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International audience; The assembly process in the aeronautical sector mostly uses mechani- cal fastening. It requires the drilling of hundreds of thousands of fastening holes. These holes are critical areas for the assembly fatigue life, thus he drilling oper- ation is a key point. The fatigue performance of a machined surface can be cor- related to the surface integrity, but this correlation seems very broad and com- plex, especially for the drilling of aluminum alloys (commonly used in the air- craft industry). Moreover, different drilling processes may be used, such as axial drilling or orbital drilling. Orbital drilling is particularly interesting for limiting burrs and improving chip evacuation. But the process can impact also the fatigue life of drilled parts. To study this impact, the hole surface integrity needs to be analyzed and its influence on fatigue strength must be investigated. The main challenge concerns the interactions between the surface integrity factors. More- over, the affected layer in drilled aluminum parts is particularly thin, and the ac- cess to the hole surface is geometrically restricted. The characterization of the surface integrity is thus a complex task. In this paper, the correlations between the hole surface integrity and fatigue life of drilled 2024-T351 aluminum alloy are addressed. The study considered both orbital and axial drilling processes. The generated surface integrity was characterized in terms of surface roughness, metallographic observations, hardness, and residual stress. Fatigue tests were performed and correlated to the surface integrity results.

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.

International audience

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.

Designed for aeronautical and automotive applications, the split sleeve cold expansion process is used to improve the fatigue life of bolted metallic parts. Although its application has been well tested on aluminum assemblies, hard metal applications are still being studied. This paper presents experimental results of double bolt joint assemblies under double shear fatigue tests after stacked split sleeve cold expansion. The behaviors of two sizes of assemblies with different degrees of expansion are investigated. S-N curves are the main indicators of this study but thermal aspects are also investigated to observe fretting in the specimens as bolts are preloaded. Bolt tension is a major parameter in assembly regarding fatigue life. Interference between those two phenomena is at the heart of this paper. The first results show that stacked cold expansion has a negative effect on mechanical performances, as it deteriorates the fatigue life of the assembly. However, an examination of these results provides a coherent explanation for the loss of performance that occurs.

International audience; This research deals with the development of a new semi-analytical model for composite-metal eccentric tensile bolted joints of casing structures. The model is based on a direct stiffness method considering structural elements, where flanges are modeled with axisymmetric shell elements, the bolt with a beam element and contact behavior with a specific number of compressive linear springs, identified by means of convergence tests. A hybrid element is added in order to set the link between flange and bolt. Since the model is intended for the preliminary design of bolted joints including composite materials, we propose an extension of the stiffness matrix formulation of isotropic axisymmetric shell elements to orthotropic ones. This formulation is validated with respect to a 2D FE axisymmetric model. Then, a semi-analytical model is established for a casing assembly approximately corresponding to a real casing of an aircraft engine. Design criteria such as normal stress on the bolt, bending moment and axial load at a specific section of the composite flange, given by the semi-analytical model are compared to a 3D FE computation. Results are in very good agreement and the semi-analytical model offers considerable time saving where the computation time was 120 times faster than the 3D FE model. Moreover, relative error does not exceed 2% for normal stress of the bolt and 9% for the flange bending moment which is satisfactory in a preliminary design approach. However, due to the formulation of hybrid element, less accurate results were observed for axial displacement of the composite flange and it would be advisable to consider a new formulation. This will be detailed in future work. Finally, a parametric study showed the ability of the semi-analytical model to predict the physical behavior of joints and proved its usability for running optimization studies and its reliability as a preliminary design tool.

The main aim of the present work is to study the effect of axial preload on the durability of aerospace fastened joints. For that purpose, experimental and numerical approaches are adopted to demonstrate the beneficial effect of axial preload on the fatigue life of representative aerospace fastened joints. In the experimental approach, an innovative tightening technique, based on iterative tightening, is used to reduce the scatter habitually induced on preload when the conventional torque tightening technique is employed. The main benefit, besides the use of this alternative tightening technique, is to allow an accurate preload to be introduced without geometry or stiffness modification, thus enabling the preload effect to be determined precisely. The results show the effect of preload on the static resistance, fatigue life and failure mode of fastened joints. In the numerical approach, a 3D finite element model is developed and the analysis of its behavior confirms and explains experimental observations. Analysis of longitudinal stresses through critical paths shows that the effect of high preload is beneficial because it reduces hydrostatic pressure and shear stress amplitude, thus leading to longer fatigue lives. The correlation between experimental observations and the numerical results is studied using SWT and Crossland multiaxial fatigue criteria.

