This paper is a master thesis written at NTNU for the Co-Patch project. The Co-Patch project is an EU-funded consortium of 15 organizations from 8 different countries, developing a standard for composite patch repair of steel structures in fire hazard areas. The standard includes methods for dimensioning against delamination. This thesis looks at the fatigue properties in delamination of patches from their substrates. By testing Double Cantilever Beam and End-Notched Flexure specimens in fatigue loading, G-N curves were made for Mode I and Mode II fatigue fracture. This gives an indication of the lifetime of a patch given a specific cyclic load condition and a pre-existing crack. The longest cycle life recorded was 4.35E5 cycles at 104 J/m2 for Mode I and 1.23E5 cycles at 122 J/m2 for Mode II.Furthermore, a relation for crack propagation rate and energy release rate in Mode I was found for low cycle life estimation. From this relation, the crack front position may be estimated for a given set of cycles through calculations using Paris’ law. Based on the quasi-static data of a previous report, and the fatigue data found in this thesis, a numerical simulation procedure for crack propagation was proposed. Using direct cyclic fatigue analysis and Virtual Crack Closure Technique based on linear elastic fracture modelling, the crack propagation may be estimated for patches exposed to combined conditions, such as plane strain/plane stress and different fracture modes, as well as on advanced geometries. The procedure was validated as a viable simulation, but more work needs to be done on the model for accurate estimates.