Use of two behaviour laws for the determination of the forming-limit stress diagram of a thin steel sheet: results and comparisons

In this paper two behaviour laws used for the determination of the forming-limit stress curves of orthotropic steel sheets are compared. The first is the classical quadratic Hill’s criterion and the second is the 3G theory proposed by CRM of Liege, which assumes that the straining mechanism is described by sliding in the three planes showing maximum shear stresses, i.e. the planes at 45° from the principal stress directions in the isotropic case. Both of the theories enable the determination of the forming-limit stress states by means of a step-by-step plastic calculation along the strain paths determined in an experimental way on laboratory-drawn parts. For the two cases it is shown that the forming-limit stress curves are independent of the shape of the strain path. The interest of the forming-limit stress curve is to be a very good test stop enabling the prediction of the onset of necking in the numerical simulation of industrial drawing operations showing non-linear strain paths.