Influence of curvature variation on edge stretchability in hole expansion and stretch flanging of advanced high-strength steel

Abstract

Advanced high-strength steels (AHSSs) have been widely used in automotive structural components to reduce the vehicle weight and improve the safety, while edge stretchability is an important characteristic besides traditional material property parameters. In the presented work, a new method is proposed to investigate the influence of curvature variation on the edge stretching performance by introducing an evaluation index, i.e., strain localization intensity index (SLII), which is strain-based and defined as the strain ratio of the theoretical edge failure strain (first appearing at the location with maximum curvature) and the strain appearing at the same time at the location with minimum curvature. The algorithm to calculate SLII based on elliptical hole expansion simulation was developed in details. Numerical and experimental studies of a typical AHSS-DP780 on elliptical hole expansion and semi-ellipse stretch flanging were conducted to investigate the influence of geometric features on edge stretching performance. Results showed that strain localization got more severe with the increasing of hole eccentricity. More severe strain localization caused declined edge stretchability. Fractures were prone to appearing at the position with the largest curvature. Comparative studies showed that strain localization during stretch flanging was more severe than that during elliptical hole expansion under the same geometric eccentricity. Perimeter elongation (PE) and flanging limit height (FLH) for hole expansion and stretch flanging were calculated and compared, showing that stretching performance of stretch flanging in terms of PE and FLH was more sensitive to geometric eccentricity than that of hole expansion.