Increasing Remaining Fatigue Life of Steel Plates Using Bonded Ultrahigh-Modulus CFRP Plates: The Effect of Cyclic History and Bond Length

Fatigue cracking in steel structures poses a significant threat to their long-term performance and reduces their fatigue life. This study examines the effect of a repair intervention using ultrahigh-modulus carbon fiber–reinforced polymer (CFRP) bonded plates applied at various stages of fatigue crack propagation and its impact on extending the remaining fatigue life. The study also investigates the effect of CFRP bond length on crack growth rates and strain distributions. Bond lengths ranged from 0.5 to 2.4 times the effective development length (75–350 mm). Machined notches simulated initial defects, and cyclic loading was applied before and after retrofitting to control damage levels. Crack growth rates were quantified using beach marking, while strain concentrations and profiles were measured using distributed fiber optic sensing. The results show that CFRP repair is effective in increasing the fatigue life of a structure. For a CFRP plate with a 225 mm bond length, the remaining fatigue life extension ratio increased from 3.2 when intervention occurred at the onset of crack propagation (i.e., 25% area loss in the notched plate) to 7.2 when intervention occurred late in life, at 95% of the total cycles to failure (59% area loss) of the control samples. However, if assessment is based on increase in total cycles (i.e., pre- plus postrepair), results show a reduction in effectiveness at late intervention.