In sheet forming, inhomogeneous through-thickness deformation, (e.g. due to a combined deformation of simultaneous stretching and bending when sheet material is drawn over a defined tool radius), is known to have influence on materials formability (i.e. onset of necking). Thus, it is preferable to take this influence into account when assessing the formability of sheet metal in a forming process. For that reason, in 2003 Tharett and Stoughton introduced the so-called "Concave-Side Rule" (CSR) approach to assess the formability of stretch-bent steel sheets. In this study the predictive quality of the CSR approach is analyzed. Therefore a series of Angular Stretch Bend Tests (ASBT) is performed. H340LAD (micro-alloyed steel) sheet specimens, with a thickness of 1.5 mm are stretch-bent over punches of various radii from 1 mm to 20 mm to produce different severity of bending in the test specimens. Results of optical on-line surface strain measurements of the test specimens are used to calibrate Finite-Element (FE) simulations of the ASBTs. From these FE-simulations, numerical results are used to assess the predictive quality of the CSR approach for H340LAD. The rule is found to be valid for H340LAD sheet material stretch-bent with punch radii R ≥ 10 mm. Whereas predictive quality decreases for more pronounced inhomogeneous through-thickness deformation (i.e. stretchbending deformation using punch radii R < 10 mm).
In sheet forming, stretch-bending deformation, (i.e. combined deformation of simultaneous stretching and bending when sheet material is stretched over a defined tool radius), is known to enhance material formability. In order to use this forming potential of sheet material, current research put its emphasis on the experimental characterization of the formability of sheet material, subjected to stretch-bending deformation and on a more reliable formability prediction in complex shaped parts. Nevertheless, significant limitations exist in the current available experimental setups for the characterization of the stretch-bending formability of sheet material. Furthermore, the predictive quality of existing formability prediction models is not sufficient to meet current industrial requirements. In this work results of an experimental characterization of influences on stretch-bending formability using existing and newly developed stretch-bending test setups are presented and a phenomenological concept that uses the experimental results to predict formability in Finite-Element simulations of complex shaped parts involving stretch-bending deformation is proposed.
Abstract. Bending effects, especially for Advanced High Strength Steels (AHSS), are known to influence the material formability when stretching and bending is combined in sheet forming. Traditional formability measures (e.g. the conventional forming limit curve (FLC)) fail to reliably predict formability when bending is added. Consequently, in order to take full advantage of the available forming potential of AHSS sheets in industrial applications and to ensure a reliable failure assessment at the same time, current research is focusing on the experimental characterization and modeling of the influence of bending effects on the AHSS sheets formability in forming scenarios of combined stretching and bending. It is expected that aside parameters such as bending radius or strain ratio, individual deformation scenarios of combined stretching and bending may affect the material formability too. Due to tool geometry and the resulting material flow in deep drawing various complex scenarios of combined stretching and bending can occur. For example, a material element is subjected to a complex deformation history of in-plane stretching with subsequent stretch-bending over a cylindrical tool contour, followed by unbending under tension. Another material element of the same drawing part presumably starts also with in-plane stretching but is consequently stretch-bent over a doubly curved tool geometry. Consequently, comprehensive knowledge on the stretch-bending deformation scenarios prevailing in deep drawing is crucial for a more reliable formability assessment. This work aims to identify and characterize the stretch-bending deformation scenarios to occur in different complex deep drawing parts (i.e. B-pillar, cross-die test) and small scale tests (i.e. Angular Stretch-Bend Test (ASBT)). For this reason, this investigation uses an innovative approach recently developed by some of the authors and published elsewhere to categorize the stretch-bending scenarios in industrial deep drawing processes. The approach consists of a stretch-bending categorization schema and a procedure to categorize the forming scenarios in deep drawing parts using data of finite element (FE) simulations. Results of the categorization can directly be plotted on the FE mesh of the deep drawing part (i.e. map type plot of deformation scenarios). The categorization approach mentioned uses results of conventional shell-type FE forming simulation and is therefore applicable in an industrial environment. The FE forming simulation results of the parts selected were analyzed using the stretch-bending categorization approach to identify which stretch-bending scenarios occur in deep drawing parts, to quantify which scenarios to prevail and to show that the conventional ASBT does not represent all the relevant deformation scenarios that prevail in typical deep drawing parts. Furthermore, with the use of experimental observations from real part forming, the stretch-bending scenarios which are the most critical (i.e. the scenarios under which failure occurs) in forming the cross-die geometry are identified. Results of these analysis are presented and discussed in detail.
Under stretch-bending conditions, a significant tensile stress gradient through sheet thickness is induced, especially for a small punch radius. The traditional instability theories were developed assuming a uniform tensile stress / strain distribution through thickness; hence, may lead to unreliable prediction of stretch-bending formability. In this study, the instability behavior of sheet metal under stretch-bending is analyzed via FE-simulation of an Angular Stretch-Bend Test (ASBT). In order to reflect the influence of bending, contact normal stress etc., solid elements are used in the simulation. Three deformation stages are identified: (a). stable deformation; (b). strain localization through sheet thickness; (c). localized necking. Based on the instability characteristics, a localized necking criterion is proposed for predicting forming limits of sheet metal under stretch-bending. By combining the proposed criterion and solid element simulation, good agreement between numerical and experimental results is indicated. This work provides a new approach for predicting stretch-bend formability with sufficient accuracy and convenience.
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