Vers une optimisation de la tenue à la fatigue de pièces forgées intégrant l'histoire mécanique des matériaux

International audienceLes pièces métalliques forgées présentent un fibrage qui leur confère des propriétés mécaniques anisotropes. L'objectif de notre étude consiste à définir la notion de fibrage dans le cadre de la simulation numérique, et à l'utiliser dans l'estimation de la durée de vie de pièces en fatigue. Nous nous focalisons ici sur la fatigue à haut nombre de cycles, et nous traitons les aciers contenant des inclusions malléables supérieures à une taille caractéristique. Un nouveau critère de fatigue multiaxial est proposé, sur la base du critère de Murakami, et a été implémenté dans le logiciel éléments finis FORGE3®. Une variable représentant le fibrage est calculée lors de la simulation de forgeage, puis utilisée lors du calcul de fatigue anisotrope. Les contraintes résiduelles obtenues en fin de forgeage sont elles aussi prises en compte dans notre modèle. Enfin une chaîne de simulation virtuelle, intégrant les simulations de forgeage et de fatigue, est réalisée. Les résultats obtenus montrent que l'on peut améliorer la tenue à la fatigue des pièces forgées en modifiant les conditions de mise en forme, telles que le fibrage initial ou encore la géométrie de la pièce. = Forging of metal components induces grain flow orientation, which gives rise to anisotropic mechanical properties. Our study deals with the definition and the use of this grain flow anisotropy into fatigue life calculation. We have focused on high cycle fatigue, and we deal with steels containing malleable inclusions above a critical size. A new multiaxial criterion has been developed on the basis of the Murakami equation. This criterion has been implemented in the finite element code Forge 3®. This computational tool uses a new variable to represent the grain flow orientation provided by forging simulation to account for subsequent fatigue anisotropic behaviour. Moreover, residual stresses resulting from forging are also taken into account to perform the fatigue calculation. Finally, a virtual simulation chain, including Forging and Fatigue calculation is carried out. The final goal is to improve fatigue properties by modifying forging conditions

Approche multi-échelles pour l'étude de l'anisotropie induite par le forgeage en fatigue à grand nombre de cycles = A multiscale finite element approach to deal with high cycle fatigue anisotropy induced by forging

Cet article est basé sur l'exposé présenté lors des Journées annuelles de la SF2M, JA 2010, Paris, 22-24 juin 2010. La publication originale est disponible sur le site http://www.revue-metallurgie.orgInternational audienceLes pièces de forge sont universellement reconnues pour leurs bonnes propriétés mécaniques, notamment en fatigue. L'approche proposée ici consiste à intégrer la simulation du forgeage dans le dimensionnement en fatigue des pièces forgées. Le fibrage et le taux de corroyage sont deux caractéristiques principales du forgeage. À l'aide du logiciel FORGE®, le fibrage est calculé tout au long des opérations de mise en forme. Ce fibrage, ainsi que les contraintes résiduelles, sont ensuite introduits dans des outils de dimensionnement pour améliorer la prédiction du calcul de durée de vie en fatigue. Les critères de fatigue actuels, basés sur des modèles isotropes, ne permettent pas de valoriser le sens long de ce fibrage, et par là les pièces de forge, car c'est en général dans ce sens que ces pièces sont le plus sollicitées. Une extension anisotrope du modèle de Papadopoulos est proposée sur la base d'une campagne expérimentale effectuée sur des éprouvettes prélevées à 0°, 45° et 90° par rapport à la direction de fibrage. Une modélisation à l'échelle micro (DIGIMICRO) permet également de mieux comprendre le rôle des inclusions sur les sollicitations en fatigue. La mise en place d'une chaîne de simulation virtuelle globale à l'échelle d'un composant industriel, associée à des modélisations microstructurales, permet de mieux comprendre et de quantifier le rôle du fibrage sur les propriétés en fatigue à grand nombre de cycles des pièces forgées = Forged components are recognized for their good mechanical strength and fatigue properties. The methodology presented here consists in improving fatigue analyses of forged components by accounting for the forging simulation stage. Kneading rate and grain flow orientation are two consequences of the forging process. Using the FORGE® software, grain flow orientation is computed all along the forming process simulation. This grain flow orientation, as well as residual stresses, are then used as input data to enhance the fatigue analysis. Usual isotropic fatigue criteria are usually unable to demonstrate the higher fatigue properties of forged components along the grain flow orientation. An anisotropic extension of the Papadopoulos fatigue criterion is defined here. This extension is based on experimental fatigue results obtained on samples extracted at 0°, 45° and 90° with respect to the grain flow orientation. A numerical modelling is performed at the microscale using the DIGIMICRO software. These simulations give a better understanding on the influence of elongated particles and cluster of particles on high cycle fatigue mechanisms. A virtual simulation chain is set-up to work on real industrial components. This simulation chain, together with microscale numerical modelling demonstrate the positive influence of the grain flow orientation of forged components on high cycle fatigue properties of industrial parts

Oxidative stress biomarkers are associated with visible clinical signs of a disease in frigatebird nestlings

