[en] This paper deals with the improvement of concentration relations when some basic hypothesis of homogenization techniques are no longer valid inducing important errors in the deduced local fields. In the first part of this work we propose a new formalism of the standard concentration rules developed in the homogenization of periodic media framework. In a second part, we insist on the fact that a homogeneous equivalent medium replacing a coarse grain material is in fact expected to be a generalized continuum. Additional stiffnesses must be attributed to the unit cell and special non-homogeneous boundary conditions and their periodic counterparts are proposed. The resulting HEM turns out to be a Cosserat continuum. (orig.)

[en] In this work, we try to answer the following question: What is the effective characteristic length of a mixture of Cosserat continua? More generally, homogenization methods for heterogeneous Cosserat media are proposed and applied to the case of linear elasticity. They first application deals with a beam network regarded as a discrete Cosserat medium and the second with a continuous heterogeneous Cosserat continuum. (orig.)

[en] Recently, several proposals were made for the enlargement of the classical field equations in order to solve locally inhomogeneous deformation problems (e.g. at shear banding and damage localization or of composites). In the present paper basic issues of this topic will be treated : (i) the implicit dependence within the gradient plasticity theory, (ii) the more open micromorphic view-point of the gradient of internal variable approach, (iii) a derivation of an elastic-plastic decomposition of the Cosserat strain measures, (iv) its application to the description of lattice curvature in crystals. Finally, finite element simulations of strain localization in Cosserat single crystals are presented. (orig.)

[en] Simulations of torsion of copper single, multi- and polycrystals are presented. The stress is laid on an accurate description of intragranular behaviour, which requires the use of parallel computing. If the wire contains up to 1000 grains, the deformation is shown to localize within some weak sections of the specimen. A lower bound of a critical grain number for a multi- to polycrystal plasticity transition is therefore established. (orig.)

[en] Models allowing the prediction of the failure of structures by crack propagation were first introduced in the 50's using linear fracture mechanics whose principles were first proposed by Griffith (1920). This approach was extended to non linear cases (plasticity and visco-plasticity) in the 70's based on the work of Rice (J or C* integrals); it has been largely adopted by the industry. However this so called global approach cannot deal with all practical cases and cannot explain all experimental observations as, for instance, the warm pre-stress effect (WPS). The local approach to fracture, which relies on a fine analysis of strains, stresses and damage of highly solicited regions (cracks, notches...) of structures is an alternative which allows to solve problems encountered while applying the global approach. It has been developed since the 80's in particular in France. Important research efforts are currently undertaken in this field in Europe (France, Germany), United States and Japan. This book presents several aspects of the local approach to fracture: damage mechanisms, experimental techniques, damage evolution law and failure criteria, modelling of damage, numerical simulation. This work is the result of a collective work carried out by the best french specialists (Ecole des Mines de Paris, Ecole Centrale Paris, ENS Cachan, Universite de Louvain, INSA Lyon, ONERA, EDF). (authors)

[en] In polycrystals, stress-driven vacancy diffusion at high homologous temperatures leads to inelastic deformation. In this work, a novel continuum mechanics framework is proposed to describe the strain fields resulting from such a diffusion-driven process in a polycrystalline aggregate where grains and grain boundaries are explicitly considered. The choice of an anisotropic eigenstrain in the grain boundary region provides the driving force for the diffusive creep processes. The corresponding inelastic strain rate is shown to be related to the gradient of the vacancy flux. Dislocation driven deformation is then introduced as an additional mechanism, through standard crystal plasticity constitutive equations. The fully coupled diffusion-mechanical model is implemented into the finite element method and then used to describe the biaxial creep behaviour of FCC polycrystalline aggregates. The corresponding results revealed for the first time that such a coupled diffusion-stress approach, involving the gradient of the vacancy flux, can accurately predict the well-known macroscopic strain rate dependency on stress and grain size in the diffusion creep regime. They also predict strongly heterogeneous viscoplastic strain fields, especially close to grain boundaries triple junctions. Finally, a smooth transition from Herring and Coble to dislocation creep behaviour is predicted and compared to experimental results for copper. (paper)

[en] We have developed a nozzle source that delivers a continuous beam of atomic helium or molecular hydrogen having a high degree of transverse coherence and with adequate optical brightness to enable new kinds of experiments. Using this source we have measured single slit diffraction patterns and the first ever speckle-diffraction patterns using atomic and molecular de Broglie waves. Our results suggest fruitful application of coherent matter beams in dynamic scattering and diffractive imaging at short wavelength and with extreme surface sensitivity

[en] A lot of mechanical damages of aging phenomena (solute dislocations-atoms interactions) in the temperature range of 20 - 600 C have been observed in zirconium alloys. The systematic influence of the addition elements (oxygen, niobium) and the exact characterization of the aging domain stay nevertheless badly known. In this study, new zirconium alloys have been specially prepared from a zirconium - 2.2% hafnium bar with a low amount in oxygen (80 ppm), and characterized by different mechanical tests. The dynamic aging phenomena are bound to weakly positive values of sensitivity of the deformation velocity stress. These phenomena are then modelled for the standard alloy called Zr702 containing a high amount of oxygen (1300 ppm): hole of sensitivity of the stress at the deformation velocity around 300 C, creep and relaxation stops at 200 and 300 C and tensile plastic deformation heterogeneities along the specimen barrel. The behaviour model used in finite elements is based on the dynamic aging phenomenological model proposed by MacCormick, validated on aluminium alloys and on steels, and applied here at the Zr702 alloy in the temperature range of 20-400 C. (O.M.)

[en] Laser scanning extensometry was used to detect and characterize propagating plastic instabilities such as the Luders bands at the millimeter scale. Spatio-temporal plastic heterogeneities are due to either static or dynamic strain ageing (SSA and DSA) phenomena. Regarding zirconium alloys, different type of heterogeneities were observed: their features strongly depended on mechanical test conditions. In one case, they appeared to be non propagating but preserved along the stress-strain curve and were associated with SSA effects such as stress peaks after relaxation periods or after unloading steps with waiting times. In other case, they appeared as non propagating but were not associated with SSA effects. (authors)

[en] Coherent x-ray diffraction is used to investigate the mechanical properties of a single grain within a polycrystalline thin film in situ during a thermal cycle. Both the experimental approach and finite element simulation are described. Coherent diffraction from a single grain has been monitored in situ at different temperatures. This experiment offers unique perspectives for the study of the mechanical properties of nano-objects.