RO1 : Multiscale approaches to deformation, damage and failure

This RO aims at understanding, simulating and predicting the response of materials in fatigue or in severe conditions of use. The activities of the RO are based on 3 issues     
- to reproduce in the laboratory the stresses seen by the structure, which requires developments adapted to particular stresses (fretting fatigue, etc.) or to structures with specific shapes (wires and cables)
- characterise the plasticity and damage mechanisms at the relevant scales     
- simulate the mechanical response using multi-scale approaches tailored to the category of problem (cracking, contact, diffuse damage)    
To develop predictive models for the durability of the structures concerned.
Fatigue, damage and failure of materials and structures are governed by their microstructural heterogeneities, surface or volume defects and stress gradients present in the structure. The understanding of the mechanisms requires a fine analysis of the microstructure, in connection with the elaboration process, at the relevant scales. The studies are therefore based on characterisations of the properties at the micrometre scale. Examples include the measurement of residual stresses by FIB cutting, in-situ mechanical or thermal tests in the SEM or TEM on FIB samples, as well as the characterisation of the microstructure in 2D or 3D by microscopy or tomography.
Fatigue life prediction requires an accurate assessment of the problem in the vicinity of the crack front and how the loading history (multi-axiality, amplitude variations) and the non-linearities of the material behaviour (plasticity, texture, work hardening, process...) modify the speed and direction of crack propagation or initiation. An incremental crack propagation model has been developed to facilitate its use in structural design.
Most of the studies carried out by the RO are in partnership with the transport industry, with in particular strong interactions with Michelin within the framework of the joint SteeRLab laboratory or Safran within the framework of the Cognac project.