板料成形极限理论与实验研究进展

成形极限是板材成形领域中重要的性能指标和工艺参数。文章在阐述成形极限在板料成形中的意义的基础上,综述并分析了成形极限在理论和实验方面的研究进展。成形极限图受应变路径的影响,给工业生产应用带来极大不便。以极限应力构成的成形极限应力图不受应变路径的影响,作为复杂加载路径的成形极限判据更加方便和实用。FLSD研究与FLD相结合,成为精确地确定破裂判别准则的主要途径之一,是近来研究的热点。十字形双向拉伸是实现复杂加载路径有效实用的试验方法。最后对成形极限应力图和十字形双向拉伸试验需要解决的关键问题作了阐述。     

汽车结构件内高压成形应力极限分析

由极限应力构成的应力成形极限图(FLSD)独立于应变路径,可作为复杂应变路径下成形极限的判据。通过标准成形极限实验获得3A21铝合金板材的成形极限图(FLD);由极限应力应变转换关系,将极限应变转换至主应力空间,建立对应的FLSD;采用LS-DYNA软件对方截面汽车结构件内高压成形过程进行了模拟,应用FLSD预测胀形过程中破裂的发生及极限成形压力。模拟结果与物理实验结果相吻合,证明FLSD可作为管材内高压成形等复杂应变路径下成形极限的判据。     

Failure of high strength steel sheets: Experiments and modelling

Failure in sheet metal structures of ductile material is usually caused by one of, or a combination of, ductile fracture, shear fracture or localised instability. In this paper the failure of the high strength steel Docol 600DP and the ultra high strength steel Docol 1200M is explored. The constitutive model used in this study includes plastic anisotropy and mixed isotropic-kinematic hardening. For modelling of the ductile and shear fracture the models presented by Cockroft–Latham and Bressan–Williams have been used. The instability phenomenon is described by the constitutive law and the finite element (FE) models. For calibration of the failure models and validation of the results, an extensive experimental series has been conducted including shear tests, plane strain tests and Nakajima tests. The geometries of the Nakajima tests have been chosen so that the first quadrant of the forming limit diagram (FLD) were covered. The results are presented both in an FLD and using prediction of force–displacement response of the Nakajima test employing element erosion during the FE simulations. The classical approach for failure prediction is to compare the principal plastic strains obtained from FE simulations with experimental determined forming limit curves (FLCs). It is well known that the experimental FLC requires proportional strains to be useful. In this work failure criteria, both of the instability and fracture, are proposed which can be used also for non-proportional strain paths.     

Numerical Investigations on the Influence of Superimposed Double-Sided Pressure on the Formability of Biaxially Stretched AA6111-T4 Sheet Metal

Lightweight materials have been widely used in aerospace, automobile industries to meet the requirement of structural weight reduction. Due to their limited plasticity at room temperature, however, lightweight materials always exhibit distinctly poor forming capability in comparison with conventional deep drawing steels. Based on the phenomenon that the superimposed hydrostatic pressure can improve the plasticity of metal, many kinds of double-sided pressure forming processes have been proposed. In the present study, the Gurson-Tvergaard-Needleman (GTN) damage model combined with finite element method is used to investigate the influence of double-sided pressure on the deformation behavior of biaxially stretched AA6111-T4 sheet metal, including nucleation and growth of microvoids, evaluation of stress triaxiality, and so forth. The Marciniak-Kuczynski (M-K) localized necking model is used to predict the right-hand side of the forming limit diagram (FLD) of sheet metal under superimposed double-sided pressure. It is found that the superimposed double-sided pressure has no obvious effect on the nucleation of microvoids. However, the superimposed double-sided pressure can suppress the growth and coalescence of microvoids. The forming limit curve (FLC) of the biaxially stretched AA6111-T4 sheet metal under the superimposed double-sided pressure is improved and the fracture locus shifts to the left. Furthermore, the formability increase value is sensitive to the strain path.     

