Failure modes and rules related to the yield-to-tensile strength ratio in steel structures

Upper limits on the ratio of the yield strength to the tensile strength (σ_y/σ_u ratio) and lower limits on the fracture elongation ε_f 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 σ_y/σ_u ratio exceeds a certain limit. Such requirements deter the use of high strength steels (nominal σ_y higher than 690 MPa), which inherently have a high σ_y/σ_u ratio. To guide subsequent efforts towards optimised and scientifically grounded σ_y/σ_u limits and wider application of high strength steels, this paper first presents an overview of the current provisions in engineering practice relating to the σ_y/σ_u 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.