AbstractVarious carbon fiber–reinforced polymer (CFRP) strengthening systems have been developed to increase the fatigue lifetime of existing aging steel structures. According to the literature, the results of fatigue tests on CFRP-strengthened steel plates showed a significant increase in the fatigue lifetime of the specimens, compared with the bare ones. In particular, the fatigue lifetime extension was more pronounced for short initial crack sizes (i.e., low initial damage level). In this study, the fatigue-crack growth curves in bonded CFRP-strengthened single edge notched tension (SENT) specimens were numerically investigated. The proposed numerical approach adopted a cyclic cohesive-zone model (CCZM), which enabled the simulation of crack growth in steel plates, through the definition of a scalar damage variable (k). The selected model contained some parameters that did not possess a precise physical meaning and therefore were not amenable to direct measurement. Therefore, a robust identification procedure was proposed to calibrate the model parameters that governed fatigue behavior, which was based on the response of unstrengthened specimens in the crack propagation curves. The successful identification was then validated by comparison with the measured response of the strengthened SENT specimens, which were modeled by adopting the same identified model parameters. The adhesive between the CFRP laminate and steel substrate was simulated by defining an elasto–brittle surface-to-surface contact model, whose properties were selected based on well known approaches that were proposed in the literature.