成形极限图的测试、应用和可信度分析

成形极限图通常是通过钢模胀形试验测得，实际测量的成形极限图与ASAME自动应变测试分析系统模拟计算的成形极限曲线吻合较好；应用成形极限图分析冲压零件成形的安全裕度和进行选材预测时，对以平面应变和胀形为主的成形零件具有较高的可信度，而对以深拉延变形为主的成形零件可信度不高。     

镁合金板材成形极限图(FLD)的实验研究

首次利用电蚀网格法,在BCS-30D板材成形性试验机上进行镁合金板材成形实验,利用先进的ASAME自动应变测量系统进行应变测量分析,测试镁合金板材的成形极限图(FLD).实验表明,室温下AZ31B镁合金冷轧态板材的力学性能和冲压性能不佳,难以完成成形极限图的测试,不具备成形加工能力;热轧态镁合金板材具有一定的塑性和成形性能,并测试了其成形极限图.成形极限曲线FLC的测试对制订镁合金板材的冲压成形工艺提供了理论依据.     

基于弧长法和减薄率判据研究金属镀层的成形极限

用ABAQUS有限元软件结合弧长法对镀层薄板冲压成形进行数值模拟,研究了镍镀层薄板在不同应变路径下的平面应变,得到了镀层的冲压成形极限,并用RG2000型微机控制电子万能试验机结合ARGUS板金成形网格应变测试系统对镍镀层薄板进行冲压成形实验验证。结果表明,用弧长法得到的失稳减薄率可以较好地预测镀层的成形极限,而以微裂纹产生时的减薄率为失稳依据在一定程度上可以满足工程需要。     

大型车车身覆盖件冲压成形特征分析及选材研究

为满足大型车身覆盖件冲压成形要求并实现钢板与零件的对路供应,必须在深入解析零件成形特征的基础上,确定高应变部位的成形敏感材料参数.在材料力学性能测试和成形极限分析的基础上,对侧围外板的冲压成形特征和成形敏感参数进行了分析.结果表明:侧围外板零件高应变部位的变形方式为胀形-深拉延变形,n值和r值为影响侧围外板零件成形的主要材料敏感参数.     

国产轿车用冷轧薄钢板的成形性能评价

就评价国产轿车用冷轧薄钢板成形性力学性能参数做了详细的分析;并就轿车零件冲压成形的特点,将材料的FLD图和冲压件应变图合成为零件应变极限图,从而对零件的冲压安全裕度进行评价,用于指导国产轿车用冷轧薄钢板的冲压生产.     

国产轿车用冷轧薄钢板的成形性能评价

本文就评价国产轿车用冷轧薄钢板成形性力学性能参数做了详细的分析;并就轿车零件冲压成形的特点,将材料的FLD图和冲压件应变图合成为零件应变极限图,从而对零件的冲压安全裕度进行评价,用于指导国产轿车用冷轧薄钢板的冲压生产.同时就冷轧薄板的表面粗糙度、表面微观形貌及表面润滑条件等因素对钢板成形性的影响作了探讨,认为润滑有利于金属薄板在冲压成形过程中的流动,可使整个零件的应变分布趋于均匀,从而提高零件的冲压成形能力,降低冲压件的废品率.     

AA5052板材准静态/动态平面应变成形极限试验研究

为建立磁脉冲辅助冲压成形(EMAS)工艺的有效性,采用准静态平面应变预拉伸和动态磁脉冲成形相结合的方法对5052-O铝合金板材的准静态/动态平面应变状态复合成形极限进行了试验研究.结果表明:准静态/动态复合加载过程能显著改善该铝合金板材的室温成形性;准静态/动态平面应变复合成形极限比准静态平面应变成形极限有显著提高,相似或者略高于完全磁脉冲平面应变成形性,且随着准静态预应变水平的增加,准静态/动态复合变形成形极限变化不大.预变形的存在不会削弱复合成形过程的极限变形能力.     

汽车翼子板零件的成形与工艺分析

通过对汽车翼子板零件投料冲压试验,对零件进行了应变测试分析,评价试验钢板的成形效果,分析了冲压工艺、表面粗糙度和板厚等条件对零件冲压生产的影响,得到翼子板零件的最佳用材方案及冲压工艺条件.     

温成形中铝合金板成形极限图的预测研究

在铝合金板温成形数值仿真中,成形极限图是判断材料颈缩失效和评价温冲压成形能力的基础.提出了一种温成形条件下铝合金板成形极限图的理论预测方法.采用曲线拟合方法建立了Al5083-O铝合金板应变硬化指数、应变率硬化指数随成形温度的变化规律;采用M-K理论模型,结合Logan-Hosford屈服函数计算获得温成形条件下铝合金板的成形极限图.计算结果与实验数据吻合较好,证实了温成形条件下铝板成形极限图的理论预测方法是正确的.     

