Papers by Author: Nobusuke Hattori

Abstract: Three kinds of materials, pure aluminum (Al1050), carbon steel (S25C) and eutectoid steel (NHH) with V-notch are used in this study to compare the effect of roller working on the fatigue properties among the test materials. As the result, all of the fatigue strengths are improved after roller working and the optimum deformation values exist for every test materials. Fiberized microstructure at the notch bottom, work hardening and compressive residual stress caused by roller working are considered as the main reasons for the fatigue strength improvement. The highest fatigue strength improvement of Al1050, S25C and NHH was 30%, 118% and 155%, respectively. The optimum fatigue improvement of NHH is the highest and it is much higher than that of Al1050, and the optimum deformation value of Al1050 is much lower than that of the other materials. Moreover, the roller working effect is much remarkable for the materials with better hardenability.

Abstract: The authors have studied and clarified that ion nitriding was able to improve the fatigue properties of tool steel. Five kinds of ion nitriding methods (ion nitriding condition is different) were used in this study. The fatigue test had been performed using a rotating bending fatigue testing machine to investigate the effects of ion nitriding on fatigue properties of tool steel. The fractography was analyzed using a scanning electron microscope (SEM), and hardness distribution was also investigated using a microhardness tester. As a result, the fatigue strength and hardness of the ion nitrided specimen increased after ion nitriding processing. It is considered that the compressive residual stress which produced by ion nitriding processing in the layer reduced fatigue fracture, and the altered surface composition improved surface hardness. According to the results of the fatigue test, the optimal ion nitriding method on improving the fatigue limit of tool steel was determined. The hardness of the specimens remarkably increased after ion nitriding processing.

Abstract: This paper is aimed to investigate the effect of roller working on the torsional fatigue properties of a typical low carbon steel referring by the resutls of FEM analysis. Three types of specimens had been prepared with plastic deformation value of 0, 0.5 and 1.0mm, respectively. The main results obtained in this study are as follows: (1) The torsional fatigue strength of roller-worked specimen with 1.0mm plastic deformation is increased more than twice of that of non-roller
worked specimens. (2) Surface hardness of roller worked specimen is much higher than that of the non-roller worked one, and compressed and elongated structure is formed at the notch bottom of the specimen. (3) Crack length along the axial direction of roller worked specimens is longer than that of the non-roller worked specimen, and plural cracks initiated and propagated in the roller worked specimens. (4) The residual stress distribution at the specimen’s notch bottom was analysed using FEM analysis and it is verified the same tendency as the experimental results.

Abstract: In order to study the effect of plastic deformation on fatigue behaviors of plastically deformed specimen, bending fatigue tests had been performed on notched deformed stainless steel specimens. Also pulsating fatigue tests were done on notched non-deformed specimens to evaluate the influence of mean stress on fatigue behavior of notched non-deformed specimens. The result showed that according to increase of deformation value, the fatigue limits of these specimens also significantly increase. Fatigue limit of rolled specimen does not linearly increase with increase in plastic deformation value. Based on fatigue limit diagram, the effect of compressive residual stress on fatigue limit improvement of stainless steel is higher than that of work-hardening. In case of non-deformed specimen, when the compressive mean stress increases, the fatigue limit and the number of cycles to failure increase. In case of tensile mean stress, this kind of mean stress decreases the fatigue limit.

Abstract: Nearly 90% of failures of machines and mechanical parts are caused at the area of stress concentrated in the structural components [1]. Hence, it is important to investigate the method of improving fatigue strength for notched parts, and method of evaluation the fatigue strength of notched parts for mechanical engineering. In this paper, the tests were carried out for improving the fatigue strength of specimens, with the hole notches that have been chamfered by die-pressing. The fatigue strength of notched part is enhanced to around 100% by die-pressing, comparing with notched specimen without die-pressing. Based on the above results, the new method for evaluation the fatigue strength with both the effects of work hardening and residual stress caused by the cold working. The estimated results by the method revealed good agreement with the tested results.

Abstract: The object in this study is to investigate the pulsating fatigue properties of ferritic and martenstic stainless steels using replica method. The main results obtained in this test are as follows; two kinds of ferrritic stainless steels show the same fatigue strength, even if there is difference in micro-structures due to charge of heat treatment condition. In addition, the fatigue limit of martenstic stainless steel shows higher than that of ferritic one by 230%. It is considered that the static mechanical properties relate to the mean value of micro-structures and the fatigue strength relates to the weakest portion of structures. In addition, the ratio between fatigue limit and hardness number becomes considerably smaller than that of conventional structural steels.

Abstract: The binding energy between 3d transition metals (TM) such as iron (Fe), nickel (Ni) and copper (Cu), and boron (B) in Si are studied using first-principles molecular dynamics method. The binding energies of between each TM for Fe, Ni, Cu and B are 0.64,0.57,and 0.44eV respectively, and the binding energy of Fe and B is the largest, on the other hand, binding energy of Ni and B is the smallest. This result is well in agreement with the experiment fact that Fe and Cu exist as a positive charge in P+ silicon, so it is easy to combine with the B, which has a negative charge, on the other hand, Ni exists in the state of neutrality electrically in P+ silicon, so it can not combine with B atom.

Abstract: Prevention of slack in fastening bolt has been an important researching subject since fatigue failure can be generally caused by loosening nut. According to the above reason, the super lock bolt with double screw mechanism, which is non-loosening, has been developed for preventing such kind of bolt failure. The objects of the present study are as follows: (1) To clarify the fatigue strength characteristics. (2) To investigate the initiation and propagation behaviour of fatigue crack regarding to the thread root of first pitch. In addition, the mechanism of “fatigue strength of super lock bolt” is also discussed in this paper and it is confirmed that this bolt shows excellent fatigue strength and non-loosening properties.

Abstract: This study is focused to the effects of plastic working on the fatigue strength of Ti-6Al-4V alloy with notch. In general, the fatigue strength of plastic worked specimen is higher than that of non-worked one. However, the potential of hardening ability of Ti-6Al-4V alloy is very limited. Accordingly, the effect of work hardening on fatigue strength about this material is very small. In addition, the surface of the worked part becomes rougher with increasing plastic deformed value and the fatigue cracks initiate at this part. Consequently, the fatigue limit of the plastic worked specimen is lower than that of the non-plastic-worked one.