Plastic anisotropy of hexagonal closed packed metals.

다국어 초록 (Multilingual Abstract)

Due to the effects of twinning and texture evolution, the yield surface for hexagonal closed packed (hcp) metals displays an asymmetry between the yield in tension and compression, and significantly changes its shape with accumulated plastic deformation. Traditional initial yield criteria or hardening assumptions such as isotropic or kinematic hardening cannot accurately model these phenomena. In this dissertation, a macroscopic anisotropic model that can describe both the initial yielding and influence of evolving texture on the plastic response of hexagonal metals is proposed. Initial yielding is described by a newly developed macroscopic yield criterion that accounts for both anisotropy and asymmetry between yielding in tension and compression. The coefficients involved in this proposed yield criterion as well as the size of the elastic domain are then considered to be functions of the accumulated plastic strain. Viscoplastic self-consistent polycrystal simulations and a newly developed interpolation technique are then used to determine the evolution laws. The proposed model was implemented into the implicit finite element code ABAQUS and used to simulate the three-dimensional deformation of a pure zirconium beam subjected to four-point bend tests along different directions with respect to the texture orientation. Comparison between predicted and measured macroscopic strain fields and beam sections shows that the proposed model describes very well the contribution of twinning to deformation.
The proposed model is then extended to include the effects of strain rate and the temperate increase within the material due to mechanical work. The proposed rate and temperature dependent model was implemented into ABAQUS/EXPLICIT and used to simulate the three-dimensional Taylor impact experiment for specimens made from pure zirconium and from a tantalum alloy. The post experiment data and simulation results are shown to be in very good agreement.