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As more Advanced High-Strength Steels (AHSS) are heavily used for automotive body structures and closures panels, accurate springback prediction for these components becomes more challenging because of their rapid hardening characteristics and ability to sustain even higher stresses. In this paper, a modified Mroz hardening model is proposed to capture realistic Bauschinger effect at reverse loading, such as when material passes through die radii or drawbead during sheet metal forming process. This model accounts for material anisotropic yield surface and nonlinear isotropic/kinematic hardening behavior. Material tension/compression test data are used to accurately represent Bauschinger effect. The effectiveness of the model is demonstrated by comparison of numerical and experimental springback results for a DP600 straight U-channel test. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The impact of material models on the numerical simulation of springback is investigated. The study is focused on the strain-path sensitivity of two hardening models. While both models predict the Bauschinger effect, their response in the transient zone after a strain-path change is fairly different. Their respective predictions are compared in terms of sequential test response and of strip-drawing springback. For this purpose, an accurate and general time integration algorithm has been developed and implemented in the Abaqus code. The impact of several numerical parameters is also studied in order to assess the overall accuracy of the finite element prediction. For some test geometries, both material and numerical parameters are shown to clearly influence the springback behavior at a large extent. Moreover, a general trend cannot always be extracted, thus justifying the need for the finite element simulation of the stamping process. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The present work aims at analyzing a comparative study between the strain-based forming limit criterion (FLD) and the stress-based forming limit criterion (FLSD), under linear and complex strain paths. The selected material is an AA5182-0 aluminium alloy. Some relevant remarks about stress-based forming limit criterion concept are presented. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The effects of changing strain paths on forming limit stresses of sheet metals are investigated using the Marciniak-Kuczynski model. Forming limits are analyzed for proportional loading and two types of combined loadings: combined loading which includes unloading between the first and second loadings and that which includes an abrupt strain path change without unloading between the first and second loadings. The forming limit stress curves in stress space calculated for the combined loading with unloading are in good agreement with those calculated for the proportional loading, while the forming limit curves in strain space are strongly dependent on the strain paths. The forming limit stresses calculated for combined loading with an abrupt strain path change, however, do not coincide with those calculated for proportional loading. The strain path dependence of the forming limit stresses is discussed in detail. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

An increasing use of combining more than one process step is noticed for coupling crash simulations with the results of forming operations — mostly by inheriting the forming history like plastic strain and material hardening. Introducing a continuous failure model allows a further benefit of these coupling processes; it sometimes can even be the most attractive result of such a work. In this paper the algorithm CrachFEM for fracture prediction has been used to generate more benefit of the successive forming and crash simulations — especially for ultra high strength steels. The choice and selection of the material grade in combination with the component design can therefore be done far before the prototyping might show an unsuccessful crash result; and in an industrial applicable manner. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

This paper reports a novel extension of the stress-based flow limit curve approach. A conventional strain-based FLC is converted into a stress-based FLC using the method proposed by Stoughton. This stress-based FLC is then transformed into the XSFLC by casting the stresses in terms of the invariants — effective stress and mean stress that are the variables that constitute the XSFLC. This curve is used in conjunction with LSDYNA finite element simulations that use eight-noded solid elements to predict the onset of necking in tubular hydroforming. The particular case of straight tube hydroforming of ENAW 5018 aluminum alloy tubes is considered. The XSFLC is able to predict the internal pressure at the onset of the neck and the final failure location in the tube with remarkable accuracy. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

In stamping practice, springback of the blank is often compensated by developing the shape of the blank, which provides the targeted shape after its springback. In this paper we attempt to address the root cause of springback phenomenon dictated by residual stresses in the blank after completion of the stamping process. New process of springback calibration has been developed based upon the idea of elimination of elastic residual stresses in stamped blank. This technology attempts to address the issues of sheet material variability and stamping process variations, which make geometrical compensation more difficult. Simple set of experiments demonstrated that formed blank can be clamped to the targeted shape and then subjected to pulsed electromagnetic field. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

Forming Limit Diagram (FLD) has been proved to be a powerful tool for assessing necking failures in sheet metal forming analysis for majority of stamping operations over the last three decades. However, experimental evidence and theoretical analysis suggest that its applications are limited to linear or almost linear strain paths during its deformation history. Abrupt changes or even gradual deviations from linear strain-paths will shift forming limit curves from their original values, a situation that occurs in vast majority of sequential stamping operations such as where the drawing process is followed by flanging and re-strike processes. Various forming limit models have been put forward recently to provide remedies for the problem, noticeably stress-based and strain gradient-based forming limit criteria. This study presents an alternative path-independent forming limit criterion. Instead of traditional Forming Limit Diagrams (FLD) which are constructed in terms of major - minor principal strains throughout deformation history, the new criterion defines a critical effective strain [variant_greek_epsilon]* as the limit strain for necking, and it is shown that [variant_greek_epsilon]* can be expressed as a function of current strain rate state and material work hardening properties, without the need of explicitly considering strain-path effects. It is given by [variant_greek_epsilon]* = f(β, k, n) where β = d[variant_greek_epsilon]2/d[variant_greek_epsilon]1 at current deformation state, and k and n are material strain hardening parameters if a power law is assumed. The analysis is built upon previous work by Storen and Rice [1975] and Zhu et al [2002] with the incorporation of anisotropic yield models such as Hill’48 for quadratic orthotropic yield and Hill’79 for non-quadratic orthotropic yield. Effects of anisotropic parameters such as R-values and exponent n-values on necking are investigated in detail for a variety of strain paths. Results predicted according to current analysis are compared against experimental data gathered from literature and good agreements are achieved. This provides a powerful validation for this approach. The new criterion retains all the advantages of the traditional FLDs since it’s still constructed in the strain space. Its relationship with stress-based forming limit criteria is discussed. It is believed that the approach is especially suitable for production stamping CAE analysis since the strain rate state at any material point during forming deformation is readily available from FEA simulations. It can be easily embedded in forming simulation software or alternatively deployed as a post-processing function for forming simulations. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]