Дисертації з теми "Matériaux à gradient fonctionnel – Vibrations"
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Koutoati, Kouami. "Modélisation numérique du comportement statique et vibratoire des poutres sandwich viscoélastiques à gradient de propriétés." Electronic Thesis or Diss., Université de Lorraine, 2020. http://www.theses.fr/2020LORR0290.
This thesis proposes a numerical tool for the static and dynamic study of viscoelastic structures made of Functionally Graded Materials (FGM) for vibration control by passive damping. The objective is to make available to engineers a generic code based on the finite element approach for sizing calculations on FGM sandwich beam with viscoelastic core for applications requiring lightness and good thermal and mechanical resistance such as aerospace, automotive and nuclear. To reach this objective we first proposed a numerical model for the static and free vibration study of FGM sandwich beams with elastic behavior. This finite element model is implemented in the Matlab code environment. Using this code, we compare different beam theories for different geometric properties and boundary conditions. Thus, the limit of the classical beam theory in the study of short structures is highlighted. Also with this numerical model, the study of axial-bending and axial-rotation coupling is possible. From this, it is shown that FGM structures are very sensitive to spatial coupling and warping effects because of the non-symmetrical distribution of the material in their cross sections. In the proposed code, the resolution of the vibration problem is possible using classical eigenvalue and eigenvector problem solving methods. Then to introduce passive damping in the FGM sandwich structure, we proposed a sandwich beam model with FGM materials faces and viscoelastic materials core. This model is also implemented in the Matlab language and proposed as a generic tool. The interest of this numerical tool lies in its ability to compute the modal properties as well as the behavior of the viscoelastic FGM sandwich beam while taking into account the frequency dependence of the viscoelastic behavior, the boundary conditions and the axial-bending and axial-rotation coupling specific to FGM materials. The free vibration problem is non-linear in this case due to the material non-linearity induced by the soft layer. In the proposed code, the resolution of this problem is possible thanks to the coupling of the homotopy technical, the asymptotic numerical method and the automatic differentiation. Through this work, the contribution of FGM materials in the improvement of the damping power of structures is highlighted. In the continuation of the work, we propose a finite element formulation to compute the amplitude of forced vibrations of viscoelastic FGM sandwich structures. The resolution of the forced vibration problem is possible by using the bandwidths method. A study on the contribution of FGM materials in the reduction of vibration amplitudes is carried out for different viscoelastic laws. It is proved in this study that by a direct control of the composition gradient of FGM materials it is possible to optimize the damping power of structures even for low frequency modes for which classical composite materials have a damping power requiring improvement
Dammak, Yosra. "Caractérisation numérique et expérimentale par ultrasons de matériaux à gradient fonctionnel." Thesis, Le Mans, 2016. http://www.theses.fr/2016LEMA1039/document.
This thesis focuses on the study of multilayered and FGM systems (FGM : Functionnally Graded Materials). The main purpose of this type of materials is to obtain deposits with new and innovative features and to increase the fracture toughness. From now on, FGM have been used in various high technology applications.A multilayer system with a composition gradient of copper and nickel was studied experimentally by the application of the laser ultrasonics (LU) technique which was coupled to a theoretical study based on the ordinary differential equations (ODE) and the Stiffness Matrix Method (SMM). This PhD thesis is organized around four chapters. The first chapter is dedicated to a theoretical study of the propagation behavior of surface acoustic wave (SAW) in a multilayer system with à gradient of properties. Thus, the numerical methods developped for the piezoelectric materials (FGPM) are presented. The second chapter is devoted to describe the setup for making the samples used in this study which were obtained by sputtering technique. The third chapter presents the experimental study dedicated to the measurement of surface wave velocities in many crystal orientations. The last chapter of the manuscript presents experimental results, compared to the theoretical results, describing the dispersive behavior of submicrometer multilayers
Froehly, Luc. "Nouveaux matériaux à gradient d'indice pour l'optique : étude des procédés de fabrication existants et développement de méthodes de caractèrisation." Saint-Etienne, 2000. http://www.theses.fr/2000STET4005.
Audouard, Lisa. "Conception et caractérisation de matériaux ultra haute température à gradient de propriétés." Electronic Thesis or Diss., Bourgogne Franche-Comté, 2023. http://www.theses.fr/2023UBFCA019.
