Presentation of KI-COF, a phenomenological model of variable friction in fretting contact

International audienceIn this paper, a new phenomenological model, called KI-COF is developed to account for variable coefficient of friction (COF) in space and time. The COF is no longer considered as a global value valid for the whole contact area. A local value is introduced instead, the evolution of which is governed by the local history of the contact and the amount of slip

Evaluation of local stress and strain state at notch root by means of a new method valid for multiaxial random loadings

International audienceNeuber type methods are widely used to predict the local stress-strain behaviour at notch root in specimens or industrial components. Some limitations of these methods are pointed out in the present paper, especially when the global loading is multiaxial and/or random. A fully new approach is then presented: it introduces a phenomenological model describing the development of residual stresses that can be calibrated by reference to Eshelby's type approaches. A tensorial variable is used for this purpose. Its evolution rule allows us to represent the stress redistribution at the surface of the component. Isotropic and anisotropic constitutive equations are accepted for the description of the material behaviour. It has been successfully used for several complex situations (cyclic, multiaxial, random loadings)

Modélisation de la fabrication directe de pièces par projection laser : application au Ti-6Al-4V

International audienceLa projection laser permet de fabriquer de façon relativement simple des pièces complexes, dont les dimensions sont proches des cotes finales. Le procédé s'accompagne néanmoins d'importantes variations locales de température, à l'origine d'une microstructure hétérogène, et de contraintes résiduelles difficiles à maîtriser. Une bonne manière d'aider au développement du procédé est de mettre en place sa simulation numérique, afin de prévoir la microstructure et les contraintes résiduelles. Des conditions laser adaptées et une stratégie de balayage optimisée doivent permettre d'obtenir la microstructure désirée et des contraintes résiduelles minimisées

A strategy for the estimation of the fatigue life of notched components under random multiaxial fatigue

International audienceThis paper describes three new models that can be used (1) to determine the stress-strain response in stress concentration zones for components submitted to complex multiaxial loading paths, (2) to extract relevant cycle sequences from a general threedimensional loadin path, (3) to find the hypersphere enclosing any type of cyclic load history. These three models can be combined to post-process an elastic Finite Element Analysis, and provide a fast estimation of the fatigue life of the components

Cyclic calculations and life estimation in thermomechanical fatigue

International audienceThe present paper describes a new numerical procedure devoted to the calculation of aeronautic engines and to the fatigue life assessment evaluation. The original aspects consists (1) in a new method used to determine the mechanical steady state of a structure under thermomechanical loading, (2) in a short review of an advanced fatigue model which can reproduce the Woehler curve, including the influence of the mean stress

Z-set/ZeBuLoN : une suite logicielle pour la mécanique des matériaux et le calcul de structures

National audienceLa suite Z-set/ZéBuLoN est un ensemble d'outils dédiés à la mécanique des matériaux et au calcul de structures co-développés par l'Onera, le Centre des Matériaux (Mines ParisTech) et par la société NW Numerics. Elle intègre différents modules tels qu'un solveur éléments finis, une bibliothèque d'intégration de loi matériaux, un optimiseur permettant de déterminer les paramètres matériaux d'une loi à partir de données d'essais. Certains de ces modules sont indépendants et peuvent être utilisés par des codes commerciaux (Abaqus, Ansys, Samcef, . . .) sous la forme de plugins. Des développements récents ont été faits dans les domaines de la mécanique de la rupture et du calcul haute performance et multimodèle

Préconditionneur grille grossière pour la résolution par solveur itératif de problème éléments-finis fortement non-linéaires de grandes tailles

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Préconditionnement des solveurs itératifs par résolution d’un problème sur grille grossière

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Combining porous plasticity with Coulomb and Portevin-Le Chatelier models for ductile fracture analyses

International audienceFor large void volume fraction, the Rousselier porous plasticity model transforms naturally into a coalescence model, in the whole range of stress triaxiality, in agreement with the necessary kinematic coalescence condition (NKCC). It enables to model slant fracture in notched tensile or cracked specimens. Nevertheless, void coalescence is not the single mechanism involved in slant fracture. That is why it is necessary to combine porous plasticity with other models. In this paper, the Coulomb fracture model and the Portevin-Le Chatelier (PLC) model (or dynamic strain aging: DSA) are formulated at the slip system scale. The Coulomb model combines the resolved normal and shear stresses for each slip plane and direction. For DSA, we postulate that each slip system has its own history of dislocation pinning and unpinning by solute atoms. The models are fully coupled in the framework of classical polycrystalline plasticity. A Reduced Texture Methodology (RTM) is used to provide the computational efficiency needed for numerical applications. The RTM approach involves a significant reduction of the number of representative crystallographic orientations. The models are applied to a notched tensile specimen taken from a 6260 aluminum alloy thin-walled extrusion. Fractographic examinations show a combination of dimples and large smooth areas on the slant fracture surface (mixed fracture). It highlights the need for combined fracture models. The PLC model gives very sharp oscillations of the macroscopic plastic strain rate, associated with moving plastic strain rate bands. It leads to a significant reduction of ductility compared to porous plasticity alone

Numerical modeling of fatigue crack growth in single crystals based on microdamage theory

International audienceProper life-time prediction modeling of single crystalline components is of increasing importance due to their common use in turbine industry. Viscoplastic damage approaches are of great interest in that context. However, mechanical properties of single crystals are strongly anisotropic and nonlinear in service conditions, bringing certain complexity into constitutive and numerical modeling. The aim of this work is to develop a thermodynamically consistent constitutive model based on generalized continua in order to simulate fatigue crack initiation and growth in single crystals. For that purpose, a standard crystal plasticity model is taken as a basis and coupled with the continuous damage model developed by Marchal et al. (2006a) [Marchal, N., Forest, S., Remy, L. and Duvinage, S. (2006a). Simulation of Fatigue Crack Growth in Single Crystal Superalloys Using Local Approach to Fracture, In: Moinereau, D., Steglich, D. and Besson, J. (eds.), Local Approach to Fracture, 9th European Mechanics of Materials Conference, Euromech-Mechamat, Moret-Sur-Loing, France, Presses de l'Ecole des mines de Paris, pp. 353-358]. As a variant of micromorphic theory, microdamage approach is applied to the model in order to obtain a regularized continuum damage formulation which solves mesh dependency problem by introducing an intrinsic length scale. A detailed finite element implementation procedure and its validation for monotonic crack growth areshown. Fatigue crack growth analyses have been performed on a single edge notched geometry and a comparison between numerical and experimental results is presented