On the cementation of iron by means of carburetted hydrogen

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Influence des transformations surfaciques induites par traitements thermomécaniques sur la tenue en fatigue du Ti-10V-2Fe-3Al

Titanium alloys are widely used in aeronautics industries. However, these alloys are highly sensitive to the method for elaboration and transformation processes. These processes impact on the surface integrity of products. Therefore, they have to be controlled in order to predict life time of structures.In this work, the study of surface integrity is focused on the description of microstructal evolution, residual stresses and micro-geometry. Four points bending tests were performed in order to determined the fatigue limit of the different studied processes. We find out a good consistency between models used to determinate lifetime from surface integrity investigation and experimental results.Moreover, microstructures of titanium alloys are highly complex and they often are two-phased materials. Consequently, we studied the heterogeneous behaviour of such material by X-ray diffraction investigation. We found out that these heterogeneities have an impact on residual stresses determination. Therefore, a non-standard methodology was defined and validate by simulation using a micro-mechanic model: a self-consistent model.Micro-mechanical models are interesting for the residual stresses determination using X-ray diffraction because they allow to take into account heterogeneous and anisotropic behaviours through anisotropic elasticity and anisotropic texture.Les alliages de titane sont largement utilisés dans le secteur de l'aéronautique. Cependant, ces alliages sont très sensibles aux gammes d'élaboration et de mises en forme. Ces dernières ont une grande influence sur l'intégrité de surface des produits finis, ce qui a un impact sur la durée de vie des pièces en service. Il est donc nécessaire de maîtriser les procédés afin de pouvoir définir la tenue en service des pièces.Dans ces travaux, les différentes intégrités de surface sont décrites en termes d'évolution microstructurale, de contraintes résiduelles et de micro-géométrie. Des essais de fatigue en flexion 4 points sont réalisés pour tester les performances mécaniques de ces surfaces. Nous avons remarqué une bonne cohérence entre les modèles reliant l'intégrité de surface et la tenue en fatigue déterminée expérimentalement.De plus, étant donné que les alliages de titane ont des microstructures complexes et peuvent être multiphasés, nous nous sommes aussi intéressés au caractère hétérogène de ces matériaux principalement dans l'analyse de contraintes résiduelles déterminées à l'aide de la diffraction des rayons X. Nous avons remarqué que ces hétérogénéités ont un impact sur la détermination des contraintes résiduelles. De fait une méthodologie de détermination de contraintes non standard a été mise en place et validée par des simulations utilisant un modèle d'homogénéisation auto-cohérent.Ces modèles d'homogénéisation sont intéressants d'un point de vue de la détermination des contraintes résiduelles par diffraction des rayons X car ils permettent la prise en compte de l'aspect multiphasé des matériaux ainsi que leur caractère anisotrope

Experimental and numerical investigation of thermomechanical couplings and energy balance in metallic polycrystals

Les critères de localisation et d endommagement sont généralement basés sur un cadre dissipatif et ce travail s intéresse aux couplages thermomécaniques accompagnant les micromécanismes de déformation. Il repose en partie sur des données expérimentales obtenues précédemment dans le laboratoire par Bodelot pour un polycristal d acier A316L. Ce travail tire profit d'une combinaison de techniques différentes, en particulier de mesures in situ de champs cinématiques et thermiques ainsi que de l Orientation Imaging Microscopy, de la profilométrie et d une micrographie de surface. Différents outils ont été développés afin (1) d'identifier automatiquement les systèmes de glissement activés, (2) d estimer l émissivité de la surface permettant ainsi une détermination des champs thermiques avec une précision de 30 mK, (3) de projeter les champs bruts expérimentaux sur la microstructure et (4) de permettre la modélisation du polycristal et de ses conditions aux limites thermomécaniques réelles dans un cadre de plasticité cristalline dans le code EF Abaqus. Il a notamment été montré que les variations de température fournissent une estimation précise et aisée de la limite d'élasticité macroscopique ainsi que la détermination de la contrainte de cisaillement critique à l'échelle granulaire. En outre, les mesures cinématiques ont permis l'identification des systèmes de glissement activés. Des bilans énergétiques expérimentaux et numériques ont été réalisés et une grande influence de l'hétérogénéité polycristalline sur les mécanismes de stockage d énergie a été soulignée. Les méthodes proposées contribueront à améliorer les critères d endommagement basés sur un cadre dissipatifStrain localization and damage criteria of materials and structures are commonly based on a dissipative framework and this work investigates the thermomechanical couplings accompanying the deformation micromechanisms. It is partly based on experimental data obtained previously in the laboratory by Bodelot for a A316L austenitic stainless steel polycrystal. This work takes profit of a multi-technique approach combining, in particular, in-situ kinematic and thermal fields measurements as well as Orientation Imaging Microscopy, profilometry and surface micrography. Different tools have been developed (1) to automatically identify the activated slip systems directly from the surface micrography, (2) to approach the surface emissivity field allowing an accurate determination of the thermal fields with a 30 mK precision, (3) to project raw experimental fields on the microstructure and (4) to allow the modeling of the polycrystal aggregate and its real thermomechanical boundary conditions by using a crystal plasticity framework within the Abaqus FE code. It has notably been shown that the temperature variations provides an easy and accurate estimation of the macroscopic yield stress at the specimen scale as well as the determination of the Critical Resolved Shear Stress at the intragranular scale. In addition, the local kinematic measurements allow the in-situ identification of the activated slip systems. Experimental and numerical energy balances have been conducted and a great influence of the polycrystalline heterogeneity on the energy storage mechanism has been underlined. The proposed methods would help improving physical based dissipative criteria for damage analysisVILLENEUVE D'ASCQ-ECLI (590092307) / SudocSudocFranceF

