Extremely low cycle fatigue tests on structural carbon steel and stainless steel

Cyclic material tests in the low and extremely low cycle fatigue regime were carried out to study the properties of structural carbon steel and stainless steel. A total of 62 experiments were performed in cyclic axial and bending configurations, with strain amplitudes up to ±15%. Materials from hot-rolled carbon steel (S355J2H), cold-formed carbon steel (S235JRH) and cold-formed austenitic stainless steel (EN 1.4301 and EN 1.4307) structural sections were tested and the results were compared. The strain–life data from the axial tests were used to derive suitable Coffin–Manson parameters for the three materials; two further extremely low cycle fatigue life prediction models were also considered. The results revealed that the three materials exhibit similar strain–life relationships despite significantly different elongations at fracture measured in monotonic tensile tests. The hysteretic responses of the materials at different strain amplitudes were used to calibrate a combined isotropic/kinematic cyclic material hardening model which can be incorporated into numerical models of structural members. The stainless steel specimens displayed significantly greater levels of cyclic hardening than the corresponding carbon steel samples. A relationship between the results obtained from axial and bending test arrangements was established through consideration of energy dissipation, enabling strain–life models to be derived from either means of testing.