International audience; This article is devoted to the development of a calculation macro-model for the sizing of aircraft wheel fasteners. By their design, airliners tires are too rigid to be mounted on the wheel like car or motorcycle tires. Therefore, it is necessary to design a wheel in two separate parts so as to plug the tire on a first half-wheel and then “close” the assembly with the second. Therefore, the two half-wheels must be held together. Inmost cases, a bolted joint plays this essential role. The aim of this work is to develop a methodology related to the generation of semi-analytical model representative of the assembly. The tool is designed with a local approach that models an angular wheel sector including the most solicited bolt. It makes possible to evaluate tensile and bending stresses in the bolt, taking into account the roundness effect of the wheel. The objective is to obtain results close to finite element 3D modeling but with a very limited computation time. To achieve this goal, the principle of this model is to represent all parts of the wheel by 1D axisymmetric elements such as plate and shell elements, beam, tube, and spring elements (to model the contact) ensuring a description close to the actual wheel. Contact between the two half-wheels is also taken into account in order to detect the location of any unsticking after application of external loads. Different cases will be simulated (inflation case, straight line, and cornering case). The mechanical strength of the most solicited bolt will be analyzed for these different configurations. The results in tensile and bending stresses will be observed at the screwlevel.Acomparison of the results from the proposed digital macro-model and the 3D digital simulations will be analyzed. The developed tool will be used in the early design phase which allows the design engineer to define the mechanical and geometric properties of the wheel and bolts to find the optimal assembly.

International audience; This paper proposes the use of 1D basic models to build a design assistance tool capable of evaluating the heat transfer between a third-level electronic packaging and its support, considering a conventional configuration where a thermal interface material is placed between these two parts. Using this kind of tool early in the design process may facilitate choices concerning geometry and material. The packaging is modelled by a stepped beam (the equipment) and the interface layer by a nonlinear elastic foundation (the thermal interface material). Considering that the electronic equipment bends under the effect of the forces exerted by the fasteners, the tool makes it possible to determine the contact zone remaining operative after deformation, and the pressure distribution at the interface. Mechanical results are then used to calculate the steady-state heat transfer between the equipment and its support, taking into account the diffusion within the equipment and the thermal interface material, and also the thermal contact resistances, the latter being dependent on the contact pressure. A detailed case study is used to illustrate the utility of the approach. The 1D models are exploited to illustrate the interest of the design assistance tool. The influence of different parameters on the thermal performance is studied and a new innovative proposal is analyzed, which could lead to a significant increase in thermal performance.

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.

International audience; This paper presents the experimental and numerical work achieved in the aim of evaluating the heat transfer at the interfaces of threaded spacecraft assemblies, where Thermal Interface Materials (TIMs) are placed between two surfaces to improve the thermal performance. Developing a model to predict thermal resistance for such an assembly is a serious challenge, which has to take various influencing parameters into account. First, mechanical and thermal experiments used to characterise TIMs are summarised. Second, a numerical model capable of representing the behaviour of these materials is built. To verify the mechanical model, the preload of a single fastener assembly is measured and compared with a simulation. The thermo-mechanical model is verified by an assembly heated by a power resistor to evaluate the thermal aspects. The proposed material model is able to predict the loss of preload caused by creep/relaxation of the TIM and the temperature distribution of the assembly. This work is part of a broader study that seeks to develop a multi-physics approach to evaluate the heat transfer at interfaces of space application assemblies.

