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International audience; The work presented here is an experimental and numerical study on a new technology called friction shims. Those are thin shim made of steel with a shape of washer. Their coating made of nickel and micro diamonds aim to increase the friction coefficient when located at the interface of two parts. The mechanical behaviour as well as the static and fatigue strengths of titanium bolted joints with this specific technology are presented in this study. Specimens are representative of hole-to-hole aeronautical joints, involving large clearance and misalignments. Specimen geometrical and material definition are presented as well as test set up. The experimental test results show limited impact on the static strength and a large increase in the fatigue strength. A numerical analysis based on the finite element method was developed and correlated with the experiments to provides complementary information and to understand the mechanical behaviour observed during the static tests and the gain in fatigue strength.

International audience; In today's business environment, the trend towards more product variety and customization is unbroken. Due to this development, the need of agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the similarity between product families by providing design support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach.

Smart tightening development is part of the Industry 4.0 transformation with the introduction of smart tools, and preload in bolted assemblies is of major interest in today’s aircraft manufacturing process. So far, it has been difficult to estimate the importance of each parameter for tightening process quality, mainly because of the large number of combinations and configurations that exist.The present work aims at evaluating the effects and the interactions between different parameters that have to be taken in consideration in future torquing strategy. Many experimental tests have been conducted on an Automatica test bench using a Taguchi strategy and an analysis of the first main results is now presented, highlighting the complexity of the phenomena studied.All these points will help us to better understand tightening, so as to improve performance during installation, maintenance and repair.

International audience; The influence of the hole surface integrity on the fatigue life of 2024‐T351 aluminum drilled parts was investigated. Fatigue tests were conducted on open‐hole specimens, and a large campaign was carried out to characterize the hole surface integrity (roughness measurements, hardness measurements, metallographic observations, and residual stress analysis). An innovative technique, the Hole Opening Comparative Technique, was set up in order to analyze the residual stress state of the parts. In this study, strain hardening of the hole subsurface seems to be the main factor influencing the fatigue behavior, associated with a residual stress state of the part.

Fatigue strengths of aluminium 2024-T351 open-hole specimens drilled by axial and orbital drilling processes are compared. Two drilling diameters (Ø) are studied: 6.35 mm and 9.53 mm. Surface integrity characterization tests are conducted in order to study the link between drilling processes, surface integrity and fatigue life. Fatigue test results show an increase of the fatigue life for specimens drilled by axial drilling for Ø = 9.53 mm and no significant difference in fatigue life between the two drilling processes for Ø = 6.35 mm. Surface integrity results show no impact of the roughness on the fatigue strength but a potential positive influence of the hole microhardness on the fatigue life.

International audience

International audience; The self-loosening of a joint through unscrewing of the bolt is a phenomenon mainly occurring when the assembly is solicited by transverse repeated loads. Previous works highlighted that the transversal sliding of the bearing surfaces, either in the threads or underneath the bearing surfaces of bolts and/or nuts, is its root cause and that this phenomenon begins during the first loading cycles. In order to study the early stage of this self-loosening, a simplified numerical model has been developed. The latter factors the bearing surfaces of the bolt, the preload, the friction coefficient, the amplitude of the shear-load and the fastener's material. Through measurements and interpretation of the results, the shearing of the fastener has been identified as the main deformation leading to the self-loosening of the assembly, while the bending of the fastener shank limits the self-loosening. Moreover, according to the values of preload and shear-load, the behaviors were identified, and an interpretation has been proposed.

ISBN 978-3-030-12346-8; International audience; In aircraft assemblies, holes for fastening are critical areas from where fatigue damage can be initiated, especially for metal parts. Depending on the operating conditions of the various stages of the machining process of the hole (drilling, reaming, cold expansion , second reaming), the manufacturers observe significantly different fatigue strength of the structures. To optimize the behavior of the aircraft assemblies, the manufacturers want to understand the impact of the whole machining process of the hole on the material characteristics of the part. They investigate particularly the residual stress state in the hole edge zone which can be significantly different depending on the operating conditions. The study, in collaboration with Airbus, focuses specifically on the impact of the reaming on the final residual stress state in parts with initial material state not clean of mechanical stress. Indeed, before the final reaming, the parts undergo at least a drilling operation, and sometimes a cold expansion operation. Therefore, they do not present a zero stress state. This paper investigates the influence of reaming in an aluminum 2024-T351 part which has been pre-stressed by the cold expansion process. Both processes are simulated using a finite element model with Abaqus. The implemented strategy to simulate the reaming process is based on the progressive deactivation of mesh elements. The obtained results show a low relaxation of the circumferential and radial residual stresses in the part during the reaming simulation.

