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Lugon-et-l'Île-du-Carnay, France

Laouafa F.,INERIS | Prunier F.,CNRS Grenoble Laboratory for Soils, Solids, Structures, and Risks | Daouadji A.,Laboratoire Of Physique Et Mecanique Des Materiaux | Gali H.A.,Laboratoire Of Physique Et Mecanique Des Materiaux | Darve F.,CNRS Grenoble Laboratory for Soils, Solids, Structures, and Risks
International Journal for Numerical and Analytical Methods in Geomechanics

One of the main onsequences of the nonassociative character of plastic strains in geomaterials is the existence of failure states strictly inside the Mohr-Coulomb plastic limit surface. This point is first emphasized by considering proportional strain loading paths as generalizations of the classical undrained triaxial path. It is shown that the sign of the second-order work is a proper criterion for analyzing these particular failure states. Then, experimental results and theoretical curves (obtained from an incrementally nonlinear constitutive relation) are compared for the case of proportional stress paths in axisymmetric conditions. Main features of the second-order work criterion are identified, such as the existence of a bifurcation domain together with a number of instability cones inside the Mohr-Coulomb surface. Furthermore, decomposing the second-order work into its isotropic and deviatoric parts makes it possible to compare each of the respective contributions to material instability. Finally, a heuristic boundary value problem is simulated via finite element modelling. A spectral analysis of the symmetric part of the stiffness matrix is conducted to extract the first vanishing eigenvalue and its associated eigenvector. It is found that the displacement field related to this eigenvector appears to be very close to the displacement computed just before global numerical breakdown signalling an effective failure. A plausible explanation is that, considering a material point, the flow rule at the boundary of the bifurcation domain almost coincides with the one describing the failure mechanism on the Mohr-Coulomb plastic limit surface. © 2010 John Wiley & Sons, Ltd. Source

Templier C.,University of Poitiers | Stinville J.C.,Laboratoire Of Physique Et Mecanique Des Materiaux | Villechaise P.,Laboratoire Of Physique Et Mecanique Des Materiaux | Renault P.O.,University of Poitiers | And 4 more authors.
Surface and Coatings Technology

Crystallographic structure and lattice rotation of the 'expanded' austenite produced by means of low temperature plasma nitriding is investigated. The microstructure of the nitrogen enriched layer has been investigated by means of X-ray diffraction (XRD) while the lattice rotations in the nitrided zone were assessed by electron backscattered diffraction (EBSD). The nitrogen depth profiles have been determined by means of glow discharge optical emission spectroscopy and nuclear reaction analysis. XRD shows the presence of the 'expanded' austenite or γN phase in all the nitrided samples characterized by average larger lattice spacing in relation to non-nitrided steel matrix. EBSD investigation demonstrates that in addition to the lattice expansion nitrogen incorporation into the stainless steel matrix induces significant lattice rotations. The amount and direction of these crystallographic plane rotations are function of the initial orientation. An unusual evolution of the 220 γN line as a function of the nitriding duration is observed together with an anomalously high intensity ratio of the 111 and 200 γN and matrix lines. The XRD results are interpreted on the basis of the lattice rotations of diffracting planes, nitrogen concentration gradient, nitrogen diffusion anisotropy and residual stress. It shows that these rotations are a pertaining feature for the understanding of the γN microstructure. © 2010 Elsevier B.V. Source

Stinville J.C.,Laboratoire Of Physique Et Mecanique Des Materiaux | Stinville J.C.,University of Poitiers | Villechaise P.,Laboratoire Of Physique Et Mecanique Des Materiaux | Templier C.,University of Poitiers | And 2 more authors.
Surface and Coatings Technology

AISI 316L austenitic stainless steel is widely used in nuclear and chemical industries for structural components that are submitted to cyclic deformation and stresses. The influence of low temperature (~ 400 °C) plasma nitriding on the crystallographic texture and fatigue durability of 316L has been investigated. Electron back-scattering diffraction measurements show that nitriding enhances the <001> and <111> texture components while the <110> one vanishes. Series of fatigue tests carried out in air at room temperature in the low cycle fatigue range show a significant improvement of the fatigue life. Results are discussed taking into account the residual compressive stresses in the nitrided layer. © 2009 Elsevier B.V. All rights reserved. Source

Nicaise N.,Laboratoire Of Physique Et Mecanique Des Materiaux | Berbenni S.,Laboratoire Of Physique Et Mecanique Des Materiaux | Wagner F.,Laboratoire dEtude des Textures et Application Aux Materiaux | Berveiller M.,Laboratoire Of Physique Et Mecanique Des Materiaux | And 2 more authors.
International Journal of Plasticity

Micro-macro scale transition theories were developed to model the inelastic behaviour of polycrystals starting from the local behaviour of the grains. The anisotropy of the plastic behaviour of polycrystalline metals was essentially explained by taking into account the crystallographic textures. Issues like plastic heterogeneities due to grain size dispersion, involving the Hall-Petch mechanism at the grain scale, were often not taken into account, and, only the role of a mean grain size was investigated in the literature. Here, both sources of plastic heterogeneities are studied using: (i) experimental data from EBSD measurements and tensile tests, and, (ii) a self-consistent model devoted to elastic-viscoplastic heterogeneous materials. The results of the model are applied to two different industrial IF steels with similar global orientation distributions functions but different mean grain sizes and grain size distributions. The coupled role of grain size distributions and crystallographic textures on the overall tensile behaviour, local stresses and strains, stored energy and overall plastic anisotropy (Lankford coefficients) is deeply analyzed by considering different other possible correlations between crystallographic orientations and grain sizes from the measured data. © 2010 Elsevier Ltd. All rights reserved. Source

Jallouli I.,Laboratoire Of Physique Et Mecanique Des Materiaux | Jallouli I.,Gafsa University | Kallel T.,Laboratoire Of Physique Et Mecanique Des Materiaux | Kallel T.,University of Sfax | And 4 more authors.
Physical and Chemical News

An experimental characterization of various defects extrusion, carried out using a capillary rheometery, is presented in this paper. We present the extrusion conditions influence on the flow of two types of highdensity polyethylene. The extruded beyond a certain speed present a distortion oscillating. Furthermore, the periodic and dynamic state of instability of the defect is quantified, based on the viscoelastic properties of the used polymer. In this work, with the mixture of two polyethylenes, we are able to facilitate the extrusion and delaying the onset of extrusion defects. Source

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