非线性应变路径下金属薄板的成形极限研究

发布时间：2018-04-22 20:23

本文选题：非线性应变路径 + 成形极限　；参考：《湖南大学》2015年博士论文

【摘要】：金属薄板冲压成形是一种广泛用于汽车、航空航天、装备制造业零部件生产的塑性成形工艺,成形极限是评价成形零件质量好坏和工艺设计优劣的重要指标,准确与否对成形工艺设计至关重要。传统的成形极限图是基于线性应变路径假设进行理论计算或试验测试获取的,然而,实际冲压成形过程中的材料所经历的多为非线性应变路径状态,在复杂的非线性加载路径条件下,由晶体塑性变形理论,一个滑移方向的塑性变形会影响晶体其它方向的后续变形,应变路径变化对材料行为尤其是强化特性和成形极限有着显著的影响。因此,对非线性应变路径条件下的材料本构模型和成形极限进行研究,探索预应变加载条件下的成形极限理论,分析热冲压工艺条件下的成形极限,对于提高成形性评价准确度及其在金属塑性成形工艺中的工程应用具有重要意义。本文围绕非线性应变路径变化条件下金属薄板的成形极限,开展了预应变加载条件下的成形极限理论与计算方法、连续应变路径变化条件下的成形极限试验测试方法和数值模拟分析,热冲压工艺条件下的成形极限及其在热成形工艺中的应用等方面的研究。本论文完成的主要研究内容和所取得的研究成果如下:(1)针对Swift分散性失稳理论和Hill集中性失稳理论受到线性应变路径假设条件约束的问题,建立了变应变路径条件下材料失稳的约束方程,并基于该约束方程,提出了适用于含预应变加载条件的双段线性应变路径成形极限理论与计算方法。在理论计算中,当采用复杂的材料本构模型时,往往难以得到直接的解析结果,此时,采用基于沟槽缺陷假设的M-K模型成为计算非比例加载条件下的板料成形极限的合理选择,然而,材料本构模型中高次屈服函数的引入易使得M-K模型中力平衡条件和几何相容条件组成的非线性方程组的求解计算产生振荡,造成塑性应变率比达不到临界收敛值的问题,为了克服这一问题,在非线性方程组的迭代求解中引入了一个回溯算子,对迭代步长进行相应的缩放,有效的抑制和消除了迭代计算过程中的振荡和不收敛问题,使得基于M-K模型的成形极限计算更易于得到收敛的结果。同时,采用文中的方法对铝合金板Al 2008-T4在双段线性应变路径条件下的成形极限进行了计算,结果表明,计算结果与成形极限试验曲线吻合良好,较为准确的预测了预应变对材料成形极限曲线的影响。(2)提出了包含应变路径加载历史的全新成形极限评价指标Fsp,为准确判断连续应变路径变化条件下的金属薄板成形性提供了科学的依据,为验证该成形性评价指标的准确性,基于拉延成形工艺中材料的变形模式,设计了拉延-反拉延成形极限试验装置,通过试验研究了采用DIC技术获取板料成形过程中实时应变路径的方法和成形极限应变状态的判断方法。测量得到的板料初始失效点的应变路径数据表明,拉延-反拉延成形极限试验装置成功的实现了薄板成形过程中应变路径的典型变化,得到的连续应变路径变化模式包括:单轴拉伸-平面应变转变模式、等双轴拉伸-平面应变转变模式、平面应变-等双轴拉伸-平面应变转变模式。基于拉延-反拉延成形极限试验中板料的变形特点,修正了极限拱顶高评价指标,定量地分析了试样形状和工艺条件对拉延-反拉延成形极限试验过程中应变路径和修正的极限拱顶高评价指标的影响,依据测量得到的试样初始失效点应变路径试验数据验证了所提出的连续应变路径变化条件下的薄板成形性评价指标的准确性。(3)建立了考虑非线性应变路径引起的材料潜在硬化(Latent Hardening)效应的各向异性本构模型,该模型通过引入反映材料变形过程加载路径变化的内变量,分别考虑了非线性应变路径加载中材料微结构变化对各向同性硬化和随动硬化模量的影响,反映了材料特性的应变路径相关性,能够表征材料在反向加载时的包辛格效应和正交应变路径变化时的交叉硬化效应,弥补了标准的各向同性-随动强化组合模型存在没有考虑因非比例加载路径变化引起的潜在硬化效应的缺陷。编写了二次开发材料子程序UMAT将该模型集成到薄板成形有限元数值模拟软件LS-DYNA中。采用文中建立的各向异性本构模型对拉延-反拉延成形极限试验和先进高强度钢的扭曲回弹进行了数值模拟仿真,得到的结果和相应的试验数据比较表明,考虑了潜在硬化效应的强化模型结合先进的各向异性屈服准则提高了数值模拟的精度。(4)分析了温度和应变率对22Mn B5超高强度钢的材料特性和力学行为的影响,引入Logistic方程对热成形板料塑性各向异性特性的温度相关性进行描述,构建了热成形工艺条件下的塑形各向异性材料本构模型,研究结果表明,所建立的本构方程对板料在高温奥氏体条件下的流动行为描述与试验结果一致;基于M-K模型,计算得到了恒温常应变率稳态条件下的成形极限曲线,结果表明,在考虑的工艺条件范围内,当温度升高时,板料的成形性提高,当应变率增加时,成形极限曲线有所提高;构建了适用于热冲压成形工艺过程的三维成形极限曲面,建立了B立柱的热-力耦合有限元模型,B立柱热成形数值模拟结果和试验结果的对比表明,对成形后零件截面的厚度分布二者吻合良好,有限元仿真准确的预测了工件的破裂失效位置,验证了所建立的材料本构模型和热-力耦合模型的准确性以及三维成形极限曲面在热成形工艺中应用的有效性,提出的热冲压成形极限的模拟预测方法为温热状态下薄板的成形性分析提供了重要的依据。
[Abstract]:Sheet metal forming is a plastic forming process widely used in automobile, aerospace, and equipment manufacturing parts production. The forming limit is an important index to evaluate the quality of forming parts and the advantages and disadvantages of the process design. The accuracy is very important to the design of forming process. The traditional forming limit diagram is based on the linear strain path false. It is obtained by theoretical calculation or test test. However, the material in the actual stamping process is mostly nonlinear strain path state. Under the condition of complex nonlinear loading path, the plastic deformation theory of crystal, the plastic deformation of a slip direction will affect the subsequent deformation of the other direction of the crystal and the strain path change. Therefore, the constitutive model and forming limit of materials under the condition of nonlinear strain path are studied. The forming limit theory under pre strain loading conditions is explored and the forming limit is analyzed under the conditions of hot stamping, and the accuracy of formability evaluation is improved. It is of great significance for the engineering application of metal plastic forming process. In this paper, the forming limit theory and calculation method under the condition of pre strain loading are carried out around the forming limit