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The strain-induced self-assembly of islands in heteroepitaxial systems is a promising approach to the fabrication of quantum nanostructures for optoelectronic devices. In this study, a phase-field model which can simulate the growth process of self-assembled SiGe/Si quantum dots during deposition is developed. The novel feature of this model is that it can simulate the morphological changes of islands, i.e., from single-faceted pyramid to multifaceted dome, by taking a high anisotropy and a sixteen-fold anisotropy of surface energy into account. By using the developed model, two-dimensional simulations are 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 is clarified that the bimodal distributions are largely affected by island morphological change from pyramid to dome. The variations of island size and energy variations are estimated for single island in detail. As a result, it is concluded that the island morphology transitions occur so as to reduce the elastic strain energy.

We have performed phase-field simulations during directional solidification of a binary alloy. Adaptive mesh refinement techniques, in which the degrees of freedom of additional hanging nodes which occur when a quadrilateral element is refined are eliminated by matrix operations, are introduced to the finite element analysis in order to conduct the phase-field simulations efficiently. The validity of the numerical techniques presented here is ascertained by comparing the numerical results of the absolute stability limit and the onset of instability with those calculated from the Mullins–Sekerka theory and from the good linear relationship between log(V) and log(λ), in which the simulations under the constant concentration and temperature gradient are conducted by varying the pulling velocity. Furthermore, we examine the morphology change from cellular to dendritic structure and the relationship between log(λ) and log(G) for varying the temperature gradient.

Mechanical behavior of bolted joints during the tightening process with torque control is analyzed by FEM as elastic plastic contact problems. Three-dimensional analysis was conducted employing two-dimensional model with each node having three degrees of freedom that attains high computation efficiency. Such important factors as development of plastic zones, variations of load distributions along engaged threads, the relationships between axial bolt stress and nut rotation angle were analyzed in order to provide an effective guideline when tightening critical structures with high bolt preloads. It was also shown that the proposed numerical method can be applied to evaluate the tightening process by “plastic region tightening” used extensively in recent years. The validity of the numerical method is demonstrated by comparing the calculated bolt elongation with that obtained from experiment.

It is well known that a large amount of scatter in bolt preloads is observed when bolting up a pipe flange connection, especially in the case of using a spiral wound gasket. In this study, a numerical approach is proposed, which can simulate the bolt-up process of a pipe flange connection with a spiral wound gasket inserted. The numerical approach is designed so as to predict the scatter in bolt preloads and achieve uniform bolt preloads at the completion of pipe flange assembly. To attain the foregoing purposes, the stress-strain relationship of a spiral wound gasket, which shows highly nonlinear behavior, is identified with a sixth-degree polynomial during loading and with an exponential equation during unloading and reloading. Numerical analyses are conducted by three-dimensional FEM, in which a gasket is modeled as groups of nonlinear one-dimensional elements.

A bolted joint is subjected to high stress concentration at the thread root because of Its complicated geometry involved. Plastic regions are produced around the bottom of thread root, even under relatively low axial bolt force. It is predicted that the plastic deformation thus produced might affect such important mechanical behaviors of bolted joint as load distributions along threads, the relationships between axial bolt force and bolt elongation, etc. In many practical applications, bolts are tightened with considerably high clamping force, especially when tightening critical structures. Thus, it is considered that numerical studies taking the effects of plastic deformation Into account are extremely important. However, many previous studies have only dealt with a bolted joint under pure tension. When tightening bolted joints with torque control method, shear stresses are generated in addition to the tensile stress. In this study, mechanical behaviors of bolted joint during the tightening process with torque control method are analyzed by FEM as an elasto-plastic contact problem, where three-dimensional analysis in conducted employing two dimensional model with each node having three degrees of freedom.

