微结构对金属基复合材料宏观弹塑性性能的影响

采用广义自洽有限元迭代平均化方法分析ＳｉＣ晶须增强铝基复合材料的弹塑性拉伸行为，研究纤维长径与体分比的变化对复合材料宏观弹塑性变形的影响。通过细观应力场的分析，讨论基体内塑性区的发展与复合材料宏观弹塑性变形过程之间的联系，指出纤维端头处基体塑性区的发展将对复合材料拉伸弹塑性行为有着显著影响。最后，还讨论了以名义屈服应力σ０．２来表征金属基复合材料的弹塑性特征的不足之处。     

高分子材料平面应变拉伸变形局部化

将基于应变软化玻璃状高分子材料微观特征建立的ＢＰＡ８－链分子网络模型引入ＵｐｄａｔｉｎｇＬａｇｒａｎｇｅ有限元方法，建立了适于变形局部化分析的大变形弹塑性有限元驱动应力法．在此基础上，数值模拟了初始各向同性高分子材料平面应变拉伸变形局部化的传播过程．探讨了ＢＰＡ模型对具有加工硬化特性的结晶性高分子材料变形分析的适应性；分析了局部化传播过程中颈缩截面的非均匀应力三轴效应；最后，讨论了网格尺寸以及初始几何不均匀性对颈缩扩散以及应力三轴效应的影响     

冲压板材拉伸筋阻力的一种有效数值计算方法

将弹塑性有限变形的拟流动角点本构理论和厚向各向异性屈服函数引入弹塑性动力显式有限元列式,对板材通过拉伸筋的变形过程及拉伸筋阻力进行了数值模拟,并与有关实验结果进行了比较很好的一致性,表明了该计算方法的有效性。进而,数值研究了拉伸筋形状、界面摩擦状况以及材料的厚向各向异性对拉伸筋阻力的影响,为实际覆盖件冲压成形中拉伸筋的设置提供了重要的定量依据。     

光滑拉伸试件中不同初始形状孔洞长大的有限元模拟

本文对具有不同硬化指数（ｎ＝０．０５，ｎ＝０．１，ｎ＝０．２）的幂硬化材料的光滑拉伸试样的拉伸变形过程进行了有限元模拟，通过有限元计算和经Ｂｒｉｄｇｍａｎ修正分别得到了试样变形过程中心部应力三维度随应变的变化情况；在此基础上运用控制体胞宏观应力三维度的方法，对含不同初始形状孔洞的体胞模型进行了有限元分析，计算结果表明：（１）孔洞初始形状和材料硬化特性对试样的拉伸破坏过程有重要影响；（２）Ｂｒｉｄｇ     

板材多点成形过程的有限元分析

多点成形过程采用静力隐式格式进行数值模拟是比较合适的。本文建立了用于多点成形过程分析的静力隐式弹塑性大变形有限元方法 ,给出了对稳定迭代收敛过程效果较好的板壳有限单元模型、处理多点不连续接触边界的接触单元方法以及增量变形过程中应力及塑性应变计算的多步回映计算方法。基于这些方法编制了计算软件 ,应用该软件进行了矩形板的液压胀形过程及球形模具拉伸成形过程的有限元分析 ,数值计算结果与典型的实验结果及计算结果吻合很好。最后给出了球形、圆柱形目标形状的实际多点成形过程的数值模拟结果。     

弹塑性有限变形的拟流动理论

本文提出一种弹塑性有限变形的拟流动理论。该理论从正交性法则出发，通过引入“拟弹性模量”和模量衰减函数并改进应变率的弹塑性分解，实现了由有限变形Ｐｒａｎｄｔｌ－Ｒｅｕｓｓ流动理论（Ｊ２Ｆ）向基于非正交法则的率形式形变理论（Ｊ２Ｄ）的合理的光滑过渡；并适用于初始及后继各向异性变形分析。在特殊条件下，可退化为Ｊ２Ｆ、Ｊ２Ｄ理论以及由任意各向异性屈服函数描述的流动理论。将该理论用于韧性金属平面应力／应变拉伸失稳与变形局部化的有限元模拟，并与理论分析及实验结果相比较，表明了本文理论的正确性。     

