In-plane plane strain testing to evaluate formability of sheet steels used in tubular products

Abstract

In order to effectively and efficiently hydroform new automotive components, the formability of new tubular steels must be evaluated. Standard forming limit diagrams have been used for decades to evaluate and predict the formability of sheet steel formed along linear strain paths. However, tube hydroforming can present a problem since the pre-bending stage used in many hydroforming operations causes multiple non-linear strain paths. This thesis has modified a formability test method that deforms small-scale sheet steel samples in a single plane. The sample geometries were designed such that the strain paths achieved at the center of the samples were very near the plane strain condition. The four steels chosen for this study were: a deep drawing quality (DDQ), a high strength low alloy (HSLA) and two dual phase steels (DP600 and DP780). The plane strain formability for each of the four steels was tested in both the rolling and transverse directions. Three objective criteria were employed to evaluate and directly compare the formability of the four steels tested: difference in strain, difference in strain rate and local necking. The DDQ steel showed the highest formability followed in order by the HSLA, DP600 and DP780 steels. The repeatability in determining the forming limit strains using the difference in strain, the difference in strain rate and the local necking criteria for a 95% confidence interval was ± 1.5%, ± 1.2% and ± 3.2% engineering strain, respectively. The forming limit data collected for this thesis has been compared to results from full-scale tube hydroforming operations and free expansion tube burst tests carried out by researchers at the University of Waterloo on the same four materials. It was found that local necking results could be used to predict failure of hydroformed HSLA steel tubes with low levels of end-feed. However, this same method could only predict the failure of hydroformed DP600 steel tubes at higher levels of end-feed. The three objective criteria were not found to be suitable for predicting failure of free expansion tube burst tests.