Caractérisation du comportement anisotrope en fatigue d’un acier forgé

Dans cette étude, différentes méthodes expérimentales sont employées pour déterminer le comportement anisotrope monotone et en fatigue d’un acier 25MnCrSiVB6 laminé: le METASCO MC. Ce matériau de microstructure bainitique a la particularité de posséder des inclusions allongées dans le sens du laminage. Des résultats d’essais de traction monotone et de résilience sont présentés. Concernant la caractérisation du comportement en fatigue, des résultats issus d’essais d’auto-échauffement et de fatigue classiques sont comparés et discutés

Competition between mesoplasticity and damage under HCF – Elasticity/damage shakedown concept

 Lien vers la version éditeur: http://www.sciencedirect.com/science/article/pii/S0142112306003240The aim of this paper is to present new modelling dedicated to multiaxial high cycle fatigue (HCF), and applied to polycrystalline metals. The model presented is based on the experimental characterization of damage during HCF tests, under pure tension and torsion modes. The origin of this approach is a mesoscopic model considering three plastic behaviour stages (hardening, saturation and softening) suggested by Papadopoulos [Papadopoulos I.V. Fatigue limit of metals under multiaxial stress conditions: the microscopic approach. Technical Note No. I.93.101, Commission of the European Communities, Joint Research Centre; 1993. [ISEI/IE 2495/93].], and used by Morel [Morel F. A critical plane approach for life prediction of high cycle fatigue under multiaxial variable loading. Int J Fatigue 2000;22:101–119.]. The principal evolution brought in by this study is a competition description during all the sample lifetime of the plasticity and damage effects. The plasticity mechanisms induce a hardening saturating effects (resulting from movement and accumulation of dislocations), especially significant at the beginning of the crystal life. Damage, present at the end of crystal lifetime, is considered as a degradation process inducing a strong reduction of the crystal ductility, leading to its failure (decohesion). The coexistence and the competition between these two effect (hardening and damage-induced softening) describe cyclic crystal behavior, including shakedown phase. The model is formulated in the framework of the continuum damage mechanics, according to the identified physical mechanisms during the tests. The second purpose is to compare the model predictions with experimental data, after identification of the parameter for a ferritic-pearlitic steel. The case of in-phase loading is merely studied here. In particular, the evolution of a few internal variables is discussed and correlated with the available physical features. It is shown that this model provides a complementary insight into a crystal with respect to the endurance criterion of Dang Van. The model predicts, in a particular stress amplitude range, the damage growth arrest

Microstructural heterogeneities and fatigue anisotropy

In this study, various experimental methods are employed to determine the anisotropic fatigue behavior of a 25MnCrSiVB6 forged steel (Metasco MC). This material has a bainitic microstructure and contains many elongated non-metallic inclusions in the rolled direction, which are grouped into clusters. Specimens with different orientations relative to the rolling direction have been extracted from a hot rolled bar and the ability of certain experimental techniques to capture the fatigue anisotropy has been tested. Results obtained from monotonic tensile tests and Charpy impact tests show that the material has isotropic fracture strength and anisotropic ductility. The influence of the inclusion clusters is clearly demonstrated via observation of the fracture surfaces. Concerning the fatigue behavior, results from a classical staircase experimental procedure are compared to results from self-heating fatigue tests. For specimens orientated at 0° relative to the rolled direction, microcrack initiation is controlled by the material matrix and the prediction of the fatigue strength with the self-heating method has been observed to be correct. For specimens orientated at 45° and 90°, the elongated manganese sulfide inclusion clusters are the origin of crack initiation and the fatigue strength drops significantly. For this case, it appears that the self-heating method has difficulty predicting the fatigue behavior

Modélisation multiaxiale et architecture triaxiale périodique : application à un matériau composite 3D C/C

Cette publication est dédiée à la modélisation du comportement mécanique d'un matériau composite triaxiale carbone/carbone sous fortes sollicitations mécaniques. Ce matériel se compose d'une architecture orthogonale de baguettes, elles-mêmes composées des fibres de carbone. En conséquence, selon l'état de sollicitation induit par le chargement mécanique, la ruine de ce matériau se produit soit par rupture des fibres, soit par déformation excessive de son architecture. A température ambiante, ces deux mécanismes mènent respectivement à un comportement élastique fragile, ou bien à un comportement anélastique endommageable, engendrant la dissipation d'énergie. Il est proposé une modélisation mécanique, dans le cadre de la thermodynamique des processus irréversibles, afin de rendre compte de ces comportements

