Fatigue tests of multiplanar concrete-filled steel tubular KK-joints with inner ring stiffeners

Wind turbine structures are subjected to prolonged cyclic loading, with fatigue issues being particularly prominent. This study aims to investigate the fatigue performance of multiplanar concrete-filled steel tubular (CFST) KK-joints with inner ring stiffeners. Fatigue tests were conducted on these joints, and comparison specimens were designed, including one with a circular hollow section (CHS) tubular joint and another with a CFST joint. The results indicated that the incorporation of inner concrete and ring stiffeners led to a significant reduction in the hot spot stress within tubular joints. The maximum hot spot stress concentration factor (hot-spot SCF) of CFST KK-joints with inner ring stiffeners decreased by 39.7% compared to CHS KK-joints and by 8.7% compared to CFST KK-joints. Furthermore, an increase in the brace thickness-to-chord thickness ratio tau from 0.94 to 1.24 for CFST KK-joints with inner ring stiffeners resulted in a corresponding 74.3% increase in hot-spot SCF. Additionally, fatigue tests were conducted to ascertain the failure modes, crack propagation patterns, and fatigue lives of specimens. These measured fatigue lives were compared with the S-N curves given by different codes. Notably, it was observed that the curve proposed by DNV/CCS exhibited the most suitable correlation with the experimental results.