Researcher basic information

• 博士（工学）, 北海道大学

• https://researchmap.jp/fujimura70?lang=en

• 201501057628831545

• Manufacturing Technology (Mechanical Engineering, Electrical and Electronic Engineering, Chemical Engineering), Mechanics of materials and materials

• Bachelor's degree program, School of Engineering
• Master's degree program, Graduate School of Engineering
• Doctoral (PhD) degree program, Graduate School of Engineering

Research activity information

• Jun. 2020, 日本材料学会, 令和元年度学術奨励賞
  藤村奈央
• Mar. 2017, 日本材料学会 疲労部門委員会, 第33回疲労シンポジウム・第1回生体・医療材料シンポジウム 優秀研究発表賞
  藤村奈央, Japan society
• Jul. 2013, 21st Rudy Scavuzzo Student Paper Symposium Competition at the 2013 ASME Pressure Vessels Piping Conference, Honorable Mention (Ph. D. Category)
  Nao FUJIMURA, International society
• Jul. 2011, Student Paper Competition 2011 ASME Pressure Vessels and Piping Conference, The First Runner-Up Student Paper, Student Paper Competition
  Nao FUJIMURA, International society

• Nondestructive Observation of Small Internal Cracks in Gigacycle Fatigue of Titanium Alloys
  Takashi Nakamura; Fumiyoshi Yoshinaka; Gaoge Xue; Nao Fujimura; Kosuke Takahashi
  Materia Japan, 63, 11, 770, 775, Japan Institute of Metals, 01 Nov. 2024
  Scientific journal
• Toughened single-lap joints by composite bondline of adhesive and double-sided tape
  Kosuke Takahashi; Zhantong Sun; Takuma Kikuzawa; Kounosuke Shimamura; Sourabh Jagrat; Nao Fujimura; Takashi Nakamura
  Advanced Composite Materials, 1, 17, Informa UK Limited, 20 Nov. 2023
  Scientific journal
• Experimental Approach for Clarifying Initiation and Growth Behaviors of Internal Fatigue Cracks Using Synchrotron Radiation Multiscale X-ray Computed Tomography
  Takashi Nakamura; Gaoge Xue; Yuma Kon; Nao Fujimura; Takuya Yamazaki; Nobuyuki Tonozaki; Akihisa Takeuchi; Masayuki Uesugi; Kentaro Uesugi
  Materials Performance and Characterization, 12, 2, 107, 128, ASTM International, 10 Nov. 2023
  Scientific journal, Abstract
  