WTC2005-63363 EXPERIMENTAL THERMAL STUDY OF CONTACT WITH THIRD BODY

ABSTRACT The thermal study of sliding contact is complex due to numerous physical aspects highly coupled. Heat generation mechanisms are still badly known due to the complex interactions between mechanical, thermal and physico-chemical behaviours and surface degradations. In the goal to better appreciate the third body role on the thermal aspect, an experimental set-up has been realized. It consists in two rings sliding to each other, the first one is made of sapphire (rotating ring) and the second one is made of steel (fixed ring). The temperatures are obtained by an infrared camera scanning through the sapphire and by thermocouples on the contact surface specially realized for this experimental setup. The contact surfaces of the two rings have been observed with a scanning electronic microscope. Comparison between the thermal scene and the surface observations has allowed connecting the third body accumulation with local surface heating. INTRODUCTION The study of dry friction between rubbing surfaces is generally closely related to the debris behavior of the interface. The debris particles have often been considered as worn particles from the contact area. The concept of third body was introduced in tribology by M. Godet [1] in order to define physically the components of a friction system and the mechanical interaction between them. Numerous experimental observations emphasized the complexity of the existing mechanisms in a sliding contact (third body distinction in granular form and in sheet microplates, third body flows, etc.

WTC2005-63363 EXPERIMENTAL THERMAL STUDY OF CONTACT WITH THIRD BODY

ABSTRACT The thermal study of sliding contact is complex due to numerous physical aspects highly coupled. Heat generation mechanisms are still badly known due to the complex interactions between mechanical, thermal and physico-chemical behaviours and surface degradations. In the goal to better appreciate the third body role on the thermal aspect, an experimental set-up has been realized. It consists in two rings sliding to each other, the first one is made of sapphire (rotating ring) and the second one is made of steel (fixed ring). The temperatures are obtained by an infrared camera scanning through the sapphire and by thermocouples on the contact surface specially realized for this experimental setup. The contact surfaces of the two rings have been observed with a scanning electronic microscope. Comparison between the thermal scene and the surface observations has allowed connecting the third body accumulation with local surface heating. INTRODUCTION The study of dry friction between rubbing surfaces is generally closely related to the debris behavior of the interface. The debris particles have often been considered as worn particles from the contact area. The concept of third body was introduced in tribology by M. Godet [1] in order to define physically the components of a friction system and the mechanical interaction between them. Numerous experimental observations emphasized the complexity of the existing mechanisms in a sliding contact (third body distinction in granular form and in sheet microplates, third body flows, etc.

An energy balance of blade-casing interaction

International audienc

Space- and time-resolved observation of extreme laser frequency upshifting during ultrafast-ionization

A 65-fs, 800-nm, 2-TW laser pulse propagating through a nitrogen gas jet has been experimentally studied by 90 Thomson scattering. Time-integrated spectra of scattered light show unprecedented broadening towards the blue which exceeds 300 nm. Images of the scattering region provide for the first time a space- and time-resolved description of the process leading quite regularly to such a large upshift. The mean shifting rate was as high as dk/dt3A ̊/fs, never observed before. Interferometry shows that it occurs after partial laser defocusing. Numerical simulations prove that such an upshift is consistent with a laser-gas late interaction, when laser intensity has decreased well below relativistic values (a0 1) and ionization process involves most of the laser pulse. This kind of interaction makes spectral tuning of ultrashort intense laser pulses possible in a large spectral range. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4818602

On the influence of microstructural gradients in the fatigue lifetime estimation of a railway axle

This study aims at characterizing the influence of gradients at different scales (loading, geometry, microstructure...) on fatigue strength through a multi-scale finite element modeling associated to several high cycle fatigue criteria. This is a necessary step in the perspective of conducting a relevant experimental campaign on notched specimen exhibiting a gradient of mechanical properties