Numerical quantification of damage accumulation resulting from blanking in multi-phase steel

Abstract

The high potentials of utilizing high strength steels in the automotive industry have been proved. However, there are still some unsolved challenges. Forming of a component that has been produced by blanking is one of these. Blanking is commonly used in sheet metal forming as the initial cutting process. Yet, it introduces damage into the blanked edges that in subsequent forming steps may lead to crack formation. This problem arises in particular in modern multi-phase steels and can currently not be avoided thoroughly due to a lack of understanding of the relevant influences. In the present work, the effects of the blanking process on an HCT980XD sheet were therefore numerically investigated. To achieve this, the Edge-Fracture-Tensile-Test method was simulated. By using this method, a conventional uniaxial tensile specimen is manufactured with one milled side and one blanked side. This allows to highlight the effect of blanking in a subsequent tensile test. In order to investigate the damage process, the coupled Modified-Bai-Wierzbicki model was applied to simulate first the blanking process and then the tensile test. The results revealed that during the blanking process, the failure initiated from the surface elements near the punch and propagated through the thickness. Meanwhile, another crack initiated from the opposite side of the sheet. The elements of these cracks experienced near pure-shear condition at both damage initiation and fracture moments. At the end of this step, the remaining damage, which is considered as the predamage of the next step, was higher in the middle of thickness. During the subsequent uniaxial tension, the crack in the specimen initiated at a shear cut edge and crossed the width. The possible detailed resolution of loading paths and crack formation shows that the damage mechanics simulation provides researchers with a powerful tool for assessing limit states in forming processes.