研究者データベース

中村 孝(ナカムラ タカシ)
工学研究院 機械・宇宙航空工学部門 機械材料システム
教授

基本情報

所属

  • 工学研究院 機械・宇宙航空工学部門 機械材料システム

職名

  • 教授

学位

  • 博士(工学)(東京工業大学)

ホームページURL

J-Global ID

研究キーワード

  • 材料強度学   機械材料学   

研究分野

  • ものづくり技術(機械・電気電子・化学工学) / 材料力学、機械材料 / 疲労,超高サイクル疲労,真空

職歴

  • 2007年 - 2010年 北海道大学大学院工学研究科教授(機械宇宙工学専攻) 教授
  • 2007年 - 2010年 Professor
  • 2010年 - 北海道大学大学院工学研究院教授(機械宇宙工学部門) 教授
  • 2010年 - Professor

所属学協会

  • 日本機械学会   日本材料学会   日本航空宇宙学会   

研究活動情報

論文

  • Masako Yamada, Kosuke Takahashi, Nao Fujimura, Takashi Nakamura
    Engineering Fracture Mechanics 271 2022年08月 
    The peel tests of pressure-sensitive adhesive (PSA) tapes were conducted to clarify the effect of the peel velocity, peel angle, and tape thickness. We focused on the viscoelastic deformation of the PSA layer and its relationship with the adhesion energy. It was found that, at a sufficiently low strain rate of the PSA layer, the adhesion energy has a threshold value of debonding. In addition, the increase of adhesion energy from the threshold value is proportional to the maximum stretch of the PSA layer and follows a power law relationship with the strain rate. The general relationship between the adhesion energy and the strain of PSA layer, independent from the peel angle and tape thickness, was established.
  • Gaoge Xue, Takashi Nakamura, Nao Fujimura, Kosuke Takahashi, Hiroyuki Oguma, Akihisa Takeuchi, Masayuki Uesugi, Kentaro Uesugi
    Engineering Fracture Mechanics 263 108308 - 108308 2022年03月 [査読有り]
  • 放射光 X線ナノ CT を用いたその場引張・疲労試験による炭素繊維-エポキシ樹脂の界面はく離検出
    勝谷亮介, 松尾拓磨, 髙橋航圭, 藤村奈央, 中村孝
    日本複合材料学会誌 47 5 186 - 193 2021年09月 [査読有り]
  • Akihisa Takeuchi, Kentaro Uesugi, Masayuki Uesugi, Hiroyuki Toda, Kyosuke Hirayama, Kazuyuki Shimizu, Koichi Matsuo, Takashi Nakamura
    Review of Scientific Instruments 92 2 023701 - 023701 2021年02月01日 [査読有り]
     
    In this study, high-energy x-ray nanotomography (nano-computed tomography, nano-CT) based on full-field x-ray microscopy was developed. Fine two-dimensional and three-dimensional (3D) structures with linewidths of 75 nm-100 nm were successfully resolved in the x-ray energy range of 15 keV-37.7 keV. The effective field of view was ∼60 µm, and the typical measurement time for one tomographic scan was 30 min-60 min. The optical system was established at the 250-m-long beamline 20XU of SPring-8 to realize greater than 100× magnification images. An apodization Fresnel zone plate (A-FZP), specifically developed for high-energy x-ray imaging, was used as the objective lens. The design of the A-FZP for high-energy imaging is discussed, and its diffraction efficiency distribution is evaluated. The spatial resolutions of this system at energies of 15 keV, 20 keV, 30 keV, and 37.7 keV were examined using a test object, and the measured values are shown to be in good agreement with theoretical values. High-energy x-ray nano-CT in combination with x-ray micro-CT is applied for 3D multiscale imaging. The entire bodies of bulky samples, ∼1 mm in diameter, were measured with the micro-CT, and the nano-CT was used for nondestructive observation of regions of interest. Examples of multiscale CT measurements involving carbon steel, mouse bones, and a meteorite are discussed.
  • Nakamura Takashi, Fujimura Nao, Tomoda Yuta, Xue Gaoge, Yoshinaka Fumiyoshi, Takeuchi Akihisa, Uesugi Masayuki, Uesugi Kentaro
    Proceedings of the 8th Engineering Integrity Society International Conference on Durability & Fatigue, 2021 349 - 357 2021年 [査読有り]
  • Gaoge Xue, Takashi Nakamura, Nao Fujimura, Kosuke Takahashi, Hiroyuki Oguma
    APPLIED SCIENCES-BASEL 11 1 2021年01月 [査読有り]
     
    Uniaxial fatigue tests were conducted for a beta titanium alloy Ti-22V-4Al up to a very high cycle fatigue (VHCF) regime. The initiation and propagation processes of the internal fatigue cracks were investigated using 3D fractographic analysis. Multiple facets were observed at the crack initiation site. Three facet initiation models were proposed based on the surface appearances and the 3D facet bonding patterns of the multiple facets, and the major facet was determined to be the true crack initiation site. Using the size of the major facet, a Tanaka-Akiniwa model, which can determine the material constants for the Paris law using only conventional fatigue tests, was applied to reveal the propagation process of the internal cracks. A reverse fatigue life prediction was also conducted to evaluate the accuracy of the material constants obtained using the Tanaka-Akiniwa model. When the facet initiation models were applied, the predictions showed less deviation and better agreement than when the facet initiation process was not considered. The findings of this study indicate that the formation of multiple facets in beta titanium alloys is sequential rather than simultaneous.
  • Fumiyoshi Yoshinaka, Gaoge Xue, Nao Fujimura, Takashi Nakamura
    INTERNATIONAL JOURNAL OF FATIGUE 142 2021年01月 [査読有り]
     
    In this study, small crack propagation tests are conducted on artificially defected specimens of Ti-6Al-4V under air and vacuum pressures of 100 Pa, 10(-2) Pa, 10(-4) Pa, and 10(-6) Pa. Fracture surfaces between these environments are compared, and the effect of vacuum pressure on the crack propagation process is investigated. The similarities between surface cracks in vacuum and internal cracks in the very high cycle regime are also discussed from the perspectives of crack propagation rate and fractographic characteristics. From the results, it was observed that there was a tendency for the crack initiation life to increase and the crack propagation rate to decrease with a decrease in the vacuum pressure, although no notable distinction was observed in air and under 10(0) Pa. The fractography exhibited an angular surface with sharp edges in air and under 10(0) Pa. However, a granular feature with rounded edges was observed below 10(-2) Pa and became more pronounced as the vacuum pressure decreased. The window periods, which formed an oxidized surface coverage, were believed to account for the vacuum effect on crack propagation rate and fracture surface features. Both the crack propagation rate and fractographic features of surface cracks under 10(-4)-10(-6) Pa resembled those of internal cracks; therefore, the presumably working vacuum environment around internal cracks was considered to be at a high vacuum level.
  • Nao Fujimura, Koyo Ooga, Kosuke Takahashi, Takashi Nakamura, Tatsuki Wajima
    MECHANICAL ENGINEERING JOURNAL 7 4 2020年 [査読有り]
     
    A machined material has a work-hardened layer at its surface. In this study, a surface modification technique, the scanning cyclic press (SCP), was applied to machined specimens of magnesium alloy, AZ31, to investigate whether SCP can improve its fatigue properties regardless of the surface finish. During the SCP process, a vibrating indenter reciprocally scanned the specimen's surface, and it applied cyclical low-compressive loadings to the surface for 8 x 10(6) cycles. After applying SCP, the surfaces of the specimens were observed using a laser scanning microscope, and the surface roughness was measured. The surface observation and surface roughness measurement showed that the changes in the surface state after applying SCP were relatively small and the surface roughness after applying SCP was more homogenous than before applying SCP. Uniaxial push-pull fatigue tests were conducted for SCP-treated specimens and untreated specimens. The test results showed that the fatigue life of SCP-treated specimens was longer than that of untreated specimens. To clarify the reason for the improvement effect, the fracture surfaces were observed using a scanning electron microscope (SEM). The SEM observation showed that the fracture morphology was different between the SCP-treated specimen and the untreated specimen. In the SCP-treated specimen, fatigue fracture origins were sub-surface, while the untreated specimen fractured at the surface. These results suggest that SCP could improve the fatigue properties of AZ31 regardless of the surface finish of the specimen before SCP.
  • Fumiyoshi Yoshinaka, Takashi Nakamura, Akihisa Takeuchi, Masayuki Uesugi, Kentaro Uesugi
    FATIGUE & FRACTURE OF ENGINEERING MATERIALS & STRUCTURES 42 9 2093 - 2105 2019年09月 [査読有り]
     
    Small internal fatigue cracks initiated in Ti-6Al-4V in the very high cycle regime were detected by synchrotron radiation microcomputed tomography (SR-mu CT) at SPring-8 in Japan. The initiation and growth behaviours of the cracks were nondestructively observed, and the da/dN-Delta K relationship was measured and compared with that obtained in a high vacuum environment. SR-mu CT revealed that more than 20 cracks were initiated in one specimen. The crack initiation life varied widely from 20% to 70% of the average fatigue life and had little influence on the growth behaviour that followed. The initiation site size of each internal crack detected in one specimen was comparable with the size of the fracture origins obtained in ordinary fatigue tests. These results suggest that the surrounding microstructures around the initiation site are likely a dominant factor on the internal fracture rather than the potential initiation site itself. The internal crack growth rates were lower than 10(-10) m/cycle, and extremely slow rates ranging from 10(-13) to 10(-11) m/cycle were measured in a lower Delta K regime below 5 MPa root m. The internal crack growth rate closely matched that of surface cracks in a high vacuum, and the reason for the very long life of internal fatigue fractures was believed to result from the vacuum-like environment inside the internal cracks.
  • 吉中奎貴, 中村孝
    チタン 67 3 2019年 [査読有り]
  • Takashi Nakamura, Fumiyoshi Yoshinaka
    Procedia Structural Integrity 14 978 - 985 2019年 [査読有り]
     
    In recent years, very high cycle fatigue (VHCF) has become serious issues in the field of mechanical engineering. The VHCF phenomenon is mainly characterized by an internal fracture in cycles over 10 -10 . This type of fracture has been observed not only in ferrous metals but also in Ti alloys. Especially in Ti-6Al-4V, a remarkable decrease of fatigue strength is evident in internal fractures compared with surface fractures. However, the mechanism of internal fracture is not yet fully understood. The internal cracks seem to propagate through a vacuum-like environment that is shut off from the air.Therefore, the effects of oxidation or gas adsorption on crack growth could be almost ignored in the internal crack. On the basis of this concept, we conducted small crack growth tests in air and high vacuum and observed the fracture surfaces to clarify the effects of vacuum on crack propagation. In addition, we applied a micro computed tomography (µCT) technique using synchrotron radiation to observe the internal crack growth non-destructively. By comparing the crack growth process in vacuum with those of internal crack detected by µCT, the effects of vacuum-like environment on small internal crack growth processes were investigated. 7 8
  • N. Fujimura, T. Nakamura, M. Ueno
    Mechanical Engineering Letters 4 2018年12月 [査読有り][通常論文]
  • Akihisa Takeuchi, Kentaro Uesugi, Masayuki Uesugi, Fumiyoshi Yoshinaka, Takashi Nakamura
    Microscopy and Microanalysis 24 S2 108 - 109 2018年08月 [査読有り]
  • Nao Fujimura, Takashi Nakamura, Kosuke Takahashi
    12TH INTERNATIONAL FATIGUE CONGRESS (FATIGUE 2018) 165 2018年 [査読有り]
     
    To quantitatively investigate the cause of the changes in arithmetic mean roughness R-a and arithmetic mean waviness W-a of austenitic stainless steel under low-cycle fatigue loading, precise observation focusing on persistent slip bands (PSBs) and crystal grain deformations was conducted on SU5316NG. During the fatigue tests, the specimen's surface topography was regularly measured using a laser microscope. The surface topographies were analysed by frequency analysis to separate the surface relief due to PSBs from that due to grain deformation. The height caused by PSBs and that by grain deformation were measured respectively. As a result, both of the heights rose with the increase of usage factor (UF). The amount of increase in the heights with respect to UF increased with strain range. The trend of development of both heights was similar with the trend of R-a. and W-a. A comparison between R-a and the height caused by PSBs showed that these values strongly correlated with each other. A comparison between W-a and the height caused by grain deformation also showed that these values strongly correlated with each other. Consequently, the surface texture parameters R-a. and W-a represent the changes in the heights of surface reliefs due to PSBs and grain deformation.
  • 藤村 奈央, 西塚 祐斗, 髙橋 航圭, 中村 孝
    日本機械学会論文集 84 864 18 - 00206-18-00206 一般社団法人 日本機械学会 2018年 [査読有り][通常論文]
     
    <p>To quantitatively investigate the cause of the changes in arithmetic mean roughness <i>R<sub>a</sub></i> of austenitic stainless steel under low-cycle fatigue loading, precise observation focusing on slip bands was conducted on SUS316NG. During the loading, the specimen's surface topography was regularly measured using a laser microscope. The topography was then characterized by frequency analysis to identify the surface reliefs due to persistent slip bands (PSBs), and their heights were measured. The heights increased with the usage factor (<i>UF</i>). The amount of the increase with respect to <i>UF</i> increased with the strain range. These tendencies are similar to those for <i>R<sub>a</sub></i>. A comparison between <i>R<sub>a</sub></i> and the heights of surface reliefs caused by PSBs showed that the values were strongly correlated. The number of PSBs formed on the surface was estimated from the area ratio of PSBs. The area ratio increased with <i>UF</i>, which is similar to the relationship for <i>R<sub>a</sub></i>. A comparison between <i>R<sub>a</sub></i> and the area ratio showed that these values were also strongly correlated. The product of the height due to PSBs and the area ratio was calculated and it was compared with <i>R<sub>a</sub></i>. As a result, the product was in good agreement with <i>R<sub>a</sub></i>. Consequently, the surface texture parameter <i>R<sub>a</sub></i> represent both the change in the height and the number of surface reliefs due to PSBs.</p>
  • 吉中 奎貴, 中村 孝, 髙久 和明, 上杉 健太朗
    材料 66 12 928 - 934 公益社団法人 日本材料学会 2017年12月 [査読有り]
     

    Observations of internal small fatigue crack growth in Ti-6Al-4V were conducted using synchrotron radiation μCT imaging at SPring-8 to obtain the internal crack growth rate. The just-initiated internal cracks and their growth processes were successively observed. The internal small cracks propagated very slowly with the growth rate of less than 10-10 m/cycle. The crack growth rate had a lot variability in the smaller crack length regime. In order to discuss the reason for the small growth rate of internal crack in terms of the effect of environment inside the crack, the internal crack growth rate was compared with the surface crack growth rate in air and ultra-high vacuum. As a result, the internal crack growth rate was lower than the surface crack growth rate in air, and matched with that in ultra-high vacuum in the longer crack length regime. However, the growth rate of internal crack was smaller than that of surface crack in ultra-high vacuum in the relatively smaller crack length regime. These results indicate the possibility that the differences in vacuum pressure have a dominant effect on the small crack growth length, and that the gas contained in the alloy plays an important role in the internal small crack growth.

  • 吉中 奎貴, 中村 孝, 髙久 和明, 塩澤 大輝, 中井 善一, 上杉 健太朗
    日本機械学会論文集 83 856 17 - 00104-17-00104 一般社団法人 日本機械学会 2017年 [査読有り]
     

    The initiation and growth of internal small fatigue cracks with around ten or several dozen μm in Ti-6Al-4V were nondestructively examined by using synchrotron radiation μCT at the large synchrotron radiation facility SPring-8. Lots of grain-sized internal cracks were observed roughly evenly in the observation volume in the specimen; in contrast, only one surface crack was detected. The initiation lives of the internal cracks were widely different for each crack and had no significant correlation with the crack initiation site nor the initial crack size. The internal cracks propagated microstructure-sensitively with several crack deflections, and the growth rates were very small, less than 10-10 m/cycle. The crack growth rates just after facet formations showed large variability and had no apparent relationship with the crack initiation life nor the initial crack size. This variability can likely be attributed to microstructural inhomogeneities around the crack initiation facets. The estimated facet formation rate indicated that most facets formed rapidly compared with the following internal crack growth rate.

  • 藤村 奈央, 中村 孝, 髙橋 航圭, 和島 達希
    材料試験 66 12 906 - 912 公益社団法人 日本材料学会 2017年 [査読有り][通常論文]
     
    <p>A new surface modification technique, scanning cyclic press (SCP), was developed. SCP scans a metal surface with a vibrating indenter under precise loading control based on servo fatigue testing machine and can apply a variable cyclically compressive load. This study applied SCP to magnesium alloy AZ31 to investigate the effect on fatigue properties. After applying SCP, the surfaces of specimens were observed by using a laser scanning microscope and uniaxial push-pull fatigue tests were conducted. As a result, surface roughness of SCP-treated specimens slightly increased; however, the fatigue life became longer than that of untreated specimens. To clarify the reason for the improvement effect, fracture surfaces of the specimens were investigated in detail on the basis of SEM observation. SEM observation showed differences between the fracture surfaces of the untreated and SCP-treated specimens. The origin of fracture was at the surface in the untreated specimen. In the SCP-treated specimen, however, the fracture origins were sub-surface and a band-like layer just beneath the surface was observed on fracture surface, whereas the layer did not exist on that of untreated specimen. The surface hardness of SCP-treated specimen increased to almost twice as much after applying SCP. The result suggests that SCP modified surface layer of specimen and this layer suppressed crack initiation from surface.</p>
  • Fumiyoshi Yoshinaka, Takashi Nakamura, Shinya Nakayama, Daiki Shiozawa, Yoshikazu Nakai, Kentaro Uesugi
    INTERNATIONAL JOURNAL OF FATIGUE 93 397 - 405 2016年12月 [査読有り]
     
    The propagation of an internal fatigue crack in Ti-6Al-4V was non-destructively observed by synchrotron radiation mu CT imaging to clarify the crack growth rate in very high cycle fatigue. The results show that the internal crack propagated quite slowly at a rate of less than 10(-10) m/cycle. The propagation rate of an internal crack was compared with that of a surface crack in air and in high vacuum to examine the internal fracture process in terms of the environment around the crack. The rate of the internal crack was similar to that of the surface crack in high vacuum, but was significantly lower than that in air. This led us to conclude that the low propagation rate of the internal crack is due to the vacuum-like environment inside the crack. (C) 2016 Elsevier Ltd. All rights reserved.
  • Fumiyoshi Yoshinaka, Takashi Nakamura, Kazuaki Takaku
    INTERNATIONAL JOURNAL OF FATIGUE 91 29 - 38 2016年10月 [査読有り]
     
    Small fatigue crack propagation tests were conducted on Ti-6Al-4V in ultrahigh vacuum, air, and argon to clarify the effects of vacuum on crack propagation processes. The crack propagation rate in vacuum was significantly lower than that in air. The crack propagation rate in argon agreed with that in air in the small crack regime; however, it became similar to that in vacuum in the long crack regime. This indicates that the existence of gases has significant effects in the small crack regime, even if they are chemically inert. These results led us to conclude that, in addition to chemical inactivity, the absence of physical adsorption plays an important role in the retarding effect of vacuum on small crack propagation. (C) 2016 Elsevier Ltd. All rights reserved.
  • YOSHINAKA Fumiyoshi, NAKAMURA Takashi
    Mechanical Engineering Letters 2 15 - 00730-15-00730 一般社団法人 日本機械学会 2016年 [査読有り]
     
    Fatigue cracks often initiate inside materials in the very high cycle region (VHCF), although they always initiate at the surface of materials in usual high cycle fatigue. However, the mechanism behind internal crack propagation is not yet clear due to difficulties in observation. The environment inside internal cracks is likely similar to a vacuum environment since it is shut off from air, leading to negligible effect of oxidation or gas absorption. In the present work, the effect of vacuum on fatigue fracture surfaces of high strength steel SNCM439 was investigated quantitatively to estimate the effect of the vacuum-like environment inside the internal crack on the crack propagation process. Uniaxial fatigue tests were carried out in vacuum and air environments and then 3D fractography was performed to measure surface roughness of the fracture surface. These analyses targeted three typical fracture surfaces: (a) surface fracture in air, (b) surface fracture in vacuum, and (c) subsurface fracture. Results of fatigue tests showed that fatigue lives were longer in vacuum than in air. Fracture surface roughness of surface fracture in air was greater than that in vacuum, while fracture surface roughness of subsurface fracture agreed quite well with that of surface fracture in vacuum. These results indicate that the effects of vacuum environment and the environment inside internal crack on fatigue crack propagation are almost the same. This leads us to conclude that the behavior of internal crack propagation can probably be estimated from surface crack propagation in vacuum environment.
  • NAKAMURA Takashi, YOSHINAKA Fumiyoshi, NAKAYAMA Shinya, OGUMA Hiroyuki, SHIOZAWA Daiki, NAKAI Yoshikazu, UESUGI Kentaro
    Mechanical Engineering Letters 2 16 - 00233-16-00233 一般社団法人 日本機械学会 2016年 [査読有り]
     
    The purpose of this study is to develop a method for detecting small internal fatigue cracks in Ti-6Al-4V by using synchrotron radiation provided at SPring-8. An electro-hydraulic fatigue testing machine was carried into the facility and a combination of uniaxial fatigue tests and micro computed tomography (μCT) imagings were conducted. The small specimen without size effect was newly designed for μCT imaging. The conditions of the fatigue tests were determined to observe internal crack initiations within the available time at the facility. To obtain a clear and accurate image of internal cracks, several important parameters for μCT imaging such as the crack opening load, the distance between detector and specimen, were optimized. As a result, multiple internal fatigue cracks sized around 30 μm or below were successfully detected.
  • 堀川 紀孝, 鮫島 大湖, 中村 孝, 小熊 博幸
    鋳造工学 87 6 375 - 381 公益社団法人 日本鋳造工学会 2015年 [査読有り]
     

      With increasing demands for the weight reduction of cast iron products in various industries, advanced processing techniques to make thin-wall ductile cast iron without chill structure were developed. This study investigated the tensile and fatigue properties of the thin-wall ductile cast iron and effects on the casting surface. Thin-wall ductile cast iron specimens of 2mm in thickness were prepared. The tensile test and fatigue test were carried out using test pieces with machined and casting surfaces. The tensile strength of the test pieces with casting surface was smaller than that of the machined test pieces. The measured thickness including the casting surface is thought to be larger than the effective thickness which supports the load. The authors estimated the effective thickness and it was approximately 0.25mm smaller than the thickness of the test pieces with casting surface. The S-N diagram indicated that the fatigue limit of the test piece with the casting surface was 217MPa, which is smaller than that of the machined test piece. Most of fracture origins were the concavity of the casting surface, and shrinkages and dross were also observed. The fatigue limit evaluated with the test pieces whose fracture origins were the concavity of the casting surface, was 252MPa. The surface roughness profile was evaluated by the two parameters in the altitudinal and horizontal directions, and the fatigue limit was estimated from the maximum concavity that was obtained by extreme statistics. Estimated fatigue limit was 259MPa, and it is close to the fatigue limit of the test pieces whose fracture origin was the concavity of casting surface. The authors also proposed a method to estimate the size of the fracture origin by roughness parameter.

