Lap-shear tests carried out on Externally Bonded (EB) FRP strips or, more recently, prestress forcerelease tests have established in the scientific community as experimental methods intended at determining the actual bearing capacity of EB FRP strips glued to concrete. The results of these tests are employed to scrutinize the interface behavior between FRP and supporting materials, which is generally modelled as a pure mode II fracture process. This paper shows the results of a numerical study intended at further unveiling the role of mixed-mode fracture parameters in the debonding process between FRP strips and concrete in lap-shear and prestress force releasing tests.