Papers by Author: Yoshihiro Tomita

Abstract: The novel coupling recrystallization model is proposed in this study. First, the deformation microstructure was simulated by the finite element method based on the strain gradient crystal plasticity theory. The calculated dislocation density and crystal orientation were transferred to the recrystallization phase-field simulation. The initial subgrain structures used in phase-field simulation were determined by a relationship between dislocation density and subgrain size with the dislocation density distribution calculated by crystal plasticity simulation. The so-called KWC phase-field model, which can introduce both subgrain rotation and grain boundary migration, was employed, and spontaneous nucleation and grain growth were simulated simultaneously.

Abstract: In this work, the physical aging and its effect on nonlinear creep behavior of poly(methyl methacrylate) are presented. After annealing above Tg to release the previous thermal and stress history, the samples were quenched to 60oC, aged for various times, and were then tested at three different stress levels (22MPa, 26MPa and 30MPa) at room temperature of 27oC. At each stress level, the creep strain was converted to compliance and measured as a function of test time and aging time. The test results show that higher stress accelerates creep rate of the material while physical aging plays a reverse role. The time-aging time superposition is applicable to build a master creep compliance curve at each stress level, and it is demonstrated that the shift rate deceases with increasing stress. Moreover, based on the time-stress superposition principle, a unified master curve was constructed by further shifting the sub-master curves at 30MPa and 26 MPa to a reference stress level of 22MPa.

Abstract: By using molecular dynamics simulation, misfit dislocation networks are made on semi-coherent interfaces in a laminate structure of Ni and Ni3Al single crystals. The core structure of the networks is discussed in detail, focusing on the different atomic configuration at the interfaces; e.g. with or without Al atoms on the Ni3Al side. It is revealed that the networks can be a source of partial dislocation loops under the external loading; however, the loops tend to form immobile wedge-like stacking faults, analogous to the stacking fault tetrahedron (SFT), near the interface with Al atoms. On the other hand, the loops propagate into both Ni and Ni3Al phases, from the network dislocations on the interface without Al atoms.

Abstract: The establishment of the coupled numerical model which enable to simulate the spherulite formation and its mechanical behavior continuously is our final goal. In this paper, we have developed Phase-field model for spherulte growth of polymer by generalizing the model proposed by Granasy et. al.. The numerical simulations for single spherulite and multi-sperulites have been performed with isotropic interface energy.

Abstract: The integrated simulation model for microstructural design of Fe-C alloy using the phase-field method and the homogenization method is proposed. First, the phase-field simulation is performed to simulate the morphological change of the grain boundary ferrite to Widmanstätten ferrite. Then, in order to clarify the effects of the morphology of the ferrite phase on the micro- and macroscopic mechanical properties, the finite element analysis based on the homogenization method is conducted with the representative volume element obtained from the phase-field simulation. This numerical approach provides a powerful tool to investigate systematically the micro and macroscopic mechanical behavior with the morphological change of the ferrite phase in the Fe-C alloy.

Abstract: A constitutive equation of rubber is derived by employing a nonaffine molecular chain network model for an elastic deformation behavior and the reptation theory for a viscoelastic deformation behavior. The results reveal the roles of the individual springs and dashpot, and the strain rate dependence of materials in the monotonic and cyclic deformation behaviors, particularly softening and hysteresis loss, that is, the Mullins effect, occurring in stress-stretch curves under cyclic deformation processes of carbon black filled rubber..

Abstract: We have developed a phase-field model which can simulate the growth process of self-assembled SiGe/Si islands during deposition. The novel feature of this model is that it can reproduce the morphological transitions of islands, i.e., from single-faceted pyramid to multifaceted dome and from dome to barn, by taking a high anisotropy and a sixteen-fold anisotropy of surface energy into account. Two-dimensional simulations have been performed on a large computational model. As a result, island nucleation on the surface of a wetting layer, island morphological change and Ostwald ripening due to an interaction between two neighbor islands were well reproduced. The bimodal distribution of island size, which is a very important phenomenon in self-assembled quantum dots, could also be generated. Furthermore, it has been clarified that the bimodal distributions are largely affected by island morphological change from pyramid to dome. Furthermore, in order to discuss the mechanism of island growth, a simulation of single-island growth has been conducted and the variations of island size and energies have been estimated in detail. As a result, it is concluded that the island morphological transitions occur so as to reduce the elastic strain energy.

Abstract: The constitutive equation of rubber is derived by employing a nonaffine molecular chain network model for an elastic deformation behavior and the reptation theory for a viscoelastic deformation behavior. The results reveal the roles of the individual springs and dashpot, and the strain rate dependence of materials and disentanglement of molecular chains in the monotonic and cyclic deformation behaviors, particularly softening and hysteresis loss, that is, the Mullins effect, occurring in stress-stretch curves under cyclic deformation processes.

Abstract: In present study, we clarify the micro- to mesoscopic deformation behavior of semicrystalline polymer unit cell by using large deformation finite element homogenization method. Crystalline plasticity theory with penalty method for enforcing the inextensibility of chain direction and nonaffine molecular chain network theory were applied to the representation of the deformation behavior of crystalline and amorphous phases, respectively, in composite microstructure of semicrystalline polymer. The different directional tension and compression are applied to the 2- dimensional plane strain semi-crystalline unit cell model. A series of computational simulation clarified highly anisotropic deformation behavior of microstructure of semi-crystalline polymer, which is caused by rotation of chain direction and lamella interface, and manifests as a substantial hardening/softening. This anisotropy for tensile deformation is higher than that for compressive deformation.