Deformation mechanisms and damage of oxide dispersion strengthened steels at high temperature

International audienceA ferritic oxide dispersion strengthened steel is under study for fuel cladding applications in future nuclear systems. Tensile tests and creep tests are carried out at various temperatures to determine its mechanical properties along the extrusion direction. For these two types of loading, the material exhibits a high mechanical resistance. Its ductility appears to be strongly influenced by the strain rate and the temperature. Deformation mechanisms linked to diffusion phenomena are suspected and intergranular damage is observed on fractured specimens

Modulational instability of two pairs of counter-propagating waves and energy exchange in two-component media

The dynamics of two pairs of counter-propagating waves in two-component media is considered within the framework of two generally nonintegrable coupled Sine-Gordon equations. We consider the dynamics of weakly nonlinear wave packets, and using an asymptotic multiple-scales expansion we obtain a suite of evolution equations to describe energy exchange between the two components of the system. Depending on the wave packet length-scale vis-a-vis the wave amplitude scale, these evolution equations are either four non-dispersive and nonlinearly coupled envelope equations, or four non-locally coupled nonlinear Schroedinger equations. We also consider a set of fully coupled nonlinear Schroedinger equations, even though this system contains small dispersive terms which are strictly beyond the leading order of the asymptotic multiple-scales expansion method. Using both the theoretical predictions following from these asymptotic models and numerical simulations of the original unapproximated equations, we investigate the stability of plane-wave solutions, and show that they may be modulationally unstable. These instabilities can then lead to the formation of localized structures, and to a modification of the energy exchange between the components. When the system is close to being integrable, the time-evolution is distinguished by a remarkable almost periodic sequence of energy exchange scenarios, with spatial patterns alternating between approximately uniform wavetrains and localized structures.Comment: 35 pages, 13 figure

Transient crosslinking kinetics optimize gene cluster interactions

Our understanding of how chromosomes structurally organize and dynamically interact has been revolutionized through the lens of long-chain polymer physics. Major protein contributors to chromosome structure and dynamics are condensin and cohesin that stochastically generate loops within and between chains, and entrap proximal strands of sister chromatids. In this paper, we explore the ability of transient, protein-mediated, gene-gene crosslinks to induce clusters of genes, thereby dynamic architecture, within the highly repeated ribosomal DNA that comprises the nucleolus of budding yeast. We implement three approaches: live cell microscopy; computational modeling of the full genome during G1 in budding yeast, exploring four decades of timescales for transient crosslinks between 5k bp domains in the nucleolus on Chromosome XII; and, temporal network models with automated community detection algorithms applied to the full range of 4D modeling datasets. The data analysis tools detect and track gene clusters, their size, number, persistence time, and their plasticity. Of biological significance, our analysis reveals an optimal mean crosslink lifetime that promotes pairwise and cluster gene interactions through "flexible" clustering. In this state, large gene clusters self-assemble yet frequently interact, marked by gene exchanges between clusters, which in turn maximizes global gene interactions in the nucleolus. This regime stands between two limiting cases each with far less global gene interactions: with shorter crosslink lifetimes, "rigid" clustering emerges with clusters that interact infrequently; with longer crosslink lifetimes, there is a dissolution of clusters. These observations are compared with imaging experiments on a normal yeast strain and two condensin-modified mutant cell strains, applying the same image analysis pipeline to the experimental and simulated datasets

COMBINED CONDUCTIVE-RADIATIVE HEAT TRANSFER ANALYSIS IN COMPLEX GEOMETRY USING THE MONTE CARLO METHOD

Deterministic methods are commonly used to solve the heat balance equation in three-dimensional (3D) geometries. This article presents a preliminary study to the use of a stochastic method for the computation of the temperature in complex 3D geometries where the combined conductive and radiative heat transfers are coupled in the porous solid phase. The Monte Carlo algorithm and its results are validated by a comparison with the results obtained with a conventional finite-volume method

Propriétés en fluage à haute température et caractérisations microstructurales des joints soudés P92

National audienceLa présente étude est consacrée aux propriétés en fluage des liaisons soudées en acier P92. Les objectifs sont de déterminer le lieu de rupture d'éprouvettes sollicitées en fluage à 550°C sous diffé rents niveaux de contraintes (160 à 240 MPa) et de caractériser l'évolution de la microstructure du joint soudé au cours du fluage à cette même température. L'observation des faciès de rupture et l'examen des microstructures du joint soudé avant et après fluage sont entrepris afin de caractériser le mode d'endommagement. Pour les éprouvettes observées, la rupture est ductile et se produit dans la zone affectée thermiquement, en particulier dans la zone intercritique. Une striction importante est également observée, à l'intérieur de laquelle sont présentes de multiples cavités et fissures

High temperature creep properties and microstructural examinations of P92 welds

International audienceThe present study deals with the creep properties of welded joints ...