International audience; The self-loosening of bolted assembly is a phenomenon that has been studied several times in the past. Among the existing solicitations, it has been showed that transverse loading might cause the most severe loosening. Thus, different explanations have been proposed. In spite of their similarity, as they all consider sliding as the root cause, the sliding surface distinguish them one from another. This paper will aim at giving another point of view of the problematic in order to advantage a Zadoks' theory, which highlights the transverse sliding of components. To do so, firstly a double-shear assembly will be modeled. Its specificities are the preload application by screwing the nut, and the use of a self-locking nut. Secondly, some tests will be done in order to valid the built model. Those experiments will also factor the thickness of the assembly in the self-loosening. Thirdly, the analysis of the numerical results and of the experiments, especially the interface surfaces will give us a clue in order to validate the sliding phenomena, which is according to Zadoks, the movement implying the self-loosening of the assembly.

International audience; In this paper, the mechanisms of damage of an oxide/oxide ceramic matrix composite bearing are studied with a new experimental setup, the balanced quarter hole device (BQH). This test was designed to allow direct observation of the damage development on a material subjected to bearing failure. In a standard bearing experiment, real time monitoring is extremely difficult to set up, and post mortem observations of the bearing plane can be biased by the cutting operation. High speed cameras were used to take pictures of the bearing plane so that the damage development could be studied and a damage chronology established. The validity of the setup was verified by comparing the results obtained with those of a standard bearing test. Two different stacking sequences were studied, and the influence of the material microstructure and composite machining was investigated. It was shown that the first critical damage, matrix cracks, appeared before the load drop, and then led to delamination and kink bands, causing the final failure of the material.

International audience; The paper examines the static behavior of the inverted planetary roller screw (PRS) through numerical and experimental studies. The numerical analysis of the inverted PRS is first presented to capture the global and local deformations in different configurations. Using a three-dimensional finite element (3D FE) method, a sectorial model of the mechanism is built involving an entire roller. The model describes the static behavior of the system under a heavy load and shows the state of the contacts and the in-depth stress zones. The current work also investigates the axial stiffness (AS) and the load distribution (LD) under both compressive and tensile loadings. It is shown that the LDs are not the same at each contact interface of the roller and that they depend on the configuration of the system. Also, the nut is less stressed than the screw shaft because of their contact curvatures. In parallel, complementary experiments are carried out to measure the axial deflection of the screw shaft and the rollers in five cases with different numbers of rollers. In each situation, the mechanism is under the same equivalent axial and static load. The tests reveal that rollers do not have the same behavior, the difference certainly being due to manufacturing and positioning errors that directly affect the number of effective contacts in the device. This stresses the fact that the external load is unequally shared over rollers and contacting threads. By introducing the notion of an equivalent roller, the results are used to validate the previous numerical model of an inverted PRS. As they provide a better understanding of the inverted PRS, these investigations are useful to improve the existing analytical models of the device.

International audience; The cold expansion process is a technology that is widely used to enhance the fatigue resistance of aircraft metallic parts. The issue analysed in this paper concerns the case when the expansion is carried out through an assembly composed of several sheets. The numerical work conducted was intended to understand the phenomenology of the process within a stiff assembly. In particular, it aimed to analyse the deformation of the sheets and the residual stress fields generated by the process. For this purpose, an axisymmetric finite element model of the split sleeve process was developed, simulating a single expansion performed through a stack of two titanium holes (Ti-6Al-4V). The sheets to be expanded were positioned between two steel plates to simulate the assembly. The model could predict the shape and intensities of the fields within the expanded sections and their global outer shapes. We particularly focused on the phenomena prevailing between sheets. The simulation showed that deformations at the interface were greatly reduced when the stack was stiffened axially. Moreover, high circumferential and axial residual stresses were generated in the sheets. Results were compared with a single hole expanded without axial stiffening.

Research in interactive Design – Vol. 4 presents the most recent successful developments in mechanics, design engineering and advanced manufacturing for innovation in product, process and production engineering. The readers are invited to discover new techniques, methods, models and tools based on integrated and interactive design approaches. In a context of worldwide competition, there is an unquestionable need for such approaches to improve the reactivity, flexibility and capacity for innovation of engineers faced with particular challenges in enhancing the development of transport systems, the production of energy, the manufacture of consumer goods, the building engineering or the product design. But these approaches are also of great interest to academics, to improve the content of their curriculum and education programmes and to define novel research roadmaps in the field of mechanics, design engineering and advanced manufacturing.

