板料成形反向模拟精度修正法

传统的板料成形反向模拟法(Traditional inverse analysis method, TIAM)没有考虑变形历史对成形过程的影响,使得反向模拟法计算的应力值与实际情况有较大偏差。金属在流经凹模圆角时受到强烈的弯曲—反弯曲作用,并且凹模圆角处往往是应力集中的区域,能否较为准确地描述该区域变形历史,关系到反向模拟法应力/应变计算精度。针对反向模拟法无法准确预测成形零件应力分布的缺点,提出一种快速搜索流经凹模圆角区域单元的方法,对该部分单元进行应力修正。采用一种高效的多步加载应力应变本构关系模型,避免了增量有限元法应力应变更新算法的复杂性,同时充分考虑了凹模圆角对金属变形历史的影响。NUMISHEET’93 U形件弯曲的标准考题中,通过与增量有限元分析软件ABAQUS/Standard计算结果的比较,验证了采用所提出的多步加载本构关系模型可以有效地反映变形历史的影响,获得更为接近真实情况的应力分布。     

多工步板料成形问题的多步反向模拟法

提出一种新的多步反向模拟法,该方法建立在虚功原理的基础上,为提高计算精度,采用弹塑性材料模型。提出一种适用于多步反向模拟法的本构关系,可以充分考虑弹 / 塑性变形状态、加载 / 卸载状况。提出一种“双截面线法”来快速获取中间构型,可以充分考虑塑性变形对构型的影响。提出一种快速接触搜索算法来实现中间构型各个节点在约束滑移面上的移动,采用全局搜索和局部搜索策略相结合的方法,加快搜索速度。理论成果已经在自主开发的板料成形反向模拟软件InverStamp/Multi-step模块中实现。在盒形件多步拉深实例中,坯料轮廓、等效应变分布等计算结果与基于ls-dyna的增量法模拟软件Dynaform计算的结果十分吻合,验证了多步反向模拟法相比于一步反向模拟法的优势。     

金属注射成形烧结工艺的试验与数值模拟

采用粘塑性理论建立烧结过程的本构模型,以316L不锈钢粉末注射成形产品为例,设计出烧结过程中重力作用下弯曲梁试验和膨胀计中的自由烧结试验,分别用于标定本构模型中的粘度模量和烧结应力.通过用户子程序,在有限元求解器Abaqus(r)上实现数值模拟,预测产品在烧结过程中的收缩和变形.考虑了重力、摩擦以及注射阶段粉末与粘结剂的偏析效应对于烧结工艺的影响,数值模拟结果与试验结果一致.     

多步逆成形有限元法中间构型初始解预示算法

在逆成形有限元法中,为考虑加载路径的影响,针对不同成形类型阶段引入中间构型,整个过程被划分为多阶段成形过程,即多步逆成形有限元法,一个好的中间构型初始解的构造是其中的关键问题之一。针对中间构型初始解的构造,提出一种基于滑移约束曲面的节点反向映射法：利用板材的纯几何弯曲效应构造出弯曲构型,经过伪最小面积法优化后生成空间滑移约束曲面;投影展平的最终构型网格面到展平滑移约束曲面网格中,利用面积坐标计算出所有投影节点的坐标;投影节点反向映射回到空间滑移约束曲面上,构造出对应中间构型的初始解。另外,针对中间构型初始解构造中可能出现部分节点无投影区域的问题,给出一种扩展空间滑移约束曲面轮廓的解决方法。零件的数值算例验证相应算法的可行性与有效性。     

基于变弹性模量的Ti-6Al-4V板材五点弯曲回弹预测

Ti-6Al-4V钛合金材料在弯曲成形过程中会产生较大的回弹,其弹性模量对回弹影响较大,但以往研究均未考虑材料塑性应变变化过程中弹性模量的变化。以Ti-6Al-4V钛合金为对象,进行了材料的单轴拉伸实验和循环加载-卸载实验,以揭示材料各向异性参数及材料弹性模量随塑性应变变化的规律,在此基础上建立了Ti-6Al-4V钛合金变弹性模量数学模型。基于YLD2000-2D屈服准则及变弹性模量和Mises各向同性两种不同的本构模型,对常温下Ti-6Al-4V钛合金板材的五点弯曲过程进行了数值模拟,为了验证数值模拟结果,进行了常温下Ti-6Al-4V板材的五点弯曲实验,结果显示,前者显著提高了Ti-6Al-4V钛合金弯曲回弹预测精度,预测精度相比后者提高了31.18%。     

应变梯度理论的新进展（一）：偶应力理论和SG理论

介绍两种应变梯度塑性本构模型：ＣＳ应变梯度理论－偶应力理论，ＳＧ应变梯度塑性理论，并对它们在断裂国 的应用进行了评述。给出一种考虑可压缩性的方法，并根据这种模型用薄梁弯曲的例了给出了可压缩性的影响，本文的讨论虽限制在形变理论范围内，但按照相应的方法也可以得到流动理论的形式。     

