Papers by Author: Nobusuke Hattori

Abstract: Mechanical properties, especially fatigue strength, of ferritic spheroidal graphite cast iron might depend not only on the graphite size but also on the ferrite grain size, little systematic research has been made on these factors. To clarify the influences of these structural factors as well as loading condition, fatigue tests have been carried out on ferritic spheroidal graphite cast iron with different sizes of graphite nodules and ferrite grains, under the axial loading with mean stress from -70MPa to 240MPa. The results obtained in this study are summarized as follows: (1) The fatigue limits are decreased with increasing graphite nodule and matrix grain sizes. (2) The fatigue limit decreases with increasing tensile mean stress. The fatigue limit at an arbitrary mean stress has been estimated by the modified Goodman diagram as well. (3) The parameter α is evaluated from the experimental results relating to the stress ratio R, for the mean stress acting on the tensile side and on the compressive side separately. For the case of R0, the value of α is 0.324. The result has been applied to the fatigue limit evaluation equation of Y.Murakami et al, the fatigue limit ratio falls within the range of about ±10%.

Abstract: The present investigation attempts to evaluate the improvement of working by bolt on the fatigue limit of structural steel sheets with drilling a circular hole. The material used in this study is structural steel (JIS SM400A). And the specimens are identified to two types of the non-worked specimen and the worked specimen. The results obtained in this study can be summarized as follows: (1) The fatigue limit of worked specimens increases than that of non-worked specimen. Futhermore, there has a suitable value of the torque for improving the fatigue limits. (2) The reasons of enhancing the fatigue strength of the worked specimens are due to the elastic deformation, the plastic deformation and the changing of stress concentration part. Our investigations confirm the advantages of this working method which repaired the structural steel sheets by bolt.

Abstract: Diamond-like carbon (DLC) is an amorphous hard carbon, which has very high hardness, high resistivity, and dielectric optical properties. Economically and technologically attractive properties have drawn almost unparalleled interest towards the coatings. Eutectoid steel is a kind of material that has been widely used in shafts and various kinds of industrial components. Three kinds of fatigue specimens with different DLC conditions were used in this study. Fatigue test had been performed to investigate the effects of DLC on fatigue properties of eutectoid steel. The fractography was analyzed by a scanning electron microscope (SEM), and surface hardness was also evaluated. The fatigue limits of the DLC coated specimens did notincrease after DLC process, though the compressive residual stress which produced by DLC process can prevent fatigue fracture. According to the results of fatigue test, the optimal DLC method for improving the fatigue properties of eutectoid steel is determined and the relationship between fatigue limits and coating bias are obtained.

Abstract: Nearly 90% of failures of machines and mechanical parts are caused at the area of stress concentrated in the structural components. Hence, it is important to investigate the method of improving fatigue strength for notched parts. In this paper, the fatigue tests have been performed to investigate the effect of the plastic-working on fatigue strength of specimens with notch. The main results obtained in this study are as follows: (1) The fatigue limit of notched specimen chamfered by the plastic-working increases by 45% than that of specimen without chamfer. (2) The reason of enhancing the fatigue strength may be to attributed to the work-hardening and compressive residual stress which suppress the fatigue crack initiation and propagation in early stage.

Abstract: In this research, the authors have tried to decrease the weight of circular-digging large size bucket as far as possible with keeping the reliability of whole of the construction machine. In the case of main bucket, the authors have evaluated the strength reliability of this bucket and proposed more optimum design changing the thickness and kind of the material, though the expected weight reduction ratio becomes only 5% with increasing the rigidity of the main bucket. In the case of sub-bucket, it is desirable to change the thickness of the material and to attach the reinforcement in the both side. Therefore, the designer could reduce the weight by 11% with keeping the rigidity of the sub-bucket.

Abstract: The effect of stress concentration factor on the fatigue properties of typical austenitic stainless steel SUS304 have been investigated using the circumferentially notched specimens. The notch of specimens has six kinds of radii, i.e. ρ = ∞ (i.e. plain specimen), 2.0, 1.0, 0.6, 0.3, and 0.1 mm with constant notch depth (t=0.2mm). Though the fatigue cracks in the specimens with a blunt notch initiate at one point, those in the specimens with a sharp notch initiate at several points. There exist the slip bands in the surface of the specimen under the stress amplitude of fatigue limit by 1×107 cycles, and do not exist the non-propagating micro-cracks in all kinds of the specimens. Furthermore, it has been found that notch sensitivity of austenitic stainless steels is higher than that of a typical plain carbon steels under the higher stress concentration factor region.

