电沉积镍涂层的冲压成形极限右边分析与预测

基于材料实际应力应变曲线,运用多项式拟合法计算了涂层薄板的成形极限。对冲压过程中涂层薄板的等效应力和等效应变进行了推导,通过求解Swift分散颈缩失稳条件的非线性方程得到了冲压成形极限的第一主应变,获得了涂层薄板的成形极限右边曲线。计算发现,涂层厚度、基体厚向异性指数对涂层薄板的成形极限有显著影响,镍涂层的成形性能低于钢基体的成形性能,并且基于实际应力应变曲线的涂层薄板的成形极限低于传统方法计算的成形极限。     

基于韧性断裂准则用有限元方法预测镍涂层薄钢板的成形极限

首先通过拉伸试验,得到了镍涂层薄钢板和低碳钢薄板的材料参数,然后利用有限元软件Abaqus/Explicit对镍涂层薄钢板的冲压成形过程进行了有限元模拟,得到了涂层和基体在冲压变形过程中的应力、应变以及断裂的初始位置及厚度分布,并通过Oyane韧性断裂准则预测了镍涂层薄钢板的成形极限;并将韧性断裂准则的理论计算、试验结果与有限元计算结果进行对比。结果表明:Oyane韧性断裂准则能够较好地预测镍涂层薄板的成形极限和失效的初始位置,预测值与试验值相差较小。     

激光拼焊板成形极限图的理论建立方法

由变形不均引起的颈缩或破裂是激光拼焊板的成形难题,工程中迫切需要了解拼焊板的成形性能.成形极限图是一种评价板料成形性能的有效手段,通常可以由试验法和理论计算法获得.目前试验法成本高,费时费工:理论计算法尚不完善,需进一步研究改进.提出一种新的拼焊板成形极限图理论计算模型,结合HOSFORD屈服准则,获得了高强度IF钢等厚激光拼焊板的成形极限图,计算结果与试验数据吻合较好,证实拼焊板成形极限图的理论建立方法的正确性.基于拼焊板成形极限图的理论建立方法,可在已知拼焊板焊接区材料性能参数的条件下快速获得拼焊板成形极限图.     

基于最大二阶主应变和Savitzky-Golay平滑原理对热轧酸洗板成形极限的数值模拟

通过ABAQUS有限元软件对热轧酸洗板的半球形凸模胀形试验进行数值模拟,分析了热轧酸洗板在成形过程中发生全局颈缩的位置,采用Savitzky-Golay平滑原理对主应变一阶、二阶导数进行拟合得到二阶主应变随时间的变化曲线,基于最大二阶主应变准则预测了热轧酸洗板的成形应变极限,并与试验结果进行对比。结果表明:基于有限元模拟和数据拟合得到的成形极限略低于试验测试值,主应变的最大误差为2.2%,模拟结果与试验结果吻合较好,此数值模拟方法可用于热轧酸洗板的成形极限预测。     

基于不同失稳理论的DP780双相钢成形极限预测

采用板材综合成形试验机对DP780双相钢进行极限应变试验,分别基于C-H失稳理论和M-K凹槽失稳理论搭载Yld2000屈服准则和幂指数硬化模型对DP780双相钢成形极限曲线进行预测,并与试验结果进行对比。结果表明：基于M-K凹槽失稳理论和C-H失稳理论获得的成形极限曲线对成形极限的预测精度分别为97.97%和95.82%;初始厚度不均匀度越大,钢板表面越光滑,越有利于成形;当初始厚度不均匀度为0.992时,M-K凹槽失稳理论对DP780双相钢成形极限的预测精度最高,相对误差为0.66%,在实际冲压生产中,当初始厚度不均匀度取0.992时,该理论模型可作为获取DP780双相钢成形极限曲线的一种可靠方法。     

金属板料单点渐进成形极限的数值模拟预测

渐进成形(Single point incremental forming, SPIF)有着较高的成形极限,但目前对其成形极限预测研究较少.基于数值模拟得到的应力应变数据,并结合四组具体的破裂试验,应用Oyane韧性破裂准则有效地预测了厚1.5 mm的LY12(M)硬质铝板的渐进成形极限;在Oyane韧性破裂准则中,当破裂积分值I=4时,预测的工件破裂起始点及成形极限图都与试验结果较为吻合.渐进成形极限远远高于传统成形方式,其"局部、交替、小增量"的变形特点,使小变形不断积累以获得大变形,"强制性"地实现变形的均匀分布,从而获得较高的成形极限.具体体现在成形过程中,应力路径跌宕起伏,应力三轴度(σh/(-σ))较小,不利于材料韧性破裂;局部高压力小增量叠加成形、摩擦热及良好的润滑条件保障了渐进成形件较高的塑性.     

