一种新的墙单元

本文提出了一种新的空间墙单元模型，这种墙单元把土建中常用的剪力墙作为它的一种特殊情形．本文基于四边形单元所提出的墙单元在平面内是一块膜，它除面内的两个平动自由度外还具有绕平面转动的自由度；平面外是一块弯曲板．因而它是膜和板的一个组合，它的每个节点具有空间的全部六个自由度．由于考虑了空间墙平面内和平面外的刚度，因此这种新的墙单元能够直接与三维框架的梁、柱单元连接．它也能很容易地处理墙的空间变形。     

一种新的墙单元

本文提出了一种新的空间墙单元模型，这种墙单元把土建中常用的剪力墙作为它的一种特殊情形，本文基于四边形壳单元所提出的墙单元在平面内是一块膜，它除面内的两个平动自由度外还具有绕平面转动的自由度，平面外是一块弯曲板，因而它是膜和板的一个组合，它的每个节点具有空间的全部六个自由度，由于空间墙平面内和平面外的刚度，因此这种新的墙单元能够直接与三维框架的梁、柱单元连接。它也能很容易地处理墙的空间变形。     

一种健壮的混合Roe黎曼求解器

使用Roe格式计算多维流动问题时,在强激波附近会出现数值激波不稳定现象。带有剪切粘性的HLLEC格式不仅可以捕捉接触间断,而且表现出很好的稳定性。混合Roe格式和HLLEC格式来消除数值激波不稳定性。在强激波附近,通过激波面法向和网格界面法向的夹角来定义开关函数,使得数值通量在激波面横向切换成HLLEC格式。在其余地方,数值通量依然使用Roe格式来计算。数值试验表明,混合格式不仅消除了Roe格式的数值激波不稳定性,还最大程度地减少了HLLEC格式所带来的剪切耗散,保留了Roe格式高分辨率的优点。     

管系内一维可压缩流动数值建模与特性分析

针对复杂管系内可压缩流体,基于有限体积法,采用HLLC(Harten-Lax-vanLeerContact)格式和黎曼求解器构建了有限控制体数值离散方法,引入虚拟节点用于连接有限控制体,借助虚拟节点给出控制体之间数值通量的计算格式,发展了一种管道内一维流动数值建模方法。针对含有分支管路的管系,在管道连接部位构建了分支管路拟一维流动数值计算模型。基于所发展的一维流动数值方法,建立了变径管道和含60°分支管道内流动计算模型,验证了该方法的收敛性和有效性;基于虚拟节点的数值格式处理变径管激波问题具有一定精度优势。研究了变径管和分支管模型中可压缩流体激波、稀疏波等的传播机理,分析了管径对相邻支管压力的影响,为工程管路设计提供了参考。     

对流扩散方程QUICK格式的数值摄动高精度重构格式

利用高智提出的数值摄动算法, 把对流扩散方程的常用QUICK格式(黏性和对流项分别用二阶中心和QUICK格式离散)进行了高精度重构, 包括利用离散单元内所有结点的全域重构和分别利用离散单元内上下游结点的上下游重构, 得到两类新的更高阶精度的数值摄动重构格式, 称为高的QUICK格式(G-QUICK格式). G-QUICK格式与QUICK格式相比简单性相当, 但精度更高; 全域重构G-QUICK格式和QUICK格式均为条件稳定, 上下游重构得到一些绝对稳定的G-QUICK格式. 解析分析和数值算例均证实了G-QUICK格式的优良性能, 上下游重构的G-QUICK格式为在对流扩散方程的QUICK格式中避免使用人工黏性提供了新途径.      