The strain path and forming limit analysis of the lubricated sheet metal forming process

Few previous attempts have been made to analyze numerically the strain path and the forming limit in complex lubricated sheet metal forming. Since usual approaches of solving the lubrication model are limited to axisymmetric and plane strain cases only, this paper developed a unified procedure for combining the finite element code of sheet metal forming, the current lubrication/friction model and forming limit theory, to predict the strain path and fracture strains for either a steady or an unsteady three-dimensional process including both axisymmetric and plane strain cases. The availability of the method must be proved by a published problem, and an axisymmetric stretch forming process was therefore adopted as a benchmark. Numerical results showed that the present analysis provides good agreement with the experimental data of the strain path and the fracture strain for various tribological parameters such as lubricant viscosity and composite roughness of tooling and workpiece, and the advantage of the developed model is that it can be applied to solve the complicated 3D geometric problems.     

Cruciform shape benefits for experimental and numerical evaluation of sheet metal formability

The optimization of sheet metal forming processes requires accurate evaluations of material forming abilities. This paper presents an original technique based on the use of a cruciform shape for experimental characterization and numerical prediction of forming limit curves. The whole forming limit diagram is covered with a unique geometry by controlling displacements in the two main directions of the cruciform shape. The test is frictionless and the influence of linear and non-linear strain paths can be easily studied. The modelling of the cruciform shape with the finite element method permits to plot forming limit curves without any calibration step, essential for the classical Marciniak–Kuczynski (M–K) model. Experimental and numerical results are presented for an aluminium alloy 5086. These results are respectively compared with the ones from classical techniques: Marciniak test and numerical M–K model.     

Forming limit of electrodeposited nickel coating in the left region

A uniform nickel (Ni) coating was bilaterally electrodeposited on the low-carbon steel substrate for the application of advanced battery shells. Its forming limit was investigated by Hill localized necking theory coupled with finite element simulation and scanning electron microscopy. The effective stress and effective strain in the Ni coating and steel substrate are deduced using Hill’s anisotropic yield function. The localized necking condition is derived by sandwich sheet analysis, and the forming limit strains are obtained by solving the nonlinear equation of the localized necking condition. Extensive calculations are carried out using the proposed model. This study exhibits the nickel coating thickness and the normal anisotropic coefficients of the coating and substrate have little influence on the forming limit curve (FLC) in the left region of the coated sheet, but the strain hardening exponents of the coating and substrate have much effect on it. The calculated result matches well with the measured data in uniaxial tension. This investigation is useful for the preparation of the electrodeposited Ni coating and helpful for the forming operation of the battery shells.     

应用韧性断裂准则预测不同材料的胀形极限

为了准确地预测板料成形极限 ,将韧性断裂准则引入到有限元模拟中。在有限元模拟获得的应力应变场基础上 ,应用韧性断裂准则预测板料断裂的发生。本文应用作者提出的韧性断裂准则及材料常数的确定方法预测了铝合金板和钢板的半球形凸模胀形的成形极限。与实验结果比较表明 ,该方法能在较宽的材料范围内预测胀形成形极限。     

法向应力对板料成形极限影响的研究进展

在板料成形过程中应力状态对板料(板材和管材)的成形极限有很大影响,通过对板料施加法向应力可以提高板料的成形极限。文章综述并分析了法向应力对板料成形极限影响的理论模型、有限元模型以及实验研究方面的进展。理论模型方面的主要进展,是根据经典塑性失稳理论和M-K理论,建立了考虑法向应力影响的成形极限理论模型,可以准确地预测法向应力对板料成形极限的影响;有限元分析的主要进展,是在韧性断裂准则的基础上,利用体单元建立了考虑法向应力影响的数值模型;有关实验研究方面只是初步的探索,还有待进行深入的研究;对影响法向应力提高板料成形极限的因素进行了总结分析。     

