Experimental and numerical analysis of the fields of the frozen stresses in an epoxy sphere using the stress-freezing method and mechanical slicing

The stresses induced by a mechanical contact imposed on a birefringent sphere have been studied experimentally and numerically. The birefringent sphere is machined from a birefringent parallelepiped on a high speed numerically controlled machine. The residual stresses developed in the birefringent sphere have been eliminated by a heat treatment relaxation. The technique of freezing and mechanical cutting in thin slices was used. In the case of 3D photoelasticity, the frozen stress techniques, which are very much used, may introduce residual stresses in the cut up slices. Although the means to avoid the introduction of these residual stresses are well established, these techniques remain time consuming. Slices are analyzed on a polariscope using plane polarized light and circular polarized light. The photoelastic fringes are used to determine the principal directions of the stresses as well as the values of the stresses. A numerical simulation using the finite elements made it possible to make a comparison with the experimental results. The analysis shows a good correlation between experimental measurements and numerical simulation

Experimental and numerical analysis of the fields of the frozen stresses in an epoxy sphere using the stress-freezing method and mechanical slicing

The stresses induced by a mechanical contact imposed on a birefringent sphere have been studied experimentally and numerically. The birefringent sphere is machined from a birefringent parallelepiped on a high speed numerically controlled machine. The residual stresses developed in the birefringent sphere have been eliminated by a heat treatment relaxation. The technique of freezing and mechanical cutting in thin slices was used. In the case of 3D photoelasticity, the frozen stress techniques, which are very much used, may introduce residual stresses in the cut up slices. Although the means to avoid the introduction of these residual stresses are well established, these techniques remain time consuming. Slices are analyzed on a polariscope using plane polarized light and circular polarized light. The photoelastic fringes are used to determine the principal directions of the stresses as well as the values of the stresses. A numerical simulation using the finite elements made it possible to make a comparison with the experimental results. The analysis shows a good correlation between experimental measurements and numerical simulation

Stress-assisted versus strain-induced martensites formed by cryogenic ultrasonic shot peening in austenitic stainless steels

International audienceIntroduction Severe plastic deformation (SPD) is used to create nanocrystalline metallic materials resulting in high strength but associated, generally and unfortunately, with a reduced ductility [1]. On one side, the cryogenic temperature that improves the grain refinement by preventing dynamic recrystallization or self-annealing, has been used during SPD processes such as equal channel angular extrusion (ECAE) or high pressure torsion (HPT), effectively producing significant extra grain refinement down to the nanometer scale [2-4]. On the other side, numerous research works have been done to improve the low ductility by creating multi-length scale structures [5] or grain size gradients [6]. In steels, other mechanisms can be active and lead to a significant improvement of the strength/ductility balance such as TRIP (Transformation Induce Plasticity) [7] or the TWIP (TWinning Induced Plasticity) [8] effects. In the case of the metastable austenitic stainless steel, the TRIP effect is produced through the martensitic phase transformation. The martensitic transformation requires an activation energy to be triggered which can be produced either thermally or by a mechanical loading. Two temperatures, the Ms and Md30, are used to evaluate the occurrence of the martensitic transformation. The Ms temperature represents the temperature at which the martensitic phase transformation can be triggered spontaneously without an external loading. By applying a loading, the transformation can take place at higher temperatures than Ms and the stress or strain required to activate the process will vary with the temperature [9]. The Md30 temperature, higher than the Ms, reflects the temperature at which a martensitic fraction of 50% can be formed under a true strain of 30 %. When the martensitic phase transformation is triggered slightly higher than the material Ms temperature, elastic stresses in the microstructure are enough to activate the transformation and the elastic energy induced in the material is enough to compensate the missing chemical driving force at this temperature [11]. On the other hand, when the deformation is applied close to the material Md30 temperature, the transformation will be mainly controlled by plastic deformation and the role of deformation defects will control the transformation process [10]. The so-formed martensites can then be considered as different and called Stress-Assisted Martensite (SAM) and Strain-Induced Martensite (SIM), respectively. On the other hand, TWIP can happen when Stacking Fault Energies (SFE) is in the range 18-45 m.Jm-2 for austenitic structures. Deformation twinning is especially promoted by high strain rate. The ' martensite can be produced at the intersection of mechanical twins as this volume is double-sheared, resulting in the nucleation of the phase:    (twins)  '. In the case of lower SFE (<18 mJm-2), martensitic transformation can involve the formation of a transient phase named -martensite. The formation of the -martensite is driven by the insertion of Shockley partial dislocations in every two successive {111} plans [13]. The face-centered cubic austenite is consequently transformed in the hexagonal close-packed -martensite as they share their same atomic packing factor. Thus, under increasing loading, the -martensite will act as a transient phase to produce the more stable ' martensite as follows:     '

