Modeling and Numerical Computation of Necking in Round Bars Using a Total Lagrangian Elastoplastic Formulation

International audienceNecking is a bifurcation phenomenon observed in round bars under tensile loading and has been investigated in numbers of papers. In the present work, it is modeled within the framework of finite rate-independent plasticity. The theory is based on thermody-namic foundations developed for standard materials and results in a total Lagrangian formulation for finite plasticity , where the total strain is decomposed additively according to [Green and Nagdhi 1965)] and the hardening is characterized by a nonlinear isotropic hardening law of the saturation type. The discretization and consistent linearization of the elastic-plastic equation set using the standard finite element procedure lead to a low-cost algorithm, robust enough to deal with necking problems. The numerical computations of necking are performed on cylindrical bars with various boundary condition types and the corresponding results compared with those obtained in the literature

Some new analytical results for plastic buckling and initial post-buckling of plates and cylinders under uniform compression

International audienceThis paper is devoted to the theoretical aspects of the elastoplastic buckling and initial post-buckling of plates and cylinders under uniform compression. The analysis is based oil the 3D plastic bifurcation theory assuming the J(2) flow theory of plasticity with the von Mises yield criterion and a linear isotropic hardening. The proposed method is shown to be a systematic and unified way to obtain the critical loads, the buckling modes and the initial slope of the bifurcated branch for rectangular plates under uniaxial or biaxial compression(-tension) and cylinders under axial compression, with various boundary conditions

General formulation for local integration in standard elastoplasticity with an arbitrary hardening model.

International audienceThis paper describes a general method for deriving the plastic corrections and the consistent tangent modulus for a wide range of arbitrary non-linear hardening models within the framework of standard small strains elastoplasticity. The features of the proposed formulation are: (i) the local solution is obtained through an iterative procedure. The plastic corrections are given in closed forms exhibiting one scalar function denoted by G(alg) and three fourth-order tensors D-alg, G(alg), L-alg, which are shown to be the algorithmic discrete counterparts of usual theoretical continuum quantities, (ii) the consistent tangent modulus has a symmetrical expression involving the same quantities. Finite element computations are performed using a particular non-linear kinematic hardening model and allow to exhibit the ratcheting phenomenon usually observed on mechanical components. subjected to cyclic loadings

Contribution à l'analyse du flambement et post-flambement de poutres sandwich élastoplastiques

Les structures sandwich connaissent actuellement un essor sans précédent dans un large panel d'applications industrielles, du fait du compromis réalisé entre rigidité et légèreté. La présence d'éléments minces ou élancés tels que les peaux rend cependant ces matériaux très vulnérables face à des chargements de compression prédominante, et les instabilités géométriques dont ils font l'objet représentent même leur principale source de ruine. Ce travail est donc voué à l'étude analytique et numérique du flambement et du post-flambement de poutres sandwich sous compression longitudinale. L'approche analytique développée vise à formuler des expressions explicites et précises des différents modes de flambement (globaux/locaux) observés en pratique et des chargements critiques associés. Un élément fini 1D enrichi est ensuite mis en oeuvre, dont la formulation s'inspire des prédictions modales analytiques. Le programme numérique qui en découle est pourvu de techniques robustes de longueur d'arc et de branchement pour assurer un suivi de courbe incrémental efficace en présence de fortes non-linéarités et une gestion automatique de la bifurcation (sans l'introduction de défauts). À des fins de validation, des calculs EF 2D sont enfin réalisés en utilisant un code de calcul spécifique mettant en jeu les méthodes numériques sus-citées. Ces outils permettent l'analyse du post-flambement et la mise en valeur de divers phénomènes de localisation et d'interaction modale, tant en élasticité qu'en plasticité

Nocivité des défauts et propagation de fissures dans les équipements sous pression

L'objectif de ce travail est de déterminer l'évolution du degré de nocivité d'un défaut dans un équipement sous pression durant sa propagation. L'estimation du degré de nocivité d'une fissure passe par le calcul des facteurs d'intensité de contrainte à chaque état d'avancement du front de fissure. Les fissures considérées sont semi-elliptiques. Les géométries et chargements peuvent être complexes de façon à couvrir la majorité des cas industriels. La modélisation numérique par éléments finis s'appuie sur la création d'un bloc-fissure, représentant le maillage optimisé du voisinage de la discontinuité. La loi de Paris permet de décrire le comportement en fatigue sous chargement cyclique. Un programme spécifique (Python), alliant les avantages des codes de calcul Castem et Abaqus, permet d'automatiser la démarche de propagation et facilite l'évaluation de la durée de vie résiduelle d'une structure sous pression fissurée

Synthesis of a-chlorolactams by cyanoborohydride-mediated radical cyclization of trichloroacetamides

A cyanoborohydride-promoted radical cyclization methodology has been developed to access α-chlorolactams in a simple and efficient way, using NaBH3CN and trichloroacetamides easily available from allylic and homoallylic secondary amines. This methodology allowed the synthesis of a library of αchlorolactams (mono and bicyclic), which were tested for herbicidal activity, trans-3-chloro-4-methyl-1-(3-trifluoromethyl)phenyl-2-pyrrolidinone being the most active

Evaluation of drug administration errors in a teaching hospital

<p>Abstract</p> <p>Background</p> <p>Medication errors can occur at any of the three steps of the medication use process: prescribing, dispensing and administration. We aimed to determine the incidence, type and clinical importance of drug administration errors and to identify risk factors.</p> <p>Methods</p> <p>Prospective study based on disguised observation technique in four wards in a teaching hospital in Paris, France (800 beds). A pharmacist accompanied nurses and witnessed the preparation and administration of drugs to all patients during the three drug rounds on each of six days per ward. Main outcomes were number, type and clinical importance of errors and associated risk factors. Drug administration error rate was calculated with and without wrong time errors. Relationship between the occurrence of errors and potential risk factors were investigated using logistic regression models with random effects.</p> <p>Results</p> <p>Twenty-eight nurses caring for 108 patients were observed. Among 1501 opportunities for error, 415 administrations (430 errors) with one or more errors were detected (27.6%). There were 312 wrong time errors, ten simultaneously with another type of error, resulting in an error rate without wrong time error of 7.5% (113/1501). The most frequently administered drugs were the cardiovascular drugs (425/1501, 28.3%). The highest risks of error in a drug administration were for dermatological drugs. No potentially life-threatening errors were witnessed and 6% of errors were classified as having a serious or significant impact on patients (mainly omission). In multivariate analysis, the occurrence of errors was associated with drug administration route, drug classification (ATC) and the number of patient under the nurse's care.</p> <p>Conclusion</p> <p>Medication administration errors are frequent. The identification of its determinants helps to undertake designed interventions.</p

Controlled Radical Polymerization of Vinyl Acetate Mediated by a Bis(imino)pyridine Vanadium Complex

Source type: Prin