Application of an advanced necking criterion for nonlinear strain paths to a complex sheet metal forming component

For meeting time, cost and quality targets in industrial sheet metal forming processes, correct formability prediction in a very early project stage has become a crucial factor. It is well known that the conventional Forming Limit Curve (FLC), which is most commonly used for this purpose, is only valid for linear strain paths. Yet, in most industrial sheet metal components, the occurring strain paths are nonlinear. Due to this fact, most users apply a safety margin of 10 to 20% when using the Forming Limit Diagram for formability prediction resulting in higher process robustness, but also increasing component weight and material costs in production. Reasons for the broad application of the FLC in industry is the simple experimental determination of the curve as well as the easy implementation in finite element post-processing. In this paper, an advanced failure criterion for nonlinear strain paths is presented and applied to a deep drawn door inner part. The investigated deep drawing specimen was manufactured using the aluminium alloy AA6014 T4 and the dual phase steel DP600. The sheet thickness was chosen to be closely to 1.0 mm. The calibration procedure of the criterion for arbitrary sheet materials is based purely on uniaxial tensile test data. Experiments for the calibration of the model as well as the application of the criterion to an experimental stamping part will be explained in the paper. Finally, a comparison of the newly presented model with conventional formability evaluation using standard FLC will be given.