FORMABILITY OF SHEET-METAL UNDER LATERAL PRESSURE

Abstract

An experimental investigation into the mechanics of axi· symmetrical bulge-forming is made by subjecting circular metal diaphragms to hydraulic pressure on one side. Also, the effects of using different die-shoulder radii on the bulge-forming process, bulge geometry, principal strains, membrane stresses, and formabil- ity are studied. A simple theoretical model is formulated and applied to account for the observed phenomena. For the material used in this investigation, the results reveal that bulge-forming up to the point of fracture comprises two modes of deformation. During the initial stage of deformation, the bulge shape increases in sphericity. Beyond a certain point, the second or advanced mode of bulging sets in, and the bulge deviates increasingly from sphericity until fracture occurs. Throughout deformation, the bulge surface is never spherical, but exhibits a complex geometry which has yet to be explained theoret- ical. The die-shoulder radius has significant effects on bulge forming, and these become increasingly evident as deformation progresses. A larger die-shoulder radius inhibits deformation to a greater extent, and tends to confine it to the central portion of the bulge. During initial deformation, the effects of using different die-shoulder radii are less apparent. However, as bulging proceeds, a larger die-shoulder radius causes a greater deviation of the overall bulge shape from sphericity as compared to forming with a smaller die-shoulder radius. The findings also indicate that formability is improved by the use of smaller die shoulder radii. Moreover, they point toward the necessity of specifying the die-shoulder radius when employing the bulge test to assess material formability. The membrane stresses induced during bulging exhibit complex distributions which are governed by bulge geometry. The influence of through-thickness strains on the equivalent stress distribution is found to increase as deformation progresses. The theoretical model, based on a spherical bulge, is useful in explaining the effects of the die-shoulder radius on the forming process, the polar strains, and on formability. The model also provides close approximations of the observed behaviour.