流体力学两种格式能量守恒比较

Lagrange系统下的非定常流体力学数值方法中，使用非守恒型能量方程获得的总能量(内能与动能之和)的误差大小是鉴别一种格式好坏的重要标志之一．讨论在校坐标系下两种有限元方法的离散格式及其能量守恒性．一种是采用由因子γ^-1来加权插值基函数的Galerkin有限元方法，即面平均格式；另一种是直接加权插值基函数的Galerkin有限元方法，即体平均格式．误差分析表明体平均格式具有较小的能量守恒误差，数值计算结果也显示出体平均格式能量守恒误差比面平均格式明显小．     

匀速转动内悬臂梁稳定性研究

本文采用了凯恩简化模型并提出大范围平动和转动共同作用下的连续体梁模型;通过两种模型分析了匀速转动内悬臂梁的稳定性;指出了匀速转动内悬臂梁存在动力柔化现象,动力柔化的原因在于梁的大范围平动;提出了匀速转动内悬臂梁的内转半径和外转半径的概念,指出匀速转动内臂梁的内转半径和外转半径比值存在一定关系时,系统为一阶稳定,研究表明,同时作大范围平动和转动的柔性梁存在柔化和刚化的分界点,需要在实际应用中予以注意。     

改进WENO格式及其在爆轰计算中的应用

对已有的一种改进型WENO-M格式进行了理论上的修正,得到了新的WENO权函数。与原WENO-M格式相比,新得到的WENO-M-P格式减少了约9%的CPU计算时间,同时也保证了在一阶极值点处不降低精度（经典的WENO格式在一阶极值点处精度下降）,仍然保持5阶精度。为了验证修正后的格式,采用二步反应模型数值模拟了几组一维和二维爆轰：在一维爆轰模拟中,对比了WENO-M格式和WENO-M-P格式在一定温度下的最低起爆压强以及一定压强下的最低起爆温度;在二维旋转爆轰的模拟中,对比了WENO-M格式和WENO-M-P格式的计算效果。结果表明：在同样的起爆温度下,WENO-M 格式和 WENO-M-P 格式的最低起爆压强均高于 WENO格式,但WENO-M-P格式比WENO-M格式的最低起爆压强低;三种格式在不同压强下的最低起爆温度相同;WENO-M-P格式具有和WENO-M格式一样的计算效果,优势是节省了计算时间。     

带有沙漏控制的相对自由度壳元非线性动力分析

针对八节点相对自由度壳单元，给出了单元内坐标和位移的插值公式，利用HuWashinzu变分原理，基于拟应变法，在大变形情况下推导了拟应变的表达式，构造了带有沙漏控制的动力问题的有限元求解格式。通过算例表明该文提出的基于相对自由度壳元的沙漏控制算法能够很好地解决非线性动力问题，可改善计算精度和计算效率。     

基于解析试函数三角形带旋转自由度膜元的显式格式研究

三角形单元是有限元分析中常用的单元.在平面单元内引入结点转动自由度,可以提高单元位移场的阶次,在不增加单元结点的前提下提高单元性能.论文利用问题基本解析解作为试函数来构造带旋转自由度的三角形单元ATF-R3H,采用了杂交应力函数单元模式,确保了单元优良的抗畸变性能和较高应力计算精度.论文利用直角坐标与三角形面积坐标的线性关系,以及面积坐标幂函数在三角形域内和边界上的积分公式,直接给出单元刚度矩阵的显式表达式,从而避免了大量数值积分,提高了计算效率.数值算例表明,显式格式的ATF-R3H单元具有良好的性能.     

Buckling analysis of plain knitted fabric sheets under simple shear in an arbitrary direction

Knitting structures make plain knitted fabric different from woven fabric. With the aid of a micro-constitutive model the buckling of a knitted fabric sheet subjected to simple shear in an arbitrary direction is investigated. The large deformation of the fabric sheet in its critical configuration is considered. The theory of stability for finite deformations is applied to the analysis. All the stress boundary conditions of the knitted fabric sheet are satisfied. An equation for determining the buckling direction angle is derived. It is shown that there are two possible buckling modes: a flexural mode and a barreling mode. The buckling conditions for the two modes are also obtained, respectively. A numerical calculation reveals that only the flexural mode can occur, which agrees with experimental observations.     