Bolted joints are one of the most common elements in aerostructures. Currently, the preload applied to join the parts together is achieved by applying torque to the bolt head or to the nut. In the case of shear joints, an emerging opportunity to optimise structural joints is to apply the preload more accurately. Up to now, the preload effect is most often neglected due to the large scatter on its value due to torque tightening. The aim of this article is to describe the effect of preload on shear joints, for this reason, a numerical and experimental approach has been adopted to demonstrate its influence. In the experimental approach, iterative tightening was used to reduce the scatter on preload. The benefits of preload on high load transfer double lap shear joints will be discussed. In the numerical section, an approach of predicting fatigue life of shear joints is presented. The effect of preload/friction of coefficient is also studied. Finally, practical aspects like alternative tightening techniques and a focus on an industrial application case are also presented.

International audience; The cold expansion process is a technology that is widely used to enhance the fatigue resistance of aircraft metallic parts. The issue analysed in this paper concerns the case when the expansion is carried out through an assembly composed of several sheets. The numerical work conducted was intended to understand the phenomenology of the process within a stiff assembly. In particular, it aimed to analyse the deformation of the sheets and the residual stress fields generated by the process. For this purpose, an axisymmetric finite element model of the split sleeve process was developed, simulating a single expansion performed through a stack of two titanium holes (Ti-6Al-4V). The sheets to be expanded were positioned between two steel plates to simulate the assembly. The model could predict the shape and intensities of the fields within the expanded sections and their global outer shapes. We particularly focused on the phenomena prevailing between sheets. The simulation showed that deformations at the interface were greatly reduced when the stack was stiffened axially. Moreover, high circumferential and axial residual stresses were generated in the sheets. Results were compared with a single hole expanded without axial stiffening.

International audience; In this paper, an original approach is proposed to calculate the static load distribution and the axial stiffness of a planetary roller screw (PRS) mechanism. Assuming that the external loading is shared equally over an arbitrary number of rollers, only a sector of the system is represented to save on computing time. The approach consists in using a structure of bars, beams, and nonlinear springs to model the different components of the mechanism and their interactions. This nonlinear model describes the details of the mechanism and captures the shape of the nut as well as the bending deformation of the roller. All materials are assumed to operate in the elastic range. The load distribution and the axial stiffness are determined in three specific configurations of the system for both compressive and tensile loads. Further, the influence of the shape of the nut is studied in the case of the inverted PRS. The results obtained from this approach are also compared to those computed with a three-dimensional finite-element (3D FE) model. Finally, since the calculations appear to be very accurate, a parametric study is conducted to show the impact of the bending of the roller on the load distribution.

International audience; Bolted joints are essential elements of mechanical structures and metal constructions. Although their static behaviour is fairly well known, their dynamic behaviour due to shocks and vibrations has been less studied, because of the large size of the finite element models needed for a detailed simulation. This work presents four different simplified models suitable for studying the dynamic behaviour of an elementary bolted joint. Three of them include contact elements to allow sliding of the screw head and the nut on the assembled parts, and the last one allows rotation between screw and nut. A penalty approach based on the Coulomb friction model is used to model contact. The results show that these models effectively represent the dynamic behaviour, with different accuracy depending on the model details. The last model simulates the self loosening of a bolt subjected to transversal vibrations.

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.

National audience; La méthode de modélisation discrète des plis (DPM : Discrete Ply Modelling) est une stratégie de modélisation qui a été développé initialement pour la modélisation des impacts basse vitesse/basse énergie sur stratifiés et qui s’est révélée particulièrement efficace. Sa robustesse a été démontrée et elle a été appliquée avec succès à un certain nombre de problématiques : impact sur stratifiés, calcul de l’indentation résiduelle, compression après impact, impact sur chant ou sur zone de reprise de plis. Si elle repose sur maillage complexe qui offre des chemins naturels de fissurations matricielles et de délaminage, elle présente l’avantage de ne reposer que sur 13 paramètres parfaitement objectifs : les constantes élastiques, les contraintes à rupture matrice, les déformations des fibres à rupture en traction, les taux de restitution d’énergie critiques pour les interfaces ainsi que pour les fibres en compression. Cette méthode est aussi insensible par nature aux phénomènes de localisation et permet de représenter physiquement les ruptures observées en pratique. Dans ce papier, on présentera les cas particuliers de l’arrachement et de la rupture trouée en détaillant les particularités de maillage et la corrélation des essais. Des perspectives seront données en ce qui concerne la modélisation et les essais des assemblages en lien en particulier avec l’ANR Vertex.

