Improved exponential stability for near-integrable quasi-convex Hamiltonians

In this article, we improve previous results on exponential stability for analytic and Gevrey perturbations of quasi-convex integrable Hamiltonian systems. In particular, this provides a sharper upper bound on the speed of Arnold diffusion which we believe to be optimal

The synthesis of allylic organotin compounds for the enantioselective formation of homoallylic alcohols

Allylic tins when reacted with aldehydes are known to be diastereoselective for the resulting homoallylic alcohols. It is believed that a diastereomeric transition state can be created if the allylic tin contains a chiral ligand. Enantiomerically enriched α-pinene was chosen for the chiral ligand and brominated by an electroptilic addition of HBr to form enantiomerically enriched endo-2-bromobornane. This was then reacted with various alkyltin anions prepared from the chlorides, R3SnCl (R=Ph, Bu, & Me). Of the alkyltin chlorides chosen Me3SnCl produced the desired product in the best yield; the others were either very slow in reacting or unreactive. The final step was the addition of an allylic group to the organotin compound. Crotyl and cinnamyl were the allylic groups chosen. Model compounds were prepared to investigate attaching crotyl and cinnamyl to tin. The cinnamyl group was chosen since it is known to go by way of a cyclic transition state, when attached to an organometallic compound and reacted with an aldehyde. In forming cinnamyldimethylisobornyltin, the chiral allylic tin, a rearrangement of the double bond on the cinnamyl group occurred and the product was a mixture of two isomers. Because of this and the very low yield, the final reaction with the aldehyde was not carried out

Ellipticity loss analysis for tangent moduli deduced from a large strain elastic–plastic self-consistent model

In order to investigate the impact of microstructures and deformation mechanisms on the ductility of materials, the criterion first proposed by Rice is applied to elastic–plastic tangent moduli derived from a large strain micromechanical model combined with a self-consistent scale-transition technique. This approach takes into account several microstructural aspects for polycrystalline aggregates: initial and induced textures, dislocation densities as well as softening mechanisms such that the behavior during complex loading paths can be accurately described. In order to significantly reduce the computing time, a new method drawn from viscoplastic formulations is introduced so that the slip system activity can be efficiently determined. The different aspects of the single crystal hardening (self and latent hardening, dislocation storage and annihilation, mean free path, etc.) are taken into account both by the introduction of dislocation densities per slip system as internal variables and the corresponding evolution equations. Comparisons are made with experimental results for single and dual-phase steels involving linear and complex loading paths. Rice’s criterion is then coupled and applied to this constitutive model in order to determine the ellipticity loss of the polycrystalline tangent modulus. This criterion, which does not need any additional “fitting” parameter, is used to build Ellipticity Limit Diagrams (ELDs).ArcelorMittal Researc

Systems Verification using Randomized Exploration of Large State Spaces

International audienceExhaustive verification often suffers from the state-explosion problem, where the reachable state space is too large to fit in main memory. For this reason, and because of disk swapping, once the main memory is full very little progress is made, and the process is not scalable. To alleviate this, partial verification methods have been proposed, some based on randomized exploration, mostly in the form of random walks. In this paper, we enhance partial, randomized state-space exploration methods with the concept of resource-awareness: the exploration algorithm is made aware of the limits on resources, in particular memory and time. We present a memory-aware algorithm that by design never stores more states than those that fit in main memory. We also propose criteria to compare this algorithm with similar other algorithms. We study properties of such algorithms both theoretically on simple classes of state spaces and experimentally on some preliminary case studies

Physically-motivated elasto-visco-plastic model for the large strain-rate behavior of steels

A physically based elasto-visco-plastic constitutive model is presented and compared to experimental results for a DD14 mild steel. The model requires significantly fewer material parameters compared to other visco-plasticity models from the literature while exhibiting very good accuracy. Accordingly, the parameter identification is simple and intuitive, requiring a relatively small set of experiments. The strain-rate sensitivity modeling is not restricted to a particular hardening law and thus provides a general framework in which advanced hardening equations can be adopted and compared. The model has been implemented in the commercial finite element code Abaqus/Explicit. First predictions compared to experiments are analyzed and underline the effect of hardening law and strain-rate sensitivity on 3D finite element simulations. The model has been also applied as the basis for a homogenization approach at the phase scale; preliminary investigations showed the benefits of coupling such an approach with scale-transition technique where microstructure-relevant data can explicitly enter the model and may be used for material design simulations.ArcelorMitta

