Determination of the Forming Limit Curve Using Digital Image Correlation - Comparison of Different Approaches to Pinpoint the Onset of Localized Necking

Digital image correlation (DIC) technique has been proved as a potent tool to determine the forming limit curve (FLC) of sheet metal. One of the major technical challenges using the DIC to generate FLC is to accurately pinpoint the onset of localized necking from the DIC data. In addition to the commonly applied ISO 12004-2 standard, a plethora of other DIC data analysis approaches have been developed and used by various users and researchers. In this study, different approaches, including spatial, temporal and hybrid approaches, have been practiced to determine the limit strains at the onset of localized necking. The formability of a 980GEN3 sheet steel was studied in this work using the Marciniak cup test coupled with a DIC system. The resulting forming limits determined by different approaches were compared. Strengths and limitations of each approach were discussed. In addition, the conventional finger-touch approach was excised using specimens with perceivable localized necks. Strain measurements associated with localized necks detected by the finger-touch approach were used as the basis to evaluate the effectiveness of selected approaches. Initial suggestions were made based on this case study regarding the harmonization in DIC data analysis approaches for the FLC determination.