The development of a new green ergol prototype for satellite repositioning engines requires more severe thermal and environmental conditions for combustion chamber materials than is currently the case. As a result, alternative materials known as functionally graded materials (FGM) have been developed for several years as part of an ONERA-CNES-ICB study. The aim of this thesis is to pursue the development of this type of ceramic/metal gradient material, in order to optimize its design and ensure that it can be used up to 2400 °C in the presence of water vapor. Firstly, different configurations of FGM developed by air plasma thermal spraying (APS) were tested under vacuum laser heat flux up to 2350 °C. By modelling the cracking of these materials when subjected to thermal shock, the link between the observed degradations and the FGM configurations was better established. In particular, it has been shown that increasing the thickness of the ceramic on the FGM surface is responsible for the appearance and propagation of deeper, deviated cracks.Secondly, the possibility to use such FGM under an oxidising atmosphere at ultra-high temperature was studied through two experimental set ups. The first one is a laser test bench which allowed to assure the resistance of the materials submitted to repeated thermal schocks up to 1800 °C in presence of water vapour. The tested materials presented an appropriate behaviour under the tested conditions. The degradation mechanisms related to FGM oxidation have been identified and compared from one FGM configuration to another and linked to the tested conditions. The second one permits to qualify the behaviour of FGM in the H2/O2 flame of a combustion chamber. Thus, the tested conditions were relatively close to the ones of the intended application. No major degradation was observed after the combustion chamber tests, which demonstrates the potential of this type of FGM for the application.In parallel, a study was carried out about the improvement of the ceramic part of the FGM. Indeed, the thermal expansion coefficient of the chosen metal is twice lower than the one of the chosen ceramic. Thus, and despite the presence of graded layers in-between the metal and the ceramic, high thermomechanical stresses occur at the interfaces between the different layers of the FGM. Thus, a key point of this study consisted in the understanding of the influence of the ceramic composition, and in particular of the amount and nature of the rare earth oxide, on the thermal expansion coefficient. In addition, ionic conductivity and thermal conductivity measurements most accurately reflect the role of thermal and environmental barrier coating of the pure ceramic layer upon the FGM. It has been shown that high content Lu2O3 based compositions are the most promising to be used for the ceramic composition of the FGM. The last part of this thesis was dedicated to study the possibility to heal the cracks observed in the ceramic, which came either from the thermal treatment, either from the thermal tests. Thus, an yttrium disilicate was introduced in the pure ceramic layer of the FGM directly during the elaboration process with APS. Its influence on the resistance of FGM under harsh thermal and environmental conditions was finally reported. In particular, the presence of this disilicate is responsible of chemical transformations in the FGM during high temperature tests
Madec, Clémentine. "Elaboration de matériaux à gradient de fonction céramique / métal par SPS pour la protection balistique." Thesis, Dijon, 2016. http://www.theses.fr/2016DIJOS057/document.
The objective is to improve ballistic performance of armors. A perfect armor combines ductility to resistto the impact and high hardness to stop projectile’s fragments. However, such an association of properties is inconsistent witha single material. The solution is to perform a functionally graded material (FGM) with a ductile metal at the back side of thesample and a hard ceramic on the top side. Non-conventional technologies like Spark Plasma Sintering allow joining orsintering all types of materials with different and additional properties. Furthermore, with this technique, high heating ratescan be achieved, limiting grain growth and resulting in a fine microstructure. The goal is to study joining conditions or cosinteringof such materials (in this case, Al2O3 and Ti), as well as the resulting microstructure on the ballistic efficiency.The first part of the study focused on the characterization of alumina and titanium. Five powders of alumina werestudied from a sintering point of view. Three of which were selected because of their interesting microstructures, close indensities and grain sizes. These ceramics have been characterized mechanically (hardness, toughness and strength) andballistically. One of them is adopted to realize FGM. Titanium, sintered with the same conditions, unfortunately, doesn’t haveexpected properties (absence of ductility).The second part of the work showed that the preparation of FGM without cracks from Al2O3 and Ti only ischallenging, with an interlayer with one or more layers. The strong affinity of Ti with oxygen (formation of oxides orinsertion) with C (forming carbides) and its reactivity with alumina (forming intermetallics) make the FGM brittle and enablethe release of residual stresses during the process. By adding a low amount of nickel (more ductile and less reactive withoxygen and titanium) in composites, FGMs almost without cracks were obtained. The latter were evaluated ballistically
Boulvert, Jean. "Traitements acoustiques à porosité contrôlée pour atténuation optimale." Thesis, Le Mans, 2020. http://www.theses.fr/2020LEMA1033.
