The numerical simulation of stretch flange forming

Simulations of stretch flange forming operations are undertaken using explicit dynamic finite element calculations utilizing various quadratic and non-quadratic yield criteria. Both circular and square cut-out blanks are investigated with corresponding circular and square punches. Simple stretch flanges are considered, utilizing a single punch to expand the cut-out, as well as z-flanges, which employ a back-up punch to form the second bend needed in the z-flange profile. Results from a model of an automotive inner component incorporating a cut-out with stretch flange corner features are also presented. Predictions utilizing the Barlat-89 criterion are shown to accurately capture the effect of yield anisotropy (R-value). The predicted strains from the corner regions of square cut-out stretch flange laboratory specimens are shown to be similar to those within the automotive inner panel, supporting the use of laboratory-scale stretch flange experiments to simulate the larger panels. Measured limit strains from the stretch flange formability experiments are compared to forming limit diagram (FLD) data from dome specimens. Stretch flange formability is shown to exceed allowable levels predicted using a classical FLD approach, particularly for simple stretch flanges, indicating that the FLD approach is overly conservative.