National audience; Dans les structures mécaniques, les trous percés constituent des zones critiques majeures à partir desquelles des endommagements peuvent s'initier en fatigue. Les industriels, notamment dans le domaine aéronautique, constatent des différences significatives en termes de tenue en fatigue des structures percées en fonction des paramètres et des procédés de perçage employés. L'optimisation du comportement des assemblages aéronautiques passe par la compréhension de l'impact du procédé de perçage sur les caractéristiques matériau de la pièce percée qui sont liées aux sollicitations thermomécaniques. L'objectif de l'étude, en collaboration avec Airbus, est donc de comprendre et d'identifier les phénomènes et mécanismes qui pilotent la durée de vie en fatigue des pièces percées en alliage d'aluminium. Pour cela, un plan d'expériences est conduit, dans lequel certains paramètres de perçage sont testés (perçage axial ou orbital, réalisation d'un avant-trou), avec une instrumentation en effort, couple et température. Ces acquisitions sont corrélées aux paramètres de process. Cela permettra ensuite d'envisager une modélisation numérique du procédé de perçage qui permette la simulation de l'état du matériau suite à cette opération. Une étude de la durée de vie en fatigue sera envisagée, afin de relier la performance en fatigue (quantifiée par l'IQF) aux caractéristiques matériau.

International audience

International audience; Cold expansion of metallic holes is widely used in the aerospace industry as an efficient industrial process for improving fatigue performance of mechanical assemblies. To increase the efficiency of the process, it is worth considering the possibility of carrying out the expansion twice in succession in the same hole. This paper discusses the consequences of such an operation on a single titanium hole through a numerical and experimental study. More precisely, dedicated finite element modelling of the cold expansion process has been developed in order to simulate cases where the expansion is carried out twice: in the same direction and in opposite directions. The residual fields generated are analyzed and the results compared with experimental results. Furthermore, we propose in this study to assess a new methodology for measurement of the intensity of the compressive residual stresses generated at the hole edge. Specifically, we will use the incremental hole drilling technique to characterize the expanded holes, including or not a reaming operation of the cold worked hole. Results show that double expansion has a very positive impact on the homogeneity of the compressive fields generated as long as the direction of expansion is properly selected to guarantee the efficiency of the process.

The use of composite materials in joined structures allowed reaching highperformance level thanks to its specific material properties. However, to ensure theseperformances, a complex flow-process grid is needed in order to reduce uncertaintiesand to control therefore load distribution between fasteners. For example, with largeaeronautical structures, holes are drilled in a single operation in order to reduce holelocationerrors and clearance which increase considerably manufacturing cost.Today, no robust tool is available to allow justifying or optimizing joining process interms of both cost and joint performance.This thesis proposes therefore a reliability-based approach for the tolerancing ofuncertain parameters which could affect the behavior of fastened metal-compositejoints. The aim through the developed approach is to study the effect of geometricaland material variabilities on the performance of joints and to propose a method toprovide the optimal tolerances. The probabilistic character of the approach requiredthe development of a simplified model of fastened joint in order to reduce calculationtime. This model, based on connected rigid surfaces and continuum shell elements,allows representing finely physical phenomena in multi-material fastened joints(clearance, contact, adherence induced by tightening). This model was validatedexperimentally and numerically. The output distribution of the performance criteria,calculated after running a propagation of uncertainties, is then formalized by ananalytical probability density law. The parameters of this law are identified usingMonte Carlo method and worst-case approach based on a Genetic Algorithm. Thedeveloped approach allows therefore providing the appropriate tolerance associatedto an admissible performance criteria for the required reliability of the joint., L’utilisation des matériaux composites dans les structures assemblées a permis d’atteindre des niveaux de performance très élevés grâce aux propriétés spécifiques de ces matériaux. Cependant, pour garantir ces performances, les industriels s’appuient sur des gammes d’assemblage complexes afin de réduire les incertitudes et ainsi maitriser les chemins d’effort. Par exemple pour les assemblages par fixations, le contreperçage est privilégié pour limiter les défauts de positionnement des alésages et limiter les jeux. Ces gammes d’assemblage, incompatibles avec le principe d’interchangeabilité, augmentent considérablement les coûts de production.Aujourd’hui aucun outil ne permet de justifier, de remettre en cause ou d’optimiser les gammes d’assemblage en terme de compromis coût performance.Ce manuscrit propose donc une démarche de tolérancement fiabiliste des paramètres incertains pilotant le comportement des assemblages métal-composite par fixation. L’approche développée a pour but d’étudier l’effet des variabilités géométriques et matériaux sur la tenue des assemblages et de proposer une démarche pour les tolérancer. Le caractère probabiliste de la démarche, a nécessité le développement d’un modèle simplifié de comportement des assemblages par fixations afin de maitriser les temps de calcul. Ce modèle, construit à partir de surfaces rigides connectées et des éléments plaque 3-D, permet de représenter finement les phénomènes prépondérants dans le comportement des assemblages(jeu alésage fixation, contact, adhérence induit par la précharge des fixations). Il a été validé à la fois numériquement et expérimentalement. La réponse numérique de la propagation des incertitudes sur un critère de tenue mécanique est ensuite formalisée par une loi de densité de probabilité analytique. Les paramètres de cette loi sont identifiés par une stratégie combinant la méthode de Monte Carlo et la recherche des pire-cas avec un Algorithme Génétique. Cette approche permet alors de calculer les tolérances associées à un niveau de fiabilité visé.