Amorçage d'une fissure de Fatigue dans un Grain d'Agrégat polycristallin et Propagation dans les grains adjacents – AFGRAP

The industrial objectives of the project are the definition of the economical optimal surface finish of a material in regards to the applied solicitations and the definition of parameters for the optimisation of fatigue tests programmes. The methodology set out in this project is intended to be applied to others materials and others environments. The scientific objective of the AFGRAP project is to give elements for the modelling of the first steps of initiation and propagation in a polycrystalline aggregate of a metallic material. The project proposes to develop an initiation criteria at the level of a grain and a description of the interaction between a crack and the grain boundaries, by performing modelling at several levels, that of crystalline plasticity and that of dislocations, as also experimental observations at the microscopic level. The studied material is an austenitic steel, AISI 316L, chosen for the good knowledge of its behaviour and the relative simplicity of its structure. The project aims to study the effect of a multi-axial solicitation with variable amplitude in fatigue, at first by studying the effect of a uni-axial solicitation with constant amplitude. The project leans on the modelling of aggregates developed by Ecole Nationale des Mines de Paris-ARMINES, Ecole Centrale de Paris and EDF and the modelling in dynamics of dislocations developed by Institut National Polytechnique de Grenoble, University of Savoie, CEA and EDF. In these domains, it involves major scientific enhances: - Improvement of the behaviour model of aggregates (generalized continuum mechanics, second gradient method) and use of these models on aggregates of significant volume; - Creation of a coupling between aggregates calculations and dislocations dynamics calculations, in the frame of parallel programming, which will permit to realise dislocations dynamic calculations including some tens of grains. Concerning the experimental part, the projects leans on the accumulated experience in tests and expertises on specimens and structures by Ecole Nationale Supérieure de Mécanique et Aéronautique of Poitiers, CEA and EDF. To converge to criteria's on the initiation in a first grain and on the propagation across the grain boundaries, two ways will be coupled: numerous calculations will be performed in terms of grain orientations with regards to solicitation and relative orientation of neighbour grains in order to extract statistical parameters; microscopical experimental observations will be performed to extract the favourable trends to initiation and propagation.