210 research outputs found

    Micromechanical investigation of the influence of defects in high cycle fatigue

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    This study aims to analyse the influence of geometrical defects (notches and holes) on the high cycle fatigue behaviour of an electrolytic copper based on finite element simulations of 2D polycrystalline aggregates. In order to investigate the role of each source of anisotropy on the mechanical response at the grain scale, three different material constitutive models are assigned successively to the grains: isotropic elasticity, cubic elasticity and crystal plasticity in addition to the cubic elasticity. The significant influence of the elastic anisotropy on the mechanical response of the grains is highlighted. When considering smooth microstructures, the crystal plasticity have has a slight effect in comparison with the cubic elasticity influence. However, in the case of notched microstructures, it has been shown that the influence of the plasticity is no more negligible. Finally, the predictions of three fatigue criteria are analysed. Their ability to predict the defect size effect on the fatigue strength is evaluated thanks to a comparison with experimental data from the literature

    L’Asie du Sud-Est, nouveau centre de gravité des câbles sous-marins

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    In the physical World Wild Web, now made up of more than 500 submarine communication cables, Southeast Asia plays a key role in data exchange, whether at the regional level or beyond the continent, toward North America and Europe. While the American Internet giants have been helping to relaunch cable projects on the Transpacific road since 2011, Southeast Asia is also the subject of new Beijing-developed infrastructure projects within the framework of the BRI strategy, contributing in making this region a theatre of Sino-American rivalry. By studying the evolution of regional connectivity, we analyse what the current changes reveal about the future place of this geographical space in the world’s digital scene. As an attractive region, giving rise to both new economic prospects and geopolitical tensions, Southeast Asia could thus become a new centre of gravity in digital networks

    Analysis of the multiaxial fatigue strength at the mesoscopic scale using 3D microstructure modeling and extreme value statistics

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    Fatigue life computing methods are generally based on putting into equation the mechanical quantities calculated at the micro or meso scale, the relevance of these selected quantities being validated by the capacity of the models to reproduce experimental results at the macroscopic scale. Although the scaling of the damage mechanisms involved in fatigue crack initiation processes are relatively well identified (grain scale, slip bands), their explicit consideration in fatigue criteria is still not well-developed. Furthermore, the existing methods do not consider the microstructure-sensitivity. The aim of this paper is to present the computational strategies developed to account for the microstructure-sensitivity in the calculation of fatigue strength. This work is based on three parts: (1) the development of 3D microstructure modeling tools (2) the analysis of the dispersion induced by the microstructure heterogeneities on the critical fatigue damage indicators and (3) the development of a statistical approach which provides a framework for analyzing calculation results in the HCF (High Cycle Fatigue) regime. In this background, a method of analysis based on the construction of statistical extreme value distributions from FEA calculation results was developed. The evolution of the scaling parameters of these distributions for different loading conditions informed us about the effect of non-proportional loading and microstructure. A design method based on these extreme value statistics is presented to obtain a new mesoscopic criterion sensitive to microstructure parameters. Finally, surface effects are discussed too

    Analysis of the mesoscopic high cycle fatigue strength of FCC metals with polycrystalline plasticity and extreme value probability methods

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    This study analyzes the influence of microstructure properties on the extreme value distributions of the fatigue indicator parameter (FIP) corresponding to the multiaxial HCF Dang Van criterion. The following loading cases are considered: uniaxial loading, proportional and non-proportional multiaxial loadings. The mesoscopic FIP determined from FE calculations on 2D polycrystalline synthetic aggregates using the ZeBuLoN code. The approach adopted in this work is to replace the RVE by random microstructure elements that can be considered as “statistical volume element” (SVE). A set of extreme values is constructed by determining the maximum value of the FIP for each SVE. The type of extreme value distribution is analyzed with a generalized extreme value function and is shown to follow a Gumbel type distribution. The shape factors of this distribution are compared for the different loading conditions. This comparison shows the limitations of the used criterion, especially in the case of multiaxial loadings. The effect of anisotropy on these distributions is finally investigated by comparing the results of two types of texture (isotropic and rolling). The introduction of a preferential texture reduces the shape factor and the criterion applied with the classic stress field becomes conservative, and also decreases the scatter parameter of the extreme value distributions of the FIP

    Analysis of the mesoscopic high cycle multiaxial fatigue strength of fcc metals with crystal plasticity and generalized extreme values probability

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    Multiaxial high cycle fatigue modeling of materials is an issue that concerns many industrial domains (automotive, aerospace, nuclear, etc) and in wich many progress still remains to be achieved. Several approaches exist in the litterature: invariants, energy, integral and critical plane approaches all of there having their advantages and their drawbacks. These different formulations are usually based on mechanical quantities at the micro or meso scales using localization schemes and strong assumptions to propose simple analytical forms. This study aims to revisit these formulations using a numerical approach based on crystal plasticity modelling coupled with explicit description of microstructure (morphology and texture). This work has three steps: First, 2D periodic digital microstructures based on a random grain sizes distribution are generated. Multiaxial cyclic load conditions corresponding to the fatigue strength at 107 cycles are applied to these microstructures. Then, the mesoscopic Fatigue Indicator Parameters (FIPs), formulated from the different criteria existing in the literature, are identified using the FE calculations of the mechanical fields. These mesoscopic FIP show the limits of the original criteria when it comes to applying them at the grain scale. Finally, a statistical method based on extreme value probability is used to redefine the parameters of these criteria. These new criteria contain the sensitivity of the microstructure variability

