Prédiction de durée de vie de structures sous chargement de fatigue thermique

L'objectif de cette thèse est la prévision de durée de vie de structures soumises à des chargements de fatigue thermique. Ce travail s'inscrit dans le cadre des études menées par le CEA sur les problèmes de fatigue thermique rencontrés sur le circuit RRA de Civaux. En particulier nous étudions l'essai SPLASH du CEA : une éprouvette est chauffée de manière continue et soumise cycliquement à un choc thermique. Ce chargement génère d'importants gradients thermiques en temps et en espace qui conduisent à l'initiation et la propagation d'un réseau de fissures. Nous avons proposé une nouvelle modélisation thermo-mécanique de cet essai et nous avons proposé un nouveau critère de fatigue pour prédire la durée de vie de l'éprouvette. Nous avons aussi proposé et comparé une série de techniques de coût et de complexité diverses pour estimer la réponse mécanique de l'éprouvette. Les implications pratiques de ce travail sont une remise en question des hypothèses utilisées dans les codes de dimensionnement RCC du CEA pour la simulation du choc thermique et l'interprétation en terme de fatigue de la structure. Ce travail permet d'ouvrir plusieurs perspectives industrielles et scientifiques sur l'interprétation mécanique du critère de fatigue par exempleThe aim of this work is to predict the lifetime of structures submitted to thermal fatigue loadings. This work lies within the studies undertaken by the CEA on the thermal fatigue problems from the french reactor of Civaux. In particular we study the SPLASH test : a specimen is heated continuously and cyclically cooled down by a water spray. This loading generates important temperature gradients in space and time and leads to the initiation and the propagation of a crack network. We propose a new thermo-mechanical model to simulate the SPLASH experiment and we propose a new fatigue criterion to predict the lifetime of the SPLASH specimen. We propose and compare several numerical models with various complexity to estimate the mechanical response of the SPLASH specimen. The practical implications of this work are the reevaluation of the hypothesis used in the French code RCC, which are used to simulate thermal shock and to interpret the results in terms of fatigue. This work leads to new perspectives on the mechanical interpretation of the fatigue criterionVERSAILLES-BU Sciences et IUT (786462101) / SudocMARNE-LA-VALLEE-ENPC-BIBL. (774682303) / SudocSudocFranceF

A computational lifetime prediction of a thermal shock experiment. Part II: discussion on difference fatigue criteria

The SPLASH experiment has been designed in 1985 by the CEA to simulate thermal fatigue due to cooling shocks on steel specimens and is similar to the device reported by Marsh in Ref. [1]. The purpose of this paper is to discuss the application of different fatigue criteria in this case. The fatigue criteria: dissipated energy, Manson Coffin, Park and Nelson, dissipated energy with a pressure term, are determined for the experiment using results from FEM computations presented in the first part of the paper (Part I) and compared with results from uniaxial and multiaxial experiments from literature. The work emphasizes the evolution of the triaxiality ratio during the loading cycle

A computational lifetime prediction of a thermal shock experiment. Part I: thermomechanical modelling and lifetime prediction

The SPLASH experiment has been designed in 1985 by the CEA to simulate thermal fatigue due to short cooling shocks on steel specimens and is similar to the device reported by Marsh in Ref. [1]. The purpose of this paper is to discuss the mechanical and the fatigue analysis of the experiment using results from FEM computations. The lifetime predictions are obtained using a modified dissipated energy with a maximal pressure term and agree with the experimental observations. The numerical analysis of the mechanical state shows an important evolution of the triaxiality ratio during the loading cycle. Further comparisons and discussions of the fatigue criteria are provided in the second part of the paper (Part II)

Comparaison de modélisations numériques appliquées à une expérience de fatigue sous choc thermique

International audienceThis paper is dedicated to the comparison of several numerical models for estimating the mechanical state in a thermal fatigue experiment known as the SPLASH test, in terms of their prediction capacity with respect to several fatigue criteria. The numerical methods are either completely nonlinear or combine approximate elastic solutions obtained from minimizing a potential energy or closed form solutions with a Neuber or Zarka technique to estimate directly the elastoplastic state. The fatigue criteria used are Manson, dissipated energy and dissipated energy combined with a hydrostatic pressure term. The latter had provided a best prediction over a series of anisothermal and isothermal LCF experiments in a classical fatigue analysis. The analysis shows that for this criteria we obtain a systematic and conservative error.Ce travail présente une comparaison de plusieurs modèles numériques pour déter-miner le cycle mécanique de l'expérience de fatigue thermique SPLASH. On s'intéresse en particulier à la précision des différents modèles lorsqu'on les utilise pour une analyse de fatigue. Les techniques numériques étudiés sont : (i) la résolution du problème élasto-plastique non-linéaire complet par calculs Eléments Finis, (ii) des solutions élastiques approchées cou-plées à une règle de Neuber modifié (iii) la méthode de calcul simplifié des structures de Zarka et al. Les critères de fatigue utilisés sont : (1) le critère de Manson, (2) un critère sur l'énergie dissipée par cycle, (3) un critère sur l'énergie dissipée modifiée par la pression hydrostatique. Ce dernier est le plus pertinent du point de vue de la fatigue au regard des résultats obtenus sur une série d'essais multiaxiaux et uniaxiaux. L'analyse présentée ici montre que ce critère est conservatif et ne propage pas les erreurs dues à la technique de calcul du cycle stabilisé utilisé

A comparison of lifetime prediction methods for a thermal fatigue experiment

This paper is dedicated to the comparison of several numerical models for estimating the lifetime in a fatigue experiment. The models simulate the SPLASH experiment, which produces thermal fatigue by locally quenching stainless steel specimens. All models predict first a stabilized mechanical state (plastic shakedown) and then a lifetime prediction using several fatigue crack initiation criteria.The numerical methods are either completely nonlinear or combine approximate elastic solutions obtained from minimizing a potential energy or closed form solutions with a Neuber or Zarka technique to estimate directly the elastoplastic state.The fatigue criteria used are Manson, dissipated energy and dissipated energy combined with a hydrostatic pressure term. The latter had provided a best prediction over a series of anisothermal and isothermal LCF experiments in a classical fatigue analysis.The analysis shows that for fatigue criteria taking into account the triaxiality of the mechanical response we obtain a systematic and conservative error. As a consequence of this work, we show that simplified models can be used for lifetime prediction. Moreover the paper provides a general technique to asses from the point of view of the design engineer the combination between a numerical method and a fatigue criterion