Static and fatigue behavior of CFRP strengthened spiral welded steel tubes subjected to four-point bending

Carbon fiber reinforced polymer (CFRP) has proven to be effective for repairing and strengthening existing structures. Spiral welded steel tubes (SWTs), extensively utilized for steel tube piles in bridges and offshore wind turbines, frequently experience fatigue loadings. This paper explores the spiral welded steel tube's static bending and fatigue behavior with initial damage before and after CFRP strengthening. An initial crack was introduced at the weld toe in the tensile area. Under static loading, CFRP strengthening increased the tube's stiffness and bending capacity by 175.4% and 27.45%, respectively. Under fatigue loading, crack growth was observed in two directions with different rates: circumferentially along the loading direction and along the weld seam, penetrating through the weld and continuing along the loading direction. This highlights how weld geometry and residual stress significantly influence the fatigue crack growth. CFRP strengthening reduced the crack's stress intensity, considerably decreasing the crack growth rate and extending fatigue life by two to three times. Moreover, the CFRP wrapping orientation had significantly impacted fatigue strengthening, with inverse-diagonal wrapping proving to be most effective. The debonding area of the strengthened specimens is also estimated. These findings provide valuable insights for practical engineering applications.