Effect of local ductility on crash performance of automotive structures considering press forming strain of advanced high strength steels

<div class="section abstract"><div class="htmlview paragraph">A fundamental study on the ductility of high strength steels in crash deformation is carried out to investigate the effect of the local ductility of various materials on automobile crashworthiness, considering the prestrain induced by press forming in the manufacturing process. In this study, a newly developed 980 MPa-grade steels [<span class="xref">1</span>], ‘jetQ^TM’, is investigated to clarify its advantage in term of crashworthiness in comparison with the conventional DP (Dual Phase) and TRIP steels.</div><div class="htmlview paragraph">Quasi-static axial crushing tests are performed to evaluate the crashworthiness of the different types of steel. Based on the experimental results, the effect of the local ductility of high-strength steel on the risk of material fracture is discussed.</div><div class="htmlview paragraph">In this paper, a new bending test method, orthogonally reverse bending, (ORB), is proposed to simulate the fracture that occurs during crash deformation considering press-forming strain. The test method is developed using a combination of the V-bending process and the tight radius bending method (based on VDA 238-100). The V-bending process is used to induce the pre-strain in the test sample, simulating the strain caused by press-forming, and the second bending process is performed to evaluate the fracture angle and local ductility of the high strength steel after the press-forming process. The proposed ORB method shows that the local ductility of TRIP steel is decreased by the press-forming due to transformation of the microstructure, while jetQ steel displays higher local ductility even after press-forming.</div><div class="htmlview paragraph">A numerical simulation model is also developed to understand the fracture behavior in the ORB process. The simulation shows that fracture strain occurs in the local bending area with prestrain by the V-bending process.</div><div class="htmlview paragraph">Finally, the applicability of the developed high strength steel to automobile body structures to realized light-weight body structures is discussed.</div></div>