一种建立板料成形极限应力图的新方法

基于塑性理论建立了比例加载条件下双向拉伸应力应变关系,结合Swift分散性失稳准则,提出了一种建立板料成形极限应力图的方法。分别应用Hill 48和Hosford屈服准则以及单向拉伸性能参数,建立了铝合金板(r<1)和薄钢板(r>1)两种材料的成形极限应力图(FLSD),分析表明,不同的屈服准则的选取对于成形极限应力曲线有不同的影响,对于不同类型的材料屈服准则的影响程度也不同。与由通常的成形极限图(FLD)转换所得到的成形极限应力图(FLSD)进行了对比分析,结果表明,所提出的方法计算过程更为简便,并能较为准确地建立成形极限应力图,可以作为复杂加载路径下的成形极限破裂判据。     

复杂加载路径下板料屈服强化与成形极限的研究进展

本文在阐述塑性变形行为与成形极限对于解析板料成形过程的作用与意义的基础上 ,针对板料屈服准则、强化模型、成形极限及复杂加载路径的影响规律的研究进展进行了综述与分析 ,得出 :建立符合实际板料成形特点的复杂加载路径的实验方法 ,验证理论研究结果的准确程度及适用范围 ,确定复杂加载路径下的解析描述及实用判据 ,是目前该领域主要的研究方向。最后对实现复杂加载路径的实验方法及其可行性进行了分析。     

中心区减薄的十字形试件拉伸性能研究

中心区减薄的十字形试件拉伸,是实现板料双向拉伸变路径条件下,达到大变形以至破裂的可行试验方法,对于复杂加载路径板料屈服行为及成形极限研究,有重要的试验意义。通过标准单向拉伸试验,对比研究了板材减薄前后单向拉伸性能的变化。对于中心区方形减薄的十字形试件,进行了单臂试件和十字形试件的单向拉伸试验,验证了中心区减薄后应力计算的正确性。     

Forming limit diagram of auto-body steel sheets for high-speed sheet metal forming

This paper is concerned with the uniaxial tensile properties and formability of steel sheets in relation to the strain rate effect. The elongation at fracture for CQ increases at a high strain rate while the elongation at fracture for DP590 decreases slightly in relation to the corresponding value for a quasi-static strain rate. The uniform elongation and the strain hardening coefficient decrease gradually when the strain rate increases. The r-value of CQ and DP590 was measured with a high-speed camera in relation to the strain rate. The r-value is slightly sensitive to the strain rate. Static forming limit curves (FLCs) and high-speed FLCs were constructed with the aid of punch-stretch tests with arc-shaped and square-shaped specimens. In addition, a high-speed crash testing machine with a specially designed high-speed forming jig was used for the high-speed punch-stretch tests. Compared with the static FLC, the high-speed FLC of CQ is higher in a simple tension region and lower in a biaxial stretch forming region. The high-speed FLC for DP590 decreases in relation to the static FLC throughout the entire region. The elongation at fracture appears to be closely related to the simple tension region of the FLC. The shear fracture is observed from SEM images of specimens tested in the biaxial stretch forming region under the high-speed forming condition. The dimples indicating the shear fracture have elongated horseshoe shape. The high-speed FLC is lower than the static FLC in the biaxial stretch forming region because the shear fracture induces the decrease of ductility. The results confirm that the strain rate has a noticeably influence on the formability of steel sheets. Thus, the forming limit diagram of high-speed tests should be considered in the design of high-speed sheet metal forming processes.     

A theoretical and experimental study on forming limit diagram for a seamed tube hydroforming

The purpose of this work is to establish the forming limit diagram (FLD) for a seamed tube hydroforming. A new theoretical model is developed to predict the FLD for a seamed tube hydroforming. Based on this theoretical model, the FLD for a seamed tube made of QSTE340 sheet metal is calculated by using the Hosford yield criterion. Some forming limit experiments are performed. A classical free hydroforming tool set is used for obtaining the left hand side forming limit strains, and a novel hydroforming tool set is designed for the right hand side of FLD. The novel device required the simultaneous application of lateral compression force and internal pressure to control the material flow under tension–tension strain states. Furthermore, the suitable loading paths for the left hand side of FLD by theoretical formulas and for the right hand side of FLD by finite element (FE) simulations are calculated. Finally, a comparison between the theoretical results and experimental data is performed. The theoretical predicting results show good agreement with the experimental results.     

