For characterising formability of sheet metal materials several testing methods have been developed during recent years. The most common way to estimate formability of sheet metal parts is the so called Forming Limit Diagram (FLD) method. But forming limits under bending cannot be described by the conventional FLC. This contribution reveals possibilities of an experimental characterisation of material's bendability. Furthermore, an approach concerning the characterisation of formability under simultaneous drawing and bending for various sheet thicknesses is given.

The knowledge of material behaviour under certain load conditions, occurring in metal-forming processes, urgently calls for the development of robust production processes. For characterisation of materials behaviour in metal forming processes in the automotive industry, Forming Limit Diagrams (FLD) are mainly used to characterise formability under certain in-plane load conditions . But in industrial forming processes not only in-planeloads, but a mixture of in-plane, bending, pure-shear and combined shear-and tensions loads can occur. But today there is a lack in testing equipment for those combined load conditions because with conventional testing set-ups no superposed shear rates can be realized. For this purpose three novel sample geometries are presented in this article. Further first experimental results show the application of the specimen geometries. On the one hand maximal achievable strains under combined shear and tension loads are measured and on the other hand a first approach to get real stress-strain curves is shown.

The investigation of bendability of sheet metal alloys is of great importance for the evaluation of process robustness in production of hemmed sheet metal assemblies. Furthermore, material cracks due to bending can also occur at deep drawing operations. This contribution focuses on the experimental determination of material's formability at deep drawing, the characterisation of bendability especially for hemming operations and the determination of formability under combined deep drawing and bending loads, respectively.

For characterising formability of sheet metal materials the so called Forming Limit Curve (FLC) due to the ISO 12004-2 standard to predict material potential, subjected to static multi-axial in-plane load conditions has been developed during recent years [1]. Experimental results have shown that forming limits under bending loads cannot be described by the conventional FLC at all. This is due to the fact that the FLC describes first occurrence of membrane instability and no material failure in consequence of an inter-crystalline fracture at bending. However, first experimental and simulative approaches concerning the Bending Limit Curve (BLC) describe the material formability under pure bending conditions, such as hemming [2]. Thus, the investigation of bendability of sheet metal alloys is of great importance for the evaluation of process robustness for the production of mechanically joined sheet metal assemblies. Furthermore, material cracks due to a dominant bending load can also occur during deep drawing. The so called combined load FLC (cFLC) enables the failure prediction for such load cases under the assumption of an isotropic damage evolution at drawing [3]. This contribution focuses on the development of an improved failure criterion based on the cFLC concept to take the influence of curvature and anisotropic damage evolution on failure prediction at deep drawing into account.

KurzfassungDieser Beitrag zeigt auf der Basis bestehender Ansätze Möglichkeiten neuer Duktilitätsdefinitionen auf. Die dabei neu erstellten Ansätze bieten im besonderen Maße für umformkritische Leichtbaublechwerkstoffe die Möglichkeit zur differenzierten Werkstoffcharakterisierung. Die Beschreibung der Duktilität eines Werkstoffes dient nicht nur der Versagensvorhersage in der Bauteilsimulation, sondern zeigt sowohl dem Karosserieentwickler weitere Optimierungspotenziale als objektives Bewertungskriterium, als auch der Qualitätssicherung zusätzliche Prüfkriterien auf.