受冲击作用弹塑性圆板动力响应的弹性效应

利用有限差分离散微分方程进行计算分析,研究冲击载荷作用下弹塑性圆板的早期动力响应,通过对瞬态径向弯矩分布规律的细致分析,阐明弹塑性固支圆板响应过程中弹性效应对其变形历史的影响.研究表明:弹塑性响应过程可划分为八个阶段,对应的变形模式为:“单铰圆模式”,“双铰圆模式”,“五铰圆模式”,“四铰圆模式”,“三铰圆模式”,“双铰圆模式”,“双驻定铰圆模式”,“弹性振动模式”.与刚塑性分析所假定的三相的变形模式比较,弹塑性响应分析证实了固支边界“驻定塑性铰圆”的存在性.虽然刚塑性分析所假定的第一相位移响应模式并不存在,但第二相和第三相响应模式则得到了证实.由于这两相及相应弹塑性分析的两个阶段持续时间都较长,因而也肯定了刚塑性分析所假定变形模式的主要特征.弹性效应对于板内“移行铰圆”的影响比较大,它不但使“移行铰圆”出现“回退”现象,还使得“移行铰圆”的个数增加到三个;对于圆心处的“塑性铰圆”,弹性效应则使得它的符号出现由负向到正向的反复变化.因此,弹性效应对弹塑性板的变形历史影响十分明显.     

预延伸非晶聚合物材料各向异性变形局部化

给出一种可描述预延伸各向异性特性的背应力张量三维表达式，引入大变形弹塑性有限元驱动应力法，结合ＢＰＡ８ 链细观分子网络模型，模拟了预延伸各向异性非晶聚合物材料平面应变拉伸变形局部化力学行为．详细讨论了预延伸比（ＩｎｉｔｉａｌＤｒａｗｉｎｇＲａｔｉｏ；ＩＤＲ）和预延伸方向（ＩｎｉｔｉａｌＤｒａｗｉｎｇＤｉｒｅｃｔｉｏｎ；ＩＤＤ）对变形抗力、颈缩规律、剪切带方向以及试件中心部位链延伸比的影响．     

带裂纹板单向拉伸极限承载力有限元研究

一般金属材料的板式构件,在其使用的过程中有时会出现裂纹,而构件出现裂纹后的承载力问题是人们比较关心的.由于金属材料一般为韧性材料,在极限承载力计算中有两种模型,一种是弹塑性屈服模型,一种是弹塑性断裂力学模型.本文利用通用软件分别采用弹塑性和基于扩展有限元的裂纹扩展分析模型对单向拉伸的带裂纹板进行数值模拟,并将数值模拟的极限承载力与试验结果进行比较;结果表明,裂纹扩展分析模型计算出的极限承载力与实验结果更吻合;在此基础上,本文对裂纹角度不同的单向拉伸带裂纹板进行分析,得到其极限承载力跟裂纹与拉伸方向夹角的关系,其趋势与理论分析结果吻合.     

中厚圆柱壳在局部荷载下有限变形与屈曲分析

在柱壳的有限元计算中，采用Mindlin八结点杂交壳单元和增量荷载法，基于选择积分，缩减积分及完全积分三种积分模式编制了分层计算各种厚度板壳的有限元程序FEAM，并对中厚圆柱壳在局部法向均布荷载作用下的弹塑性有限变形和屈曲问题进行了分析和计算，算例表明，利用FEAM可对壳体屈曲的临界荷载及屈曲后结构的承载与形状改变作定性与定量的分析.     

夹层梁总体屈曲及皱曲的有限元计算

皱曲是夹层结构的一种短波屈曲模式,通常发生于夹心较厚或夹心刚度较低的情况。由于模型规模的限制,在常规有限元建模时通常将夹层板模拟为二维板单元,这种方法忽略了面板和夹心在厚度方向上的相互作用,无法计算出皱曲模式。针对上述问题,本文首先介绍了一个计算夹层结构总体屈曲和皱曲的统一理论,并将此理论的计算结果作为理论解。为了同时...     

