Maxime Melchior

Simulation of the lattice strains developed during a tensile test on a multiphase steel

This work investigates the micro-mechanics of a multiphase steel sheet during a uniaxial tensile test. Based on crystal plasticity theory, one assesses how the distribution of strain and stress is influenced by the presence of a soft b.c.c. phase and a strong f.c.c. phase. The two phases have been characterized by neutron diffraction. Initial textures are used as input in crystal plasticity simulations. Lattice strains measured in the tensile direction serve to fit hardening parameters. Three modeling hypotheses are tested: the Taylor model assumes uniform strain, the ALAMEL model considers the interaction of pairs of adjacent grains, and a finite element mesh is used to distribute strain and stress over the complete aggregate. The accuracy of each modeling is evaluated based on experimental measurements of the macroscopic stress, the heterogeneity of plastic strain, and the texture development in the two phases.

Crystal-plasticity-based FE modelling of a dual-phase microstructure in which grains have non-uniform shape and size

Different modelling approaches are tested for the prediction of plastic heterogeneity of a dual-phase polycrystalline microstructure. A novel technique is proposed for the generation of finite element (FE) meshes offering a more realistic representation of the grain topology in 3D. The model microstructure consists of a primary phase with regular grain shapes, and a secondary phase, characterised by a smaller average grain size, located preferentially at the triple junctions. In order to reduce computational cost, the FE mesh is coarse within the grains and fine at the interfaces. A specifically designed coarsening procedure is used to ensure that the mesh remains periodic in 3D. Initial grain orientations are generated such that they constitute a statistically representative sampling of the global texture while accounting for the non-uniform grain size. Compared to other modelling strategies including the conventional Taylor model and FE modelling with grains shaped as bricks or as truncated octahedrons, the new method yields a more accurate prediction of the strain partitioning between the two phases and the macroscopic texture development in a multiphase steel.

Strain heterogeneity and local anisotropy in TWIP steels

Deformation twinning is known to play a major role in the huge work hardening and formability of TWinning Induced Plasticity steels (TWIP steels). Twins carry a part of the plastic deformation and they act as barriers to dislocation motion. However, their interaction with their parent grain is already anisotropic and they strongly influence the development of texture and macroscopic anisotropy of the material. Twinning in TWIP steels is investigated in this study by means of experimental observations as well as an advanced crystal plasticity model. Individual twins, as they are formed, are treated as separate crystal entities co-deforming with the parent grain. The behavior of the polycrystalline aggregate is assessed by means of a multisite model of short-range grain interactions in comparison with the crystal plasticity finite element method (CPFEM) and the Taylor FC model. This approach allows validating the occurrence of twinning in grains with preferred orientations as well as the orientation relation between a twin and its parent grain with evolving deformation. Moreover, the results demonstrate that a combined prediction of macroscopic work hardening, texture and twin volume fraction requires both an appropriate description of strain heterogeneity in the polycrystal and a proper account of the anisotropic twin-grain interactions at the crystal level.