  • Tomoki Shinko, Takashi Nakamura, Nao Fujimura, Yasutaka Nakata
    Zairyo/Journal of the Society of Materials Science, Japan 64 11 910 - 917 2015年 [査読有り][通常論文]
     
    Low-cycle fatigue tests of SUS316NG austenitic stainless steel were conducted for several strain ranges (0.76, 1.26, 2.1, 4.1, and 8.1%) in which the specimen’s surface topography was regularly measured using a laser scanning microscope. The surface topographies obtained were analyzed by frequency analysis to separate the surface relief due to persistent slip bands (PSBs) from that due to crystal grain deformation. The PSBs-induced surface relief evolution and the grain-deformation-induced one were quantitatively evaluated by using arithmetic mean roughness Ra and arithmetic mean waviness Wa, respectively. The Î"Ra and Î"Wa, the increments in Ra and Wa from the initial values, increased with the usage factor (UF) for each strain range. Moreover, the rates of increase in Î"Ra and Î"Wa with respect to UF increased with the strain range. Î"Wa/Î"Ra was larger for higher strain ranges. This tendency of Î"Wa/Î"Ra indicates that the surface relief due to grain deformation develops more prominently than that due to PSBs for larger strain ranges. The results for Ra and Wa agree with the results of surface topography observation. On the basis of these results, a method was developed for estimating the UF of fatigued material and the applied strain range by using only the measured Ra and Wa.
  • Hiroyuki Oguma, Takashi Nakamura
    11TH INTERNATIONAL FATIGUE CONGRESS, PTS 1 AND 2 891-892 1436 - 1441 2014年 [査読有り][通常論文]
     
    In Ti-6Al-4V alloy, fatigue properties have been widely investigated, and the origin of fatigue fracture is usually at the surface in the high stress and lower fatigue life region, whereas in low stress and longer fatigue lifetimes origins are generally sub-surface in nature. Very high cycle fatigue tests were conducted, and observation of fracture surfaces revealed that a unique fine concave and convex agglutinate (hereinafter called Granular Region) formed on the fracture surface of sub-surface fractures. The granular region was not observed on the fracture surface of surface fractures. To clarify the formation mechanism and process of forming the granular region, which is a unique phenomenon in the very high cycle fatigue, fatigue tests using specimens with an artificial surface defect were conducted in air and vacuum. The fatigue tests were based on the idea that the environment around a sub-surface fatigue crack is a vacuum-like environment. During the tests, fracture surfaces were intentionally contacted in air and vacuum under different loading conditions. Fracture surface observations revealed that repeated contact of the fracture surfaces and a vacuum environment are necessary for the formation of the granular region. A mechanism for the formation of the granular region will be proposed.
  • Takashi Nakamura, Hiroyuki Oguma, Shingo Nukaya
    PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE - 2013, VOL 1B: CODES AND STANDARDS 2014年 [査読有り][通常論文]
     
    Uni-axial fatigue tests up to 10(9) cycles were conducted at two cyclic frequencies (10 and 250 Hz) under several stress ratios (R= -1, 0.1, and 0.5) to investigate the effects of strain rate on the very high cycle fatigue properties of SFVQ1A, which is equivalent to ASTM A508 C1.3. Longer fatigue lives and 5% higher fatigue limit were measured at 250 Hz than those at 10 Hz for R= -1. Under R= 0.1 and R=0.5, however, the fatigue properties did not differ between 10 and 250 Hz. Observations of fracture surfaces clarified that all fractures under R=-1 and R=0.1 were caused by a general crack growth process from non-metallic inclusion(s) at the specimen surfaces. In contrast, the entire fracture surface under R=0.5 was covered with a dimpled pattern. The fatigue mechanism was considered to be due to ductility exhaustion through ratcheting behavior under high mean stress. The longer fatigue lives and larger fatigue limit at 250 Hz under R= -1 was explained by the increase in crack growth resistance at a high strain rate based on crack growth behaviors and the da/dN-Delta K relation. No differences in fatigue properties between different frequencies under R=0.1 and 0.5 were likely caused by the negligible effect of strain rate compared with the increase material strength during fatigue loading, which resulted from strain hardening induced by maximum cyclic stress that was larger than yield stress.
  • Nao Fujimura, Takashi Nakamura, Hiroyuki Oguma
    PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE - 2013, VOL 5 2014年 [査読有り][通常論文]
     
    Changes in the surface roughness of SUS316NG during cyclic loadings were investigated, and the relations between those changes and the crack initiation and propagation processes are discussed on the basis of microscopic observations and cellulose acetate replica observations. Strain-controlled fatigue tests were conducted at three constant strain ranges. Surface roughness was measured periodically during the tests, and three roughness parameters were calculated: arithmetic mean roughness R-a, surface profile maximum height R-max, and maximum valley depth R-v. Until the middle of fatigue life, all three increased linearly with the number of cycles regardless of the strain range, and their rates of increase became smaller with decreasing strain range. Surface observation revealed that small cracks initiated very early in fatigue life, propagated slowly until the middle of fatigue life, and then grew rapidly. Changes in surface roughness are therefore sensitive to fatigue loading even when cracks are very small and crack detection is difficult. The results suggest that surface roughness can probably be used to assess fatigue damage because until the middle of fatigue life it increases linearly with the number of cycles. The definition of each roughness parameter and the changes show that R-max and R-v are suitable for damage assessment.
  • Hiroyuki Oguma, Takashi Nakamura
    Zairyo/Journal of the Society of Materials Science, Japan 62 12 731 - 737 2013年12月 [査読有り][通常論文]
     
    To reveal influence factors on the very high cycle fatigue properties of Ti-6Al-4V alloy, uni-axial fatigue tests and fatigue crack propagation tests were conducted. In the uni-axial fatigue tests, specimens with artificial defects of several sizes were used. Both tests were performed in air and vacuum environments. The effects of environments around fatigue cracks on crack propagation properties were investigated. As a results, fatigue crack propagation rates in vacuum were lower than those in air, and the results correspond to the fact that the sub-surface fractures occur in longer life regime than surface fractures. The threshold stress intensity factor range showed size dependencies, in addition, the difference of environments affected the size dependencies. The dependence in vacuum was stronger than in air. The result indicates that a small crack (size of a crack origin) can propagate in lower stress intensity factor ranges in vacuum, and this can relate to the reason why the sub-surface fractures occur in lower stress levels than the surface fractures. Fracture surface observations revealed that fine convex-concave granular pattern (Granular region) was formed around the origins of surface fractures in the vacuum environment, and the aspect resembled the fracture surface of sub-surface fractures. The result showed that the crack propagation mechanism in vacuum and sub-surface of materials were similar. The effects of vacuum environment are almost same as those of environment around the sub-surface crack. The effects of environments around fatigue cracks can well explain the very high cycle fatigue properties of the material. © 2013 The Society of Materials Science, Japan.
  • Oguma H., Nakamura T.
    International Journal of Fatigue 50 89 - 93 Elsevier 2013年05月 
    To determine the effects of vacuum environment on fatigue crack propagations in a Ti-6Al-4V alloy, K-decreasing tests were conducted in air and vacuum. The fatigue crack propagation rate became slower and threshold stress intensity factor range became larger with decreasing vacuum pressure. The tendency cannot be fully explained by the crack closure. Based on fracture surface observations, granular region of a few micrometer size asperities was observed on the fracture surface only in high vacuum and ultra high vacuum. The high vacuum environment is one of the necessary conditions for the formation of the granular region, and the fraction of surface coverage of adsorbed gas on fracture surfaces relates to the phenomenon. The formation of the granular region represents the difference of the crack propagation mechanism between vacuum and air environments. A new mechanism for the formation of the granular region was proposed, and that is one of the phenomena which can explain the reduction of crack propagation rate in vacuum. (C) 2012 Elsevier Ltd. All rights reserved.
  • Nao FUJIMURA, Takashi NAKAMURA, Hiroyuki OGUMA
    Journal of Solid Mechanics and Materials Engineering 7 3 372 - 380 The Japan Society of Mechanical Engineers 2013年 [査読有り][通常論文]
     
    Changes in surface roughness during low cycle fatigue loading were investigated on austenitic stainless steel, SUS316NG, commonly used in the piping systems of nuclear power plants. The fatigue damage process, including crack initiation and propagation, was observed using cellulose acetate replicas, and the relationship between the process and changes in surface roughness was discussed. Strain-controlled fatigue tests were conducted on mirror polished specimens at constant strain range conditions Δε = 8, 4, and 1%. During the cyclic loadings, the surface roughness was measured at cycles determined with respect to the usage factor UFpre. As a result, the surface roughness was found to increase roughly linearly until approximately UFpre = 0.4 regardless of the strain range conditions. The rate at which surface roughness increased with UFpre became smaller with decreasing applied strain range. In the damage process, small cracks were observed to initiate very early in fatigue life. The cracks propagated slowly until about the middle of fatigue life, however it grew rapidly after around UFpre = 0.6. These results showed that the change in surface roughness is sensitive to fatigue loading even when cracks are very small and crack detection is difficult. The obtained results suggest that surface roughness can probably be used to assess fatigue damage until the middle of fatigue life because of its linear increase with respect to the number of cycles.
  • 藤村奈央, 中村孝, 小熊博幸, 中島智広, 信耕友樹
    材料 62 12 750 - 755 The Society of Materials Science, Japan 2013年 [査読有り][通常論文]
     
    Changes in the surface roughness of SUS316NG during cyclic loadings were investigated, and the relationship between those changes and the fatigue damage processes, including evolution of surface relief due to active slip systems, were discussed on the basis of microscopic observations. Strain-controlled fatigue tests were conducted at three constant strain ranges Δε = 8, 4, and 1%. During the tests, surface roughness was measured periodically at cycles determined with respect to the usage factor UF, and then the surface was observed directly to clarify the surface morphology. As a result, until the middle of fatigue life, the arithmetic mean roughness R<font size="-1">a</font> increased linearly with the number of cycles regardless of the strain range conditions. Clear changes in surface roughness were obtained even at a strain range as small as 1%. The results suggest that the surface roughness measurement can probably be used to assess fatigue damage because it increases linearly with the number of cycles until the middle of fatigue life. The rate at which surface roughness increased with UF became smaller with decreasing applied strain range. In the damage process, the surface became uneven because of active slip systems and the asperity became larger when the number of cycles increased. Surface observations revealed that the number of slip bands decreased and the development of the asperity became more difficult with decreasing applied strain range. The change tendencies of surface roughness were similar to those of the density of slip bands at each strain range. This shows that the rate at which surface roughness increased with UF is affected by the number of slip bands at each strain range condition.
  • 小熊 博幸, 中村 孝, 糠谷 慎吾
    日本機械学会論文集 A編 78 789 599 - 603 The Japan Society of Mechanical Engineers 2012年 [査読有り]
     
    To investigate effects of cyclic frequency on high cycle fatigue properties of SFVQ1A, uni-axial fatigue tests were conducted under several stress ratios and cyclic frequencies. In the case of stress ratio of R = -1, longer fatigue lives and higher fatigue strength were observed under higher frequency condition in comparison to those under lower frequency condition. Strain ranges during the fatigue tests were measured, and the difference of range was not observed at the same stress amplitude in both cyclic frequency conditions. The result indicated that the strain rate can affect fatigue properties. However, the difference of fatigue lives and strength did not appear under R = 0.1. The strain ranges were relatively small, and the effects of strain rate probably became smaller than those under R = -1. Effects of cyclic loading were observed under R = 0.5, and strength in the cyclic loading condition became higher than that in the static loading condition. Strain rate should play an important role in the fatigue properties of the material.
  • Nao Fujimura, Hiroyuki Oguma, Takashi Nakamura
    PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE VOL 1 CD-ROM 1061 - 1067 2012年 [査読有り][通常論文]
     
    The effects of cyclic pre-strain on low cycle fatigue properties of austenitic stainless steel were investigated, and the fatigue damage was assessed based on several parameters such as the full width at half maximum (FWHM) of diffracted X-ray profile and surface roughness of specimens. The strain-controlled tests were conducted under strain ratio R epsilon = -1 and various constant total strain ranges. Also the change in remnant fatigue lives were investigated when the cyclic pre-strain were applied to the specimens under the different number of cycles which were determined with reference to the usage factor UFpre ranged from 0.2 to 0.8. As a result, the remnant fatigue life of the pre-strained samples became shorter than that of the sample without pre-strain as the UFpre increased. The relationship between the pre-strain damage expressed in UFpre and the remnant fatigue damage in UFpost was roughly described by the cumulative linear damage law: UTpre + UFpost = 1. Namely, the cyclic pre-strain affected the remnant fatigue lives. In order to evaluate the effects of cyclic pre-strain on fatigue lives more precisely, the damage in the cyclic pre-straining processes was estimated by using FWHM and surface roughness. The FWHM of the specimens with pre-strain once decreased with increase in UFpre, and then increased after showing a minimum value. The surface roughness of specimens increased linearly with an increase of the number of pre-straining cycles. These results suggested that the damage due to pre-strain can be assessed by means of FWHM and surface roughness of specimens.
  • 小熊 博幸, 中村 孝
    材料 60 12 1072 - 1078 2011年 [査読有り][通常論文]
     
    To determine the influential factors on the formation of initial crack propagation region in the very high cycle fatigue (VHCF), uniaxial fatigue tests and K-decreasing tests were conducted with Ti-6Al-4V alloy. The test data indicated that the crack growth rate in vacuum was much slower than that in air, which corresponded to longer fatigue lives of sub-surface fractures. The fracture surface of sub-surface fracture and that obtained in the crack propagation test in vacuum showed similar features that are characterized by a unique granular concavo-convex microasperity. Since the granular fracture surface was never observed in surface-initiated cracks, the granular concavo-convex pattern was considered as clues to an elucidation of the initial crack propagation mechanism in the sub-surface fracture. The granular features also resembled so-called ODA (Optically Dark Area) in high strength steels. Based on this observation, a common mechanism of sub-surface crack growth regardless of materials was discussed for the VHCF phenomena. © 2011 The Society of Materials Science, Japan.
  • 中村 孝, 小熊 博幸
    材料 60 12 1066 - 1071 2011年 [査読有り][通常論文]
  • Vacuum fatigue tests of high strength steel to simulate sub-surface fractures
    Proc. of the 5th International Conference on Very High Cycle Fatigue 251 - 256 2011年 [査読有り][通常論文]
  • Influence of environment on the formation of unique morphology on fracture surface in sub-surface fractures
    Proc. of the 5th International Conference on Very High Cycle Fatigue 257 - 262 2011年 [査読有り][通常論文]
  • H. Oguma, T. Nakamura
    SCRIPTA MATERIALIA 63 1 32 - 34 2010年07月 [査読有り][通常論文]
     
    Uniaxial tension fatigue tests were conducted on a Ti-6A1-4V alloy that underwent different heat treatments to reveal the effect of microstructures on the very high cycle fatigue properties. The size dependency of the threshold stress intensity factor range is indicated for the sub-surface crack, and a possible mechanism is presented by which sub-surface fractures occur at lower stress levels in the very high cycle regime than do surface-initiated fractures. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • 澤田 健太, 中村 孝, 小熊 博幸, 山下 悟, 森 一高, 竹林 仁
    日本機械学会論文集 A編 76 764 413 - 415 一般社団法人 日本機械学会 2010年 
    The objective of this study is to investigate the effect of tensile stress on polymer materials in gaseous fluorine atmosphere. Poly-Ether-Ether-Ketone 0.4mm thin films were exposed to fluorine gas under various tensile stresses. As a result, specimens were fractured during this treatment, and lower applied stress brought longer time to fracture. This trend is typical delayed fracture. Fracture surface observed by Scanning Electron Microscope clearly showed the existence of tiny flat part at fracture origin. The analysis of Electron Dispersive X-ray Spectroscopy demonstrated the fracture origin was caused by the gaseous fluorine. The use of stress intensity factor evaluates the transition condition in crack propagation quantitatively.
  • Hiroyuki Oguma, Takashi Nakamura, Mina Wakita
    日本機械学会論文集(A編) 76 764 428 - 430 2010年 [査読有り][通常論文]
     
    To determine the effects of the vacuum environment on fatigue crack propagations, K-decreasing tests with compact tension (CT) specimens were conducted in air and vacuum environments. The test data clarified the fatigue crack propagation rate (da/dN) and threshold stress intensity factor range (ΔKth) for both environments. The da/dN becomes lower and ΔKth becomes larger with decreasing vacuum pressure. However, this tendency was not explained only by the crack closure. Based on fracture surface observations, the crack propagation properties in vacuum were compared with those in air. A few micrometer size granular region was observed on the fracture surface only in the high vacuum (∼10-6 Pa) and ultra high vacuum (∼10-7 Pa), but not in the air and medium vacuum (∼10-1Pa). The high vacuum environment is one of the necessary conditions for the formation of the granular region, and the fraction of surface coverage of fracture surfaces probably relates to the phenomenon. The formation of the granular region represents the difference of the crack propagation mechanism between in vacuum and in air environments, and the tendency observed in da/dN and ΔKth can be accounted for by the different mechanism.
  • Satoshi Kisugi, Takashi Nakamura, Hiroyuki Oguma, Yoshinori Iwai
    日本機械学会論文集(A編) 76 764 437 - 439 2010年 [査読有り][通常論文]
     
    The surface of color and mechanical properties of Poly-ether-ether-ketone (PEEK) films are changed under ultraviolet light (UV) irradiation in low earth orbit (LEO). In this study, PEEK composites dispersed with titanium oxide (TiO2) were developed to improve the mechanical properties and UV resistance of PEEK. The composites were compressed, being melted when they were produced. The performances and properties of the PEEK/TiO2 nanocomposites were examined in terms of hardness and surface color change after UV irradiation tests. The results showed that PEEK/TiO2 nanocomposites enhanced the hardness and improved the UV resistance of PEEK.
  • Kouichi Murakami, Takashi Nakamura, Hiroyuki Oguma, Hiroyuki Shimamura
    日本機械学会論文集(A編) 76 764 434 - 436 2010年 [査読有り][通常論文]
     
