An Integrated Methodology for HER Determination in Hole Expansion Test

Abstract

Advanced high-strength steels (AHSS) are increasingly being used in the automotive industry due to their high yield and ultimate tensile strengths, allowing the production of lighter components for the car body structure without compromising safety standards. However, they present setbacks when compared with conventional steels, such as the edge cracking, which is caused by forming components with shear-cut edges. The hole expansion test (ISO 16630:2017) is a standard method used in industrial applications to evaluate the edge stretchability of sheet materials. However, this test requires being accurate on the visualization of the first cracking to occur, which is usually challenging and can be a subjective choice, often leading to inconsistent results and low reproducibility. Another problem is related to the possible influence of springback on the hole dimension. Due to this phenomenon, the hole diameter can be different when the punch is removed, thus giving inconsistency on measurements. In this work, a new methodology was used to improve the diameter measurement in the hole expansion test. Therefore, an instrumented device was developed to acquire the real-time evolution of the hole dimension throughout the test. This improvement helps in the detection of the appearance of the first edge crack and, consequently, allows a more accurate determination of the hole expansion ratio (HER). The proposed method was evaluated for different materials, and the corresponding HER values obtained with the developed method were compared with the traditional method. Additionally, numerical simulation was performed to complement the analysis of developed strategies.