【作者】 郑刚；

【导师】 李光耀；

【作者基本信息】 湖南大学 ， 车辆工程， 2008， 博士

【摘要】 汽车车身覆盖件成形中,若工艺参数和模具几何参数设置不当则易引起起皱、破裂等成形缺陷。为保证成形质量,通常在拉延模压料面合理设置拉延筋调整拉延阻力大小和分布,以改善板料流动的不均匀性,提高覆盖件成形质量。近几十年来很多学者在拉延筋相关方面开展了较为广泛的研究,并已取得诸多成果,但拉延筋相关研究仍存在较多问题。本文在前人研究的基础上对拉延筋分析模型、数值模拟中的等效拉延筋仿真模型、拉延筋几何参数优化反演以及真实拉延筋仿真模型等方面进行了深入的研究。并将研究成果应用到自主开发的板料冲压成形软件CADEMⅡ中,开发了拉延筋相关功能模块,提高了CAE软件的工程实用性。研究具体内容如下:(1)建立了一种考虑混合硬化的拉延筋分析模型。目前的拉延筋阻力模型大多忽略板料弯曲时中性层的偏移,并假设板料在复杂的循环加载条件下仍然保持等向强化,不考虑包辛格效应的影响。本文将板料经过拉延筋的复杂塑性变形过程简化为一根平面应变板条在拉伸力作用下的反复弯曲与校直过程,根据Love Kirchhoff假设及平面应变假设,建立了一种拉延筋数值分析模型。该模型不仅考虑了板材厚度变化、材料各向异性,而且通过应变记忆因子考虑板料的混合强化,构建一种简易循环加载本构关系,并能够比较准确地描述了板料的加工硬化特性和包辛格效应。通过与试验结果和仿真结果进行对比,验证模型的合理性和有效性。(2)建立了一种基于快速接触搜寻的等效拉延筋仿真模型。将点到块的接触算法应用于等效拉延筋的接触搜寻,快速建立与更新等效拉延筋节点与板料单元的接触对关系,减少仿真过程接触搜寻时间。(3)提出一种基于近似模型的拉延筋几何参数优化反演方法。针对覆盖件的起皱、拉裂等成形缺陷,建立了相应的目标评价函数。采用均匀拉丁方试验设计方法提取适当的设计参数样本构造响应面近似模型,并不断通过移动和缩放设计兴趣域优化响应面模型优化反演拉延筋参数。该方法能有效克服常规响应面法在整个设计空间进行逼近导致精度低的缺陷;大量减少调用有限元正问题模拟调用次数,提高计算效率,而且能够克服板料成形数值模拟中因单元计算或接触计算产生问题而使整个优化过程无法进行的瓶颈问题。此外,将多目标粒子群优化方法应用到了板料成形优化研究,拓展了多目标粒子群优化方法的应用领域,为其它复杂系统的优化研究起到重要的借荐作用。数值算例表明该方法可在设计域内快速寻优,有助于加快模具设计进程,减少生产成本。(4)建立了一种基于自适应和子循环的真实拉延筋仿真模型。目前主要的等效拉延筋模型,对变截面过渡拉延筋、拉延端部效应、斜拉延筋以及材料特性的局部变化未给予考虑,因而需要通过真实拉延筋模型进行有限元仿真分析获取板料经过拉延筋后更为准确的应力应变信息。然而由于流经拉延筋的板料单元尺寸限制,导致临界时间步长过小,极大地降低了真实拉延筋模拟的计算效率。针对该问题,本文建立了一种基于自适应和子循环方法的真实拉延筋模型。自适应方法针对拉延筋的几何尺寸问题,对流经拉延筋的板料单元进行自适应网格划分限制计算单元总数,并将整个板料单元网格按时间步长划分区域;采用子循环算法对不同区域单元采用各自的时间步长进行积分,以提高真实拉延筋模拟的计算效率。更多还原

【Abstract】 In forming field, defects such as wrinkles and fractures are often caused owing to non-uniform material flow in the complex surfaces. In the forming operations of an automotive cover panel, the setting of drawbeads in the blank holder is an effective technique to supply a frictional resistance force in the sheet contact area and produce tensile stress in the sheet, which controls the material flow to obtain a more uniform deformation. In the past several decades, works of the drawbead have been studied by many researchers, but there still exist many problems in this field. In this study, the drawbead analytical model, the equivalent drawbead model in FEM and the optimization of drawbead parameters, have been proposed based on the former researches. All of the research results have been applied successfully in forming self-devolped software CADEMⅡ. This paper includes the following topics:1. A numerical analysis drawbead model with isotropic-kinematic harding law is proposed to calculate drawbead restraining force. The equivalent drawbead restrained force models are used to describe the effects on the mechanics of the deformation of the sheet experiencing cyclic bending/unbending with isotropic cyclic hardening rule, and does not consider the incline of neutral layer and the Bauschinger effect. A numerical analysis model is proposed to calculate drawbead restraining force based on the Love-Kirchhoff assumption and plane strain assumption. The proposed model adequately considers the valuation of sheet thickness, the anisotropy and the incline of neutral layer. A Stress-strain law and a mixed isotropic-kinematic harding law under cyclic bending are described through the introduction of a strain memory factor to take into account the Bauschinger effect. The proposed model is compared with experimental ones and the validity of this model is verified.2. An equivalent drawbead model is established, which is based on the particle to segment contact algorithm. The particle to segment contact algorithm is applied to quickly build and update the contact pair between the equivalent drawbead nodes and sheet metal elements, to reduce the time consuming for contact search.3. A metamodel based inverse method of the drawbead geometrical parameters is suggested. The building of reasonable criterions to form objective functions such as fracture, wrinkle and insufficient stretching, which can be used to evaluate the formability of automobile panels. The first approach was for an optimization of the design variables of sheet forming with the region of the interest moved across the design space and the uniform latin square designs of experiment. The method can effectively overcome the shortcoming of low calculation accuracy compared with the conventional response surface method. And the obtained optimization procedure is very efficient due to decreasing the number of FEM evaluations. In addition, Multi-objective particle swarm optimization method is applied in the research on the forming optimization. It extends the applications of the particle swarm optimization method. numerical examples are presented to demonstrate the usefulness of the proposed optimization procedure for the design of mould.4. Based on the adaptive method and subcycling algorithm, The FEM model with true drawbeads is established. Because the variable cross-section of the transition drawbead, the effect of drawbead end, the cable-stayed extension drawbead and the material properties change of local sheet are not be taken into account in the equivalent drawbead models usually used in numerical simulation, The strain of the sheet metal having passed the true drawbead is accurate access by the true drawbead simulation model compared with the equivalent drawbead models. However, the sheet element must be enough small to run through the drawbead, leading to the critical time step too small to significantly reduce computational efficiency. To solve the problem, we establish the true drawbead model based on the adaptive method and the subcycling algorithm. We first refine the elements accessing the drawbead and coase these having passed over the drawbead, the sheet metal elements mesh is divided into subdomains with the different time-steps, and then the elements in different domains are updated with their time-steps to increase the simulation efficiency. numerical examples are presented to demonstrate the usefulness of the proposed procedure for the finite element simulation with true drawbeads.更多还原