Improved Ultra-Low Cycle Fatigue Fracture Models for Structural Steels Considering the Dependence of Cyclic Damage Degradation Parameters on Stress Triaxiality

The ultra-low cycle fatigue (ULCF) fracture initiation caused by cyclic large plastic strain in structural steels is often the governing limit state in steel structures when subjected to strong earthquake actions. Based on the author’s previous work, this paper presents the improved cyclic void growth model (CVGM) and degraded significant plastic strain (DSPS) model considering the dependence of cyclic damage degradation parameters on stress triaxiality. To this end, tests on circular notched specimens and coupon specimens made of Q345qC steel were conducted, and scanning electron microscopy studies were performed on fracture surfaces of specimens, analysis results of which show that the ULCF fracture of Q345qC steel exhibits the typical behaviour of “void nucleation, growth, and coalescence.” The cyclic damage degradation parameters of CVGM and DSPS model were calibrated at different stress triaxialities based on experimental results of specimens and complementary finite element analysis, and empirical formulas were subsequently established between cyclic damage degradation parameters and stress triaxiality. Finally, detailed finite element analysis results demonstrate that the improved CVGM and DSPS model can predict ULCF fracture behaviour with higher accuracy in comparison with original models.