Fracture, deformation route, and mechanical performance of welded cold-formed ultra-high strength steel S1100 S. Afkhami1,∗, T. Skriko2, K. Lipiäinen1, T. Björk1 1 Laboratory of Steel Structures, LUT University, Lappeenranta 53850, Finland 2 Laboratory of Welding Technology, LUT University, Lappeenranta 53850, Finland ∗ shahriar.afkhami@lut.fi Keywords: fracture, weld, ultra-high strength steel Cold forming has a crucial role in fabricating hollow sections commonly used in steel structures. Further, assembling steel structures requires arc welding as a joining process to leverage the full potential of steel to achieve economic structures with relatively acceptable strength-to-weight ratios [1]. However, the mechanical performance and fracture behavior of ultra-high strength steels, especially when they are simultaneously subjected to cold-forming and welding, still require further research. Hence, further study is crucially required to fill these knowledge gaps to avoid catastrophic failures in steel structures [2]. Consequently, quench and tempered ultrahigh strength steel S1100 was selected for this study to investigate the influence of cold-forming and pre-strains on the mechanical performance and fracture behavior of the new generation of ultra-high strength steels after arc welding. To do so, base metals with different bending radii, representing various levels of cold-forming, were welded via gas-metal arc welding. Next, the welded joints were subjected to uniaxial quasi-static tensile loads to evaluate their mechanical performance after welding and calculate their strength, ductility, and toughness parameters. In addition, digital image correlation was also used with the tensile tests to study the plasticity and final failure. Further, macrographs by optical microscopy and fractography via scanning electron microscopy were utilized to investigate the fracture locations and mechanisms of the welded joints. The results confirmed the reliability of the welded joints in steel structures. In addition, the fracture of the welded material was identified as ductile, regardless of its cold-forming degree. Heat-affected zone softening, as a common drawback associated with welded ultra-high strength steel, did not adversely affect the fracture mechanism of welded S1100. References [1] Aksel, H., Eren, Ö. (2016). A discussion on the advantages of steel structures in the context of sustainable construction. Int. J. Contemp. Archit., 2(3), 46–53. [2] Afkhami, S., Björk, T. Larkiola, J. (2019). Weldability of cold-formed high strength and ultra-high strength steels. J. Constr. Steel Res., 158, 86–98. 33
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