薄板冲压回弹仿真计算及应用技术研究
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摘要
回弹现象是冷冲压成型过程中不可避免的物理现象。回弹问题的存在造成零件的形状及尺寸与设计要求不符,直接影响冲压件的品质,包括外观质量、装配性能和使用可靠性等。如何准确的预测回弹后零件的形状、设计出准确的型面以补偿回弹,在目前还是模具工业中的实际难题。采用数值方法准确的预测零件的回弹量对最终解决回弹问题至关重要。有限元方法是一种广泛应用于冲压成型仿真的数值方法,该方法已经被证实可用于回弹仿真计算,但由于回弹计算结果受到很多因素的影响,目前有限元方法对回弹预测精度还不理想,回弹问题有待进一步的研究。
本文针对影响回弹预测精度的三个主要因素:仿真模型中的单元尺寸、材料参数和等效拉延筋模型,开展了薄板冲压回弹仿真计算及其应用技术的研究,研究的目的在于提高回弹预测的精度。主要研究工作如下:
(1) 研究了翻边回弹仿真中板料过模具圆角时的单元尺寸问题,证实了在翻边仿真中板料过模具圆角时采用较小的单元尺寸是取得合理的回弹预测结果的必要条件之一。通常认为板料过模具圆角时采用的单元数目在模具圆角大的情况下可以采用较大的单元尺寸,本文研究中发现该观点并不能应用于翻边回弹仿真。本文建立了凹模圆角半径不同的三个翻边试验的仿真模型,比较了在模型中采用不同单元尺寸时的回弹仿真与试验结果。研究表明,不论模具圆角半径大小,板料在过圆角的部分采用较小的单元尺寸时,回弹的预测结果才能够与试验结果较为一致。
(2) 研究了在U型冲压仿真中单元尺寸对回弹的影响机理,解释了令许多研究者困惑的回弹预测结果偏小问题。本文进行了U型冲压的试验,并把U型冲压仿真分为成型中期与成型术期两个阶段,分别研究了两个阶段中单元尺寸对回弹的影响机理。研究表明,在成型仿真过程中期,板料弯曲部分出现“应力松弛”现象,即板料中的应力逐渐下降。在不同单元尺寸下,“应力松弛”程度不同,导致了板料在回弹计算前的应力不同,最后导致了在回弹仿真中预测结果不同。在这种情况下,单元尺寸越大,“应力松弛”越严重,回弹预测结果越小。在成型仿真过程术期,由于板料和模具之间的间隙小,板料和模具之间不可避免的发生“穿透现象”,这种“穿透现象”导致的接触力对回弹值所起的效果与有底凹模弯曲中的“校正力”所起的效果相同。有底凹模弯曲中的“校正力”大小不同,零件的回弹量就有较大差异。仿真中不同单元尺寸下该“校正力”大小不同,导致了回
Springback is a common physical phenomenon in the metal sheet forming. The springback problem, which make the shape and dimension diverge from design requirements, direct affects the quality of forming product, including the appearance quality, the assembly quality and the final product performance. How to predict the shape of products after forming, and design the required die/punch surface to compensate springback is a difficult problem in the metal sheet forming industry. It is essential to use numerical methods to predict the springback of complex products. Finite element method (FEM) is a widely used numerical method in sheet forming simulation, and is approved to predict springback successfully, but the springback prediction result is affected by many factors, and the accuracy of springback prediction is not satisfactory, deep research on the springback is still necessary.Element sizes, material parameters in simulation model and equivalent drawbead model are three main factors influencing the accuracy of the springback prediction. Focusing on these factors, methods to improve the accuracy of springback prediction in the metal sheet forming are studied. The main work is listed as follows:(1) The size of elements of blank passing through the die/punch shoulder in flanging process is studied, and it is verified that the element size should be enough small when the element passing the die/punch shoulder to get reasonable springback prediction of flanging process. Many researchers haven't reached an agreement on the number of element passing throng the shoulder, but it is generally accepted the element size can be larger if the radius of the shoulder is larger. Author has found this opinion is not suitable when it is used on the flanging springback simulation. In this paper, models of three flanging process are built, and springback predicted by models with different element size are compared with the experimental results. It is shown that springback predicted by the models using small element size of blank in simulation of flanging processes with different die radius are consistent with the experimental results.(2) The influence of element size on springback in U-shape forming is investigated, and why the springback is always under predicted by FEM is explained, which is a problem many researchers have been puzzled with. Experiments of U
shape forming are carried out. Simulations of U shape forming are divided into two phases: at the process and at the end of forming process. The influence of element size on springback prediction is discussed individually for these two phases. Results show that at the process of forming simulation, "stress relaxation" ,which low down the stress, incurs at the bending part of the blank, and the "stress relaxation" is more obvious when the element size is larger. The stresses, which determine the springback, are affected by the "stress relaxation", and thus the springback prediction is affected by the element size. In this situation, springback is smaller in models with big element size for more "stress relaxation" incurs. At the end of forming simulation, penetration is unavoidable between the blank and die/punch. The contact force caused by the penetration has same effect of the force in shape correction at the bending process with die bottom, which will reduce the springback. For the force in shape correction is varied with different element size, so as the springback predicted. In this situation, springback is smaller in models with big element size for big force in shape correction is applied. In this paper, the methods to low down the influence of element size on springback prediction are also suggested.