A novel constitutive model under various cyclic loading protocols with large strain ranges considering strain memory effect and loading history dependence

This study conducted a series of ultra-low cyclic fatigue (ULCF) tests and investigated the cyclic responses of structural steel Q345 under various cyclic loading cases at a relatively large inelastic strain range whose maximum amplitude reached ±10 %. Test results reveal that Q345 steel exhibits pronounced strain memory effects (SME), transient Bauschinger effect (TBE), and loading history dependence. The evolution methods of typical internal variables and stresses are examined. It is noteworthy that the SME and loading history dependence are observed in both effective and back stresses. Based on the analysis, a novel constitutive model was proposed. A scaling factor $\Phi$ was introduced into the kinematic hardening component to consider the SME and loading history dependence. Besides, an improved memory surface with shrinkage features was employed to track the applied strain ranges under various loading protocols. Furthermore, the virtual boundary surface was employed to account for the TBE. Finally, The proposed model was validated by comparing the simulated results with test data. The validation was discussed from the hysteresis loop and peak stress evolution. The comparison results showed that the developed model could capture the cyclic plasticity characteristics of Q345 steel under various loading protocols during the entire loading procedure until fracture.