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.

当结构钢遭受强烈地震作用时，由周期性大塑性应变引起的超低周疲劳（ULCF）断裂通常是钢结构的控制极限状态。基于作者先前的工作，本文提出了改进的循环空隙生长模型（CVGM）和退化显着塑性应变（DSPS）模型，其中考虑了循环损伤退化参数对应力三轴性的依赖性。为此，对Q345qC钢制成的圆形缺口试样和试样块进行了测试，并对试样的断裂表面进行了扫描电子显微镜研究，分析结果表明Q345qC钢的ULCF断裂表现出“空核，生长和聚结。根据样品的实验结果和互补有限元分析，在不同的应力三轴方向上对CVGM和DSPS模型的循环损伤退化参数进行了校准，随后建立了循环损伤退化参数与应力三轴性之间的经验公式。最后，详细的有限元分析结果表明，与原始模型相比，改进的CVGM和DSPS模型可以更准确地预测ULCF断裂行为。