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Maxime Melchior's PhD Thesis
Modelling of texture and hardening of TWIP steel - Advanced finite element representation of polycrystalline aggregates
by Maxime Melchior (Public defense : December 11th, 2009, 17h00, BARB93)
Metallic alloys with enhanced mechanical properties are required for example by the car industry. The development of micromechanical models and their use in numerical simulations of forming processes (e.g. rolling or deep drawing) as well as mechanical tests (e.g. uniaxial tension) helps drawing the link between, on the one hand, physical and microstructural properties and, on the other hand, the macroscopic mechanical response.
The first part of the thesis proposes a new crystal plasticity model adapted to TWinning Induced Plasticity (TWIP) steel. The latter steel grade combines high strength and ductility because of the formation of microscopic lenticular regions (called « twins ») which are very hard and which act as obstacles to dislocation glide. The mathematical model accounts for this as well as the induced anisotropy and the rotation of the crystal lattice. It is demonstrated that, by taking proper account of the interaction of adjacent grains, one obtains improved predictions of the macroscopic texture development and hardening in tension and in plane strain compression. In the second part of the thesis, an advanced numerical representation of single and multi-phase polycrystalline aggregates is presented; relying, first, on a novel algorithm for texture discretisation valid when grains have non uniform sizes and shapes; second, on the generation of finite element meshes selectively refined along grain boundaries. A systematic sensitivity study about the design of such crystal plasticity based finite element modelling (CPFEM) is conducted. Predictions obtained with the new model microstructures offer an improved compromise between accuracy and computation time.
Pdf-file of thesis (local server)
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