Papers by Author: Masanori Kikuchi

Abstract: Due to more complex and severe design restrictions, more effective and faster finite element analyses are demanded. There are several ways to compute FE analysis efficiently: parallel computing, fast iterative or direct solvers, adaptive analysis and so on. One of the most effective analysis ways is the combination of adaptive analysis and multigrid iterative solver, because an adaptive analysis requires several meshes with difference resolutions and multigrid solver utilizes such meshes to accelerate its computation. However, convergence of multigrid solver is largely affected by initial shape of each element. An effective mesh improvement method is proposed here. It is the combination of mesh coarsening and refinement. A good mesh can be obtained by the method to be applied to an initial mesh, and better convergence is achieved by the improved initial mesh.

Abstract: Recent 3-Dimensional Atom Probe (3DAP) experimental observations showed the formation of spherical γ-precipitates at a central region of γ’ phases in nickel based superalloys. The γ precipitates should give a significant effect to the material strength. Whereat, in this work, we first developed a simulation technique for the investigation of the formation of γ-precipitates in γ’ phases, which is based on a combination of an Embedded Atom Method (EAM) interatomic potential designed for Molecular Dynamics (MD) and the kinetic lattice Monte Carlo (KLMC) method. As a demonstration of the KLMC simulation, the formation of the L12 structure was simulated from an initial state with randomly distributed aluminum atoms (25at.%Al). Finally, the γ’ phase with the L12 structure is formed in the entire simulation volume. To understand the fundamental mechanism of the formation of the L12 structures, the binding energies between a pair of aluminum atoms in a nickel single crystal were calculated using MD method. The result gave a clear picture of the mechanism. The stability of the γ-precipitate was also investigated using the MD method. Finally, the formation of γ-precipitates was simulated using the KLMC method. The dependence of the formation on the temperature condition is discussed.

Abstract: In this paper, the mathematical model for nonlinear multi-shaped randomly distributed domains in revealable and non-revealbale fields is established by using waveform mesh generation (WMG) method with damp, and made the application function of finite element pre and post handling system. The results show, this method can conduct mesh transformation to any given nonlinear function or given shapes in different scale randomly distributed domain in revealable and non-revealable fields, and obtained some smoothly mesh with matching each boundary and neighboring domains interaction.

Abstract: This article presents a modified FEM Superpostion method (S-FEM) for composite material analysis. Around the reinforcement body, failure and interface fracture may occur in the matrix. So the S-FEM was employed to detect the stress distribution around the reinforcement. One particle in big matrix is studied. Area of twice of particle radium is selected as local field.First, the feasibility of modified S-FEM is verified. And by symmetric analysis, geometric distribution of particle which may influence on the strength of composite material were discussed.

Abstract: This paper describes phase field simulations of the rafting behavior of γ’ phase with a simple interfacial dislocation network model. The interfacial dislocation network model accounts for the effect of the network on the lattice misfit between γ and γ’ phases and the subsequent rafting behavior. The model is implemented into the phase field simulation to see the dependence of the rafting behavior of γ’ phases on the interfacial dislocation network. Without the dislocation network model, the amount of the rafting was negligibly small. On the other hand, with the dislocation network model, the γ’ phases shows a large amount of rafting, which is in good agreement with the results of the experimental observations. Therefore, the combination of the phase field method and the simple interfacial dislocation network model developed in this work is appropriate for the simulation of the rafting of γ’ phases.

Abstract: The Mode I and Mode II dynamic fracture toughness (DFT) values, KId and KIId for high strength steels 40Cr and 30CrMnSiNi2A were evaluated using a combined experimental-numerical method. The tests were performed on three point bend (3PB) and shear specimens with Hopkinson pressure bar. The time of crack initiation was determined by a strain gauge. With the assistance of 3-D transient finite element analysis, the temporal evolution of the dynamic stress intensity factor under different loading rates was obtained, and the DFT was determined by the fracture initiation time. In Mode I tests, a brittle transgranular fracture was found in 40Cr, while evidence for ductile fracture was observed on the failure surface of 30CrMnSiNi2A. In Mode II tests, both tensile cracks and adiabatic shear bands were found for the two steels. The effect of ligament size on fracture toughness was discussed. The correlations of KId and KIId with loading rate were also investigated. The micromechanisms of loading rate effect on the DFT were discussed for both modes.

Abstract: Fatigue crack growth under mixed mode loading conditions is simulated using S-FEM. By using S-FEM technique, only local mesh should be re-meshed and it becomes easy to simulate crack growth. By combining with auto-meshing technique, local mesh is re-meshed automatically, and curved crack path is modeled easily. Plural fatigue crack problem is solved by this technique. For two parallel crack problem, criteria of crack coalescence are proposed. By simulating this problem by S-FEM, it is verified these criteria are conservative ones.

Abstract: In the virtual crack closure method (VCCM), the energy release rate is computed based on the results of finite element calculation, and the stress intensity factor (SIF) is computed from the energy release rate. In this paper, the stress intensity factor of mixed-mode surface cracks under three point bending is studied by using the three dimensional modified virtual crack closure method (MVCCM). The modified virtual crack closure method is required to open one element face area whose shape is arbitrary and finite element widths are unequal across the crack front. The effect of the distance between the location of load and crack face, crack shape and crack depth to the stress intensity factor is also discussed, along with practical results and conclusions.

Abstract: Dimple fracture tests are conducted under mode I and mixed mode lading conditions. Dimple fracture zone and shear-lip fracture zone are observed by scanning electron microscope precisely. It is found that crack growth direction is affected largely by the change of loading condition. It is also found that the differences of fracture pattern between mid-plane and at free surface are very large. Void diameter and crack growth direction are measured. Numerical simulation is conducted to simulate fracture tests in three-dimensional field. Gurson’s constitutive equation is used and large deformation analyses are conducted. It is assumed that void nucleation is controlled by both plastic strain and stress. Numerical results are compared with those of experiments. It is found that results of numerical simulation agree well with those of experiment qualitatively.

Abstract: Thickness effect is studied experimentally. At free surface of the specimen, shear lip fracture pattern appears, though dimple fracture pattern is observed inside of the specimen. The area of shear-lip fracture changes due to the change of the specimen thickness. In this study, experimental study is conducted by changing specimen thicknesses. Fracture surfaces are precisely observed using SEM, and dimple patterns on them are observed. At the free surface, very narrow no-void area is observed. It is also found that many voids are nucleated in shear-lip fracture area. FEM simulation is carried out using Gurson’s constitutive equation. It is found that shear-lip type fracture is simulated near free surface area by this method. The results show similar tendency with the experimental observation.