An energetic approach in thermomechanical fatigue for silicium molybden cast iron

The purpose of this paper is to define a low cycle fatigue criterion in order to predict the failure of engineering structures. The major problem in defining a predictive fatigue criterion is that it should be applicable for structures submitted to complex multiaxial thermo-mechanical loadings but should be identifiable from simple experiments on specimens. After a short critical review of the principal criteria used in low cycle fatigue it will be shown that the dissipated energy per cycle permits a correlation of isothermal and anisothermal results obtained on silicon molybdenum cast iron in the case of specimens and also on structures

Rolling contact fatigue of railways wheels: Influence of steel grade and sliding conditions

AbstractThe aim of this work is to develop a numerical approach which is able to compare the different steel grades influence on rolling contact fatigue of railways wheels according to practical conditions. The main stages are the identification of the material behavior, the determination of the stress-strain fields and the application of a fatigue criterion. Two steels usually used for the manufacturing of wheels have been studied, R9T and 50CrMo4. Their influence has been numerically studied. Results show that the threshold of elastic and plastic shakedown differs depending on the steel grades and consequently the risk of damage can be affected. This methodology allows a classification of the material grades face the risk on rolling contact fatigue

Détermination par analyse thermique des seuils d'amorçage des fissures sous chargement de Fretting

La sollicitation de Fretting est un phénomène associé à des micro-déplacements de faibles amplitudes entre deux solides en contact inférieur à la dimension de contact. L’endommagement généré par le Fretting sur la surface de contact est contrôlé par l’amplitude de glissement et dans le cas du glissement partiel l’endommagement prépondérant est la fissuration. La détermination des conditions d’amorçages passe par des méthodes expérimentales destructives très couteuses en temps et matériaux. Le but de notre étude est de développer une nouvelle méthode expérimentale basée sur la réponse thermique des matériaux, à l’image de celles s’appuyant sur les courbes d’auto-échauffement en fatigue. L’étude présentée ici a été réalisé sur un acier et nous nous sommes placés dans la configuration Cylindre/Plan. Un essai de Fretting consiste à appliquée une force normale constante sur le Plan. Un déplacement cyclique relatif est imposé, donnant naissance à une force tangentielle macroscopique. Une caméra thermique maintenue fixe et perpendiculaire à la surface latérale de l’échantillon lors des essais est utilisée. Les mesures thermiques ont été couplées à une méthode de suivi des marqueurs afin d’éliminer les mouvements de solide rigide. Une méthode de lissage local de la température est utilisée. Lors d’un test de Fretting à paramètres de chargement constants, l’évolution de la température moyennée sur une zone d’intérêt(ZI) peut être décomposée en trois parties, une dérive thermique globale, un signal périodique de même fréquence que la sollicitation et un signal périodique à deux fois cette fréquence. La dérive et les amplitudes des signaux périodiques atteignent rapidement une valeur stabilisée. Les résultats montrent que la valeur stabilisée de la dérive et les amplitudes des signaux périodiques de la température moyenne sur la ZI peuvent être empiriquement lié au seuil d’amorçage des fissures déterminée par les méthodes destructives. Les différences entre les deux méthodes restent inférieures à 10%

Plastic zone evolution during fatigue crack growth: Digital image correlation coupled with finite elements method

International audienceNonlinearities effects at the crack tip, due to the elastic-plastic material behavior , impact the crack growth rate and path. This paper is devoted to the study of the plastic zone evolution in the crack tip region. The methodology relies on coupling an elastic-plastic Finite Elements Method (FEM) model and experimental displacements measured by Digital Image Correlation (DIC). These latter are introduced as Dirichlet boundary conditions in the finite elements analysis. The considered FEM domain is constant, i.e. the same mesh with a centered crack is moved to each new crack tip position deduced from DIC. The new boundary conditions are updated and the residual stresses and plastic strains of the former computation are interpolated and actualized on the mesh shifted to the new crack tip position in order to incorporate them in the numerical model. The coupling method allowed applying experimental boundary conditions in order to be as close as possible to real experimental conditions and to observe the plasticity evolution from small to large scale yielding conditions. A fatigue test was conducted to validate the proposed approach. The identification residues are proved to be lower than those obtained with an experimental displacements projection onto Williams' series basis, which is a method commonly used with DIC. The coupling results present an attractive similarity with Irwin's model regardless of the crack length. Thus, the definition of the mask needed for the displacements fields projection on Williams' model can be deduced with a reliable estimate of Irwin's plastic radius

A multi-scale crystal plasticity model for cyclic plasticity and low-cycle fatigue in a precipitate-strengthened steel at elevated temperature

peer-reviewedIn this paper, a multi-scale crystal plasticity model is presented for cyclic plasticity and low-cycle fatigue in a tempered martensite ferritic steel at elevated temperature. The model explicitly represents the geometry of grains, sub-grains and precipitates in the material, with strain gradient effects and kinematic hardening included in the crystal plasticity formulation. With the multiscale model, the cyclic behaviour at the sub-grain level is predicted with the effect of lath and precipitate sizes examined. A crystallographic, accumulated slip (strain) parameter, modulated by triaxiality, is implemented at the micro scale, to predict crack initiation in precipitate-strengthened laths. The predicted numbers of cycles to crack initiation agree well with experimental data. A strong dependence on the precipitate size is demonstrated, indicating a detrimental effect of coarsening of precipitates on fatigue at elevated temperature. (C) 2016 Elsevier Ltd. All rights reserved.ACCEPTEDpeer-reviewe

Dissipation and Fatigue Damage

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Energie dissipée: un critère de dimensionnement â la fatigue thermomécanique

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Life-time prediction of aluminium automotive components under thermo-mechanical cyclic loadings

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A macroscopic crack initiation criterion in multiaxial thermomechanical fatigue

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