    The strength evaluation method of thin polymer films less than 1 mm thick containing defects is not established. The purpose of this research is to establish the strength evaluation method of thin polymer films based on fracture mechanics. An initial notch was introduced at thin film specimen of Poly-Imide and Poly-Ether-Ether-Ketone. Then, specimens were stretched and the fracture stress was measured. Fracture surface of specimens were observed to analyze the fracture behavior. Stress intensity factor at fracture were measured and the trend with regard to notch length was investigated. As a result, those values were roughly constant when plastic zone at crack tip was small compared with ligament size.
  • Kazuya Takahashi, Takashi Nakamura, Hiroyuki Oguma, Toshikatu Takagi
    日本機械学会論文集(A編) 76 764 431 - 433 2010年 [査読有り][通常論文]
     
    To clarify the water absorption properties of CFRP and resin, water immersion tests of resin and CFRP were conducted. The cross-section of CFRP was observed to understand fiber placement inside CFRP. To analyze the water absorption of CFRP with different fiber placements, a new software program which used a diffusion coefficient and a maximum water concentration in resin was developed. The calculated data were coincident with the experimental water absorption. Therefore, the adequacy of this program was confirmed. The analysis using this program suggested that fiber should be placed as close as the surface to improve the resistant properties of CFRP against aqueous environment.
  • Y. Kikuchi, T. Nakamura, H. Oguma, K. Takahashi
    日本機械学会論文集(A編) 76 764 404 - 406 2010年 [査読有り][通常論文]
     
    Recently, carbon fiber reinforced plastic (CFRP) is developing its range of application especially into construction and architectural engineering fields using resins cured at room temperature. Due to long-term outdoor use in such fields, CFRP is exposed in aqueous environment such as rain, moisture in air and soil. For this reason, degradation of CFRP becomes a strong concern. In addition, our previous tensile tests clarified that the CFRP immersed in deionized water decreased its tensile strength about 10%. To reveal the degradation mechanism of room-temperature cured CFRP in aqueous environment, it is important to figure out water absorption properties of CFRP. So, this research focused on the following two points : (1) the effect of carbon fiber direction in laminate structures on water absorption properties, and (2) the effect of carbon fiber on water diffusion in matrix. Two kinds of CFRP with different laminate structures ([0, O]5 and [0, 90]s) as well as the epoxy resin itself were immersed in deionized water, and those water absorption properties were examined through periodical mass measurements. As a result, it was clarified that the difference in laminate structure hardly influenced the water absorption properties, and that carbon fiber within CFRP blocked water diffusion.
  • Noritaka Horikawa, Daigo Sameshima, Takashi Nakamura, Hiroyuki Oguma
    日本機械学会論文集(A編) 76 764 425 - 427 2010年 [査読有り][通常論文]
     
    This report deals with fatigue properties of the thin wall ductile cast iron and effect of casting surface. Fatigue tests were conducted with as-cast and surface machined test pieces made of ductile cast iron in the shape of thin plate 2 mm thick. The fatigue limit of as-cast test piece was smaller than that of machined ones. Fracture origin and its size were investigated by SEM observation. Most of the fracture origins were concavity of casting surface, and others were micro shrinkages and dross. Measurement manner of concavities on the casting surface as the fracture origin was examined. Fracture origin size was also estimated by the profile of the casting surface to predict fatigue properties. The size of the fracture origins were evaluated well with the depth and the radius at the bottom of the concavity. Evaluation with the depth and the width of the concavity is overestimate, compare with the experimental results. Estimated fatigue limit by the profile of the casting surface was 259 MPa, and it was agreed well with the fatigue limit of the experimental results of 252 MPa. Fatigue limit can be predicted by the profiles of the casting surface.
  • Nozomu Sasaki, Takashi Nakamura, Hiroyuki Oguma, Hiroyuki Shimamura
    日本機械学会論文集(A編) 76 764 422 - 424 2010年 [査読有り][通常論文]
     
    Ultraviolet (UV) irradiated tests with four kinds of fluence were carried out on PEEK films. After UV irradiation tests, tensile tests were conducted to clarify the effect of UV on mechanical properties. As a result, the UV fluence more than 8×102 J/cm2 decreased elongation at break of PEEK drastically. Surface observation of specimens after tensile tests clarified that cracks were initiated on the irradiated surface during necking deformation. The number of cracks tended to increase with increasing UV fluence. These cracks can be the main reason of degradation of PEEK after UV irradiation.
  • Eri Suimon, Takashi Nakamura, Hiroyuki Oguma, Shintaro Ikeda
    日本機械学会論文集(A編) 76 764 440 - 442 2010年 [査読有り][通常論文]
     
    Uniaxial tension-compression fatigue tests of Ni-Cr-Mo steel "SNCM439" tempered at different temperatures were conducted in air environment. Effects of tempering temperature on fatigue properties were investigated. As a result, the following conclusions were obtained : (1) Fatigue limit of surface fracture was proportional to Vickers hardness, and the relation was in good agreement with the expression σ≈1.6Hv. ( 2 ) In very high cycle region, the fatigue strength of the material tempered at 433 K was lower than that tempered at 573 K and 773 K. In other words, the harder material showed lower fatigue strength. ( 3 ) The kinds of inclusions, which could be interior fracture origins, were probably affected by different tempering temperatures.
  • Takashi Nakamura, Hiroyuki Oguma, Yuto Shinohara
    FATIGUE 2010 2 1 2121 - 2129 2010年 [査読有り][通常論文]
     
    Very high cycle fatigue (VHCF) of high strength steel has become an important issue for mechanical engineers in recent years. In VHCF regime over 107 cycles, fatigue crack initiates not from surface but from sub-surface of materials. The sub-surface fractures even occur in a lower stress than surface-originating fractures; therefore, to clarify its mechanism is strongly needed for the safety use of high strength steel. In sub-surface fractures, a typical fracture surface with a fine concavo-convex pattern called "ODA" was discovered. To reveal the formation mechanism of ODA is regarded as a key point for sub-surface crack growth because it is never observed in surface-originating fractures. This study focuses on a special environment inside sub-surface crack. The sub-surface crack is not exposed to atmosphere and the adsorption of gaseous molecules on fresh-surface at crack tip seems to be negligible. With this in mind, fatigue crack growth tests in high vacuum were conducted to simulate sub-surface crack propagation, and fracture features were thoroughly investigated by SEM analyses. As a result, it was clarified that high vacuum is closely similar to the environment inside sub-surface crack, and is a necessary condition to form ODA in VHCF. (C) 2010 Published by Elsevier Ltd.
  • Fatigue crack propagation properties of Ti-6Al-4V in ultra high vacuum environment
    Proceedings of the European Conference of Fracture, ECF18, CD-ROM CD-R  2010年 [査読有り][通常論文]
  • Hiroyuki Shimamura, Takashi Nakamura
    POLYMER DEGRADATION AND STABILITY 95 1 21 - 33 2010年01月 [査読有り][通常論文]
     
    The degradation of the mechanical properties of polyimide films was evaluated by means of tensile tests after exposure to a low earth orbit(LEO) environment. Polyimide films irradiated with atomic oxygen (AO), ultraviolet (UV) light, and electron beam (EB) rays using ground simulation facilities were also evaluated similarly and compared. In these experiments tensile stress (7.0 MPa or less) was applied to the samples in order to assess its effects on mechanical properties. The mechanical properties of the flight samples decreased concomitantly with increased exposure duration. The fracture surfaces exhibited characteristic radiated patterns initiating from the exposed surfaces which showed a rough texture. In the AO-irradiated samples the mechanical properties degraded and the surface texture developed as the AO fluence increased; similar fracture surfaces appeared in the flight samples. In contrast, UV and EB irradiation had little impact on mechanical properties. Based on these results, the eroded surfaces by AO irradiation served as the starting points of the rupture, resulting in degradation of mechanical properties of polyimide films exposed to a LEO environment. The tensile stress states induced no difference in evaluations. (C) 2009 Elsevier Ltd. All rights reserved.
  • Hiroyuki Shimamura, Takashi Nakamura
    POLYMER DEGRADATION AND STABILITY 94 9 1389 - 1396 2009年09月 [査読有り][通常論文]
     
    Mechanical properties of polyimide films are degraded by exposure to a low earth orbit environment. The main environmental factor for that degradation is atomic oxygen (AO). Using tensile tests, AO-irradiated surface topography observations, and fracture surface analyses, this study investigated the degradation behavior of polyimide films' mechanical properties by increased AO fluence and its accompanying degradation mechanisms. Tensile strength and elongation of polyimide films were reduced concomitantly with increased AO fluence. Furthermore, AO-irradiated polyimide films fractured from the AO-irradiated surfaces, of which roughness became marked as AO fluence increased. These results reflect that reduction of mechanical properties is attributable to the roughness increase in AO-irradiated surfaces. Polyimide films coated with indium tin oxide (ITO) were also evaluated to confirm the degradation behavior of AO protective films. Surfaces of ITO-coated polyimide films remained smooth even after AO irradiation. However, undercut cavities were formed at ITO coating defect sites. Rupture of ITO-coated polyimide films initiates from the undercut cavities, engendering large reduction of tensile strength and elongation. The degradation of the mechanical properties of ITO-coated polyimide films increased substantially until the undercut cavities fully penetrated the film. (C) 2009 Elsevier Ltd. All rights reserved.
  • 堀川 紀孝, 鮫島 大湖, 小熊 博幸, 中村 孝
    鋳造工学 全国講演大会講演概要集 155 101 - 101 公益社団法人 日本鋳造工学会 2009年
  • 鮫島 大湖, 野口 徹, 堀川 紀孝, 中村 孝, 桃野 正
    鋳造工学 全国講演大会講演概要集 154 9 - 9 公益社団法人 日本鋳造工学会 2009年
  • Hiroyuki Shimamura, Takashi Nakamura
    PROTECTION OF MATERIALS AND STRUCTURES FROM SPACE ENVIRONMENT 1087 75 - 82 2009年 [査読有り][通常論文]
     
    Effects of atomic oxygen (AO) on mechanical properties of polyimide films were investigated using surface topography observations and tensile tests. The ITO-coated polyimide films, which are expected to have a high durability against AO attacks, were also evaluated. Surfaces of polyimide films irradiated by AO were deeply eroded, exhibiting a rough texture. The roughness developed as the AO fluence increased. Tensile strength and elongation of the polyimide films degraded after AO irradiation tests. In addition, that degradation became marked concomitantly with increased AO fluence. These results indicate that development of surface roughness can degrade mechanical properties of polyimide films. In contrast, surfaces of ITO-coated polyimide films remained flat and were hardly affected by AO irradiation. Nevertheless, the tensile strength and elongation of ITO-coated polyimide films were decreased by AO irradiation. Such degradation resulted from undercut cavities formed by AO erosion at the defects sites of ITO coating.
  • Takashi Nakamura, Hiroshi Nakamura, Hiroyuki Shimamura
    PROTECTION OF MATERIALS AND STRUCTURES FROM SPACE ENVIRONMENT 1087 127 - + 2009年 [査読有り][通常論文]
     
    To clarify the effects of space environment on mechanical properties of polymer, exposure experiments were conducted utilizing the International Space Station Russian Service Module. Poly-ether-ether-ketone (PEEK) films under tensile stress were exposed to low Earth orbit (LEO) environment, and reference samples were irradiated with atomic oxygen (AO), electron beam (EB), and ultraviolet light (UV) in ground facilities. By comparing the results of flight and ground tests, the degradation behavior and the influential factors in LEO were investigated. The following results were obtained. (1) UV was found to be the harshest factor in LEO on tensile properties, since it decreased elongation to 15% of pristine sample after 46-months exposure. (2) AO in LEO eroded the specimen surface with a cone-like morphology and reduced the thickness; however, it had no significant effect on tensile properties. (3) EB irradiation in LEO had no measurable effects on the material properties.
  • SAMESHIMA Daigo, NAKAMURA Takashi, HORIKAWA Noritaka, OGUMA Hiroyuki, ENDO Takeshi
    Journal of Solid Mechanics and Materials Engineering 3 3 440 - 448 The Japan Society of Mechanical Engineers 2009年 [査読有り][通常論文]
     
    Reducing the weight of a machine structure is an increasingly important consideration both for the conservation of resources during production and for the energy saving during operation. With these objectives in mind, thin-walled ductile cast iron has recently been developed. Because rapid cooling could result in brittle microstructure of cementite (chill) in this cast iron, it is necessary to investigate the effect of cementite on the fatigue properties. Therefore, fatigue tests were carried out on a ductile cast iron of block castings which contained a relatively small amount of cementite. Fracture surface observation indicated that the fracture origins were located at graphite clusters and cast shrinkage porosity, not at cementite. It appears that when the size of the cementite is smaller than that of the graphite, the cementite does not affect the fatigue properties of ductile cast iron. Not surprisingly, the fatigue lives were found to increase with decrease in the size of the fatigue fracture origin. The threshold initial stress intensity factor range ΔKini,th for fatigue failure was found to be about 3-4MPa√m, independent of microstructure.
  • 鮫島 大湖, 李 成燮, 堀川 紀孝, 野口 徹, 中村 孝
    鋳造工学 81 2 63 - 69 日本鋳造工学会 2009年 [査読有り][通常論文]
  • Effects of tempering temperature on very high cycle fatigue properties of high strength steel
    Proceedings of the International Conference of Fracture, ICF12 (CD-ROM) T12-068  2009年 [査読有り][通常論文]
  • A. J. McEvily, T. Nakamura, H. Oguma, K. Yamashita, H. Matsunaga, M. Endo
    SCRIPTA MATERIALIA 59 11 1207 - 1209 2008年12月 [査読有り][通常論文]
     
    A mechanism of fatigue crack advance involving the erosion of crack closure in vacuum is proposed to account for the very high cycle fatigue of Ti-6Al-4V. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • 鮫島 大湖, 遠藤 岳志, 堀川 紀孝, 小熊 博幸, 中村 孝
    鋳造工学 全国講演大会講演概要集 153 87 - 87 公益社団法人 日本鋳造工学会 2008年
  • 脇田 実奈, 庄司 哲郎, 中村 孝, 小熊 博幸
    年次大会講演論文集 2008 157 - 158 一般社団法人日本機械学会 2008年 
    In order to specify the exposure environment of interior-originating cracks in very high cycle fatigue, crack growth tests with CT specimens were conducted in air, high vacuum (HV) and ultra high vacuum (UHV). Influence of vacuum environment on characteristics of fatigue crack propagation and fracture surfaces was investigated. As a result, the following were obtained. (1) difference of fatigue crack growth rate between HV and UHV was accounted for by ΔKeff, although difference between air and vacuum was not explained only by the crack closure, (2) Kop became larger when vacuum pressure decreased, (3) a few micrometer size granular region was observed on the fracture surface only in HV and UHV.
  • 池田 慎太郎, 中村 孝, 小熊 博幸, 中村 謙介
    年次大会講演論文集 2008 207 - 208 一般社団法人日本機械学会 2008年 
    Uniaxial tension-compression fatigue tests of Ni-Cr-Mo steel "SNCM439" tempered at different temperatures were conducted in air environment. Effects of tempering temperature on fatigue properties were investigated. As a result, the following conclusions were obtained; (1) Fatigue limit of surface fracture was proportional to Vickers hardness, and the relation was in good agreement with the expression σ≈ 1.6HV. (2) In very high cycle region, the fatigue strength of the material tempered at 433K was lower than that tempered at 573K. In other words, the harder material showed lower fatigue strength. (3)The kinds of inclusions, which can be interior fracture origins, probably change between different tempering temperature.
  • Statistical duplex S-N characteristics of high carbon choromium bearing steel in rotating bending in very high cycle regime
    Proc. of the Materials Science & Technology 2008 Conference, MS&T'08, CD-ROM CD-ROM  2008年 [査読有り][通常論文]
  • Effects of vacuum like environment around interior crack on gigacycle fatigue properties
    Proceedings of the Materials Science & Technology 2008 Conference, MS&T'08,CD-ROM, (2008) CD-ROM,論文番号489561  2008年 [査読有り][通常論文]
  • 鋳造工学 80 3 170 - 176 2008年 [査読有り][通常論文]
  • 堀川 紀孝, 中村 孝, 荻内 佑季子, 東 宏行, 野口 徹
    材料 56 12 1163 - 1169 2007年12月 [査読有り][通常論文]
     
    This paper deals with the compressive strength of Terfenol-D type giant magnetostrictive materials fabricated with the powder metallurgy. Static and fatigue compressive tests for the specimens with different porosity were performed. Static compressive tests indicated that the compressive strength of this material can be estimated with the porosity and strength of dense material with same components. Compressive fatigue tests with the same specimens showed that the compressive fatigue strength of this material is 90% to the static compressive strength. Observations of cracks in the specimens were carried out before and after compressive loading. The effects of the compressive stress to the numbers and length of the cracks were examined. Observations showed significant increase of the number of the cracks with length of 5 to 20μm after static and fatigue compressive loading. Fracture processes for static and cyclic loading were proposed. © 2007 The Society of Materials Science.
  • 中村孝, 山下亮平, 小熊博幸, 脇田実奈, 野口徹
    材料 56 12 1111 - 1117 2007年12月 [査読有り][通常論文]
     
    In order to simulate interior-originating cracks of Ti-6Al-4V in very high cycle fatigue, crack growth tests using CT specimens were conducted in a high vacuum. Morphogenetic factors on granular fracture surface of interior-originating fatigue were investigated based on the effect of vacuum on crack growth process. As a result, the following were obtained. (1) Vacuum decreased da/dN and increased △Kth, compared with air. This results from a stronger closure effect induced by plasticity in a vacuum. (2) Fine concavo-convexo patterns consisting of a few micrometer size granular features were observed on the fracture surface only in a vacuum. This granular region is same as the region observed in the interior-originating fatigue. (3) The magnitude of da/dN or △K had no relation with the formation of granular region. (4) To clarify the effect of crack closure on granular region, an additional test, which makes one of fracture surfaces contact with other, was conducted. This clarified that the area of granular region increased and the fine concavo-convexo patterns became more distinct. Therefore, granular fracture surface of interior-originating fracture in Ti-6Al-4V results from repeating contacts of crack surfaces in a high vacuum or similar environment. © 2007 The Society of Materials Science,.
  • 中村 孝
    鋳造工学 = Journal of Japan Foundry Engineering Society 79 2 58 - 69 Japan Foundry Engineering Society 2007年02月25日
  • 堀川 紀孝, 小美浪 真仁, 中村 孝, 野口 徹
    年次大会講演論文集 2007 595 - 596 一般社団法人日本機械学会 2007年 
    Giant magnetostrictive materials have been used for actuators and sensors. The materials show hysteresis loss when compressive load is applied and removed, and the hysteresis properties vary with magnetic field. This characteristics can be used as a energy/vibration absorber. This report deals with the damping properties of the giant magnetostrictive materials fabricated by the powder metallurgy process and effects of the stress intensity and the bias magnetic field on the damping properties. This material showed hysteresis clearly with the stress smaller than 30MPa. The magnetostriction which causes the energy absorption during loading/unloading of the stress, was generated in low stress. With stress larger than 50MPa, the magnetostriction saturated and did not affect the damping property. The bias magnetic field increased the magnetostriction induced by the compressive stress and reduced the residual strain and Young's modulus.
  • 鮫島 大湖, 中村 孝, 堀川 紀孝
    年次大会講演論文集 2007 311 - 312 一般社団法人日本機械学会 2007年 
    Thin wall ductile cast iron is a promising material to reduce the weight of industrial products. Because chill structures may be produced by rapid cooling in the process of making thin wall ductile cast iron, we have to examine the effect of chill structures on the fatigue properties. Fatigue tests were carried out on the ductile cast iron containing a little cementite. Fracture surface observation clarified that fracture origins were not cementite, but graphite and cast shrinkages. Fatigue lives were well arranged by the fracture origin size with the initial stress intensity facture range. When the size of cementite is the same as or smaller than that of graphite, the cementite does not effect on the fatigue properties of the ductile cast iron.
  • A Mechanism of Interior-Originating Fatigue of Ti-6Al-4V Based on Crack Growth Behavior in a High Vacuum
    Ti-2007, Science and Technology 2 1225 - 1228 2007年 [査読有り][通常論文]
  • 中村 孝
    材料 55 4 445 - 446 社団法人日本材料学会 2006年04月15日 
    筆者は2004年10月から2005年7月まで,カナダのアルバータ大学機械工学科で在外研究を行う機会を得た.アルバータ大学はアルバータ州の州都エドモントンにある学生数3万人以上の総合大学であり,世界45ヵ国170校と提携を結んでいる.特に,機械工学科は筆者の所属する北海道大学機械系専攻と実質的な交流を続けている.筆者は約10ヶ月の間,Advanced Composite Materials Engineering (ACME) Groupに在籍し,Fernand Ellyin教授とZihui Xia助教授の指導の下でCFRPの環境強度に関する研究に携わった.研究内容については,論文等で紹介する機会もあると思うので,本稿では印象に残った身の回りの出来事を綴ってみたい.
  • H Nakamura, T Nakamura, T Noguchi, K Imagawa
    POLYMER DEGRADATION AND STABILITY 91 4 740 - 746 2006年04月 [査読有り][通常論文]
     
