板材冲压成形的晶体塑性有限元模拟

将率相关晶体塑性本构理论引入Mindlin曲壳单元模型与动力显式有限元法,采用切线系数法计算剪切应变率,根据晶体取向正态分布规律在单元积分点处分配晶体取向,按各晶体取向体积分数的加权平均计算多晶体应力,开发晶体塑性动力显式有限元程序,实现板材冲压成形过程模拟和晶体取向演化预示.以主要初始织构为铜织构和S织构的轧制铝板为对象,对方盒件冲压成形过程及织构演化进行数值模拟,计算结果与试验结果呈现出较好的一致性.通过晶体弹塑性有限元法不仅可以预示板材宏观成形构形变化,而且能够预测板材织构的演化情况.模拟结果显示在方盒件冲压成形过程中,铜织构和S织构为不稳定取向,变形后逐渐转到其他取向.     

板材成形局部塑性失稳与起趋的数值模拟

以Ｈｉｌｌ关于弹塑性材料唯一性的充分性条件为理论基础，采用虚功率增率型原理和Ｍｉｎｄｌｉｎ曲壳单元的弹塑性大变形有限元模型成功地模拟了方盒与圆筒拉深成形凸缘起皱现象，以及圆锥件拉深成形侧壁起皱现象。方盒拉深成形凸缘起皱的个数、形状和位置等起皱模式与试验结果完全吻合，并且实现了起皱过程的３维激光动态仿真演示     

Drawability assessment of BCC steel sheet by using elastic/crystalline viscoplastic finite element analyses

Elastic/crystalline viscoplastic finite element (FE) analyses were carried out to asses the drawability of three kinds of BCC steel sheets, such as mild steel, dual-phase steel and high-strength steel, in the cylindrical cup deep drawing processes. In this study, the crystal orientations were obtained by X-ray diffraction and orientation distribution function (ODF) analyses. The measured ODF results have revealed clearly different textures of sheets, featured by orientation fibers, skeleton lines and selected orientations in Euler angle coordinate space, which can be related to the plastic anisotropy. An orientation probability assignment method, which can be categorized as an inhomogenized material modeling, was used in this FE modeling. The orientations were determined from the measured ODF and assigned to FE integration points one by one. Numbers of integration points, which represent crystallites and can rotate individually, are employed to represent textures of the sheet metals for taking account of the initial and evolutional plastic anisotropy without introducing Taylor or Sachs homogenization assumption. The FE analyses showed how the fiber textures affect the strain localization and earing in the deep drawing operation. It was confirmed by comparison with experimental results that this FE code could predict the extreme strain localization and earing with good accuracy and assess the sheet drawability.     

The role of plastic anisotropy deformation in fretting wear predictions

The deformation occurring under fretting conditions occurs over length scales of the same order as the grain size, so the plastic anisotropy plays a significant role in the very local region near the contact edge during fretting process. The present study first describes plastic anisotropy by unified anisotropy plastic model coupling with Archard's wear law on the fretting behavior incorporating the effect of wear debris into such a quantitative model. The finite element method, utilizing this model, is used to analyze gross slip fretting conditions. The implementation of the wear simulation tool together with anisotropy cyclic plasticity analysis during fretting process is applied to the wear depth simulation. The present study validates the experiment phenomena from numerical simulation that failure location of the specimens under the flat-on-flat configuration is very close to the trailing edge. The scar at the trailing edge is much deeper than any other locations and the larger relative slip range resulted in considerably deeper surface damage. Another interesting discovery is that when material with different orientations the degree of wear also develops differently and the quantitative prediction is given.     

Microscopic approach of powder compaction using finite element method

An approach for simulating microscopic densification behaviour of powder particles in compaction using a finite element method is proposed. In this method, the contacts between powder particles during the compaction are detected, and plastic deformation of the particles is calculated by the finite element method for a porous metal. The finite element mesh is generated by connecting the centres of the particles in contact. It is assumed that the finite elements are porous metals having an average relative density calculated from the volumes of the powder and pore inside the element. The elements are classified into the triangular and quadrilateral ones used in the conventional finite element methods and a linear one for the simple compression. The accuracy of the stiffness for plastic deformation of the particles is improved by applying the finite element method. The calculated plastic deformation of powder particles in plane-strain compaction is compared with that for a model experiment using aluminium rods.     

