ANR project MICROSTRESS

                         

Stress field heterogeneities at (sub)micrometer scales in elasto-plastic polycrystals (MICROSTRESS)

2012-2015 (AAP 2011 Blanc SIMI 9, # ANR-11-BS09-030-MICROSTRESS)

Participants

  • PIMM : Olivier CASTELNAU , Véronique FAVIER, Thierry BRETHEAU, Wilfrid SEILER, Frédéric VALES,
    Fennguo ZHANG (PhD), Emeric PLANCHER (PhD), Jean-Baptiste MARIJON (PhD)
  • CEA-INAC : Odile ROBACH, Jean-Sébastien MICHA, Olivier ULRICH
  • SMS-LCG : Claire MAURICE, Romain QUEY
  • EDF : Julien STODOLNA, Dominique LOISNARD, Elodie BOSSO, Nicolas RUPIN
  • LMS-X : Eva HERIPRE, Alexandre TANGUY
  • Navier : Michel BORNERT

and also

  • LEME : Johann PETIT
  • CEA-Cadarache : Hervé PALANCHER, Etienne CASTELIER

Contact : Olivier CASTELNAU (project P.I.)

Intranet : GLOUTON

Abstract

During the last decade, both experimental and theoretical micromechanical analyses have been significantly improved. Many efforts tend to understand the systematic heterogeneity of strain at the micrometer or even finer scale (intra- and inter-granular strain heterogeneities) that are highlighted experimentally on elastoplastic polycrystalline materials, and to reproduce it by means of theoretical or numerical models. However, making the link between strain heterogeneities and material microstructure is at present strongly limited by the poor knowledge of the material behavior at the micrometric scale, essentially due to the lack of experimental data at that scale. The goal of the project is to develop new experimental micromechanical techniques to measure stress fields with (sub)micrometer spatial resolution. These new experimental data will be coupled with other mechanical fields measured at similar spatial resolution, namely crystal orientation (EBSD) and local strain (elastic + plastic) measured by microextensometry based on Digital Image Correlation (DIC). Development of measurement techniques for thermal and kinematic fields has given rise to a revolution in experimental mechanics since a decade. We expect that the additional measurement of the static field, as proposed in this project, will lead to another significant transition.

To reach this goal, we will essentially develop simultaneously two experimental techniques and apply them to materials with increasing microstructure complexity (mono-, bi-, and poly-crystals of 316L austenitic steel). This proposal includes academic partners (labs PIMM, LMS-X, Navier, EMSE), the CEA/INAC team in charge of a synchrotron beamline at ESRF, and an industrial partner (EDF); they bring together a recognize expertise in experimental micromechanics, material sciences, synchrotron techniques, images processing, electron microscopy, and share common interests on polycrystal plasticity studies. The present study on a (relatively simple) industrial material is important to keep in mind potential future applications.

The techniques that will be developed (High Angular Resolution EBSD – HR-EBSD – and Laue microdiffraction) are based on diffraction analyses; they are complementary in terms of spatial resolution (~30 nm vs. ~micrometers), access (Electron Microscopy vs. synchrotron beamline), setup versatility (limited for HR-EBSD), etc. For both, the diffraction pattern (Kikuchi vs. Laue) formed by the highly focused incoming beam (electron vs. X-ray) and acquired on the 2-D detector contains information on the local lattice orientation but also on the local elastic strain. However, estimation of elastic strain with a resolution adapted to micromechanical studies (+/-0.0001) requires in practice a very accurate image analysis procedure that can track image distortions of hundredth of pixel. All partners are involved in that research direction since several years; the present project aims at pushing ongoing efforts a large step further to reach a stage in which we feel confident with applications. Image analyses methods based on DIC will be further developed and applied to both Kikuchi and Laue patterns. With both techniques, stress fields will be measured on in situ deformed steel specimens. With Laue microdiffraction, in-depth (near surface) stress gradients will be estimated. Experimental data will be also interpreted in terms of dislocation structure, compared to TEM analyses, and tested against numerical results obtained by Finite Element polycrystal calculations. A major aspect of the project will be the cross-validation of these complementary techniques and the estimation of experimental accuracies.

An important corollary of the project will be the direct determination of the local constitutive relation at the local (micron) scale, a necessary step for providing to micromechanical models the status of predictive tools.