Crack growth is an important failure mechanism in many engineering materials. Numerical models for crack growth have been developed within the framework of damage mechanics. All these models aim for the same goal, obtaining accurate results for crack growth under various loading conditions. Many damage models are based upon the cohesive crack approach. However, level set based models provide advantages compared to the cohesive crack models with regard to fatigue analysis. An alternative method to the existing thick level set (TLS) method was proposed, the interfacial thick level set (ITLS) model. The use of interface elements made the model more suitable to simulate several failure processes. In this thesis it is demonstrated that this method suffers from a dependency on the initial interfacial stiffness for the global response of a system by conducting a parameter study under quasi-static loading conditions. This parameter study proves that for a varying value of the initial interfacial stiffness parameter K the global response varies as well. This gives motivation to conduct further research on the removal of the initial interfacial stiffness from the current ITLS model. Two methods are developed to overcome the initial interfacial stiffness dependency. The first method assumes that the initial interfacial stiffness dependency is caused by the current formulation of the constitutive law of the interface. A new expression for the interfacial stiffness is adapted after which all constitutive relations are updated. The new method shows a perfect agreement with the current ITLS model, which validates the method as an accurate alternative. Compared to the current ITLS model, method 1 allows for the control of the initial stiffness of the undamaged part of the interface by changing the lower bound damage without affecting the global response. However, when executing simulations for a varying initial interfacial stiffness K the same problem as for the current ITLS is observed. It can be concluded that in a way this method removes the dependency on the initial interfacial stiffness but the initial interfacial stiffness parameter K should then remain constant. Furthermore, the calibration process is not simplified compared to the current ITLS model. For this reason method 1 is rejected as the final solution to the problem and a second method is proposed. The second method assumes a direct relation between the damage parameter c1 and the initial interfacial stiffness parameter K. This damage parameter is responsible for the steepness of the damage profile. The results from the parameter study showed that an increase of one of these parameters results in opposite behaviour for the initial stiffness of the global response. The leading hypothesis becomes that an increase in the stiffness parameter K can be neutralized by an increase in the damage parameter c1. Proportionality between both parameters is assumed. By trial and error the proportionality is found to be one to one. When using this proportionality condition, simulations with different values for the initial interfacial stiffness showed a perfect agreement for the load-displacement and the crack growth responses. Moreover, the proportionality condition is accurate for both a linear elastic (LE) material and an elastic-plastic (EP) material. The proportionality condition is proven by elaborating the interfacial stiffness over the damaged zone. This elaboration is done both numerically and analytically and proves that there is a one to one proportionality between c1 and K, which validates the replacement of c1 by a constant c multiplied with K. Lastly, the method should be compared to a response obtained through a different type of analysis. The method is compared with the solution of an analytical analysis resulting in a very good agreement for both a LE and an EP material. Therefore, method 2 can be approved as a solution to the initial interfacial stiffness dependency problem.

Offshore wind turbines are designed to persist harsh environmental conditions, including corrosion. Pitting corrosion is one of the most dangerous forms of corrosion. In combination with a fatigue loading, a corrosion pit could result into the failure of a structure, due to a minimal weight reduction. Of the corrosion fatigue process, the transition of a corrosion pit to a fatigue crack is one of the less understood topics. In this thesis, a combined analytical/numerical model is presented that implements mechanisms of both the fatigue, and the corrosion process to determine the moment in time and location in the corrosion pit of the pit-to-crack transition. The hypothesis used to build the model, is that the pit-to-crack transition occurs at the moment that a fatigue crack initiates. Assumptions are made to be able to predict the level of stress and strain at the surface and below the surface of a corrosion pit using a FEM model. Using the stress and strain levels, damage is accumulated until the failure criterion is reached, i.e. when a fatigue crack will initiate. In this research study, elliptical pit shapes are considered, with and without a certain roughness at the surface of the corrosion pit. From the results it is found that a more sharp shaped corrosion pit is a more favourable location for fatigue crack initiation, than a more blunt shaped corrosion pit. The number of cycles to fatigue crack initiation resulting from the developed model are higher, than the number of cycles to failure found by the DNV GL B1 SN-curve for free corrosion conditions. Also, the DNV GL SN-curve is more steep, than a fitted curve through the data points resulting from the developed model. Through a sensitivity study and by considering the number of assumptions and simplifications made in this research study, it is concluded that the spread and uncertainty of the outcome of the developed model is too large to be able to draw conclusions based on the comparison between the results and the DNV GL SN-curves. More research on the input parameters of the developed model will give more certainty and a better validity to the model. It is found that the effect of surface roughness to the fatigue crack initiation life is not substantial, because it affects the stress and strain levels only up to a relatively small depth. The location of fatigue crack initiation is in the pit bottom for blunt shaped corrosion pits, and in the pit wall for sharp shaped corrosion pits. For circular shaped corrosion pits, the difference between the fatigue crack initiation life at the pit wall and the pit bottom is negligible. It is concluded that the model gives a comprehensive basis for the prediction of the pit-to-crack transition, but more (experimental) research on the input parameters will be necessary to be able to give a substantiated estimate of structural life of offshore wind turbine foundations.