影响板料成形回弹数值模拟精度的因素分析

分析了影响板料回弹精度的数值模拟因素：屈服准则、硬化模式、单元技术及有限元数值计算方法。研究结果表明各向异性屈服准则Barlat89更接近于材料的实际屈服行为；对于具有Bauschinger效应的材料及复杂加载问题，采用非线性混合强化材料模型预测板料回弹量的精度最高；由于实体壳单元具有实体单元和壳单元的优点，预测回弹模拟结果精度高。研究还表明，在时间允许的条件下，采用较小单元尺寸模拟精度高。     

Comparison of Material Models for Spring Back Prediction in an Automotive Panel Using Finite Element Method

Springback is a crucial factor in sheet metal forming process. An accurate prediction of springback is the premise for its control. An elasto-plastic constitutive model that can fully reflect anisotropic character of sheet metal has a crucial influence in the forming simulation. The forming process simulation and springback prediction of an automobile body panel is implemented by using JSTAMP/LS-DYNA with the Yoshida-Uemori, the 3-parameter Barlat and transversely anisotropic elasto-plastic model, respectively. Simulation predictions on spingback from the three constitutive models are compared with experiment measurements to demonstrate the effectiveness and accuracy of the Yoshida-Uemori model in characterizing the anisotropic material behavior of sheet metal during forming. With an accurate prediction of springback, it can provide design guideline for the practical application in mold design with springback compensation and to achieve an accurate forming.     

Prediction and inline compensation of springback in roll forming of high and ultra-high strength steels

With the rising interest in lightweight construction, the usage of high and ultra-high strength steels has increased remarkably during the last years. Unfortunately, these steel grades show more springback than mild steels do, which leads in consequence to lower dimensional accuracy. To improve the bended part’s geometry considerable trial-and-error work is necessary since the influence of different bending-parameters (e.g. the bending radius, sheet thickness, yield strength, Young’s modulus, the material’s strain hardening coefficient, …) on the amount of springback is still unknown. The aim of the paper at hand is therefore the investigation of springback for different parameter combinations. Furthermore, an online calibration system for occurring springback during roll forming is presented to compensate springback independent of material or process parameters.     

Prediction of springback in sheet forming by a new finite strain model with nonlinear kinematic and isotropic hardening

The paper discusses the application of a finite strain model to predict springback in sheet forming. The concept combines both isotropic and kinematic hardening and utilizes a new algorithm based on the exponential map. A special focus is put on the simulation of the draw-bend test of DP600 dual phase steel sheets and the comparison of the numerical results with experimental data. Further, we investigate the sensitivity of the results with respect to geometry parameters as the tool radius and the sheet thickness as well as the ratio between isotropic and kinematic hardening. In addition, the model is applied to the S-rail benchmark problem and the thermoforming of thermoplastic blends, where large elastic strains occur. The simulation results match the experiments very well.     

Formability Investigations of High-Strength Dual-Phase Steels

Car manufacturing is always regarded as the key industry behind sheet metal forming, and thus, the requirements of and developments in car manufacturing play a decisive role in the development of sheet metal forming. The automotive industry is faced with contradictory demands and requirements: better performance with lower consumption and less harmful emissions, more safety and comfort; these are extremely difficult to supply simultaneously with conventional materials and conventional manufacturing processes. The fulfillment of these often contradictory requirements is one of the main driving forces in the automotive industry and thus in the material and process developments in sheet metal forming, as well. In recent years, significant developments can be observed in the application of high-strength steels. In this respect, the application of various dual-phase steels is one of the best examples. However, the application of these highstrength steels often leads to formability and manufacturing problems. One formability problem is the springback occurring after sheet metal forming. In the current research, we have dealt mainly with advanced high-strength steels, primarily with dual-phase steels. When applying them, the springback phenomenon is one of the most critical issues. To reduce the tremendous amount of experimental work needed, we also applied numerical simulation using isotropic–kinematic hardening rules. The isotropic–kinematic hardening behavior of a given material in the applied Auto Form numerical package may be characterized with three independent material parameters c, v and K(a detailed explanation of their meaning will be given in the main part of this paper). However, we found that the material data included in simulation packages for these new high-strength steels are not fully adequate. For the determination of more reliable material parameters and to achieve better simulation results, a new testing device was developed. Numerical simulations were performed using the material parameters determined by the new device to show the sensitivity of springback behavior to these material parameters.     

