Extremely Low-cycle Fatigue Testing of Structural Carbon Steel for a Novel Self- Centring System

In this study, laboratory experiments have been conducted in order to investigate the material behaviour of coldform square hollow section bracing members under monotonic tensile loading and low-cycle fatigue loading. Two different materials, namely S275 and S235, were tested at room temperature. The tested coupons were cut from the bracing members of a novel concentrically braced self-centring frame. The low- and extremely low- cycle fatigue tests were performed at various strain amplitudes. Cyclic hardening and cyclic softening to different strain amplitudes are studied to find material properties. The buckling and Bauschinger effects in the specimens seemed to have an impact on the results. From the tensile test results, the mechanical properties of the specimens such as ultimate tensile strength, yield strength, percentage of elongation and area reduction, fracture strain and young's modulus were derived. In addition, the parameters for a mixed strain hardening model of isotropic and kinematic are determined from each hysteresis loop with different strain amplitudes. The strength and ductility hardening parameters were calibrated for the Coffin-Mason relationship. The comparison between the experimental results and calibrated material model showed a good agreement. The obtained test results and material model provides important data for the performance analyses of the self-centring structures.