De nos jours, les politiques environnementales sont devenues plus strictes envers l’industrie automobile pour réduire les émissions de CO2, donc les structures légères utilisant des matériaux de haute résistance sont d’un grand intérêt. Deux modèles différents EF, à savoir "Solid Refine Model" (SRM) et "Shell Coarse Model " (SCM) ont été développés et sont utilisés comme modèles standard par Faurecia Automotive Seating (Caligny). Le SRM est capable de prédire avec précision le comportement de soudage local, mais malheureusement, en raison de son coût de calcul élevé, le SRM n’est pas adapté à une modélisation de si`ge de voiture complète. D’autre part, le SCM est efficace sur le plan numérique, mais il ne peut pas prédire le comportement de la ligne de soudure. L’objectif de la présente thèse est de développer un modèle EF multi-matèriel dans le logiciel commercial Ls-dyna, qui améliorera le SCM pour permettre une prédiction précise du comportement de la ligne de soudure jusqu’à l’échec avec un coût de calcul raisonnable. Le modèle FE quadrilatère standard est développé et enrichie à l’aide d’une méthode récemment développée appelée "Interpolation Covers Method" (ICM) pour capturer les gradients de la solution avec précision sans raffinement de maillage. Un modèle de matériau élasto-plastique est développé dans le logiciel commercial Ls-dyna qui prend en compte deux matériaux différents à savoir BM et HAZ dans un seul élément de coque. Le modèle généralisé d’endommagement dépendant de l’état de contrainte a été implémenté comme UMAT dans le logiciel commercial Ls-dyna pour prédire l’échec de la ligne de soudure dans SCM. Les différents développements ont permis au SCM de prédire avec précision le comportement complexe de la ligne soudée jusqu’à l’échec, à faible coût de calcul compatible avec les besoins industriels. / Nowadays environmental policies have become more strict towards the automotive industry to reduce the CO2 emission, therefore lightweight structures using high strength materials have become of great interest. Two different FE models namely “Solid Refine Model” (SRM) and “Shell Coarse Model” (SCM) have been developed and are being used as standard models by Faurecia Automotive Seating (Caligny). The SRM is capable to predict accurately the local welding behavior but unfortunately, due to its high computational cost, the SRM is not suitable for a full car seat modeling. On the other hand, the SCM is computationally efficient but it cannot predict the weld line behaviour. The aim of the present thesis is to develop a multimaterial FE model within the Ls-dyna commercial software, which will enhance the SCM to allow the accurate prediction of weld line behavior until failure with a reasonable computational cost. The standard quadrilateral shell FE is developed and enriched using a recently developed method called the “Interpolation Covers Method” (ICM) to capture the solution gradients accurately without mesh refinement. An elasto-plastic material model is developed within Ls-dyna commercial software which takes into account two different materials namely BM and HAZ inside a single shell element. The Generalized Incremental Stress State dependent damage Model has been implemented as a UMAT within Ls-dyna commercial software to predict the weld line failure in SCM. The different developments have allowed the SCM to become able to predict the complex behavior of the welded line accurately until failure, at low computational cost compatible with the industrial needs.

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The reduction of fuel consumption and carbon dioxide emissions are currently a key factor for the aviation industry due to major concerns about climate change and more restrictive environmental laws. One way to reduce both fuel consumption and CO2 emissions is by significantly decreasing the vehicle’s weight while increasing engine's efficiency. In order to meet these requirements, the European aero engine industry is continuously focusing on alternative manufacturing methods for load carrying structures in advanced materials, such as titanium and nickel-based superalloys. Alternatively to traditional large-scale single castings, new manufacturing methods involve sheet metal parts, small castings and forgings assembled by welding. These new manufacturing methods allow more flexible designs in which each part is made of the most suitable material state, leading to several advantages such as reduction of product cost and weight while increasing engine's efficiency. Nickel-based superalloys are widely used in the aero engine industry, typically constituting up to 50% of the total weight of the aircraft engine. Due to their excellent material properties at high temperatures in severe corrosive environments, these superalloys are employed most extensively in the hot sections of gas turbine engines for both military and civil aircrafts with running temperatures up to 650°C. In this thesis, a manufacturing process chain including forming and welding in the nickelbased superalloy 718 is studied. The main focus in the work lies on determining the thermomechanical properties, modelling and simulation of cold forming, study forming limits based on Nakazima tests for forming limit curves (FLC) and applying a damage and failure criterion. The work also comprises a brief study on hot forming. Finally, modelling of a subsequent welding procedure is included where residual stresses from the forming simulation are used to predict shape distortions due to the welding procedure. The results are compared with experimental observations. The cold forming procedure of a double-curved component made of alloy 718 is studied using FE-analyses and forming tests. The same geometry was used to produce a hot forming tool. During forming tests at room temperature, micro cracks and open cracks were observed in the draw bead regions, not indicated when formability is assessed using a forming limit curve (FLC). Standard material models such as von Mises or Barlat Yld2000-2D were not capable of accurately predict the behaviour of the material after the point of diffuse necking, making the prediction of damage and failure during forming a challenge. The GISSMO damage model was therefore calibrated and used to predict material failure in forming of alloy 718. Tensile, plane strain, shear and biaxial tests at room temperature are performed up to fracture and continuously evaluated using Digital Image Correlation (DIC) by ARAMIS™. In this work, the GISSMO damage model is coupled with the anisotropic Barlat Yld2000-2D material model for forming simulations in alloy 718 at room temperature using LS-DYNA. Numerical predictions are able to accurately predict failure on the same regions as observed during the experimental forming tests. Comparisons of the distribution of damage on one of the draw beads between simulations and damage measurements by acoustic emission indicate that higher damage values correspond to bigger micro cracks. Numerical FE-predictions of the cold forming and subsequent welding procedure shows that the welding procedure further increases the shape distortions. This was found to agree with experimental observations. / Virtuell processkedja för plåtformade flygmotorstrukturer i superlegeringar – Validering och demonstrator

Finite element simulation of thermoplastic components is gaining importance as the companies aim to avoid overdesign of the components. Cost of the component can be minimized by using an adequate amount of material for its application. Life of the component, in a particular application, can be predicted as early as during its design phase with the help of computer simulations. To achieve reliable simulation results, an accurate material model which can predict the material behaviour is vital. Most material models consist of a number of material parameters that needs to be fed into them. These material parameters can be identified with the inputs from physical tests. The accuracy of the data extracted from the physical tests, however, remains the base for the aforementioned process. The report deals with the implementation of optical measurement technique such as Digital Image Correlation (DIC) in contrast with the conventional extensometers. A tensile test is conducted on a glass fibre reinforced thermoplastic specimen, according to ISO 527-2/1A, to extract the experimental data with the help of DIC technique. The material behavior is reproduced within a finite element analysis software package LS-DYNA, with the combination of elastoplastic model called *MAT_024 and stress state dependent damage and failure model called GISSMO. The tensile test is performed under quasi-static condition to rule out the strain rate dependency of the thermoplastic material. The mesh sensitivity of the damage model is taken into account with the element size regularization. The thesis concerns setting up a routine for material parameter identification of thermoplastics by full-field calibration (FFC) approach. Also, comparison of the strain field in the specimen, obtained through the newly set up routine against the regular non-FFC i.e. extensometer measurement routine. The major objective being, through the comparisons, a qualitative assessment of the two routines in terms of calibration time vs. gain in simulation accuracy. Material models obtained through both the routines are implemented in three-point and four-point bending simulations. The predicted material behaviors are evaluated against experimental tests.

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