Cold expansion of metallic holes is widely used in the aerospace industry for improving the 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, numerical modeling is developed in order to simulate cases where the expansion is carried out twice: in the same direction and in opposite directions. The residual fields are analyzed and the results compared with experimental results using incremental hole-drilling technique. Results show that double expansion may have a very positive impact on the homogeneity of the compressive fields.

International audience; Self-loosening of bolts is a problem encountered in many industrial fields, and especially in aviation. The purpose of this paper is to study the self-loosening phenomenon numerically through an assembly consisting of two plates assembled by a bolt and subjected to a transversal dynamic load. First, a 3D finite element model is developed, with contact elements and all geometrical details, including the real thread geometry with its helix angle. This leads to many degrees of freedom and long calculation time. Then a 1D simulation is made, based on a simplified model with only 4 degrees of freedom and few contact conditions. Calculation is nearly instantaneous. Both models describe the succession of sticking and slipping on contact surfaces and predict loosening of the assembly. They provide close results. The next step will be to compare their behaviour with an experimental approach, in order to validate and fit the models.

International audience; This paper presents an evaluation of the influence of the cold expansion process on the fatigue performance of holes in hard alloys, as such materials are involved in an increasing number of aeronautical applications. Fatigue enhancement could bring significant savings for numerous possible industrial applications. However, although the cold expansion of aluminium holes has been studied widely, there have been few publications concerning this process in hard alloys and there is currently very few research activities on technique dedicated to high strength metallic holes. Thus, this work aims to define an approach to understand why processes and methodologies are suitable for obtaining effective expansion of these materials. In this article, the response of expanded holes is studied, considering various experimental parameters and for a wide range of expansion ratios. The influence on the fatigue strength of Ti-6Al-4V tensile specimens is also reported for various expansion ratios, more precisely \textquotedblleftstandard\textquotedblright expansion ratios and high expansion ratios. Then, a specific numerical modelling of the process is presented, which enables us to understand the impact of high expansion ratios in titanium holes and the influence on fatigue performance. A very good correlation between experimental and numerical results is observed.

In the space industry, the current industrial design method for screw connections involves the use of simplified numerical models in which the connections are modeled by 6 linear springs. Several improvements of the current approach are suggested in this work. For this, non-linear models of real behavior of screw connections are developed. Priority is given to the shear model because this type of loading is the main cause of slip. The study focuses on an isolated connection. The contacts behavior is modeled by a coupled phenomenological / constitutive approach which allows to take the roughness of the contact surfaces into account. The development of new analytical models allows to build a new design method for screw connections. This new method is confronted to the current one on real application cases, which include a quasi-static case of double shear plates, a thermal case of a bimetallic specimen and a dynamic case of space equipment type. Each case includes a dedicated test bench, a detailed numerical model and an industrial numerical model. In conclusion, perspectives are given in the light of the results obtained in this work.; Dans l'industrie spatiale, la méthode de dimensionnement actuelle des liaisons vissées implique l’utilisation de modèles numériques simplifiés, dans lesquels les liaisons sont modélisées par 6 ressorts linéaires. Plusieurs améliorations de la méthode actuelle sont proposées dans ces travaux. Pour cela, des modèles de comportements réels non-linéaires des liaisons vissées sont développés. La priorité est donnée à la sollicitation de cisaillement car elle est la principale cause de glissement des pièces. L’étude se focalise sur une liaison isolée. Le comportement des contacts est modélisé par une approche couplée phénoménologique / constitutive qui permet de prendre en compte le caractère rugueux des surfaces de contacts. Le développement des modèles analytiques permet de mettre en place une nouvelle méthode de dimensionnement des liaisons vissées. Cette nouvelle méthode est confrontée à la méthode actuelle sur des cas d'application concrets, dont un cas quasi-statique de plaques en double cisaillement, un cas thermique de type bilame et un cas dynamique de type équipement spatial. Chaque cas d’étude comprend un banc d’essais dédié, un modèle numérique détaillé et un modèle numérique industriel. En conclusion, des perspectives sont données au regard des résultats obtenus dans ces travaux.

