Edge fracture of AHSS sheets under out-of-plane loading followed by in-plane loading

In recent years edge fracture has become a challenge in the manufacturing industry with the dramatical increase in the application of light-weight materials such as Advanced High Strength Steels (AHSS) and aluminum alloys. A premature edge cracking is observed in components with blanked/trimmed/pierced edges during the subsequent metal forming process. To understand the underlying physical mechanism and to establish a reliable CAE model, the thesis carries out a comprehensive experimental and numerical investigation on edge fracture of a commercially available DP780 steel sheet. The study reveals that it is the substantial plastic deformation introduced during the out-of-plane sheet blanking process that compromises the material ductility within the Shear Affected Zone (SAZ), and subsequently causes the edge to fracture prematurely under the in-plane edge stretching. To simulate the fracture behavior under such a complex a loading path, the fracture initiation is modeled using the concept of a scalar damage indicator that consists of two parts which are accumulated at different stages. The first one is accumulated during the sheet blanking process, referred to as pre-damage, while the second part is produced by the following metal forming. In such a modeling frame work, a corner stone is to determine the first part of pre-damage within the SAZ. This is achieved by a hybrid experimental and numerical method. Aided by microscopic examinations on cracked surfaces, the study shows that the pre-damage distribution within the SAZ follows an exponential function that drops from the critical value of unity at the edge surface to zero over the width of the SAZ. The obtained pre-damage is then introduced as the initial damage value during the following metal forming process. Based on a detail experimental characterization, a plasticity and fracture model is introduced to describe the material behavior under investigation. The model is based on the von Mises yield condition, a non-associated Hill'48 flow potential and an isotropic hardening law, together with the MMC fracture locus description with dependence on both stress triaxiality and Lode angle parameter. The proposed model successfully predict edge fracture in numerical simulation.

Department dc:contributor.department

Massachusetts Institute of Technology. Department of Mechanical Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2015

Author dc:creator

• Wang, Kai, Ph. D. Massachusetts Institute of Technology

Advisor dc:contributor.advisor

• Tomasz Wierzbicki.

• Mechanical Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/100151

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/100151