Formability of aluminium alloy subjected to prestrain by rolling
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The purpose of this thesis was to improve the understanding and the accuracy of the description of metal sheet formability following the needs of the automotive industry. In particular, the effects of plastic anisotropy and prestrain by rolling on the formability of AA6016 sheets were investigated. The formability of metal sheets is traditionally described by strain-based Forming Limit Curves (FLCs) which are known to be path-dependent. In this thesis the merits of the alternative descriptions by means of stress-based and equivalent plastic strain based FLCs were investigated. First, the mechanical properties of the AA6016 sheets were examined through a series of material tests. Uniaxial tension tests in seven different directions in the plane of the sheet along with the disc compression test were used for calibration of the plasticity model. In addition, plane-strain tension, in-plane shear and cyclic shear tests were performed for validation of the plasticity model. Then, the formability properties were investigated by means of Marciniak-Kuczynski and Nakazima tests. The investigated material showed a tendency to form multiple local necks in biaxial tension making the standard method for experimental detection of forming limit strains inapplicable. In order to enable the construction of FLCs a method capable of handling multiple local necks had to be developed. The new method improved the accuracy and robustness of experimental detection of forming limits. In addition, this method was developed to differentiate between local necking and fracture strains without involvement of the user. It was shown that for the tested AA6016 the sheet formability in biaxial tension is limited by local necking. The possible influence of fracture on the forming limits was excluded both by improving the experimental procedure to differentiate between local necking and fracture strains and by applying predictions of calibrated fracture models. The experimentally detected FLCs at necking were compared with the predictions of the Marciniak- Kuczynski model. The comparison revealed that the investigated material displayed significantly higher anisotropy in formability than could be explained by the plastic anisotropy alone. The formability proved to be affected by roping phenomenon. Roping is likely to be the cause of the multiple local necking that made accurate experimental detection of the forming limit strains challenging in the first place. The effect of prestrain by rolling on the forming limit strains proved to be similar to the effect of prestrain in plane-strain tension. On the other hand, the forming limit stresses and the equivalent plastic strain at necking proved to be much less affected by the prestrain. This confirmed the advantages of the description of formability by means of stress-based and equivalent strain based FLCs in comparison with the traditional strain-based FLCs.