Contribution a la modélisation du dommage en fatigue multiaxiale d un acier C36 (confrontation à l expérience)

Le premier objectif est la caractérisation expérimentale du dommage, pour des sollicitations simples (traction et torsion), mais également l étude du cumul de dommage, en particulier par des séquences de chargement reposant sur des blocs de sollicitations différentes. Le second objectif est l utilisation du contexte de la mécanique de l endommagement afin de proposer une nouvelle modélisation des phénomènes observés. Etablies dans un cadre thermodynamique, à une échelle mésoscopique, c est en fait trois modélisations qui sont proposées. L augmentation graduelle de leur complexité est la conséquence de la prise en compte d un nombre croissant de phénomènes : nucléation, propagation en mode II, bifurcation, puis bifurcation en mode I, et enfin ruine du cristal. De plus, en utilisant une description considérant la direction privilégié du glissement au sein du système activé, il est possible d établir une description orientée de l évolution des caractéristiques mécaniques du cristal. Une approche statistique est également proposée au sein de ces modélisations.The basic objective of this thesis is the experimental characterization of deterioration mechanisms for simple (tension and torsion) loading cycles as well as for complex sequential loading programmes. The study of cumulative damage development for corresponding (tension and torsion) loadings blocks is put forward. This experimental investigation is a basis for coupled damage-plasticity mesomechanical modelling of the multiaxial fatigue phenoma. Established within a thermodynamic framework, there are in fact three modular and ascending damage models proposed. The gradual increase in their respective complexity is a consequence of accounting for increasing number of phenomena : nucleation, propagation in mode II, bifurcation, propagation in mode I, combination of the both and finally ruin of the crystalline aggregate. Moreover, by using an approach considering privileged slip direction within the activated system, it is possible to establish a description of damage intensity accounting for multitude of orientations in physical space of the material. A statistical approach is also proposed within the above approach.POITIERS-BU Sciences (861942102) / SudocCACHAN-ENS (940162301) / SudocSudocFranceF

Essential requirement for -arrestin2 in mouse intestinal tumors with elevated Wnt signaling

International audienceβ-Arrestins (Arrb) participate in the regulation of multiple signaling pathways, including Wnt/β-catenin, the major actor in human colorectal cancer initiation. To better understand the roles of Arrb in intestinal tumorigenesis, a reverse genetic approach (Arrb(-/-)) and in vivo siRNA treatment were used in Apc(Δ14/+) mice. Mice with Arrb2 depletion (knockout and siRNA) developed only 33% of the tumors detected in their Arrb2-WT littermates, whereas Arrb1 depletion remained without significant effect. These remaining tumors grow normally and are essentially Arrb2-independent. Unsupervised hierarchical clustering analysis showed that they clustered with 25% of Apc(Δ14/+);Arrb2(+/+) tumors. Genes overexpressed in this subset reflect a high interaction with the immune system, whereas those overexpressed in Arrb2-dependent tumors are predominantly involved in Wnt signaling, cell adhesion, migration, and extracellular matrix remodeling. The involvement of Arrb2 in intestinal tumor development via the regulation of the Wnt pathway is supported by ex vivo and in vitro experiments using either tumors from Apc(Δ14/+) mice or murine Apc(Min/+) cells. Indeed, Arrb2 siRNAs decreased the expression of Wnt target genes in cells isolated from 12 of 18 tumors from Apc(Δ14/+) mice. In Apc(Min/+) cells, Arrb2 siRNAs completely reversed the increased Wnt activity and colony formation in soft agar induced by Apc siRNA treatment, whereas they did not affect these parameters in basal conditions or in cells expressing constitutively active β-catenin. We demonstrate that Arrb2 is essential for the initiation and growth of intestinal tumors displaying elevated Wnt pathway activity and identify a previously unsuspected molecular heterogeneity among tumors induced by truncating Apc mutations