Influence de l'environnement et de la microstructure sur la propagation en fatigue des fissures courtes tridimensionnelles

The propagation of small surface cracks has been studied on a high strength Aluminium alloy 7075 in peak aged (T651) and overaged (T7351) conditions. Tests were performed at a load ratio R = 0.1 and a frequency of 35 Hz in ambient air and in vacuum. Small crack growth was monitored by mean of replicas. The behaviour of small cracks is compared to that of long and short through cracks and discussed on the basis of closure measurements performed on long cracks. A strong influence of environment on small crack growth has been shown which is similar to that observed for through cracks in the low rate range (< 10-8 m/cycle ). The lower resistance observed in air has been related to water vapour embrittlement as previously shown for long cracks. The influence of microstructure at the scale of grain size has been shown to be large on crystallographic stage I propagation observed in vacuum on the 7075 T651, but weak on stage II propagation observed on the 7075 T651 in air and on the 7075 T7351 in both environmental conditions. A modelisation of localised plasticity and microstructure is proposed.La propagation en fatigue de fissures courtes de surface a été étudiée dans un alliage léger à haute résistance type 7075 dans deux états de revenu, T651 et T7351. Les essais ont été effectués à l'air ambiant et sous vide, avec un rapport de charge R = 0,1 et une fréquence de 35 Hz. La longueur des fissures a été mesurée en surface à l'aide de répliques. Le comportement de ces fissures courtes tridimensionnelles (3D) a été comparé à celui de fissures bidimensionnelles (2D) traversantes courtes et longues étudiées précédemment. Les résultats sont discutés sur la base des mesures de fermeture effectuées sur les fissures longues et d'observations microfractographiques. La forte influence de l'environnement ambiant observée sur les fissures 3D s'avère comparable à celle observée précédemment pour les fissures 2D dans le domaine de vitesses exploré < 10-8 m/cycle. La moindre résistance à l'air est attribuée à un effet fragilisant de la vapeur d'eau. L'influence de la microstructure à l'échelle du grain au niveau du site d'amorçage s'avère très grande dans le cas d'une propagation cristallographique en stade I (7075 T651 sous vide) ; elle est par contre très réduite pour une propagation en stade II (7075 T651 à l'air, 7075 T7351 à l'air et sous vide). Une modélisation des effets de la plasticité localisée et de la microstructure est proposée

Influence de l'environnement sur la propagation des fissures courtes et longues dans un alliage léger type 7075

The growth in fatigue of small bidimensionnal cracks has been studied in air and in vacuum on the 7075 T651 Aluminum alloy (peak aged) and in air at two load ratio, on the 7075 T7351 alloy (overaged). The low growth rate range was investigated near threshold conditions. On the basis of crack closure measurements and long cracks propagation results, it was shown that, for given environmental conditions, the behaviour of small and long cracks can be described in terms of the effective stress intensity factor range.La propagation en fatigue des fissures courtes bidimensionnelles a été étudiée à l'air et dans le vide dans l'alliage 7075 T651 (vieilli au pic de durcissement) et à l'air pour deux rapports de charge dans l'alliage 7075 T7351 (sur-revenu). Les essais ont été effectués dans le domaine des basses vitesses de fissuration au voisinage du seuil de non-propagation. Sur la base des mesures de fermeture de fissure et des résultats relatifs à la propagation des fissures longues, il est montré que, pour un environnement donné, la propagation des fissures de petites dimensions peut être décrite à l'aide d'une seule relation liant la vitesse de fissuration au facteur d'intensité de contrainte effectif ΔKeff

Étude des domaines d'existence des instabilités plastiques du type Portevin-Le Chatelier dans l'alliage d'aluminium-magnesium AG3

Plastic instabilities associated with the Portevin-Le Chatelier (PLC) effect, were studied in an aluminium-magnesium alloy AG3, in large ranges of temperature as well as of strain and stress rates. Experimental results presented in a log $\dot\sigma - 1/T$ or log $\dot\varepsilon - 1T$ diagram, show that the region of PLC effect is closed at high loading rates. The critical strain at which the PLC phenomenon appears, increases when the loading rate increases at low temperature or at high loading rates. The “inverse” behaviour branches are observed at high temperature and at low loading rates.Les instabilités plastiques associées au phénomène Portevin-Le Chatelier (PLC) dans l'alliage aluminium-magnésium AG3, ont été étudiées dans de larges intervalles de températures, de vitesse de déformation et de vitesse de contrainte. Les résultats expérimentaux obtenus montrent que le domaine d'existence de l'effet PLC, représenté dans un diagramme log $\dot\sigma - 1/T$ ou log $\dot\varepsilon - 1T$, est fermé aux vitesses de chargement élevées. La déformation critique à laquelle apparaît le phénomène PLC augmente avec la vitesse de chargement aux basses températures ou aux vitesses de chargement élevées. Le comportement inverse est observé aux températures élevées et aux basses viteses de chargement

A new rate-type gradient-dependent viscoplastic approach for stop-and-go strain band propagation. Numercial vs. physical experiments

It is shown that the non-local rate-type constitutive equation proposed by Faciu & Molinari [1] can predict a large number of experimentally observed features of the kinetics of the Portevin-Le Châtelier (PLC) bands. Numerically obtained serrated yielding curves and staircase creep curves for strain and stress controlled problems, respectively, are compared with experimental curves. A good agreement between the phenomenological approach and microscopic aspects of the PLC effect is obtained

Short surface fatigue cracks growth under constant and variable amplitude loading

WOSInternational audienceThis paper deals with the fatigue strength of S355NL steel, of a common use within the shipbuilding industry, under uniaxial constant and variable loading. Indeed, ship structures are subjected to variable loading due to various sea states, wind and waves. As a consequence, a better knowledge of fatigue behavior under real loading conditions is needed. This study aims at analyzing the influence of loading conditions (load ratio and variable amplitude loading) on the short crack behavior and last, with a proposed model to assess the fatigue crack life. The tools used to prepare inspections in critical areas only take into account the long crack behavior. The results from the proposed model were compared to the assessments these tools are providing with

Experimental investigations on small fatigue cracks growth in naval steel.

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Distinct recovery of copper and aluminum from waste electric wires using a roll-type electrostatic separator

International audienceElectrostatic separators are commonly employed for the sorting of insulating and conducting fractions of granular mixtures. However, separation of the various metals that compose the conductive fraction still represents a major challenge for the recycling industry. The aim of this paper is to identify the conditions in which a roll-type electrostatic separator could be used for distinctly recover the copper and aluminum from waste electric wires. A numerical model of particle trajectories was employed to predefine these conditions and guide the experimental study. The electrostatic separation experiments were carried out using a reverse-S-type-plate high-voltage electrode. The visualization of particle trajectories using a high-speed camera paved the way to an original design. Thus, a metallic trap was attached to the high-voltage electrode, to facilitate the collection of the aluminum flake-like particles from a mixture that also contains copper strands. With a plastic trap replacing the metallic one, to avoid electric field modification, 68.6% of the copper was recovered with a purity attaining 99%

Identification of Upper Limb Motion Specifications via Visual Tracking for Robot Assisted Exercising

International audienceThis paper deals with the identification of upper limb motion specifications for robot assisted exercising. Motion capture system has been used to track the upper limb motion of a subject with a specific protocol. Two movements have been investigated based on the procedure done by the medical specialists on patients. The experimental analysis focused on the shoulder, leading to range of motion, has been presented and used to reconstruct the upper limb kinematic model. The ability of a cable-driven robot LAWEX, which has been designed and built at LARM to replicate the upper limb motion is discussed