  The very high cycle fatigue (VHCF) phenomenon has been recognized and extensively studied in the past quarter century. One of the most peculiar and noticeable characteristics of VHCF is the transition of the origin site from the surface to the interior of the material in long-life regimes over 107 cycles. In particular, in high-strength metals, a tiny site can become an origin of internal fatigue cracks, such as nonmetallic inclusions of several micrometers to several tens of micrometers in high-strength steels and crystal grains of several tens of micrometers in titanium alloys. However, such small cracks are difficult to detect using conventional nondestructive approaches, such as industrial X-ray computed tomography (CT) or ultrasonic CT. Given this background, we have attempted to use a synchrotron radiation multiscale X-ray CT provided by SPring-8 in Japan. This system comprises a projection CT (micro-CT) with a spatial resolution of approximately 1 μm and a phase-contrast imaging CT (nano-CT) with a spatial resolution of approximately 200 nm or higher. The present study introduces our experimental approach to clarify internal fatigue crack behaviors using the multiscale X-ray CT with in situ fatigue testing. First, the principle of material selection focusing on the VHCF study is explained with the details of the materials used: (α+β) type Ti-6Al-4V, β type Ti-22V-4Al, and 17-4 precipitation-hardened martensite stainless steel. Afterward, the outline and primary performance of the multiscale X-ray CT are described. Subsequently, important points in conducting accurate in situ fatigue tests are discussed from the viewpoints of the development policies of the testing system and preparation of the special thin specimen for CT imaging. Finally, the multiscale X-ray CT is conducted for the above materials, and the initiation and growth behaviors of the internal fatigue cracks are compared and discussed for an in-depth understanding of the VHCF phenomenon.
• Room-Temperature-Nitriding of Pure Titanium Using Scanning Cyclic Press in a Nitrogen Environment
  Yuta Funaki; Nao Fujimura; Takashi Nakamura; Kosuke Takahashi; Tatsuki Wajima
  Journal of the Japan Institute of Metals and Materials, 87, 11, 305, 308, Japan Institute of Metals, 01 Nov. 2023
  Scientific journal
• Full-life growth behavior of a naturally initiated internal fatigue crack in beta titanium alloy via in situ synchrotron radiation multiscale tomography
  Gaoge Xue; Takashi Nakamura; Nao Fujimura; Kosuke Takahashi; Hiroyuki Oguma; Akihisa Takeuchi; Masayuki Uesugi; Kentaro Uesugi
  International Journal of Fatigue, 170, 107571, 107571, Elsevier BV, May 2023
  Scientific journal
• Characterization of internal fatigue crack initiation in Ti‐6Al‐4V alloy via synchrotron radiation X‐ray computed tomography
  Fumiyoshi Yoshinaka; Takashi Nakamura; Hiroyuki Oguma; Nao Fujimura; Akihisa Takeuchi; Masayuki Uesugi; Kentaro Uesugi
  Fatigue & Fracture of Engineering Materials & Structures, Wiley, 03 Feb. 2023, [Peer-reviewed]
  Scientific journal
• Fatigue Life Distribution Analysis of α+β Titanium Alloy Based on the Initiation and Propagation Behaviors of Small Internal Cracks
  Hironori MORISHITA; Takashi NAKAMURA; Nao FUJIMURA; Kosuke TAKAHASHI
  Journal of the Society of Materials Science, Japan, 71, 12, 997, 1004, Society of Materials Science, Japan, 15 Dec. 2022
  Scientific journal
• Generalized characteristics of peel tests independent of peel angle and tape thickness
  Masako Yamada; Kosuke Takahashi; Nao Fujimura; Takashi Nakamura
  Engineering Fracture Mechanics, 271, Aug. 2022
  Scientific journal
• Detection of small internal fatigue cracks in Ti-6Al-4V via synchrotron radiation nanocomputed tomography
  Gaoge Xue; Yuta Tomoda; Takashi Nakamura; Nao Fujimura; Kosuke Takahashi; Fumiyoshi Yoshinaka; Akihisa Takeuchi; Masayuki Uesugi; Kentaro Uesugi
  Fatigue and Fracture of Engineering Materials and Structures, 45, 9, Jun. 2022, [Peer-reviewed]
  Scientific journal
• Initiation and propagation of small fatigue crack in beta titanium alloy observed through synchrotron radiation multiscale computed tomography
  Gaoge Xue; Takashi Nakamura; Nao Fujimura; Kosuke Takahashi; Hiroyuki Oguma; Akihisa Takeuchi; Masayuki Uesugi; Kentaro Uesugi
  Engineering Fracture Mechanics, 263, 108308, 108308, Elsevier BV, Mar. 2022
  Scientific journal
• Three-dimensional Observation of Small Fatigue Cracks Growth Process in a Beta Titanium Alloy Ti-22V-4Al using Multiscale Synchrotron Radiation Computed Tomography
  中村 孝; 藤村 奈央; 小熊 博幸; 竹内 晃久; 上椙 真之; 上杉 健太朗
  Proceedings of the 7th International Conference on Advanced Steels, ICAS 2022, -, 132, 135, 2022, [Peer-reviewed]
  Scientific journal
• The Formation Process of Multiple Facets in Internal Fatigue Fractures of (α+β) Ti-6Al-4V
  中村 孝; 藤村 奈央
  Proceedings of the 7th International Conference on Advanced Steels, ICAS 2022, -, 136, 139, 2022, [Peer-reviewed]
  Scientific journal
• Estimation of Fatigue life for internal cracks using crack growth rates measured in a vacuum environment
