Crash Simulation of Roll Formed Parts by Damage Modelling Taking Into Account Preforming Effects

摘要

Complex phase steels of strength levels up to 1200 MPa are suitable to roll forming. These may be applied in automotive structures for enhancing the crashworthiness, e. g. as stiffeners in doors. Even though the strain hardening of the material is low there is considerable bending formability. However ductility decreases with the strength level. Higher strength requires more focus to the structural integrity of the part during the process planning stage and with respect to the crash behavior. Nowadays numerical simulation is used as a process design tool for roll-forming in a production environment. The assessment of the stability of a roll forming process is quite challenging for AHSS grades. There are two objectives of the present work. First to provide a reliable assessment tool to the roll forming analyst for failure prediction. Second to establish simulation procedures in order to predict the part's behavior in crash applications taking into account damage and failure. Today adequate ductile fracture models are available which can be used in forming and crash applications. These continuum models are based on failure strain curves or surfaces which depend on the stress triaxiality (e. g. Crach or GISSMO) and may additionally include the Lode angle (extended Mohr Coulomb or extended GISSMO model). A challenging task is to obtain the respective failure strain curves. In the paper the procedure is described in detail how these failure strain curves are obtained using small scale tests within voestalpine Stahl, notch tensile-, bulge and shear tests. It is shown that capturing the surface strains is not sufficient for obtaining reliable material failure parameters. The simulation tool for roll-forming at the site of voestalpine Krems is Copra FEA RF, which is a 3D continuum finite element solver based on MSC.Marc. The simulation environment for crash applications is LS-DYNA. Shell elements are used for this type of analyses. A major task is to provide results of the roll forming simulation as initial conditions for the crash model, taking over the shell thickness, the variation of the plastic strain and the damage parameter over the profile. This is realized by a python [13] interface program. Profiles are manufactured by the roll forming facility in Krems with a complexphase steel grade of 980 MPa strength. The final samples are manufactured using the profiled parts with cover plates fixed to them by spotwelds. Axial crash experiments are carried out using the inhouse horizontal crash test facility. It is observed that the component shows good folding behavior with some minor failure sites at edges where there is extensive forming during roll-forming. Simulation runs are made with LS-DYNA using the GISSMO damage model. The results match reasonably well with the experimental results. The simulation tool seems to be useful in order to assess not only the integrity of the roll-forming process but also to adequately predict the crash behavior of roll-formed components. Some suggestions are made in order to improve the simulation of the evolution of damage after initiation in the future.