Dwell fatigue life dispersion of a near alpha titanium alloy

Early failure during dwell-fatigue of titanium alloys is a research of interest as it directly drives the in-service life of critical parts in turbines. A study was undertaken to examine the evolution of life and strain in six specimens made of near-alpha titanium alloy (IMI 834) during dwell-fatigue loading conditions. The strain accumulation in dwell-fatigue test for one sample was significantly different. This specimen presents a different ultrasonic response from that of the other specimens, that is ultrasonic measurements reveal a different macro-texture for this sample. When this specimen is excluded, it is found that ultrasonic speed correlates well with dwell-fatigue life for the remaining samples. This paper evaluates the usefulness of ultrasound waves to predict the strain and the dwellfatigue life of a titanium alloy.NRC publication: Ye

Traitement de surfaces par impacts: Évaluation des contraintes résiduelles induites par martelage

Ce travail a pour objectif l'étude de la technique de relaxation des contraintes résiduelles par martelage. Comme première approche, on a choisi d'étudier l'effet du martelage seul sur des plaques libres. 4 plaques d'acier inoxydable austénitique 304L ont été martelées par 1, 3, 5 et 9 couches de martelage respectivement, alors qu'une cinquième plaque issue du même lot a servi de témoin pour évaluer les contraintes initiales. Les contraintes résiduelles ont été évaluées grâce à la technique des contours, qui permet d'avoir une carte 2D de la distribution des contraintes résiduelles dans le sens perpendiculaire au cordon de soudage. Ces essais ont permis de démontrer l'effet bénéfique du martelage à induire des contraintes de compression sur une profondeur de quelques millimètres, ce qui réduit les risques de fissuration, ainsi qu'une amélioration des propriétés mécaniques locales. Aussi, nous avons réussi à identifier le nombre optimal de couches de martelage à appliquer afin d'induire un niveau maximal de contraintes de compression tout en minimisant le nombre de passes de martelage et aussi le temps de traitement

In-situ monitoring of microstructure and metallurgical processes using laser-ultrasonics

Peer reviewed: YesNRC publication: Ye

Solidification sequence of Ni-Si-Cr ~3wt% B brazing alloys

Various Ni-Si-Cr-B brazing alloys with a similar boron content (3 wt%) are investigated. Alloy compositions cover a section of the quaternary phase diagram that connects BNi-3 to BNi-9 ternary alloys for industrial use. Samples were melted and solidified at low cooling rate (1 K/min) under secondary vacuum to minimize oxidation and the metastable phase formation. Transformation temperatures, microstructures, and chemical analyses are reported. Experimental conditions were found to play a significant role on the accuracy of obtained data. Thermodynamic data were collected upon heating. Before analyzing quaternary alloys, the results of investigated ternary alloys were compared with the literature and discussed.The authors thank Etablissements Chpolansky, Marcoussis, France, which kindly provided the certified commercial brazing alloys. Dr. J. Zollinger calculated the equilibrium ternary alloy phase diagrams with Thermo-Calc® and the TTNi7 database. M. Bertrand, master student, also contributed to this study. Finally Dr. L. Peltier and P. Charbonnier realized a secondary vacuum furnace which allowed melting of these alloys. The authors thank NSERC and CRIAQ for their indirect financial support on related projects

Use of stable isotope fingerprints to assign wintering origin and trace shorebird movements along the East Atlantic Flyway

Migratory connectivity can be defined as the flux of individuals or populations among areas between stages of an animal's life cycle. Many shorebird species perform long-distance migrations and while moving between breeding and wintering grounds, they depend on a network of intermediate wetlands (stopover sites) where populations of different origins extensively overlap. The difficulty to discriminate such populations represents a serious obstacle to the identification of the links between breeding or wintering areas and stopover sites, and also precludes the estimation of demographic parameters for each population. In this study, we test if linear discriminant models based on stable carbon and nitrogen isotope ratios in toenails can he used to identify populations of several shorebird species of different wintering origins overlapping at two stopover sites of the East Atlantic Flyway. In addition, we evaluate the ability of this approach to infer migratory phenological patterns of shorebirds. Linear discriminant analyses performed overall well in distinguishing the isotopic signals of birds from wintering areas (in France, Portugal, Morocco, Mauritania and Guinea-Bissau) most species, correctly classifying over 80% (n = 542) of all wintering individuals sampled at these areas. Assignment rates of shorebirds captured during spring migration were also high (96%, n=323) at the Tejo estuary, Portugal, but lower (40%, 0 = 185) at Marennes-Oleron Bay in France, and also differed among species. A large proportion of spring migrants captured in Portugal and France were assigned to Banc d' Arguin in Mauritania, the most important wintering area in the flyway. Phenological patterns derived for dunlins (Calidris alpina), common ringed plovers (Charadrius hiaticula) and grey plovers (Pluvialis squatarola) suggest that the first northward migrants started arriving at the Tejo estuary during the second half of March, with peaking numbers occurring during April or May.</p