Initiation and growth of wrinkling due to nonuniform tension in sheet metal forming

An experimental investigation was conducted on the initiation and growth of wrinkling due to nonuniform tension using the Yoshida buckling test. The initiation of wrinkling was detected by strain gages mounted on both surfaces of the samples in the loading and transverse directions. The bifurcation of aluminum auto body sheets appeared to be smooth and much less abrupt than that observed in a steel sheet. A special fixture was designed to, perhaps for the first time, continuously measure the in situ growth of the buckle heights so that the rates of buckle growth were monitored as functions of strain and stress in the loading direction. In contrast to what is commonly believed, it was found that the buckle height is not predominantly determined by the material yield strength, and lower averager value does not increase the rate of buckle growth. Crystallographic texture components and pole figures of the test materials were also measured, and the relationship of plastic anisotropy with wrinkling behavior was investigated by experiments with specimens aligned in the rolling direction, the transverse direction and 45-deg to the rolling direction of the sheet materials.     

薄板冲压成型中板料起皱的临界应力分析和预测

针对薄板冲压成型中起皱这一常见的材料失效形式，运用板料压缩失稳理论，提出虚拟刚度变量的概念和板料产生起皱的临界应力计算方法，并借助于计算机仿真技术和有限元计算方法，搜寻并计算得到反映板料各处发生起皱难易程度的临界因子，生成起皱云图来观察材料各部分的稳定状态，预测可能出现起皱的部位，为修改冲压工艺和修模提供依据，通过分析起皱云图得出的结论与实际情况非常接近。     

Multiple bifurcations in wrinkling analysis of thin films on compliant substrates

Wrinkling phenomena of stiff thin films on compliant substrates are investigated based on a non-linear finite element model. The resulting non-linear equations are then solved by the Asymptotic Numerical Method (ANM) that gives interactive access to semi-analytical equilibrium branches, which offers considerable advantage of reliability compared with classical iterative algorithms. Bifurcation points are detected through computing bifurcation indicators well adapted to the ANM. The effect of boundary conditions and material properties of the substrate on the bifurcation portrait is carefully studied. The evolution of wrinkling patterns and post-bifurcation modes including period-doubling has been observed beyond the onset of the primary sinusoidal wrinkling mode in the post-buckling range.     

A comparative analysis of numerical approaches to the mechanics of elastic sheets

Numerically simulating deformations in thin elastic sheets is a challenging problem in computational mechanics due to destabilizing compressive stresses that result in wrinkling. Determining the location, structure, and evolution of wrinkles in these problems has important implications in design and is an area of increasing interest in the fields of physics and engineering. In this work, several numerical approaches previously proposed to model equilibrium deformations in thin elastic sheets are compared. These include standard finite element-based static post-buckling approaches as well as a recently proposed method based on dynamic relaxation, which are applied to the problem of an annular sheet with opposed tractions where wrinkling is a key feature. Numerical solutions are compared to analytic predictions of the ground state, enabling a quantitative evaluation of the predictive power of the various methods. Results indicate that static finite element approaches produce local minima that are highly sensitive to initial imperfections, relying on a priori knowledge of the equilibrium wrinkling pattern to generate optimal results. In contrast, dynamic relaxation is much less sensitive to initial imperfections and can generate low-energy solutions for a wide variety of loading conditions without requiring knowledge of the equilibrium solution beforehand.     

金属板材冲压成型过程的有限单元法模拟

用有限单元法模拟了金属板材的冲压成型过程。在模拟过程中，应用了同时考虑了薄膜力和弯矩的板壳大变形理论，考虑了板材在塑性阶段各向异性的强化性质，考虑了模具和工件的接触和摩擦条件，分析了金属板在冲压过程中的屈曲现象，建立了增量形式的变分原理。弹塑性薄壳单元被引入，它的位移模式在变分的意义上满足单元边界上一阶导数连续的条件，并有足够的秩来适应单元的有限拉伸、转动和弯曲，计算中采用了罚函数方法，即在模具和工件的接触面上，模具的表面被假设为文克尔地基，采用了修正的库伦摩擦定律，迭代法被用来决定模具与工件的接触条件和金属板的塑性行为。本文最后提供了一个算例。     

Wrinkling prediction of rectangular shell-membrane under transverse in-plane displacement

