖的适用于实时有限元分析的四面体网格生成算法,该算法在并行插入与删除网格节点的前提下,能够最大限度的保证网格生成的质量和保真度。     

工程中有限元网格生成技术的研究和应用

有限元网格生成是工程科学与计算科学相交叉的一个重要研究领域,文中简要综述了二维、三维网格生成算法。通过工程实例分析了目前工程中有限元网格生成技术应用情况,讨论了土木、水利工程中网格结点重复编号、施工过程中结点优化、随机投放骨料等特殊问题的网格特点及其处理方法。文中给出了较多的工程有限元网格实例以供借鉴。     

C/C-SiC缎纹编织复合材料孔隙缺陷的建模及其拉伸性能仿真

主要研究了随机孔隙缺陷在C/C-SiC缎纹编织复合材料中的有限元建模方法及其对拉伸性能的影响。基于C/C-SiC缎纹编织复合材料的细观结构和实验观察所得的微观形貌,得出孔隙缺陷具有随机分布特征,提出了一种三维随机碰撞算法模拟孔隙在复合材料中的分布,建立了含随机孔隙缺陷的C/C-SiC缎纹编织复合材料的有限元模型。采用有限元软件ABAQUS模拟了其在拉伸载荷下的力学行为,讨论了孔隙缺陷的尺寸和分布形式对材料拉伸性能的影响,并对试样进行了单轴拉伸实验测试,验证了数值模拟的有效性。结果表明,用本文方法建立的有限元模型符合含孔隙缺陷C/C-SiC缎纹编织复合材料的真实细观结构,相应的数值模拟结果也与试验数据吻合较好。本文的研究结果为含孔隙缺陷的缎纹编织复合材料及具有相似结构特征的复合材料的力学分析与优化设计提供了一种有效的方法。      

基于Al/PTFE真实细观特性统计模型的宏观力学性能模拟

通过细观力学有限元方法对Al颗粒增强聚四氟乙烯（Al/PTFE）复合材料的宏观力学性能进行了研究,基于Al颗粒粒径分布统计规律及通过SEM对该复合材料孔洞含量的统计分析,在细观层次上建立了考虑颗粒和孔洞位置和尺寸分布的随机代表性体积单元（RVE）,借助ABAQUS有限元软件,对二维平面应变和三维实体模型在单轴压缩载荷作用下的力学行为进行了数值仿真;并施加了周期性边界条件（PBC）以提高数值计算结果的准确性。此外,分析了不同颗粒含量下复合材料准静态应力-应变关系,重点对细观有限元模型与准静态压缩试验结果进行了对比分析。研究结果表明:二维和三维2种模型均能较好地模拟Al/PTFE的宏观力学性能,且二维模型较三维模型计算效率更高,并进一步证实了PBC的正确性和有效性。      

面向大型复杂焊接结构仿真的组合式自适应四面体网格生成算法

目的 研究高质量、高效率的网格生成技术以实现大型复杂结构的焊接工艺仿真优化。方法 提出一种组合式的自适应四面体网格划分算法,即在高效生成各个零部件四面体网格的基础上,根据焊缝中心面的几何信息自动对焊缝附近网格进行细分,再缝合成高质量的大型复杂焊接结构的整体四面体网格,并集成到自主可控的商用网格划分软件Vision Mesh中。提出了摄动几何边界的方法,解决了大型复杂结构STL几何体在存在几何错误时网格难以生成的问题。提出了基于BVH树结构表达的背景网格表达方法,解决了多条焊缝同时高效、自动细分的难题,并通过“四面体分割–四面体合并–四面体翻转–点平滑优化”方法,实现了四面体网格的高质量优化。结果 算法网格效率可以达到200万个/h,生成的四面体99%以上均接近正四面体。可以由多个零部件一步组合生成大型结构的整体网格,并可对焊缝区域进行自动细分,大幅度简化了划分流程。将生成的网格导入国产焊接仿真软件InteWeld中进行测试,验证算法可用于大型复杂焊接结构整体应力变形的计算中。结论 实现了大型复杂焊接结构的高质量自适应四面体网格划分,使用简便操作得到了高质量网格,为焊接结构件工艺仿真优化...     