(3) A new inverse method to identify parameters in material models is suggested. The accuracy of material model parameters, which is determined by the description accuracy of mechanical properties of sheet, has great influence on the accuracy prediction of springback. The material identification process can be time consuming if simulation model of experiment is complex. An inverse method, which combined FEM software ANSYS LS-DYNA with modified Levenberg Marquart (LM) optimization algorithm, is utilized to identify material parameters based on tensile test with rectangular specimens. It is time efficient and converges fast. Results show that the material parameters in Barlat 1989 and Barlat 1991 identified by this method describe the mechanical properties well. Using the same inverse method, a new approach to identify the true stress-strain curve based on the blank deformation in the necking process in tensile test. This method can get the true stress-strain curve at large strain region and avoid the shortcoming of normal method, which can get the curve at small strain region only.(4) The influence of drawbead on springback is investigated, and a modified equivalent drawbead model is proposed. The influence of using equivalent drawbead model on springback prediction is not clear. Based on drawbead test and flanging test, the influence of drawbead on springback is studied. Results show that after passing
through the drawbead, the blank become hardened, and thickness of blank is reduced, and stress is left in the blank, which affect the springback of parts. Even the equivalent model used now can describe the drawbead restraint force accurately; it isn't qualified to predict springback accurately. To avoid the shortage of the equivalent drawbead model, a modified equivalent drawbead model is suggested, which can describe the material hardening, thinning, and stress left accurately besides the drawbead restraint force. Theory method, experiment method and simulation method to calculate parameters in the modified equivalent model are suggested, and the application of this model on simple springback problem is also suggested. Results show the modified equivalent model can improve the accuracy of springback prediction.
本文针对影响回弹预测精度的三个主要因素:仿真模型中的单元尺寸、材料参数和等效拉延筋模型,开展了薄板冲压回弹仿真计算及其应用技术的研究,研究的目的在于提高回弹预测的精度。主要研究工作如下:
(1) 研究了翻边回弹仿真中板料过模具圆角时的单元尺寸问题,证实了在翻边仿真中板料过模具圆角时采用较小的单元尺寸是取得合理的回弹预测结果的必要条件之一。通常认为板料过模具圆角时采用的单元数目在模具圆角大的情况下可以采用较大的单元尺寸,本文研究中发现该观点并不能应用于翻边回弹仿真。本文建立了凹模圆角半径不同的三个翻边试验的仿真模型,比较了在模型中采用不同单元尺寸时的回弹仿真与试验结果。研究表明,不论模具圆角半径大小,板料在过圆角的部分采用较小的单元尺寸时,回弹的预测结果才能够与试验结果较为一致。
(2) 研究了在U型冲压仿真中单元尺寸对回弹的影响机理,解释了令许多研究者困惑的回弹预测结果偏小问题。本文进行了U型冲压的试验,并把U型冲压仿真分为成型中期与成型术期两个阶段,分别研究了两个阶段中单元尺寸对回弹的影响机理。研究表明,在成型仿真过程中期,板料弯曲部分出现“应力松弛”现象,即板料中的应力逐渐下降。在不同单元尺寸下,“应力松弛”程度不同,导致了板料在回弹计算前的应力不同,最后导致了在回弹仿真中预测结果不同。在这种情况下,单元尺寸越大,“应力松弛”越严重,回弹预测结果越小。在成型仿真过程术期,由于板料和模具之间的间隙小,板料和模具之间不可避免的发生“穿透现象”,这种“穿透现象”导致的接触力对回弹值所起的效果与有底凹模弯曲中的“校正力”所起的效果相同。有底凹模弯曲中的“校正力”大小不同,零件的回弹量就有较大差异。仿真中不同单元尺寸下该“校正力”大小不同,导致了回
Springback is a common physical phenomenon in the metal sheet forming. The springback problem, which make the shape and dimension diverge from design requirements, direct affects the quality of forming product, including the appearance quality, the assembly quality and the final product performance. How to predict the shape of products after forming, and design the required die/punch surface to compensate springback is a difficult problem in the metal sheet forming industry. It is essential to use numerical methods to predict the springback of complex products. Finite element method (FEM) is a widely used numerical method in sheet forming simulation, and is approved to predict springback successfully, but the springback prediction result is affected by many factors, and the accuracy of springback prediction is not satisfactory, deep research on the springback is still necessary.Element sizes, material parameters in simulation model and equivalent drawbead model are three main factors influencing the accuracy of the springback prediction. Focusing on these factors, methods to improve the accuracy of springback prediction in the metal sheet forming are studied. The main work is listed as follows:(1) The size of elements of blank passing through the die/punch shoulder in flanging process is studied, and it is verified that the element size should be enough small when the element passing the die/punch shoulder