of the metal sheet under the condition of nonlinear strain path change, and the test method and numerical simulation of the forming limit test under the condition of continuous strain path change are carried out. Analysis, research on the forming limit of hot stamping process and its application in hot forming process. The main contents and achievements of this paper are as follows: (1) according to the problem of the Swift dispersion instability theory and the Hill centralized instability theory being constrained by the linear strain path hypothesis, the changes are established. The constraint equation of material instability under the strain path condition, and based on the constraint equation, the forming limit theory and calculation method suitable for the double segment linear strain path with pre strain loading conditions are proposed. In the theoretical calculation, when the complex material constitutive model is used, it is often difficult to get the direct analytical results. At this time, the trench based on the ditch is adopted. The M-K model of the hypothesis of slot defect is a reasonable choice for calculating the forming limit of sheet metal under non proportional loading. However, the introduction of high yield function in the material constitutive model makes it easy to make the calculation of nonlinear equations composed of force equilibrium and geometric compatibility conditions in the M-K model, resulting in the ratio of plastic strain rate to the ratio. To overcome this problem, in order to overcome this problem, a backtracking operator is introduced in the iterative solution of nonlinear equations, and the iteration step is scaling accordingly. The oscillation and non convergence of the iterative calculation are effectively suppressed and eliminated, which makes the calculation of the forming limit based on the M-K model more easily convergent. At the same time, the forming limit of the aluminum alloy plate Al 2008-T4 under the condition of two segment linear strain path is calculated by the method. The results show that the calculation results are in good agreement with the forming limit test curve, and the effect of the pre strain on the forming limit curve of the material is accurately predicted. (2) the strain path loading is put forward. The new forming limit evaluation index Fsp of history provides a scientific basis for accurately judging the formability of metal sheet under the condition of continuous strain path change. In order to verify the accuracy of the formability evaluation index, based on the deformation mode of the material in the drawing process, a tensile and reverse drawing limit test device is designed, and the test is carried out through the test. The method of obtaining the real time strain path in the process of sheet metal forming with DIC technology and the method of determining the forming limit strain state are studied. The strain path data of the initial failure point of the sheet material measured shows that the typical change of the strain path in the Bo Bancheng shape process is successfully realized by the drawing limit test device. The model of continuous strain path change includes: uniaxial tension plane strain transition mode, double axis tensile plane strain transition mode, plane strain and equal biaxial tensile and plane strain transition mode. Based on the deformation characteristics of plate material in the limit test of drawing and reverse drawing, the evaluation index of limit vault height is corrected and the sample is analyzed quantitatively The influence of the shape and process conditions on the