Achieving uniform bolt preload is difficult when tightening a pipe flange with a number of bolts. Several bolt-tightening strategies have been proposed so far for achieving uniform bolt preloads. It seems, however, that effective guidelines for tightening pipe flange connections have not been established. In this study, a numerical approach is presented for estimating the scatter in bolt preloads and achieving the uniform bolt preloads when tightening each bolt one by one in an arbitrary order. Numerical analyses are conducted using three-dimensional FEM as an elastic contact problem. The analytical objects are pipe flanges specified in JIS B 2238 with an aluminum gasket inserted. The validity of the numerical procedures proposed here is ascertained by experiment.

In multi-bolt system, the scatter in bolt tension is largely dependent on the method of tightening operation and elastic interaction due to the successive tightening of bolts. In this study, focuses are placed on effects of elastic interaction and a new bolt-up strategy using three-dimensional finite element analysis, in which a gasket is modeled as a nonlinear one-dimensional gasket element, is proposed. Two types of numerical procedures are presented here. One is to predict the final scatter in bolt tension when tightening all bolts one by one in arbitray order with target preload. The other is to predict the amounts of initial bolt loads needed to create uniform final preloads with one pass operation of bolt tightening. The effectiveness of the numerical approach proposed here is substantiated by comparing numerical results with experimental ones. Contact pressure distributions on the gasket bearing surface, which significantly affect the sealing performance of flange connections, are also evaluated. The objective connection is composed of bolts, pipe flanges and a spiral wound gasket with inner and outer rings.

レーザー核融合用の均一な厚さをもつ燃料カプセルを作るためのエマルション法のモデル化を行った. エマルションが変形することによって発生するセンタリング力に対するスケーリング則は, 均一な壁が形成されていく主要なプロセスがTaylor-Wangによって示された力であることを示している. 均一な壁をもつ容器を作る上で重要なポイントを議論している.

Scatter in bolt axial stresses, when tightening a pipe flange with a number of bolts, is of critical problem. Bibel et al. recently proposed both experimental and numerical methods to give uniform bolt axial stresses at the completion of tighening operation. However, their method is not so attractive from the practical point of view because it needs preparatory experiments and it also involves some restrictions in executing numerical analysis. In this study, a numerical approach is proposed, which can efficiently evaluate the scatter in bolt axial stresses using FEM as a threedimensional elastic contact problem, and its validity is ascertained by experiment. Specifically, the scatter in bolt axial stresses generated at the completion of tightening operation is estimated, when tightening each bolt one by one with equal axial stress. In addition, it is also investigated how much initial bolt stresses are needed to achieve uniform axial bolt tension in the final state. The analytical objects are pipe flanges specified in JIS B 2238.

In the tightening process of a bolted joint using torque control, the effects of friction on axial tension of bolt and torque is an issue of great interest. In this study, the mechanical behaviors of a bolted joint during tightening, such as variations of axial tension and torque, are investigated both experimentally and numerically. Hence, friction coefficients on pressure flank of screw thread and the nut loaded surface are estimated by measuring the total torque applied to nut, axial tension and thread friction torque, and compared with variations of axial tension and torque. The validity of the finite element approach proposed here for evaluating changes in torque and axial tension is ascertained by comparing the numerical results to the experimental ones. It is also shown that the size of grip length influences, to some extent, the reduction rates of torque and axial tension.