This thesis exploits some of the new possibilities offered by additive manufacturing to design and optimize treatments for sound attenuation consisting in porous materials. Additive manufacturing allows to control individually each pore of a material. The porous treatment design process is turned upside down: instead of searching through a catalogue of existing materials to solve a problem, it is possible to directly design the right material by adjusting its microstructure. This research is part of a plan to reduce aircraft engine noise but extends beyond the aeronautical field, both theoretically and in terms of possible applications. A predicting method of the acoustic behaviour of porous materials produced by additive manufacturing and taking into account the impact of manufacturing defects is first introduced. Porous materials with controlled graded properties are then studied. A method for optimizing microstructural or manufacturing parameters is developed. The ability of graded porous materials to attenuate frequencies too low to be attenuated by non-graded materials is then proven and the optimal gradient for broadband attenuation is defined. The impact of the wall thickness of the pores along with the impact of transverse propagation inside porous materials is studied. Finally, a metaporous treatment allowing broadband and sub-wavelength absorption is developed. The results of this research can be applied to create porous treatments with a high noise attenuation. The analytical and numerical models used in this research are based on the hypothesis of porous materials acoustically behaving as equivalent fluids. The results are physically analyzed and experimentally validated through impedance tube testing of specimens produced by additive manufacturing
Le, Duc Trung. "Modèle d'endommagement à gradient : approche par homogénéisation." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066662/document.
The aim of this work is to propose a general framework to obtain a gradient damage model from the micro-structural level. It is based, firstly, on the homogenization method to derive an effective medium from the microstructure, and secondly, on the variational formulation of a damage evolution law from the homogenized medium. We propose, as a first step, an approach based on asymptotic expansion and the variational method for homogenizing a periodic elastic medium. To model the localization of damage, this approach has been extended to a quasi-periodic heterogeneous medium. From an example of quasi periodically micro-cracked solid, we obtain an elastic energy that not only depends on the gradient of the damage but also the strain gradients. Based on the principle of energy minimization, we propose the construction of a gradient damage model from the elastic energy homogenized in the second part. By adding some hypothesis to simplify the model, we can construct localized damage and strain solutions in closed form. Finally, a numerical resolution scheme, which is based on an alternate minimization algorithm, is proposed for the one-dimensional traction bar test. From the numerical results, the advantages and disadvantages of the model are discussed
Maruani, Jonas. "Contrôle actif des vibrations de structures élancées FGPM." Thesis, Paris 10, 2019. http://www.theses.fr/2019PA100062.
The aim of this thesis is to show the feasibility and the efficiency of active vibration control by structures made of functionally graded piezoelectric materials (FGPM). One bloc structure, made of FGPM, with piezoelectric properties embedded, is used to replace classical intelligent structures (a host structure equipped with piezoelectric patches) and to remove their disadvantages (stresses concentrations near interface, delamination of patches, …).This study focuses on the FGPM’s modelization, in particular on the graded behavior laws and on the development of finite elements of FGPM beams and plates. Two finite element are implemented, a beam element based on Timoshenko’s kinematics and a plate element based on an adaptive kinematics. Both elements have a numerical layers approximation for the electrical potential. These two elements are used for active vibration control simulations. In the beam case, the system is governed by a linear quadratic regulator. Otherwise, for the plate a fuzzy decentralized regulator is developed and used. Both systems beam and plate are observed thanks to a Luerberger’s observer. Static studies show the behavior of FGPM depending on the material gradation. In addition, active vibration simulations show the feasibility of control with both systems and the ability of fuzzy regulator to accommodate to sudden changes on external perturbations
Gozdecki, Nicolas. "Nouveaux alliages de titane à gradient de propriétés pour l'implantologie dentaire : approches expérimentale et numérique." Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066710.