    The photochemical reaction of poly(ether ether ketone) (PEEK) sheets under tensile loads has been investigated. Two types of UV irradiation tests were carried out in a vacuum environment: with and without a cooling apparatus. Chemical structures, thermal properties, and mechanical properties were measured to clarify photo-deterioration. Chemical analysis based on Fourier Transform Infrared Spectrometry (FT-IR) and X-ray Photoelectron Spectroscopy (XPS) showed photochemical scission caused by UV exposure. Thermal properties, measured by Differential Scanning Calorimetry (DSC), indicated that a crosslinking reaction occurred during the radiation tests. Tensile properties of PEEK sheets after UV radiation clearly showed a tendency to embrittlement affected not only by crosslinking but also by the orientation of molecular chains resulting from the temperature rise of the specimens. Furthermore, applied tensile stress during exposure accelerated molecular scission and disturbed the crosslinking effects of the tensile properties. (c) 2005 Elsevier Ltd. All rights reserved.
  • 中村 孝, 中村 寛, 藤田 修, 今川 吉郎, 井上 利彦
    年次大会講演論文集 2005 451 - 452 一般社団法人日本機械学会 2005年 
    PEEK (Poly-ether-ether-keton) sheets with 0.4mm thickness were exposed to the real space environment for 315 days by using the International Space Station (ISS). Surface observations, mass measurements, and monotonic tensile tests were carried out to investigate the degradation after space exposure. As a result, the sample showed: (a) color change into dark brown, (b) thickness reduction, and (c) decrease of elongation at break. By comparison with results obtained by the ground control tests which irradiated atomic oxygen (AO), ultraviolet ray (UV), and electron beam (EB) to the same kind of PEEK samples, it was clarified that (a) and (c) were caused by UV in the ISS orbit, and that (b) was by AO.
  • 中村 寛, 中村 孝, 野口 徹, 藤田 修, 今川 吉郎, 井上 利彦
    日本機械学会論文集(A) 71 710 1327 - 1332 一般社団法人日本機械学会 2005年 [査読有り][通常論文]
     
    We irradiated 5 eV Atomic Oxygen (AO) to PEEK sheets with 0.4 mm thickness under tensile load, and estimated strength of the specimen. Three phenomena caused by exposure were observed : (1) appearance of numerous conical pits a few I-μm deep on the specimen surface; (2) reduction of thickness at the irradiated area; (3) change of chemical structure on the surface, which is probably caused by UV generated slightly during the AO irradiation test. As a result of tensile tests, tensile properties after AO irradiation were almost same as those of pristine samples regardless of applied stress. The depth of conical pits was so small and the thickness of layer changed after AO irradiation test was so thin that they did not affect mechanical properties of PEEK sheets. Consequently, residual strength of this material after AO exposure can be estimated by consideration of thickness decrease.
  • T Noguchi, N Horlkawa, H Nagate, T Nakamura, K Sato
    INTERNATIONAL JOURNAL OF CAST METALS RESEARCH 18 4 214 - 220 2005年 [査読有り][通常論文]
     
    This report examines an application of a flow-solidification simulation system in the cast-in insertion process. This process is effective for producing composite castings, but the bonding state is determined by thermal conditions at the interface, which alter the geometrical condition of the inserts and castings. The bonding state is estimated approximately by geometrical and thermal parameters. However, in practical applications, it is quite difficult to predict the bonding state over the whole interface area. This report applies a How-solidification simulation system, combined with criteria the authors have established previously, to estimate the bonding state and to optimise the shape and size of insert/casting/gating systems. Several types of gates and insertion settings were examined and bonding and uniformity compared. The effects of the temperature distribution during pouring on the bonding states and ways to optimise the gating system are also discussed. Application of the process to automobile components is also described.
  • T Nakamura, H Nakamura, O Fujita, T Noguchi, K Imagawa
    JSME INTERNATIONAL JOURNAL SERIES A-SOLID MECHANICS AND MATERIAL ENGINEERING 47 3 365 - 370 2004年07月 [査読有り][通常論文]
     
    To find out the degradation behavior of polymer in the real space, space exposure experiments utilizing the International Space Station (ISS) were scheduled. PEEK sheets under tensile stresses were exposed to the environment around the ISS orbit, and were irradiated by atomic oxygen (AO), ultraviolet ray, and electron beam (EB) in the ground test facility., This study introduces the outline of these experiments, and shows the results of AO and EB pilot irradiation tests as follows: (1) Test piece surfaces after AO exposure exhibited significant morphological damages characterized by micron-sized conical pits. (2) Thickness reductions of the test pieces by AO exposure increased with increasing tensile stress. (3) Residual strength after AO exposure could be estimated by taking account of thickness reduction. (4) No significant change was observed on surface morph, mass, chemical structure, and tensile properties of the test pieces after EB exposure regardless of tensile stress.
  • 横山 秀治, 中村 孝, 椎名 貴弘, 野口 徹
    年次大会講演論文集 2004 303 - 304 一般社団法人日本機械学会 2004年 
    Uniaxial tension fatigue tests were carried out in air and in high vacuum environments using test pieces made of Ti-6Al-4V with small artificial defects. We examined environmental effects of high vacuum on S-N properties and fatigue crack growth limit. As a result, the following were obtained, (i) Regardless of defect sizes, fatigue lives in high vacuum were much longer than those in air. The slope of S-N curve showed different tendencies between in air and in high vacuum corresponding to defect sizes, (ii) The downward trend of ΔK_ with decreasing crack size existed not only in air but also in high vacuum, and the tendency was stronger in high vacuum than in air.
  • 中村 寛, 中村 孝, 野口 徹, 藤田 修, 今川 吉郎, 井上 利彦
    年次大会講演論文集 2004 445 - 446 一般社団法人日本機械学会 2004年 
    This paper describes the changes of mechanical properties on PEEK after electron beam (EB), atomic oxygen (AO) and ultraviolet ray (UV) exposure under tensile stress. Tensile tests showed that mechanical properties after EB exposure were almost same as those of pristine PEEK. Radiation resistance of PEEK was so high that EB radiation gave no effect on material properties. Although test peace surface was removed by AO attack, tensile strengths after AO exposure were almost same as those of pristine sample. It was clarified that residual strength of PEEK sheet after AO exposure can be estimated by consideration of thickness reduction. PEEK was embrittled by crosslinking resulting from UV radiation. Applied tensile stress during exposure inhibited the crosslinking reaction.
  • 中村 孝, 野口 徹, 堀川 紀孝, 荻内 佑季子
    年次大会講演論文集 2004 149 - 150 一般社団法人日本機械学会 2004年 
    Compressive fatigue tests and static compression tests were carried out ontwo types of giant magnetostrictive materials with different porosities. The observation of crack initiations and the classification of fragments after fractures clarified a mechanism of fatigue failure. As a result, the following were obtained: (1) Both the static and fatigue strengths showed a clear dependence on porosity. (2) The static strength σ_c was well expressed by an equation "σ_c=σ_0exp(-4_p)" with the porosity p and the strength of a pore-free material σ_0. (3) The S-N plots standardized by σ_c of the former equation distributed in a narrow band regardless of different porosities. This means that the compressive fatigue strength can be evaluated by using the porosity. (4) Many small intergranular cracks initiated from pores were observed in the fatigued test peice. The fatigue fracture results from the chain coalescence of these cracks.
  • T Nakamura, H Nakamura, O Fujita, T Noguchi, K Imagawa, T Inoue
    ADVANCES IN FRACTURE AND FAILURE PREVENTION, PTS 1 AND 2 261-263 1617 - 1622 2004年 [査読有り][通常論文]
     
    Atomic Oxygen (AO) is a main constituent of the atmosphere on low earth orbit where the International Space Station (ISS) goes around, and is also known as the matter which deteriorate many kinds of polymers. However, the strength properties of polymers suffered from AO have not been fully clarified. To investigate this problem, we irradiated AO to Poly-Ether-Ether-Keton (PEEK) films under three kinds of tensile stresses. Based on the analysis of irradiated samples, the effects of AO fluence (total amount of AO per unit area) and tensile stresses on damage properties were discussed with regard to reaction efficiency Re, surface morph, and tensile strength properties. As a result, the following were obtained: (1) Test piece surfaces exhibited considerable damage covered by conical pits of 1 mum sizes with a few mum depths. (2) Test piece thickness of irradiated area decreased almost proportionally to AO fluence. (3) Re and thickness reduction was accelerated by tensile stress. (4) Strength properties after AO irradiation were almost same as those of a pristine sample considering the decrease of specimen thickness.
  • The Effect of Defect Sizes on Initial Crack Propagations of Ti-6Al-4V in High Vacuum Environment
    Proc. of the 3rd Int. Conf. on Very High Cycle Fatigue VHCF-3 569 - 576 2004年 [査読有り][通常論文]
  • H Oguma, T Nakamura
    ADVANCES IN FRACTURE AND FAILURE PREVENTION, PTS 1 AND 2 261-263 1227 - 1232 2004年 [査読有り][通常論文]
     
    Uniaxial fatigue tests of (alpha+beta) Ti-6Al-4V alloy were carried out at various stress ratios up to very high cycle region. The effects of stress ratios (R=sigma(min)/sigma(max)) on very high cycle fatigue behavior were investigated based on SEM fractography. The S-N curves in positive stress ratios showed a complex folding shape that corresponded to the locations of the fracture origins; i.e. surface-originating fractures in high stress and interior-originating fractures in low stress. The S-N curves of negative stress ratios had a tendency to increase the fatigue lives of interior fractures. SEM observation revealed that an interior crack was formed by linking of small cracks propagated from many interior origins located closely, in contrast to almost all the surface cracks growing from a single origin. This type of interior crack was particularly observed in low stress under positive stress ratios, which was a remarkable characteristic of this material since multiple cracks observed in single specimen were usually limited in high stress. In addition, the positive stress ratio may play an important role in initiation of interior origins. Accordingly, the evaluation of fatigue strength by the modified Goodman diagram under positive stress ratio is considered as inappropriate in a long life region where interior-originating multiple cracks initiate and link together.
  • Development of a Compact Digital Servo Testing Machine for Very High Cycle Fatigue
    Proc. of the 3rd Int. Conf. on Very High Cycle Fatigue VHCF-3 499 - 505 2004年 [査読有り][通常論文]
  • The Space Exposure Experiment of Tension Loaded PEEK Sheets Utilizing the International Space Station
    Proceedings of the 24th International Symposium on Space Technology and Science (Selected Paper) 756 - 759 2004年 [査読有り][通常論文]
  • A Fractographic Comparison Between Fatigue Crack Propagation of Surface-originating Fractures in Vacuum and Interior-originating Fractures on High Strength Steel
    Proc. of the 3rd Int. Conf. on Very High Cycle Fatigue VHCF-3 48 - 55 2004年 [査読有り][通常論文]
  • Charecteristics of Initial Fatigue Crack Propagation Process of Ti-6Al-4V in Very High Cycle Fatigue
    Proc. of the 3rd Int. Conf. on Very High Cycle Fatigue VHCF-3 201 - 208 2004年 [査読有り][通常論文]
  • 椎名貴弘, 中村孝, 野口徹
    日本機械学会論文集(A) 70 696 1042 - 1049 一般社団法人日本機械学会 2004年 [査読有り][通常論文]
     
    In order to clarify effects of stress ratios (R=σmin/σmax) on surface- and interior-originating fatigue properties, uniaxial fatigue tests of high strength steel (SNCM 439) were carried out under three different stress ratios, R=0.3, 0.1 and -1. Based on tests results and fractographic analysis in initial crack propagation regions (Optically Dark Area (ODA) and Stage 2 a), the following results were obtained, (1) Surface-originating fractures occurred in the same fatigue life region regardless of the stress ratio. On the other hand, interior-originating fractures occurred at a longer fatigue life region, as the stress ratio increased. It is unsure to apply the modified Goodman diagram to high strength steels of which main fracture mode was the interior-originating fracture. (2) In initial crack propagation regions, the fracture surface of the interior-originating fracture was strongly affected by the stress ratio, even though that of the surface-originating fracture was scarcely affected. The formation of a fine bumpy pattern which was characteristic of the ODA's fracture surface was affected by a multiplier effect of the inner environment of material and the compression load.
  • 小熊博幸, 中村孝, 横山秀治, 野口徹
    日本機械学会論文集(A) 70 696 1116 - 1123 2004年 [査読有り][通常論文]
     
    Uniaxial fatigue tests of (α + β) Ti-6Al-4V alloy were carried out at various stress ratios (R= σ min/σ max) up to very high cycle region. The effect of stress ratio on very high cycle fatigue behavior was investigated based on SEM fractography. The S-N curves under positive stress ratios showed a complex folding shape with a steep incline in a long life region where interior-originating fractures occurred. On the other hand, the S-N curves under negative stress ratios had a general feature a monotonic increase of the fatigue lives with decreasing stress. SEM observations revealed that an interior crack was formed by linking of small cracks propagated from many interior origins located closely. This type of interior crack was particularly observed in low stress under positive stress ratios. The positive stress ratios facilitated the initiations of cracks and subsequent linkage of the cracks. This tendency may relate with the steep inclination of S-N curves under positive stress ratios. A clear difference was observed on the fracture surfaces between surface- and interior-originating fractures. A few micro-meter size granular substances existed only on the fracture surfaces of interior-originating fractures. The formation of the granular region was accelerated by negative stress ratios. Compressive loads may play an important role in the phenomenon. © 2004, The Japan Society of Mechanical Engineers. All rights reserved.
  • 中村 孝, 小熊 博幸, 野口 徹, 園生 義人
    年次大会講演論文集 2003 173 - 174 一般社団法人日本機械学会 2003年 
    Uniaxial tension fatigue tests and fracture surface observations were carried out on Ti-6Al-4V. The effects of stress ratios R(=σ_/σ_) on fatigue properties and initial crack propagation processes on interior-originating fractures were investigated. As a result, the following were obtained : (1) S-N curves under R<0 showed a steep incline in a long life region where interior-originating fractures occurred although those under R<0 revealed a gentle slope in a whole life. (2) Multiple cracks initiated from plane facets were observed on the fracture surface under R<0,however those multiple facets could not be found under R<0. The result of (1) was probably explained by the idea that positive stress rations facilitated the initiations of many small cracks and the linkage of those cracks.
  • 椎名 貴弘, 中村 孝, 野口 徹
    年次大会講演論文集 2003 171 - 172 一般社団法人日本機械学会 2003年 
    Uniaxial tension-compression fatigue tests using low-temperature-tempered Ni-Cr-Mo steel "SNCN439" were conducted over (10)^7 cycles. Based on the fractographic analysis around fracture origins and the data arranged by using stress intensity factor range, following results were obtained; (1) Even though fatigue tests were carried out until (10)^8 cycles, the fatigue limit of interior-originating fractures was not found. (2) Interior cracks grew from small defects whose initial stress intensity factor ranges ((ΔK)_) were not only higher but also lower than growth limits of surface cracks. (3) ODA (Optically Dark Area) was only observed around interior origins. ODA was formed even in a short life region where surface-originating fractures usually occurred. ODA was formed around the interior origin whose (ΔK)_ was not only lower but also higher than the growth limit of the surface crack. (4) The formation of a fine bumpy pattern which was a characteristic of ODA's fracture surface was affected by cyclic load, and the fine bumpy pattern became clear as number of cyclic load increased.
  • Influential Factors on Interior-Originating Fatigue Fractures of Ti-6Al-4V in Gigacycle Region: Focusing on Stress Ratio and Internal Environment of Material
    Ti-2003, Science and Technology 3 1775 - 1782 2003年 [査読有り][通常論文]
  • N Horikawa, T Ito, T Noguchi, T Nakamura
    INTERNATIONAL JOURNAL OF CAST METALS RESEARCH 16 1-3 365 - 369 2003年 [査読有り][通常論文]
     
    In the cast-in insertion process, the thermal conditions necessary for good bonding have been estimated by volume ratio. However, even with the same volume ratio, the bonding state varies with the size of the insert and the casting as well as with the location of the insert in the casting. Estimates of the effects of these geometrical conditions are useful to improve ease and reliability of this process. This study performed solidification analysis together with experiments to explain the size effect in cast-in insertion, and the results are discussed and compared. The model used in the study is a hollow cylindrical ductile iron casting and a mild steel ring insert located at the bottom of the cylinder. The standard size is about 80mm in diameter and height, and the scale was varied from 0.33 to 3 times the standard size for three volume ratios, 20, 25, and 30. The periods where the insert is in contact with liquidus melt (liquidus period) obtained by calculations were compared as the thermal condition. The calculations showed that, the liquid contact time is longer for larger sizes until about twice the standard size, but that it then decreases rapidly with even larger sizes. This is because the thick solidified layer formed by the large cooling effect of the insert does not remelt. Large size is not always advantageous to good bonding, and it is necessary to choose volume ratios suitable to the size of a casting.
  • 堀川 紀孝, 加野 潤二, 野口 徹, 中村 孝
    鋳造工学 75 2 95 - 101 公益社団法人 日本鋳造工学会 2003年 [査読有り][通常論文]
     
      This paper discusses calculation methods with a two-dimensional model and calculating factors as interfacial heat transfer coefficient and initial temperature in end surface cast-in insertion. In this process, the temperature and solidification progress at the interface in about 100s after pouring are important for the bonding. In calculating these conditions at the interface, it is necessary that the values and boundary conditions used in the calculations are reasonable. In a previous paper, the measured temperatures were used in the calculation as the boundary condltion. To perform realistic calculations with only the initial temperature, interfacial heat transfer coefficients (IHTC) between cast iron melt, sand mold, and steel insert were obtained by temperature measurement and simple inverse method. The calculated IHTC value between melt and mold was 290 W/m2K on the average, and 116 W/m2K at the interface of the insert and mold. There was hardly any difference in the calculated results with the IHTC at the melt-insert interface from 5000 to 12000 W/m2K. These results agreed with the values used in the previous papers. The effects of the initial temperature and IHTC values on the calculated results were also examined.
  • 椎名 貴弘, 中村 孝, 野口 徹, 細川 冬樹
    日本機械学会論文集(A編) 69 678 404 - 411 一般社団法人日本機械学会 2003年 [査読有り][通常論文]
     
    Uniaxial tension fatigue tests were carried out in air and high vacuum environments using pieces made of high strength steel (SNCM 439) with various finishing conditions. Based on the results of fatigue tests and the data arranged by the initial stress intensity factor range (ΔKini), (i) effects of surface finishing and high vacuum environment on surface- and interior-originating fatigue properties, (ii) effects of high vacuum and inner environment of materials on the growth limit of the fatigue crack, were considered. The following results were obtained; (1) Interior-originating fractures were not affected by surface finishing conditions and surrounding environments. (2) The shape of the "Duplex S-N curve" depended on surface finishing conditions and surrounding environments. (3) Fatigue lives of surface-originating fractures in high vacuum and interior-originating fractures were much longer than surface-originating fractures in air. Surface cracks in high vacuum and interior cracks of materials grew from smaller defects whoseΔKini were lower than the growth limit of surface cracks in air. A lower growth limit of fatigue cracks and a longer fatigue life of interior-originating fractures were inferred from the effects of the inner environment of materials, which were similar to those of high vacuum environment.
  • Ti-6Al-4V合金の超高サイクル疲労特性に及ぼす応力比の影響
    構造物の安全性および信頼性Vol.5 JCOSSAR2003論文集 413 - 418 2003年 [査読有り][通常論文]
  • The Degradation of PEEK Sheets Accelerated by Stress in a Real Space Environment Based on the Space Exposure Experiment
    Proceedings of the International Conference on Advanced Technology in Experimental Mechanics, ATEM, (CD-ROM) W0339  2003年 [査読有り][通常論文]
  • Effects of High Vacuum Environment on High Cycle Fatigue Properties of Ti-6Al-4V alloy
    Proceedings of the International Conference on Advanced Technology in Experimental Mechanics, ATEM, CD-ROM W0355  2003年 [査読有り][通常論文]
  • 寿命分布特性からみた超高サイクル疲労の特徴
    構造物の安全性および信頼性Vol.5 JCOSSAR2003論文集 423 - 426 2003年 [査読有り][通常論文]
  • 中村孝, 泉谷仁, 椎名貴弘, 小熊博幸, 野口徹
    材料 52 11 1280 - 1284 2003年 [査読有り][通常論文]
     