基于回弹补偿的TC4钛合金薄板多工步压制成形参数优化设计

针对TC4钛合金薄板多工步压制成形参数设计优化问题,为使板料能够高效精确地压制成形,分析了TC4钛合金薄板压制成形工艺的成形过程和板料回弹的形成机理,建立了压制过程数学模型和板料回弹数学模型。为获得满足回弹控制准则、减薄控制准则和厚度偏差控制准则的TC4钛合金薄板多工步压制成形优化成形参数,构建了板料回弹控制准则、减薄控制准则、厚度偏差控制准则,提出了基于回弹补偿的TC4钛合金薄板多工步压制成形参数设计方法,建立了基于回弹补偿的TC4钛合金薄板多工步压制成形参数优化流程,以某型号大管径特种圆管的多工步压制成形为例,验证了所提方法的正确性和有效性。     

基于GTN细观损伤模型的激光拼焊板成形极限预测

建立了从细观损伤角度预测拼焊板成形极限的Gurson-Tvergaard-Needleman(GTN)损伤模型,用有限元逆向法确定了损伤模型中的各损伤参数。采用有限元软件ABAQUS耦合基于Mises屈服准则的弹塑性GTN损伤模型,对拼焊板半球凸模胀形过程进行了数值模拟。设计了拼焊板半球凸模胀形物理试验,试验过程中通过改变试件的宽度得到了不同应变状态下完整的拼焊板成形极限图,并与GTN细观损伤模型预测到的拼焊板成形极限图进行对比分析,验证了GTN细观损伤模型预测拼焊板成形极限图的准确性。     

直缝焊管液压成形极限理论预测模型

直缝焊管广泛应用于汽车车身管状零件液压成形中,焊接区影响着焊管塑性变形规律,准确评价焊管缩颈或破裂现象是工程上倍受关注的问题。基于金相分析法和显微硬度测量法分析高频感应焊管的结构特征,并根据液压成形条件下高频感应焊管的变形特点,提出一种用于计算直缝焊管液压成形极限的理论方法。基于该方法,选用Swift硬化方程和Hill屈服准则推导出直缝焊管液压成形极限理论预测模型,在已知焊管(包含焊接区和基体区)材料性能参数条件下可获得直缝焊管液压成形极限图。运用此理论预测模型,计算出QSTE340高频感应焊管的液压成形极限图。成形极限的计算结果与试验对比表明,二者吻合较好,这证明所建立的直缝焊管液压成形极限的理论预测模型是正确的。     

基于韧性断裂的汽车用铝合金板滚压包边成形开裂预测

通过对比铝合金平面直线翻边试验及基于集中性失稳模型得到的极限应变和开裂断口,研究了汽车用铝合金滚压包边的失效机理;基于韧性断裂、塑性增量法则和混合强化准则,理论推导得到了弯曲成形极限图,并通过试验对成形极限应力图进行了验证;最后,通过数值解析的方法,研究了韧性断裂准则在滚边成形中的适用范围。结果表明:基于韧性断裂准则的成形极限图,可以用来预测铝合金滚压包边过程中产生的开裂;包边变形过程中弯曲强化效应无法忽略,适用于拉弯成形极限预测的集中性失稳理论将无法应用于滚压包边成形。     