A two-level mesh repartitioning scheme for the displacement-based lower-order finite element methods in volumetric locking-free analyses

We present an approach for repartitioning existing lower-order finite element mesh based on quadrilateral or triangular elements for the linear and nonlinear volumetric locking-free analysis. This approach contains two levels of mesh repartitioning. The first-level mesh re-partitioning is an h-adaptive mesh refinement for the generation of a refined mesh needed in the second-level mesh coarsening. The second-level mesh coarsening involves a gradient smoothing scheme performed on each pair of adjacent elements selected based on the first-level refined mesh. With the repartitioned mesh and smoothed gradient, the equivalence between the mixed finite element formulation and the displacement-based finite element formulation is established. The extension to nonlinear finite element formulation is also considered. Several linear and non-linear numerical benchmarks are solved and numerical inf-sup tests are conducted to demonstrate the accuracy and stability of the proposed formulation in the nearly incompressible applications.     

Mechanical and geometrical approaches applied to composite fabric forming

In this paper, we are interested in the forming of composite fabric by deep-drawing. Two approaches (geometrical and mechanical) are proposed for the simulation of the composite fabric forming. The geometrical approach is based on a fishnet model. It is well adapted to preliminary design phase and to give a suitable estimate of the resulting flat patterns. The mechanical approach is based on a meso-structural approach. It allows us to take into account the mechanical properties of composite fabric (fibres and resin) and the various dominant modes of deformation of fabrics during the forming process. During simulation of composite fabric forming, where large displacement and relative rotation of fibres are possible, severe mesh distortions occur after a few incremental steps. Hence an automatic mesh generation with remeshing capabilities is essential to carry out the finite element analysis. Some numerical simulations of forming process are proposed and compared with the experimental results in order to demonstrate the efficiency of the proposed approaches.     

Mechanical analysis of 3-D braided composites by the finite multiphase element method

A finite multiphase element method (FMEM), in which the element comprises more than one kind of material, has been proposed to predict the effective elastic properties of 3-D braided composites. This method is based on the variational principle and our previous geometric model that assumes the existence of different types of unit cells in the three regions in a 3-D braided composite, i.e. the interior, surface and corner. The numerical procedure involved two steps. First, a fine local mesh at the unit cell level is used to analyze the stress/strain of each unit cell. Then, a relatively coarse global mesh is used to obtain the overall responses of the composite at macroscopic level. By using the stress volume averaging method, the effective elastic properties of the composite can be calculated under the prescribed uniform strain boundary conditions. Finally, the predicted stress/strain curves are compared with experimental results, demonstrating the applicability of the FME method.     

An extended advancing front technique for closed surfaces mesh generation

An extended advancing front technique (AFT) with shift operations and Riemann metric named as shifting‐AFT is presented for finite element mesh generation on 3D surfaces, especially 3D closed surfaces. Riemann metric is used to govern the size and shape of the triangles in the parametric space. The shift operators are employed to insert a floating space between real space and parametric space during the 2D parametric space mesh generation. In the previous work of closed surface mesh generation, the virtual boundaries are adopted when mapping the closed surfaces into 2D open parametric domains. However, it may cause the mesh quality‐worsening problem. In order to overcome this problem, the AFT kernel is combined with the shift operator in this paper. The shifting‐AFT can generate high‐quality meshes and guarantee convergence in both open and closed surfaces. For the shifting‐AFT, it is not necessary to introduce virtual boundaries while meshing a closed surface; hence, the boundary discretization procedure is largely simplified, and moreover, better‐shaped triangles will be generated because there are no additional interior constraints yielded by virtual boundaries. Comparing with direct methods, the shifting‐AFT avoids costly and unstable 3D geometrical computations in the real space. Some examples presented in this paper have demonstrated the advantages of shift‐AFT in 3D surface mesh generation, especially for the closed surfaces. Copyright © 2007 John Wiley & Sons, Ltd.     

Voxel‐based meshing and unit‐cell analysis of textile composites

Unit‐cell homogenization techniques are frequently used together with the finite element method to compute effective mechanical properties for a wide range of different composites and heterogeneous materials systems. For systems with very complicated material arrangements, mesh generation can be a considerable obstacle to usage of these techniques. In this work, pixel‐based (2D) and voxel‐based (3D) meshing concepts borrowed from image processing are thus developed and employed to construct the finite element models used in computing the micro‐scale stress and strain fields in the composite. The potential advantage of these techniques is that generation of unit‐cell models can be automated, thus requiring far less human time than traditional finite element models. Essential ideas and algorithms for implementation of proposed techniques are presented. In addition, a new error estimator based on sensitivity of virtual strain energy to mesh refinement is presented and applied. The computational costs and rate of convergence for the proposed methods are presented for three different mesh‐refinement algorithms: uniform refinement; selective refinement based on material boundary resolution; and adaptive refinement based on error estimation. Copyright © 2003 John Wiley & Sons, Ltd.     