    We developed a uniaxial tension hydraulic servo fatigue-testing machine optimized for obtaining gigacycle fatigue data. Main development goals of this machine were as follows : (1) to assure high accuracy of each cyclic loading, (2) to minimize a bending stress generated in a specimen without alignment before the test, (3) to bear high durability enough for a number of gigacycle fatigue tests, (4) to have good user-friendliness for changing testing conditions freely including stress ratio and frequency, (5) to get twice or three-fold higher response compared with a conventional fatigue testing machine. To accomplish these goals, we introduced a miniaturization of the hardware, an alignment free mechanism, fluid lubrication bearings without a sealing system, and a digital controller. As a result, this machine achieved the following performance : (a) high accuracy with loading errors less than ± 0.1%, (b) good alignment with bending stress in the specimen less than 3%, (d) high response with 150Hz of ± 9.8kN loading, and (e) friendly user interface for setting testing conditions.
  • 小熊博幸, 中村孝, 横山秀治, 野口徹
    材料 52 11 1298 - 1304 2003年 [査読有り][通常論文]
     
    To investigate the fatigue characteristics and the fracture feature of Ti-6Al-4V in very high cycle region, uniaxial tension fatigue tests were carried out. The S-N diagram showed a complex folding shape, which corresponded to the locations of the fracture origins i.e., surface-originating fractures in high stress level and interior-originating fractures in low stress level. In this study, observed also were interference type fractures in which the main crack was formed by the coalescence of a surface-originating crack and interior-originating cracks in low stress level. Interference type fractures occurred in shorter life region than interior-originating fractures. Therefore, it is inappropriate and risky to evaluate a fatigue life in low stress level without consideration of interference type fractures. When characteristics of this type of material are investigated, it is important to make a model in consideration of distribution of crack initiation sites. A clear difference was observed on the fracture surfaces of structure sensitive stage 2a regions between the surface-originating fracture and the interior-originating fracture. The initial interior crack propagation is characterized by unclear slip marks and a generally round ductile feature with aggregation of a few micro-meter size granular substances. This tendency was not observed on the surface cracks. This seemed to result from the different surrounding environments of surface and interior cracks. Synergistic effects of a vacuum-like environment inside of material and compressive load may cause forming of granular substances.
  • The Effect of Microstructures on Interior-Originating Fatigue Fractures of Ti-6Al-4V in Gigacycle Region
    Ti-2003, Science and Technology 3 1783 - 1790 2003年 [査読有り][通常論文]
  • 椎名 貴弘, 中村 孝, 野口 徹
    年次大会講演論文集 2002 351 - 352 一般社団法人日本機械学会 2002年 
    In order to clarify properties of the surface-originating fracture in high vacuum environments, uniaxial tension fatigue tests were carried out in air and high vacuum using pieces made of Ti-6Al-4V alloy with micro-defects by drilling. As a result, the following were obtained : (1) Compared with in air, fatigue lives in high vacuum strongly depended on stress. (2) Interior-originating fractures in high vacuum occurred from higher stress level than those in air. Therefore, we must pay attention to the effect of the interior-originating fracture in case of using this material in vacuum environment like space. (3) There was the possibility that the effect of the dependence of the &K_ on crack size in high vacuum is stronger than that in air.
  • 中村 孝, 椎名 貴弘, 野口 徹
    年次大会講演論文集 2002 357 - 358 一般社団法人日本機械学会 2002年 
    In order to clarify how compression load affects interior-originating fatigue fracture surfaces of high strength steel, uniaxial tension fatigue tests were carried out under different stress rations (R=0.3,0 and -1). The results were as follows; Interior-originating fractures under R=-1 occurred in shorter fatigue lives than those under R=0.3. Optically Dark Area (ODA) was formed even in short fatigue life of (10)^5 cycles. The morphology of ODA was closely related to the stress ratio, especially compression stress. Even though the fracture origin of the specimen was not inclusion, ODA was formed on the fracture surface.
  • 椎名貴弘, 中村孝, 野口徹, 細川冬樹
    材料 51 6 673 - 680 2002年 [査読有り][通常論文]
     
    Uniaxial tension fatigue tests of high strength steel, SNCM439, were carried out in air and high vacuum environments. In order to clarify how different environments affect the fatigue fracture surfaces and interior fatigue crack propagations, fracture surfaces were compared through fractographic analysis under air, high vacuum, and inner environment of materials. The following results were obtained (1) Compared with surface fractures in air, the fatigue life of surface fractures in high vacuum tended to be longer. (2) High vacuum environment strongly influenced the fatigue fracture surface in initial stage of crack propagation. (3) When the transition of crack propagation from Optically Dark Area (ODA) to Stage II a occurred, there was the possibility that some mechanical factors were involved in the transition. The formation of ODA was closely related to the mean stress. (4) Inner environment of materials gave more similar effects on crack propagation with high vacuum rather than air. (5) When fracture origins were non-metallic inclusions, interior fatigue cracks of materials in Fish eye were propagated in the following order ODA→Stage II a→Stage lib→Stage II c. © 2002, The Society of Materials Science, Japan. All rights reserved.
  • Gigacycle Fatigue Properties of Ti-6Al-4V Under Uniaxial Tension
    Proceedings of the 8th International Fatigue Congress FATIGUE’99 5 2971 - 2978 2002年 [査読有り][通常論文]
  • The Effect of Atomosphere, High Vacuum, and Inner Environment of Materials on Surface and Interior Originating Fatigue Fractures of High strength Steel
    Proceedings of the 8th International Fatigue Congress FATIGUE’99 4 2517 - 2524 2002年 [査読有り][通常論文]
  • 小熊 博幸, 中村 孝, 杉野 淳一, 野口 徹
    材料力学部門講演会講演論文集 2001 247 - 248 一般社団法人日本機械学会 2001年 
    To investigate the interrelation between high-cycle fatigue behavior and stress ratio (R=σ_/σ_), uniaxial fatigue tests were carried out under two different stress ratios, R=-1 and -0.5. The results are as follows: (1)The difference of stress ratios had a great influence on mechanism of interior fracture. (2)When stress ratios were negative, softening type fracture sometimes occurred. (3)Stress ratios had significant effect on aspects of fracture surface. (4)It is inappropriate and risky to apply modified Goodman diagram to the evaluation of a long fatigue life where interior fractures occur.
  • 椎名 貴弘, 中村 孝, 細川 冬樹, 野口 徹
    材料力学部門講演会講演論文集 2001 241 - 242 一般社団法人日本機械学会 2001年 
    Uniaxial tension fatigue tests of high strength steel, SNCM439, were carried out in air and high vacuum environment. As a result, the following were obtained : (1) Compared with surface fractures in air, the fatigue life of surface fractures in high vacuum tended to be longer. (2) High vacuum environment strongly influenced the fatigue fracture surface in the initial stage of crack propagation. (3) High vacuum environment was similar to inner environment of materials. (4) The formation of Optically Dark Area (ODA) closely related to mean stress.
  • 中村 孝, 野口 徹, 寺崎 友巳, 岡島 英昭
    年次大会講演論文集 1 311 - 312 一般社団法人日本機械学会 2001年 
    The effect of ball-burnishing process on surface conditions and uniaxial fatigue properties was investigated as compared with lathe turning process using S35C normalized test pieces. As a result, the following were obtained: (1) Surface roughness was improved from 2.2 Ra to 0.3 Ra, and surface hardness was increased from 200 Hv to 300 Hv. (2) Residual stress was changed from tensile value of 90 MPa to compressive of -700 MPa. (3) Surface improvement was limited to near the surface because both hardness and residual stress distributions showed steep decreases within 100 fμ. m from the surface. (4) Both fatigue life and fatigue strength were improved by 3〜10 times and 5〜10% respectively.
  • 椎名 貴弘, 中村 孝, 細川 冬樹, 野口 徹
    年次大会講演論文集 1 171 - 172 一般社団法人日本機械学会 2001年 
    In order to investigate the properties of interior originating fractures, uniaxial tension fatigue tests were carried out in air and high vacuum environments using pieces made of high strength steel (SNCM439) with various finishing conditions. As a result, the following were obtained: (1) Compared with surfaces fractures in air, the fatigue life of surface fractures in high vacuum tended to be longer. (2) The interior originating fractures were not affected by surface finishing conditions and environment. (3) The shape of the stepwise S-N diagram depends on surface finishing conditions. (4) Surface cracks in high vacuum grew from defects whose ΔK_ were lower than threshold of surface cracks growth in air.
  • T Noguchi, J Kano, K Noguchi, N Horikawa, T Nakamura
    INTERNATIONAL JOURNAL OF CAST METALS RESEARCH 13 6 363 - 371 2001年 [査読有り][通常論文]
     
    Insertion bonding of mild steel and titanium pipes in aluminium castings was performed in two ways; either by holding paired pipe and aluminium cylinder specimens at constant high temperatures, or by pouring molten aluminium into sand moulds containing inserts. The surfaces of the inserts were electroplated using Ag or Au, or spray coated with Al-Si alloy. The bonding was evaluated by shear strength measurements and microstructure observations. To verify the effect of surface coatings, the contact between the molten aluminium and the specimens was observed. At constant high temperatures, coated steels showed good metallurgical bonding when they were maintained for about 200 s in contact with molten aluminium. The time necessary for bonding was shorter with titanium inserts. In bonded specimens, alloyed microstructures were observed at the interfaces. There was no bonding with uncoated specimens. All coatings used were effective when poured, but preheating of the moulds was necessary to allow sufficient solidification time for the melt/insert volume ratios which were 32 and 36, Process analysis by the finite difference method showed that bonding required a period of liquid phase contact of the melt with the inserts. Observation of aluminium droplet contact with the inserts showed that uncoated surfaces of steel and titanium are not wetted by the melt due to stable oxide skins. With Ag electroplating, aluminium droplets can make wet contact with the specimens. The contact angle decreased from about 140 degrees in uncoated specimens to 15 degrees -45 degrees with Ag plating. The plating prevents surface oxidation of the inserts and promotes wet contact when the active aluminium liquid surface is exposed. The Al-Si alloy spray coating has some effect in promoting wettability, but it is inferior to Ag plating.
  • H. Oguma, T. Nakamura, T. Noguchi
    材料 50 10 1074 - 1079 2001年 [査読有り][通常論文]
     
    The main purpose of this study is to investigate the mechanism of transition from the surface fracture to the interior fracture in Ti-6Al-4V alloy. During the investigation, we paid attention to the relation between the fracture mode and fatigue life distribution. We analyzed fatigue life distribution using a competing risk model, that regards surface fracture and interior fracture as independent fracture modes. As a result, the distribution of the surface fracture shows a tendency similar to that of nonferrous metals, and the interior fracture distribution is similar to that of low and medium carbon steel that shows a saturated failure probability. In this case, the latter (saturated failure probability) was caused by arrest of interior crack propagations. The fatigue life distribution shows a peculiarity when we do not consider fracture modes. However, there is no peculiar aspect when using the competing risk model to analyze fatigue life distribution individually. It shows that the competing risk model, which consists of two fracture modes, describes well the fatigue characteristics of Ti-6Al-4V alloy.
  • 坂 忠明, 中村 孝, 野口 徹
    年次大会講演論文集 2000 243 - 244 一般社団法人日本機械学会 2000年 
    Compressive strength tests of Giant Magnetostrictive Materials fabricated by powder metallurgy were carried out and the effect of aspect ratio of test pieces and elastic modulus of compression plates were examined. Influential factors on compressive strenght were investigated through observation of fracture behavior, SEM fractography and stress analysis of a specimen near its contact surface. As a result, it was clarified that compressive strength decreased as aspect ratio decreases when elastic modulus of compression plate was high whereas it remained constant without being affected by aspect ratio when elastic modulus was low. This result was well explained by very low tensile strength of Giant Magnetostrictive Materials and tensile stress distribution near the contact surface, which was generated by the difference of elastic deformation between compression plate and test piece.
  • 中村孝, 金子真, 風見志郎, 野口徹
    材料 49 10 1148 - 1154 2000年 [査読有り][通常論文]
     
    Uniaxial tension fatigue tests of (α + β) Ti-6Al-4V were carried out in vacuum and air environments to clarify how these environments affect the surface and the interior originating type fatigue properties. Then, the effects of the environments on both fracture surface of stage 2a region and the condition of transition from stage 2a to stage 2b region were also investigated through fractographic analysis. As a result, the following conclusions were obtained :(1) The vacuum environment improved the fatigue lives of surface originating fractures although it gave no effect on those of interior originating fractures. (2) Crack propagation mechanism of surface fractures in the vacuum was different from that in the air. (3) The effect of the internal environment of materials on interior crack propagation was similar to that of the vacuum environment on surface crack propagation. (4) S-N properties were determined by the competition of surface fracture mode in high stress and interior fracture mode in low stress regardless of the environments.
  • 酒井 達雄, 武田 光弘, 塩澤 和章, 越智 保雄, 中島 正貴, 中村 孝, 小熊 規泰
    材料 49 7 779 - 785 2000年 [査読有り][通常論文]
     
    In fatigue tests of high strength steels and surface hardened steels, a characteristic fatigue behavior such that SN curve tends to come down again in the long life region of N> 107 was often observed and reported by many researchers. When the mechanical design is based on the fatigue limit of the material, the above aspect introduces a typical difficulty to provide the reliability of the mechanical structures. In order to clarify such S-N characteristics in wide life region, a series of fatigue tests were performed by means of same type fatigue testing machines and same type of fatigue specimens in a definite high carbon chromium steel for the use of bearing as a collaborative study by the authors. Thus the complicated S-N property of this steel was tentatively interpreted as duplex S-N characteristics given by superposition of S-N curves for the respective fracture modes of the surface-originated fracture and the inclusion-originated fish-eye fracture.
  • T Noguchi, K Shimizu, N Takahashi, T Nakamura
    WEAR 231 2 301 - 309 1999年07月 [査読有り][通常論文]
     
    Repeated drop tests were performed on Ni-hard and high-Cr cast iron grinding balls with material toughness varied by heat treatment. Instrumented impact tests and bending fatigue tests were also performed on bar specimens with the same heat treatment, and correlation between drop strength and other strength characteristics were discussed. In the drop tests from various heights, balls fractured by breakage or spalling, with longer life (Nf) at lower drop heights (H) giving H-Nf curves similar to the S-N curves in fatigue tests. Experiments show that drop strength correlated better with fatigue strength and hardness than with impact toughness (K-Id) in both irons. The stress causing spalling by repeated drops was inferred to be repeated contact stress, and internal tensile stress caused by surface plastic deformation assists the fracture. Breakage from the bail center is caused by cyclic tensile radial stress by impact body force, and is assisted by residual casting stress. Breakage type fracture is possible in as cast or very brittle balls, and with drops from high heights. (C) 1999 Elsevier Science S.A. All rights reserved.
  • T Nakamura, T Noguchi, M Kaneko, S Kazami
    FATIGUE '99: PROCEEDINGS OF THE SEVENTH INTERNATIONAL FATIGUE CONGRESS, VOLS 1-4 4 2365 - 2370 1999年 [査読有り][通常論文]
     
    Uniaxial tension fatigue tests of Ti-6Al-4V were conducted in vacuum and air environments. S-N curves were determined by the competition of fractures originating on the surface under high stress and fractures originating in the interior under low stress. The vacuum environment improved the fatigue life of surface fractures, but had no effect on interior fractures. A clear difference was shown in the fracture surfaces of fractures originating on the surface between the environments. Clear slip marks were observed in air, but in the vacuum they were unclear, and rebonding of crack surfaces may have occurred. There was no difference in the fracture surfaces of fractures originating in the interior between the environments, and they resembled surface fractures in the vacuum. The-influence of the internal condition of material on crack propagation was considered to be similar to that of the vacuum environment.
  • T Sakai, M Takeda, K Shiozawa, Y Ochi, M Nakajima, T Nakamura, N Oguma
    FATIGUE '99: PROCEEDINGS OF THE SEVENTH INTERNATIONAL FATIGUE CONGRESS, VOLS 1-4 1 573 - 578 1999年 [査読有り][通常論文]
     
    High strength steels and surface hardened steels indicate characteristic fatigue behavior such that S-N curve tends to come down again in the long life region of N>10(7). From a viewpoint of safety design of machines and structures, it is important to clarify such a characteristic S-N property of each component material. In this study, gigacycle fatigue tests were systematically performed by means of same type of six testing machines and same type of specimens of high carbon chromium steel for the use of bearing. Experimental data thus obtained were pooled altogether and a duplex S-N property in wide life region was confirmed.
  • T Nakamura, T Noguchi
    EXPERIMENTAL MECHANICS, VOLS 1 AND 2 1039 - 1043 1998年 [査読有り][通常論文]
     
    The influence of fabrication processes on static and fatigue strength properties of Giant Magnetostrictive Materials were investigated. Test results with Bridgman materials (BM) and powder metallurgy materials (PM) showed: (1) Compression strength of PM was about 2/3 of BM, and the strength scatter of PM was much smaller than BM. The smaller compression strength can be explained by the porosity of PM, and the smaller strength scatter by the microstructure anisotropy of BM. (2) Radial compression strength was about ii 20 of the compression strength. (3) Both kinds of materials showed stress dependence on Young's modulus especially at low stresses, and this may be considered a Delta E effect. At high stresses Young's modulus of PM showed about 3/4 of BM because of its porosity. (4) There was an apparent fatigue limit at 60% of the compression strength. This ratio is almost the same in both kinds of materials so fatigue Limits can be evaluated by static strengths.
  • M Kaneko, T Nakamura, T Noguchi
    EXPERIMENTAL MECHANICS, VOLS 1 AND 2 1117 - 1120 1998年 [査読有り][通常論文]
     
    Fatigue tests of Ti-6Al-4V alloy up to a very high numbers of cycles were performed. The S-N curve showed a change from gentle to steep near 10(7) cycles. This feature is different from ordinary non-ferrous alloys. In the short life, high stress region fractures originated from the surface of specimens while in the long life, low stress region they originated from the interior. The SEM observations of fracture surfaces indicated that the changes in the S-N slope and fracture orgin site resulted from a competition between the two fracture modes, surface and interior originated fractures. The fatigue life distribution in two modes can be described by the competing-risk model.
  • 中村 孝, 野口 徹, 山口 篤, 中林 正史
    日本機械学会論文集(A編) 64 627 2819 - 2825 一般社団法人日本機械学会 1998年 [査読無し][通常論文]
     
    The strength and mechanical properties of giant magnetostrictive materials fabricated by powder metallurgy and Bridgman method were investigated. The effect of the shape and volume of test pieces on compressive strength and compressive fatigue properties were examined for the powder metallurgy specimens. Test results showed : (1) The compressive strength of the powder metallurgy material specimens (PM) was about 2/3 of the Bridgman material specimens (BM), while the strength scatter of PM was smaller than BM. (2) The radial compressive strength was about 1/20 of the compressive strength, and both kinds of specimens showed brittle fracture features. (3) Specimens of both materials showed nonlinear stress-strain curves, and the modulus of elasticity rapidly increased with increasing stress in the low stress region. (4) The compressive strength of PM was influenced by the shape of specimens : the compressive strength of prism type specimens was lower than that of rod type specimens. However, the strength was not influenced by the volume of specimens. (5) There was an apparent fatigue limit at about 60% of the compressive strength for PM.
  • 中村 孝, 金子 真, 野口 徹, 神保 勝久
    日本機械学会論文集(A編) 64 623 1820 - 1825 一般社団法人日本機械学会 1998年 [査読無し][通常論文]
     
    Uniaxial tensile fatigue tests were carried out on high strength steel to investigate the relation between fatigue characteristics and fracture origins. The examination compared two surface finishing conditions, one grinding polished by the emery paper and the other electro-chemically polished. The emery paper finished specimens showed a stepwise S-N diagram while the electro-chemically polished specimens had a straight S-N diagram. The sites of fracture origins changed from surface to interior of test pieces as the stress level was reduced on both kinds of test piece. An examination of the initial stress intensity factor range ΔKini of fracture origins was interpreted to show that large surface defects in the grinding condition lowers the fatigue strength of surfaceoriginating fractures, resulting in the stepwise S-N diagram. Interior-originating fractures showed that cracks originate and propagate from inclusions with much smaller ΔKini than the surfaceoriginating cracks.
  • Makoto Kaneko, Takashi Nakamura, Toru Noguchi
    材料 47 12 1216 - 1220 1998年 [査読有り][通常論文]
     