Forming limit diagrams of strain-rate-dependent sheet metals

This paper presents a predictive model of localized necking for strain-rate-dependent sheet metals. This is achieved by the development of a modified vertex theory for use as a localized necking criterion. The vertex theory is von Mises based and isotropic. The study is motivated by the apparent discrepancies between the measured forming limit diagrams (FLDs) of strain-rate-dependent sheet metals and those predicted using the conventional vertex theory. The modified theory considers the rate-dependent power-hardening material rule. A novel form of quasi-linear stress-strain relation based on the power-hardening rule is analytically derived to model the constitute behavior of the rate-dependent material. The localized necking criterion of the sheet metals is based on the vertex theory, which assumes that localized necking occurs simultaneously with the initiation of a vertex on the yield surface. The stress-strain relation of rate-dependent material is coupled into the vertex theory to deduce the critical conditions for localized necking on both sides of the FLD. Numerical results indicate that the forming limits of rate-dependent sheet metals follow quite different rule as that for rate-independent sheet metals. The ratio between the strain-rate-hardening index and the strain-hardening index plays an important role in the strain-rate effect on FLDs. A typical strain-rate-dependent metal, AKDQ steel, is chosen for validation of the modified vertex theory. The tensile stress-strain curves of AKDQ are tested on both regular and high-speed MTS, covering a wide range of strain rate from 10⁻⁵ to 10 s⁻¹. The hardening law of AKDQ is found to depend greatly on the strain rate, while the strain-rate-hardening index is not a material constant but dependent on the strain rate. Forming limit tests are also performed for AKDQ sheets to measure the FLD. The measured FLD of AKDQ is compared with the prediction based on the developed theory and good agreement is observed.     

基于成形应力极限的管材液压成形缺陷预测

基于塑性应力应变关系及Hill79屈服准则,推导出极限应力与极限应变间转化关系,进而建立2008T4铝合金的成形应力极限图(Forming limit stress diagram,FLSD)。采用LS-DYNA软件对三通管液压胀形过程进行模拟,应用FLSD预测胀形过程中破裂的发生及成形压力极限,并与传统成形极限图(Forming limit diagram,FLD)结果进行了对比。研究表明,FLD与FLSD预测结果中破裂缺陷位置相同,但极限内压力值存在很大差别,而FLSD预测结果与物理试验结果较吻合。考虑到FLD受应变路径影响显著的因素,将FLSD作为管材液压成形等复杂应变路径下的成形极限的判据更加方便可靠。     

Forming limits of AL 6022 sheets with material damage consideration—theory and experimental validation

Characteristics of localized necking in sheet metals are examined with anisotropic plasticity as well as anisotropic damage developed progressively after load application. The vertex theory is employed to describe basic mechanisms of localized necking. Anisotropic plasticity is accounted for by Hill's quadratic yield criterion. An anisotropic damage model based on Continuum Damage Mechanics is reviewed and expanded. The anisotropic damage model is combined with a modified vertex theory to generate damage-coupled localized necking criteria on both sides of forming limit diagram (FLD). The criteria lead to explicit expressions of critical hardening modulus on both sides of FLD. It is shown that the damage-coupled FLD model can be readily reduced and used to predict the forming limit strains of damage-free materials satisfying power hardening law given by other researchers (Hill, J. Mech. Phys. Solids. (1952)1,19; Zhu et al., ASME J. Eng. Mater. Tech. (2001) 123, 329). Critical damage value at localized necking can be computed from the damage-coupled localized necking criteria as a function of stress/strain states and strain paths. Tests on the formability and material properties of Al 6022, such as hardening and damage law, anisotropic plasticity parameter, have been performed. The measured FLD of the material are compared with the predictions based on the damage-coupled localized necking criteria for validation of the proposed FLD model. Material damage is observed to have a definite effect on the forming limits of Al 6022, thus providing a more accurate prediction than that of the conventional models.     

Al-Mg-Si基合金车身板材成形极限及数值应用

采用成形极限图（FLD）试验,运用网格应变自动测试分析系统ASAME,对Al-Mg-Si基合金车身板的成形极限进行了检测,同时,将FLD应用于有限元分析作为成形时出现破裂缺陷的判据,并分析车门外板在冲压成形过程中破裂危险部位主应变在FLD上分布。结果表明：Al-Mg-Si基合金板材的成形极限高于目前常用的AA6111铝合金车身板;拉延筋的形状和位置对成形过程的影响很大,采用双曲面法设计车门可以改善车门腹部拉深不充分引起的刚度问题。     

Forming severity concept for predicting sheet necking under complex loading histories