In the field of biomaterials, titanium alloys are among the most attractive materials for osseointegrated implants due to their high biocorrosion resistance, increased general biocompatibility and specific mechanical properties. Among these properties, low elastic modulus of titanium alloys has attracted much attention regarding the transmission of functional loads from the implant to the surrounding bone. The aim of this work consists in developing functionally graded materials, with careful attention to the thermomechanical treatments. In one hand, this allows us to obtain a gradient of elasticity in bulk materials and in the other hand, a gradient of grain sizes. This is possible thanks to the reversible martensitic transformation β ↔ α’’ and also to the α phase dissolution during flash treatments. The microstructural scale is also controlled in order to develop homogeneous materials at the nanoscale, thermally stables, and exhibiting superplasticity at low temperatures. These results are thought to be good strategy to avoid the use of SPD processes. A complete characterization of these new materials is performed with the combination of SEM, TEM and XRD analyses to appreciate the modifications of the microstructures and grain sizes. Values of elastic modulus are obtained by tensile tests, and locally determined with the use of instrumented microindentation measurements
Leblond, Timothée. "Calcul de gradient sur des paramètres CAO pour l’optimisation de forme." Thesis, Université Paris-Saclay (ComUE), 2017. http://www.theses.fr/2017SACLC017/document.
In this manuscript, we present a shape optimization method based on CAD parameters such as lengths, angles, etc. We rely on gradient-based optimization techniques. The sensitivity of the objective function, with respect to the mesh nodes position, is provided by an adjoint solver considered here as a black box. To optimize with respect to CAD parameters, we focus on computing the sensitivity of the nodes positions with respect to these parameters. Thus, we propose two approaches based on finite differences. The first method uses a harmonic projection to compare in the same space the initial mesh and the one obtained after a change of the set of CAD parameters. The developments presented in this manuscript open up new doors like the application to shapes with multiple borders such as exhaust manifolds. We also developed an interpolation method suitable for this comparison. The entire process is automated, and we demonstrate the entire effectiveness on internal aerodynamics industrial applications. The second method is directly based on the CAD geometries to assess this sensitivity. To perform this comparison, we use the intrinsic definition of the patches in the parametric space (u;v). Through the use of the exact coordinates at any point on the surface provided by the CAD, we avoid using an interpolation to get the best calculation accuracy possible. However, unlike the first method, it requires to identify the correspondence between patches from one shape to another. An application on an external aerodynamics academic case was made. The relevance of the first method is demonstrated on a representative multi-objective case, which facilitate its deployment use in an industrial environment. Regarding the second method, we showed its great potential. However, further developments are needed to handle more advanced cases. Because they are independent of the mechanical solver and the number of parameters, these methods significantly reduce product development time, particularly by allowing large and multiphysics optimization
Diallo, Alpha Ousmane. "Modélisation hyperfréquence de problèmes multi-échelles appliquée au cas des antennes à métamatériaux diélectriques." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066356/document.
This work focuses on the improvement of the antennas compactness used primarily for embedded systems while respecting the performance and competitiveness requirements. The approach explored consists in using artificial materials operating in transmission and designed by structuring the dielectric material on a scale smaller than the wavelength (sub-wavelength). This structuring makes it possible in practice to achieve a variation in the effective refractive index in order to produce diffractive elements capable of performing a microwave function. However, the particularity of this type of structured element is to mix several physical scales generating complexity in their study. The largest dimension of a structured component can reach several tens of wavelength, for example 20λ, while the minimum size of the sub-wavelength structures may be less than a fraction of the wavelength, as than λ / 20. This multi-scale aspect increases the simulation times of antenna devices integrating these structured elements, thus preventing any possibility of multi-parameter optimization in reasonable times. In order to exploit fully the potential of these structured materials, a numerical model of computation has been developed on the basis of optical paths. This model gives results on the maximum gain of structured diffractive lens antennas with an accuracy of 0.5 dB. The computation time of the model is of the order of the minute compared to more than 6 hours for a complete simulation with the electromagnetic calculation software CST Microwave Studio. The speed and precision of this model have been used to optimize the design of a structured diffractive lens. To illustrate the relevance of this structured approach, its performances were compared with those of Fresnel lens antenna and hyperbolic lens antenna. This comparison was carried out under identical footprint conditions with a length to diameter ratio L / D of 0.5. The gain of the structured lens was found to be 1.6 dB higher than the Fresnel lens and 2.7 dB higher than the hyperbolic lens
Schneider-Maunoury, Catherine. "Application de l’injection différentielle au procédé de fabrication additive DED-CLAD® pour la réalisation d’alliages de titane à gradients de compositions chimiques." Electronic Thesis or Diss., Université de Lorraine, 2018. http://www.theses.fr/2018LORR0260.
