The influence of dent imperfections on the elastic stress behavior of cylindrical shells is explored. This problem is of central importance to the prediction of fatigue failure due to dents in petroleum pipelines. Using an approximate technique called the Equivalent Load Method, a semi-analytical model of two-dimensional dent stress behavior is developed. In the three-dimensional situation, decreased dent localization, in particular dent length, and increased dent depth are confirmed to cause dent stress concentration behavior to shift from having a single peak at the dent center to having peaks at the dent periphery. It is demonstrated that the equivalent load method does not predict this shift in stress behavior and cannot be relied upon to analyze relatively small, deep imperfections. The two stress modes of dents are associated with two modes of dent fatigue behavior that have significantly different fatigue lives. A method for distinguishing longer lived Mode P dents from shorter lived Mode C dents based on two measured features of dent geometry is developed and validated. An approach for implementing this analysis in the evaluation of real dents is also suggested.

Afin d’améliorer la qualité des solutions proposées par l’optimisation dans les processus de conception, il est important de se donner des outils permettant à l’optimiseur de parcourir l’espace de conception le plus largement possible. L’objet de cette Thèse est d’analyser différentes méthodes de paramétrage de formes surfaciques d’une automobile en vue de proposer à Renault un processus d’optimisation efficace. Trois méthodes sont analysées dans cette Thèse. Les deux premières sont issues de l’existant, et proposent de mélanger des formes, afin de créer de la diversité. Ainsi, on maximise l’exploration de l’espace de conception, tout en limitant l’effort de paramétrage des CAO. On montre qu’elles ont un fort potentiel, mais impliquent l’utilisation de méthodes d’optimisation difficiles à mettre en œuvre aujourd’hui. La troisième méthode étudiée consiste à exploiter la formulation de Koiter des équations de coques, qui intègre paramètres de forme et mécanique, et de l’utiliser pour faire de l’optimisation de forme sur critères mécaniques. Cette méthode a par ailleurs pour avantage de permettre le calcul des gradients. D’autre part, nous montrons qu’il est possible d’utiliser les points de contrôles de carreaux de Bézier comme paramètres d’optimisation, et ainsi, de limiter au strict nécessaire le nombre de variables du problème d’optimisation, tout en permettant une large exploration de l’espace de conception. Cependant, cette méthode est non-standard dans l’industrie et implique des développements spécifiques, qui ont été réalisés dans le cadre de cette Thèse. Enfin, nous mettons en place dans cette Thèse les éléments d’un processus d’optimisation de forme surfacique. / To improve optimized solutions quality in the design process, it is important to provide the optimizer tools to navigate the design space as much as possible. The purpose of this thesis is to analyze different parametrization methods for automotive surface shapes, in order to offer Renault an efficient optimization process. Three methods are analyzed in this thesis. The first two are closed to the existing ones, and propose to blend shapes to create diversity. Thus, we are able to maximize the exploration of the design space, while minimizing the effort for CAD setting. We show their high potential, but they involve the use of optimization methods difficult to implement today. The third method is designed to exploit the formulation of Koiter shell equations, which integrates mechanical and shape parameters, and to use it to perform shape optimization with respect to different mechanical criteria. This method also has the advantage of allowing the gradients calculation. On the other hand, we show that it is possible to use the Bezier’s control points as optimization parameters, and thus control the minimum number of variables necessary for the optimization problem, while allowing a broad exploration of the design space. However, this method is non-standard in the industry and involves specific developments that have been made in the context of this thesis. Finally, we implement in this thesis essentials elements of an optimization process for surface shapes.