This thesis aims to contribute to the development of innovative processes for mechanical components made of hard metals (Titanium, steels and superalloys). In the aeronautical field, the design of ever more efficient and reliable structures remains a technical challenge. In mechanical assemblies, hole edges are the seats of high stress concentrations and are a major risk site for crack initiation. To fight against this damage, manufacturing technologies such as cold expansion are widespread for aluminium applications but not for hard metal. The objective of this study is to provide answers concerning the impact of the cold expansion on the fatigue performance of holes and understand the mechanisms of the process in these high performance alloys. In the present work, the methodology proposed is to carry out both experimental and numerical studies of the response of hard alloy holes subjected to cold expansion. An extensive experimental campaign has been set up. It includes several materials (Ti-6Al-4V αβ and β annealed, 15-5PH & Inconel 718) and aims to test many parameters. The process has proved very effective on the fatigue performance in these high performance alloys. On the other hand, the main technical and scientific challenge lies in determining the stress fields generated within the material after cold expansion, especially in hard metals, the behaviour of which can be diverse and complex. The numerical modelling strategy chosen has led to the development of polyvalent axisymmetric models that are dedicated to simulation of the split sleeve process. Experimental and numerical results were compared using various methodologies, such as the incremental hole drilling technique. Measurements have validated the finite element simulations, with the purpose of mapping the residual fields in the expanded metallic section and the proposal of optimisation techniques.; Ces travaux de thèse ont pour objectif de contribuer au développement de procédés innovants pour les assemblages contenant des métaux durs. Le procédé d’expansion à froid, dont l’efficacité a été prouvée à de nombreuses reprises sur les alliages d’aluminium et dont les qualités sont nombreuses, a attiré notre attention. L’objectif de la thèse est d’apporter des éléments de réponse quant à l’intérêt de l’utilisation du procédé d’expansion au sein des métaux durs, mesuré notamment par son impact sur la performance en fatigue. Plus généralement, il s’agit de comprendre son fonctionnement et son mode d’action au sein de ces alliages à hautes performances. Dans les travaux réalisés, les challenges scientifiques et techniques s’articulent autour de plusieurs thématiques d’étude. Des essais expérimentaux ont été effectués en vue de prouver la faisabilité du procédé mais aussi de mesurer de son impact sur la tenue fatigue d’alésages en alliages de titane (Ti-6Al-4V), en acier inoxydables à durcissement structural (15-5PH) mais aussi en superalliages à base Nickel (Inconel 718). L’efficacité du procédé a été prouvée et les gains apportés par la technologie sont importants. D’un autre côté, la problématique de la détermination des champs résiduels en bord de trou reste un réel défi pour la communauté intéressée par l’expansion à froid. La considération cette fois-ci des métaux durs apporte un degré supplémentaire de nouveauté et d’inconnu. La méthodologie générale employée réside dans la considération d’études numériques et expérimentales afin d’étudier la réponse des métaux durs à l’expansion à froid. Des modèles éléments finis axisymétriques ont été développés en vue de simuler le procédé complet d’expansion à froid réalisée au sein d’alésages et d’empilages de divers métaux durs. Les résultats numériques fournis ont été mis en parallèle avec ceux issus de mesures expérimentales, telles que la méthode du trou incrémental. Le but étant ainsi d’obtenir les cartographies les plus fiables possibles des contraintes résiduelles triaxiales générées en bord de trou mais aussi de rechercher de stratégies d’optimisations du procédé