汽车覆盖件用 5 B003 A 板拉伸实验及成形性的有限元分析

目的研究汽车车身用5B003A板的成形性能。方法在进行单轴拉伸试验的基础上,利用软件eta/DYNAFORM模拟了板成形极限曲线、破裂点应变路径、圆筒拉深过程,并进行了相应变形参数的优化。结果 5B003A板单向拉伸和有限元模拟均出现"交叉颈缩"现象,并且在与轧制呈45°角方向上的冲压性能优于0°和90°方向;5B003A板成形极限破裂点的应变路径漂移倾向较明显,双拉区中均呈ε2=const的应变状态;在与拱顶高实验相近变形条件下,优化得到最佳凸模圆角半径为20 mm时,与极限拉深系数0.43对应的无凸缘拉深的最大板坯尺寸为233 mm,相应最佳压料力为68 kN。结论 5B003A板在45°方向上有较好的冲压性能,且凸模圆角半径、板坯直径、压料力等工艺参数对其拉深成形性能影响较大。     

Experimental and theoretical formability analysis using strain and stress based forming limit diagram for advanced high strength steels

In this study, experimental and numerical analyses of Forming Limit Diagram (FLD) and Forming Limit Stress Diagram (FLSD) for two Advanced High Strength Steel (AHSS) sheets grade DP780 and TRIP780 were performed. Initially, the forming limit curves were experimentally determined by means of the Nakazima forming test. Subsequently, analytical calculations of both FLD and FLSD were carried out based on the Marciniak–Kuczinsky (M–K) model. Additionally, the FLSDs were calculated using the experimental FLD data for both investigated steels. Different yield criteria, namely, von Mises, Hill’s 48, and Barlat2000 (Yld2000-2d) were applied for describing plastic flow behavior of the AHS steels. Both Swift and modified Voce strain hardening laws were taken into account. Hereby, influences of the constitutive yield models on the numerically determined FLDs and FLSDs were studied regarding to those resulted from the experimental data. The obtained stress based forming limits were significantly affected by the yield criterion and hardening model. It was found that the forming limit curves calculated by the combination of the Yld2000-2d yield criterion and Swift hardening law were in better agreement with the experimental curves. Finally, hole expansion tests were conducted in order to verify the different failure criteria. It was shown that the stress based forming limit curves could more precisely describe the formability behavior of both high strength steel sheets than the strain based forming limit curves.     

A New Curve Fitting Method for Forming Limit Experimental Data

The forming limit curve (FLC) can be obtained by. means of curve fitting the limit strain points of different strain paths. The theory of percent regression analysis is applied to the curve fitting of forming limit experimental data. Forecast intervals of FLC percentiles can be calculated. Thus reliability and confidence level can be considered. The theoretical method to get the limits of limit strain points distributing region is presented, and the FLC position can be adjusted according to practical requirement. Method for establishing FLC with high reliability using small samples is presented at the same time. This method can make full use of the current experimental data and the previous data. Compared with the traditional method that can only use current experimental data, fewer specimens are required in the present method to obtain the same precision and the result is more accurate with the same number of specimens.     

Dedicated linear – Voce model and its application in investigating temperature and strain rate effects on sheet formability of aluminum alloys

This paper proposes a method to investigate the effects of temperature and strain rate on the forming limit curves (FLCs) by combining a modified Voce constitutive model (Lin-Voce model) with the numerical simulation of Marciniak test. The tensile tests are firstly carried out at different forming temperatures (20, 230 and 290 °C) and strain rates (2.5, 120 and 150 s⁻¹) for AA5086 sheet. A modified Voce constitutive model (named Lin-Voce model) is proposed to describe the deformation behavior of AA5086 and its material parameters are identified by inverse analysis technique. Then, the proposed constitutive model is verified by comparing numerical and experimental results obtained by tensile tests and Marciniak test, respectively. Finally, the numerical simulation of Marciniak test is carried out at different temperatures (100, 200 and 300 °C) and strain rates (2.5, 120 and 150 s⁻¹), and the effects of temperature and strain rate on the FLCs of AA5086 are investigated and discussed.     

Analysis of the thermomechanical shear behaviour of woven-reinforced thermoplastic-matrix composites during forming

Shear behaviour of a glass fibre/polypropylene composite is characterized over a wide range of strain rates and forming temperatures using the bias extension test. A temperature- and rate-dependent material model is here introduced to describe the observed behaviour. The model is based on a continuous approach and formulated considering a stress objective derivative based on the warp and weft yarns rotation. The effects of temperature and strain rate on the shear behaviour are analysed via bias extension test simulations. Temperature change in the sheet during forming was measured. This data is used to model cooling during forming. Isothermal and transient forming simulations were performed in order to show the effects of temperature and forming speed on the obtained shear angle distribution. It was found that at low forming speeds the assumption of isothermal forming is not valid anymore since the cooling of the sheet affects the shear behaviour.     