A new method of predicting neck formation in sheets under non-proportional loading is proposed, based on the concept of “cumulative forming severity”. This concept is borrowed from a macroscopic model of ductile fracture where the crack initiation is governed by the accumulated equivalent plastic strain modified by the stress triaxiality and the Lode angle parameter. Such an approach necessitates a representation of the forming limit diagram (FLD) in the space of the equivalent strain to neck and the Lode angle parameter.Another new factor is the assumption of the non-linear accumulation of forming severity for non-proportional and complex loading histories. A class of non-linear weighting function is proposed with only one free parameter. A starting point in the derivation is the known FLD corresponding to proportional loading. This can be determined from Hill's and Stören and Rice analytical solutions, from numerical simulation, or else taken directly from experiments. In the case of proportional loading, necking depends on the final state of stress or strain, so it does not matter if necking severity index is accumulated in a linear or non-linear way. For non-proportional loading, the unknown free parameter of the non-linear accumulation rule must be determined from a test.Experimental data on FLDs under complex strain paths for two types of material, aluminum alloy 6111-T4 [Graf A, Hosford W. The influence of strain-path changes on forming limit diagrams of A1 6111 T4. International Journal of Mechanical Sciences 1994;36(10):897–910.] and aluminum-killed sheet steel [Muschenborn W, Sonne HM. Influence of the strain path on the forming limits of sheet metal. Archiv fur das Eisenhuttenwesen 1975;46:597–602], found in the literature are revisited by the proposed model. Calibrated from only one test with non-proportional loading condition, the model is able to predict the remaining tests of complex loading paths with good accuracy.     

板料成形极限应力图及其应用研究进展

简要介绍了板料成形极限判据的可靠性对于板料成形质量预测与控制的重要性;分析了传统的基于应变的成形极限(成形极限应变图)判据所存在的只适于在线性加载板料塑性成形中应用的缺陷;介绍了基于应力的成形极限判据(成形极限应力图)的应用前景;综述了国内外成形极限应力图的研究进展,并指出了目前成形极限应力图研究所面临的主要问题.     

Influences of material properties of components on formability of two-layer metallic sheets

Formability of two-layer metallic sheet is constrained by plastic instability and localized necking. Forming limit diagram (FLD) is an accepted measure of sheet metal formability. The formability of two-layer sheets depends on the material properties of their components such as strain hardening exponent, strain rate sensitivity coefficient, stiffness coefficient, and grain size. In this paper, the effects of the mentioned parameters on the FLD of two-layer sheets are investigated with a theoretical model which has been verified with an experimental approach. The results show that the forming limit of two-layer sheet lies between the forming limits of its components depends on their material properties.     

Determination of forming limit diagrams of sheet materials with a hybrid experimental-numerical approach

A new methodology is proposed to obtain the forming limit diagram (FLD) of sheet materials by utilizing routinely obtained experimental punch load versus displacement traces from hemispherical punch stretching experiments and by analyzing strain history of the test samples from finite element simulations of the experiments. The simulations based characteristic points of diffuse and localized necking are utilized to obtain the limit strains. The proposed method for FLD determination considers out-of-plane displacement, punch-sheet contact and friction, and avoids using experimental strain measurement in the vicinity of the neck on the dome specimens and the rather arbitrary inhomogeneity factor to trigger localization such as in the commonly used Marciniak-Kuczynski method. A criterion of maximum in the ‘pseudo’ major strain acceleration for the onset of localized necking, proposed earlier by the present authors, is utilized to determine the limit strain in FE simulations as well as in FLD verification experiments. The proposed overall approach for obtaining FLD is rapid and accurate and could be implemented for routine FLD generation in a laboratory setting with significant reduction in cost, effort and subjectivity.     

Calculation of forming limit diagrams using Hill's 1993 yield criterion

Based on the analysis proposed by Jones and Gillis (JG), forming limit diagrams (FLDs) are calculated from idealization of sheet deformation into three stages: (I) homogeneous deformation up to maximum load, (II) deformation localization under constant load, and (III) local necking with precipitous drop in load. In the calculation, Hill's 1993 yield criterion is used. Using this yield criterion and the JG model, effects of materials parameters such as ratio of uniaxial to equi-biaxial yield stress, strain hardening, strain rate sensitivity and plastic anisotropy on the shape and level of forming limit curves are studied. In addition, the capability of the JG model to predict limit strains is demonstrated through comparison of calculated results with experimental data for the interstitial free (IF) steel and aluminum alloys 3003-O and 8014-O. It is concluded that although the model predicts the effect of material parameters reasonably well, the calculated limit strains are higher than the experimental FLDs. The observed discrepancy may be attributed to the assumption of planar isotropy, cavitation and the nature of texture present in the sheets. Due to the overestimation of the predictions, care must be taken when using this approach for industrial purposes.