Papers by Author: Naoya Nishimura

Abstract: The method for prescribing the site of the first buckling lobe in the axial impact of the tubular structure is proposed. The inertia force induced by the solid mass attached to the tube during the impact is exploited to trigger the first buckling lobe. When the tube with the solid mass undergoes a large acceleration, the inertia force of the solid mass is expected to bend on the tube wall. In the experiment, the rectangular solid mass was attached to the aluminum alloy square tube. The tube fixed to the drop-hammer was impacted against the stationary rigid plate at the velocity of 5 or 7.7 m/s. For the case of the tube without the solid mass, the site of the first buckling lobe varied and the slight wavy plastic deformation remained further than the buckling lobes. On the other hand, when the tube with the solid mass was impacted, the onset of the first buckling lobe was observed at the portion where the solid mass was attached and the wavy deformation stated above was suppressed. The corresponding computation was also conducted using the dynamic explicit finite element method. The result showed a good agreement with the experimental one.

Abstract: In this paper, we have evaluated the sliding and the loosening behavior of thread joints under transverse loading. Firstly, the critical relative slippage (Scr), less than which the thread joints can keep the fastening, is obtained by the cyclic loading tests. Then, this critical relative slippage is estimated according to the theoretically obtained equation considering the bending deformation of bolt and the geometrical constraint condition. The inclination compliance (kw) of the bolt head used in this equation is evaluated by comparing the experimental result with the corresponding analytical one. In consideration of the nonlinearity of kw with respect to the bolt axial tension, the Scr is well estimated by this equation.

Abstract: In this paper we present the estimation methods of fretting wear process and fretting fatigue life using this wear process. Firstly the fretting-wear process was estimated using contact pressure and relative slippage. And then the stress intensity factor for cracking due to fretting fatigue was calculated by using contact pressure and frictional stress distributions, which were analyzed by the finite element method. The S-N curves of fretting fatigue were predicted by using the relationship between the calculated stress intensity factor range ( #K) with the threshold stress intensity factor range ( #Kth) and the crack propagation rate (da/dN) obtained using CT specimens of the material. Finaly fretting fatigue tests were conducted on Ni-Mo-V steel specimens. The S-N curves of our experimental results were in good agreement with the analytical results obtained by considering fretting wear process. Using these estimation methods we can explain many fretting troubles in industrial fields.

Abstract: Considering the productivity and the maintenance, most of machine and product has many joints (fastening, welding and adhesive joint). Especially, the thread joint has been used the most mainly as a machine element. However, many troubles on strength and reliability of these machine and product are generating in these joints. In this paper, we have evaluated the sliding and the self loosening behavior of thread joints under transverse loading, where the entire response of the thread joint portion exhibits an inelastic one. The finite element analysis using a simple model of thread joints with respect to the relative sliding motion between the parts was performed and compared with the experimental results under quasi-static loading. The critical relative slippage (Scr), less than the value which thread joints can keep the fastening, was obtained by the result of cyclic loading tests. In addition, this critical relative slippage was estimated according to the theoretically obtained equation considering the bolt bending and the geometrical constraint condition. The inclination compliance (kw) of the bolt head used in this equation was evaluated by the comparison of experimental and analytical results from theoretical equation. In consideration of the nonlinearity of kw with respect to the bolt axial tension, the Scr is well estimated by this equation.

Abstract: Various polygonal tubes were compressed in the axial direction under quasi-static and dynamic loading conditions. The effect of the polygonal shape and the wall thickness on the crush behavior is investigated, in which the cyclic buckling takes place. The numbers of polygonal edges were 3, 4, 5, 6 and 7 in the experiment. A circular tube was also tested for comparison. The tubes were machined from aluminum alloy A5056 bar. Crush strength is estimated as an index of the energy absorption capacity of the tube. It increases with increasing the number of polygonal edges of the tube, although it almost saturates when the number of polygonal edges is more than 6. For the wider variety of polygonal tubes than that in the experiment, numerical simulation is performed using the dynamic explicit finite element code DYNA3D. The computed crush behavior well agrees with the corresponding experimental one, however, the difference in collapse mode arises due to the slight imperfections in experiment. The deformation pattern becomes more irregular for the thinner-walled tube. Further, it is presumed that the large hardening exponent in the plastic property of the material could prevent the buckling switching from the symmetric mode to asymmetric one in the crushing of circular tube.

Abstract: Magnetic, dielectric properties and magnetoelectric effect of ceramics with a composition of 0.9 BaTiO3-0.1 LaMnO3 have been investigated. X-ray diffraction measurements showed that the sintered ceramics are composites containing a small amount of (La, Ba)MnO3 phase in the BaTiO3 matrix. These composites were multiferroic, having ferromagnetic and ferroelectric Curie temperatures of 330 K and 392 K respectively. We found that the composite sintered at 1150 °C exhibits a reduction of spontaneous magnetization as large as 55 % at room temperature when an electric field of 1.4 KV/mm is applied. This reduction is probably ascribed to a change of hole concentration distribution in the precipitated ferromagnetic (La, Ba)MnO3 and the resultant decrease of ferromagnetic Curie temperature.