Microstructure modelling and prediction of effective elastic properties of 3D multiphase and multilayer braided composite

Abstract

This paper is focused on the microstructure modelling and evaluation of effective elastic properties of three-dimensional multiphase and multilayer braided composite. Regarding the multiscale characteristics of the composite, the microstructure modelling is carried out sequentially from fibre to tow scale. The geometrical configuration of the microstructure is first analysed, and mathematical relations among different geometrical parameters are derived on each scale. Second, effective elastic properties are obtained based on the sequential homogenisation from fibre to tow scale. A strain energy based method is proposed to evaluate effective elastic properties with specific boundary conditions imposed on the microstructure. Numerical results obtained by the proposed method and the microstructure model show a good agreement with the results measured experimentally.     

The generation of triangular meshes for NURBS‐enhanced FEM

This paper presents the first method that enables the fully automatic generation of triangular meshes suitable for the so‐called non‐uniform rational B‐spline (NURBS)‐enhanced finite element method (NEFEM). The meshes generated with the proposed approach account for the computer‐aided design boundary representation of the domain given by NURBS curves. The characteristic element size is completely independent of the geometric complexity and of the presence of very small geometric features. The proposed strategy allows to circumvent the time‐consuming process of de‐featuring complex geometric models before a finite element mesh suitable for the analysis can be produced. A generalisation of the original definition of a NEFEM element is also proposed, enabling to treat more complicated elements with an edge defined by several NURBS curves or more than one edge defined by different NURBS. Three examples of increasing difficulty demonstrate the applicability of the proposed approach and illustrate the advantages compared with those of traditional finite element mesh generators. Finally, a simulation of an electromagnetic scattering problem is considered to show the applicability of the generated meshes for finite element analysis. ©2016 The Authors. International Journal for Numerical Methods in Engineering published by John Wiley & Sons Ltd.     

BOLJAT: a tool for designing composite bolted joints using three-dimensional finite element analysis

This paper discusses the development of a software tool for design of composite bolted joints, using three-dimensional finite element analysis. The tool allows the user to create the joint geometry through a menu-driven interface and then generate a customised mesh according to the user's needs. Contact parameters are defined automatically, which shields the user from the most difficult part of the process. Boundary conditions, bolt pre-loads, and material properties can also be set. Only a few manual steps are necessary to complete the finite element code generation process. By automating the time-consuming model creation process, the tool facilitates the increased use of three-dimensional finite element analysis in the design of composite bolted joints. A case study is shown to demonstrate the usefulness of the tool.     

Mesh superposition-based multiscale stress analysis of composites using homogenization theory and re-localization technique considering fiber location variation

In this article, the accuracy of multiscale stress analysis of heterogeneous materials (unidirectional fiber-reinforced composite) was improved by considering microscopic geometrical variation using the mesh superposition method. When analyzing the stress distribution in a composite with the finite element method considering the variation of the fiber location, updating the mesh significantly is necessary; however, generating an appropriate mesh for a large geometrical variation is difficult. Therefore, we focused on the mesh superposition method, which can easily generate a numerical FE model, even if the internal structure is complex, because the matrix and inclusion can be expressed by the global mesh and local mesh independently. However, in the original mesh superposition method, the analysis accuracy may be degraded owing to the mesh overlap conditions. Therefore, an improved method was applied to the homogenization theory-based analysis. In this article, the effectiveness of the proposed approach was discussed by comparing the numerical results of this method with those of conventional mesh superposition method and standard finite element method. From the numerical results, accuracy improvement by the proposed approach for the multiscale stochastic stress analysis is confirmed.     

Multiphase hybrid stress finite element analysis of heterogeneous media by simple mesh: One element with one interface

In this paper, a finite element method is proposed to analyze the microscopic and macroscopic mechanical behaviors of heterogeneous media with randomly distributed inclusions. A simple mesh partitioned the domain into regular quadrilateral or triangular elements, where one element may contain two phases. An assumed stress hybrid formulation is implemented in the finite element model and the functional is derived for an element containing two phases. Numerical examples were used to study the microscopic and macroscopic properties of the composites, such as the effective modulus, to validate of the proposed model. The results show that the proposed multiphase hybrid stress finite element model can accurately measure the stress fields of materials with arbitrary microstructural distributions and improve computational efficiency by about 30 to 1500 times in comparison with the traditional displacement based finite element method.