Surface Damage and Treatment by Impact of a Low Temperature Nitrogen Jet

International audienceNitrogen jets under high pressure and low temperature have been introduced recently. The process consists in projecting onto a surface a low temperature jet obtained from releasing the liquid nitrogen stored in a high pressure tank (e.g. 3000 bars) through a nozzle. It can be used in a range of industrial applications, including surface treatment or material removal through cutting, drilling, striping and cleaning. The process does not generate waste other than the removed matter, and it only releases neutral gas into the atmosphere. This work is aimed at understanding the mechanisms of the interaction between the jet and the material surface. Depending on the impacted material, the thermo-mechanical shock and blast effect induced by the jet can activate a wide range of damage mechanisms, including cleavage, crack nucleation and spalling, as well as void expansion and localized ductile failure. The test parameters (standoff distance, dwell time, operating pressure) play a role in selecting the dominant damage mechanism, but combinations of these various modes are usually present. Surface treatment through phase transformation or grain fragmentation in a layer below the surface can also be obtained by adequate tuning of the process parameters. In the current study, work is undertaken to map the damage mechanisms in metallic materials as well as the influence of the test parameters on damage, along with measurements of the thermo-mechanical conditions (impact force, temperature) in the impacted area

Dislocation transport and intermittency in the plasticity of crystalline solids

International audienceWhen envisioned at the relevant length scale, plasticity of crystalline solids consists in the transport of dislocations through the lattice. In this paper, transport of dislocations is evidenced by experimental data gathered from high-resolution extensometry carried out on copper single crystals in tension. Spatiotemporal kinematic fields display spatial correlation through characteristic lines intermittently covered by plastic activity. Intermittency shows temporal correlation and power-law distribution of avalanche size. Interpretation of this phenomenon is proposed within the framework of a field dislocation theory attacking the combined problem of dislocation transport and long-range internal stress field development. Intermittency and transport properties show remarkable independence from sample size, aspect ratio, loading rate, and strain-rate sensitivity of the flow stress

Effects of the impact of a low temperature nitrogen jet on metallic surfaces

Lien vers la version éditeur: http://rspa.royalsocietypublishing.org/content/468/2147/3601.abstractThe technology of nitrogen jets impacting surfaces at low temperature has recently been introduced for surface cleaning/stripping. Under the impact of the jet, the material surface undergoes a thermomechanical shock inducing complex transformation mechanisms. Depending on the material and test parameters such as standoff distance, dwell time, upstream pressure, the latter include cleavage, cracking, spalling, blistering, grain fragmentation, phase transformation and ductile deformation. Quite often, these modes are superimposed in the same test, or even in the same material area. In this study, an overview of these mechanisms is proposed for metallic materials. Measurements of thermomechanical variables in the impacted area are presented and the influence of the test parameters on surface transformation is investigated. Grain fragmentation and ultrafast transport of nitrogen in a deep layer below the surface are explored

Etude micromécanique du comportement thermomécanique des alliages à mémoire de forme

Shape memory alloys (SMA) present a superthermoelastic behavior, i.e. they are able to be strained one hundred times more than classical alloys. This strain appears by mechanical loading (isothermal superelasticity) or by temperature variations (non isothermal creep). SMA's very particular properties are linked to a diffusionless solid-solid phase transformation (martensic transformation) during which a praent phase (austenite)becomes a product phase (martensite). This transformation is cristallographically reversible. Determination of constitutive law and behavior's models are an important step for industrial applications of these materials. Aim of this work is to model the superthermoelastic behavior using a micromechanical approach. Physical micromechanisms responsible of the behavior are taken into account in the description of the behavior at the microscopic level. Using an adapted scale transition method (self-consistent one), the macroscopic behavior is determined. An experimental study performed on a cupper-aluminium-berylium alloy allows to characterize in stress-temperature diagram some single and polycristal materials having the same composition. Experimental and numerical results are in good agreement. Furthermore, it's possible to get informations on the microstructure evolution in all the grains that are constituing the polycristalLes alliages à mémoire de forme (amf) sont superthermoélastiques, c'est-à-dire qu'ils sont capables de subir un allongement plus de cent fois supérieur à celui des alliages courants. Cette déformation a lieu lors d'application de contrainte mécanique (superélasticité isotherme) ou de variations de température (fluage anisotherme). Les propriétés très particulières des amf sont liées à une transformation de phase solide sans diffusion (la transformation martensitique) au cours de laquelle une phase mère (austénite) donne naissance d'une manière cristallographiquement réversible à une phase appelée martensite. L'élaboration de lois et de modèles de comportement est une étape nécessaire dans le processus de développement industriel de ce type de matériau. Ce travail a pour but d'élaborer un modèle du comportement superthermoélastique base sur une approche micromécanique. La prise en compte des micromécanismes physiques responsables du comportement aboutit à la description du comportement à l'échelle microscopique. L'utilisation d'une méthode de transition d'échelle adaptée (la méthode autocohérente) rend compte du comportement macroscopique. Une étude expérimentale réalisée sur un alliage cuivre-aluminium-bérylium permet de caractériser thermomécaniquement des monocristaux et polycristaux de même composition. Les résultats expérimentaux et numériques ainsi obtenus sont en bon accord. De plus, l'approche utilisée permet d'accéder à des résultats numériques microstructuraux relatifs aux différents grains constituant le polycrista

Place des femmes en mécanique : les Chiffres

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Etude des phénomènes d'auto-organisation des ensembles de dislocations dans un alliage au vieillissement dynamique

