Simulation numérique du soudage appliqué au cas d'une traverse arrière d'automobile

L'objectif de ce travail est de proposer une méthodologie robuste (définition de la source, du maillage, du cordon, etc.) pour supprimer toute étape de validation par mesures lors de l'application aux cas industriels. Deux cas tests (T et clin) sont utilisés pour figer les paramètres du modèle et valider la méthodologie. Ensuite, une simulation d'une traverse comportant 88 cordons est réalisée afin de prévoir sa distorsion. Les résultats ainsi obtenus sont comparés avec des essais et sont en accord avec les observations expérimentales

Etude de l'angle de propagation d'une fissure carrée dans un milieu tridimensionnel à comportement élastique non local

On s'intéresse au comportement d'une fissure dans un milieu tridimensionnel dont le comportement est décrit par un modèle élastique non local tel que celui proposé par Erigen. On sait que pour ce type de comportement, les contraintes en pointe de fissure sont finies. Nous étudions la possibilité de prendre avantage de ce fait pour s'affranchir de l'utilisation des facteurs d'intensité de contrainte dans la prédiction du développement de la fissure

Experimental and Numerical Modeling of Segregation in Metallic Alloys

International audienceElectromagnetic levitation (EML) has been used as an experimental technique for investigating the effect of the nucleation and cooling rate on segregation and structure formation in metallic alloys. The technique has been applied to aluminum-copper alloys. For all samples, the primary phase nucleation has been triggered by the contact of the levitated droplet with an alumina plate at a given undercooling. Based on the recorded temperature curves, the heat extraction rate and the nucleation undercooling for the primary dendritic and the secondary eutectic structures have been determined. Metallurgical characterizations have consisted of composition measurements using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometry and the analysis of SEM images. The distribution maps drawn for the composition, the volume fraction of the eutectic structure, and the dendrite arm spacing (DAS) reveal strong correlations. Analysis of the measurements with the help of a cellular-automaton (CA)-finite-element (FE) model is also proposed. The model involves a new coupling scheme between the CA and FE methods and a segregation model accounting for diffusion in the solid and liquid phases. Extensive validation of the model has been carried out on a typical equiaxed grain configuration, i.e., considering the free growth of a mushy zone in an undercooled melt. It demonstrates its capability of dealing with mass exchange inside and outside the envelope of a growing primary dendritic structure. The model has been applied to predict the temperature curve, the segregation, and the eutectic volume fraction obtained upon single-grain nucleation and growth from the south pole of a spherical domain with and without triggering of the nucleation of the primary solid phase, thus simulating the solidification of a levitated droplet. Predictions permit a direct interpretation of the measurements

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Intégration de l’historique du procédé de soudage dans les calculs de tenue en fatigue

Ce document présente la simulation du soudage par apport de matière (MIG/MAG) en utilisant la méthode des éléments finis. Nous soulignons l’intérêt de prendre en compte l’historique du procédé de soudage, c’est-à-dire les contraintes résiduelles, les déformations plastiques cumulées et les pourcentages de phases, sur les calculs de tenue en fatigue. Le groupe PSA Peugeot Citroën travaille depuis plusieurs années sur la simulation numérique du soudage, avec deux objectifs : la réduction du nombre d’essais nécessaire à la mise au point d’une gamme de soudage et la prise en compte du procédé en phase de conception des pièces mécano soudées. Ce deuxième point nécessite de déterminer précisément les contraintes résiduelles et les changements de caractéristiques matériaux liés aux opérations de soudage. La méthode mise en place chez PSA Peugeot Citroën est construite sur la base d’essais simplifiés pour lesquels nous établissons une corrélation entre résultats expérimentaux et numériques. L’objectif de cette étude est de construire une méthodologie suffisamment robuste pour supprimer toute étape de validation par des mesures lors de l’application aux cas industriels et de montrer l’intérêt de prendre en compte l’historique du soudage lors des calculs de tenue en fatigue. Les résultats obtenus par simulation sont en accord avec les observations expérimentales

Impact of solute flow during directional solidification of a Ni-based alloy: In-situ and real-time X-radiography

International audienc