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Forming Technology Forum

5 - 6 June 2012, Zurich, Switzerland

Advanced failure prediction methods in sheet metal forming

The nowadays applied FEM-based methods for virtual layout and optimization of sheet metal forming processes are mostly based on a post-processing-oriented failure prediction, where the computed strain state is compared against a limit curve in order to predict the beginning of localized necking. These Forming Limit Curves (FLC), which are widely used in practice are well known to be restricted only to linear strain paths, a condition which is mostly fulfilled only during the first deep-drawing step. Furthermore the FLC’s also do not clearly distinguish between the state of localized necking and the later state of crack nucleation. This is why failure types such as those of hemming induced by surface cracks or by edge cracks, which occur on blank inner holes, cannot be accurately predicted. If additional factors such as temperature, increasing martensite volume fraction or strain rate effects influence the failure occurrence, those influences are not described by the FLC-based analysis either.

In order to solve this unsatisfactory situation for the virtual planning, failure criteria alternative to the M-K-models will be discussed. Such are damage based failure criteria (FC), FC based on critical principal and shear-stress-limits as well as multi-scale FC concerning the material microstructure.
At the level of direct FEM-based modeling of the necking phenomena the problem of mesh dependency in the modeling of shear bands and the correct modeling of crack propagation is not satisfactorily solved.

A further important point of physically based FC is the experimental identification of critical values. These are, among others, the damage parameters describing the nucleation and growth of pores, in case of damage based FC and the critical stresses for the stress limit criteria.

The aim of the conference is to demonstrate the applicability of new criteria for industrially relevant examples. As such, complex multi-stage deep drawing processes, cracking at panel edges and cracks in hemming processes have to be considered. Regarding the material the applicability of modern lightweight HSS and UHSS as well as of  Al-sheet materials is of interest.

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Please find more informationhere.