Numerical simulations of impacts during Surface Mechanical Attrition Treatment using crystal plasticity model in finite element method

Surface Mechanical Attrition Treatment (SMAT) is a process, which transforms the top surface layer of materials from coarse grains to nano-sized grains by severe plastic deformation. SMAT is based on multidirectional mechanical impacts between shot and the surface of material. As the strain rate is high and the accumulated plastic strain is large, a great number of defects such as dislocations and deformation twins can be generated at the top surface, which progressively lead to the formation of a nanostructured layer. Simultaneously, high compressive residual stresses may be introduced in the SMAT affected layer. This nanostructured layer coupled with compressive residual stresses induced by SMAT allow to significantly improve the mechanical properties of materials. Due to excellent mechanical properties of SMATed materials, it is necessary and useful to investigate the SMAT process both experimentally and numerically in order to obtain a better understanding and a better control of the process. Experimental studies, extensively performed previously and recorded in the literature, have shown that the mechanical properties of SMATed materials are highly influenced by the microstructure such as grain size and work hardening. From a modeling perspective, it would be highly beneficial to establish accurate numerical models of SMAT in order to consider the influence of the different parameters of this process at the different scales. In this work, a crystal plasticity model introduced in finite element analysis, taking into account the microstructure, was used to investigate the plastic activity due to the impacts between shot and the surface of material. To do this, the shape-controllable 3D Voronoï geometries as well as meshes were first generated using Neper software. A phenomenological crystal plasticity model implemented through user-defined ABAQUS subroutines was used in this work to perform numerical simulations. A number of parameters are studied such as shot size, impact velocity, incident angle, etc. The influences of these different parameters on slip systems and stress fields were analyzed. The first results demonstrated the interests of numerical simulations for this specific process

Mécanismes locaux de déformation de l'acier inoxydable austénitique 316L

International audienceDans cette étude, nous présentons une approche nouvelle pour comprendre les mécanismes locaux de déformation.Les champs cinématiques obtenus à partir d’une grille nanoparticules d’orsont combinés aux mesures cristallographiques de la région de la grille. Les déformations dans les grains et aux joints de grains et l’apparition des glissements et leurs évolutions sont suivies. Ces résultats locaux sont comparés au comportement macroscopique fourni lors d’un essai de traction

Study of Mechanical Behaviour of Polycrystalline Materials at the Mesoscale Using High Energy X-Ray Diffraction

International audienceOwing to its selectivity, diffraction is a powerful tool for analysing the mechanical behaviour of polycrystalline materials at the mesoscale, i.e. phase and grain scale. In situ synchrotron diffraction (transmission mode) during tensile tests and modified self-consistent elastoplastic model were used to study elastic and plastic phenomena occurring in polycrystalline specimens during deformation. The evolution of stress for grains which belong to different phases of duplex stainless steel and pearlitic steel was analyzed

Damage in duplex steels studied at mesoscopic and macroscopic scales

Different experimental approaches have been performed in order to extract damage at several scales. In this paper two experimental methods are treated. Neutron diffraction coupled with tensile test has been performed to study damage at mesoscopic scale. At macroscopic scale, classical tensile test has been used to extract damage effects, from material hardening evolution. Optical measurements and particular data treatment have been used in order to correct data for the necking phenomenon at large deformation, for each experimental method. Damage process in duplex steels has then been analysed at both macroscopic and mesoscopic scales using scale transition models. Eventually, investigations at those scales have been compared to understand correlation between mesoscopic and macroscopic behaviour of our material

Elastoplastic deformation and damage process in duplex stainless steels studied using synchrotron and neutron diffractions in comparison with a self-consistent model

In situ time of flight neutron diffraction and X-ray synchrotron diffraction methods were applied to measure lattice strains in duplex steels during a tensile test. The experimental results were used to study slips on crystallographic planes and the mechanical effects of damage occurring during plastic deformation. For this purpose the prediction of an elastoplastic self-consistent model was compared with the experimental data. The used methodology allowed to determine the elastic limits and parameters describing work hardening in both phases of studied polycrystalline materials. In the second part of this work the developed elastoplastic model was applied to study damage occurring in the ferritic phase. The theoretical results showed a significant reduction of stresses localized in the damaged phase (ferrite) and confirmed the evolution of the lattice strains measured in the ferritic and austenitic phases

Contraintes « de croissance » et cinétiques d'oxydation dans des couches d'oxydes thermiques de Fer et de Nickel ;<br />Etude in-situ par Diffraction des Rayons X et modélisation