    Many high strength materials, including high strength steels and high strength titanium alloys, show the complex characteristics of fatigue life distribution in the high cycle region. It is caused by the two competing fracture modes, fractures from surface and from interior of the specimen. A method to describe such a complex fatigue life data was proposed based on the competing-risk model. The characteristics of fatigue life distributions and S-N relations of each mode were successfully separated and extracted from whole data. These separated data was analyzed and the complex fatigue life distributions was well described by the competing-risk model. The method is helpful to understand the fatigue mechanisms because the fatigue life distribution of different two modes results from and reflects the fatigue mechanism of each mode.
  • 野口 徹, 佐伯 彰一, 島崎 敏樹, 高橋 典夫, 中村 孝
    鋳造工学 68 9 739 - 745 日本鋳造工学会 1996年 [査読有り][通常論文]
  • 中村 孝, 野口 徹, 神保 勝久, 金子 真, 金子 正吾
    鋳物 : 講演大会講演概要集 127 118 - 118 1995年10月01日 [査読有り]
  • 中村 孝, 金子 真, 田辺 智明, 神保 勝久, 永井 文雄
    日本機械学会論文集(A) 61 582 441 - 446 一般社団法人日本機械学会 1995年 [査読有り][通常論文]
     
    We throughly observed fracture surfaces of austempered ductile iron (ADI) cylinders which fractured in the very high cycle region under internal pressure fatigue tests by SEM, and investigated the reason for the unique fatigue behavior, i.e. the fracture origins changed from casting defects to small graphite with decreasing applied stress, and the S-N diagram drops again after levelling off at l06∼107 cycles (two drop S-N diagram). As a result, it was clarified that (1) ADI has special spheroidal graphites which have the possibility to induce stress concentration on the micron order because of the wedgewise precipitation of carbon into the ferrite layer at the interface of graphite and matrix ; (2) at low stress levels, there exists a highly structure-sensitive crack propagation initiated from this type of spheroidal graphite, and its crack growth rate is estimated to be very low compared with that at high stress levels ; and (3) the "two drop S-N diagram" is caused by both phenomena that cracks from casting defects are arrested and that the propagating mechanism of (2)appears at low stress levels.
  • 金子 真, 中村 孝, 神保 勝久, 永井 文雄
    日本機械学会論文集(A) 60 570 305 - 308 一般社団法人日本機械学会 1994年 [査読有り][通常論文]
     
    We carried out internal pressure tests on a large number of thin-wall cylinders and CT tests of austempered ductile iron (ADI). As a result of internal pressure tests, we observed that the fatigue life distribution at the low-pressure level was divided into two groups : a short-life group below l x 106 cycles and a long-life group fracturing at near l x 107 cycles. At the high-pressure level and in the short-life group of the low-pressure level, the fatigue crack origins were the casting defects, and the √area distribution of these was fitted with the extreme value distribution. On the other hand, it was observed that the crack growth rate obeyed the Paris law in CT tests. Thus we explained the form of the fatigue life distribution using the √area distribution of the fatigue crack origins and the crack growth rate based on the Paris law.
  • T NAKAMURA, K KISHIMA, K JINBO, Y MATSUO, F NAGAI
    JSME INTERNATIONAL JOURNAL SERIES A-MECHANICS AND MATERIAL ENGINEERING 36 4 348 - 353 1993年10月 [査読有り][通常論文]
     
    We carried out a repeating internal pressure test on a thin-wall cylinder of austempered ductile iron (ADI) which has been noted for its good combination of high static strength and elongation. We examined the effect of the austempering and surface hardening treatments by means of roller burnishing, which affects fatigue strength. For the lathe turned specimen (not roller burnished), the fatigue life of ADI was longer than that of the as-cast sample at a high pressure. But this difference of fatigue life became less clear with decreasing pressure. At the lowest pressure, most of the specimens were unfractured at 1 X 10(7) cycles; the fatigue life of the ADI sample was shorter than that of the as-cast specimen. On the other hand, for a roller burnished specimen, the fatigue life of ADI was longer than that of the as-cast specimen at all pressures. After the scanning electron microscopic (SEM) observation of the surface, it was clarified that most of the fracture origins were 0.1 approximately 0.15 mm casting defects.
  • 浦井 隆宏, 杉山 高洋, 中村 孝, 神保 勝久
    日本機械学会論文集(C) 59 563 2112 - 2115 1993年 [査読有り][通常論文]
  • 中村 孝, 菅野 明弘, 神保 勝久, 永井 文雄
    日本機械学会論文集(A) 59 568 2817 - 2824 一般社団法人日本機械学会 1993年 [査読有り][通常論文]
     
    We carried out uniaxial tension fatigue tests on a large number of austempered ductile iron (ADI) specimens. As a result, it was observed that the fatigue life distributions at low stress levels were divided into two groups : a short-life group below 2×106 cycles and a long-life group fractured at near 1×107 cycles. Weibull plots of these fatigue data showed the distinctive feature that the gradient of distribution rose at near 1×107 cycles. After Scanning Electron Microscopy observation of the fracture surface, it was clarified that the short-life group was mainly characterized by stage 2b crack propagation from casting defects and that the long-life group was characterized by fish-eye-type crack propagation. Using a competing risk model composed of these two fracture modes, we could well explain the characteristic fatigue life distributions at low stress levels.
  • 中村 孝, 東郷 善之, 神保 勝久, 永井 文雄
    日本機械学会論文集(A) 59 567 2487 - 2494 一般社団法人日本機械学会 1993年 [査読有り][通常論文]
     
    We carried out high-cycle internal pressure fatigue tests using a large number of thin-wall cylinders of austempered ductile iron (ADI). As a result, we observed a distinctive feature often seen in high-strength steel, that the SN diagram inclined again after about 107 cycles, in spite of once revealing a horizontal part at 106- 107 cycles, and that an obvious fatigue limit could not been observed. Corresponding to this characteristic SN diagram, Weibull plots of the fatigue data were dramatically changed between high-and low-pressure levels. The thorough SEM observation of the fracture surfaces clarified that the fracture origins changed from casting defects to relatively small graphite particles between high-and low-pressure levels. This means that different fracture mechanisms exist between high-and low-pressure levels. These two mechanisms competed in the high-cycle region and brought about the distinctive fatigue behavior of ADI.
  • Mechanism of Very High Cycle Fatigue Behavior on Austempered Ductile Iron (General Characteristics of Fatigue Life Distributions)
    Proceedings of the International Conference on Mechanical Behavior of Ductile Cast Iron and Other Cast Metals 178 - 183 1993年 [査読有り][通常論文]
  • Mechanism of Very High Cycle Fatigue Behavior on Austempered Ductile Iron (Mechanism)
    Proceedings of the International Conference on Mechanical Behavior of Ductile Cast Iron and Other Cast Metals 184 - 189 1993年 [査読有り][通常論文]
  • 中村 孝, 木島 公一朗, 神保 勝久, 加藤 直, 松尾 陽太郎, 永井 文雄
    日本機械学会論文集(A) 58 549 683 - 687 一般社団法人日本機械学会 1992年 [査読有り][通常論文]
     
    We carried out a repeating internal pressure test on the thin wall cylinder of austempered ductile iron (ADI) which has been noted for its good combination of high static strength and elongation. We examined the effect of the austempering and surface hardening treatments using roller burnishing which affects fatigue strength. As for the lathe-turned specimen (not roller-burnished), the fatigue life of ADI was longer than that of the as-cast sample at high pressure level. But this difference of fatigue life became less clear with decreasing pressure level. At the lowest pressure level, most of the specimen was unfractured at 1×107 cycles ; the fatigue life of the ADI sample was shorter than that of the as-cast specimen. On the other hand, for a roller-burnished specimen, the fatigue life of ADI was longer than that of the as-cast specimen at all pressure levels. After the SEM observation of the fracture surface, it was clarified that most of the fracture origins were 0.10.15 mm microshrin-kages produced under the casting process.
  • 中村 孝, 川上 充郎, 神保 勝久, 松尾 陽太郎, 永井 文雄
    日本機械学会論文集(A) 58 555 2046 - 2050 一般社団法人日本機械学会 1992年 [査読有り][通常論文]
     
    We carried out internal pressure tests on a large number of thin wall cylinders of austempered ductile iron (ADI), and investigated the influence of the casting defects on fatigue life distributions. As a result, it was observed that the fatigue life distributions at low-pressure levels were divided into two groups : a short-life group below 1×106 cycles and a long-life group fractured at near 1×107 cycles. After the SEM observation of the fracture surface around the casting defects, the short-life group showed significant roughness, and the long-life group tended to be flat ; it was supposed that the fracture mechanisms of the two were different. Specifically, in the short-life group, a proportional relationship was observed between the initial stress intensity factor of the casting defect and the fatigue life on the double log scale. It is apparent that crack propagation of the short-life group was caused under the Stage 2 process in which the Paris law was effective.
  • 中村 孝, 浦井 隆宏, 岩堀 正, 神保 勝久, 永井 文雄, 松尾 陽太郎
    日本機械学会論文集(A) 55 510 198 - 204 一般社団法人日本機械学会 1989年 [査読有り][通常論文]
     
    We carried out the repeated internal pressure test for an A 7075-T 73 cylindrical specimen, and obtained accurate fatigue data in large quantities. According to the fractography by SEM analysis, the most important factors which affect fatigue life are the Stage 1 and Stage 2 a crack propagation processes. In this paper, we could well explain the statistical feature of fatigue life data using stand-by distribution which consists of two kinds of weibull distributions (two-parameters).
  • 浦井 隆宏, 中村 孝, 神保 勝久, 永井 文雄, 松尾 陽太郎
    日本機械学会論文集(A) 55 511 460 - 466 一般社団法人日本機械学会 1989年 [査読有り][通常論文]
     
    Using the three-parameter Weibull distribution function, we analyzed two kinds of fatigue life data. Three parameters estimated by the Maximum Likelihood Method are dependent on the stress, and have strong relationships between each other. Using these relationships, we proposed a new formula of Weibull distribution, which has only one parameter, shape parameter m. and three material constants. We clarified the meaning of each material constants included in the formula of the distribution function by numerical simulation. We applied this new distribution to the actual data, namely, the rotating bending test data of S 45 C and repeated internal pressure test data of A 7075-T 73 cylinders, which proved the validity of the new equation.
  • 赤城 協, 神保 勝久, 中村 孝, 浦井 隆宏, 永井 文雄, 松尾 陽太郎
    材料 36 408 1005 - 1011 1987年 [査読有り][通常論文]
     
    In this work, a proto-type high-pressure fatigue testing machine capable of generating high internal pressure and high frequency up to 15 Hz with high efficiency was constructed first, and then it was used to carry out a repeated internal pressure test for A2017-T4 cylindrical specimens. It was found through Weibull statistics that there are three peaks in the time-strength distribution and each peak is closely connected with the fractographical feature. A new estimating method of fatigue limit using Weibull's location parameter was suggested. © 1987, The Society of Materials Science, Japan. All rights reserved.

書籍

  • 機械材料工学(改訂版)
    工学図書 2006年
  • 機械材料工学
    工学図書 2001年

その他活動・業績

  • 学薛 高格, 藤村 奈央, 中村 孝, 小熊 博幸, 髙橋 航圭 M&M材料力学カンファレンス 2019 (0) OS0908 2019年 

    Uniaxial fatigue tests of Ti-22V-4Al alloy were carried out up to very high cycle regime at various stress ratios. In case of internal crack, so-called facets regarded as a crack initiation site were found from the fracture surface. To figure out the internal crack initiation and propagation process, fractographic analysis using 3D SEM was conducted. Material constants "C" and "m" in the Paris law were obtained by comparing the characteristics of facets around the fracture origins to estimate fatigue life caused by internal crack, and a new model of facet initiation process was proposed. As a result, the internal crack fatigue life estimation using the proposed model was clarified to be effective to evaluate experimental results.

  • 中村 孝, 友田 悠太, 藤村 奈央, 吉中 奎貴, 小熊 博幸, 竹内 晃久, 上椙 真之, 上杉 健太朗 M&M材料力学カンファレンス 2019 (0) OS0909 2019年 

    The initiation process of internal fatigue cracks in Ti-6Al-4V in very high cycle regime was investigated via synchrotron radiation X-ray CT. A projection-type CT with a resolution of one micrometer or below and a phase contrast imaging-type CT with a resolution of sub-micrometer were used. The projection-type CT revealed that 1) more than twenty cracks initiated in one specimen, 2) fatigue life varied widely from 20% to 70% of the average fatigue life, and 3) whether the initiated crack can smoothly propagate or not was determined by the nonuniformity of surrounding microstructure. The imaging-type CT clarified that the crack initiation was caused by slip deformation in α phase and that the shape of internal crack tip was blunter than that of surface crack tip.

  • 藤村 奈央, 中村 孝, 信耕 友樹 M&M材料力学カンファレンス 2015 "OS0913 -454-1"-"OS0913-454-3" 2015年11月21日 
    To statistically investigate the changes and the distributions in surface roughness, strain controlled low cycle fatigue tests of austenitic stainless steel were conducted under strain range Δε=8, 4%. During the tests, the specimen's surface topography was regularly measured using a laser scanning microscope. The surface topographies obtained were analyzed by frequency analysis to separate the surface relief due to persistent slip bands. The persistent-slip-bands-induced surface relief evolution was quantitatively evaluated by using arithmetic mean roughness R_a. As a result, R_a at Δε=8, 4% increased with the increase of UF (Usage Factor). The R_a data were plotted roughly linearly on a log-normal probability paper. It suggests that R_a likely follows a log-normal distribution. On the probability papers, the slope of R_a plots and the average value at Δε=8, 4% were clearly different. It shows that the distribution form of R_a varies between different Δε. The slopes of R_a plots at each UF were similar on the paper. It showed that the dispersion of R_a data didnot change even if UF increases.
  • Yoshinaka Fumiyoshi, Nakamura Takashi, Shiozawa Daiki, Nakai Yoshikazu, Uesugi Kentaro Abstracts of ATEM : International Conference on Advanced Technology in Experimental Mechanics : Asian Conference on Experimental Mechanics 2015 (14) 55 -55 2015年10月04日
  • 中田 康敬, 西塚 祐斗, 信耕 友樹, 藤村 奈央, 中村 孝 年次大会 : Mechanical Engineering Congress, Japan 2015 "G0301502 -1"-"G0301502-5" 2015年09月13日 
    To investigate the effects of change in loading amplitude on surface topography, two-step loaded low-cycle fatigue tests of SUS316NG austenitic stainless steel were conducted. During the tests, the specimen's surface topography was regularly measured using a laser scanning microscope. The surface topographies obtained were analyzed by frequency analysis to separate the surface relief due to persistent slip bands from that due to crystal grain deformation. The persistent-slip-bands-induced surface relief evolution and the grain-deformation-induced one were quantitatively evaluated by using arithmetic mean roughness R_a and arithmetic mean waviness W_a, respectively. As a result, the rates of increase in R_a and W_a changed obviously after changing loading amplitude. The results for R_a and W_a agree with the results of surface observation, the increases in the numbers of slip bands and deformed crystal grains changed after changing loading amplitude.
  • 中田 康敬, 信耕 友樹, 藤村 奈央, 中村 孝 北海道支部講演会講演概要集 2014 (53) 27 -28 2014年09月27日
  • 堀川 紀孝, 濱口 萌愛, 藤村 奈央, 中村 孝 M&M材料力学カンファレンス 2014 "OS2126 -1"-"OS2126-3" 2014年07月19日 [査読無し]
     
    Austenite stainless steel is widely used in power plants and nondestructive damage estimation method is required to determine the damage on the structure due to fatigue or earthquake. Eddy current method is one of the nondestructive inspection technique, and it reflects the surface condition of the metal. Surface roughness of the specimen increased by the deformation and the roughness change is used to estimate the damage by the low cycle fatigue. This report deals with the eddy current signal change due to the surface roughness. Test pieces of SUS316 with 6mm in diameter were prepared, and the surface was ground with emery paper. Eddy current testing was carried out with two types of the probe and testing frequency of 200kHz and 1MHz. Eddy current signal, which is output of the eddy current instrument, were measured. Eddy current signal was changed by surface grinding. Grinding on the surface showed significant effect on the eddy current signal. It is caused by removal of residual stress near the surface of the test piece that induced by the drawing process. Difference in the eddy current signal by the surface roughness within 1μm in Ra is quite small and higher testing frequency is required to distinguish the difference.
  • 高久 和明, 中村 孝, 小熊 博幸 M&M材料力学カンファレンス 2014 "OS2105 -1"-"OS2105-3" 2014年07月19日 
    Sub-surface fatigue fractures in titanium alloys are frequently observed in very high cycle region. However, the mechanism of sub-surface fractures has many unclear points. This study focused on an idea that the environment around sub-surface cracks resembles vacuum environment. To reveal the effects of vacuum environment on small fatigue crack propagation, uni-axial fatigue tests were conducted in air and vacuum environments using specimens with an artificial small defect. Crack propagation rates were measured by observation of crack propagation behaviors. As a result, fatigue crack propagation rates in vacuum were much lower than those in air. And the distinctive characteristics of small fatigue crack growth behaviors were more remarkable in vacuum than in air.
  • 吉中 奎貴, 中村 孝, 小熊 博幸 M&M材料力学カンファレンス 2014 "OS2104 -1"-"OS2104-3" 2014年07月19日 
    The effects of vacuum environment on fatigue fracture surfaces of high strength steel were investigated quantitatively. Uni-axial fatigue tests were carried out on SNCM439 in vacuum and air environments. 3D-fractography was then performed using 3D-SEM to determine the fracture surface roughness and to clarify the effects of environments on fracture surface. These analyses targeted on three typical fracture surfaces as follows: (a)surface fracture in air environment, (b)surface fracture in vacuum environment and (c)subsurface fracture. As a result, fracture surface roughness of surface fracture in air environment was large compared to that in vacuum environment. On the other hand, fracture surface roughness of subsurface fracture agreed quite well with that of surface fracture in vacuum environment. These results show that the effects of vacuum environment and environment inside subsurface crack on fatigue crack propagation are exactly similar. Therefore the behavior of subsurface crack propagation can probably be estimated from surface crack propagation in vacuum environment.
  • 藤村 奈央, 中村 孝, 小熊 博幸, 新沼 賢基 M&M材料力学カンファレンス 2014 "OS0820 -1"-"OS0820-3" 2014年07月19日 
    Changes in surface roughness of SUS316NG during cyclic variable loadings were investigated. Strain controlled two-step fatigue tests were conducted under two different strain ranges. In the tests, strain range was switched from Δε_H=4% to Δε_L=2% or from Δε_L=2% to Δε_H=4% at the usage factor UF=0.2. The specimen's surface was observed by a laser scanning microscope to clarify the surface profile and surface roughness R_a. As a result, R_a increased with UF during the first and second steps in the two-step tests. The increasing rate of R_a at the second step became smaller after strain range was changed from 4% to 2%, whereas it became larger after strain range was changed from 2% to 4%. The increasing tendencies of Ra at each step were similar to those in fatigue test under constant strain ranges Δε=4% and 2%, respectively. The results suggest that changes in surface roughness under two-step variable loadings can be described by superimposing R_a values in order of loading, which were obtained under constant strain range tests. Surface observation revealed that the height change in surface profile during each loading step varied depending on the strain range. This shows that the history of strain range affected the changes in surface profile, and it is the reason that the increasing rate of R_a changed in the two-step tests.
  • 信耕 友樹, 藤村 奈央, 中田 康敬, 小熊 博幸, 中村 孝 M&M材料力学カンファレンス 2014 "OS0819 -1"-"OS0819-3" 2014年07月19日 
    Low-cycle fatigue tests of SUS316NG were conducted under conditions of strain range Δε=2, 4 and 8%. During the fatigue tests, changes in surface relief of specimen were measured by a laser scanning microscope. The surface roughening due to slip bands and deformation of grains was quantitatively evaluated by using arithmetic average roughness R_a and arithmetic average waviness W_a respectively. As a result, R_a and W_a increased with the increase of UF (Usage Factor), and the increasing rates of R_a and W_a with respect to UF became larger in larger strain ranges. The ratio of increasing rate of W_a to R_a also became larger in larger strain ranges. This result indicates that the surface roughening due to deformation of grains is caused more significantly than that due to slip bands is in larger strain ranges. Based on the different tendencies of R_a and W_a, we suggested a new damage evaluation method which can estimate UF and applied strain range of fatigued material.
  • 新沼 賢基, 藤村 奈央, 信耕 友樹, 中村 孝, 小熊 博幸 M&M材料力学カンファレンス 2013 "PS15 -1"-"PS15-3" 2013年10月12日 
    A strain controlled two step test was conducted to investigate changes in the surface features of SUS316NG during cyclic variable loading. In the two step test, strain range was switched from Δ_<εH> = 4% to Δ_<εL> = 2% at the usage factor UF= 0.2. The specimen surface was observed by a laser microscope to clarify the surface morphology and surface roughness R=a. Initiation and propagation of the main crack was also observed by using cellulose acetate replica The results in the two step test were compared with those obtained in fatigue tests under constant strain ranges. As a result changes in surface features such as slip bands and surface asperities clearly depended on the value of strain ranges even under the same UF value. Small cracks initiated very early in the fatigue life, however, they hardly propagated until the middle of fatigue life. In contrast, Ra clearly increased from the beginning to the middle of fatigue life. These results suggest that surface roughness can be an effective parameter for fatigue damage assessment. In addition, the increasing rates of Ra during the first and second steps in two step test were similar to those in fatigue tests under constant strain ranges Δ_ε = 4% and Δ_ε = 2%, respectively. Therefore, the history of strain range under cyclic variable loadings can probably be described from the value of R_a and the increasing rate of R_a in the fatigue tests under constant strain ranges.
  • 鈴木 千慶, 中村 孝, 小熊 博幸 M&M材料力学カンファレンス 2013 "PS11 -1"-"PS11-3" 2013年10月12日 [査読無し]
     
