This paper describes an experimental investigation of the deformation characteristics of SN490B, a construction steel with good weldability and an excellent yield ratio, under multiaxial non-proportional cyclic loading. Steel beams in buildings are plastically deformed under severe loading during heavy earthquakes, resulting in multiaxial and non–proportional loading owing to the complexity of the external loading and the discontinuities in the geometry of the structure. The stress response of the SN490B hollow cylinder specimens was investigated by conducting circular– and diamond–shaped strain waveform control tests using an axial–torsional fatigue testing machine. Using Mises–type equivalent stresses and strains, it was possible to obtain stress–strain relationships in a unified manner, even for multiaxial non–proportional loading. For equivalent strain amplitudes of 0.3% and 0.5%, most of the total strain range for both the circular and diamond shapes was plastic strain. The occurrence of plastic strain was found to be larger than that in the uniaxial tension–compression test. For both the circular and diamond shapes, the stress amplitude in the strain–controlled tests increased significantly from the initial cycles to approximately 10 cycles, indicating cyclic hardening. The subsequent increase in the stress amplitude slowed, but hardening continued until the specimen failed. An expression for the change in the stress amplitude under multiaxial non–proportional loading of SN490B was proposed.
