Evaluation of the forming limit curve of medium steel plate based on non-constant through-thickness normal stress

The forming limit curve (FLC) provides a convenient and useful way of presenting the necking or facture in sheet forming process. This work focused on determining the FLC of the medium steel plate with a thickness of 5 mm. The Nakazima test and uniaxial tension test were conducted to determine the limit strain pairs and the mechanical properties of the QSTE600TM medium plate, respectively. The M-K theory was extended under non-constant through-thickness normal stress condition to predict the FLC of the medium plate with higher accuracy. Meanwhile, the numerical Nakazima test was implemented to validate the non-constant through-thickness normal stress and determine the forming limit strains. The criterion for evaluating the limit strain pairs in simulation process was presented. The comparison showed that the experimental, theoretical and numerical FLCs well coincided, and demonstrated that the non-constant through-thickness normal stress was a critical factor in the FLC prediction of medium plate.