A novel shell-membrane concept is presented in this paper to define the wrinkling analytical object. A stress field model is established and applied to determine the different regions (taut, wrinkled and slack regions) of the shell-membrane. An analytical model based on the bifurcation theory of thin-plate is introduced to predict the wrinkling wavelength and amplitude. Numerical simulation incorporating nonlinear post-wrinkling analysis is employed to simulate the detailed nonlinear wrinkling behavior. To stabilize the finite element model, the initial prestress is introduced to the model at first. The out-of-plane disturbing forces are then applied to the model to initiate the wrinkling and complete the post-wrinkling analysis. Results from numerical simulation show good agreement with the analytical prediction.     

含胶接缺陷的椭球泡沫夹层结构破坏机理与承载能力研究

采用试验和有限元方法,对含胶接缺陷的椭球泡沫夹层结构在外压作用下的破坏模式和承载能力进行了研究。针对国内在研的大尺寸椭球泡沫夹层结构,实施了全尺寸静力外压试验,发现初始胶接缺陷破坏了夹层结构界面应力传递的连续性;随着载荷增加,面板发生皱褶且脱胶界面继续扩展,从而降低结构整体承载能力。通过红外无损检测确定了缺陷的类型和形貌;采用预留相应初始脱粘面积、脱粘间隙以及内聚力单元模拟界面脱粘的有限元分析方法,对含胶接缺陷的椭球泡沫夹层结构承载能力进行预测。数值结果表明:含胶接缺陷结构易发生面板皱褶,且结构顶部和根部区域较易发生界面脱粘扩展。数值和试验结果取得了较好的一致性,本文结果可为同类结构设计提供参考。     

Mechanical behavior and wrinkling of lined pipes

The paper focuses on wrinkling of lined pipes (sometimes referred to as clad pipes) under bending loading, where a corrosion-resistant thin-walled liner is fitted inside a carbon–steel outer pipe. The problem is solved numerically, using nonlinear finite elements to simulate liner pipe deformation and its interaction with the outer pipe. Stresses and strains are monitored throughout the deformation stage, detecting possible detachment of the liner from the outer pipe and the formation of wrinkles. The wrinkling behavior of elastic and elastic–plastic (steel) lined pipes under bending is examined. The results indicate that the lateral confinement of the liner pipe due to the deformable outer pipe and its interaction with the outer pipe has a decisive influence on the wrinkling behavior of the lined pipe. It is also shown that the behavior is characterized by a first bifurcation in a uniform wrinkling pattern, followed by a secondary bifurcation. The values of corresponding buckling curvature are determined and comparison with available experimental results is conducted in terms of wrinkle height development and the corresponding buckling wavelength. The results of the present research can be used for safer design of lined pipes in pipeline applications.     

The effect of plastic anisotropy on compressive instability in sheet metal forming

The wrinkling behavior of a thin sheet with perfect geometry is associated with compressive instability. The compressive instability is influenced by many factors such as stress state, mechanical properties of the sheet material, geometry of the body, contact conditions and plastic anisotropy. The analysis of compressive instability in a plastically deforming body is difficult considering all the factors because the effects of the factors are very complex and the instability behavior may show a wide variation for a small deviation of the factors. In this study, the bifurcation theory is introduced for the finite element analysis of puckering initiation and growth of a thin sheet with perfect geometry. All the above mentioned factors are conveniently considered by the finite-element method. The instability limit is found by the incremental analysis and the post-bifurcation behavior is analyzed by introducing the branching scheme proposed by Riks. The finite-element formulation is based on the incremental deformation theory and elastic–plastic material modeling. The finite-element analysis is carried out using the continuum-based resultant shell elements considering the anisotropy of the sheet metal. In order to investigate the effect of plastic anisotropy on the compressive instability, a square plate that is subjected to compression in one direction and tension in the other direction is analyzed by the above-mentioned finite-element analysis. The critical stress ratios above which buckling does not take place are found for various plastic anisotropic modeling methods and discussed. Finally, the effect of plastic anisotropy on the puckering behavior in the spherical cup deep drawing process is investigated. From the results of the finite-element analysis, it is shown that puckering behavior of sheet metal is largely affected by plastic anisotropy.