ESSC & DUPLEX 2019

Forming limit curves (FLCs) are widely used for predicting failure of sheet metals in press shop operations. An FLC is a borderline between safe and unsafe regions in the principal strain space. In other words, this curve defines the extent by which a specific material can be deformed without developing localized necking and incipient failure. In Europe, FLCs are commonly determined in the laboratory using the Nakajima test according to ISO 12004-2. In the testing procedure, a series of dog-bone shaped blanks is stretched with a hemispherical punch until failure occurs. Optical strain measurement equipment is used to measure the strain field on the surface of the specimen. The limit strains are then calculated based on an objective mathematical criterion. Measurement of FLCs is extremely time-consuming. Therefore, a variety of empirical and statistical methods have been proposed for predicting FLCs based on parameters that can be measured with relative ease. Unfortunately, almost all proposed methods have been developed using carbon steel and alloy steel as test material. Stainless steel possesses somewhat different forming characteristics than carbon steel. Therefore, it is questionable whether the methods proposed for prediction of FLCs can be applied as such to stainless steel. In the present work, an empirical method is developed for predicting the entire FLC of stabilized ferritic stainless steels. Predictive equations are derived based on statistical correlations between measured FLCs and the strip properties. The accuracy of the new method in comparison with the experimental uncertainly of forming limit curve points obtained with the Nakajima test according to ISO 12004-2 is discussed.