A formability evaluation method for sheet metal forming with non-linear strain path change

The strain path dependence of forming limit curve (FLC) restricts its practical application for formability evaluation as the real strain path of material point in true industrial forming processes is often non-linear. In this work, the constraint equation for material necking was extended to the loading condition with non-linear strain path change and an analytical approach for forming limit calculation under combined-linear strain path loading with pre-strain was developed. The approach developed in this work was used to predict the forming limit strains after pre-strained in uni-axial tension, plane strain, and bi-axial tension for Aluminium alloy sheet Al 2008-T4. The comparison between the predicted results and the past experimental data showed that the effect of pre-strain on the material FLC can be reflected by the developed approach. Furthermore, a formability evaluation index accounted for the strain path history was put forward and used for the formability evaluation of a drawing reverse-drawing process, in which the blank underwent a continuous strain path change. Based on the measured data of strain path history and the forming limit strains at the initial failure location in specimens, the validity of the proposed formability evaluation index for sheet metal forming with non-linear strain path change was verified.