strain path and the revised limit vault height evaluation index during the drawing limit test of drawing and drawing, and the accuracy of the evaluation index of the Sheet Formability under the condition of the continuous strain path change were verified by the test data of the initial failure point strain path test obtained. (3) An anisotropic constitutive model for the potential hardening (Latent Hardening) effect of material caused by nonlinear strain paths is established. By introducing an internal variable reflecting the variation of the loading path in the process of material deformation, the model considers the isotropic hardening and the dynamic hardening modulus of the material microstructure change in the nonlinear strain path loading. The effect, reflecting the strain path correlation of material properties, can characterize the cross hardening effect of the Basinger effect and the change of the orthogonal strain path in the reverse loading, and make up for the defects of the standard isotropic and servo intensifying combination model without considering the potential hardening effect caused by the change of non proportional loading path. The two development material subprogram UMAT was compiled to integrate the model into the finite element numerical simulation software LS-DYNA of thin sheet forming. By using the anisotropic constitutive model established in this paper, the numerical simulation of the stretch back drawing forming limit test and the distorted springback of advanced high strength steel was simulated, and the results obtained and the corresponding test data were compared. It is shown that the enhancement model considering the potential hardening effect and the advanced anisotropic yield criterion improve the accuracy of numerical simulation. (4) the effect of temperature and strain rate on the material properties and mechanical behavior of 22Mn B5 super high strength steel is analyzed, and the temperature dependence of the plastic anisotropy of the thermally formed sheet is introduced by the Logistic equation. The constitutive model of plastic anisotropic material under the condition of hot forming is described. The results show that the flow behavior of the plate under the condition of high temperature austenite is consistent with the experimental results. Based on the M-K model, the forming limit curve of the constant temperature constant strain rate steady condition is calculated. The results show that the formability of the sheet is increased when the temperature is raised, and the forming limit curve is improved when the strain rate is increased. A three-dimensional forming limit surface suitable for the hot stamping process is constructed. The thermal force coupling finite element model of the B column is established, and the numerical simulation results of the B column hot forming are obtained. The comparison of the experimental results shows that the thickness distribution of the sections of the parts after forming is in good agreement. The finite element simulation can accurately predict the fracture failure position of the workpiece. The accuracy of the constitutive model and the thermal force coupling model of the material and the validity of the application of the three-dimensional forming limit curve in the hot forming process are verified. The simulation and prediction method of hot stamping forming limit provides an important basis for the formability analysis of thin sheet under warm and hot condition.

【学位授予单位】：湖南大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG306

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