In this thesis extensive finite element simulations of ductile fracture in pipeline steels were analyzed using the cohesive zone model (CZM). The analyses focused on the effect of loading mode under quasi-static loading conditions on the CTOA, and higher loading rates on the CTOA. The results of the loading mode simulations demonstrate that the CTOA has a weak dependence on the loading mode (constraint). The results of the loading rate study demonstrate that the CZM must be re-calibrated to produce an experimental parameter, such as the dynamic CTOA, for higher loading rates. A relation between the cohesive energy and crack velocity is developed to provide insight into the effect of loading rate on the CZM based on a constant CTOA model. Lastly, this study provided data which supports the notion that the upswing in the two curve method (TCM) is due to the effect of inertia.