Deformation Scenarios of Combined Stretching and Bending in Complex Shaped Deep Drawing Parts

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.