    Evidence for an optimal level of connectivity for establishment and colonisation

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    International audienceDispersal is usually associated with the spread of invasive species, but it also has two opposing effects, one decreasing and the other increasing the probability of establishment. Indeed, dispersal both slows population growth at the site of introduction and increases the likelihood of surrounding habitat being colonized. The connectivity of the introduction site is likely to affect dispersal, and, thus, establishment, according to the dispersal behaviour of individuals. Using individual-based models and microcosm experiments on minute wasps, we demonstrated the existence of a hump-shaped relationship between connectivity and establishment in situations in which individual dispersal resembled a diffusion process. These results suggest that there is an optimal level of connectivity for the establishment of introduced populations locally at the site of introduction, and regionally over the whole landscape

    Micro-mechanical modelling of high cycle fatigue behaviour of metals under multiaxial loads

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    An analysis of high cycle multiaxial fatigue behaviour is conducted through the numerical simulation of polycrystalline aggregates using the finite element method. The metallic material chosen for investigation is pure copper, which has a Face Centred Cubic (FCC) crystalline microstructure. The elementary volumes are modelled in 2D using an hypothesis of generalised plane strain and consist of 300 equi-probability, randomly oriented grains with equiaxed geometry. The aggregates are loaded at levels equivalent to the average macroscopic fatigue strength at 1E7 cycles. The goal is to compute the mechanical quantities at the mesoscopic scale (i.e., average within the grain) after stabilization of the local cyclic behaviour. The results show that the mesoscopic mechanical variables are characterised by high dispersion. A statistical analysis of the response of the aggregates is undertaken for different loading modes: fully reversed tensile loads, torsion and combined in-phase tension–torsion. Via the calculation of the local mechanical quantities for a sufficiently large number of different microstructures, a critical analysis of certain multiaxial endurance criteria (Crossland, Dang Van and Matake) is conducted. In terms of material behaviour models, it is shown that elastic anisotropy strongly affects the scatter of the mechanical parameters used in the different criteria and that its role is predominant compared to that of crystal plasticity. The analysis of multiaxial endurance criteria at both the macroscopic and mesoscopic scales clearly show that the critical plane type criteria (Dang Van and Matake) give an adequate estimation of the shear stress but badly reflect the scatter of the normal stress or the hydrostatic stress

    A two-scale finite element model for the fatigue design of large welded structures

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    Weld toes and weld roots of continuously welded structures subjected to cyclic loading are critical zones in terms of the fatigue resistance. The finite element method coupled with a fatigue criterion is commonly used to ensure the correct sizing and fatigue design of welded structures. However, weld geometries are often simplified or idealized to limit computational cost. In this work, a numerical two-scale approach is proposed in order to calculate a non-local multiaxial equivalent stress at the weld toe and the weld root from a global finite element shell model. The influence of the parameters of the proposed model on the stiffness behaviour is investigated for three welded structures and for different loading cases. A comparison in terms of stiffness with other models from the literature is also proposed. The results show that the stiffness behaviour is not affected by the parameters of the proposed approach and that it is the most robust model for the different geometries and loading cases studied. The variation in the non-local multiaxial equivalent stress as a function of the parameters of the proposed approach was also studied. The comparison with full solid finite element models makes it possible to define minimum values for the different parameters studied and validates the potential of the proposed approach for the fatigue design of welded structures

    Separated representation of incremental elastoplastic simulations

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    Forming processes usually involve irreversible plastic transformations. The calculation in that case becomes cumbersome when large parts and processes are considered. Recently Model Order Reduction techniques opened new perspectives for an accurate and fast simulation of mechanical systems, however nonlinear history-dependent behaviors remain still today challenging scenarios for the application of these techniques. In this work we are proposing a quite simple non intrusive strategy able to address such behaviors by coupling a separated representation with a POD-based reduced basis within an incremental elastoplastic formulation

    L’Asie du Sud-Est 2023 : bilan, enjeux et perspectives

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    Chaque année, l’Institut de recherche sur l’Asie du Sud-Est contemporaine (IRASEC), basé à Bangkok, mobilise une vingtaine de chercheurs et d’experts pour mieux comprendre l’actualité régionale de ce carrefour économique, culturel et religieux, au cœur de l’Indo-Pacifique. Cette collection permet de suivre au fil des ans l’évolution des grands enjeux contemporains de cette région continentale et archipélagique de plus de 680 millions d’habitants, et d’en comprendre les dynamiques d’intégration régionale et de connectivités avec le reste du monde. L’Asie du Sud-Est 2023 propose une analyse synthétique et détaillée des principaux événements politiques et diplomatiques, ainsi que des évolutions économiques, sociales et environnementales de l’année 2022 dans chacun des onze pays de la région. Ce décryptage est complété pour chaque pays par un focus sur deux personnalités de l’année et une actualité marquante en image. L’ouvrage propose également cinq dossiers thématiques qui abordent des sujets traités à l’échelle régionale sud-est asiatique : les ressorts institutionnels de l’approche de santé intégrée One Health, le vieillissement de la population et sa prise en compte par les politiques publiques, les câbles sous-marins au cœur de la connectivité sud-est asiatique, l’aménagement du bassin du Mékong et ses multiples acteurs, et les enjeux politiques et linguistiques des langues transnationales. Des outils pratiques sont également disponibles : une fiche et une chronologie par pays et un cahier des principaux indicateurs démographiques, sociaux, économiques et environnementaux
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