Development of a new model for plane strain bending and springback analysis

A new mathematical model is presented for plane strain bending and springback analysis in sheet metal forming. This model combines effects associated with bending and stretching, considers stress and strain distributions and different thickness variations in the thickness direction, and takes force equilibrium into account. An elastic-plastic material model and Hill’s nonquadratic yield function are incorporated in the model. The model is used to obtain force, bending moment, and springback curvature. A typical two-dimensional draw bending part is divided into five regions along the strip, and the forces and moments acting on each region and the deformation history of each region are examined. Three different methods are applied to the two-dimensional draw bending problems: the first using the new model, the second using the new model but also including a kinematic directional hardening material model to consider the bending and unbending deformation in the wall, and the third using membrane theory plus bending strain. Results from these methods, including those from the recent benchmark program, are compared. University of Michigan, Dept. of Mechanical Engineering and Applied Mechanics, Ann Arbor, Mi 48109, USA.     

车用铝合金板材回弹规律研究

在车用铝合金材料的成形加工过程中,回弹是主要的成形缺陷之一并且较难控制。本文对车用6061铝合金板材进行了室温拉伸试验获得其应力-应变曲线并建立改进的Johnson-Cook本构模型。该模型被应用于V形弯曲试验的有限元仿真中,研究不同各向异性屈服准则对板料回弹预测精度的影响,仿真结果表明应用YLD2000-2d屈服准则时其预测精度较高,同时也验证了该模型用于回弹分析的有效性。进一步探究不同因素如变形程度,冲压速度,摩擦条件,压边力等对铝合金板材回弹行为的影响规律,并应用于铝合金发罩内板的冲压成形过程,能够有效减小工件的回弹。     

冲压板料回弹中的失稳研究

回弹是材料成型结束后的一个复杂的弹塑性变形过程 ,它直接影响到冲压件的尺寸精度。本文采用随动强化材料模型考虑了Bauschinger效应 ,指出回弹卸载中可能会出现局部失稳的现象 ,并应用有限元方法和能量原理 ,提出了回弹失稳的临界应力计算方法 ,对于提高回弹的计算精度、改善冲压件的产品质量有重要意义。     

变压边力下高强度钢板的回弹研究

回弹是影响板料成形精度的缺陷之一,特别是对高强度钢板控制回弹是板料成形中研究的重要课题。通过对高强度钢板的回弹仿真,证明合适的变压边力能够减小成形后的回弹,并通过实验验证了这一结论的正确性。     

基于试验和有限元方法的橡皮囊液压成形回弹规律

橡皮囊液压成形中常见的缺陷包括破裂、起皱和回弹等成形缺陷,其中回弹缺陷直接影响钣金件的尺寸精度。文章针对带下陷直弯边和凸弯边钣金件进行橡皮囊液压成形试验。结果表明,随着压力的增加,回弹值减小;弯曲线取向为90°的回弹值,大于弯曲线取向为0°的回弹值;弯曲成形性能与轧制方向的选取有关。有限元模拟结果与试验结果进行对比分析表明,模拟所得回弹值与试验结果相比偏小。     

基于Yoshida-Uemori硬化模型的高强钢冲压件回弹预测研究

基于LS-DYNA软件对高强钢薄板零件的冲压成形进行仿真分析与回弹预测,研究了不同材料硬化模型对回弹预测精度的影响。在仿真分析中应用各向同性硬化模型和考虑包申格效应的Yoshida-Uemori随动硬化模型,将模拟回弹值同试验回弹量进行比较,验证了Yoshida-Uemori材料模型在高强钢回弹预测中的可靠性,为实际生产中模具设计及修正提供理论指导。     

The effect of rate sensitivity on history dependent forming limits of anisotropic sheet metals

history dependent forming limits of sheet metals are computed by examining the role of rate sensitivity of flow stress in forming limits for anisotropic materials. Assuming that necking or nonuniform deformation is caused by initial heterogeneity, the forming limits are computed by considering the flow theory and incorporating an isotropic hardening model for anisotropic materials. Representative results show some of the trends for changing forming limits with different parameters under complex nonproportional loading histories. The computed results agree well with published experimental data for different loading history, anisotropy, and rate sensitivity of sheet materials.     

