An approach to describe edge ductility

Abstract

One of the challenges of utilizing advanced high-strength steels is their limited ability to withstand the forming of cut edges. Large production quantities of parts often lead to mechanical punching/shearing processes in blank preparation, providing a challenging starting point for forming processes. The most commonly used edge ductility test is the ISO 16630 hole-expansion test to describe stretch-flangeability properties. However, this method has been widely criticized for its large-scatter, unreliable results and the fact that it covers only a certain stress-strain state of cut edge forming. In addition, it does not provide enough data to be reliably used in forming simulations to predict edge failures. This paper presents an approach to create a more comprehensive way of describing overall edge ductility. Multiple edge forming test methods coupled with digital image correlation (DIC) were selected in order to investigate different edge loading scenarios in both open and closed trim line forming situations. Data regarding limiting local strain before cracking was collected for several steel grades, and results are gathered in 2D and 3D “Edge FLC”-figures. Attempts to utilize these results for simulation purposes are also presented. Results indicate that this approach can be useful to evaluate overall edge forming limits.

Effect of pre-strain on edge formability of high strength steels for chassis applications

“…Nevertheless, it is important to remark that sheared edge forming limits may depend on several factors, including cut edge quality and loading scenario (stretch flanging, bending, in-plane stretching, open or closed trim line, etc.) [11,13]. Therefore, the results obtained in this study must be considered only representative of the specific load case investigated.…”

“…Furthermore, it only represents a specific case of edge formability; a closed edge cut with a 12 ± 1% cutting clearance and expanded with a conical tool of 60º. To overcome such limitations, alternative edge crack tests based on optical strain measurements and Digital Image Correlation (DIC) techniques covering a wide range of loading cases and strain gradients have been proposed in the literature [3,[11][12][13]. All these test configurations provide relevant strain data that may help to define edge forming limits and improve the accuracy of forming simulations.…”