[in Japanese]

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[in Japanese]

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Vanadium contents in the β phase in the Ti-6Al-4V alloy (Ti-64) changes by the heat treatment, plastic working, and so on, and its mechanical properties are also changed; it would affect the deformation mechanisms of microstructures in Ti-64; however, it is not fully understood. In this study, a sandwiched microstructure of α and β phases, where the crystal orientations satisfied Burgers orientation relation, was assumed, and the uniaxial tensile analysis was conducted using a crystal plasticity finite element method (CPFEM). The results showed that the elastic constants of the β phase was higher than that of α phase when the vanadium content of the β phase was high and the tensile direction corresponded to the direction perpendicular to the c-axis; it resulted in higher stress within the β phase in the range of elastic deformation. In contrast, there was a possibility that such relationship of stresses between α and β phases was not established by conditions in the range of plastic deformation.

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Theoretical prediction of glass forming ability (GFA) of metallic alloys is one of the keys to explore metallic alloy compositions having excellent GFA and thus having potential to form a large-sized bulk metallic glass. Molecular dynamics (MD) simulation is a promising tool to archive the theoretical prediction. However, direct MD prediction is still challenging because of the time-scale limitation of MD. For the practical bulk metallic glass alloys, the time necessary for quenching at typical cooling rate is five or more orders of magnitude longer than the MD time-scale. In this study, therefore, I propose a method to depict Time-Temperature-Transformation (TTT) diagram of the alloys based on the classical nucleation theory, which is widely employed to understand nucleation phenomena, informed by MD simulations. The TTT directly provides the prediction of the critical cooling rate and GFA. Using the method, the TTT diagrams and critical cooling rates of two Cu-Zr alloy compositions (Cu₅₀Zr₅₀ and Cu₂₀Zr₈₀) were computed. I found that the proposed method reasonably predicts the critical cooling rate.

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In modern applications such as in the modeling of electric currents in magnetic fields, solving complex symmetric systems are increasingly important. For the non-Hermitian symmetric case, orthogonality cannot be achieved using a standard Hermitian Krylov subspace method. In this study, iterative methods based on CS-MINRES-QLP are proposed. Same as the MINRE method, the proposed methods are reliable not only on positive-definite systems but also on indefinite systems. And the proposed methods can also solve ill-conditioned or incompatible singular symmetric systems. The performance of the convergence of the methods is evaluated by applying them for electromagnetic field simulations. The results indicate that the proposed methods give more accurate solutions than the MINRES-based method and show a smoother and faster convergence behavior.

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[in Japanese]

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Many industrial automotive or aerospace applications involve the analysis of the sound propagation, either radiated by vibrating structures or induced by a flow. Numerical simulation can be used as an efficient and robust alternative to experimental prototyping for both analysis and design. The finite element method has been widely used for tackling such NVH problems. In this paper, acoustic analysis originated by Lighthill’s analogy, mechanism of standing wave, calculation results of acoustic analysis tool ACTRAN, and comparison with analytical solution are reported.

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[in Japanese]

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Metallic glass has excellent properties such as high corrosion resistance, high strength, and large elastic elongation. However, its brittle nature and sudden fracture owing to its poor plastic deformability are critical problem for its practical use. Although the brittle nature attributes to the nucleation of shear bands and the shear fracture along shear bands, the whole mechanism of the fracture of metallic glass is still unclear. In this study, our molecular dynamics study tells the shear fracture of metallic glass is owing to the friction in the shear band region. We found the stress serration occurs during the shear fracture which happened in the shear band region. This is quite similar to famous stick slip phenomenon of the friction.

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High accuracy of the turbulent transition process plays an important role in aerodynamic design. Therefore computational fluid dynamics must also accurately predict the boundary transition process. In this study, two turbulence models, Spalart-Allaras (SA) and Delayed Detached Eddy Simulation (DDES) the laminar flow separation point is evaluated together with the latest transition gamma ( γ ) model by Menter. This model is verified by the simplified mirror model of the car and evaluates the accuracy of written simulation efficiency and method.

