Mechanics of Materials group.
This research group develops a multiscale approach of the mechanical behaviour of a wide range of structural, functional or geological materials, in order to provide physically-based constitutive equations and relevant damage and fracture models for all kinds of thermo-mechanical loadings. An important goal is also to establish clear connections between microstructures and mechanical behaviours as a key for the optimization of materials performances.
The group makes use of a large set of experimental facilities for both macro-scale mechanical testing at various temperatures (including biaxial/triaxial loadings, DMA, static or dynamic loading and simple or stereo-DIC measurements of strain fields) and in situ testing (in tension, compression, bending or torsion, at room or high temperature, with controlled moisture) inside two scanning electron microscopes (with SEM-image based high resolution DIC, EBSD and EDS), under an atomic force microscope, or on a synchrotron line or a laboratory tomograph for 3D DIC.. Crystal plasticity, dislocation dynamics or finite element computations are current modelling tools.
Using a recently acquired Direct Laser Melting type 3D printing machine, a PhD student funded by SNCF and DGA, Y. Balit, investigates the influence of the process parameters on the microstructure and mechanical properties of an austenitic stainless steel, and the fatigue performance of steel or Inconel made structures repaired by additive manufacturing.
The PhD thesis of F. De Francqueville, funded by Airbus Safran Launchers and conducted in cooperation with PIMM aims at clarifying the relations between the microstructure and mechanical properties of new solid propergols, with a very high content in micron-size particles. A model including a cohesive zone at the particle-matrix interface was developed and will be enriched to take into account the visco-hyperelastic behaviour of the elastomer and its dependence on the temperature.
The effect of a prestrain on the fracture of carbon black-filled elastomers is investigated in D. Roucou's PhD thesis, funded by Michelin and conducted in cooperation with LML.
A current project focuses on the analysis and modelling of the role played by grain boundary sliding in the viscoplastic flow of ultrafine grained metals produced at LMS by severe plastic deformation with the ECAP process (PhD thesis of A. Goyal) or in conventional metals, at high temperature (PhD Thesis of A. El Sabbagh, supported by the Chaire Energies Durables EDF-Ecole Polytechnique).
The role of mechanical twinning, responsible for the high hardening rate and ductility of TWIP steels in tension is not so clear under cyclic loadings. This motivates the PhD thesis of C. D'Hondt, conducted in cooperation with ICMPE, with a support from Arcelor Mittal. The nano size of the twins requires fine scale testing, observations and measurement techniques (in situ tests in a SEM, an AFM or a TEM, high resolution DIC)
Fatigue crack growth of rail steel under mixed-mode non-proportional loadings induced by the repeated contact with train wheels is being investigated in partnership with SNCF. A particular attention is paid to loading path effects due to rough crack face interactions and crack tip plasticity.
The fatigue performance of a ultrafine-grained Al alloy is investigated, from the low-cycle regime to the gigacycle regime, in cooperation with PIMM and ICMPE (PhD thesis of A. Goyal and internship of M. Li, from Shangai Jiatong University).
The PhD thesis of M. Ruiz de Sotto, funded by Safran Aircraft Engines and conducted in cooperation with ICA-ISAE focusses on the fracture of aircraft turbines parts due to bird impact. To identify an anisotropic thermo-visco-plastic constitutive equation taking the tension-compression asymmetry of the titanium alloy, a large set of quasi static and dynamic tests had to be run. The challenge is now to formulate and identify a damage model suitable for a wide range of stress states, loading rates and temperatures.
Investigations on the deformation and damage mechanisms of clay rock are funded by ANDRA in relation with the future Cigeo nuclear waste underground repository. Mechanical tests are run in a SEM or coupled with MCT, with controled moisture, as well as dessication/hydration cycles during which 2D and 3D strain mappings are made to analyse the associated deformation and damage processes.