Fracture Toughness to Understand Stretch-Flangeability and Edge Cracking Resistance in AHSS
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CUTTING or shearing operations are widely used in metal sheet forming industries to produce final components. It is well known that cut or sheared edges may present damage in terms of surface irregularities, microvoids and microcracks. The degree of such damage at the cut edge is known as edge integrity. It is known to influence part quality in materials with limited ductility when sheared edges are subjected to bending or stretching in subsequent forming operations. This is the case for the so-called advanced high strength steels (AHSS) that are extensively implemented in the automotive industry to reduce weight and increase crashworthiness (a modern vehicle body contains about 30-50 pct of AHSS[1]).
DANIEL CASELLAS, Director of Materials Technology Area, is with Fundacio´ CTM-Centre Tecnolo`gic, Plac¸a de la Cie`ncia 2, 08243 Manresa, Spain, and also with Lulea˚ University of Technology, 971 87 Lulea˚, Sweden. Contact e-mail: [email protected] ANTONI LARA, DAVID FRO`META, and SI`LVIA MOLAS, Researchers at Materials Technology Area, are with Fundacio´ CTM-Centre Tecnolo`gic. DAVID GUTIE´RREZ, formerly Ph.D. Student with Fundacio´ CTM-Centre Tecnolo`gic, is now R&D Manager with Vac-tron S.A., C. Joan de la Cierva 6, 08420 Canovelles, Spain. LLUI´S PE´REZ, formerly Junior Researcher with Fundacio´ CTM-Centre Tecnolo`gic, is now Ph.D. Student with IUC Olfstro¨m, Va¨llaregatan 30, 293 38 Olofstro¨m, Sweden. JOHANNES REHRL and CLEMENS SUPPAN, Researchers, are with Voestalpine Stahl GmbH, Voestalpine-Straße 3, 4020 Linz, Austria. Manuscript submitted October 10, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
Although AHSS have contributed to the huge improvement of today’s vehicles, they have also introduced new challenges which are still only solved partially. In the last years, many works focused on springback and formability prediction of these steels.[2–6] However, less attention has been given to the cracking phenomena observed for some AHSS grades at cut or sheared edges (Figure 1). Edge cracking is associated with sheared areas that expands during forming operations involving stretch flanging or hole expansion. This process increases the flange edge length during the deformation.[7] Typical examples of stretch flanges in the automotive industry include cut-outs in automotive inner panels and corners of window panels, hub-holes of wheel disks, hidden joints, etc. Edge cracking compromises part quality and it is a serious production problem, because if it is not accounted for in the overall design, the load paths through the vehicle frame in a crash situation can be misdirected and the resultant intrusion levels can exceed target levels. Such problem was not observed in mild steels, whose high ductility prevents the cut edge from cracking, and less knowledge and expertise are available to immediately solve it in industrial parts.[8,9] AHSS are more sensitive to the crack edge integrity, so their crack edge resistance depends on the hole preparation method (punching, laser cutting waterjet cutting…), as well
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