An Effective Experimental Technique for Determining Necking Limit Strains of Ultrahigh-strength Steel Sheets

Abstract

Fracture is a major issue in the press forming of ultrahigh-strength steel (UHSS) sheets. To avoid fracture, it is helpful to accurately predict necking that appears before fracture. Forming limit diagrams (FLDs) have been used to predict necking. However, conventional methods often fail to predict necking because UHSS sheets have different necking behaviors from mild steel sheets. In this study, three types of FLD are compared, namely, those determined by conventional time-dependent (TD) and position-dependent (PD) methods, and by the developed time- and position-dependent (TPD) method. The results of conventional methods indicate that the necking limit determined by the TD method is very large, and that the accuracy of the necking limit determined by the PD method is unstable under a strain path with high-degree peaks in its strain distribution. Alternatively, the necking limit determined by the TPD method is less affected by high-degree peaks, suggesting that the accuracy of the necking limit is stable. Furthermore, the necking limit determined by the TPD method can also be precise based on the thickness distribution in a panel formed immediately before fracture. From the above reasons, the TPD method is considered to be more appropriate in accurately predicting necking in UHSS panels.