    The thin polyimide (PI) films have been widely used for the spacecraft polymeric material. However, small defects often initiate at the surface of PI films by the impingement of atomic oxygen present in the space environment such as low earth orbit Mechanical strengths of polyimide films are significantly reduced by these defects. To evaluate the material strength of PI films for space use, therefore, it is important to consider the crack propagation from the defect in the direction of film thickness. In this paper, tensile tests were carried out using PI films with a surface short crack and the fracture toughnesses in the film thickness direction were compared with the results of FEM numerical analyses. From the experimental results, R-curves of PI film were obtained using J-integral. These R-curves had a similar trend of R-curves obtained from the standard specimens. However, the fracture toughness measured by R-curves was about five times higher than the calculated value by the numerical analyses for each initial crack depth. The reason for this discrepancy was discussed based on the following two factors; one is a considerable plastic deformation at the tip of pre-crack introduced by a razor blade, and the other is an underestimation of crack extension Δa.
  • 中谷 建太朗, 植野 正大, 中村 孝, 小熊 博幸, 大久保 賢二, 渡辺 精一 北海道支部講演会講演概要集 2013 (52) 93 -94 2013年09月28日
  • 吉中 奎貴, 中村 孝, 小熊 博幸 北海道支部講演会講演概要集 2013 (52) 11 -12 2013年09月28日
  • 信耕 友樹, 藤村 奈央, 新沼 賢基, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2013 (52) 15 -16 2013年09月28日
  • 高久 和明, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2013 (52) 13 -14 2013年09月28日
  • 中村 孝, 静 宏幸, 小熊 博幸 年次大会 : Mechanical Engineering Congress, Japan 2013 "J191026 -1"-"J191026-3" 2013年09月08日 [査読無し]
     
    This study carried out a trial production of Epoxy/TiO_2 nancomposites to improve ultraviolet light resistance. Two kinds of TiO_2 particles with a diameter of 30nm and 270nm were dispersed in the epoxy resin and the ultraviolet light resistance was evaluated based on UV irradiation tests, tensile tests, and SEM observations of specimen surfaces and fracture surfaces. As a result, elongation at break of epoxy resin and epoxy/TiO_2(270nm) composite decreased after UV irradiation although that of epoxy/TiO_2(30nm) composite remained unchanged. Epoxy/TiO_2 (30nm) composite likely to suppress the embrittlement of surface layer caused by UV irradiation, and tend to reduce the crack initiations which deteriorate strength properties.
  • 中島 智広, 藤村 奈央, 新沼 賢基, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2012 (51) 65 -66 2012年10月20日
  • 藤村 奈央, 中村 孝, 小熊 博幸 M&M材料力学カンファレンス 2012 "OS2109 -1"-"OS2109-3" 2012年09月22日 
    The effects of surface roughness on low cycle fatigue properties were investigated by strain-controlled fatigue tests using specimens of austenitic stainless steel finished by two different methods: machining and mirror polishing. During fatigue tests, the surface roughness of each specimen was measured, and crack initiation and propagation were observed using cellulose acetate replicas. As a result, the fatigue life of mirror polished specimen was longer than that of machined one. It was shown that the surface roughness affected the fatigue life. The change of surface roughness of each specimen showed the same increasing tendency with respect to the number of cycles. However, around the end of fatigue life, the surface roughness of mirror polished specimen became larger than that of machined one. In the damage process, the crack initiated at the very early stage of fatigue life. The crack growth rate was small until the middle of fatigue life; however, it increased rapidly after that. The surface roughness affected the crack initiation, and that is the reason why the fatigue life and the change of surface roughness were different between each specimen.
  • 中村 孝, 小熊 博幸, 杉村 優輝 年次大会 : Mechanical Engineering Congress, Japan 2012 "J191013 -1"-"J191013-5" 2012年09月09日 
    This study investigated fracture criteria of thin Polyimide films with a surface short crack propagating into the direction of film thickness In order to estimate fracture toughness, deformation behaviors at crack tip were observed by SEM and R-curves were measured with different crack depths Fracture characteristics of PI films were extracted by the comparison of several results with those obtained in PEEK films As a result, PI films were fractured by unstable crack growth whereas PEEK films were by a process similar to tensile fracture of smooth specimen due to a large blunting of the surface crack tip R-curves of PI films revealed that crack growth amount and the J-integral at fracture increased with increasing the depth of surface crack This tendency corresponded with that of general R-curves of bulk specimens
  • 齊藤 聡太, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2011 (50) 75 -76 2011年10月01日
  • 杉村 優輝, 村上 晃一, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2011 (50) 71 -72 2011年10月01日
  • 中島 智広, 藤村 奈央, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2011 (50) 77 -78 2011年10月01日
  • 静 宏幸, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2011 (50) 73 -74 2011年10月01日
  • 高田 貴大, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2011 (50) 69 -70 2011年10月01日
  • 中村 孝, 小熊 博幸 北海道支部講演会講演概要集 2011 (50) 65 -66 2011年10月01日
  • 中村 孝, 小熊 博幸, 佐々木 望, 島村 宏之 年次大会 : Mechanical Engineering Congress, Japan 2011 "J191021 -1"-"J191021-5" 2011年09月11日 
    The effects of UV fluence on strength properties of PEEK films were investigated based on tensile tests. Elongation at break clearly decreased with increasing UV fluence although yield strength and elastic modulus were almost same before and after irradiation. Surface observations during tensile tests and fracture surface observations clarified that small cracks with a depth of a few ttm initiated at irradiated surface, and that true stress at which crack initiated decreased with increasing UV fluence. The analyses of chemical structure at UV irradiated surface by using FT-IR method confirmed the increase of carbonyl bonding at the depth corresponding to the small cracks, which caused the embrittlement of the surface layer. Small cracks initiated at the thin embrittled layer propagated and coalesced each other during tensile tests. This phenomenon can be the main reason for the decrease of elongation at break of UV irradiated PEEK films.
  • 「宇宙環境曝露によるポリイミドフィルムの機械特性劣化とその予測法」 宇宙航空研究開発機構研究開発報告 JAXA-RR-10-009
    宇宙航空研究開発機構 2011年 [査読無し][通常論文]
  • 糠谷 慎吾, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2010 (49) 121 -122 2010年11月07日
  • 藤村 奈央, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2010 (49) 145 -146 2010年11月07日
  • 中村 孝, 小熊 博幸, 橋向 隆太 M&M材料力学カンファレンス 2010 1000 -1001 2010年10月09日 
    In order to clarify the formation factors of fine concavo-convexo region on the fracture surface of sub-surface fractures in very high cycle fatigue, crack growth tests of Ti-6Al-4V were conducted in ultra high vacuum and the effects of vacuum environment and repeating number of cycles on fracture surfaces were investigated. As a result, the fine concavo-convexo regions were formed in vacuum and the area ratio of them increased with increasing number of cycles. The fine concavo-convexo regions; however, were not formed in crack growth tests in air. These results coincided with the formation factors of ODA in high strength steel, therefore, it was concluded that long term cyclic loadings in vacuum-like environment around sub-surface crack is the main reason for the formation of fine concavo-convexo fracture surfaces regardless of materials.
  • 篠原 雄人, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2009 (48) 127 -128 2009年11月28日
  • 福地 亮平, 中村 孝, 小熊 博幸 北海道支部講演会講演概要集 2009 (48) 131 -132 2009年11月28日
  • 水門 恵理, 中村 孝, 小熊 博幸, 池田 慎太郎 M&M材料力学カンファレンス 2009 425 -427 2009年07月24日 
    Uniaxial tension-compression fatigue tests of Ni-Cr-Mo steel "SNCM439" tempered at different temperatures were conducted in air environment. Effects of tempering temperature on fatigue properties were investigated. As a result, the following conclusions were obtained; (1)Fatigue limit of surface fracture was proportional to Vickers hardness, and the relation was in good agreement with the expression 6 z 1.6HV. (2) In very high cycle region, the fatigue strength of the material tempered at 433K was lower than that tempered at 573K and 773K. In other words, the harder material showed lower fatigue strength. (3)The kinds of inclusions, which can be interior fracture origins, probably change between different tempering temperatures.
  • 菊池 雄治, 中村 孝, 小熊 博幸, 高橋 和也 M&M材料力学カンファレンス 2009 434 -436 2009年07月24日 
    Recently, carbon fiber reinforced plastic (CFRP) is developing its range of application especially in construction and architectural engineering fields by the use of room-ternperature cured resin. However, CFRP is exposed in aqueous environment such as rain, moisture in air and soil due to long-term outdoor use. Degradation of CFRP is concerned about due to long-term use in aqueous environment. As a result of tensile test of CFRP immersed in deionized water, degradation of CFRP was confuined. To reveal the degradation mechanism of room-temperature cured CFRP in aqueous environment, coupons of CFRP and epoxy resin were immersed in deionized water and those water absorption properties were examined. As a result, it was showed that carbon fiber within CFRP blocked water diffusion.
  • 佐々木 望, 中村 孝, 小熊 博幸, 島村 宏之 M&M材料力学カンファレンス 2009 491 -493 2009年07月24日 
    Ultraviolet (UV) inadiation tests with four kind of fluence were carried out on PEEK films. After UV irradiation tests, tensile tests were conducted to clarify the effect of UV on mechanical properties. As a result, the UV fluence more than 5.1×1C^2J/cm^2 decreased elongation at break of PEEK drastically. Surface observation of specimen after tensile tests clarified that cracks initiated on the irradiated surface during necking deformation. The number of cracks tended to increase with increasing UV fluence. These cracks can be the main reason of degradation of PEEK after UV irradiation.
  • 橋向 隆太, 中村 孝, 小熊 博幸, 篠原 雄人 M&M材料力学カンファレンス 2009 428 -430 2009年07月24日 
    In our previous studies, crack growth tests have been conducted in a high vacuum to simulate interior-originating cracks of Ti-6Al-4V in very high cycle fatigue. As a result, granular region which is very similar to fracture surface of interior-originating fracture was observed. In addition, we inferred that repeating compressive stress on fracture surface is involved in the formation of granular region. So, crack growth tests under R=-1, 0.1 using MT specimens were conducted in air to reveal effects of repeating compressive stress on the formation of granular region and on fatigue crack growth behavior. As a result, repeating compressive stress in air had no significant effect on fatigue crack growth behavior, and did not cause the formation of granular region. The existence of both repeating compressive stress and vacuum environment can be the main reason of granular region.
  • 村上 晃一, 中村 孝, 小熊 博幸, 島村 宏之 M&M材料力学カンファレンス 2009 488 -490 2009年07月24日 
    The strength evaluation method of thin polymer films less than lmm thick containing defects is not established. The purpose of this research is to establish the strength evaluation method of thin polymer films based on fracture mechanics. An initial crack was introduced at thin film specimen of Poly-Imide and Poly-Ether-Ether-Ketone. Then, specimens were stretched and the fracture stress was measured. Fracture surface of specimens were observed to measure the length of the initial crack and to analyze the fracture behavior. Stress intensity factor at fracture were measured and the trend with regard to crack length was investigated. As a result, those values were roughly constant when plastic zone at crack tip was small compared with ligament size.
  • 来生 聡, 中村 孝, 小熊 博幸, 岩井 佳紀 M&M材料力学カンファレンス 2009 59 -61 2009年07月24日 
    The surface of color and mechanical properties of Poly-ether-ether-ketone(PEEK) films are changed under ultraviolet light (UV) irradiation in low earth orbit (LEO). In this study, PEEK composites dispersed with titanium oxide (TiO_2) were developed to modify the mechanical properties of PEEK. The composites were compressed, being melted when they were produced. The performances and properties of the PEEK/TiO_2 nanocomposites were examined in terms of hardness and color change of surface after UV irradiation test. The results of hardness and UV irradiation test showed that PEEK/TiO_2 nanocomposites enhanced the hardness and improved the UV resistance of PEEK.
  • 高橋 和也, 中村 孝, 小熊 博幸, 高木 敏勝 M&M材料力学カンファレンス 2009 54 -56 2009年07月24日 
    To clarify the water absorption properties of CFRP and resin, water immersion tests of resin and CFRP were conducted. The cross-section of CFRP was observed to understand fiber placement inside CFRP. To analyze the water absorption of CFRP with different fiber placement, a new software program which only used a diffusion coefficient and a maximum water concentration in resin was developed. The calculated data were coincident with the experimental water absorption. Therefore, the adequacy of this program was confirmed. The analysis using this program suggested that fiber placement as close as the surface improved the resistant properties of CFRP against aqueous environment.
  • 澤田 健太, 中村 孝, 小熊 博幸, 山下 悟 M&M材料力学カンファレンス 2009 62 -64 2009年07月24日 
    The objective of this study is to investigate the effect of tensile stress on polymer materials in gaseous fluorine atmosphere. In this study, Poly-Ether-Ether-Ketone (PEEK) 0.4mm thin films were used. Tensile stress was given by tensile jig, which can apply various stresses to specimens. Direct fluorination was carried out by Takamatsu Teisan Co., Ltd. Fracture surface was observed by Scanning Electron Microscope (SEM). As a result, specimens were fractured during this treatment. Moreover, lower applied stress brought longer time to fracture. This trend is typical delayed fracture. Fracture surface clearly showed the existence of fracture origin. The analysis of Electron Dispersive X-ray Spectroscopy (EDS) demonstrated the fracture origin was caused by gaseous fluorine. The use of stress intensity factor evaluates quantitatively whether the delayed fracture would be occurred or not.
  • 堀川 紀孝, 鮫島 大湖, 中村 孝, 小熊 博幸 M&M材料力学カンファレンス 2009 277 -279 2009年07月24日 
    This report deals with fatigue properties of the thin wall ductile cast iron and effect of casting surface. Fatigue tests were conducted with as-cast and surface machined test pieces made of ductile cast iron in the shape of thin plate 2mm thick. The fatigue limit of as-cast test piece was smaller than that of machined ones. Fracture origin and its size were investigated by SEM observatioii Most of the fracture origins were concavity of casting surface, and others were micro shrinkages and dross. Measurement manners of fracture origins at the concavity on the casting surface were examined. Fracture origin size was also estimated by the profile of the casting surface to predict fatigue properties. The sizes of the fracture origins were evaluated well with the depth and the radius at the bottom of the concavity. Evaluation with the depth and the width of the concavity is overestimated, compared with the experimental results. Estimated fatigue limit by the proffle of the casting surface was 259MPa, and it was agreed well with the fatigue limit of the experimental results of 252MPa. Fatigue limit can be predicted by the profiles of the casting surface.
  • Effects of LEO environment on mechanical properties of PEEK films
    Proceedings of the 5th Japan-Taiwan Workshop on Mechanical and Aerospace Engineering (CD-ROM) B06 2009年 [査読無し][通常論文]
  • Effects of LEO Environment on Mechanical Properties of PEEK Films under Tensile Stress
    Proceedings of International Symposium on "SM/MPAC&SEED Experiment", JAXA Special Publication JAXA-SP-08-015E 73 -80 2009年 [査読無し][通常論文]
  • 高木 敏勝, 辻 敏郎, 高橋 和也, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2008 (47) 39 -40 2008年09月27日
  • 高橋 和也, 高木 敏勝, 辻 敏郎, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2008 (47) 37 -38 2008年09月27日
  • 遠藤 岳志, 鮫島 大湖, 堀川 紀孝, 小熊 博幸, 中村 孝 北海道支部講演会講演概要集 2008 (47) 41 -42 2008年09月27日
  • 小熊 博幸, 中村 孝 M&M材料力学カンファレンス 2008 "OS1503 -1"-"OS1503-2" 2008年09月16日 
    異なる熱処理を施したチタン合金を用いて,超高サイクル域までの軸荷重疲労試験を実施した.そして,組織の違いが疲労強度特性に与える影響について検討した.疲労試験の結果,時間強度は粒径の小さい方が高くなった.その傾向は内部破壊が生じる超高サイクル域で明瞭に見られた.また,内部破壊の結果から初期き裂の小さい方が低い疲労き裂進展下限界値ΔK_を示すことが明らかになった.このことは材料内部においてΔK_は寸法依存性を有することを示している.
  • 中村 孝, 小熊 博幸 M&M材料力学カンファレンス 2008 "OS1504 -1"-"OS1504-2" 2008年09月16日 
    高強度鋼やチタン合金は超高サイクル域において材料内部を起点とする疲労破壊を生じる.一般に内部破壊の疲労強度は表面破壊の疲労強度より低いことが知られている.この要因を検討するために,本研究では内部き裂が大気から遮断されていることに注目し,人工微小欠陥を有する試験片を用いてTi-6Al-4VおよびSCM435の高真空疲労試験を行った.その結果,真空中の疲労限度は,欠陥寸法が大きい場合には大気中より大幅に高い値を示すのに対し,欠陥寸法が小さくなると大気中と同程度かそれより低下することが明らかとなった.この理由は,両素材ともに真空中におけるΔK_の欠陥寸法依存性が大気中のそれより大きくなるためであると考えられた.
  • Statistical Analysis of Very High Cycle Fatigue Property of High Carbon Chromium Bearing Steel in Rotating Bending
    Proceeding of the First Japan-China Joint Symposium on Fatigue of Engineering Materials and Structures 64 -67 2008年 [査読無し][通常論文]
  • Very High Cycle Fatigue Properties of High Strength Steel SNCM439 - Part 1: General Characteristics based on Round Robin Tests -
    Proceeding of the First Japan-China Joint Symposium on Fatigue of Engineering Materials and Structures 68 -71 2008年 [査読無し][通常論文]
  • Very High Cycle Fatigue Properties of High Strength Steel SNCM439 - Part 2: Influential Factors -
    Proceeding of the First Japan-China Joint Symposium on Fatigue of Engineering Materials and Structures 72 -75 2008年 [査読無し][通常論文]
  • Damping Properties of Giant Magnetostrictive Materials Fabricated by Powder Metallurgy
    Proceedings of the 4th Japan-Taiwan Workshop on Mechanical and Aerospace Engineering 217 -222 2007年 [査読無し][通常論文]
  • A Study of Interior Crack Growth Process on Ti-6Al-4V Simulated by High Vacuum Fatigue Tests
    Proceedings of the 4th Japan-Taiwan Workshop on Mechanical and Aerospace Engineering 332 -339 2007年 [査読無し][通常論文]
  • 野口 徹, 堀川 紀孝, 佐々木 真, 中村 孝 鋳造工学 = Journal of Japan Foundry Engineering Society 78 (12) 668 -674 2006年12月25日
  • 小美浪 真仁, 中村 孝, 堀川 紀孝, 野口 徹 北海道支部講演会講演概要集 2006 (45) 89 -90 2006年09月25日
  • 堀川 紀孝, 中村 孝, 辻 敏郎, 荻内 佑季子, 野口 徹 北海道支部講演会講演概要集 2006 (45) 95 -96 2006年09月25日
  • 野口 徹, 中村 孝, 堀川 紀孝 工学・工業教育研究講演会講演論文集 15 27 -28 2003年09月04日
  • 中村 孝, 中村 寛, 藤田 修, 野口 徹, 今川 吉郎 学術講演会講演論文集 52 143 -144 2003年
  • 中村 寛, 中村 孝, 野口 徹, 藤田 修, 今川 吉郎, 舘 義昭 北海道支部講演会講演概要集 2002 (42) 122 -123 2002年10月05日 
    The relationship between the polymeric material degradation by atomic oxygen (AO) and tensile load was investigated. AO irradiation experiments were carried out at the facility of National Space Development Agency (NASDA). AO was irradiated to PEEK sheets under tensile stress. After AO exposure, surface observation, reaction efficiency (Re) measurements, and tensile tests were made on the sample. As a result, the following were obtained : (1) The circular irradiated region can be observed on all exposed samples. (2) Re was increased by AO exposure under tensile stress. (3) Conical pits were observed allover on all exposed specimens. (4) Elastic modulus was decreased by AO irradiation and this tendency was enhanced by applied stress. It was clear that stress affected polymer degradation by AO.
  • 澤谷 真澄, 中村 孝, 野口 徹, 藤田 修, 今川 吉郎, 舘 義昭 北海道支部講演会講演概要集 2002 (42) 124 -125 2002年10月05日 
    A polymer, PEEK, was investigated how tensile load would effect on its characteristics when irradiated by electron beam (EB). PEEK sheets under different tensile stresses were irradiated to EB by using the Combined Space Effects Test Facility of NASDA. After EB radiati on, mass measurements, surface observation and tensile tests were made on the samples. As a result, the following were obtained : (1) No major changes were not observed in mass measurements. (2) Irradiated surface showed no difference from non-irradiated surface. (3) Decreases in tensile strength and elongation were detected for the sheets irradiated under tensile stress.
  • 泉谷 仁, 中村 孝, 野口 徹 北海道支部講演会講演概要集 2001 (41) 16 -17 2001年09月25日 
    In this study, we developed a compact digital electrohydraulic servo testing machine to acquire fatigue test data in very high cycle region efficiently. To improve response and load accuracy, we designed a high rigidity load frame, developed a special gripping jig for decreasing bending load, and used a high-speed servo valve. As a result, this machine achieved the following performance : (1) ±9.8kN cyclic uniaxial load could be generated at 200Hz. (2) Bending that applied to specimen was suppressed within ±3%. (3) Digital proportional control made it possible to keep error of load less than ± 1%.
  • Space Exposure Experiment of PEEK Sheets Under a Tensile Load Utilizing an International Space Station
    Proc. of the First Taiwan-Japan Workshop on Mechanical and Aerospace Engineering 1 107 -113 2001年 [査読無し][通常論文]
  • 中村 孝 電子情報通信学会技術研究報告. ED, 電子デバイス 99 (670) 37 -42 2000年03月08日 
    強誘電体メモリーは高速性・高集積性・低消費電力性などに非常に優れており, 既存半導体メモリーの大部分を置き換える可能性を持つメモリーデバイスとして注目されている.強誘電体材料にPZT(Pb(Zr, Ti)O_3)系を用いて256kbitまでのメモリーは既に製品化が行われており, 今後市場が拡大していくものと予想される.しかし, さらなる市場拡大のためには克服するべき課題も残れている.そのなかで, 本稿では高集積化に必須技術となるSTC構造作成技術, システムLSIに搭載していくには不可欠となる成膜低温化, 動作電圧低減について述べていく.
  • 中村 孝, 野口 徹 学術講演会講演論文集 49 399 -400 2000年
  • 山口 篤, 中村 孝, 中林 正史, 野口 徹 学術講演会講演論文集 47 343 -344 1998年
  • 風見 志郎, 金子 真, 中村 孝, 野口 徹 学術講演会講演論文集 47 45 -46 1998年
  • 松崎 拓治, 野口 徹, 中村 孝, 五家 政人 学術講演会講演論文集 45 25 -26 1996年

