Improvement of the Forming Limit of Sheet Metals by Removal of Surface Roughening with Plastic Strain

Surface roughening of sheet metals with plastic strain is removed at a given strain during the forming process, and its effect on the improvement of the forming limit is investigated. As a preliminary examination to estimate how the onset of localized necking is delayed by the removal of surface roughness, a rigid-plastic FEM simulation is first made. Experiments for soft aluminium and copper sheets are also carried out, and the increase in forming limit strain is examined for a few strain paths. It is shown that the improvement of the forming limit is considerable in the stretch forming region where the major and minor strains in the plane of a sheet are positive. For balanced biaxial stretching, for example, the forming limit strain can be increased up to about 1.3 to 1.4 times as large as that for ordinary in-plane stretching.