The main objective of this study is to examine the three-dimensional surface crack problems in orthotropic materials subjected to mechanical or thermal loading. The cracks are modeled and embedded in the orthotropic material by considering semielliptical crack front geometry. In the model special elements are embedded in the crack front region, in this way it is possible to include crack tip singular fields along the crack front. Three-dimensional finite element analyses are conducted to obtain mode I stress intensity factors. The stress intensity factor is calculated by using the displacement correlation technique. In the analysis, collapsed 20-node iso-parametric elements are utilized to simulate strain singularity around the semi-elliptical crack front. The surface crack problem is analyzed under both mechanical and thermal stresses. In the case of mechanical loading, uniform tension and fixed grip tension loading cases are applied on the model. In thermal analysis, thermal boundary conditions are defined. Comparisons of the results generated to those available in the literature verify the developed techniques.

The objective of this study is to develop crack growth analysis methods for functionally graded materials under mode I cyclic loading by using finite element technique. The study starts with the analysis of test specimens which are given in ASTM standard E399. The material properties of specimens are assumed to be changing along the thickness direction according to a presumed variation function used for the modeling of functionally graded materials. The results of the study reveal the influence of different material variation functions on the crack growth behavior. In the second part, the growth of an elliptical crack which is a common case in engineering applications is analyzed. First, mode I cycling loading is applied perpendicular to the crack plane and crack growth profiles for a certain number of cycles are obtained for homogeneous materials. Then, the code is extended for the analysis functionally graded materials. The material properties are assumed to vary as an exponential function along the major or minor axis direction of the crack. The results can be used to examine the crack profile and material constants&rsquo / influence for a certain number of cyclic loading.

Wind power, especially offshore, is considered to be one of the most promising sources of ‘clean’ energy towards meeting the EU targets for 2020 and 2050. However, its popularity has always fluctuated with the price of fossil fuels since nowadays wind electricity production cannot compete with nuclear or coal electricity production. Support structures are thought to be one of the main drivers for reducing costs in order to make the wind industry more economically efficient. Foundations and towers should be fit for purpose, extending their effective service life but avoiding costs of oversizing. An exhaustive review of the background and state of the art of the Fatigue-Life assessment approaches has been carried out, combining analysis of the gathered experimental data and the development of Finite Element models based on contemporary 3D solid models with diverse Regression Analyses, in order to identify their weakness and evaluate their accuracy. This research shows that the guides and practices currently employed in the design and during the operation of the offshore wind turbine support structures are obsolete and not useful for optimisation, which generally leads to conservationism and an unnecessary increase in costs. The basis for a comprehensive update of the Girth Weld and Tubular Joint S-N curves and the Stress Concentration Factors of Tubular Joints has been set out. Furthermore, a reliable methodology for deriving the Stress Intensity Factor at the deepest point of a semi-elliptical surface saddle crack in a tubular welded T-joint has been proposed.

Une méthode améliorant le calcul des facteurs d’intensité de contrainte en mode I par hybridation de deux fonctions de poids est présentée et appliquée aux cas de fissures elliptiques sous différents chargements. L'hybridation consiste à utiliser l'une ou l'autre des deux fonctions dans la zone de la fissure où la fonction est la plus efficace. La délimitation des zones est faite après optimisation des paramètres géométriques de la fissure. Afin d’étendre l’utilisation de cette approche à la modélisation des fissures semi-elliptiques, son couplage avec la PWFM (Point Weight Function Method) pour tenir compte de l’effet de la surface libre a été réalisé. L’utilisation des lois de propagation de fissure (de Paris et de Sih) a permis l’extension de l’application de l’approche d’hybridation aux problèmes de fatigue. La qualité des résultats trouvés pour les fissures elliptiques et semi-elliptiques est bonne aussi bien pour les chargements quasi-statiques que pour ceux de fatigue. / A method improving the evaluation of the stress intensity factor by hybridization of two weight functions is presented and applied for embedded elliptical cracks under various loadings. The hybridization consists in using one or another function in the zone of the crack where it is the most efficient. The delimitation of the zones is achieved after optimizing the geometrical parameters of the crack. In order to extend the use of this approach to the modeling of semi-elliptical surface cracks, its coupling with the PWFM (Point Weight Function Method) to take account of the free edge effect were carried out. The use of the fatigue crack growth laws (Paris law and Sih law) allowed the extension of the application of the hybrid approach to the fatigue problems. The quality of the results found for the elliptical and the semi-elliptical cracks is good for the static loads as well as for the fatigue ones.