分野別セミナー

Uncoupled analyses have been implemented, in which the code Zebulon is used for crystal plasticity analyses including a fully anisotropic elasticity tensor and an in-house finite element code is written for the hydrogen advection diffusion computations. After completion of the structural analysis, the diffusion code utilizing a Mass-Conservative Characteristic Finite Element (MCC FE) scheme is programmed. The required structural parameters - displacements, hydrostatic stresses and equivalent plastic strains - are transferred step by step from the previously completed structural analysis to the hydrogen code.

This work indicates that even without assuming some special properties for grain boundaries, which is usually the case in research on grain boundary engineering, the non-homogeneous stress field itself results in either depletion or enrichment of hydrogen, depending on the direction of stress driven flux produced by grain misorientation during straining. The segregation of hydrogen is observed mainly at the grain boundaries and some hydrogen perturbations appear. Results show that the grain size has a significant effect not only on the amount of hydrogen perturbation but also on the span of region under perturbation. A decrease in the grain size by a factor of 1.5 times induces an increase in maximum NILS concentration by a factor of 1.76. Results of similar analyses indicate that an increase in maximum NILS concentration occurs by a factor of 3.13 in the case of a decrease in the grain size by a factor of 2.
Moreover, the effect of crystallographic orientations on the hydrogen redistributions has been investigated. Different hydrogen redistributions have evolved from the uniform initial concentration due to dissimilar hydrogen fluxes resulted from different crystallographic orientations. The contour of trap site concentration changes due to variations in the contour of equivalent plastic strain at different crystallographic orientations. Progress has been made in that a one-way coupled crystal plasticity-transient hydrogen diffusion analysis is employed to solve a boundary value problem of an elasto-plastic deformation in meso-scale. The hydrogen distributions all over the polycrystal, including the hydrogen concentration within the grains and close to the grain boundaries, are determined. The results of this work may be useful for controlling and optimizing the material microstructure, eventually providing safe and reliable hydrogen transport and storage systems.