A model for process-based crash simulation

Manufacturing of a bumper system from aluminium extrusions often involves series of forming operations performed in the soft W-temper condition, and then artificially age-hardening of the components to the material's peak hardness T6 condition. It is probable that proper finite element (FE) modelling of the crash performance of the resulting systems must rely upon a geometry obtained from an FE model following the process route, i.e., including simulation of all major forming operations. The forming operations also result in an inhomogeneous evolution of some internal variables (among others the effective plastic strain) within the shaped components. Results from tensile tests reveal that plastic straining in W-temper leads to a significant change of the T6 work-hardening curves. In addition, the tests show that the plastic pre-deformation causes a reduction of the elongation of the T6 specimens. In the present work, these process effects have been included in a user-defined elastoplastic constitutive model in LS-DYNA incorporating a state-of-the-art anisotropic yield criterion, the associated flow rule and a non-linear isotropic work hardening rule as well as some ductile fracture criteria. A first demonstration and assessment of the modelling methodology is shown by ‘through-process analysis’ of two uniaxial tensile test series. The industrial use and relevance of the modelling technique is subsequently demonstrated by a case study on an industrial bumper beam system.     

An experimental and numerical study of the effects of length scale and strain state on the necking and fracture behaviours in sheet metals

Within sheet metal forming, crashworthiness analysis in the automotive industry and ship research on collision and grounding, modelling of the material failure/fracture, including the behaviour at large plastic deformations, is critical for accurate failure predictions. In order to validate existing failure models used in finite element (FE) simulations in terms of dependence on length scale and strain state, tests recorded with the optical strain measuring system ARAMIS have been conducted. With this system, the stress–strain behaviour of uniaxial tensile tests was examined locally, and from this information true stress–strain relations were calculated on different length scales across the necking region. Forming limit tests were conducted to study the multiaxial failure behaviour of the material in terms of necking and fracture. The failure criteria that were verified against the tests were chosen among those available in the FE software Abaqus and the Bressan–Williams–Hill (BWH) criterion proposed by Alsos et al, 2008. The experimental and numerical results from the tensile tests confirmed that Barba's relation is valid for handling stress–strain dependence on the length scale used for strain evaluation after necking. Also, the evolution of damage in the FE simulations was related to the processes ultimately leading to initiation and propagation of a macroscopic crack in the final phase of the tensile tests. Furthermore, numerical simulations using the BWH criterion for prediction of instability at the necking point showed good agreement with the forming limit test results. The effect of pre-straining in the forming limit tests and the FE simulations of them is discussed.     

Effect of reverse dome stretching on dome height and forming limits of sheet materials

Reverse bending and stretching of sheet materials is often employed in press forming of complex automotive components. In this work, hemispherical dome stretching tests were followed by reverse dome tests on automotive aluminum sheet specimens to assess the influence of the strain and shape on dome height at neck formation and limit strains. The above test scheme offers a means of subjecting the sheet material to reverse bending and stretching and thereby changing its strain path during the process. The results indicate significant improvements in dome height as a function of pre-strain (or initial dome height) compared to the simple dome stretching process. The forming limit strains, on the other hand, are lowered. The reason is attributed to the redistribution of strain (more uniform deformation) through the punch/specimen contact during the reverse bending and stretching process.     

Analysis of the extended stress-based forming limit curve considering the effects of strain path and through-thickness normal stress

In this study, an approach based on the modified Marciniak–Kuczynski (M–K) method for computation of an extended stress-based forming limit curve (FLC) is presented. The extended stress-based FLC is built based on equivalent plastic stress versus mean stress. This curve has some advantages in comparison with the conventional FLC. This new criterion is much more strain path independent than the conventional FLC. The effect of strain path on the predicted extended stress-based FLC is reexamined. For this purpose, two types of pre-straining on the sheet metal have been loaded. Moreover, the plane stress state assumption is not adopted in the current study. The influence of a through-thickness compressive normal stress is also investigated theoretically. The verifications of the theoretical FLCs are performed by using some available published experimental data.     

Experimental and numerical studies on the forming behavior of tailor welded steel sheets in biaxial stretch forming

Stretch forming is an important process in making complex stampings for autobody components. In the present work formability of three different types of tailor welded blanks (TWBs) in biaxial stretch forming modes has been studied by conducting limiting dome height (LDH) tests. The TWBs are laser-welded samples of low carbon and ultra low carbon steel sheets with difference in thickness, grade and surface conditions. In TWBs with difference in thickness, the LDH decreases as the thickness ratio increases and the thickness of the thinner side is also crucial. A high thickness ratio causes two major strain peaks on thinner side and fracture takes place due to strain localization at the peak close to the pole. The weld ductility and the extent of difference in properties are the two crucial parameters for formability in TWBs with difference in properties. In both these TWBs, the fracture takes place perpendicular to the weld line and propagates towards the stronger side. Significant weld line movement occurs towards the thicker/stronger side in biaxial stretch forming. The maximum weld line movement occurs at the pole and it increases with increase in thickness ratio and becomes constant beyond a certain thickness ratio. The peak load required to deform the TWB specimens is less compared to the corresponding parent sheets. In case of TWBs with difference in thickness, as the thickness ratio increases, the peak load reduces due to decreasing punch-blank contact area.