Yielding of HCP Metals
[ vendredi 05-06-2009 16:00
| anglais ]Yielding of HCP Metals
Dept of Mechanical and Aerospace Engineering,
University of Florida/REEF - Shalimar, FL 32579
Hexagonal closed packed (hcp) metals show a particular deformation behaviour which is quite different from that of materials with cubic crystalline structure. Rolled or extruded products exhibit a strong anisotropy and an unlike yielding in tension and compression. The underlying mechanisms, dislocation gliding on various slip systems and deformation twinning, are manifold and hence not completely understood. The present contribution intends to illustrate the relations between the microstructural mechanisms, modelled by visco-plastic constitutive equations of crystal plasticity and the macroscopic mechanical behaviour of hcp metals, described by macroscopic anisotropic yield surfaces.
In this talk, recent macroscopic level anisotropic formulations for hcp materials are presented. The anisotropy coefficients as well as the size of the elastic domain are considered to be functions of the accumulated plastic strain. The specific expressions for the evolution laws are determined based on experimental data. Applications to the simulation of the three-dimensional deformation of a hcp beam subjected to four4-point bend tests along different directions are presented. Comparison between predicted and measured macroscopic strain fields and beam sections shows that the proposed model describes very well the difference in response between the tensile and compressive fibers and the shift of the neutral axis. Furthermore, an overstress approach is used to account for rate effects. The very good agreement between the simulated and experimental post-test geometries of the Taylor impact specimens in terms of major and minor side profiles and impact-interface footprints shows the ability of the model to describe the evolution of anisotropy as a function of the strain rate.