Modèles d'écrouissage pour flexion pure de tôles d'acier : de l'identification à l'estimation des contraintes résiduelles

National audienceLes composants pour la liaison au sol sont classiquement dimensionnés à la fatigue polycyclique. Le comportement du matériau est donc très simple à prédire car élastique. Néanmoins, la connaissance du comportement plastique ne peut être écartée pour deux raisons principales : tout d'abord, les pièces de liaison au sol sont mises en forme par déformation plastique (emboutissage), opération générant des états mécaniques locaux complexes qu'il est nécessaire de prendre en compte dans les étapes de dimensionnement ultérieures ; ensuite car le dimensionnement doit permettre d'assurer l'intégrité du composant lorsque celui-ci est soumis à des coups forts, pouvant générer localement de la plasticité. Dans ce contexte, les travaux exposés ici portent sur deux points. Dans un premier temps, nous nous sommes intéressés à l'identification de modèles élastoplastiques (isotrope, cinématique et mixte [3]) réalisée sur une base expérimentale constituée d'essais de pliage/dépliage sur tôle. Une attention toute particulière est portée à la compétition entre écrouissage cinématique et isotrope. Les essais sont réalisés sur un dispositif de flexion pure développé au CEMEF. L'identification est permise par un algorithme spécifique d'identification présenté en figure 2-b. Dans un second temps, les modèles identifiés sont testés sur une base expérimentale enrichie combinant des essais de pliage/dépliage cyclique à des mesures de contraintes résiduelles après relâchement. Un indicateur d'erreur est mis en place afin de hiérarchiser les capacités de prédiction des modèles. Celui-ci nous permet de conclure à la supériorité du modèle mixte

Dispositif d'essai de fatigue d'une éprouvette

L'invention porte sur un dispositif d'essai de fatigue d'une éprouvette comprenant un châssis pourvu d'au moins une matrice de réception de l'éprouvette. Le dispositif d'essai de fatigue comprend un système de délivrance à l'éprouvette de sollicitations à fréquence ultrasonoer comprise entre 10 kHz et 70 kHz

Simulation of the Kitagawa-Takahashi diagram using a probabilistic approach for cast Al-Si alloys under different multiaxial loads

This article describes a microstructural-based high cycle fatigue strength modelling approach applied to different cast Al-Si alloys used in an automotive context. Thank to different casting processes (gravity die casting and lost foam casting), associated with several heat treatment (T7 and Hot Isostatic Pressing-HIP), three alloys with very different microstructures have been obtained. In a vast experimental campaign undertaken to investigate the fatigue damage mechanisms governing these alloys under different multiaxial loading conditions, it was shown that the principal crack initiation mechanisms for the porosity-free alloy are either the formation of persistent slip bands (PSB) or the rupture and/or debonding of eutectic particles. For the porosity-containing alloys, the fatigue damage is always controlled by crack growth from pores. In order to take into account these fatigue damage mechanisms, a probabilistic model using a combination of the Dang Van and a modified LEFM criteria is proposed. The modified LEFM criterion is able to take into account the influence of the grain size on the threshold of the stress intensity factor. It is shown that for the porosity-free alloy, the predictions are good for combined tension-torsion loads with R = - 1. However, because the crack initiation mechanisms are not the same depending on the hydrostatic stress, the predictions are non-conservative for the uniaxial and equibiaxial tension oads with R = 0,1. For the porosity-containing alloys, the predictions are very good for the uniaxial, combined tension-torsion and equibiaxial tension loads with both R = - 1and R = 0,1. As observed experimentally, the proposed model can also predict a more pronounced effect of casting porosity for the uniaxial and combined tension-torsion loads, when compared to pure torsion loads

Scatter and size effect in High Cycle Fatigue of cast aluminum-silicon alloys: A comprehensive experimental investigation

