The failure prediction in sheet metal forming is typically realized by evaluating the so called forming limit curves (FLC). The standard experimental method is the Nakajima test, where sometimes also the Marciniak test setup is used. Up to now, the FLC determination was performed with failed specimens and an one-directional intersection line method or by manually analyzing the estimated strains before cracking. Both methods determine the failure by considering the occurrence of cracking and do not consider the possibility of time continuous recording of the Nakajima test. Consequently forming limit curves which have been evaluated in such way are often "laboratory dependent" and deviate for identical materials significantly. This paper presents an algorithm for a fully automatic and time-dependent determination of the beginning plastic instability based on physical effects. The algorithm is based on the evaluation of the strain distribution based on the displacement field which is evaluated by optical measurement and treated as a mesh of a finite element calculation. The critical deformation states are then defined by 2D-consideration of the strain distribution and their time derivates using a numerical evaluation procedure for detecting the beginning of the localization. The effectiveness of the proposed algorithm will be presented for different materials used for the Numisheet'08 Benchmark-1 with Nakajima test.

In order to perform the theoretical evaluation of Forming Limit Curves (FLC), the Modified Maximum Force Criterion (MMFC) has been proposed. This paper investigates the mechanism of the fracture of ductile sheet metals and introduces the MMFC model. The evaluation process and the simplified formulations are presented. The influences of hardening behavior and the yield loci are discussed as well. Comparisons with the experimental data of different materials showed generally satisfactory agreement.

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In this study, a model based on a strain localization level to overcome the shortcomings of the wellestablished Forming Limit Diagram (FLD) in predicting the physical phenomenon of necking is introduced. An optical measurement system was used to capture the strain history of the Nakazima experiment until rupture occurred. In order to measure the fracture strain more accurately, a further method is introduced, which is based on the microscopic measurements of ruptured regions. This model is validated using a 3-point bending test. The results show the ability of the method to predict failure under bending conditions as well. Additionally, failure is investigated based on the pressure sensitivity and the Lode dependency. The results show that the triaxiality at the failure point is independent of the loading path.