Since 1984, the Functionally Graded Material (FGM) allow to create a thermal barrier and to reduce the strong discontinuities of properties between two materials of different composition. These multimaterials,whose consist of an intentional variation in the chemical composition and, consequently, modify the microstructural, chemical, mechanical and thermal properties, lead to a smooth distribution of the thermal stress. The in-situ development of these custom-made alloys is made possible by the use of additive manufacturing processes such as the DED-CLAD® powder deposition process. These processes have grown substantially since the 1980s and are optimal for the manufacture of FGM. During this industrial thesis, technical developments have been carried out to adapt the DED-CLAD® process and to allow the manufacturing of FGM. Thanks to two industrial collaborations, a full study was carried out on titanium-molybdenum and titanium-niobium alloys. These alloys make it possible, in the first case, to produce parts resistant to strong thermal stress (space sector), and in the second case to combine mechanical properties and biocompatibility (biomedical sector). The originality of this thesis rests on the study of a complete gradient, that is the addition in alloy element varied from 0% to 100%. In fact, studies reported in the literature do not mention titanium-refractory material for high levels of refractory element. Microstructural (XRD, crystallographic analysis by EBSD technique), chemical (EDS) and mechanical (microhardness, tensile test and instrumented indentation) analyses revealed an evolution of the properties along the chemical gradient. The mechanical characterization of the sample by instrumented indentation has also proved particularly relevant in the case of these multi-materials
Baudoin, Pierre. "Caractérisation et identification de propriétés de matériaux métalliques à gradients de microstructure." Thesis, Lille 1, 2015. http://www.theses.fr/2015LIL10015/document.
The main objective of this thesis is to design a consistent methodology for the characterization and simulation of functionally graded metals. This approach should allow the assessment of the high cycle fatigue response of forged railway axles produced by Valdunes, in the context of the Innovaxle project. The tests conducted on the forged material reveal a very heterogeneous microstructure, whose grain size varies in the width of the axle. A procedure based on recrystallisation is designed to reproduce this grain size gradient on a smaller scale, on a reference material (ARMCO iron). The characterization of the obtained graded microstructure shows heterogeneities in the local elasto-plastic response of the specimen. This behaviour is tentatively described by a heterogeneously distributed elasto-plastic law over the microstructure, the local yield strength being obtained from the local grain size through a Hall-Petch formulation. This model is used to simulate the response of graded microstructures under heterogeneous loadings in the high cycle fatigue regime. The interests of functionally graded materials are outlined by these simulations. The finite element simulations run in this work make use of the Code Aster software, and the digital image correlation program YADICS is used for image registration purposes
Marteau, Julie. "Caractérisation multi-échelle et analyse par essai d'indentation instrumentée de matériaux à gradient générés par procédés mécaniques et thermochimiques de traitement de surface." Phd thesis, Université de Technologie de Compiègne, 2013. http://tel.archives-ouvertes.fr/tel-00937956.
Schneider-Maunoury, Catherine. "Application de l’injection différentielle au procédé de fabrication additive DED-CLAD® pour la réalisation d’alliages de titane à gradients de compositions chimiques." Thesis, Université de Lorraine, 2018. http://www.theses.fr/2018LORR0260/document.
Since 1984, the Functionally Graded Material (FGM) allow to create a thermal barrier and to reduce the strong discontinuities of properties between two materials of different composition. These multimaterials,whose consist of an intentional variation in the chemical composition and, consequently, modify the microstructural, chemical, mechanical and thermal properties, lead to a smooth distribution of the thermal stress. The in-situ development of these custom-made alloys is made possible by the use of additive manufacturing processes such as the DED-CLAD® powder deposition process. These processes have grown substantially since the 1980s and are optimal for the manufacture of FGM. During this industrial thesis, technical developments have been carried out to adapt the DED-CLAD® process and to allow the manufacturing of FGM. Thanks to two industrial collaborations, a full study was carried out on titanium-molybdenum and titanium-niobium alloys. These alloys make it possible, in the first case, to produce parts resistant to strong thermal stress (space sector), and in the second case to combine mechanical properties and biocompatibility (biomedical sector). The originality of this thesis rests on the study of a complete gradient, that is the addition in alloy element varied from 0% to 100%. In fact, studies reported in the literature do not mention titanium-refractory material for high levels of refractory element. Microstructural (XRD, crystallographic analysis by EBSD technique), chemical (EDS) and mechanical (microhardness, tensile test and instrumented indentation) analyses revealed an evolution of the properties along the chemical gradient. The mechanical characterization of the sample by instrumented indentation has also proved particularly relevant in the case of these multi-materials
Nguyen, Anh Quan. "Evaluation non destructive du gradient de teneur en eau dans les structures en béton armé par résistivité électrique." Thesis, Toulouse 3, 2016. http://www.theses.fr/2016TOU30380/document.