to get reasonable springback prediction of flanging process. Many researchers haven't reached an agreement on the number of element passing throng the shoulder, but it is generally accepted the element size can be larger if the radius of the shoulder is larger. Author has found this opinion is not suitable when it is used on the flanging springback simulation. In this paper, models of three flanging process are built, and springback predicted by models with different element size are compared with the experimental results. It is shown that springback predicted by the models using small element size of blank in simulation of flanging processes with different die radius are consistent with the experimental results.(2) The influence of element size on springback in U-shape forming is investigated, and why the springback is always under predicted by FEM is explained, which is a problem many researchers have been puzzled with. Experiments of U
shape forming are carried out. Simulations of U shape forming are divided into two phases: at the process and at the end of forming process. The influence of element size on springback prediction is discussed individually for these two phases. Results show that at the process of forming simulation, "stress relaxation" ,which low down the stress, incurs at the bending part of the blank, and the "stress relaxation" is more obvious when the element size is larger. The stresses, which determine the springback, are affected by the "stress relaxation", and thus the springback prediction is affected by the element size. In this situation, springback is smaller in models with big element size for more "stress relaxation" incurs. At the end of forming simulation, penetration is unavoidable between the blank and die/punch. The contact force caused by the penetration has same effect of the force in shape correction at the bending process with die bottom, which will reduce the springback. For the force in shape correction is varied with different element size, so as the springback predicted. In this situation, springback is smaller in models with big element size for big force in shape correction is applied. In this paper, the methods to low down the influence of element size on springback prediction are also suggested.(3) A new inverse method to identify parameters in material models is suggested. The accuracy of material model parameters, which is determined by the description accuracy of mechanical properties of sheet, has great influence on the accuracy prediction of springback. The material identification process can be time consuming if simulation model of experiment is complex. An inverse method, which combined FEM software ANSYS LS-DYNA with modified Levenberg Marquart (LM) optimization algorithm, is utilized to identify material parameters based on tensile test with rectangular specimens. It is time efficient and converges fast. Results show that the material parameters in Barlat 1989 and Barlat 1991 identified by this method describe the mechanical properties well. Using the same inverse method, a new approach to identify the true stress-strain curve based on the blank deformation in the necking process in tensile test. This method can get the true stress-strain curve at large strain region and avoid the shortcoming of normal method, which can get the curve at small strain region only.(4) The influence of drawbead on springback is investigated, and a modified equivalent drawbead model is proposed. The influence of using equivalent drawbead model on springback prediction is not clear. Based on drawbead test and flanging test, the influence of drawbead on springback is studied. Results show that after passing
through the drawbead, the blank become hardened, and thickness of blank is reduced, and stress is left in the blank, which affect the springback of parts. Even the equivalent model used now can describe the drawbead restraint force accurately; it isn't qualified to predict springback accurately. To avoid the shortage of the equivalent drawbead model, a modified equivalent drawbead model is suggested, which can describe the material hardening, thinning, and stress left accurately besides the drawbead restraint force. Theory method, experiment method and simulation method to calculate parameters in the modified equivalent model are suggested, and the application of this model on simple springback problem is also suggested. Results show the modified equivalent model can improve the accuracy of springback prediction.
引文
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