A ductile failure criterion for predicting sheet metal forming limit

This study proposes a new Ductile Failure Criterion (DFC) to predict forming limits for sheet metal forming. In this criterion, damage growth is defined by a modified McClintock formula, stretching failure is defined at Localized Necking (LN) or Fracture without LN, and the critical damage is defined as a function of strain path and initial sheet thickness. The failure definition removes the uncertainty of Post-Necking (P-N) deformation while the critical damage definition reflects the findings of many micromechanical studies. The observed strain path and sheet thickness effects on the critical damage are represented by an effect function, which can be phenomenologically determined by a set of critical tests. With the help of the determined effect function, the proposed criterion only needs a calibration at uniaxial tension. The proposed criterion is validated by calculating limit strains under linear, bilinear and strongly non-linear strain paths for different sheet materials, which include data from Numisheet 2008 and 2014 Benchmark studies. Comparison shows that the calculations and failure predictions are in good agreement with the experimental observations and comparable with the best results from benchmark participants.