Infectious diseases are one of the most common threats for both domestic and wild animals, but little is known about the effects on the physiological condition and survival of wild animals. Here, we have tested for the first time in a wild vertebrate facing a viral disease possibly due to herpesvirus (i) whether nestlings with either low levels of oxidative damage or high levels of antioxidant protection are less susceptible to develop visible clinical signs, (ii) whether the disease is associated with the nestlings' oxidative status, (iii) whether the association between the disease and oxidative status is similar between males and females (iv), and whether cloacal and tracheal swabs might be used to detect herpesvirus. To address our questions, we took advantage of a population of Magnificent frigatebirds (Fregata magnificens) whose nestlings have experienced high mortality rates in recent times. Our work shows that (i) blood lipid oxidative damage is associated with observable clinical signs and survival probabilities of nestling frigatebirds, and (ii) that high glutathione levels in red blood cells are associated with the emergence of visible clinical signs of the disease. Our work provides evidence that differences in the oxidative status of nestlings might underlie individual health and survival

Finite Element Modeling of Tube Piercing and Creation of a Crack

From the issue entitled "Proceedings of the 11th ESAFORM Conference on Material Forming, Lyon (France), 23-25 April 2008, edited by P. Boisse, F. Morestin, E. Vidal-Sallé, LaMCoS, INSA de Lyon)" - http://esaform2008.insa-lyon.fr/proceedings/MS06/p_Ch_571.pdfInternational audienceA 3D simulation of Mannesmann tube piercing is performed using the finite element software Forge 2005®. The sensitivity of the simulation results to numerical methods and physical parameters is discussed. Advanced numerical schemes and refined time discretizations are required to obtain correct descriptions of the material flow. In this study, one concentrates on the stress state and damage development before the material comes in contact with the plug. Indeed, the crack is to appear prior to the action of the plug. The description of the material behaviour is found to be a key information to predict the crack development. Predictions based on a modified Lemaitre damage law and a normalised Latham and Cockroft criterion are compared

Multiaxial fatigue criterion accounting for anisotropy in forged components

International audienceNumerical modelling of fatigue behavior for anisotropic structures has become critical for design applications. This is particularly true for forged components due to the intrinsic anisotropy of the material resulting from the process. The aim of this study is to relate the microstructure scale to the process scale, i.e. the engineer scale. Anisotropy induced by the forming process and the most relevant feature which results from forging, is the preferential orientation of structural defects and grains in the direction of the deformation. Grain flow is modelled using a fiber vector at the level of the representative elementary volume. It can then be used to improve and refine the Papadopoulos fatigue criterion by taking into account fatigue limits for each direction of anisotropy. In practice, it is very tedious to determine precisely these fatigue limits and impossible to obtain experimentally all of them for each direction of uniaxial loading. To circumvent this difficulty, we simulate the problem at the microstructure scale by considering fiber vector as the result of the inclusion and grain orientation. Microstructures are then precisely modelled using DIGIMICRO software. A representative elementary volume including inclusions is meshed and high cycle fatigue simulation is performed. The results can be used in order to optimize the preform of the component before simulation

Advanced numerical method for generation of three-dimensional particles and its application in microstructure-based simulation of fatigue behavior

International audienceThe topology of representative elementary volumes (REV) generated to model materials microstructure is getting more and more complex. This paper presents advanced mesh generation methods used to improve the description of 3D microstructural particles. The goal is to adapt easily the shape of the elements at the interface between the isotropic matrix and embedded inclusions. Two methods are described in this work to generate inclusions: an analytical method based on statistical experimental data and a reconstruction approach, based on tomographic imaging. Sensitivity analyses on meshing parameters are performed to obtain efficient data in order to reconstruct the most representative volume and to perform subsequent accurate numerical computations. As an example of calculations, fatigue tests are chosen to validate the proposed approach

Behavioral and physiological responses to male handicap in chick-rearing black-legged kittiwakes

Parental investment entails a trade-off between the benefits of effort in current offspring and the costs to future reproduction. Long-lived species are predicted to be reluctant to increase parental effort to avoid affecting their survival. We tested this hypothesis in black-legged kittiwakes Rissa tridactyla by clipping flight feathers of experimental males at the beginning of the chick-rearing period. We analyzed the consequences of this handicap on feeding and attendance behavior, body condition, integument coloration, and circulating levels of corticosterone and prolactin in handicapped males and their mates in comparison to unmanipulated controls. Chicks in both groups were compared in terms of aggressive behavior, growth, and mortality. Handicapped males lost more mass, had less bright integuments, and attended the nest less often than controls. Nevertheless, they fed their chicks at the same rate and had similar corticosterone and prolactin levels. Compared with control females, females mated with handicapped males showed a lower provisioning rate and higher nest attendance in the first days after manipulation. Their lower feeding rate probably triggered the increased sibling aggression and mortality observed in experimental broods. Our findings suggest that experimental females adaptively adjusted their effort to their mate's perceived quality or that their provisioning was constrained by their higher nest attendance. Overall, our results suggest that kittiwake males can decrease their condition for the sake of their chicks, which seems to contradict the hypothesis that kittiwakes should be reluctant to increase parental effort to avoid affecting their surviva