This report contains the MSc thesis of Matteo Cornalba, an Italian internship candidate visiting Trondheim and the HyLINE project from Politecnico di Milano for six months during the period April – December 2019. The thesis is based on the work performed during this stay under the co supervision of Philippe Mainçon, Antonio Alvaro and Vigdis Olden, all SINTEF. The work is performed within the HyLINE project's WP 4 Model development and validation. From the thesis abstract: "This document presents a preliminary study that, in the absence of hydrogen, tries to evaluate the fatigue crack propagation rate as a function of ΔK in a C(T) specimen made of X70 steel. The phenomenon has been simulated in the Finite Element Software Abaqus, by placing specifically designed Cohesive Zone Elements along the crack ligament of the specimen. The results have been used to construct the Paris Law, and a coefficient m close to 3 is obtained, which is similar to the one achieved in the experimental tests. A sensitivity study of the mesh dimension and the material stiffness is also presented and discussed: its role in setting up the correct level of stress and plastic strain along the tip is described."