Abstract: This study is developed to investigate the effect of monotonic plastic deformation on the torsional fatigue properties of a structural steel. Five different kinds of tensile pre-strain, 2%, 5%, 8%, 12% and 22%, were applied to the specimens, respectively. And the maximum pre-strain value is near to the necking strain of the test material. The effects of tensile pre-strain on surface hardness, fatigue crack initiation and propagation behaviors, and the behavior of non-propagating cracks. The main results obtained are: The fatigue limits are 145, 160,175, 200 and 215MPa for specimens with tensile pre-strain of 2% 5%, 8%, 12% and 22%, which are improved to 104%, 114%, 125%, 143% and 153% of the fatigue limit for non-pre-strained specimens, respectively. The torsional fatigue limit increases with the tensile pre-strain increasing, until the pre-strain value being near to the necking strain ratio. However, the fatigue limit increase becomes more slowly for high tensile pre-strained specimens than the lower ones. The fatigue cracks of the tensile pre-strained specimens initiated earlier than that of the non-pre-strained specimens, and the propagation is also accelerated, but there is no effect on the fatigue crack initiation point and the branch point. Non-propagating crack length becomes shorter with increasing of tensile pre-strain until the value near the necking strain, and the quantity of non-propagating crack increases at the same time.

Abstract: In order to study the effect of plastic working on fatigue strength of notched specimen, pulsating fatigue tests had been performed on notched deformed stainless steel specimens including on notched non-deformed specimens in order to evaluate the influence of mean stress on fatigue strength. The test results showed that the fatigue limits of plastic worked specimens are higher than that of non-worked one. This difference value would be caused by residual stress, work hardening and fiber texture due to plastic working. When degree of plastic deformation equal zero ("t=0), the fatigue limit ratio (σw /σB) of SUS430 is the higher than that of SUS304. On the other hand, the fatigue limit of worked specimen for SUS304 increases as the plastic deformation value increases to 0.5 mm and then it does not significantly increase from 0.5 mm to 1 mm. It is necessary to investigate an optimal deformation value.

Abstract: In this study, the authors have investigated and compared the effects of roller working and ion nitriding on fatigue properties of eutectoid steel. Five kinds of roller worked and two kinds of ion nitrided specimens were used in this test. The fatigue test was performed using a rotating bending fatigue testing machine to evaluate the fatigue strength of roller worked and ion nitrided eutectoid steel. The fatigue test result shows that roller working is more effective on improving the fatigue strength of the material than ion nitriding. On the other hand, the ion nitriding can much greatly increase the surface hardness than the roller working. In the case of roller working, the fatigue properties are improved by the three main factors which are compressive residual stress, work hardening and fiberized micro-structure.

Abstract: Structural steel is widely used in engineering even in nowadays and many new materials have been developed or are under developing. As one of the most common type, torsional fatigue properties are normally calculated from the rotating bending fatigue properties. In this study, the torsional fatigue properties of a structural carbon steel (S45C) with different tensile pre-strain ratio is investigated, especially on the effects of Mode II crack before it branches under different stress amplitudes. Based on the experimental results and microstructure observation, the main results obtained are: (1)Torsional fatigue strength is increased after tensile pre-strain deformation, and the fatigue limits increase with increasing of tensile pre-strain ratio, the highest improvement can be up to 125% of that of the plain specimen. (2) Torsioanl fatigue cracks initiate from the torsional slip lines in ferrite grains, and there is no obvious effect by the tensile pre-strain. With increasing of tensile pre-strain ratio, the fatigue crack initiation life ratio becomes later and the growth rate becomes faster. (3) The mode II crack length along the axial direction becomes longer with increasing of tensile pre-strain ratio, and the crack branch direction does not affected by the tensile pre-strain. Moreover, the length along axial direction is not affected by the stress amplitude change for specimens with the same tensile pre-strain ratio. (4) The length of non-propagating crack becomes shorter with increasing of tensile pre-strain ratio.