A micro-mechanical model of knitted fabric and its application to the analysis of buckling under tension in wale direction： buckling analysis

With the aid of the micro-mechanical model of knitted fabric proposed in Part 1 we analyze the buckling of a knitted fabric sheet when it is subjected to a tension along the wale direction. The large deformation of the fabric sheet in the critical configuration is considered and, to avoid possible deviation due to the approximation of the theory of thin plate, the three-dimensional theory of instability is used. The fabric sheet is considered as a three-dimensional body and all boundary conditions are satisfied. It is shown that the buckling of the fabric sheet is possible, two buckling modes and the corresponding buckling conditions are obtained, but only the flexural mode is physically possible as observed in experiments.The project supported by the National Natural Science Foundation of China (10272079)     

Strain tensor for large three-dimensional surface deformation of sheet metal from an object grating

Grating techniques are used to determine the three-dimensional deformation and the tangential strain of sheet metal. A grating is fixed on the surface and taken by stereo CCD cameras in different deformation states. By suitable line-following software, the grating coordinates in the images are determined with subpixel accuracy. Using photogrammetric methods, the three-dimensional coordinates are calculated from the image coordinates. The strain usually is determined by means of a deformation gradient, which is calculated from every deformed triangle. In this paper, the gradient is determined in the center of four neighboring meshes using a polynomial approximation of the displacement function in a reference position. The influence of the nontangential deformation is considered. By simulation, a flat sheet metal is deformed to a rotational symmetric surface. The difference of the known exact strain is compared with the numerically derived strain with respect to different grating pitches. The proposed method yields good results even in the case of large spatial deformation. It is applied to the deformation of a hatlike test specimen.     

Drapage géométrique des compositesGeometrical draping of composite fabrics

This paper presents a study devoted to the composite fabric shaping simulation in finite strain analysis. We introduce a new geometrical approach based on the fishnet method for which the deformation of a fabric mesh element consists in a pure trellis effect. Such a fabric mesh element is then defined by a curved quadrilateral whose edges are geodesic lines with the same length plotted onto the surface to drape. Given three vertices of the fabric mesh element on the surface, we propose an optimization algorithm to define the fourth vertex of the fabric mesh element. This algorithm allows us to drape the surface using an advancing front approach from the data of an initial impact point between the fabric and the surface and the initial fibre directions at this point. A numerical draping simulation example using this approach is given. To cite this article: H. Borouchaki, A. Cherouat, C. R. Mecanique 331 (2003).     

Detection of plastic deformation patterns in a binary aluminum alloy

A whole-field, in-plane strain-mapping technique is evaluated for in situ monitoring of plastic deformation patterns in aluminum sheet metals. This technique is built on the recent developments in digital image correlation and improved data reduction procedures. The sensitivity and accuracy of the measured local strain variations are critically examined in terms of random and systematic experimental errors, free-surface roughening due to large plastic deformation and microscopic surface grain deformation. Tensile specimens made from an annealed Al−Mg alloy sheet metal are subjected to a large plastic and macroscopically uniform deformation, and no visible deformation patterns can be identified by direct surface observation. Using an incremental strain-mapping approach, the existence of nonstationary deformation bands in the annealed Al−Mg alloy is uncovered. The developed technique can be used to study the formation and evolution of plastic deformation patterns and their effect on tensile ductility, formability and surface finish of sheet metals.     