特許

  • 材料試験機の試験片把持構造
    4468118
  • 疲労試験機
    4172544
  • 疲労試験機
    特開2002-243606
  • Fatigue Testing Machine
    4172544
  • Fatigue Testing Machine
    特開2002-243606

受賞

  • 2012年05月 日本材料学会 平成23年度学術貢献賞
     
    受賞者: 中村 孝
  • 2010年 日本鋳造工学会平成21年度優秀論文賞
     スモールパンチ試験による薄肉球状黒鉛鋳鉄のじん性評価 
    受賞者: 鮫島大湖;野口徹;堀川紀孝;中村孝;桃野正
  • 1999年05月 日本材料学会 日本材料学会平成10年度学術奨励賞
     
    受賞者: 中村 孝

共同研究・競争的資金等の研究課題

  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2021年04月 -2025年03月 
    代表者 : 中村 孝, 戸田 裕之, 藤村 奈央, 竹内 晃久, 上杉 健太朗, 上椙 真之, 小熊 博幸, 古谷 佳之
  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2018年04月 -2022年03月 
    代表者 : 中村 孝, 藤村 奈央, 古谷 佳之, 戸田 裕之, 竹内 晃久, 上杉 健太朗, 小熊 博幸
     
    本研究では,4年間の研究期間の中で,①放射光X線顕微CT解析の高度化,②放射光X線顕微CT解析による内部き裂発生・進展プロセスの解明,③異なる真空環境における表面き裂発生・進展プロセスの解明,④Kitagawa diagramに基づく超高サイクル疲労強度評価法の構築,の4項目を実施する.昨年度は,主として①および②の準備を行なった.本年度は①を継続するとともに②,③に関する本格的な実験を開始した.具体的な実施内容を以下に示す. ①放射光X線顕微CT解析の高度化 昨年度に構築したイメージングシステムをTi-6Al-4Vの内部微小き裂の観察に適用し,投影型CT(解像度0.5μm/pixel)と位相コントラスト結像型CT(解像度65nm/pixel)を随時切り替えることで「試験片内に発生する内部き裂の探索⇒対象とするき裂の詳細観察」を効率よく行える手法を構築した. ②放射光X線顕微CT解析による内部き裂発生・進展プロセスの解明 ①で開発したシステムを用いてTi-6Al-4Vにおける内部き裂の初期進展過程を観察した.その結果,発生直後の数~数十μmの微小内部き裂の進展速度の計測に成功し, 10^(-13)~10^(-11) m/cycleという極めて低速な値であることを明らかにした. ③異なる真空環境における表面き裂発生・進展プロセスの解明 Ti-6Al-4Vを対象に種々の真空圧力の下で表面き裂の進展試験を行った結果,真空圧力が低くなるほどき裂進展速度は低下することがわかった.さらに,②で計測した内部き裂の進展速度は,10^(-4)~10^(-6)Paの真空圧力下のき裂進展速度とほぼ一致し,内部き裂内の環境は高真空レベルにあることが明らかとなった. 以上のように,本年度の取組みにより,今後の研究遂行に寄与する成果を得ることができた.
  • 日本学術振興会:科学研究費助成事業 挑戦的萌芽研究
    研究期間 : 2014年04月 -2016年03月 
    代表者 : 中村 孝
     
    金属に低圧縮負荷を振動的に与え,転位の非可逆成分を累積させることにより,表層をnmオーダーに微細化する技術を開発する.大気,N2,Ar,真空中で実験を行い,結晶粒微細化に及ぼす雰囲気,負荷荷重,繰返し数の影響を調べた.炭素鋼およびアルミ合金を用いた実験の結果,両者ともに粒径1マイクロメートル以下の微細層が形成された.微細化層の粒径や深さは.負荷の大きさや振動回数によって制御できることがわかった.微細化の程度は大気やN2中では顕著であるが,Ar中では小さかった.真空中では微細化がほとんど生じなかった.以上から,本手法によるナノ微細化には雰囲気と転位の相互作用が不可欠であることが明らかとなった.
  • 日本学術振興会:科学研究費助成事業 基盤研究(A)
    研究期間 : 2012年04月 -2015年03月 
    代表者 : 中村 孝, 柴山 環樹, 塩澤 大輝, 中井 善一, 上杉 健太朗, 小熊 博幸, 渡辺 精一
     
    超高サイクル域における軸荷重疲労試験が行われ,放射光μCTイメージングにより材料内部に発生する微小き裂が観察された.内部き裂周囲の環境がき裂進展に及ぼす影響を明らかにするため,da/dNと応力拡大係数範囲の関係が測定され,真空中表面き裂のそれと比較された.その結果,内部き裂は1.0E-10m/cycleの極めて低速で進展するのに対し,表面に到達した後のき裂は1.0E-7m/cycle 程度の高速で進展することがわかった.内部き裂の進展速度は真空中表面き裂の進展速度に一致し,内部き裂周囲の真空に類似した環境が超高サイクル疲労における内部き裂進展過程に大きな影響を与えることが明らかとなった.
  • ナノ微粒子複合化による宇宙用高分子膜材の耐紫外線性・靭性の向上法
    日本学術振興会:科学研究費助成事業 萌芽研究
    研究期間 : 2007年 -2008年 
    代表者 : 中村 孝
     
    本研究の目的は,熱可塑性耐熱高分子PEEKに紫外線遮断性能を有する金属酸化物ナノ微粒子を分散させることで,耐紫外線性・靭性を兼ね備えた宇宙用複合膜材の開発指針を得ることにある.本研究では添加する金属酸化物として酸化チタンと酸化亜鉛の2種類を選択し,種々の条件(金属酸化物の材質,粒子径,粒子濃度,成形圧力,成形温度等)で複合材の試作を試みた. 平成19年度の研究により,金属酸化物微粒子の濃度が増加するにつれ,マイクロバブルが基地に残存し硬さを低下させることがわかった.このため,PEEKと金属酸化物の複合粉末を溶融させながら任意の圧力で成形する圧縮成形機を新たに開発し,複合膜材の製作に適用した.その結果,本研究で選択したいかなる条件下においてもマイクロバブルが発生しないこと,金属酸化物粒子の濃度を上昇させることにより硬さを増加できること等が明らかとなった.一方,紫外線照射前後の材料表面の色差を利用することで,耐紫外線性を簡易に評価する新たな手法を開発し,本試作材の評価に適用した.試作した複合膜材に,異なる量の紫外線を照射した後,EPMA分析,SEM観察,硬さ試験,色差測定を行うことで,耐紫外線性・機械特性を向上させる条件を調べた.これらの評価を総合すると,金属酸化物微粒子としては,酸化亜鉛より酸化チタンを選択した方が複合材としての耐紫外線性が高くなること,種々の製造条件のうち,平均粒径270nmの酸化チタンを濃度20%で分散させた場合に,耐紫外線性・機械特性が最も向上すること等が明らかとなった.
  • 日本学術振興会:科学研究費助成事業 基盤研究(B)
    研究期間 : 2006年 -2008年 
    代表者 : 中村 孝
     
    超高サイクル疲労の原因である内部起点型破壊において, き裂の初期伝播過程で生じる微細な凹凸状破面(粒状破面)の形成機構を明らかにすることを目的とした. 「粒状破面は, 気体成分の吸着が著しく少ない内部き裂の伝播過程において, き裂新生面が長期間繰返し接触をすることで形成される」という仮説を提案し, 新たに開発した超高真空疲労試験機を用いて, 種々の真空圧力や応力比の下で得られた疲労破面を解析することにより, その検証を試みた. その結果, 内部起点型疲労破壊を生じるチタン合金および高強度鋼において, 上記の仮説が検証され, 内部き裂伝播に及ぼす影響因子が明らかにされた.
  • 極薄肉球状黒鉛鋳鉄の強度特性評価および信頼性向上法の確立
    日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2006年 -2007年 
    代表者 : 野口 徹, 中村 孝, 堀川 紀孝
     
    球状黒鉛鋳鉄鋳物の薄肉化軽量化は、鋳物の高機能化、省エネルギー化の観点から喫緊の課題である。その実用化には、強度を評価し信頼性を保証するが要があるが、薄肉鋳物からは標準寸法の試験片が採取できない。本研究は肉厚2mm以下の小寸法の試験片で、静的強度、じん性、疲労強度を評価する手法を確立しようとするものである。 (1)引張特性の評価 薄肉鋳鉄用試験片として、厚さ2mm、幅4.5mm、標点距離12mmの板状試験片を提案した。これによって、直径14mmの試験片と同等の引張強さ、伸びを求めることができる。伸びは応力-伸び曲線の計測による。引張強さはロックウエル硬さHRBからも推定できる。 (2)じん性の評価法 じん性評価法として10×10×0.5(mm)の試験片によるスモールパンチ試験(SP試験)を用いた。荷重-変位曲線からき裂発生点変位δ^*を決定し、算定される破壊等価ひずみε_から破壊じん性値J_を求めることができる。J_はHRBからも推定できる。 (3)疲労特性の評価 薄板状の試験片で、応力比R=-1.0の軸力(引張圧縮両振り)疲労試験を実施する必要があるとの結論に達し、これを実施するための試験機と掴み治具を開発した。これにより、鋳肌からの亀裂の発生-初期伝播-定常伝播の過程を詳細に検討することができる。また、通常の溶湯処理で薄肉球状黒鉛鋳鉄を得る鋳造方案を考案した。 非破壊評価 渦電流法により、非破壊で硬さと組織を評価する可能性を検討した。計測の精度を高め、また厚肉の場合と異なる評価基準を定めることにより、非破壊で評価できる可能性がある。
  • 疲労き裂伝播下限界の欠陥寸法依存性に及ぼす真空圧力の影響(高強度材における超高サイクル疲労破壊機構の解明に向けて)
    日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2003年 -2004年 
    代表者 : 中村 孝
     
    高強度鋼やTi合金などの高強度材料の疲労では,10^7〜10^8cycleを越える超高サイクル・低応力域で,材料内部を起点とする破壊が生じる.研究代表者はこれまでの研究により,この理由を内部き裂先端の環境が真空環境に近いと考えることで説明できる可能性を示した.本研究では,この考えの妥当性を検証するために,(1)微小欠陥を起点とする疲労挙動を真空中で調べ,ΔK_に及ぼす真空圧力の影響を明らかにすること,(2)真空中の微小欠陥近傍の初期伝播破面と内部き裂のそれを比較することにより,内部き裂周囲の環境の効果を調べること,を目的とした.このために,新たに真空回転曲げ疲労試験機を開発し,大気中および真空中で微小欠陥材の疲労試験を行った. 直径30μm×深さ30μmの微小欠陥を加工した高強度鋼試験片を用いて,10^<-3>-10^<-4>Paの真空圧力下で疲労試験を行った.その結果,真空中の疲労寿命は大気中に比べ約3-4倍,長くなった.一方,真空中の疲労限度は大気中に比べて約50MPa低下した.介在物の大きさ程度の小さな欠陥を起点とする場合,真空中のΔK_は大気中に比べて低下することが明らかとなった. 微小欠陥周囲の初期き裂伝播破面をSEMにより解析した.従来,内部き裂の起点周囲には組織単位より微小な凹凸から成る特殊な破面領域が存在することが知られているが,真空圧力10^<-3>-10^<-4>Paの高強度鋼の破面ではこの特殊な破面は明瞭には認められなかった.一方,昨年度に行なった実験(真空圧力2×10^<-5>Pa)では,特にTi合金において,この特殊な破面が顕著に認められた. 以上から,内部破壊が低応力・長寿命で生じる現象は,上記下線部に示した考えによって説明できることが示された。しかし,内部き裂の初期伝播破面に及ぼす環境,特に真空圧力の影響は材質によって異なることが推察された.
  • Ti合金の高真空中での表面微小き裂進展特性とこれに基づく内部疲労破壊機構の推測
    日本学術振興会:科学研究費助成事業 基盤研究(C)
    研究期間 : 2000年 -2001年 
    代表者 : 中村 孝, 野口 徹
     
    近年,Ti合金において,10^7〜10^8を越える超高サイクル域でも疲労限度が現れず,材料内部から破壊する挙動が知られつつある.しかし,内部破壊が長寿命となる理由や,内部破壊と表面破壊の疲労機構の差はほとんど明らかにされていない.本研究では材料内部の大気に曝されない環境に着目し,Ti-6Al-4V微小欠陥材の高真空中疲労試験を行った.そして大気中/真空中/材料内部における初期き裂進展形態を比較し,内部疲労破壊機構における環境の効果を推測した.始めに実験精度の向上と微小き裂の挙動を計測可能とするために試験機のデジタル制御化と真空機器の改造を行なった.次に微小欠陥材の疲労試験を大気/高真空下で行い,初期き裂進展・停留特性や,初期破面に及ぼす環境の影響を調べた.得られた主な結果を以下に示す. 1.高真空中での疲労寿命は大気中より大きく増加した. 2.大気中でのき裂進展の下限界はき裂の発生によって決まるが,高真空中ではき裂の停留によって決まる.後者は内部破壊の疲労限度の支配要因と一致する. 3.高真空中でのstage2aからstage2bへの遷移応力拡大係数は,大気中に比べて低下した.これは高真空中でき裂先端の降伏強さが低下し,塑性変形領域が増大するとの考えで説明できる.高真空中での疲労寿命の増加は,き裂面に吸着する気体分子量の低下が,き裂先端の塑性変形やき裂面の再結合を促したためと考えられる. 4.内部破壊が長寿命となることは,表面破壊の寿命が高真空中で延びる傾向と一致する.しかし高真空の寿命は内部破壊の寿命よりさらに増大するという差があった.内部破壊の初期破面は大気中より高真空中に似ていたが,細部では異なる部分があった.材料内部環境は大気よりも真空に近いが,真空度はさほど高くなく,疲労き裂進展に影響を与えるき裂新生面への気体分子の吸着量は,高真空より材料内部の方が大きいと予想される.
  • 高強度鋼の超高サイクル疲労強度特性に及ぼす高真空および不活性環境の影響
    日本学術振興会:科学研究費助成事業 奨励研究(A)
    研究期間 : 1997年 -1998年 
    代表者 : 中村 孝
     
    低温焼戻鋼やADI,Ti合金などの高強度金属の疲労において,繰返し数10^7以上の超高サイクル域で疲労限度が消失する挙動が報告されつつある.これは材料の内部を起点とする特殊な破壊形態に起因するとされているが,その機構はわかっていない.本研究では,内部疲労破壊への影響因子として,材料内部の環境に注目した.即ち,材料内部環境を模擬するために高真空中での疲労試験を行ない,真空中の疲労特性および破壊挙動と内部破壊の類似点,相違点をフラクトグラフィを基に検討した.初年度は高真空疲労試験装置の開発と評価を行なった.その結果,真空圧力0.9×10^<-6>Pa,荷重容量50kN,繰返し速度40Hzの性能を確認した.次年度は,2000MPa級高強度鋼(SNCM439,SCM435)およびTi-6Al-4Vを用いて,真空および大気中での疲労試験と破面解析を行なった.いずれの材料も高応力短寿命域で表面破壊を生じた後,低応力長寿命域で内部破壊を生じ,明瞭な疲労限度を示さなかった.表面破壊の寿命は,真空中では大気中に比べ大幅に改善されたが,Ti-6Al-4Vの内部破壊の寿命は環境によって変わらなかった.つまり真空環境は表面破壊には影響を与えるが,内部破壊には影響を与えないことがわかった.破面解析によれば,組織敏感なステージ2aき裂進展領域に,真空中の表面破壊と大気中の表面破壊で明瞭な差が確認され,内部破壊と真空中の表面破壊で顕著な類似が認められた.大気中では破面粗さが粗く,組織の痕跡が明瞭なのに対し,真空中および内部破壊ではそれらは不明瞭であった.ステージ2aからステージ2bへの遷移条件も内部破壊は真空中の表面破壊とほぼ等しいことがわかった.以上から内部破壊が超高サイクル域で生じる理由の一つとして,材料内部の環境が真空環境に近く,き裂進展速度が大気中より遅いためであることが推測された.
  • ADIの極高サイクル疲労におけるフィッシュアイ型き裂進展挙動の解明
    日本学術振興会:科学研究費助成事業 一般研究(C)
    研究期間 : 1994年 -1995年 
    代表者 : 中村 孝
     
    本研究はADIの10^7回を越える領域でのフィッシュアイ疲労破壊機構について検討したものであり、高応答引張疲労試験装置の開発、10^8回までの多数の疲労データの取得、フィッシュアイ破壊した試験片についての破面および表面観察、の3項目を中心に行なった。開発した疲労試験装置は、負荷荷重精度±1%以内、応答性120〜200Hzの性能を達成した。この装置により、10^8回の試験が6〜10日の短期間で取得可能となった。S-N曲線は10^6回〜10^7回にかけて水平部を生じた後、さらに長寿命域で再び下方に折れ曲がる挙動を示した。10^6回以下の短寿命域は表面を起点とする破壊であり、10^7回以上の長寿命域は内部を起点とするフィッシュアイ破壊であった。競合リスクモデルによる信頼性解析により、両者の寿命分布特性が全く異なることが示された。表面観察により、10^7回以上の領域でフィッシュアイ破壊した試験片には表面き裂の停留が見出された。破面観察により、フィッシュアイき裂はこの材料のΔK_より小さいΔK領域でも進展していることが明らかとなった。特にこの破面には、表面を起点とする破面には見られないオーステナイトとフェライトの微細層状組織の痕跡が強く残されていた。起点の欠陥投影面積から求めたΔKを使い、従来のき裂進展則に基づいて寿命を推定した結果、表面破壊の寿命はほぼ予測できるが、フィッシュアイ破壊の寿命は実測値よりはるかに小さくなった。以上から、ADIにおける表面破壊からフィッシュアイ破壊への移行は表面き裂の停留によるものであること、フィッシュアイき裂はかなり低い応力でも停留しないこと、フィッシュアイ形成には表面き裂とは異なる破面形成機構が存在することなどが明らかとなった。特にフィッシュアイき裂進展は従来のき裂進展則から類推されるより、はるかに低速である可能性が見出された。

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