A multi-step assembly process: drawing, flanging and hemming of metallic sheets

International audienceThis paper presents hemming tests on complex geometries, combining curved surfaces and radii of curvature in the plane. The samples are firstly prestrained in order to obtain a strain history prior to flanging and hemming. The choice of the sample geome-tries as well as prior plastic strains is based on a survey of current geometries hemmed in automotive doors. A device has been designed to hem these samples both by classical and roll-hemming processes and to allow a comparison between both technologies. Roll-in, which characterizes the change of geometry of the hemmed zone between flanging and hemming, and loads are obtained during this multistep process. Results show that roll-in observed in roll-hemming is lower than in classical hemming and that its evolution greatly differs between the two processes. The analysis of the results on different samples shows that it is difficult to establish rules on the variation of other parameters in such a complex multistep process and that it requires an intensive use of numerical simulation

Occurrence and numerical prediction of surface defects during flanging of metallic sheets

International audienceSurface defects can develop on automotive exterior panels after drawing and flanging steps, during springback and may alter significantly the vehicle quality. These defects are characterized by a small depth below 0.5 mm and are then difficult to detect numerically. This study focuses on the surface defects for two parts: an industrial upper corner of a front door and a L-shaped part designed on purpose to reproduce at a small scale surface defects that occur after flanging. Dimensions of these defects are measured from profiles obtained either with a non-contacting method or a tridimensional measuring machine. Numerical simulations of the processes are performed with the commercial codes PamStamp2G or Abaqus and deformed meshes are analyzed in the same way as the experimental data. There is a good correlation between experiments and simulations concerning the spatial position of the defect and their dimensions. Moreover, a buckling analysis during springback is performed for the L-shaped part, showing that the position of the defect corresponds to one of the buckling mode

Crystal plasticity and phenomenological approaches for the simulation of deformation behavior in thin copper alloy sheets

In the expanding context of device miniaturization, forming processes of ultra thin sheet metals are gaining importance. Numerical simulation of these processes requires accurate material modeling. In this study, both the phenomenological modeling approach and the crystal plasticity finite element method (CPFEM) are considered. Theoretical definitions of both models, numerical implementation as well as their parameter identification procedures are outlined. Subsequently they are compared on a one to one basis, mainly with regards to their ability to predict mechanical responses for a variety of strain loading paths.Agence Nationale de la Recherche, ANR-12-RMNP-0009-0

Finite element simulation of 3D mechanical behaviour of NiTi shape memory alloys

International audienceA three-dimensional finite element model of the isothermal deformation of shape memory alloys has been developed in order to analyze and predict the mechanical behaviour of NiTi alloys. A general 3D kinematic has been studied. The constitutive behaviour is written using an elastohysteresis tensorial scheme; it is based on the splitting of the Cauchy stress tensor into two fundamental stress contributions of hyperelastic and pure hysteresis types respectively. The equilibrium equations are then discretized by the finite element method. The validity of this formulation is established in the case of three-dimensional plate bending behaviour of NiTi shape memory alloys

Multi-stage hot forming process of shackles

This study deals with the multi-stage forming process of a sling shackle, investigated with the use of finite element simulation. Used as a lifting connector during Floating Production Unit assembly and provision of supplies for petroleum platforms, Wide Body (WB) shackles endure severe working conditions in hostile environment. Providing high level of strength and toughness as well as outstanding fatigue behavior, hot forming is one of the main production method of shackles. Aiming towards process improvement and stronger shackles development, the main purpose of this study is, with the help of numerical simulation, to understand, predict and control all aspects that can be involved in the manufacturing of a WB 125 tons shackle. In close collaboration with forging industry Le Béon Manufacturing in Lorient area, forging experiments were carried out on a 0.3MJ friction drive screw press and at temperatures higher than 900°C. In order to form both jaws, two successive sequences of closed-die forging and deflashing are performed. It follows an open-die forging stage to form the body. Finally, the crown is obtained using a ram bending process. The numerical model presented herein is developed using a fully coupled 3D thermomechanical analysis and is conducted on Abaqus software. Tools are made of AISI H13 with constant thermophysical properties. Mechanical properties of the 34CrNiMo6 used for the workpiece at temperatures ranging from 900to 1200°C and strain rates ranging from 0.1 to 1/s were extracted from the literature. To describe the compression behavior, the Johnson-Cook flow stress model has been used. Temperatures, duration and kinematic movements have been recorded over the production of 80shackles, and forging loads were evaluated by instrumenting strain gauges directly on the mechanical press. A particular methodology has been developed to perform successively the numerical simulation of the various forming operations with a unique workpiece. It combines a sequence of Abaqus/Implicit-HT, Abaqus/Dynamic-Explicit and Abaqus/Static-Implicit. In all simulations, convective and radiative cooling is taken into account and heat transfer between the tools and the workpiece is considered. As in the industrial process, workpiece stability and forging accuracy are obtained with the use of a manipulator modeled as an analytical rigid surface and a combination of springs to account for the recoil and shock absorption. A remeshing method has also been developed to thwart excessive distortion in the flash and the body