Cast Al-Si alloys have been widely used in automotive applications with regard to their low density and excellent thermal conductivity. Many components made of these alloys are subjected to cyclic loads which can lead to fatigue failure. For these materials, the well-known size effect in fatigue, whereby the fatigue strength is reduced when the size is increased, can be significant and needs to be properly evaluated. This paper analyses the role of casting defects on the fatigue scatter and size effect. A uniaxial fatigue testing campaign (R=0.1) has been conducted using two cast aluminum alloys, fabricated by different casting processes (gravity die casting and lost foam casting), associated with the T7 heat treatment, and with different degrees of porosity. The fatigue response of different specimens (smooth and notched) with different stressed volumes has been investigated. The first part of this article is about the experimental characterization of the size effect and scatter in both alloys via the concept of the Highly Stressed Volume. The second part investigates the effect of the Highly Stressed Volume on the critical defect size and the establishment of Kitagawa-Takahashi diagram. It is shown that the alloy B, with a population of defects of large size, shows a slight size effect and low scatter. In comparison, alloy A that exhibits a population of defects of relatively small size manifests significant size effect and high scatter

Shot Peening Analysis on Trip780 Steel Exhibiting Martensitic Transformation

In the last years, due to increasing ecology and environmental constraints, a search for lightweight structures has been carried out, leading to the use of more complex geometries and new materials. In that context, TRIP (TRansformation Induced Plasticity) steels are of particular interest as the due to their good mechanical properties to weight ratio [1]. They are often used in the automotive industry for reinforcement parts of the vehicle (bumper, door beam…). To increase life duration,critical parts are shot peened. The specific mechanical behaviour of TRIP steels is due to their microstructure: they contain residual austenite that can transform into martensite when a stress is applied. This mechanism is responsible for hardening of the steels. The beneficial effect of shot peening on metastable austenitic steels was recently established by Fargas et al. [2], resulting in extensive austenite to martensite phase transformation. Numerous analytical models and numerical approaches based on finite element analysis have been developed to simulate the shot peening process. Reviews of the wide variety of numerical models can be found in Rouhaud et al. [3] and Sherafatnia et al. [4]. Only few experimental studies have been published up to now dealing with the impact of shot peening on metastable austenitic steels [5-7]. The first model taking into account the TRIP effect during peening was proposed very recently by Halilovic et al. [8]; it was developed for an austenitic steel AISI 304 peened steel with a single laser shot using a large strain formulation of transformation plasticity

The XMM-LSS survey: the Class 1 cluster sample over the initial 5 square degrees and its cosmological modelling

We present a sample of 29 galaxy clusters from the XMM-LSS survey over an area of some 5deg2 out to a redshift of z=1.05. The sample clusters, which represent about half of the X-ray clusters identified in the region, follow well defined X-ray selection criteria and are all spectroscopically confirmed. For all clusters, we provide X-ray luminosities and temperatures as well as masses. The cluster distribution peaks around z=0.3 and T =1.5 keV, half of the objects being groups with a temperature below 2 keV. Our L-T(z) relation points toward self-similar evolution, but does not exclude other physically plausible models. Assuming that cluster scaling laws follow self-similar evolution, our number density estimates up to z=1 are compatible with the predictions of the concordance cosmology and with the findings of previous ROSAT surveys. Our well monitored selection function allowed us to demonstrate that the inclusion of selection effects is essential for the correct determination of the evolution of the L-T relation, which may explain the contradictory results from previous studies. Extensive simulations show that extending the survey area to 10deg2 has the potential to exclude the non-evolution hypothesis, but that constraints on more refined ICM models will probably be limited by the large intrinsic dispersion of the L-T relation. We further demonstrate that increasing the dispersion in the scaling laws increases the number of detectable clusters, hence generating further degeneracy [in addition to sigma8, Omega_m, L(M,z) and T(M,z)] in the cosmological interpretation of the cluster number counts. We provide useful empirical formulae for the cluster mass-flux and mass-count-rate relations as well as a comparison between the XMM-LSS mass sensitivity and that of forthcoming SZ surveys.Comment: Accepted for publication by MNRAS. Full resolution images as well as additional cluster data are available through a dedicated database at http://l3sdb.in2p3.fr:8080/l3sdb

Self-similar scaling and evolution in the galaxy cluster X-ray Luminosity-Temperature relation