Sélection des sites et des proies par les Barges à queue noire Limosa limosa islandica s’alimentant sur les herbiers de zoostères et les vasières du centre de la côte atlantique française

Characeae surveys and water physico-chemical studies have been performed during four years (2007-2011) on 41 wetlands of Numidia (NE Algeria). Among the 12 species inventoried in the region, four are rare in North Africa and one, Nitella batrachosperma, is new for Algeria. Comparison between Characeae communities and phosphate contents shows that (1) Characeae have nearly disappeared from habitats that contain high levels of orthophosphates (> 780 μg.l-1), (2) three species of Chara (C. globularis, C. gymnophylla and C. vulgaris) characterize medium-polluted habitats (213-780 μg.l-1), where they are often present as discontinuous populations, and (3) the Nitella (N. opaca and N. translucens) are good indicators of low water pollution (< 70 μg.l-1). In addition to the sole presence of a given species, the physiological appearance of the plants has to be taken into consideration as a sign of a healthy population.Dans une population en expansion, certains individus sont voués à explorer et exploiter de nouveaux habitats de moindre qualité. Sur la façade ouest européenne, la Barge à queue noire Limosa limosa islandica, qui se reproduit exclusivement en Islande, est l’une des rares espèces de limicoles avec une tendance démographique positive. Les côtes françaises accueillent 28 % de la population au milieu de l’hiver, le reste se distribuant sur les îles britanniques et la péninsule ibérique. Contrairement à la Grande Bretagne et l’Irlande, la population française se concentre sur un nombre restreint de sites mais avec de fortes concentrations. Les Pertuis Charentais accueillent plus de 65 % de la population nationale (ca 18 000 individus). Dans cette étude, nous décrivons comment la sélection des proies et les stratégies d’alimentation en milieux intertidaux sont liées à la sélection des sites à l’échelle locale and peuvent expliquer la distribution des oiseaux en période de non reproduction. La sélection des sites et des espèces-proies par les barges a été étudiée par la description du régime alimentaire et de la ressource trophique sur six zones d’alimentation sur les quatre principaux sites hivernages. Dans les Pertuis Charentais, il a été confirmé que ces barges occupent une niche écologique herbivore sur le nouveau site d’hivernage de l’île de Ré, où elles s’alimentent de Zostères naines Zostera noltii. La barge garde un régime carnivore sur les sites continentaux, où elle s’alimente principalement du bivalve Macoma balthica. Sur ces sites, les barges ciblent la proie la plus rentable. À l’exception du dérangement humain, la distribution des individus dans les Pertuis Charentais semble être déterminée par la distribution, l’abondance et l’accessibilité de seulement deux espèces-proies parmi un large éventail d’espèces benthiques

Bird migration in space and time: chain migration by Eurasian curlew Numenius arquata arquata along the East Atlantic Flyway

Migration patterns in birds vary in space and time. Spatial patterns include chain, leapfrog and telescopic migration. Temporal patterns such as migration duration, number, and duration of stopovers may vary according to breeding latitude, sex, and season. This study aimed to verify these patterns in a long-distance migrant, the Eurasian curlew Numenius arquata arquata, and to provide a synopsis of spatio-temporal migration patterns in this species of concern throughout the East Atlantic Flyway. We tagged 85 adults with GPS-data loggers in Germany, Poland, France and Estonia between 2013 and 2019. We computed the distance flown, linear loxodromic distance, duration, stopover number, total stopover duration, mean stopover duration, departure time and arrival time for 177 out of 187 tracks. On average (± standard deviation), spring migration occurred from 4 to 14 April (10.2 ± 8.4 days), curlews flew 3.623 ± 1.366 km, and had 5.8 ± 3.6 stopovers, with a duration of 29.4 ± 38.2 h per stopover, while autumn migration occurred from 18 to 29 June (10.9 ± 9.9 days), curlews flew 3.362 ± 1.351 km, and had 5.4 ± 4.0 stopovers, with 31.8 ± 32.3 h per stopover. Curlews displayed chain migration because wintering curlews maintained the latitudinal sequence to their breeding sites. Southern curlews had a longer nesting period due to their earlier arrivals. While spring arrival at breeding sites did not differ between the sexes, in autumn females departed earlier than males. Migration duration and distance, as well as stopover number and duration, showed a significant increase with breeding site latitude but did not differ between the sexes or between spring and autumn migrations, suggesting that curlews took a comparable amount of time migrating during both seasons. The high site faithfulness in curlews suggests that rapid autumn migration allows them to return to defend their winter foraging areas