Effect of the non-uniform pressure on the deformation and instability of the sheet in flexible-die bulge forming

Abstract

In recent years, the application of non-uniform pressure in flexible-die forming, such as viscous pressure forming and magnetorheological pressure forming, has been utilized in manufacturing many thin-walled components. However, the non-uniform pressure is mainly applied to change the shape of the sheet. The effects of the non-uniform pressure on thickness reduction and fracture are still intricate. In this study, the mechanism of the non-uniform pressure to decrease thickness reduction and delay fracture instability in the bulge test is revealed. The non-uniform pressure changes the principal stresses, causing distinct deformation behaviors. There are two opposite effects on thickness reduction and fracture instability. If the pressure at the bulged pole decreases, the shape is more prolate, leading toa decreased average thickness and a decreased polar curvature. The thickness reduction tends to be severe, and the diffuse necking is easy to produce. If the pressure at the bulged pole increases, the shape is more oblate, leading to an increased strain gradient. The thickness reduction tends to be severe, and the localized necking is easy to produce. Characteristics of the pressure to decrease thickness reduction and delay instability are presented. Viscous pressure bulge tests are carried out. Experiment results of the shape, thickness uniformity, polar strain, and fracture verify the theory. The fracture strain increases by 5.6 % compared with the uniform pressure condition. This study proves that non-uniform pressure can promote sheet formability, indicating that non-uniform pressure has broad application prospects in sheet metal forming.