Numerical study of springback using the split-ring test: influence of the clearance between the die and the punch

The split-ring test consists in cutting a ring from the wall of a drawn cylindrical cup, which is split to measure the springback. This springback measure can also be used to estimate the circumferential residual stresses in the ring. It is known that the distribution of the residual stresses in the cup depends on the specific combination of forming parameters selected to perform the deep drawing operation, which include the depth of the cup and the clearance between the die and the punch. Moreover, the forming parameters also seem to affect the distribution of the axial and circumferential through-thickness stress profiles along the cup height, since different ring opening trends have been observed, when rings are cut from the cup wall at different heights. The focus of this numerical study is the analysis of the impact of an ironing stage on the residual stress distributions in the cylindrical cup and, consequently, in the split-ring test. The analysis is performed considering a 6016-T4 aluminium alloy, for which experimental results are available. The results show that the ironing of the vertical wall changes the characteristic distribution of the axial and circumferential residual stresses in all locations of the cup wall, even for relatively small ironing strains. This affects the trend observed for the ring opening value, when rings are cut at different heights.