We propose a new measurement procedure that allows drawing a resistivity curve as a function of the spacing between current electrode and potential electrode which is linked to the investigation depth and the water content gradient. The resistivity measurements above the rebars are performed and an inversion procedure to assess the resistivity gradient is carried out. For the experimental part two kinds of measurement are performed on concrete slabs (reinforced and unreinforced). Two gradient types are studied: drying and wetting. The presence of the reinforcement significantly decreases the measurement of the resistivity. The measurements show the interest of the proposed procedure for the study of the water content gradient. Two measurement results were considered in numerical analyses. The simulation shows that it is possible to assess the resistivity gradient and the electrochemical parameters of the rebar via the Butler-Volmer model
Ling, Chao. "Simulation de la rupture ductile intragranulaire des aciers irradiés. Effets de l'anisotropie cristalline et du gradient de déformations." Thesis, Paris Sciences et Lettres (ComUE), 2017. http://www.theses.fr/2017PSLEM018/document.
Irradiation causes drastic modifications of mechanical properties of austenitic stainless steels and a decrease in the fracture toughness with irradiation has been observed. Ductile fracture due to void growth and coalescence remains one dominant fracture mechanism for doses in the range of 0-10 dupa. Voids may have different origins : nucleated at inclusions or irradiation-induced precipitates during mechanical loading, or produced directly by irradiation. The present work is to investigate ductile fracture of irradiated steels due to growth and coalescence of intragranulaire voids. Based on continuum crystal plasticity theory, FE simulations are performed on unit cells for studying effects of lattice orientation and stress triaxiality on void growth and coalescence. The influence of post-irradiation hardening/softening on void growth ans coalescence is evaluated with a physically based crystal plasticity model. Besides, an elastoviscoplastic model at finite strains is proposed to describe void growth up to coalescence in single crystals, and is assessed based unit cell simulations. The model is then applied to simulate ductile damage in single crystals ans polycrystals. As voids in irradiated steels may have different origins, they may have different sizes, which potentially have an influence on ductile fracture process and fracture toughness of irradiated steels. In order to assess the size effect, a micromorphic crystal plasticity model is proposed and applied to simulate growth and coalescence of intragranular voids of different sizes
Guichard, Cindy. "Schémas volumes finis sur maillages généraux en milieux hétérogènes anisotropes pour les écoulements polyphasiques en milieux poreux." Phd thesis, Université Paris-Est, 2011. http://tel.archives-ouvertes.fr/tel-00674503.
Dureau, Clément. "Fatigue d'aciers inoxydables austénitiques traités par grenaillage ultrasonore sévère : contributions expérimentales et numériques à l'étude de l'amorçage et la propagation des fissures." Electronic Thesis or Diss., Université de Lorraine, 2022. http://www.theses.fr/2022LORR0143.
