Cette thèse traite de la prise en compte de l'impact des procédés d'assemblage thermomécanique sur la géométrie de la caisse en blanc lors de la phase de conception d'un véhicule, chez PSA Peugeot-Citroën. Pour cela, on souhaite développer un outil de prévision des déformations induites par les procédés de soudo-brage - plasmatron et laser - lors de l'assemblage du pavillon sur le côté de la caisse. Les pièces - de dimensions métriques - sont composées de tôles fines d'épaisseur 0,67 mm en acier XES. Afin de réaliser une simulation numérique du procédé, il est nécessaire d'identifier un certain nombre de modèles, objets de cette thèse. Dans un premier temps des essais de soudo-brasage en laboratoire ont été réalisés afin de définir les domaines de validité de l'étude, d'identifier des modèles de sources de chaleur, et de déterminer les coefficients d'échange thermique avec l'extérieur. Une étude spécifique d'identification des lois de comportement des matériaux mis en jeu, sur toute la gamme de température, a également été réalisée. Pour cela, une méthode de mesure de champs par corrélation d'images numériques à haute température a été développée, qui a permis d'identifier complètement les paramètres de la loi de comportement de l'acier XES. Pour finir, de façon à réaliser la validation de la simulation numérique, ces travaux de thèse ont été l'occasion de développer une nouvelle méthode de calibration pour la stéréocorrélation d'images basée sur la connaissance a priori de la géométrie de l'objet observé, en considérant sa forme théorique via son modèle CAO. Cette méthode est tout particulièrement adaptée aux dimensions des structures étudiées ici. / This study is about the impact of thermo-mechanical assembly processes of metal sheets, in the automotive industry context. The aim is to predict thermally induced deformations by using a numerical tool. We are particularly interested in Plasmatron and laser brazing processes that are used to assemble an automotive roof and the body side of the vehicle. Parts are made from 0,67 mm thickness XES thin metal sheet of about one meter length. To validate such complex non-linear numerical simulations with experimental observations, it is necessary to develop well-controlled and highly instrumented tests. In a first step, experimental welding brazing tests are carried out in order to identify validity domains, heat source model and thermal coefficients of exchange. In a second step, high temperature tension tests are performed to identify the behaviour of each material. These tests are instrumented with a new DIC protocol in order to measure displacement fields from 20°C to 1000°C. Finally, in order to validate the thermo-mechanical simulation, in-situ 3D Digital Image Correlation (stereo-correlation) measurements were performed during the welding-brazing assembly. For this purpose and due to the geometry of the involved parts, a new calibration method, based on the CAD part geometry has been developed.

La nécessité de protéger les véhicules de rentrée atmosphérique a conduit à la sélection des matériaux carbonés pour les applications les plus extrêmes [1, 2]. Au vu du coût prohibitif des essais en conditions réelles [1], la simulation permet de prédire et comprendre le comportement du matériau lors de la rentrée atmosphérique. Les modèles de l’ablation du carbone n’ont cessé de se complexifier [1, 3, 4, 5, 6]. Cependant, ils ne donnent pas de descriptions fines de la surface du matériau. C’est pour remédier à cela que plusieurs études ont été lancées au Laboratoire des Composites Thermostructuraux (LCTS) [7, 8]. Le Plasmatron du VKI est le moyen à haut flux de chaleur et à haute vitesse utilisé pour cette thèse [9, 10]. Dans cette étude, des échantillons sont testés sous des flux de chaleur allant jusqu’à 2800 kW/m2. Ces essais sur le composite 3D carbone/carbone (Cf/C) ont mené à des résultats très intéressants sur la morphologie du composite lors de l’ablation. Le four à image d’arc est le moyen à très haut flux de chaleur et à convection nulle utilisé au cours de cette thèse. Des flux de chaleurs de 10 MW m-2 ont pu être appliqués à des échantillons de 3D Cf/C. Les observations MEB couplées à la reconstruction numérique du four ont permis de noter les différences entre l’oxydation et la sublimation du composite. Un code de diffusion/ablation est développé depuis plusieurs années au LCTS [7, 11, 12, 13, 14] : AMA. Il permet de simuler l’ablation d’un matériau en carbone. Ce code a été validé par J. Lachaud sans prise en compte de l’écoulement [7]. Un nouveau couplage avec le code Open-Foam [15] a été développé et validé. Le code AMA a été utilisé afin de déterminer les paramètres influençant la rugosité du 3D Cf/C sous un écoulement laminaire. / Carbonaceous materials have been selected to protect atmospheric re-entry vehicles under the most extremes conditions [1, 2]. Because real flight experiments are very expensive [1], simulation is an important means to predict and understand the material behaviour during atmospheric re-entry. The models of carbonaceous materials ablation have become more and more complex [1, 3, 4, 5, 6]. However, they are not accurate to describe finely the material surface. To find a solution to this problem, several studies have been conducted at the Laboratoire des Composites Thermostructuraux (LCTS) [7, 8]. The Plasmatron of the VKI is the experimental means which has been used in this doctoral thesis. It can reach moderately high heat flux under high speed flow [9, 10]. In this study, several samples have been tested under an heat flux up to 2800 kW/m2. These tests on the 3D carbon/carbon (Cf/C) composite have given very interesting results about the material morphology during ablation. An arc image furnace, which can reach very high heat flux but without important flow field, has been used during this doctoral study. 3D Cf/C samples have been ablated under heat flux up to 10 MW m-2. SEM observations with the help of a numerical simulation of the furnace, have put the differences between oxidation and sublimation forward. A diffusion/ablation code has been developed at LCTS for several years [7, 11, 12, 13, 14] : AMA. This piece of software is designed to simulate the ablation of a carbonaceous material. It has been validated without flow field by J. Lachaud [7]. A new coupling method between AMA and Open-Foam [15] has been developed and validated. Finally, AMA has been used to point out the parameters which drive the 3D Cf/C surface recession under laminar flow field.

