This papers presents a damage mechanics model, which was proposed recently by the authors for predicting the localized necking and final failure of sheet metals. It is generalized to take into account the continuous directional change of principal strain plane and damage plane, and their hysteretic effect on damage evolution. The generalization is a critical requirement to predict accurately the forming limit strains, as most stamped components are subjected to multistage forming process. This type of forming process produces non-proportional stamping force/load and a complex strain history in the stamped part causing directional change of principal strain plane and damage plane. The damage model is implemented into the large-scale finite element package LS-DYNA that then was applied to predict the forming limit strain and fracture limit strain of VDIF under non-proportional loading condition. Satisfactory predictions have been achieved for several case examples where the complex strain histories are prescribed from a practical stamping operation.
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