In the high cycle fatigue, cracks initiate most of the time at the surface of workpieces. Therefore, in addition to the overall mechanical properties, surface and sub-surface characteristics such as roughness and residual stresses affect the fatigue life. Roughness essentially influences the cracks initiation phase. Indeed, the presence of surface irregularities induces stress concentrations producing high local strains potentially leading to the formation of crack-like defects. Besides, the presence of residual stresses affects both the crack initiation and propagation phase. They affect the plasticity close to the surface and influence the crack initiation. Moreover, superposed with the macroscopic loading, they locally modify the stress field and therefore may affect the crack propagation behavior as well.The ultrasonic shot peening (SMAT) is a surface mechanical treatment which consists in impacting a sample with shots put in motion by a vibrating device operating at frequencies up to 20 kHz. The repeated impacts lead to a surface plastic strain allowing the formation of compressive residual stresses as well as a microstructure gradient characterized by highly deformed zones in the sub-surface and submicronic grain size just below the surface. Such treatment was carried out to austenitic stainless steels in order to study the fatigue crack initiation and propagation in the complex microstructure and residual stress field induced by the SMAT.Uniaxial high cycle fatigue tests have been conducted for two different load ratios (under tension-compression at RTC=-1 and under tension-tension at RTT=0.1). They allowed to highlight the variable effectiveness of the SMAT with regard of the cyclic loading conditions. Indeed, at RTC, an increase of the fatigue limit was measured whereas for RTT a reduction of the fatigue limit was observed. In order to explain this difference, an in-depth study of the initial state and SMAT treated broken and run-out samples was carried out. It turns out that under the studied loading conditions the modifications of residual stress state can be considered as the primary factor governing the varying fatigue performances and the observed triggering at different initiation sites. Considering the stabilized surface residual stress after fatigue loading, the use of a Crossland criterion allowed to explain both the effects of load ratio and SMAT on the high cycle fatigue behavior of the stainless steel.A modelling method of the mechanical properties and residual stresses gradients was then developed using the finite elements method (via ABAQUS) in order to understand and predict the residual stresses redistributions. The results of the simulations were compared to the experimental measurements, and a good agreement was observed. The capacity of the model to simulate both the strain- and stress-controlled fatigue behavior was evaluated and a good consistency between the numerical and experimental results were obtained.Specimens with an artificial surface defect were then prepared in order to evaluate the surface anomalies sensitivity, and also to study the fatigue crack propagation behavior using the surface replication method. It was shown that the SMAT samples do not exhibit an increased sensitivity to the presence of defects compared to the initial state when loaded at RTT, whereas for RTC a slightly increased sensitivity was identified. Studying the crack propagation at the surface of the specimens highlighted different behaviors for different load ratios. Also, despite the presence of a defect, the crack initiation phase remained important. Finally, crack fronts were marked by different methods which permitted plotting the fatigue crack growth curves. It was then shown that at RTC, fatigue crack growth behavior in the SMAT layer is drastically different from the initial material, whereas at RTT no difference was revealed
Karolak, Cyprien. "CarboFrac : Analyse et modélisation de l'engrenage (d'un siège auto) en acier à faible teneur en carbone carbonitruré." Thesis, Paris Sciences et Lettres (ComUE), 2016. http://www.theses.fr/2016PSLEM057/document.
This work aims at a better understanding and modeling of the failure of gradient metallic materials. It is applied to carbonitrided pinions made out of 20MnB5 steel, inserted in a "recliner", a safety mechanism of automotive seats. Carbonitriding induces high surface hardness while preserving significant core ductility. The experimental analysis of the fracture behavior of seat recliners in an industrial test confirmed the dual failure behavior of the component : brittle external layer, ductile core material. A test bench has been specifically designed for the project: one tooth is submitted to a lateral force until complete failure. In situ observations are performed and the load-displacement curve recorded, showing a variety of behaviors as a function of the teeth engagement depth and of the presence or not of the carbonitrided layer. Experimental tests with various tress states were conducted to measure plastic properties as well as to calibrate fracture criteria, for the carbonitrided layer and for the core steel. Von Mises plasticity and a simple strain hardening curve fit very well all these experiments. As fracture criteria from the literature were unable to predict failure correctly for all the mechanical tests, an adapted criterion has therefore been proposed as an outcome of this extensive mechanical testing campaign. Fracture simulation in LS Dyna has been performed using the element erosion technique, the limitations of which are discussed. Comparison with the experimental tooth fracture measurements allows evaluation of the proposed failure criteria, and enables to stress out and discuss the present limits of the simulation, concluding that it will be necessary in future work to account more finely for the mechanical property gradient together with the compressive residual stresses in the carbonitrided layer
Jeyabalan, Karthikeyan. "Experimental study and prediction of microstructures and internal stresses during heat treatment of carburized and carbonitrided low-alloyed steels." Electronic Thesis or Diss., Université de Lorraine, 2022. http://www.theses.fr/2022LORR0379.
