Approximate Formability Analysis of Two-Step Stretch Flanges

Many stretch flanges in sheet metal parts are formed from material which has been significantly strained in a previous forming operation. This paper uses the assumption of axisymmetry and an engineering approximation to develop a formula for the final flange length, given the shape of the intermediate configuration and the trim location. The maximum flange length is limited by splitting of the metal at the free edge, and the strain level at which splitting takes place depends strongly on the strain state and magnitude of the intermediate configuration. Using the above-mentioned formula with experimentally determined forming limits, a computer program is developed to calculate maximum flange lengths for balanced biaxial prestrain. Comparison is made to some limited experimental data.