【作者】 李文平；

【导师】 聂绍珉； 付宝连；

【作者基本信息】 燕山大学 ， 材料加工工程， 2006， 博士

【摘要】 冲压是金属板料主要的成形方法,而弯曲又是其中重要的成形工艺。板料成形后的回弹是很多冲压成形工艺特别是弯曲成形中常出现的缺陷。近年来,高强度薄钢板与铝合金薄板在汽车车身制造中得到大量应用,使回弹问题更加突出。回弹对产品的尺寸精度影响较大,必须在模具设计阶段加以补偿才能保证产品的尺寸精度。传统的方法主要是通过反复地修模、试模来修正和补偿回弹,花费大量的人力、物力和财力。如果在模具制造以前能准确预测出补偿量的大小,就可以大大缩短修模、试模的过程,从而降低模具制造的成本和周期。因此,如何找到一种可靠的预测回弹方法,提高冲压件成形精度,是金属板料冲压成形中的重要课题。影响回弹的因素非常复杂,目前还没有其精确的理论表达式,国际上广泛采用有限元法对板料冲压过程进行数值模拟来预测回弹。板料成形的数值模拟是综合金属塑性加工工艺、有限元理论、CAD等多学科的一门交叉学科。其计算过程是包含几何非线性、材料非线性以及接触非线性的强非线性问题,一直是金属塑性成形数值模拟中的难点。回弹是在成形模拟后进行的,成形模拟中的累积误差使得要精确预测回弹更加困难。因此,目前对板料冲压成形的回弹预测还不准确,提高板料成形的数值模拟精度仍是回弹研究的热点和难点。本文对板料成形数值模拟动力显式算法和时间积分法进行推导分析,得出进行板料成形回弹数值模拟采用动力显式/静力隐式综合算法最为合适。通过讨论vonMises各向同性屈服准则、Hill各向异性屈服准则及3参数Barlat各向异性屈服准则,分析了它们各自的特点。分析了常用的罚函数法接触算法和用于板料成形数值模拟的修正库仑摩擦模型。本文从研究有限元法的理论基础—变分原理出发,根据回弹反耦联系统和反耦联方程的概念,推导了小变形回弹势能原理和广义回弹势能原理;进而又推导了小挠度直梁弯曲回弹势能原理和大挠度直梁弯曲回弹变分原理。建立了计算直梁弯曲回弹有限元法,编制了有限元程序。并应用编制的有限元程序计算简支梁弯曲回弹量,计算结果与现有公式和商用软件的计算结果、实验结果相对照,证明直梁弯曲回弹势能原理及其有限元法是正确的。另外,利用余能原理计算回弹时,仅需要成形的位移和应变,避免了由于材料模型不准确所带来的误差,所以对提高回弹计算精度较势能原理更有优势。因此,本文又推导了板材成形回弹余能原理,为进一步计算板料的复杂变形回弹奠定了理论基础。本文分析了在ANSYS/LS-DYNA软件环境下,板料成形回弹模拟过程中的一些主要数值参数,如虚拟凸模速度、模具圆角处单元密度、板料单元大小和板壳单元厚度积分点数等对回弹预测精度的影响。通过大量的计算,并与实验结果对照,确立了进行有限元数值模拟时这些参数的选取范围。在此基础上,对确定U形件的回更多还原

【Abstract】 Stamping is one of the most common sheet metal forming technique, and bendingtypes of forming operation is the most important process of sheet metal stamping. Partshape error due to springback is common manufacturing defect in sheet metal formingprocesses, specially in bending operation. Recently, more and more high strength steel aswell as aluminum alloys sheets are used by the automobile makers in making automobilebody. The springback problem becomes more prominent. Because springback is one ofthe key factors to influence on the quality of stamped sheet metal parts, it is must be doneto compensate shape deviation caused by springback in mould designing stage in order toget target part. To compensate shape deviation caused by springback, the traditionalpractice in shop floor is using the trial-and-error approaches to obtain desired part. Itseconomic impact in terms of delayed production, tooling revision costs, and rejection ofunqualified parts is tremendous. If accurate springback prediction was available forcompensate shape deviation before designing mould, we can reduce the expensive andtime consuming trial-and-error method. Therefore, finding an accurate method forspringback prediction following forming of arbitrary shapes to get desired parts is animportant task in sheet metal forming fields. The accuracy of springback prediction isaffected by many parameters, there is no accurate theoretical expression by now, the finiteelement method (FEM) is popularly used to predict the springback in sheet metalstamping.The simulation of sheet metal forming is a crossover subject made up of technique ofplastic forming of metals, theory of finite element, CAD, and so on. It is difficult tosimulate sheet metal forming for its strong nonlinear factors, which include geometry,material characters and contact. Springback occurs when sheet undergoes deformation,the accumulated error of forming simulation will make it more difficult to improve theprecision of the springback prediction. Therefore, the results of springback prediction ofsheet metal forming are not satisfied and research on improving springback prediction ofsheet metal forming is still hotspot and nodus at present.In this paper, the dynamic explicit method and time integration schemes forsimulating sheet metal forming were analyzed, the result is that the method of thedynamic explicit approach in loading and the static implicit one in unloading is mostsuitable for springback simulation. This thesis analyses the material models used in sheetmetal forming simulation such as von Mises isotropic yield criterion, Hill anisotropicyield criterion and 3-Parameter Barlat anisotropic yield criterion. This thesis analyses thecommon penalty function method and modified coulomb friction model used for contactanalysis in sheet metal forming simulation.In this paper, in order to investigate springback variational principle which istheoretical basis of springback finite element methods, based on anti-coupled systems andequations of springback the potential energy principle and generalized potential energy ofspringback for small forming are established. And the springback principle of potentialenergy for small flexibility bending and springback variational principle for largeflexibility bending of straight beam are developed. Solving formulae of the finite elementmethod for calculating springback of straight beam bending are established. The finiteelement method programs are compiled for calculating springback of straight beambending, the beam was applied single point load. The calculated results of the program arecontrasted with the results calculated by existing equations, commercial software andthose of three-point bend experiment. It is concluded that the principle of potential energyand finite element method for calculating springback of straight beam bending are right.Otherwise, for the excellence of high precision of which only needs displaces and strainsof forming body for calculating springback and avoids influence brought by untruematerial models, the complementary energy principle of springback for sheet bending areestablished. The theoretical basis is established for calculating complicated springback ofsheet forming using variational principles in the future.In this paper, the springback in U-bending is simulated by ANSYS/LS-DYNA codes.Sensitive factors, such as dumpy velocity of punch, number of element on punch and dieradius, the blank mesh size and number of integration points, are studied. From a greatnumber of calculating examples and results compared with experiment, the reasonablevalue of them for the explicit method in springback simulation is obtained. Based on these,the definition of arc bottom radius which compensated springback method of U-bendingis investigated.Two methods for designing sheet metal stamping dies to produce desired final partshape, based on springback prediction, are compared. The method for designing generalsheet forming dies to produce a desired final part shape, based on reverse compensatingspringback, was developed. Firstly, the CAD model of stamping dies is established basedon the CAD model of desired part. Secondly, the CAE analysis of stamping process iscarried out. Then, according to the springback deviation obtained by numerical simulation,the initial FEM model of dies is modified to be able to compensate springback error.Finally, based on the final FEM model of tools, the CAD model of them could be gainedby reverse compensating method. Hence, the method for die shape design of smallcurvature parts based on springback prediction is established. The die design of bendingof a small part of three dimensional geometries was considered, the correspondingexperiment was carried out, the result is that the designed tools obtained by repeateddisplace compensating method can produce the desired part shapes with small error.更多还原