【作者】 陈志英；

【导师】 阮雪榆； 董湘怀；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 博士

【摘要】 板料冲压成形技术在制造业中占有极其重要的地位,被广泛应用于航空航天、汽车、船舶、电器等工业领域。但由于变形条件、工艺参数及材料选择等因素,常产生破裂、回弹和起皱等缺陷,使得零件的废品率和试错次数显著增加。在板料成形工艺分析中,引入数值模拟技术是解决上述问题的有效手段,但目前对破裂和回弹缺陷的预测精度仍不可靠。如何准确而又迅速地预测冲压成形中可能出现的这两种缺陷,对有效控制产品质量,降低成本,缩短产品开发周期,提高产品的竞争力有重要意义。本构模型是影响数值模拟可靠性一个重要方面。实际材料由于内部存在微缺陷,因而在变形过程中其力学性能是逐渐劣化的,但目前数值模拟分析少有考虑该因素,为了提高数值模拟预测精度,将损伤引入本构模型显得十分必要。为此,本文基于GTN细观损伤理论,考虑了板料的塑性各向异性性能和包申格效应,建立了符合冲压成形特点的损伤模型,并对数值实现方法进行了改进。将新的本构模型应用于成形和回弹分析,提出了与应变路径无关的韧性失稳判据和韧性断裂判据。本文的主要研究内容如下:针对板料冲压成形问题,提出并建立了Hill’48-GTN和Barlat’89-GTN细观损伤模型。对原来GTN细观损伤模型进行了改进,将Hill’48和Barlat’89各向异性屈服准则表示的宏观等效应力引入到模型中,反映了材料的各向异性性能,推导了相应的本构方程及数值实现表达式。通过数值算例表明,新模型Hill’48-GTN和Barlart’89-GTN相对原来的GTN模型可以同时反映板料成形中的塑性各向异性行为和损伤发展过程。针对原有本构方程数值算法在求解板料成形,尤其是破裂问题时计算效率低、收敛困难的问题,改进了本构积分算法的数值实现方法。将完全隐式的向后Euler本构积分算法与显式有限元法相结合,既保证了算法的准确性和稳定性,又提高了计算效率。基于有限元软件ABAQUS编写了几种GTN损伤模型的VUMAT材料子程序,实现了对板料成形过程中损伤演化的可视化。通过圆杯拉深成形试验验证了GTN损伤模型的可靠性。研究比较了目前板料成形中3种常用流动应力模型:Hollomon、Swift和Voce方程的描述准确性。为了对韧性损伤机理有更深入直观的理解,利用扫描电镜观测了拉断试样断口形貌并进行了微观分析。采用逆向方法,将单向拉伸试验与有限元数值模拟相结合来确定GTN模型损伤参数。由于GTN模型中各损伤参数的组合不是固定不变的,而是在一定范围内变化的。基于正交试验设计和有限元数值模拟,研究了各参数变化对板料损伤行为的影响。为了准确预测板料回弹,提出建立了遵循线性随动强化、非线性随动强化和混合强化法则的几种GTN模型,通过在模型中考虑了背应力的影响来反映回弹中的包申格效应。将几种遵循不同强化法则的GTN模型采用显隐式有限元求解相结合的方法来分析板料回弹,为准确预测回弹提供了理论方法和计算依据。分析比较了几种GTN损伤模型在采用不同强化法则时对板料回弹量的影响规律。考察了本构模型在考虑损伤因素前后的回弹量变化。从工程应用角度考虑,提出了以GTN模型中损伤变量孔洞体积分数作为判别板料冲压成形中韧性失稳和破裂的依据。由于孔洞体积分数自然而然地考虑了材料的成形历史,为判断缩颈现象不明显的韧性较差板材(如铝合金板)或非线性应变路径条件下板料的成形性能提供了另一条途径。结合有限元数值模拟计算,分析了镀锌钢板和AA5052-O铝合金板圆杯件拉深的成形极限,并与试验比较。更多还原

【Abstract】 Stamping plays a very important role in manufacturing industry. It is widely used in aviation, aerospace, automobile and other areas of industry. Fracture, springback and wrinkling are three main defects occurred in sheet metal forming because of some factors, such as forming condition, process parameters and material selection. These defects increase the reject ratio and try and error. Numerical simulation technology is a valid means to solve the above problems. But the prediction precision of the fracture and springback defects is still unreliable. Predicting the two defects accurately and rapidly is very important to the product quality control, cost reduction, product development cycle shortening and competitive power improving.Constitutive model is a crucial factor affecting the reliability of numerical simulation. Owing to internal micro defects existing in actual metallic materials, the material mechanical property degrades gradually during the forming process. But up to present, which is seldom considered in numerical simulation analysis. Therefore, to improve the numerical simulation accuracy, introducing the damage factor into the constitutive relationship is very essential. In this paper, on the basis of GTN mesotropic damage theory, the damage model considering the plastic anisotropy and Bauschinger effect is established suitable for the sheet metal forming. The numerical realization of the damage model is improved. The model is applied to analyze forming process and springback. The ductile instability and fracture criteria are proposed. The main work in this paper is as follows.Hill’48-GTN and Barlat’89-GTN mesotropic damage models are proposed and established based on the initial GTN model. The macroscopic equivalent stresses denoted by Hill’48 and Barlat’89 yield criteria take the place of the original von Mises equivalent stress to reflect the plastic anisotropy. The constitutive equations and numerical realization equations are derived. The results of numerical example show that the Hill’48-GTN and Barlat’89-GTN models can describe both the damage evolution and plastic anisotropy in sheet metal forming.Because the original method is inefficient and converge difficultly when dealing with sheet metal forming, especially fracture problem. The constitutive numerical realization method is improved. Implicit back Euler constitutive integration algorithm is introduced into the explicit dynamic solver. The computational efficiency of the algorithm is improved and the accuracy is ensured. The corresponding GTN models user-defined subroutines VUMAT are developed based on the commercial software ABAQUS. The damage parameter, void volume fraction is displayed clearly in the sheet metal forming simulation. The reliability of the above damage models is validated through the circular cup drawing experiment.The precision of three main flow stress models: Hollomon, Swift and Voce equations are studied and compared. To comprehending the mechanism of ductile damage intuitively and thoroughly, the fracture sections of cracked specimens are analyzed using SEM. Combining the experimental tensile test and finite element simulation, the inverse method is carried out to identify the GTN damage parameters. No matter which method is used, the parameters of GTN model are variational. The effect of parameters variation on damage behavior is analyzed based on orthogonal experimental design and FEM numerical simulation.Several hardening laws are proposed based on GTN damage model. The back stress is considered into the model to reflect the Bauschinger effect. Springback analysis are simulated using the explicit-implicit algorithm using the above GTN models. The effects of different material hardening rules on springback are studied and compared. The effect of damage on the springback is also investigated.From the engineering point of view, the ductile instability criterion and ductile fracture criterion are proposed based on the damage variable, void volume fraction in GTN model. Because the void volume fraction can take account of the material forming history, these criteria can deal with the forming limit of worse ductile sheet, for example, the alumimum alloy, and the sheet forming limit under the non-proportional loading strain path. The forming limits of steel and AA5052-O cup drawing processes are analyzed. The simulation results agree with the experimental data well.更多还原