Investigation of Fracture Behavior of Deep Drawn Automotive Part affected by Thinning with Shell Finite Elements 2020-01-0208

In the recent decades, tremendous effort has been made in automotive industry to reduce vehicle mass and development costs for the purpose of improving fuel economy and building safer vehicles that previous generations of vehicles cannot match.

An accurate modeling approach of sheet metal fracture behavior under plastic deformation is one of the key parameters affecting optimal vehicle development process. FLD (Forming Limit Diagram) approach, which plays an important role in judging forming severity, has been widely used in forming industry, and localized necking is the dominant mechanism leading to fracture in sheet metal forming and crash events.

FLD is limited only to deal with the onset of localized necking and could not predict shear fracture. Therefore, it is essential to develop accurate fracture criteria beyond FLD for vehicle development. To enhance the accuracy of crash simulations, forming results from stamping process are generally introduced to consider work hardening and thinning/thickening of a stamped part during the simulations. However, fracture criteria are only applied to the original design thickness, not the change in the gage thickness after forming process. It causes inaccurate fracture prediction on 2D Shell elements.

How to create HC(Hosford-Coulomb) fracture model [1] from existing FFLD (Fracture Forming Limit Diagram) [2] curves is demonstrated and developed in this paper, which covers ultimate cracks beyond local necking based on traditional FLD. The fracture prediction model on deep drawn seat belt anchor plate made of CR340HSLA steel under tensioning load is investigated and compared to corresponding GISSMO(Generalized Incremental Stress State dependent damage Model) using Triaxial Limit by a new keyword (*DEFINE_CURVE_TRIAXIAL_LIMIT_FROM_FLD) in LS-DYNA3D [3], which is being converted from FFLD and developed by HC(Hosford-Coulomb) fracture model. And the thinning effect on 2D-shell elements by deep drawing that affects fracture behavior is also investigated.

Finally, a proposed approach to predict accurate fracture considering thickness changes during forming on deep drawn parts by FFLD is validated by corresponding experiments.