Dislocation core field. I. Modeling in anisotropic linear elasticity theory

Aside from the Volterra field, dislocations create a core field, which can be modeled in linear anisotropic elasticity theory with force and dislocation dipoles. We derive an expression of the elastic energy of a dislocation taking full account of its core field and show that no cross term exists between the Volterra and the core fields. We also obtain the contribution of the core field to the dislocation interaction energy with an external stress, thus showing that dislocation can interact with a pressure. The additional force that derives from this core field contribution is proportional to the gradient of the applied stress. Such a supplementary force on dislocations may be important in high stress gradient regions, such as close to a crack tip or in a dislocation pile-up

Dislocation core field. II. Screw dislocation in iron

The dislocation core field, which comes in addition to the Volterra elastic field, is studied for the screw dislocation in alpha-iron. This core field, evidenced and characterized using ab initio calculations, corresponds to a biaxial dilatation, which we modeled within the anisotropic linear elasticity. We show that this core field needs to be considered when extracting quantitative information from atomistic simulations, such as dislocation core energies. Finally, we look at how dislocation properties are modified by this core field, by studying the interaction between two dislocations composing a dipole, as well as the interaction of a screw dislocation with a carbon atom

Services rendus par les foraminifères benthiques dans l’étude de l’influence des forçages naturels (e.g. changement climatique) et anthropiques sur l’écosystème estuarien. Exemple de la Loire.

Due to its intermediate position between ocean and continent, estuary is located in the heart of the economic, social and cultural activities. Awareness of the need to manage this vulnerable environment, has led in recent years, to an increase in surveillance activities of the environmental quality. Physico-chemical methods, although dominant and indispensable, reach their limits as a tool for environmental management. This is why other ways are being explored, such as evaluating the environmental quality by bio-indicators. It is in this perspective that SEMHABEL project is subscribed (Suivi Environnemental des Micro-HAbitats Benthiques de l’Estuaire de la Loire - Plan Loire Grandeur Nature 2007-2013 - FEDER). This is the first study in the Loire incorporating the use of benthic foraminifera as a new biological approach. Following a sampling cruise in September 2012, 320 samples of surface sediments were collected from Nantes to de Saint-Gildas. The data obtained allowed to represent and analyze the spatial distribution of communities of benthic foraminifera, along the upstream-downstream continuum and following geochemical and sedimentary evolutions. These results are the first support for the establishment of a database that will allow a better understanding of the actual functioning of this ecosystem. Ultimately, we hope to assess the evolution of the estuarine ecosystem according to climate change and catchment area management and to develop a biotic index for routine monitoring of the health of the estuary

Screw dislocation in zirconium: An ab initio study

Plasticity in zirconium is controlled by 1/3 screw dislocations gliding in the prism planes of the hexagonal close-packed structure. This prismatic and not basal glide is observed for a given set of transition metals like zirconium and is known to be related to the number of valence electrons in the d band. We use ab initio calculations based on the density functional theory to study the core structure of screw dislocations in zirconium. Dislocations are found to dissociate in the prism plane in two partial dislocations, each with a pure screw character. Ab initio calculations also show that the dissociation in the basal plane is unstable. We calculate then the Peierls barrier for a screw dislocation gliding in the prism plane and obtain a small barrier. The Peierls stress deduced from this barrier is lower than 21 MPa, which is in agreement with experimental data. The ability of an empirical potential relying on the embedded atom method (EAM) to model dislocations in zirconium is also tested against these ab initio calculations

Transmission RZ de 40x40 Gbit/s sur 3 pas de 40 dB de Fibre Standard avec 27.4 dB de Gain Raman contra-propagatif et un EDFA de 27 dBm

session V « Systèmes de télécommunication » [502]National audienceLes amplificateurs EDFA forte puissance semblent de bons candidats pour des liaisons festons DWDM. Nous démontrons expérimentalement la transmission de 40 canaux à 40 Gbit/s sur la bande C sur 3 pas de 40 dB de pertes avec une modulation RZ classique

