Formability of tube hydroforming under various loading paths

Finite-element simulations of the tube hydroforming process with rectangular dies are carried out for various loading paths, and the reasons why the loading paths of the so-called pressure advanced type, in which the hydraulic pressure is raised to a certain magnitude in advance of the axial pushing, are effective to obtain good formability are investigated in detail. It is clarified that, in the early stage of the deformation process by the pressure advanced type, the growth of local wrinkling is prevented and the effect of axial pushing from the tube ends is attained at the whole part in the axial direction. Consequently the desirable biaxial stress condition of the circumferential tensile and longitudinal compressive stresses is obtained in the subsequent stages, thus achieving large expansion without the occurrence of local wrinkling and local thinning.