This thesis aims to contribute to the development of innovative processes for mechanical components made of hard metals (Titanium, steels and superalloys). In the aeronautical field, the design of ever more efficient and reliable structures remains a technical challenge. In mechanical assemblies, hole edges are the seats of high stress concentrations and are a major risk site for crack initiation. To fight against this damage, manufacturing technologies such as cold expansion are widespread for aluminium applications but not for hard metal. The objective of this study is to provide answers concerning the impact of the cold expansion on the fatigue performance of holes and understand the mechanisms of the process in these high performance alloys. In the present work, the methodology proposed is to carry out both experimental and numerical studies of the response of hard alloy holes subjected to cold expansion. An extensive experimental campaign has been set up. It includes several materials (Ti-6Al-4V αβ and β annealed, 15-5PH & Inconel 718) and aims to test many parameters. The process has proved very effective on the fatigue performance in these high performance alloys. On the other hand, the main technical and scientific challenge lies in determining the stress fields generated within the material after cold expansion, especially in hard metals, the behaviour of which can be diverse and complex. The numerical modelling strategy chosen has led to the development of polyvalent axisymmetric models that are dedicated to simulation of the split sleeve process. Experimental and numerical results were compared using various methodologies, such as the incremental hole drilling technique. Measurements have validated the finite element simulations, with the purpose of mapping the residual fields in the expanded metallic section and the proposal of optimisation techniques.; Ces travaux de thèse ont pour objectif de contribuer au développement de procédés innovants pour les assemblages contenant des métaux durs. Le procédé d’expansion à froid, dont l’efficacité a été prouvée à de nombreuses reprises sur les alliages d’aluminium et dont les qualités sont nombreuses, a attiré notre attention. L’objectif de la thèse est d’apporter des éléments de réponse quant à l’intérêt de l’utilisation du procédé d’expansion au sein des métaux durs, mesuré notamment par son impact sur la performance en fatigue. Plus généralement, il s’agit de comprendre son fonctionnement et son mode d’action au sein de ces alliages à hautes performances. Dans les travaux réalisés, les challenges scientifiques et techniques s’articulent autour de plusieurs thématiques d’étude. Des essais expérimentaux ont été effectués en vue de prouver la faisabilité du procédé mais aussi de mesurer de son impact sur la tenue fatigue d’alésages en alliages de titane (Ti-6Al-4V), en acier inoxydables à durcissement structural (15-5PH) mais aussi en superalliages à base Nickel (Inconel 718). L’efficacité du procédé a été prouvée et les gains apportés par la technologie sont importants. D’un autre côté, la problématique de la détermination des champs résiduels en bord de trou reste un réel défi pour la communauté intéressée par l’expansion à froid. La considération cette fois-ci des métaux durs apporte un degré supplémentaire de nouveauté et d’inconnu. La méthodologie générale employée réside dans la considération d’études numériques et expérimentales afin d’étudier la réponse des métaux durs à l’expansion à froid. Des modèles éléments finis axisymétriques ont été développés en vue de simuler le procédé complet d’expansion à froid réalisée au sein d’alésages et d’empilages de divers métaux durs. Les résultats numériques fournis ont été mis en parallèle avec ceux issus de mesures expérimentales, telles que la méthode du trou incrémental. Le but étant ainsi d’obtenir les cartographies les plus fiables possibles des contraintes résiduelles triaxiales générées en bord de trou mais aussi de rechercher de stratégies d’optimisations du procédé

This thesis focuses on the study of the self-loosening of bolted joints under transverse dynamic loads. Experimental and numerical work was carried out in order to reveal this phenomenon under conditions close to those of actual assemblies. An experiment allowed the loosening of a bolt to be caused by subjecting the assembly to transverse vibration with respect to the axis of the screw. Effects of preload, surface treatment and disposal of the screws were explored. These results were compared with those obtained by a detailed numerical model using the finite element method. However, due to the complexity of the geometry and modeling challenges at the local level, the computation time was very long and may be prohibitive for an industrial study. To remedy this problem, two simplified models were developed, one with a finite element approach associated with a kinematic connector, and one by unidimensional modeling. These simplified models correctly reproduce the self-loosening phenomenon and confirm their efficiency in terms of computation time.; Cette thèse porte sur l’étude du dévissage spontané d’assemblages vissés, sous sollicitations dynamiques transversales. Un travail expérimental et numérique a été réalisé pour caractériser ce phénomène, dans des conditions proches de celles des assemblages réels. Une expérimentation a permis de provoquer le dévissage d’un boulon, en soumettant l’assemblage à des vibrations transversales par rapport à l’axe de la vis. Les effets de la précharge, du traitement de surface et de la disposition des vis ont été explorés. Ces résultats ont été confrontés avec ceux obtenus par un modèle numérique détaillé utilisant la méthode des éléments finis. Cependant, en raison de la complexité de la géométrie et des difficultés de modélisation à l’échelle locale, le temps de calcul est très important et peut être prohibitif pour une étude industrielle. Afin de remédier à cette difficulté, des modèles simplifiés ont été développés, l’un avec une approche par éléments finis associés à un connecteur cinématique, et l’autre par modélisation unidimensionnelle. Ces modèles simplifiés reproduisent correctement le phénomène du dévissage spontané et confirment leur efficacité en gain de temps de calcul.