Choice alters Drosophila oviposition site preference on menthol

Food choice and preference relies on multiple sensory systems that are under the control of genes and sensory experience. Exposure to specific nutrients and nutrient-related molecules can change food preference in vertebrates and invertebrates. For example, larval exposure of several holometabolous insects to menthol can change their adult response to this molecule. However, studies involving Drosophila melanogaster exposure to menthol produced controversial results due maybe to methodological differences. Here, we compared the ovipositionsite preference of wild-type D. melanogaster lines freely or forcibly exposed to menthol-rich food. After 12 generations, oviposition-site preference diverged between the two lines. Counterintuitively, menthol 'forced' lines showed a persistent aversion to menthol whereas 'free choice' lines exhibited a decreased aversion to menthol-rich food. This effect was specific to menthol since the 'free choice' lines showed unaltered responses to caffeine and sucrose. This suggests that the genetic factors underlying Drosophila oviposition site preference are more rapidly influenced when flies have a choice between alternative sources compared to flies permanently exposed to the same aversive substance. (C) 2013. Published by The Company of Biologists Ltd

Atomic-scale grain boundary engineering to overcome hot-cracking in additively-manufactured superalloys

There are still debates regarding the mechanisms that lead to hot cracking in parts build by additive manufacturing (AM) of non-weldable Ni-based superalloys. This lack of in-depth understanding of the root causes of hot cracking is an impediment to designing engineering parts for safety-critical applications. Here, we deploy a near-atomic-scale approach to investigate the details of the compositional decoration of grain boundaries in the coarse-grained, columnar microstructure in parts built from a non-weldable Ni-based superalloy by selective electron-beam melting. The progressive enrichment in Cr, Mo and B at grain boundaries over the course of the AM-typical successive solidification and remelting events, accompanied by solid-state diffusion, causes grain boundary segregation induced liquation. This observation is consistent with thermodynamic calculations. We demonstrate that by adjusting build parameters to obtain a fine-grained equiaxed or a columnar microstructure with grain width smaller than 100 $\mu$m enables to avoid cracking, despite strong grain boundary segregation. We find that the spread of critical solutes to a higher total interfacial area, combined with lower thermal stresses, helps to suppress interfacial liquation.Comment: Accepted version at Acta Materiali

Modélisation Elasto-Viscoplastique du Comportement des Aciers

Dans cet article, une modélisation des différentes phases (ferrite, perlite etc.…) d’un acier multiphasé est proposée moyennant une loi phénoménologique à base physique. La description unifiée du comportement viscoplastique peut être décrite à partir de lois de type sinus hyperbolique. Dans cette optique, une loi en sinus hyperbolique est utilisée afin de décrire de façon physiquement acceptable l’ensemble des comportements asymptotiques du comportement viscoplastique. L’écrouissage de chacune des phases est de type combiné (isotrope et cinématique) incluant l’effet de vitesse de déformation propre au comportement visqueux. A partir d’un modèle de comportement monotone unidimensionnel d’inspiration métallurgique, nous développons une formulation tridimensionnelle incrémentale, en déterminant l’expression du potentiel dont découlent les lois de comportement correspondantes. La capacité du modèle ainsi obtenu à décrire le comportement élasto-viscoplastique d’une phase est testée au travers de simulations de tests rhéologiques décrivant différents chemins de déformation à différentes vitesses de déformation. Une discussion basée sur la comparaison de ces simulations avec des résultats expérimentaux sur un acier 100% ferritique est présentée.In this work, an elastic-viscoplastic behaviour modelling of multi-phase steels at large strain-rates is presented. For each phase (ferrrite, perlite etc…), an advanced physically based viscoplastic constitutive model is adopted. Viscoplastic behaviour can be described by hyperbolic sine function. A new physically based constitutive law suitable to capture all the main features of viscoplasticity is used. In this study, the model has been extended to three dimensional framework in view of numerical implementation in a finite element code. The ability of this modelling framework to describe the behaviour of steels at high strain-rates is explored by means of simulations of rheological tests at various strain-rates and involving different strain paths. The model is applied to a 100% polycrystalline ferritic steel. Numerical results in terms of strain rate sensitivity and Bauschinger effect are discussed and compared with experimental ones.ArcelorMittal & Région Lorrain