基于非线性弹性卸载的L形弯曲成形回弹研究

为提高板料成形数值模拟回弹预测的精度,将弹性模量随塑性变形的改变引入到数值模拟回弹分析中。通过静力拉伸实验,研究了AA2024-T3铝合金板料的塑性变形对弹性模量的影响,给出了弹性模量与等效塑性应变之间的数学关系。建立了基于Holloman型等向强化和von.Mises屈服的非线性弹塑性本构模型,并利用ABAQUS/Standard的用户自定义接口UMAT实现了该弹塑性本构模型。设计了L形弯曲实验装置,利用基于非线性弹性的本构模型,对实验过程进行了数值模拟,实验与模拟结果的对比表明,在数值模拟中考虑了弹性模量的非线性能够提高回弹预测精度。     

基于包辛格效应的回弹仿真分析

金属材料在一个方向上的应变硬化降低了反方向的屈服强度,材料包辛格效应的存在对车身成形仿真精度产生了重要影响,尤其现今高强钢和铝合金的大量应用,使车身成形件的回弹问题日益突出,车身模具制造对有限元回弹预测的准确性提出更高的要求。为了提高回弹的仿真精度有必要对材料的包辛格效应进行研究,利用一套夹具对DC06和DP600两种材料的薄板进行拉伸压缩试验,获得不同预应变下的位移加载曲线,通过拉伸压缩试验结果与仿真结果的对比分析,得到能反映材料包辛格效应的非线性混合硬化模型材料参数。开展U形件成形试验,并建立试验的仿真模型,计算DC06和DP600薄板的U形件成形回弹量,分析等向强化、混合强化和随动强化本构模型对回弹预测精度的影响,针对回弹仿真结果和试验结果的差别,对影响仿真精度的材料模型因素进行分析。结果表明,DC06和DP600的包辛格效应大小存在差别,考虑包辛格效应有助于回弹仿真精度的提高,但小曲率弯曲成形回弹计算对材料本构模型的敏感性,限制了回弹仿真精度的提高。     

数控弯管精确下料长度的确定和下料方案优化

根据数控弯管加工理论和大量现场实践研究,考虑了回弹、延伸及工艺限制等因素对弯曲成形精度的影响,制定了精确下料长度的确定方法;并制定了一种实现简单、实用的管子下料优化组合解决方法,克服了以往下料算法中的困难和不足.实践证明,给出的方案具有较好的实用价值.     

弯曲成形回弹研究进展

回弹是影响弯曲件成形质量的主要因素之一,是弯曲模具和弯曲工艺设计要考虑的关键问题之一。如果不充分了解弯曲回弹规律,要想获得预期精度的弯曲件,需要反复试模、修模,优化模具尺寸,或者在成形结束后还要对弯曲件进行校形,这样将会导致生产周期的延长,生产成本的提高。本文综述了前人对弯曲回弹问题的研究工作和研究成果,简述了弯曲回弹控制的方法,最后基于目前的研究现状,提出了弯曲成形回弹研究有待于进一步解决的问题。     

有限元逆算法与板料成形工艺的评价

依据理想形变理论，研究开发了冲压成形过程模拟的有限元逆算法，根据变形体的整体塑性功取相对极值的条件，导出了塑算法有限元方程。提出了求逆算法初始解以及求解与给定形状的毛坯相对应的冲压件形状的迭代计算方法。采用有限元塑算法预测了与冲压件形状相对应的冲压件毛坯的展开形状，根据给定的板坯形状计算了冲压件最终构形及应变分布。分析计算实例表明，逆算法可用于对板料成形工艺方案进行快速评价，对冲压工艺参数进行优化。     

Friction coefficient of upsetting with a procedure of combining the inverse model and the Tikhonov method

The purpose of this paper is to develop an inverse procedure, which is used to obtain the history of the Coulomb's friction coefficient on the contact surfaces between the workpiece and the die during the AA1050 aluminum upsetting process. This procedure is based on the experimental upsetting normal loadings and combines the matrix-presentation linear least-squares errors method of inverse elastic–plastic large deformation finite element model developed in this paper with the regularization of Tikhonov method. The square of the difference of the friction coefficients between successive stages is adopted as the stabilizing function of the regularization of Tikhonov method. In this paper, the Coulomb's friction model is adopted before the sticking effect occurs. After the sticking effect occurrence, the shear strength friction model is adopted. From the results of this paper, it can be shown and demonstrated that the inverse procedure proposed in this paper can make the history of the Coulomb's friction coefficient more stable and reasonable.     

