Stacking sequence design is a combinatorial problem when the fiber orientations in each layer are restricted to certain angles. In addition, there often exist many optimal or nearoptimal designs for the stacking sequence of a composite pressure vessel under different loading conditions. Genetic algorithms are quite well suited for finding the optimal designs for such a combinatorial problem. In this thesis, a genetic algorithm code is developed in Matlab, optimizing the stacking sequence of a composite pressure vessel subjected to internal and external pressures, axial load and body force due to rotation. For testing of the code and identification of the effects of optimization parameters, a problem, whose optimum solution is obvious, is defined and the optimum design is tried to be found by using the developed code. The results have shown that the code was quite successful in finding the best design. Afterwards, the code is used for the optimization of the stacking sequence of a composite pressure vessel under different loading conditions. Again the code has proven its reliability in finding the optimal designs. The developed genetic algorithm optimization code also has the infrastructure to be easily adapted to the solution of different combinatorial problems.

A l'heure actuelle le stockage de l'hydrogène sous forme gazeuse, comprimée à haute pression, apparaît comme la solution le plus mature présentant le meilleur compromis en termes de masse, de pression de service mais aussi de volume des réservoirs. Cependant pour un développement plus large et sécurisé, l'amélioration des performances et la réduction des coûts des réservoirs restent des enjeux prioritaires. C'est dans ce contexte que nous avons étudié le stockage de l'hydrogène dans des réservoirs de type IV, en composites fibres de carbone/époxy. Ce travail a eu pour objectif d'accroitre la fiabilité du dimensionnement. Dans un premier temps, une étude expérimentale de caractérisation des matériaux constitutifs du réservoir a été réalisée. Pour améliorer la fiabilité des calculs, un modèle probabiliste a été proposé pour décrire le comportement de la partie composite du réservoir, principalement la rupture des fibres. Des calculs multiéchelles ont été mis en place basés sur les propriétés mécaniques et physiques des fibres. Les autres modes de dégradations, décollement entre plis, liaison embase-liner ont aussi été pris en compte dans les calculs de comportement du réservoir jusqu'à son éclatement. Enfin des recommandations de dimensionnement du réservoir ont été proposées afin d'améliorer les performances tout en minimisant la masse de composite dans un objectif de réduction des coûts. / Presently, the compressed hydrogen storage under high pressure appears to be the most sophisticated solution regarding to a compromise of mass, service pressure and also volume of pressure vessels. However, the challenges of pressure vessels nowadays are their performance improvement as well as their cost reduction. In this context, we studied the type IV hydrogen storage pressure vessel in carbon fibre/epoxy composites. This work aims to obtain a reliable pressure vessel design. Firstly, an experimental study of associated materials and pressure vessel characterisation has been carried out. Then, we proposed a probabilistic model for a composite which is dedicated in particular to fibre breakage using multi-scale simulations in accordance with its mechanical and physical properties. Once this model joined with damage criteria dedicated separately to the others damage mechanisms are integrated into the pressure vessel simulations. Finally, recommendations on composite pressure vessels have been proposed in order to improve their performances and to decrease the mass of composite directly corresponding to the reduction of composite pressure vessels cost.

M. Tech. (Mechanical Engineering, Faculty of Engineering and Technology), Vaal University of Technology. / The purpose of this study was to design a composite overwrapped pressure vessel by combining the best optimal structural options. This study investigated the effects of constituents such as fibre and shell thickness, on the bursting strength. Thereafter, these constituents were combined in order to achieve optimization of strength for an improved sustainable composite pressure vessel. The analytical method was carried out using the Tsai-wu failure theorem. The developed analytical equations were solved with Matlab 2016 software to determine composite fibre and shell thickness. With variation of the vessel’s liner, a total of 56 parts were created on two different profiles with purpose of generating of vessels resistant to bursting failure. Henceforth, the structural integrity of fibre imparted into the design was optimally analyzed at an angle of 55⁰, through the negative and positive directions. The shell thickness overwrapping the liner, being as well an influential factor to this optimization, was, therefore, analyzed on symmetrical and asymmetrical lamination patterns. The optimal fibre and shell thickness range were thereafter determined on a first ply failure and hoop stress threshold approach. Additionally, the identified optimal range of pressure vessel constituents were numerically validated, on Abaqus/CAE software, to have a degree of reassurance on the result generated, using Hashin failure criteria. Optimal design with improved strength and weight factor was therefore achieved by combining the generated optimal vessel constituents yielded from Minitab software version 2016. The generated results of the study revealed no change on the fibre thickness determined with respect to direction. For shell thickness on the other hand, asymmetrical pattern was identified as the desired sequence of lamination. In addition, with two profiles considered in the research, the composite constituents were found for a p value of 0.066, to be optimal on profile 1 at 0.0048 mm of liner, 0.0005 mm of fibre and 0.0027 mm of shell. The profile 2 on the other hand, revealed optimization of liner at 0.0095 mm, fibre at 0.0021 mm and shell at 0.0055 mm. Through combination of these ultimate constituents the response optimizer on Minitab software generated optimal bursting strength with factor of 4% improvement with a weight reduction of 33% compared to the stainless steel vessel. It was, therefore, concluded that profile 1 was the most optimal with hoop strength of 123.43 MPa, Von Mises of 178.56 MPa and Tresca of 179.48 MPa.

It was found that a significant improvement of mechanical properties of CFRPs can be achieved by the adjustment of the matrix properties such as toughness and CF/matrix adhesion via the chemical modification, as well as the physical modification by a small amount of cheap and environment-friendly nano fibers. Based on investigation of fracture mechanisms at macro/micro scale, the effects of matrix properties and nano fiber on the mechanical properties of CFRP have been discussed. Subsequently, the relationship has been characterized by a numerical model to show how to modulate the parameters of the matrix properties to achieve excellent fatigue properties of CFRP. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University