Impact of woven fabric: Experiments and mesostructure-based continuum-level simulations

Woven fabric is an increasingly important component of many defense and commercial systems, including deployable structures, restraint systems, numerous forms of protective armor, and a variety of structural applications where it serves as the reinforcement phase of composite materials. With the prevalence of these systems and the desire to explore new applications, a comprehensive, computationally efficient model for the deformation of woven fabrics is needed. However, modeling woven fabrics is difficult due, in particular, to the need to simulate the response both at the scale of the entire fabric and at the meso-level, the scale of the yarns that compose the weave. Here, we present finite elements for the simulation of the three-dimensional, high-rate deformation of woven fabric. We employ a continuum-level modeling technique that, through the use of an appropriate unit cell, captures the evolution of the mesostructure of the fabric without explicitly modeling every yarn. Displacement degrees of freedom and degrees of freedom representing the change in crimp amplitude of each yarn family fully determine the deformed geometry of the mesostructure of the fabric, which in turn provides, through the constitutive relations, the internal nodal forces. In order to verify the accuracy of the elements, instrumented ballistic impact experiments with projectile velocities of 22-550 m/s were conducted on single layers of Kevlar^® fabric. Simulations of the experiments demonstrate that the finite elements are capable of efficiently simulating large, complex structures.     

织物力学研究的新进展

简要地回顾了织物力学的发展历史,重点介绍近10几年来,在服装CAD核心------"试衣系统"的技术需求的刺激下,在织物悬垂、屈曲方面研究的进展.特别是关于织物屈曲研究的开展,涉及到了织物本构理论这个织物力学中最基础的问题,织物细观本构理论的建立和对织物一系列屈曲现象的成功的解析分析是织物力学研究的最新进展,构成了近代织物力学的主要内容.最后,对今后织物力学研究的方向进行了展望.      

Nonlinear stability of timber column strengthened with fiber reinforced polymer

By taking into account the effect of the bi-modulus for tension and compression of the fiber reinforced polymer (FRP) sheet in the reinforcement layer, a general mathematical model for the nonlinear bending of a slender timber beam strengthened with the FRP sheet is established under the hypothesis of the large deflection deformation of the beam. Nonlinear governing equations of the second order effect of the beam bending are derived. The nonlinear stability of a simply-supported slender timber column strengthened with the FRP sheet is then investigated. An expression of the critical load of the simply-supported FRP-strengthened timber beam is obtained. The existence of postbuckling solution of the timber column is proved theoretically, and an asymptotic analytical solution of the postbuckling state in the vicinity of the critical load is obtained using the perturbation method. Parameters are studied showing that the FRP reinforcement layer has great influence on the critical load of the timber column, and has little influence on the dimensionless postbuckling state.     

Modèles discrets de structures tissées : Analyse de stabilité et de drapé

A discrete model of a woven fabric structure is established, whereby nodes endowed with a mass and a rotational rigidity are connected by rigid bars to form a two-dimensional truss. The set of four bars that delineate a quadrilateral area is further endowed with a torsion deformation mode. The kinematics of the truss reproduces the large rotations and displacements encountered for real tissues. The equilibrium shape of such a structure is obtained as the minimum of its total potential energy versus the whole set of kinematic translational and rotational variables, accounting for eventual kinematic constraints due to contact with a rigid surface by the Lagrange multipliers method. A stability analysis is conducted, and the potentiality of the model is illustrated by fabric draping simulations. To cite this article: B. Ben Boubaker et al., C. R. Mecanique 330 (2002) 871–877.     

Modèles discrets de structures tissées : Analyse de stabilité et de drapéDiscrete models of woven structures: stability and draping analysis

A discrete model of a woven fabric structure is established, whereby nodes endowed with a mass and a rotational rigidity are connected by rigid bars to form a two-dimensional truss. The set of four bars that delineate a quadrilateral area is further endowed with a torsion deformation mode. The kinematics of the truss reproduces the large rotations and displacements encountered for real tissues. The equilibrium shape of such a structure is obtained as the minimum of its total potential energy versus the whole set of kinematic translational and rotational variables, accounting for eventual kinematic constraints due to contact with a rigid surface by the Lagrange multipliers method. A stability analysis is conducted, and the potentiality of the model is illustrated by fabric draping simulations. To cite this article: B. Ben Boubaker et al., C. R. Mecanique 330 (2002) 871–877.