Time reversal methods in acousto-elastodynamics

The aim of the article is to solve an inverse problem in order to determine the presence and some properties of an elastic “inclusion” (an unknown object, characterized by elastic properties discriminant from the surrounding medium) from partial observa- tions of acoustic waves, scattered by the inclusion. The method will require developing techniques based on Time Reversal methods. A finite element method based on varia- tional acousto-elastodynamics formulation will be derived and used to solve to solve the forward, and then, the time reversed problem. A criterion, derived from the reverse time migration framework, is introduced, to help use to construct images of the inclusions to be determined. Our approach will be applied to configurations modeling breast cancer detection, using simulated ultrasound waves

Longitudinal association of biomarkers of pesticide exposure with cardiovascular disease risk factors in youth with diabetes

Background: Cardiovascular disease (CVD) is the leading cause of death among individuals with diabetes, but little is known about the role of exposures to environmental chemicals such as pesticides in the early development of CVD risk in this population. Objectives: To describe changes over time in concentrations of pesticide biomarkers among youth with diabetes in the United States and to estimate the longitudinal association between these concentrations and established risk factors for CVD. Methods: Pesticide biomarkers were quantified in urine and serum samples from 87 youth with diabetes participating in the multi-center SEARCH cohort study. Samples were obtained around the time of diagnosis (baseline visit, between 2006 and 2010) and, on average, 5.4 years later (follow-up visit, between 2012 and 2015). We calculated geometric mean (95% CI) pesticide biomarker concentrations. Eight CVD risk factors were measured at these two time points: body mass index (BMI) z-score, HbA1c, insulin sensitivity, fasting C-peptide (FCP), LDL cholesterol, HDL cholesterol, total cholesterol, and triglycerides. Linear regression models were used to estimate the associations between each pesticide biomarker at baseline and each CVD risk factor at follow-up, adjusting for baseline health outcome, elapsed time between baseline and follow up, sex, age, race/ethnicity, and diabetes type. Results: Participants were, on average, 14.2 years old at their baseline visit, and most were diagnosed with type 1 diabetes (57.5%). 4-nitrophenol, 3-phenoxybenzoic acid, 2,4-dichlorophenoxyacetic acid (2,4-D), 3,5,6-trichloro-2-pyridinol, 2,2-bis(4-chlorophenyl)-1,1-dichloroethene, and hexachlorobenzene were detected in a majority of participants at both time points. Participants in the highest quartile of 2,4-D and 4-nitrophenol at baseline had HbA1c levels at follow-up that were 1.05 percentage points (95% CI: −0.40, 2.51) and 1.27 percentage points (0.22, 2.75) higher, respectively, than participants in the lowest quartile of these pesticide biomarkers at baseline. These participants also had lower log FCP levels (indicating reduced beta-cell function) compared to participants in the lowest quartile at baseline: beta (95% CI) for log FCP of −0.64 (−1.17, −0.11) for 2,4-D and −0.39 (−0.96, 0.18) for 4-nitrophenol. In other words, participants in the highest quartile of 2,4-D had a 47.3% lower FCP level compared to participants in the lowest quartile, and those in the highest quartile of 4-nitrophenol had a 32.3% lower FCP level than those in the lowest quartile. Participants with trans-nonachlor concentrations in the highest quartile at baseline had HbA1c levels that were 1.45 percentage points (−0.11, 3.01) higher and log FCP levels that were −0.28 (−0.84, 0.28) lower than participants in the lowest quartile at baseline, that is to say, participants in the highest quartile of trans-nonachlor had a 24.4% lower FCP level than those in the lowest quartile. While not all of these results were statistically significant, potentially due to the small same size, clinically, there appears to be quantitative differences. No associations were observed between any pesticide biomarker at baseline with BMI z-score or insulin sensitivity at follow-up. Conclusions: Exposure to select pesticides may be associated with impaired beta-cell function and poorer glycemic control among youth with diabetes