Fundamental behaviour of high strength and ultra-high strength steel subjected to low cycle structural damage

Along with the rising application of high strength steel in civil engineering practice, it has become imperative to gain comprehensive understanding of the elastic and plastic behaviour of these materials under given strain or stress histories. For seismic analysis of structures in earthquake prone areas, this paper aims to analytically and theoretically study the cyclic behaviour of high strength steel tubes, with individual applications or incorporated in fabricated structural elements. Several low cycle tension-compression tests are conducted on high strength (grade 800) and ultra-high strength (grade 1200) steel coupons extracted from tubes. Parameters such as number of cycles, strain/stress amplitude and increment size are studied in the behaviour of cyclically strained material and its preserved mechanical properties. Numerical analysis is also conducted incorporating combined nonlinear hardening models. As opposed to conventional structural mild steel both grades of steels considered in this study exhibit cyclic softening with plastic straining having a more prominent strength reduction in higher strengths of steel. Cyclical damage applied on high tensile steel evidently influences the preserved mechanical properties of microstructure at fracture. Combined nonlinear plastic hardening and relevant parameters proposed in this study for two grades of high strength steel materials are calibrated and verified against hysteretic experimental results and proposed for further analytical and numerical modelling.