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In order to investigate a self-excited oscillatory phenomenon of a two-dimensional confined jet with a cylinder as a downstream target, the authors conduct two-dimensional numerical analyses based on vorticity ζ and stream function ψ using a finite-difference discretisation method. At first, confirming the reliability and accuracy of the present analyses, the authors examine two kinds of preliminary tests prior to the main test. As a result, the authors reveal (1) the two-dimensionality of the phenomenon by comparing computations with experiments, and (2) the importance and complexity of the upstream of the downstream target.

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[in Japanese]

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Threaded fasteners using bolts is widely applied in industrial field as well as various fields. However, threaded fasteners using bolts have loosening problems and cause many accidents. A nut or the like with a spring washer is used as a countermeasure against loosening of the screw fastening body. However, there remains a problem that an efficient loosening prevention effect can’t be obtained in the case where an increase in the kind of parts and a lock nut can’t be used. In this study, the purpose is to obtain self-loosening preventable threaded fasteners by applying spring characteristic effects on bolt structures. Helical-cutting applied bolt structures is introduced through three dimensional CAD modeling tools. Analytical approaches for evaluations on the spring characteristic effects helical-cutting applied bolt structures and self-loosing preventable performance of threaded fasteners were performed using finite element method and results are reported. Comparing slackness test results with analytical results and more details on evaluating mechanical properties will be executed in future study.

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Utilization of medical stents is one of effective treatments for stenosis and occlusion occurring in a living body’s lumen indispensable for maintenance of human life such as superficial femoral artery (SFA) occlusion. There are concerns about the occurrence of fatigue fractures caused by stress concentrations in the conventional stents, and a stent having high strength and high flexibility is required. Therefore, in this research, applicable mesh structures for medical stents based on the design concepts of high strength, high flexibility are interested to solve various problem of conventional stent. Shape design of the meshed stent are performed using 3-dimensional CAD software Solid Works first. Then analytical examination on storage characteristics and compression characteristics of such mesh structure applied stent models were carried out through finite element analysis software ANSYS Workbench. Meshed stent models with higher strength and higher flexibility are investigated analytically. From the model analysis results, it was found that the storage characteristics and compression characteristics of meshed stent modles are highly dependent on the basic mesh shapes.

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In this study, the mechanical properties of thermoplastic elastomers were investigated for uniaxial tensile tests and uniaxial cyclic loading tests for three strain rates to obtain material parameters of viscoelastic model and damage model. Material parameters which were obtained by the tests results were introduced to viscoelastic model and damage model.

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Coupling of grain growth by multi-phase-field model and elastic deformation by fast Fourier transform numerical scheme has been developed. The model comprises the equivalent-inclusion method predicting the elastic strain energy change in conjunction with the grain boundary migration. The model is applied to a simple grain growth problem of a bicrystal and a comparison with the analytical solution is made.

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Foam material like polyurethane shows the nonlinear stress strain relationships which are determined by the interaction of mechanical characteristics of matrix and microscopic structure. In this study, FEM analysis considering microscopic structure of polyurethane foam was carried out to investigate reasonable simulation area to represent the mechanical property of polyurethane foam. Rectangle area of a part of CT scan analysis results were applied to the finite element mesh for 3-dimentional simulation. Calculation of relative density and FEM analysis were carried out in different area sizes and positions. The area size which showed the converged results in the both of CT-scan analysis and FEM analysis was almost same.

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[in Japanese]

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This paper presents topology optimization, for acoustic cloaking devices, based on covariance matrix adaptation evolutioin strategy (CMA-ES). Acoustic cloaking devices can minimize the amount of scattering wave by controlling the flow of acoustic wave. The objective function is defined as the integrated intensity of scattering wave in the outer domain of the fixed design domain. Level set method is used to express structure with its explicit boundaries and level set functions are optimized by CMA-ES as design variable. The optimal configurations realizing good performances are obtained under the explicit perimeter constraint.

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[in Japanese]

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[in Japanese]

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In fluid-structure interaction simulation based on the particle method, ISPH method is used to resolve the problem of fluid particles’ motion and impact load on the structure in the meanwhile DEM based on the penalty method is commonly applied to deal with the collision contact rigid bodies. In this paper, the impulse-based rigid body dynamics is applied to deal with the collision contact problem instead of the conventional penalty method for robust and faster computation.

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