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

In recent years, the application of non-uniform pressure in flexible-die forming, such as viscous pressure forming and magnetorheological pressure forming, has become increasingly prevalent in manufacturing thin-walled components. Non-uniform pressure is primarily employed to change the shape of the sheet, while its effects on thickness reduction and fracture behaviors remain intricate. In this study, the mechanisms of non-uniform pressure to decrease thickness reduction and delay fracture instability in bulge forming are revealed. Non-uniform pressure changes the principal stresses, resulting in distinct deformation behaviors. Numerical simulations indicate that both too large and too small pressure weights lead to severe thickness thinning and increase the risk of fracture instability. Theoretical equations of the polar strain and instability strain are derived to reveal the mechanisms. The characteristics of non-uniform pressure to decrease thickness reduction and delay instability are outlined. Viscous pressure bulge tests are conducted, and the experimental results of bulged shapes, thickness uniformity, polar strains, and instability behaviors validate the theories. The fracture strain under the non-uniform pressure condition shows a 5.6% increase compared with the uniform pressure condition. This study proves that non-uniform pressure can promote sheet formability, suggesting promising applications in sheet metal forming.