Durant ces dernières années la déformation plastique est de plus en plus souvent étudiée du point de vue de la dynamique des systèmes complexes, caractérisée par l'auto-organisation et impliquant plusieurs échelles. Le présent travail est une étude multi-échelles de la dynamique collective de dislocations dans un alliage AlMg sujetà l'effet Portevin-Le Chatelier (PLC). Pour atteindre cet objectif nous avons réalisé l'enregistrement simultané des courbes de traction, de l'émission acoustique (EA) et du champ de déformations locales, ainsi que la caractérisation quantitative de la complexité grâce aux analyses statistique et multifractale. Le travail de thèse a permis de montrer que la manifestation apparente des processus de déformation plastique dépend de l'échelle d'observation. L'analyse de l'EA a révélé un comportement intrinsèquement intermittent et invariant d'échelles dans toutes les conditions expérimentales. Ces résultats suggèrent qu'aux échelles associées à l'EA, la plasticité peut être gouvernée par une dynamique universelle, qu'elle soit liée à l'instabilité PLC ou à un écoulement macroscopiquement homogène. Malgré le caractère ubiquitaire de cette observation, le champ de déformations locales montre des processus de déformation ondulatoires, et l'analyse statistique des chutes de contrainte révèle des échelles caractéristiques. Une hypothèse de synchronisation des avalanches des dislocations est avancée afin d'expliquer dans certaines conditions expérimentales le passage de l'échelle mésoscopique à l'échelle macroscopiqueIn recent years, the plastic deformation more and more often studied in terms of the dynamics of complex systems, which is characterized by self-organization and involves various scales. This work presents a multi-scale investigation of the collective behavior of dislocations in an AlMg alloy prone to the Portevin-Le Chatelier (PLC) effect. To achieve this goal we have performed simultaneous recording of tensile curves, acoustic emission (AE), and local strain field, as well as quantitative characterization of the complexity through statistical and multifractal analyses. The results obtained proved that the apparent manifestations of the plastic deformation processes depend on the scale of observation. The analysis of the AE data revealed an inherently intermittent and scale-invariant behavior in all experimental conditions. These results suggest that at the scales pertaining to the AE, the plasticity may be governed by a universal dynamics, be it related to the PLC instability or macroscopically homogeneous flow. Despite the ubiquitous nature of this observation, the local strain field uncovers wave-like deformation processes, and the statistical analysis of stress serrations reveals characteristic scales. Synchronization of dislocation avalanches is conjectured to explain (under some experimental conditions) the transition from the mesoscopic to the macroscopic scaleMETZ-SCD (574632105) / SudocSudocFranceF

Strain rate sensitivity in superelasticity

International audienceThis paper deals with the influence of martensitic transformation latent heat on the superelastic behaviour modelling. Exothermic and endothermic effects are responsible for a strong evolution on the temperature field inside the material that modifies its mechanical response. This is responsible for a strain rate effect that is taken into account by introducing a coupling equation between the production rate of martensite and the temperature change, into a micro–macro modelling of the superthermoelastic behaviour with the assumption that the temperature field remains uniform but different to the test temperature imposed. Numerical simulations so obtained show a good agreement with experimental results performed on Cu-based superelastic alloys

Scaling in the Local Strain-Rate Field during Jerky Flow in an Al-3%Mg Alloy

International audienceJerky flow in alloys, or the Portevin-Le Chatelier effect, presents an outstanding example of self-organization phenomena in plasticity. Recent acoustic emission investigations revealed that its microscopic dynamics is governed by scale invariance manifested as power-law statistics of intermittent events. As the macroscopic stress serrations show both scale invariance and characteristic scales, the micro-macro transition is an intricate question requiring an assessment of intermediate behaviors. The first attempt of such an investigation is undertaken in the present paper by virtue of a one-dimensional (1D) local extensometry technique and statistical analysis of time series. The data obtained complete the missing link and bear evidence to a coexistence of characteristic large events and power laws for smaller events. The scale separation is interpreted in terms of the phenomena of self-organized criticality and synchronization in complex systems. Furthermore, it is found that both the stress serrations and local strain-rate bursts agree with the so-called fluctuation scaling related to general mathematical laws and unifying various specific mechanisms proposed to explain scale invariance in diverse systems. Prospects of further investigations including the duality manifested by a wavy spatial organization of the local bursts of plastic deformation are discussed

Crossover in the scale-free statistics of acoustic emission associated with the Portevin-Le Chatelier instability

International audienceThe acoustic emission accompanying plastic deformation obeys scale-free statistics reflecting avalanche-like dis-location motion. This feature holds out for the macroscopically unstable deformation of alloys. However, stress serrations display peaked distributions at low enough strain rates. This occurrence of a characteristic macroscopic scale was supposed to result from the synchronization of dislocation avalanches. In the present work, the synchronization mechanism is studied using statistical analysis of different subsets of acoustic events. A crossover in the power-law exponents is detected for the events occurring during deep stress drops. It is ascribed to a transition from chaining to overlapping dislocation avalanches

Auto-organisation des ensembles de dislocations dans la plasticité cristalline

L’effet Portevin – Le Chatelier est un phénomène d’instabilité qui résulte du comportement collectif et de l’interaction de défauts cristallins tels que les dislocations et les atomes de soluté. L’analyse statistique des variations de la contrainte appliquée (comportement temporel) et les mesures de déplacement 1D ultra-rapides (structuration spatiale) lors d’essais de traction d’alliages Al-Mg, révèlent l’organisation de ces défauts à des échelles intermédiaires. On montre la nécessité de prendre en compte ces effets collectifs dans les lois de comportement