Prediction and Experimental Validation of Path-Dependent Forming Limit Diagrams of VDIF Steel

Strains in most stamped parts are produced under non-proportional loading. Limit strains induced during forming are, therefore, path dependent. Experimental Forming Limit Diagrams (FLDs) are usually determined under proportional loading and are not applicable to most forming operations. Experimental results have shown that path dependent FLDs are different from those determined under proportional loading. A number of analytical methods have been used to predict FLDs under proportional loading. The authors have recently introduced a new method for predicting FLDs based on the theory of damage mechanics. The damage model was used successfully to predict proportional FLDs for VDIF steel and Al6111-T4. In this paper, the anisotropic damage model was used to predict non-proportional FLDs for VDIF steel. Experiments were conducted to validate model predictions by applying pre-stretch in plane strain followed by uniaxial and balanced biaxial tension. Model predictions showed that pre-stretch in plane strain increased the level of the FLDs in the biaxial stretch region. Also, pre-stretching was found to decrease the major uniaxial and to increase the major biaxial limit strains. Although experimental limit strains were found to be consistently lower than the predicted ones, the correlation between experimental results and model predictions was good. Pre-strain paths and levels used in this investigation did not result in significant changes to the FLDs of VDIF steel.