We investigate the form and evolution of the X-ray luminosity-temperature (LT) relation of a sample of 114 galaxy clusters observed with Chandra at 0.1<z<1.3. The clusters were divided into subsamples based on their X-ray morphology or whether they host strong cool cores. We find that when the core regions are excluded, the most relaxed clusters (or those with the strongest cool cores) follow an LT relation with a slope that agrees well with simple self-similar expectations. This is supported by an analysis of the gas density profiles of the systems, which shows self-similar behaviour of the gas profiles of the relaxed clusters outside the core regions. By comparing our data with clusters in the REXCESS sample, which extends to lower masses, we find evidence that the self-similar behaviour of even the most relaxed clusters breaks at around 3.5keV. By contrast, the LT slopes of the subsamples of unrelaxed systems (or those without strong cool cores) are significantly steeper than the self-similar model, with lower mass systems appearing less luminous and higher mass systems appearing more luminous than the self-similar relation. We argue that these results are consistent with a model of non-gravitational energy input in clusters that combines central heating with entropy enhancements from merger shocks. Such enhancements could extend the impact of central energy input to larger radii in unrelaxed clusters, as suggested by our data. We also examine the evolution of the LT relation, and find that while the data appear inconsistent with simple self-similar evolution, the differences can be plausibly explained by selection bias, and thus we find no reason to rule out self-similar evolution. We show that the fraction of cool core clusters in our (non-representative) sample decreases at z>0.5 and discuss the effect of this on measurements of the evolution in the LT relation.Comment: 21 pages, 15 figures. Submitted to MNRAS. Comments welcom

The XMM Large Scale Structure survey: The X-ray pipeline and survey selection function

We present the X-ray pipeline developed for the purpose of the cluster search in the XMM-LSS survey. It is based on a two-stage procedure via a dedicated handling of the Poisson nature of the signal: (1) source detection on multi-resolution wavelet filtered images; (2) source analysis by means of a maximum likelihood fit to the photon images. The source detection efficiency and characterisation are studied through extensive Monte-Carlo simulations. This led us to define two samples of extended sources: the C1 class that is uncontaminated, and the less restrictive C2 class that allows for 50% contamination. The resulting predicted selection function is presented and the comparison to the current XMM-LSS confirmed cluster sample shows very good agreement. We arrive at average predicted source densities of about 7 C1 and 12 C2 per deg2, which is higher than any available wide field X-ray survey. We finally notice a substantial deviation of the predicted redshift distribution for our samples from the one obtained using the usual assumption of a flux limited sample.Comment: 16 pages, 11 figures, MNRAS accepted. The paper with full resolution cluster images is available at http://vela.astro.ulg.ac.be/themes/spatial/xmm/LSS/rel_pub_e.htm

Panmicrobial Oligonucleotide Array for Diagnosis of Infectious Diseases

To facilitate rapid, unbiased, differential diagnosis of infectious diseases, we designed GreeneChipPm, a panmicrobial microarray comprising 29,455 sixty-mer oligonucleotide probes for vertebrate viruses, bacteria, fungi, and parasites. Methods for nucleic acid preparation, random primed PCR amplification, and labeling were optimized to allow the sensitivity required for application with nucleic acid extracted from clinical materials and cultured isolates. Analysis of nasopharyngeal aspirates, blood, urine, and tissue from persons with various infectious diseases confirmed the presence of viruses and bacteria identified by other methods, and implicated Plasmodium falciparum in an unexplained fatal case of hemorrhagic feverlike disease during the Marburg hemorrhagic fever outbreak in Angola in 2004–2005

High-resolution laser system for the S3-Low Energy Branch

In this paper we present the first high-resolution laser spectroscopy results obtained at the GISELE laser laboratory of the GANIL-SPIRAL2 facility, in preparation for the first experiments with the S$^3$-Low Energy Branch. Studies of neutron-deficient radioactive isotopes of erbium and tin represent the first physics cases to be studied at S$^3$. The measured isotope-shift and hyperfine structure data are presented for stable isotopes of these elements. The erbium isotopes were studied using the $4f^{12}6s^2$ $^3H_6 \rightarrow 4f^{12}(^3 H)6s6p$ $J = 5$ atomic transition (415 nm) and the tin isotopes were studied by the $5s^25p^2 (^3P_0) \rightarrow 5s^25p6s (^3P_1)$ atomic transition (286.4 nm), and are used as a benchmark of the laser setup. Additionally, the tin isotopes were studied by the $5s^25p6s (^3P_1) \rightarrow 5s^25p6p (^3P_2)$ atomic transition (811.6 nm), for which new isotope-shift data was obtained and the corresponding field-shift $F_{812}$ and mass-shift $M_{812}$ factors are presented