Failure Modes and Rules Related to the Yield-to-Tensile Strength Ratio in Steel Structures

Abstract

Abstract Upper limits on the ratio of the yield strength to the tensile strength (σy/σu ratio) and lower limits on the fracture elongation εf are present in various offshore, maritime and civil engineering rules, standards and specifications for steel structures as a provision for the minimum material ductility and toughness which ensures sufficient structural ductility. In other instances, the design yield stress to be adopted in strength calculations is reduced from its nominal value if the σy/σu ratio exceeds a certain limit. Such requirements deter the use of high strength steels (nominal σy higher than 690 MPa), which inherently have a high σy/σu ratio. To guide subsequent efforts towards optimised and scientifically grounded σy/σu limits and wider application of high strength steels, this paper first presents an overview of the current provisions in engineering practice relating to the σy/σu ratio and structural ductility, and it then discusses the key underlying failure mechanisms to which these ductility requirements are relevant: tensile strain localization, yielding and localization precipitated by stress concentrations, localization of plastic bending hinges and ductile fracture. The reasoning behind the current provisions, the findings of previous research concerning the requirements, and the key potential areas for future research are highlighted.