The Dassault Aviation Group designs and builds a wide range of aircraft models each year, for both private and military use. Some structural parts of the aircraft, in the form of reinforced stiffener panels machined directly into the bulk of Al2024T351 thick plates are studied. As part of the shaping process, these elements must be cold formed to give their final curvature. This process severely plastified some regions of the structure. In addition, these zones are likely to constitute sites of initiation and propagation of fatigue cracks.This study aims to analyze the interactions between the plastic phenomena induced by cold forming and the propagation of a fatigue crack during fatigue service loading. This thesis is conducted according to the following approach:An instrumented cold forming test has been performed on two self-stiffening technological specimens. From an experimental base of tensile tests, a model of anisotropic plastic behavior of the plate was proposed allowing the development of a cold forming structure simulation and an analysis of the residual state of the material following the process. Using all the results about residual plastic state, a compression and traction test on large test pieces has been performed (54 mm diameter slugs and flat tensile test pieces of 75x55 mm section) whose purpose is to produce a homogeneous volume of pre-deformed material in compression and traction (+/-10 % of plasticity) .A large campaign of fatigue tests has been realized on oligocyclic specimens and specimens of propagation (Compact Tension) extracted from the volumes of original and pre-deformed material allowing the characterization of the effects of the pre-deformation on the fatigue properties. A modified Forman propagation model has been identified from the velocity curves.Finally, self-stiffened structures formed have been subjected to a cyclic loading and the fatigue initiation and propagation behavior has been observed experimentally. A simulation of the fatigue loading of the structure after ...
The Dassault Aviation Group designs and builds a wide range of aircraft models each year, for both private and military use. Some structural parts of the aircraft, in the form of reinforced stiffener panels machined directly into the bulk of Al2024T351 thick plates are studied. As part of the shaping process, these elements must be cold formed to give their final curvature. This process severely plastified some regions of the structure. In addition, these zones are likely to constitute sites of initiation and propagation of fatigue cracks.This study aims to analyze the interactions between the plastic phenomena induced by cold forming and the propagation of a fatigue crack during fatigue service loading. This thesis is conducted according to the following approach:An instrumented cold forming test has been performed on two self-stiffening technological specimens. From an experimental base of tensile tests, a model of anisotropic plastic behavior of the plate was proposed allowing the development of a cold forming structure simulation and an analysis of the residual state of the material following the process. Using all the results about residual plastic state, a compression and traction test on large test pieces has been performed (54 mm diameter slugs and flat tensile test pieces of 75x55 mm section) whose purpose is to produce a homogeneous volume of pre-deformed material in compression and traction (+/-10 % of plasticity) .A large campaign of fatigue tests has been realized on oligocyclic specimens and specimens of propagation (Compact Tension) extracted from the volumes of original and pre-deformed material allowing the characterization of the effects of the pre-deformation on the fatigue properties. A modified Forman propagation model has been identified from the velocity curves.Finally, self-stiffened structures formed have been subjected to a cyclic loading and the fatigue initiation and propagation behavior has been observed experimentally. A simulation of the fatigue loading of the structure after forming has been proposed. The Z-crack module of the Z-set finite element code associated with the propagation model allowed to simulate the propagation of a fatigue crack within the structure. ; Le groupe Dassault Aviation conçoit et construit chaque année un large choix de modèles d'avions, à la fois à usage privé comme à usage militaire. Nous nous intéressons ici à certaines pièces structurelles de l'avion, se présentant sous la forme de panneaux renforcés de raidisseurs usinés directement dans la masse de tôles épaisses en alliage Al2024T351. Dans le cadre du processus de mise en forme, ces éléments doivent être formées à froid afin de leur donner une courbure définitive. Ce procédé a pour conséquence de pré-déformer sévèrement certaines régions de la structure. De plus, ces zones sont susceptibles de constituer des sites d'amorçage et de propagation de fissures de fatigue.Cette étude vise à analyser les interactions entre les phénomènes plastiques induits par le formage à froid et la propagation d'une fissure de fatigue lors du chargement en fatigue de la structure. Cette thèse se déroule selon la démarche suivante:Un essai de formage à froid instrumenté a été réalisé sur deux éprouvettes technologiques auto-raidies. A partir, d'une base expérimentale d'essais de traction un modèle de comportement plastique anisotrope de la tôle a été proposé permettant l'élaboration d'une simulation du formage des deux structures ainsi que l'analyse fine de l'état résiduel du matériau suite au procédé. Après connaissance de l'état plastique résiduel, un protocole d'essais de compression et de traction sur éprouvettes grandes dimensions a été mené (des lopins de diamètre 54 mm et des éprouvettes de traction plate de section 75x55 mm) dont le but est de produire un volume homogène de matériau pré-déformé en compression et en traction (jusqu'à +/-10 %).A partir des volumes de matériaux d'origine et pré-déformés, une large campagne d'essais de fatigue a été réalisée sur éprouvettes oligocycliques et éprouvettes de propagation (Compact Tension) permettant en autres la caractérisation des effets de la pré-déformation sur les propriétés en fatigue. Un modèle de propagation de Forman modifié a pu être identifié à partir des courbes de vitesse.Enfin, les structures auto-raidies formées ont été soumises à un chargement cyclique et le comportement à l'amorçage et en propagation a pu être observé expérimentalement. Une simulation du chargement en fatigue de la structure après formage a été proposée. Le module Z-crack du code éléments finis Z-set associé au modèle de propagation à permis de simuler la propagation d'une fissure de fatigue au sein de la structure.Une conclusion sur la contributions des contraintes, de l'angle de formage et de la plasticité sur le trajet et la vitesse de propagation a pu être apportée numériquement et confrontée aux résultats des essais.
