A novel test method for continuous nonlinear biaxial tensile deformation of sheet metals by bulging with stepped-dies

Abstract

In this paper, a novel test method named bulging with stepped-dies is proposed to overcome the difficulty of traditional test methods in realizing continuous nonlinear loading paths from initial yield up to fracture on a sheet metal. To achieve this aim, the section shape of a stepped-die cavity is varied with increasing depth. During bulging with a stepped-die, the stress state at the pole of bulging area of the sheet changes continuously with the increase in bulging height, which results in a specific nonlinear loading path. A theoretical model is established to calculate the stress components at the pole based on the assumption that the bulged surface near the pole is approximated by a rotational ellipsoid. Bulging experiments with three different stepped-dies are performed by using ST16 steel sheet. Stress and strain paths up to fracture and equivalent stress-strain curves at the pole are analyzed and compared with the results of bulging with elliptical dies. It is shown that continuous nonlinear loading paths can be effectively realized through bulging with stepped-dies and the stress ratio at the pole changes from 0.5 up to 2.0 at most in one bulging experiment. The feasibility of the novel test method is validated successfully. And the experimental data obtained are useful to determine constitutive and forming limit models suitable for complex loading conditions.