A note on the MK computational model for predicting the forming limit strains

The Marciniak and Kuczynski computational model, [1], for predicting the forming limit strains during biaxial stretching of thin sheets with positive strain ratios assumes tangential strain continuity at the interface between the groove and the bulk zones. We question here whether this continuity holds true for the entire loading history in the case of strictly negative strain ratios. An extension of the MK model is proposed for this case. The equations can be integrated in explicit form leading to a more efficient procedure for predicting the left-hand side of the forming limit diagram. Applications to aluminum alloys are presented.