Papers by Author: Hidekazu Murakawa

Abstract: In order to simulate the large deformation surrounding the rotational tool of friction stir welding (FSW) precisely, the moving particle semi-implicit (MPS) method was employed and the temperature distributions near the tool were obtained. Also, the temperature distributions in the whole model except for the area computed by MPS were calculated by the finite element method (FEM) and then the elastic-plastic analysis was conducted using the temperature distributions obtained by MPS and FEM. The inhomogeneous temperature distributions through the thickness near the joint line could be simulated and the maximum temperature distributions computed had a good agreement with the experiments. In addition, the longitudinal plastic strain distributions indicates that this plastic strain near the tool is not governed by only the temperature distributions and the influence of plastic flow should be taken into account.

Abstract: In order to save fuel consumption by reducing the weight of automobile body, the use of aluminum alloys has a great advantage. However, how to join aluminum alloys with steels becomes a big problem in the assembly lines. Cold metal transfer (CMT) is a promising joining process for steel/Al dissimilar materials. To evaluate the shear strength and to investigate the failure modes of CMT brazed lap joints of dissimilar materials, both experimental observation and numerical simulation are performed. A numerical model for the failure criteria of the interface layer failure between steel and aluminum is developed. The interface layer of CMT brazed lap joint can be modeled by the interface element. The failure stress and failure energy at the interface element are proposed as the failure criteria for the prediction of shear strength of CMT lap joints. If steel sheet thickness becomes thicker, stress distribution and concentration at interface layer elements have some change and shear strength at the interface layer can be improved. Then the failure occurring at the interface element may transfer to the fusion line at the side of the aluminum alloy sheet.

Abstract: As a numerical tool for examining the microstructural fracture behavior in the smart and intelligent materials, a finite element method with the interface element was developed and the applicability of this method was studied through the serial computations using virtual polycrystalline models. As the results assuming the influence of grain orientation on the grain boundary, it was found that the anisotropic mechanical property of grain boundary (interaction between opening and slipping deformations) would be a dominant factor of the fracture process. Also, by employing the theory of crystal plasticity for the mechanical property of grain, it was revealed that the stress concentrations caused by both the mismatch between neighbor grains and the slipping at grain boundary could be demonstrated by using this method.

Abstract: There are few reports that evaluated quantitatively the effect of the PWHT (Post Weld Heat Treatment) on the weld residual stress relaxation. We studied the residual stress for butt welding of thick plate. We conducted the experiment and the thermo-elastic-plastic finite element analysis. We became to be able to estimate the residual stress distribution of the butt weld. Also we clarified the effect of the PWHT on the weld residual stress.

Abstract: The magnetic pressure seam welding is one of the candidate methods to join thin sheet smart and multifunctional materials. In this research, to examine the mechanism of magnetic pressure welding from a dynamic viewpoint, numerical simulation of the impact was carried out by using a commercial Euler-Lagrange coupling software MSC.Dytran (MSC.Software) as a first step of the computational studies, where the joint between Fe and Al was employed according to the previous experimental researches. From the serial numerical results, it was found that the increase of temperature at the joint interface was not enough to melt Al or Fe in the range of collision velocity and angle studied in this report. Also, it was revealed that the very large mean stress occurred at the interface which could be considered as the pressure at joint interface and Al moved with high velocity along the interface. Moreover, it was found that there were two patterns of plastic strain distribution near the joint interface depending on the collision velocity and collision angle. Finally, it can be concluded that the plastic strain pattern might be related to the success of magnetic pressure seam welding.

Abstract: An in-house developed FE code, based on the idea of Iterative Substructure Method, was used to evaluate the effectiveness of fixture and pre-strain for reduction of welding distortion in a fillet welded structure. Comparison between the simulation results and experimental results has been performed to validate the basic FE model. Effects of two fixture conditions and various prestrain schemes were analyzed. The simulation results show that appropriate fixture can greatly reduce angular distortion of the work-piece and results in an appreciable reduction in residual deformation and the application of pre-strain can significantly reduce the residual distortion.

Abstract: As examples of the most typical methods to determine the shear strength of SiC/SiC composite joints, the tensile test of lap joined composite and the asymmetrical four point bending test of butt joined composite were analyzed by using finite element method with the interface element. From the calculation results, it was revealed that the strength in the tensile test was strongly influenced by the residual stress as the increase of the joint layer thickness. In the case of asymmetrical bending test, it was found that the crack initiation point would move due to the residual stress and the strength was also affected by the joint layer thickness.

Abstract: As examples of the most typical methods to determine the shear strength of SiC/SiC composite joints, the asymmetrical four point bending test of butt joined composite, the tensile test of lap joined composite and the compressive test of double-notched composite joint were analyzed by using finite element method with the interface element. From the calculation results, it was found that the shear strength in the asymmetrical bending test was controlled by both the surface energy and the shear strength at the interface regardless of their combination although the strength in the tensile test or the compressive test was governed by the surface energy when the shear strength was large. Also, it was revealed that the apparent shear strength of the composite joint obtained experimentally might be affected by the combination of the surface energy and the shear strength at the interface.

Abstract: The distortion and the residual stress are the inevitable consequences of the welding and these may reduce the strength and the quality of the welded structures. To prevent the harmful influence of the welding distortion and the residual stress, their theoretical prediction is necessary. However the existing methods are mostly developed for the study of specimens in laboratories. Compared to the specimens, the structures manufactured in the industry are larger in size and more complex in geometry. For the theoretical prediction applicable to industrial structures, more powerful and practical methods are necessary. To achieve this goal, the authors developed a fast computational method for thermal elastic plastic analysis and an elastic finite element method using inherent deformation.

Abstract: In order to examine mode-I & II type fracture behavior of ceramic joints, the interface element was proposed as a simple model which represents the mechanism of failure in an explicit manner. It was applied to the analyses of four point bending test and asymmetrical four point bending test for SiC/SiC composite specimen joined by ARCJoinTTM. By using a new type interface potential, which is a coupled function of opening and shear deformations, both the bending and asymmetrical bending tests were simulated. From comparison with experiments, surface energy at the interface between the joint and composite was estimated to be about 30 N/m regardless of the fracture mode. Also, from the comparison between the predicted strength and the experiments for the tensile test of lap joint of SiC/SiC composites, it was found that the proposed method was considered to have a great potential as a tool to study the failure problems whose fracture type was a mixture of mode-I & II.