“Fixing a heart”: the game of electrolytes in anorexia nervosa

A 25-year-old woman with chronic anorexia nervosa and depression presented with sudden weakness and fatigue. Psychosocial history was notable for binge-starve cycles over the past year and a decline in overall well-being. Vitals on presentation were notable for hypothermia, hypotension, and bradycardia. Initial exam was significant for emaciation, lethargy, and lower extremity edema. Laboratory work-up revealed markedly elevated LFTs, hypoglycemia, thrombocytopenia and elevated INR and lipase. ECG showed sinus bradycardia with prolonged QTc. Ultrasound revealed normal liver and biliary tree. Serum acetaminophen, alcohol level, and urinary toxicology were unremarkable. Work up for infectious, autoimmune, and genetic causes of hepatitis was negative. Echocardiogram revealed left ventricular hypokinesis and EF 10-15%. Nutritional support was begun slowly, however electrolyte derangements began to manifest on hospital day 2, with hypophosphatemia, hypokalemia, hypocalcemia, and hypomagnesemia. Multiple medical and psychiatric disciplines were consulted, and aggressive electrolyte monitoring and repletion were done. The patient’s overall clinical status improved slowly during her hospital course. Her liver enzymes trended down, and her QTc interval eventually returned toward the normal range. Repeat echocardiogram following treatment revealed improvement of her EF to 40%. Anorexia nervosa is an eating disorder characterized by extremely low body weight, fear of gaining weight or distorted perception of body image, and amenorrhea. Anorexia can lead to life threatening medical complications, and thus constitutes a major challenge to manage. Central to the pathogenesis of the refeeding syndrome is a weakened cardiopulmonary system, electrolytes abnormalities, hepatic dysfunction, liver hypoperfusion and failure. Given the clinical presentation, this patient likely presented on the brink of developing frank refeeding syndrome, with cardiac dysfunction and hypovolemia, leading to hepatic hypoperfusion and ischemic hepatitis. Subsequently, she developed electrolyte disturbances characteristic of refeeding syndrome, which were managed without major complication. Her hospital course is encouraging not only for her recovery, but for the collaboration of the different teams involved in her care, and it highlights the importance of a multidisciplinary approach to caring for patients with the potential dire complications of a complex psychiatric illness

Ellipticity loss analysis for tangent moduli deduced from a large strain elastic–plastic self-consistent model

In order to investigate the impact of microstructures and deformation mechanisms on the ductility of materials, the criterion first proposed by Rice is applied to elastic–plastic tangent moduli derived from a large strain micromechanical model combined with a self-consistent scale-transition technique. This approach takes into account several microstructural aspects for polycrystalline aggregates: initial and induced textures, dislocation densities as well as softening mechanisms such that the behavior during complex loading paths can be accurately described. In order to significantly reduce the computing time, a new method drawn from viscoplastic formulations is introduced so that the slip system activity can be efficiently determined. The different aspects of the single crystal hardening (self and latent hardening, dislocation storage and annihilation, mean free path, etc.) are taken into account both by the introduction of dislocation densities per slip system as internal variables and the corresponding evolution equations. Comparisons are made with experimental results for single and dual-phase steels involving linear and complex loading paths. Rice’s criterion is then coupled and applied to this constitutive model in order to determine the ellipticity loss of the polycrystalline tangent modulus. This criterion, which does not need any additional “fitting” parameter, is used to build Ellipticity Limit Diagrams (ELDs).ArcelorMittal Researc