To evaluate the prediction accuracy of the anisotropic yield function, we propose an original cruciform hole expansion test. Displacements on two axes were applied to the cruciform specimens with a hole in the center. The thickness strain in the region near the hole was compared to the simulation results. Because this forming test is free of friction and bending, it is an appropriate method to assess the anisotropic yield function without the influences of friction or the Bauschinger effect, or the need to consider the stress-strain curve in high-strain region. Hill1948, YLD2000-2D, and spline yield function which is an improved version of the Vegter model were selected, and 6000 series aluminum alloy sheets (A6116-T4) were used in this study. The parameter identification method of the spline yield function also proposed in this paper using the pseudo plane strain tensile test and optimization software. As a result, the spline yield function has better predictive accuracy than the conventional anisotropic yield functions Hill1948 and YLD2000-2D.

The material strength of rolled steel sheet for car bodies is increasing considerably. It has been experientially known that the die deflection affects the springback, but there are few experimental verification cases. In this study, two bending dies with different rigidity but the same shape were manufactured. The springback of panels bended by each die was compared in detail. Using 590MPa class and 980MPa class steel sheets, the deformation of the dies due to bending also measured. The springback and deflection of the die were clearly large using the low rigidity die. From these results, it was suggested that it is necessary to consider the die deflection for accurate springback prediction of bending.

The material strength of cold-rolled steel sheets for car bodies is increasing considerably. It has been experientially known that die deflection affects springback, but there are only a few experimental verification cases. In this work, two bending dies with different rigidities but the same shape were manufactured. The springback of panels bent by each die was compared in detail. Using 590 MPa class and 980 MPa class steel sheets, the deformation of the dies due to bending also measured. The springback and deflection of the die were clearly significant, when the less rigid die was used. From these results, finite element analysis was carried out to investigate the effect of die deflection on springback. Material modeling for the Bauschinger effect was used for the FE analysis. The prediction results were reasonable and showed the same trend as the experimental results. Experiments and FE analysis clarified the mechanism of springback due to the effect of die deflection.