Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce a crack closure effect. Moreover, it modififies the crack geometry by bridging the crack faces and so reduces the stress intensity range at the crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A crack growth induced debonded region in the adhesive-plate interface was observed using an optical technique. Moreover, the size of the debonded region signifificantly inflfluences the effificiency of the crack repair. Debond crack total strain energy release rate is computed by the modiffied virtual crack closure technique (MVCCT). A parametric analysis is performed to investigate the inflfluence of some design parameters such as the composite patch Young's modulus, the adhesive thickness and the pretension level on the adhesive-plate interface debond.