In the automotive industry a commonly used material is thin steel sheets coated with a thin layer of zinc for corrosion resistance purposes. Welding of this material, with the high temperatures involved, causes problems with zinc burn-off leading to reduced corrosion resistance. The zinc evaporation also causes arc disturbances leading to spatter formation, pores and difficulties achieving good visual weld quality. The joints are in many cases visible or semi-visible (e.g. visible after opening a door) and “A-grade” quality is demanded, meaning no visible discontinuities are allowed. It also implies a smooth and generally appealing appearance of the joint.An alternative to welding is brazing, and laser brazing meets the requirements but the process is associated with high costs. In the national project “LEX-B”, funded by Vinnova, the possibilities of using arc brazing, in particular one MIG/MAG brazing and two TIG brazing processes, for some automotive applications are investigated. This master thesis is connected to the first part of the project and aims at compiling data for making a selection of the most promising process to investigate and optimise further. LEX-B is conducted in cooperation between Swerea KIMAB, Volvo Trucks, Scania CV and University West. The joint of interest is a lap joint between bottom and upper sheets of 1.2 mm and 0.8 mm respectively that represents a joint on the side of a truck cabin. The requirements are visual A-grade quality and tensile shear strength of 300 MPa.In the project the MIG/MAG process Cold Metal Transfer (CMT) was compared to the TIG processes forceTIG and Plasmatron. CMT was experimentally investigated while the results for the other two processes were obtained partly from a previous study and partly from Volvo Trucks where tests were performed simultaneously. A system for measuring data for the process was developed, test specimens were brazed and examined visually and mechanical destructive testing was performed to ensure the tensile shear strength. Parameter studies were done for further process optimisation. / Inom fordonsindustrin är ett ofta använt material stål i tunna ark belagda med zink p.g.a. zinkets korrossionsskyddande egenskaper. Vid svetsning av detta material uppstår problem med zink som förångas vilket leder till minskad korrosionsbeständighet. Förångningen av zink orsakar också störningar av ljusbågen vilket leder till sprut, porer och svårighet att uppnå god visuell svetskvalitet. Fogarna är i många fall synliga eller delvis synliga (t.ex. synlig efter att ha öppnat en dörr) och "A-kvalitet" efterfrågas, vilket innebär att inga synliga diskontinuiteter tillåts. Fogen ska upplevas jämn och tilltalande.Lödning är ett alternativ till svetsning, och laserlödning är en process som uppfyller kraven men är förknippad med höga kostnader. I det nationella projektet "LEX-B", som finansieras av Vinnova, undersöks möjligheterna att använda båglödning, specifikt en MIG/MAG-process och två TIG-processer, för dessa applikationer. Detta examensarbete är anslutet till den första delen av projektet och syftar till ta fram underlag för att välja den mest lovande av dessa processer för vidare utredning och optimering. LEX-B sker i samarbete mellan Swerea KIMAB, Volvo Lastvagnar, Scania CV och Högskolan Väst. Den undersökta fogen är en överlappsfog mellan en undre plåt med 1,2 mm tjocklek och en övre plåt med 0,8 mm tjocklek. Detta representerar en fog på sidan av en lastbilshytt. Kraven är visuell A-kvalitet och en drag-skjuvhållfasthet av 300 MPa.I projektet jämfördes MIG/MAG-processen Cold Metal Transfer (CMT) med TIG-processerna forceTIG och Plasmatron. CMT testades experimentellt medan resultaten för de övriga två processerna erhölls dels från tidigare tester och dels från Volvo Lastvagnar där tester utfördes parallellt. Ett mätsystem for att dokumentera processdata utvecklades och de framställda lödfogarna undersöktes visuellt och med mekanisk förstörande provning för att säkerställa drag-skjuvhållfastheten. Parameterstudier gjordes för vidare optimering av processen.

A numerical model for inductive plasma wind tunnels is developed. This model provides the flow conditions at the edge of a boundary layer in front of a thermal protection material placed in the plasma jet stream at the outlet of an inductive torch. The governing equations for the hydrodynamic field are derided from the kinetic theory. The electromagnetic field is deduced from the Maxwell equations. The transport properties of partially ionized and unmagnetized plasma in weak thermal nonequilibrium are derived from the Boltzmann equation. A kinetic data base of transport collision integrals is given for the Martian atmosphere. Multicomponent transport algorithms based upon Krylov subspaces are compared to mixture rules in terms of accuracy and computational cost. The composition and thermodynamic properties in local thermodynamic equilibrium are computed from the semi-classical statistical mechanics. The electromagnetic and hydrodynamic fields of an inductive wind tunnel is presented. A total pressure measurement technique is thoroughly investigated by means of numerical simulations.

A numerical model for inductive plasma wind tunnels is developed. This model provides the flow conditions at the edge of a boundary layer in front of a thermal protection material placed in the plasma jet stream at the outlet of an inductive torch. The governing equations for the hydrodynamic field are derided from the kinetic theory. The electromagnetic field is deduced from the Maxwell equations. The transport properties of partially ionized and unmagnetized plasma in weak thermal nonequilibrium are derived from the Boltzmann equation. A kinetic data base of transport collision integrals is given for the Martian atmosphere. Multicomponent transport algorithms based upon Krylov subspaces are compared to mixture rules in terms of accuracy and computational cost. The composition and thermodynamic properties in local thermodynamic<p>equilibrium are computed from the semi-classical statistical mechanics.<p>The electromagnetic and hydrodynamic fields of an inductive wind tunnel is presented. A total pressure measurement technique is thoroughly investigated by means of numerical simulations.<p> / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished