| Summary: | The goal of this thesis is to determine the relationship between the macroscopic stress and the macroscopic strain for a variety of complex multiphase materials exhibiting rate-independent non-linear response at the micro-scale, based on experimental data obtained both at the local and macroscopic scales. A micro-macro secant mean field model (SMF model) based on the result of Eshelby and the approach of Mori and Tanaka is developed to model the behaviour of three particular systems which we have worked out by ourselves:
1) a ferrite-martensite steel produced by rolling in which we quantify the plastic anisotropy due to the morphological texture in terms of the Lankford's coefficient and pseudo yield surface;
2) a composite made of two continuous and interpenetrating phases: an aluminium matrix reinforced by a preform of sintered Inconel601 fibres. We quantify the coupled effects of temperature and phases co-continuity on the phases and overall stresses;
3) a TRIP-aided multiphase steel, in which the dispersed metastable austenite phase transforms to martensite. We derive the relationship between the overall uniaxial elastoplastic response and the progress of phase transformation, itself influenced by the thermodynamical, microstructural and mechanical properties. The stress-state dependence of the martensitic transformation is enlightened and explained. We demonstrate the existence of thermomechanical treatments leading to optima of ductility and strength-ductility balance. Finally, we show that the formability of TRIP-aided multiphase steels depends on the stability criterion.
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