Carbonitriding thermochemical treatments are used in automotive industry for improving fatigue and wear resistance of mechanical parts. These treatments aim to generate gradients of carbon and nitrogen in the surface aera of the piece by diffusion in the austenitic field, and with the following quenching the desired gradients of microstructures, mechanical properties, and compressive residual stresses on the surface are obtained. The objective of the PhD thesis is to better understand the effects of carbon and nitrogen gradients on the development of internal stresses during cooling in relationship with the phase transformations as well as on the residual stresses distributions after cooling. The approach consists first of the elaboration of laboratory samples with controlled carbon and nitrogen gradients representative of the parts. Then, the kinetics of phase transformations and the internal stresses evolutions have been analysed experimentally by in situ High Energy (synchrotron) X-ray Diffraction throughout the chemical composition gradients thanks to a specific new methodology. To our knowledge, it is the first time internal stresses can be measured in situ during cooling in gradient specimens. Unexpected results have been obtained in nitrogen enriched samples; it has been shown that the chronology of phase transformations between core and surface is inversed as compared to the more classical case of carburizing leading to completely inversed residual stress profiles with tensile stresses in the nitrogen enriched layer and compression in the core. It has been related to the acceleration of transformation kinetics in the nitrided layer that decreases its hardenability. A coupled thermal-metallurgical-mechanical model has been developed too to predict temperature evolutions, phase transformations kinetics, internal stresses evolutions as well as final microstructure, hardness and residual stress distributions in the gradient samples. The metallurgical model developed in a previous study has been implemented in the finite element code Zebulon. The thermomechanical behaviour law of the material is thermoelastoviscoplastic including transformation strains (volumic variations and transformation plasticity strains). All material parameters (thermomechanical and thermophysical parameters) are considered as temperature, phase and carbon and nitrogen dependent; they have been determined from experimental characterizations on carbon and nitrogen homogeneously enriched specimen and in house data. The simulations allowed to confirm and understand more in details the complex microstructure and internal stresses evolutions due to combined nitrogen and carbon gradients. The comparison between calculated and experimental results shows that the simulation gives the main tendencies of the experimental observations. The main discrepancies on the level of residual stresses are attributed to the underestimation of the retained austenite fractions as the present model does not take into account its stabilization during cooling
Crisafulli, Daniela. "Advanced modelling of multilayered composites and functionally graded structures by means of Unified Formulation." Thesis, Paris 10, 2013. http://www.theses.fr/2013PA100055/document.
Most of the engineering problems of the last two centuries have been solved thanks to structural models for both beams, and for plates and shells. Classical theories, such as Euler-Bernoulli, Navier and De Saint-Venant for beams, and Kirchhoff-Love and Mindlin- Reissner for plates and shells, permitted to reduce the generic 3-D problem, in onedimensional one for beams and two-dimensional for shells and plates. Refined higher order theories have been proposed in the course of time, as the classical models do not consent to obtain a complete stress/strain field. Carrera Unified Formulation (UF) has been proposed during the last decade, and allows to develop a large number of structural theories with a variable number of main unknowns by means of a compact notation and referring to few fundamental nuclei. This Unified Formulation allows to derive straightforwardly higher-order structural models, for beams, plates and shells. In this framework, this thesis aims to extend the formulation for the analysis of Functionally Graded structures, introducing also the thermo-mechanical problem, in the case of functionally graded beams. Following the Unified Formulation, the generic displacements variables are written in terms of a base functions, which multiplies the unknowns. In the second part of the thesis, new bases functions for shells modelling, accounting for trigonometric approximation of the displacements variables, are considered
Lascaud, Julie. "Elaboration de couches minces atténuantes en silicium poreux : Application aux transducteurs ultrasonores capacitifs micro-usinés." Thesis, Tours, 2017. http://www.theses.fr/2017TOUR4026/document.
Capacitive micromachined ultrasonic transducers (CMUT) have emerged as a potential alternative to traditional piezoelectric transducers for ultrasound imaging. Along the years, CMUT processes have been evolved to enhance the device performances. In the meantime, no particular attention was paid on the silicon substrate, even if it is well-known that it could contribute to the transducer efficiency. The aim of this PhD thesis was to use porous silicon as a backing material for ultrasonic transducers to absorb a piece of the acoustic wave propagating in the substrate and which induce crosstalks in the acoustic signal. We show that porous silicon layer can be obtained on the rear side of already processed wafers without any damage on the performances of capacitive micromachined ultrasonic transducers. Finally, by means of acoustic characterizations and the transducer electroacoustic responses, we reveal the potential interest of porous silicon as backing material for ultrasonic transducers