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How can you evaluate the material formability under complexity deformation condition or multiple steps forming?

Usually, the FLD is obtained from linear strain path. However, for complex deformation or multiple step forming processes, the strain paths are no longer linear, and then it is improper to use the originally FLD to evaluate the formability of this material. Hydroforming parts cannot be analysed using FLD’s in sheet metal stamping stretch flange forming is another strain path which often exceed the predicted FLD’s

Strain history will have significant effect on the overall material formability. If the strain path is non-linear, sometimes, strain localization might have already occurred even when the strain state is below the FLD.