Numerical restrictions of the modified maximum force criterion for prediction of forming limits in sheet metal forming

This article addresses a theoretical analysis of the 'modified maximum force criterion' (MMFC) introduced by Hora et al (1996 A prediction method for ductile sheet metal failure in FE-simulation Proc. Numisheet' 96 Conf pp 252-6). In this work a computational framework for the MMFC is given and it will be shown that the MMFC contains an important singularity which emerges if the yield locus used in the necking simulation exhibits straight line segments and leads to singularities in the predicted forming limit curve or even in total failure in forming limit diagram prediction. It is shown that the singularity emerges in commercial sheet alloys and, therefore, the general reliability of the MMFC appears questionable. It is thus concluded that an extension of the MMFC is required in order to fix the singularity problem.