Prediction Method of Void Distribution near Punched Surface of Medium-Carbon Steel Sheet using Scrap

The objective of this study was to confirm the validity of the prediction method of void distribution near a punched surface of a blank (holder side) by observing the void distribution of scrap (hole side). It is important to know the exact void number density and void area fraction through an appropriate evaluation method such as that mentioned, just where a crack occurs owing to stretch-flange deformation in the same individual sample because the crack and the void behaviors fluctuate from sample to sample and with position, even under the same punching condition. This study investigated the correlation of void distribution near punched fracture surfaces of the blank and scrap in medium-carbon steel. The voids near the punched fracture surfaces of the blank and the scrap were measured using SEM images. The voids of the blank and the scrap were distributed point-symmetrically with respect to the center of the fracture surface. The equivalent plastic strain and the stress triaxiality that were analyzed with FE analysis were also distributed point-symmetrically. The void distribution of scrap was shifted to the sheared surface side, compared to that of the blank. To predict the void distribution of the blank using the scrap, the void distribution of the scrap with the burr side as a reference point was approximated by a cubic function. Furthermore, the void distribution of the scrap shifted to the burr side. The prediction method of void distribution near punched surface by scrap was validated by considering stress and strain during punching.