A metal oxidation leads to a specific strain link to the oxide growth. This growth strain is supposed to be proportional to the oxide layer thickness. We study the « growth » stresses in some metals : nickel, untreated iron and phosphated iron. The first two materials oxidation is parabolic. On the other hand, the phosphated iron oxidation shows several features : phases inversion in the oxide layer, kinetic transition. Complementary analyses led by X – Rays Diffraction showed that the layer of phosphate seemed to play the role of a diffusion barrier. The stability of the phosphate layer during the oxidation and the oxide layers morphology confirmed that the kinetic accident has a mechanical origin. The measure of the stresses in the oxides has been achieved by XRD, with Synchrotron Radiation. In the first time, the stresses modelling in thermal oxides on metals allowed us to reproduce experimental results. Finally, we develop an identification method of the system parameters.La formation d'une couche d'oxyde thermique sur un métal entraîne l'apparition d'une déformation spécifique liée à la croissance de l'oxyde. Nous considérons que cette déformation est proportionnelle à l'épaisseur de la couche d'oxyde. Nous avons analysé les contraintes de « croissance » associées, dans les oxydes se développant sur des substrats de nickel, de fer et enfin de fer phosphaté. L'oxydation des deux premiers métaux est parabolique. En revanche, l'oxydation du fer phosphaté montre plusieurs caractéristiques. Des analyses menées par Diffraction des Rayons X (évolution des pourcentages de phases) ont montré que le film de phosphate se comportait comme une barrière de diffusion. La mesure des contraintes dans les oxydes a été réalisée in-situ par thermodiffraction avec l'apport du Rayonnement Synchrotron. Enfin, la modélisation de l'évolution des contraintes dans les systèmes oxydes thermiques sur métal nous a permis de reproduire qualitativement les résultats expérimentaux

Application of Clifford algebra Cℓ3(C){C \ell_3(\mathbb{C})} to continuum and engineering mechanics

International audienceMultivectorial algebra is of both academic and technological interest. Its application, however, is not always easy. A distinction must be made between polar and axial vectors and between scalars and pseudo-scalars. Eight element types are often considered even if they are not always identified as multivectors. In some cases, for simplicity’s sake, only vectorial algebra or quaternion algebra is explicitly used for physical and mechanical applications. It would, however, be more convenient to use more complex algebra directly in order to have a wider range of mechanical applications. The aim of this paper is to examine one particular type of Clifford algebra that could solve this problem. The present study focusses on showing how these quantities can be used to model mechanical and engineering problems. First, continuum mechanics in a Cauchy medium is investigated for elastic transformations. Second, a specific type of shot-peening application is studied. Applications are then used to illustrate the scope and efficiency of this type of modeling based on geometric algebra

Clifford Algebra Cℓ 3(ℂ) for Applications to Field Theories

International audienceThe multivectorial algebras present yet both an academic and a technological interest. Difficulties can occur for their use. Indeed, in all applications care is taken to distinguish between polar and axial vectors and between scalars and pseudo scalars. Then a total of eight elements are often considered even if they are not given the correct name of multivectors. Eventually because of their simplicity, only the vectorial algebra or the quaternions algebra are explicitly used for physical applications. Nevertheless, it should be more convenient to use directly more complex algebras in order to have a wider range of application. The aim of this paper is to inquire into one particular Clifford algebra which could solve this problem. The present study is both didactic concerning its construction and pragmatic because of the introduced applications. The construction method is not an original one. But this latter allows to build up the associated real algebra as well as a peculiar formalism that enables a formal analogy with the classical vectorial algebra. Finally several fields of the theoretical physics will be described thanks to this algebra, as well as a more applied case in general relativity emphasizing simultaneously its relative validity in this particular domain and the easiness of modeling some physical problems

Description eulérienne pour les matériaux anisotropes hyper-élastiques

National audienceWe propose to use a geometric description to construct covariant Eulerian models for anisotropic hyper elastic solids. After kinematic considerations we propose a relation between the rate of deformation and a deformation tensor. We obtaincovariant state equations and hyper-elastic constitutive models for finite transformations. The covariant context ensures invariancewith respect to the changes of observer and enables us to derive these equations and relations on any configuration, without referring to a Lagrangian description;this is of particular interest for the case of anisotropic behaviors to construct hyper-elastic models valid for anisotropic materials in an Eulerian configuration. The geometric treatment, in particular of time derivatives and the rateof deformation ensures that the quantities, operators and equations are genuinely intrinsic

Derivation of Cosserat’s medium equations using different multi-dimensional frameworks

International audienceCosserat’smedium is of both academic and technological interest. Thismedium offers a wider range of possible applications allowing a better description of the mechanical effectswithin materials; it should thus be used preferentially. It should be noted that the advantages of this kind ofmedia are nevertheless counterbalanced by the simplicity of Cauchy’s medium.One objective of this article is to investigate the correspondence between different multi-dimensional representations of Cosserat’s medium and hence advocate the advantages of this representation. It is demonstrated that the equations representing Cosserat’s medium may be derived from different frameworks. The comparison between these different approaches shows that Cosserat’s medium can be seen as the natural medium to describe material behaviors accurately. As an illustration, constitutive models for isotropic linear elastic behaviors are then derived using different multi-dimensional mechanical theories