Prediction of the forming limit curves of sheet materials using the rigid-plastic finite element method

An incremental rigid-plastic finite element method is used to predict the forming limit curves (FLCs) of sheet materials. In this analysis, the deforming sheet materials is assumed to obey Hill's anisotropic yield criterion and its associated flow rule. To obtain the critical strain paths in both the positive and negative minor strain regions, simulations are performed using hemispherical punch stretching of a circular blank with various circular cutoffs and various friction conditions at the tool-sheet interface. A critical slope condition derived from the computed load-displacement curve is employed as a criterion to determine the limiting major and minor strains. FLCs of several sheet materials are predicted and the results are compared with the existing experimental data.     

板料单点渐进成形数值模拟研究

板料单点渐进成形过程变形复杂、影响因素多,工艺参数选择是提高成形质量和成形效率的关键。对单点渐进成形过程进行数值模拟,分析了成形件的应力分布和厚度变化以及成形路径、进给量等工艺参数对成形过程的影响。结果表明:成形件最大应力和最大厚度减薄发生在底面附近;采用螺旋进给方式可有效提高成形质量和成形效率。实验结果显示,实验结果与数值模拟结果基本吻合。     

Sheet metal forming limit prediction based on plastic deformation energy

For sheet metals, the endurance to fracture under different strain paths may be different. Based on plastic deformation energy, the sheet metal forming limit is calculated, and the relationship model between maximum allowable integral value of the general plastic work criterion and the strain path is built. In addition, the strain-hardening exponent, anisotropy coefficient and the initial thickness of the material are also taken into account to consider their effects on forming limit. In order to simplify the process of parameter determination, only uniaxial tension test is used to calculate the material property parameters and necessary limit strain, and the expression of the criterion is determined finally. Then the limit strains under other strain paths between uniaxial tension to equi-biaxial tension are predicted by the criterion combined with numerical simulation of the forming process. The criterion is also applied to limit strain prediction under bilinear strain path.     

An overview of stabilizing deformation mechanisms in incremental sheet forming

In incremental sheet forming (ISF) strains can be obtained well above the forming limit curve (FLC) that is applicable to common sheet forming operations like deep drawing and stretching. This paper presents an overview of mechanisms that have been suggested to explain the enhanced formability. The difference between fracture limit and necking limit in sheet metal forming is discussed. The necking limit represents a localized geometrical instability. Localized deformation is an essential characteristic of ISF and proposed mechanisms should stabilize the localization before it leads to fracture. In literature six mechanisms are mentioned in relation to ISF: contact stress; bending-under-tension; shear; cyclic straining; geometrical inability to grow and hydrostatic stress. The first three are able to localize deformation and all but the last, are found to be able to postpone unstable growth of a neck. Hydrostatic pressure may influence the final failure, but cannot explain stability above the FLC.     

Combined experimental and numerical analysis of bulge test at high strain rates using split Hopkinson pressure bar apparatus

High strain rate bulge test technique which is introduced in this paper adopts a rubber-pad as pressure carrying medium to bulge a sheet metal at high velocity using split Hopkinson pressure bar (SHPB) system. The experimental set-up is based on conventional hydraulic bulge test which is modified to mount on SHPB. The thickness thinning of the sheet metal during the test will be considered as a measure of true strain of the bulged sheet. The theoretical approach is developed in this study to attain pressure–strain curves of sheet metals during high strain rate bulge forming process. This approach is followed by a finite element simulation of the process in ABAQUS/Explicit software. To verify the developed method, analytical and finite element methods are compared with experiments.