Axial preload is a fundamental parameter that needs to be correctly mastered in order to ensure the mechanical strength of bolted joints. In the case of bolted shear joints under fatigue solicitations, a low preload does not allow to exploit the load carrying capacity of the friction interfaces. A high preload is undesirable as it can generate fretting fatigue phenomena or excessive under head contact pressure that may lead to decrease fatigue lives of aeronautical joints and structures. Controlled preload application generates and maintains equilibrium of load transfer mechanisms, inducing a full use of their potential and leading to an optimal fatigue life of the joint. This study focuses on aeronautical preloaded joints with the objective of understanding the effects of controlled preload over the mechanics of shear joints and exploiting the contribution of preload to enhance their fatigue lives. For this purpose, a hybrid approach has been adopted to demonstrate the beneficial effect of controlled preload. Numerical modelling has been carried out to better understand the preload influence but also to predict the mechanical strength of bolted joints. The experimental database has been generated using exclusively alternative tightening techniques, in order to reduce the scatter on final preload and therefore evaluate the effect of controlled preload in a more accurate way. The fatigue strength of bolted shear joints has been significantly improved for most of tested configurations due to the application of accurate axial preload. The effects of many other parameters, as the nature of interface, the stiffness of fasteners or the radial adjustment has also been assessed and permits to identify an additional potential optimization of bolted joints.; La maîtrise de la pré-tension revêt d’une grande importance pour garantir la tenue mécanique des assemblages boulonnés, et ce quel que soit le mode de sollicitations auquel ils sont soumis. En particulier, pour les chargements transverses et sous sollicitations de fatigue, un niveau de pré-tension insuffisant ne permet pas d’exploiter la capacité de transfert d’effort des interfaces entre pièces assemblées. Un niveau de pré-tension très important n’est pas souhaitable non plus, car cela génère des phénomènes de fatigue de contact ou de matage excessif sous tête qui ont un effet néfaste sur la durée de vie des assemblages et des structures aéronautiques. Le serrage contrôlé des fixations permet ainsi de produire et de maintenir un équilibre entre les modes de transfert d’effort, permettant ainsi d’exploiter pleinement leur potentiel et obtenir une tenue optimale de l’assemblage. L’objectif de ce travail est d’apporter une contribution à la compréhension et à l’exploitation des assemblages boulonnés à serrage contrôlé. Une approche hybride a été adoptée afin de démontrer l’effet bénéfique d’une pré-tension contrôlée. La modélisation numérique a permis de comprendre les mécanismes de transfert d’effort et de prédire la tenue mécanique des assemblages aéronautiques de façon satisfaisante. La démarche expérimentale a été exclusivement basée sur des techniques dites de serrages alternatifs. L’objectif est d’avoir une faible incertitude sur la valeur de la précontrainte installée, permettant d’évaluer la performance mécanique des assemblages boulonnés. La tenue en fatigue des assemblages boulonnés, travaillant en matage-cisaillement, est largement améliorée grâce à l’application d’une précontrainte précise. La maîtrise des paramètres intrinsèques au maintien de la force axiale de serrage tel que la nature de l’interface entre pièces assemblées, la rigidité des fixations et la nature de l’ajustement, permet également d’esquisser un potentiel d’optimisation non négligeable des assemblages boulonnés.