Etude de la dechirure ductile : application aux cordons de soudure

SIGLECNRS TD Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Mechanical and thermo-physical characterization of mortars made with uncontaminated marine sediments

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Numerical study of the optimum degree of cold expansion: Applicationnext term for the previous termpre-cracked specimen with the expanded hole at the crack tipnext term

International audienceIt has been known that the cold expansion repair technique is very beneficial for the fatigue life of damaged structures. A numerical investigation has been realised to analyse the optimum degree of cold expansion (DCE) to avoid the great number of fatigue test and to achieve the maximal fatigue lifetime. The numerical results have been validated by some experimental results of fatigue tests about steel and an aluminium alloy. This work proposes a method to determine the optimum degree of cold expansion for different materials using the finite element method. The high equivalent von Mises stresses around the expanded hole during the process of cold expansion can cause local damage. The position of instability, the ultimate stress corresponding to the point of instability on the true stress–true strain curve were used to determine the optimum degree of cold expansion

Contribution à l'étude de la prédiction de la durée de vie en fretting-fatigue (application à un contact acier-alliage d'aluminium)

Le but de cette étude est la prédiction de la durée de vie à l'initiation de la fissure d'un contact acier/aluminium sous la sollicitation de fretting fatigue. La prédiction des durées de vie a été déterminée à l'amorçage des fissures en fonction des différents chargements appliqués. Un dispositif expérimental adapté sur un banc d'essai multiaxial de fatigue a été mis au point. Des essais ont été réalisés sur un alliage d aluminium de la série 6082T6. Les résultats obtenus montrent un abattement de la durée de vie par rapport à la fatigue classique. La modélisation par éléments finis a été menée en utilisant un code de calcul Abaqus. Les champs de contraintes et de déformations identifiés ont été utilisés sur différents critères de fatigue multiaxiale basés sur les plans critiques. Les résultats de la prédiction donne une sous estimation de la durée de vie en fretting fatigue. Ceci est dû aux forts gradients présents dans la zone de contact. La prédiction a été améliorée par couplage d une méthode de champs moyens avec les critères de fatigue multiaxiale. Les estimations obtenues présentent une bonne corrélation avec les résultats expérimentaux. Néanmoins, la taille de la zone d'élaboration optimale semble dépendre du critère de fatigue multiaxiale utilisé et des conditions de chargement appliquées. Cette zone ne semble pas être liée directement avec la microstructure pour le matériau étudié.Dans une structure les sollicitations sont variables. La démarche ci-dessus a été couplée avec deux lois d'endommagement: loi de Miner et loi dite de la contrainte endommagée pour la prédiction avec des chargements par blocs. Les prédictions obtenues sont satisfaisantes.The purpose of this study is to predict fretting fatigue crack nucleation of a mono contact steel/aluminum at different applied loads.An experimental device was first designed and adapted on a multiaxial fatigue apparatus. Tests were conducted aluminum alloy at constant amplitude loading under both different load levels and load ratios. As expected, the fretting fatigue life was found lower compared to uniaxial fatigue under the same loading conditions. Finite element analysis was conducted using \textit{Abaqus} software. The computed stress and strain fields were used to estimate the parameters of different multiaxial fatigue criteria based on the critical plane approach. When using stress and strain values corresponding to the material point exhibiting the maximum value of the considered parameter, we have found that, whatever the multiaxial fatigue parameter, conservative fatigue life estimates due to the severe gradients in the contact zone. An averaging method of the mechanical quantities over a given reference volume was then used to attenuate these gradients effects. The estimates show a good correlation with experimental results. However, the size of the reference volume depends on both the multiaxial fatigue criteria and on loading conditions applied. So that it could not be directly linked to the grain size for the material studied. Finally, we made an attempt to extend these criteria and the developed method to variable loadings. Fretting fatigue tests using two and four blocks loadings were performed and the previous criteria were coupled with two damage laws. The estimates we have obtained seems very promising.LILLE1-Bib. Electronique (590099901) / SudocSudocFranceF

DETERMINATION OF THE MECHANICAL PROPERTIES OF CEMENTITIOUS MIXTURES WITH SEDIMENTS: EXPERIMENTAL AND NUMERICAL APPROACHES

International audienceSediment management is increasingly oriented towards valorization processes in several fields. Many researches have been conducted on the characterization of sediments and the possibility to be used as raw materials in the field of civil engineering. The overall objective of this contribution is to setup a numerical modeling strategy with a validation on the basis of experimental results. The mechanical strengths (flexural and compressive) and the elastic modulus were investigated. Five mortars were formulated with different substitution of sand by sediments (from 0% to 40%). A numerical model was built using the finite element method (FEM) based on homogenization technique. This model allows the prediction of the global mechanical behavior of the mortars based on the knowledge of the mechanical behavior of each phase constituting these samples. The model was validated through an example of a microstructure of a BOF (Basic Oxygen Furnace) slag from literature work. The microstructure has been observed using Scanning Electron Microscope SEM in order to identify the major phases in presence. Besides, the elastic properties of these phases have been determined using an instrumented indentation technique coupled with SEM. The obtained results show a good agreement between numerical and experimental. The model is validated and can then be applied on the mortars. The mechanical properties of the formulated mortars decrease with the increase of sediment substitution similarly to the obtained results in literature

Effect of contact forces on fretting fatigue behavior of bolted plates: Numerical and experimental analysis

International audienceThis paper describes the effect of contact forces on the fretting fatigue behavior of bolted plates. Both fretting fatigue experiments and numerical simulation with FEM and SEM/EDS observations and analysis were carried out. With the increase of contact forces, the sites of crack initiation change from the edge of the central hole to close the border of contact and the fretting fatigue life increases. The local stresses and the relative displacement in the contact zone are two principal factors which influence the fretting fatigue behavior. A Map of Material Response (MMP) was determined basing on obtained results can describe the behavior of the fretting fatigue damage and predict the conditions which conduct to cracking initiation and propagation