Finite Element Method on Shape Memory Alloy Structure and Its Applications

It is significant to numerically investigate thermo-mechanical behaviors of shape memory alloy(SMA) structures undergoing large and uneven deformation for they are used in many engineering fields to meet special requirements To solve the problems of convergence in the numerical simulation on thermo-mechanical behaviors of SMA structures by universal finite element software.This work suppose a finite element method to simulate the super-elasticity and shape memory effect in the SMA structure undergoing large and uneven deformation.Two scalars,named by phase-transition modulus and equivalent stiffness,are defined to make it easy to establish and implement the finite element method for a SMA structure.An incremental constitutive equation is developed to formulate the relationship of stress,strain and temperature in a SMA material based on phase-transition modulus and equivalent stiffness.A phase-transition modulus equation is derived to describe the relationship of phase-transition modulus,stress and temperature in a SMA material during the processes of martensitic phase transition and martensitic inverse phase transition.A finite element equation is established to express the incremental relationship of nodal displacement,external force and temperature change in a finite element discrete structure of SMA.The incremental constitutive equation,phase-transition modulus equation and finite element equation compose the supposed finite element method which simulate the thermo-mechanical behaviors of a SMA structure.Two SMA structures,which undergo large and uneven deformation,are numerically simulated by the supposed finite element method.Results of numerical simulation show that the supposed finite element method can effectively simulate the super-elasticity and shape memory effect of a SMA structure undergoing large and uneven deformation,and is suitable to act as an effective computational tool for the wide applications based on the SMA materials.     

Matrix-presentation linear least square error method of inverse elastic-plastic large deformation finite element model for upsetting

The main purpose of this paper is to develop the matrix presentation linear least square error method of inverse elastic-plastic large deformation finite element model for upsetting to obtain the friction coefficients during the upsetting process. This inverse model assumed the linear material and based on the modified experimental loading increments using the linear modified experimental upsetting loading standard proposed in this paper. Then the friction coefficients of contact boundary between the workpiece and the die at specific finite element analysis stages can be derived. Finally, using the cubic spline fitting, the history of friction coefficient during the upsetting process can be obtained. It is demonstrated that the workpiece profile of upsetting experiment is quite identical to the workpiece profile of simulation using the result obtained in this paper as the history of friction coefficient of contact boundary, and furthermore the distribution of stress and strain of the workpiece during upsetting process can be understood.     

基于几何映射法的钣金展开有限元逆算法

在现有线性变形逆向映射法的基础上，采用几何映射法计算变换单元，避免单元刚度矩阵出现畸形，提高钣金展开有限元逆算法的稳定性和收敛速度，对几何映射法与投影法所计算的变换单元进行了对比，数值计算结果表明用几何映射法计算的初始估算展开料形状更接近于最终的钣金件展开形状。以涡轮叶片和曲柄轴箱盖的展开为例对改进后的有限元逆算法进行了实验和数值验证，试验结果显示，该方法是有效的钣金展开算法。     

滑动接触中摩擦发热的数值分析

在滑动接触中，存在摩擦起热问题，运动机理对接触行为参数特性的影响不同于纯滚动接触，采用有限元方法，利用ANSYS软件，对2维滚滑模型进行分析，通过研究接触区的温度，接触应力和变形在运动学状态下的变化特性，可以看到接触状态的非稳定性必定会造成实际摩擦状态的不同，这也是形成接触表面不均匀磨损的原因之一。     

Combined effect of friction and macroscopic deformation on asperity flattening

The combined effect of friction and macroscopic plastic deformation on asperity flattening is studied. Crushing of a periodic array of wedge-like asperities is formulated as a rigid-viscoplastic periodic indentation problem with superimposed macroscopic deformation. A micromechanical framework is developed and the corresponding boundary value problem is solved using the finite element method. An anomalous regime of asperity flattening is predicted at low flattening rates, in which the effect of friction on asperity flattening is opposite to that observed in the absence of macroscopic deformation and also at high flattening rates. An incremental elastoplastic analysis confirms this finding.     

基于模糊的框架变形重构的应变测量修正

针对三维(three-dimensional,简称3-D)框架结构弹性变形重构中存在的应变测量误差问题,提出了一种应变测量修正的方法。通过逆有限元法(inverse finite element method,简称iFEM)推导出结构表面应变量与测量点位移量之间的应变-位移关系,实测位移量代入应变-位移的转化关系得到相应的逆解应变。由于测量应变中含有安装、测量误差等,测量应变量与逆解应变量之间非线性数学关系模型难以准确得到。基于模糊网络对任意映射关系的无限逼近特性,采用模糊网络(fuzzy network,简称FN)逼近测量应变与逆解应变的关系。在确定模糊网络后,调用网络完成测量应变的修正,利用修正后应变量求解框架的变形位移。最后在三维框架模型上进行实验验证,实验结果表明结构变形重构的误差普遍减小了60%以上,说明该测量应变修正方法能够有效提高变形重构的精度。     

NUMERICAL SIMULATION FOR LASER BENDING OF SHEET METAL

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薄板冲压成形过程的并行有限元仿真技术

根据显式有限元的计算特点和神威超级计算机的体系结构,设计了基于消息传递和区域分解的动态显式有限元的并行算法,给出了基于薄板冲压有限元仿真特点的并行化分区算法,开发了薄板冲压成形并行有限元仿真软件。算例计算结果表明：采用并行有限元计算,可以大大提高薄板冲压成形的仿真计算效率。