Uniaxial cyclic elasto-plastic deformation and fatigue failure of API-5L X65 steel under various loading conditions

The cyclic elasto-plastic deformation and fatigue failure properties of API-5L X65 steel was characterised under monotonic, high cycle fatigue, strain controlled low cycle fatigue, and stress controlled ratchetting loading regimes. The steel combines high strength, which increases with strain rate under monotonic loading, with high fatigue limit/yield strength ratio. It is found that the steel exhibits non-Masing and cyclic softening behaviours. Low cycle fatigue lifetime decreased with increase in total strain amplitude and plastic strain amplitude. It is shown that, under uniaxial fully reversed loading, the Chaboche model with three backstresses is capable of describing the cyclic hysteresis stress-strain curves of the steel. Ratchetting lifetime of the steel decreased with increase in ratchetting strain and increase in both mean stress and stress amplitude; the stress amplitude however dominates the damage process compared to mean stress.