The effect of partially cut-out blanks on geometric accuracy in incremental sheet forming

Industrial requirements for accuracy in metal sheet components are typically ±0.2 mm where current incremental sheet forming processes are capable of an accuracy of only ±3 mm. Several approaches based on process design modifications or control strategies are being developed to overcome this problem, but none has as yet been entirely successful. This paper proposes and examines a new approach in which the area to be formed within the blank is “partially cut-out” using a water jet or laser cutter. The aim of this partial cut-out is to localise deformation to the area over which the tool travels and thus reduce the difference between a part made by a “contour tool path” and the target product geometry. Several design options are considered, and the approach is evaluated with one simple and one complex part. The results indicate that partially cut-out blanks lead to slightly more accurate forming than conventional blanks when unsupported, but that the accuracy improvement is less than that which is achieved by use of a stiff cut-out supporting plate. The results include an experimental investigation of residual stresses and springback in incremental sheet forming.     

Forming limit of electrodeposited nickel coating in the left region

A uniform nickel (Ni) coating was bilaterally electrodeposited on the low-carbon steel substrate for the application of advanced battery shells. Its forming limit was investigated by Hill localized necking theory coupled with finite element simulation and scanning electron microscopy. The effective stress and effective strain in the Ni coating and steel substrate are deduced using Hill’s anisotropic yield function. The localized necking condition is derived by sandwich sheet analysis, and the forming limit strains are obtained by solving the nonlinear equation of the localized necking condition. Extensive calculations are carried out using the proposed model. This study exhibits the nickel coating thickness and the normal anisotropic coefficients of the coating and substrate have little influence on the forming limit curve (FLC) in the left region of the coated sheet, but the strain hardening exponents of the coating and substrate have much effect on it. The calculated result matches well with the measured data in uniaxial tension. This investigation is useful for the preparation of the electrodeposited Ni coating and helpful for the forming operation of the battery shells.     

DP980高强钢U形弯曲实验与数值模拟

为了评估DP980高强钢材料的成形与回弹特性,通过加载-卸载实验与单向拉伸-压缩循环加载实验,获得了考虑包申格效应的吉田-上森硬化模型参数,并对DP980高强钢的U形弯曲实验进行了研究。实验结果表明,DP980高强钢U形件在凹模圆角处易开裂,在成形过程中需减小压边力并增大凹模圆角。由于材料的回弹量与施加的压边力有关,可通过增大压边力来减小U形件的回弹,但压边力过大又可能导致开裂,因此,需合理选择压边力。同时,对U形件成形后的回弹进行仿真分析,仿真中分别采用等向硬化模型与吉田-上森硬化模型,通过将实验数据与模拟的回弹结果进行对比分析发现,采用吉田-上森硬化模型的仿真结果与实验结果吻合良好,证明了吉田-上森硬化模型模拟回弹的准确度较高,可以应用于回弹仿真中。     

高强度双相钢DP800成形件碰撞性能仿真分析

针对双相钢板DP800,在完成U型梁冲压成形及回弹仿真分析的基础上,建立了闭口帽型梁的碰撞分析模型。采用映射的方法完成冲压成形件U型梁的厚度、应力、应变向碰撞结构件的结果传递。在考虑成形、回弹及应变率等多种因素的影响下,对帽型梁的碰撞过程进行了多工况仿真分析。研究结果表明,成形后的材料厚度变化、残余应力、加工硬化以及应变率,对碰撞过程将产生直接的影响。研究结果为提高汽车碰撞仿真分析的精度提供了参考。     

板料自由弯曲成形及回弹理论解析

对宽板自由弯曲成形及弹复过程进行理论分析,将弯曲过程分为板料未包覆凸模和已包覆凸模两个成形阶段,在每个成形阶段将弯曲板料分为弹性和弹塑性变形区。基于单向应力和小变形假设,分别建立了两个成形阶段的回弹计算模型,导出了任一成形阶段中性层上任一质点卸载前后的转角和弯曲半径数学表达式。基于VC++软件平台,对回弹计算模型进行编程计算,并进行了实验验证,计算结果与实验结果吻合较好,最大误差为0.58°。