Ultra-low-cycle fatigue performance and its analysis method for partially concrete-filled steel tube pier based on ICVGM

To establish a calculation method for ultra-low-cycle fatigue (ULCF) cracking of partially concrete-filled steel tube (PCFST) piers and study their ultra-low-cycle fatigue performance, an experimentally validated prediction subroutine was developed for ULCF cracking based on an improved cyclic void growth model (ICVGM). Using this subroutine, high-precision solid element simulations and computational analyses were conducted on nine PCFST piers. The influences of key design parameters, such as concrete filling ratio, slenderness ratio, diameter-to-thickness ratio, and axial compression ratio, on ULCF damage indicators under horizontal reciprocating loads were investigated. Additionally, the relationship between ULCF failure and local buckling failure in PCFST piers was examined in conjunction with the hysteretic behavior of the models. The research results indicate that ULCF failure in PCFST piers generally occurs after local buckling failure. However, different design parameters significantly influence the ductility of PCFST piers, and local buckling deformation leads to more pronounced stress concentrations at deformation locations, thereby accelerating the occurrence of ULCF failure. As the slenderness ratio, diameter-to-thickness ratio, and axial compression ratio increase, the fatigue damage indicators of the pier models at the buckling limit state decrease, and the ULCF problem is not significant. As the concrete filling ratio increases, the fatigue damage indicators of the pier models at the buckling limit state increase, which makes the ULCF problem more pronounced. The diameter-to-thickness ratio and axial compression ratio are two key parameters influencing ULCF cracking, and PCFST piers with good ductility are more likely to experience ULCF failure before local buckling failure occurs. The established finite element model and ICVGM-based subroutine provide a quantitative evaluation method for the ULCF performance of PCFST piers.