Manifestation de l'effet Portevin-Le Châtelier en cisaillement simple

Les alliages d’aluminium de la série 5000 pourraient avantageusement remplacer les alliages de la série 6000, plus coûteux, notamment dans la fabrication des ouvrants automobiles. Cependant, l’effet Portevin-Le Châtelier (PLC) qui se manifeste dans ces alliages au cours de leur déformation plastique freine grandement leur utilisation pour les pièces d’aspects. En effet, il conduit notamment à une diminution de la formabilité et il est à l’origine de marques donnant un aspect rugueux aux pièces obtenues. Par exemple, les alliages Al-Mg sont connus pour présenter un effet PLC à température ambiante, caractérisé par des serrations sur la courbe d’écoulement et une déformation plastique localisée dans des bandes au cours de la déformation globale. Ce phénomène est dû au vieillissement dynamique, une interaction entre les atomes de soluté de magnésium et les dislocations mobiles. Bien que de nombreuses études de l'effet PLC aient déjà été réalisées, le phénomène a été observé principalement en traction. Compte tenu des différents trajets de déformation rencontrés dans les procédés de mise en forme des tôles, il semble intéressant d’élargir les connaissances sur les conditions d'apparition de l'effet PLC sous divers états de chargement. Dans ces travaux, les courbes d’écoulement d’un alliage AA 5754-O lors d’essais de cisaillement simple pour différentes vitesses de déformation et pour différentes températures sont étudiées. Il est montré que l'effet PLC peut être observé pour cet état de contrainte. Les observations directes de la surface des éprouvettes en utilisant une technique de corrélation d'images permettent l'étude de la nature et de la cinématique des bandes en fonction de la contrainte de cisaillement, de la vitesse de déformation et de la température. Enfin, des simulations numériques par éléments finis utilisant le modèle de McCormick [1] sont présentées pour expliquer la cinématique des bandes PLC observées expérimentalement. [1] McCormick P.G., Theory of flow localisation due to dynamic strain ageing, Acta Metallurgica, 1988, 36 (12) 3061-306

Micro-formage de composants à partir de tôles ultra-fines en alliages de cuivre

La miniaturisation de nombreux produits et systèmes entraine le développement permanent de micro -systèmes électro mécaniques (M.E.M.S). En raison de leurs taux de production élevés, les procédés de mise en forme demeurent la solution technologique la plus courante pour la fabrication de ces pièces miniatures. Toutefois, en raison des dimensions et épaisseurs (de l'ordre de 100microns ) en jeu, les procédés de micro-formage se révèlent instables et affectés par une grande variabilité. Nos travaux visent à mettre en place une modélisation numérique efficace et précise de ces procédés dans le but de s'en servir comme outil d'optimisation des outillages et procédés. Les deux approches de modélisation développées sont mises en oeuvre sur le cas industriel de pliage des leads d'un boitier électronique LQFP

Benchmark 3 - springback of an Al-Mg alloy in warm forming conditions

Accurate prediction of springback is a long-standing challenge in the field of warm forming of aluminium sheets. The objective of this benchmark is to predict the effect of temperature on the springback process through the use of the split-ring test [1] with an Al-Mg alloy. This test consists in determining the residual stress state by measuring the opening of a ring cut from the sidewall of a formed cylindrical cup. Cylindrical cups are drawn with a heated die and blank-holder at temperatures of 20, 150 and 240°C. The force-displacement response during the forming process, the thickness and the earing profiles of the cup as well as the ring opening and the temperature of the blank are used to evaluate numerical predictions submitted by the benchmark participants. Problem description, material properties, and simulation reports with experimental data are summarized

Lois de comportement et mise en forme des matériaux métalliques

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Mechanical behavior of fabric-reinforced elastomer straight flexible hoses

This paper presents a model applicable to large strain analysis of composite materials such as flexible hoses. A three-dimensional constitutive law, which uses the concept of convected coordinate frame, is developed for materials presenting non linear elastic and linear orthotropic behaviors in the large deformation field. The proposed model is implemented in a finite element home code and the formulation of both the hyperelastic and the orthotropic constitutive laws is presented. Finally, in order to validate our numerical results, an experimental device is developed to exhibit the mechanical behavior of straight flexible hoses in the case of elementary tests. The model is then compared to these experimental results