In this study, we aimed to develop a computer-aided simulation technique to predict the axonal extension in the neuronal network evolution processes for design new scaffolds to activate the nerve cell and promote the nerve regeneration. We developed a mathematical model of axonal extension by using phase-field method and evaluated the validity of the mathematical model by comparison with the experiments. In the previous experimental studies, the peripheral nerve scaffold has been introduced to guide the axonal extension. Damaged part of nerve was replaced by the artificial tube as the scaffold to induce the axonal growth through the artificial tube and regenerate the nerve network. However, the scaffold made of biodegradable materials has a problem that it is degraded and absorbed before the nerve regenerate, and then the nerve cannot regenerate. Therefore, there is a need for the design and development of a scaffold for nerve regeneration to promote nerve regeneration. For that purpose, it is necessary to understand the difference between the axonal extensions by the surrounding environment, such as the shape or materials of the scaffold for nerve regeneration. In particular, the numerical technique to analyze the remodeling process of the nerve in the scaffold is strongly required to be established because the in-vivo experimental observation technology at the micro scale, bioethical issues in the animal experiment and requires time and money are also remained as unresolved problems. In this study, we developed a new simulation code which employed the phase-field method to predict the two-dimensional dendritic and axonal growth processes of nerve cells on cultivation scaffolds. We curried out the phase-field analyses to make clear how the parameters of Kobayashi–Warren–Carter (KWC) phase-field model affected on the morphologic growths of dendrite and axon. Simultaneously, we had observed the axonal extension process by using the PC-12D cells with nerve growth factor (NGF) on two-dimensional cultivation dish. Based on these axonal extension observation results, we approximated the morphological changes and establish the phenomenological model for phase-field analysis. Finally, we confirmed the validity of our newly developed phase-field simulation scheme in two dimensions by comparison with the experiments.Copyright © 2013 by ASME

The mechanical properties of metal materials largely depend on their intrinsic internal microstructures. To develop engineering materials with the expected properties, predicting patterns in solidified metals would be indispensable. The phase-field simulation is the most powerful method known to simulate the micro-scale dendritic growth during solidification in a binary alloy. To evaluate the realistic description of solidification, however, phase-field simulation requires computing a large number of complex nonlinear terms over a fine-grained grid. Due to such heavy computational demand, previous work on simulating three-dimensional solidification with phase-field methods was successful only in describing simple shapes. Our new simulation techniques achieved scales unprecedentedly large, sufficient for handling complex dendritic structures required in material science. Our simulations on the GPU-rich TSUBAME 2.0 supercomputer at the Tokyo Institute of Technology have demonstrated good weak scaling and achieved 1.017 PFlops in single precision for our largest configuration, using 4,000 GPUs along with 16,000 CPU cores.

In order to investigate the effects of stress around dendrite neck cased by the convection and gravity on the dendrite fragmentation, the novel numerical model, where phase-field method, Navier-Stokes equations and finite element method are continuously and independently employed, has been developed. By applying the model to the dendritic solidification of Al-Si alloy, the maximum stress variations by melt convection and gravity with dendrite growth were evaluated.

A GPU code has been developed for a phase-field lattice Boltzmann (PFLB) method, which can simulate the dendritic growth with motion of solids in a dilute binary alloy melt. The GPU accelerated PFLB method has been implemented using CUDA C. The equiaxed dendritic growth in a shear flow and settling condition have been simulated by the developed GPU code. It has been confirmed that the PFLB simulations were efficiently accelerated by introducing the GPU computation. The characteristic dendrite morphologies which depend on the melt flow and the motion of the dendrite could also be confirmed by the simulations.

Microsegregation behaviour in a ternary alloy system has been analysed by means of quantitative phase-field (Q-PF) simulations with a particular attention directed at an influence of tie-line shift stemming from different liquid diffusivities of the solute elements. The Q-PF model developed for non-isothermal solidification in multicomponent alloys with non-zero solid diffusivities was applied to analysis of microsegregation in a ternary alloy consisting of fast and slow diffusing solute elements. The accuracy of the Q-PF simulation was first verified by performing the convergence test of segregation ratio with respect to the interface thickness. From one-dimensional analysis, it was found that the microsegregation of slow diffusing element is reduced due to the tie-line shift. In two-dimensional simulations, the refinement of microstructure, viz., the decrease of secondary arms spacing occurs at low cooling rates due to the formation of diffusion layer of slow diffusing element. It yields the reductions of degrees of microsegregation for both the fast and slow diffusing elements. Importantly, in a wide range of cooling rates, the degree of microsegregation of the slow diffusing element is always lower than that of the fast diffusing element, which is entirely ascribable to the influence of tie-line shift.