耦合温度效应晶体塑性的TWIP钢变形行为研究

为探究TWIP钢高温条件下的塑性变形机理,建立了耦合温度效应的晶体塑性本构模型,考虑温度对TWIP钢滑移和孪生的影响,提出了耦合温度效应的流动法则和硬化法则。结合在500 ℃和750 ℃条件下的原位SEM高温拉伸试验,建立了描述TWIP钢热变形过程的晶体塑性有限元模型。模拟获得不同温度条件下的应力应变曲线、应变硬化率和孪晶体积分数与试验结果相吻合,验证了该模型的正确性。进而,基于所建立的模型研究了温度对TWIP钢塑性变形过程滑移、孪生演化及应变硬化过程的影响规律,结果表明：滑移阻力、孪生阻力和应变硬化率随温度的升高呈不均匀降低的趋势,且断后伸长率呈现降低的趋势,由25 ℃时53.4%降低至750 ℃时16.5%。同时,随温度升高,孪生受到抑制,但滑移受温度的影响较小,表现为滑移主导的塑性变形机制。     

复杂冲压件成形过程的截面分析技术

基于有限变形虚功率增量型原理的弹塑性大变形有限元理论,建立了高精度的八节点四边形单元平面应变状态下的截面分析模型,并应用非均匀有理B样条曲线描述模具截面线进行接触判断。旨在用很短的计算时间,获得较理想的板料冲压成形的模拟结果。方模具圆坯料胀形和深拉延成形过程以及发动机油底壳深拉延的模拟与试验结果,表明了该截面分析技术的正确性、可行性和高效性。     

DD3镍基单晶高温合金喷丸强化后残余应力的有限元模拟

利用有限元分析软件ANSYS/LS-DYNA,通过设立局部坐标系和运用三参数Barlat模型建立了DD3镍基单晶高温合金的喷丸强化有限元模型,并对喷丸后的残余应力进行了有限元模拟和实际测试。结果表明:由于镍基单晶高温合金的各向异性,在不同取向上呈现不同的残余应力状态,且同一晶面不同取向的最小残余压应力出现在组成滑移系的晶向上;模拟结果与试验结果吻合较好。     

修井机卡瓦重要承载部件的有限元分析

研究并设计了不压井修井机卡瓦的基本结构,得出了卡瓦所能占用的最大空间、夹紧管柱所需的最大摩擦力及气缸所需要提供的最大支持力。通过有限元软件对卡瓦重要承载部件进行分析,得出卡瓦体与管柱的最大变形量和最大许用应力。     

确定结构极限载荷的有限元简化算法

基于线性硬化材料形变理论，本文提出了一种计算结构极限载荷的有限元简化算法。与传统的弹塑性增量有限元方法相比，本方法可以避免每一增量步的平衡迭代和在每个高斯点的本构方程的积分，同时该方法不仅理论简洁，而且易于编制成程序或耦合入其他有限元程序中。     

Flaring and nosing process for composite annoy tube in circular cone tool application

An elasto-plastic incremental finite element computer code based on an updated Lagrangian formulation was developed to simulate the flaring and nosing processes of a metal tube in the asisymmetric condition. The extended r _min technique was used to treat the elastic–plastic stress state and to solve contact problems at the tool–metal interface. A modified Coulomb’s friction law was introduced to treat the alternation of the sliding–sticking state of friction at the contact interface. The forming performed analysis using the finite element method and experiment. To examine the influence of the thickness ratio and the optimum punch semi-angle and friction on the forming load of the two-ply metal tubes consisting of soft aluminum, hard aluminum, and copper. The calculated tube geometries and the relationship between punch load and stroke are in good agreement with the experimental data.     

Effects of texture gradients and strain paths on localization phenomena in polycrystals

Applications of crystal plasticity theory to the numerical modelling of large strain plasticity phenomena are considered. In particular, instabilities and localized deformation phenomena for FCC polycrystals subjected to various deformation modes are investigated. In-house finite element analyses based on a rate-dependent crystal plasticity model have been developed to simulate the large strain behaviour for sheet specimens subjected to plane strain, plane stress, and simple shear deformation modes. In the formulation, the plastic deformation of an individual crystal is assumed to be due to crystallographic slip. In the simulations, polycrystalline aggregates are modelled at various scales. This formulation accounts for initial textures, as well as texture evolution during large plastic deformations. The numerical analyses incorporate parallel computing features. The results of simulations for the above-mentioned deformation modes are discussed, and the formation of localized deformation in the form of shear bands is investigated.     

