Papers by Keyword: Multi-Scale Modelling

Abstract: A multiscale model on dislocation patterning of cell structure and subgrain for polycrystal is newly developed on the basis of reaction-diffusion theory. A FD simulation for dislocation patterning and a FE one for crystal deformation are simultaneously carried out for a FCC polycrystal at large strain. Reflecting stress value on stress-effect coefficients, it is numerically predicted that the evolution of dislocation pattern in a polycrystal is different in response to the stress condition of each grain.

Abstract: We present some attempts to simulate nanoscale phenomena, which involve different length-scales and time-scales, using multiscale molecular-dynamics approaches. To simulate realistically an impurity-segregated nanostructure, we have developed the hybrid quantum/classical approach. The method can describe seamlessly both dynamical changes of local chemical bonding and nanoscale atomic relaxations. We apply the method to hydrogen diffusion in Si grain boundary. We find that the hydrogen is strongly trapped in (001)Σ5 twist boundary below 1000K, whereas it starts diffusing along the grain boundary above 1000K. For long-time processes in nanostructure formation, we apply the stochastic-difference-equation method to accelerate the simulations for microstructure evolution. The method bridges the states separated by high-energy barriers in a configuration space by optimizing an action, defined as an error accumulation along a reaction pathway. As an example, a SDE simulation is performed for Cu thin-film formation via nanocluster deposition. We show that the method can be applied effectively to search for the long-time process which involves a rare event due to a large potential barrier between two atomic configurations.

Abstract: The anisotropy of the tensile strength of plain-weave fabric is numerically evaluated by the finite element simulations. The plain-weave fabrics show complicated deformation behavior that is quite different from that of the continuum. The mechanics of woven fabric is not sophisticated yet enough to evaluate the strength and fracture mechanism in arbitrary stress conditions. The opacity of the tensile strength significantly diminishes the material reliability for the advanced use of fabrics. This study addresses the ideal tensile strength in arbitrary directions by using the pseudo-continuum model, which we have proposed to predict the deformation behavior and fiber stresses of the plain-weave fabrics. In this study, the numerical simulations of uniaxial extension in various directions are carried out by one finite element subjected to ideally uniform deformation, and we predict the breaking loads and elongations corresponding to the ultimate strength of the fiber.

Abstract: A self-organization model for repartition of dislocation cell structures and transition of subgrains on a three-stage hardening of single crystal are developed. Stress-effect coefficients models are proposed in order to introduce stress information into the reaction-diffusion equations. A FD simulation for dislocation patterning and a FE one for crystal deformation are simultaneously carried out for an FCC single crystal. It is numerically predicted that a cell structures are repartitioned and the generated dislocation pattern in stage III can be regarded as a subgrain.

Abstract: Although the Taylor-type models gives reasonable texture prediction of the monotonic cold deformation of annealed aluminum alloys both qualitatively and quantitatively, results are less satisfactory for the simple shear test when the alloy is heavily pre-deformed by cold rolling. The reason for this less good prediction originates from strain localization. A virtual stress-strain curve is proposed in which the texture aspects are dealt with by the FC Taylor simulation and the microstructure aspects by a model for the development of intragrain dislocations structure. This virtual yield law is used in a finite element simulation. A strain localization behavior is observed during the finite element simulation similar to that observed during experimental simple shear test. The strain profile of a specific global strain is discretized into a series of strain and the volume fractions of the regions deformed to these strain levels, using the statistical method of histogram. A secondary FC-Taylor simulation is performed, in order to generate the deformation textures, corresponding to this series of deformation strains. The global texture is generated by merging these textures with consideration of these volume fractions. Using this procedure of multi-level modeling, quite satisfactory texture prediction is observed, compared with the measured texture at this strain.

Abstract: The paper focuses on the multi-level character of existing or currently developed models for polycrystal deformation. A general multilevel frame is presented, which can be applied to models for the simulation of plastic anisotropy to be implemented in FE codes for the simulation of metal forming processes, or to models for the simulation of deformation textures. A short overview is presented of two-level models ranging from the full-constraints Taylor model to the crystalplasticity finite element models, including the description of a few recent and efficient models (GIA and ALAMEL). Validation efforts based on experimental cold rolling textures obtained for steel and aluminium alloys are discussed. Finally a recent three-level model which also takes the microscopic level (dislocation substructure) is discussed.