薄板件台阶孔冲压冷锻组合工艺的数值模拟

针对构想的一种薄板件台阶孔冲压冷锻组合工艺,应用金属塑性加工有限元模拟的关键技术,包括几何模型、材料断裂破坏、网格划分等,用DEFORM-3D软件对此种薄板件台阶孔的冲压冷锻组合变形进行数值模拟。得到了工件材料的等效应力、应变分布,速度场和力—行程曲线。提出细观变形过程可分为冲头小直径处冲孔、冲头大直径处压凹和闭式模锻三个阶段的认识,分析组合工艺特点和材料成形性能。用料厚为2.35 mm的AL-1100铝板材料进行工艺试验,获得了清晰的台阶孔形状,验证了数值模拟的认识。     

Consideration of geometric nonlinearity in rigid-plastic finite element formulation of continuum elements for large deformation

A rigid—plastic finite element formulation for the continuum elements employing the geometric nonlinearity during an incremental time step is presented. In sheet metal deformation, the displacement for each step is considerably large even though the effective strain increment is very small. For such large displacement problems, geometric nonlinearity must be considered. In the elastic—plastic finite element using continuum elements, general incremental formulations to include the geometric nonlinearity are available. However, in the conventional rigid—plastic finite element analysis using continuum, elements, the geometric nonlinearity has not been considered properly during an incremental time step. In this paper, in order to incorporate geometric nonlinearity to rigid—plastic continuum elements during a step, the convected coordinate system is introduced. To show the stability of strain distributions by the effect of geometric nonlinearity according to incremental step size, two sheet metal forming processes, stretching and deep drawing process, are analysed with various step sizes. Then the computed results using the derived equation are compared with those obtained without considering geometric nonlinearity.     

FINITE ELEMENT ANALYSIS AND EXPERIMENTAL STUDY OF THE METAL MULTIDIE IRONING PROCESS

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A multiscale mechanics approach for modeling textured polycrystalline thin films with nanothickness

A multiscale method is proposed for calculating elastic constants of textured polycrystalline thin films of nanothicknesses. In this method the molecular simulation and finite element method are hierarchically employed. The elastic constants for each single crystal are first calculated through the simulations of on- and off-axis tension tests of the single crystal using molecular statics. Subsequently, the constitutive relations for the single crystal are used in conjunction with a finite element code to study the macro-mechanical deformation and stresses in textured polycrystalline nanofilms. The result indicates that both film thickness and grain size influence the macro-Young's modulus and Poisson's ratio of the nanofilm. Specifically, for nickel, the value of the macro-Young's modulus decreases as film thickness decreases and increases as grain size decreases. The value of the macro-in-plane Poisson's ratio increases as the thickness decreases or grain size increases.     

Static recrystallization simulations starting from predicted deformation microstructure by coupling multi-phase-field method and finite element method based on crystal plasticity

We have developed a numerical model of recrystallization taking the inhomogeneities of the plastic deformation of a polycrystalline metal into account. Here, the plastic deformation of the polycrystalline metal is simulated by the finite element method based on crystal plasticity theory and the microstructure evolution during recrystallization is simulated by the multi-phase-field method. In primary recrystallization simulations, nucleation is the most difficult problem. In the present model, the deformation microstructure is predicted from the results of a crystal plasticity finite element simulation, and spontaneous nucleation is achieved through abnormal grain growth that is enabled by introducing the misorientation dependences of grain boundary energy and mobility. As a result of simulations under three different compression strains, it is confirmed that primary recrystallization simulations depending on the amount of deformation and taking the inhomogeneities of the plastic deformation of a polycrystalline metal into consideration can be successfully performed by employing the proposed model.     

Numerical and experimental analysis of aerostatic thrust bearings with porous restrictors

The finite element method is used to predict the performance of aerostatic thrust bearings with a complete porous surface. Results obtained by a 1D and 3D source flow model derived from D’Arcy’s law are compared for rectangular porous bearings having an infinite width. It turns out that the 1D source flow model is adequate for practical design parameters. For a circular aerostatic porous thrust bearing results calculated with several mathematical models for the source flow and slip flow are compared with experimental results. A relatively simple model incorporating unflatness and deformation of the bearing surface correlates well with the experimentally determined bearing performance.     

金属塑性成形过程CSPH无网格法数值模拟

应用微可压缩刚塑性材料的流动法则,采用修正的光滑粒子力学(Corrected smooth particle hydrodynamics, CSPH)无网格法,自行开发了求解金属方棒压缩和圆棒压缩等金属塑性成形过程应用程序。提出一种简单的求解体积应变速率的光滑技术,该技术使应力场计算结果能得到较好的改善。采用CSPH无网格法求解纯铝和Al6060铝合金材料压缩过程得到的速度场和应力场结果与有限元法计算结果以及试验数据进行了分析比较。结果表明, CSPH法能够较好地求解金属大变形过程,为今后进一步分析复杂金属变形问题提供了良好的研究手段。