  中村 孝; 藤村 奈央
  Proceedings of the 7th International Conference on Advanced Steels, ICAS 2022, -, 140, 143, 2022, [Peer-reviewed]
  Scientific journal
• In-situ Tensile and Fatigue Testing for Detection of Interfacial Debonding between Carbon Fibers and Epoxy Matrix by Synchrotron Radiation X-ray Nano-CT
  Ryosuke SHOYA; Takuma MATSUO; Kosuke TAKAHASHI; Nao FUJIMURA; Takashi NAKAMURA
  Journal of the Japan Society for Composite Materials, 47, 5, 186, 193, The Japan Society for Composite Materials, 15 Sep. 2021
  Scientific journal
• Initiation and propagation processes of internal fatigue cracks in β titanium alloy based on fractographic analysis
  Gaoge Xue; Takashi Nakamura; Nao Fujimura; Kosuke Takahashi; Hiroyuki Oguma
  Applied Sciences (Special Issue: Advances in Very-High-Cycle Fatigue), 11, 1, 2021, [Peer-reviewed]
  English, Scientific journal
• Effect of vacuum pressure on small crack propagation in Ti-6Al-4V
  Fumiyoshi Yoshinaka; Gaoge Xue; Nao Fujimura; Takashi Nakamura
  International Journal of Fatigue, 142, 2021, [Peer-reviewed]
  English, Scientific journal
• Effects of Number of Cyclic Loading in Scanning Cyclic Press on the Surface Modification of Magnesium Alloy
  ABE Suzuka; FUJIMURA Nao; OOGA Koyo; TAKAHASHI Kosuke; NAKAMURA Takashi; WAJIMA Tatsuki
  The Proceedings of Mechanical Engineering Congress, Japan, 2020, J13107, The Japan Society of Mechanical Engineers, 2020
  Japanese, A new surface modification technique, scanning cyclic press (SCP), was developed. SCP scans a metal surface with a vibrating indenter and refines the surface layer using cyclic compressive loading. This study applied SCP to the surface of magnesium alloy AZ31 under number of cyclic loading of 3.2×10⁵ and 8.0×10⁶ cycles to investigate the effect on surface structure. After applying SCP, the cross sections of specimens were observed by electron back scattering diffraction (EBSD) using SEM. As a result, a fine structure around 1－10 μm in grain size and a twin were formed in the surface layer of both specimens. The depth of these structures from the surface remained unchanged regardless of the number of cyclic loading. However, the surface structure under 8.0×10⁶ cycles was finer than that under 3.2×10⁵ cycles. This is likely caused by the increase of accumulated plastic strain during SCP process.
• A Study of Room-temperature-nitriding Process of Pure Titanium Using Scanning Cyclic Press under Controlled Atmosphere
  FUKUDA Tetsuya; KATONO Yutaka; FUJIMURA Nao; TAKAHASHI Kosuke; NAKAMURA Takashi; WAJIMA Tatsuki
  The Proceedings of Mechanical Engineering Congress, Japan, 2020, J13108, The Japan Society of Mechanical Engineers, 2020
  Japanese, A new surface modification technique, scanning cyclic press (SCP), was applied to commercially pure titanium (CP-Ti) in a nitrogen atmosphere at room temperature. During the SCP process, a vibrating indenter reciprocally scanned the specimen’s surface to apply cyclic low compressive load. In this study, a possibility of a room-temperature-nitriding technique using SCP was investigated. Before and after SCP, the surface and the cross-section of the specimens were observed using a laser microscope and scanning ion microscope (SIM), respectively. Then, elemental and composition analyses were conducted by using EPMA and XRD. As a result, a pale golden color, which is known as a characteristic of titanium nitride coating, appeared at the surface of SCP-treated specimen. The SIM observation of the cross-section showed that a nanocrystalline structure was created in the surface layer, and the nitrogen element was detected in the same layer. The XRD analysis showed a strong possibility to form titanium nitride at the surface of SCP-treated specimen. These results suggested that nitrogen element surrounding the specimen diffused into the microstructure beneath the surface and likely created titanium nitride during SCP even at room temperature.
• Damage Simulation of Mortar Materials by Multi-scale Analysis
  OHYAMA Kota; TAKAHASHI Kosuke; FUJIMURA Nao; NAKAMURA Takashi
  The Proceedings of Mechanical Engineering Congress, Japan, 2020, J03323, The Japan Society of Mechanical Engineers, 2020
  Japanese, In this study, multi-scale analysis was carried out by designing a porous cement paste model, and a mortar model using glass spheres as fine aggregates. The tensile simulation of porous cement paste model was conducted by RVE model that reflects the distribution of pore size in a cement specimen. It was shown that the elastic modulus and strength of cement have a linear relationship with porosity. The finite element model of cylindrical samples was then created to simulate the splitting tensile test. For the cylindrical mortar model, spatial variation of the physical properties due to the porosity distribution in cement was reflected. When cement had a distribution of physical properties, the strength of the cement model was similar to that of mortar model as it was observed from the experiment, whereas the strength of the cement model was higher by 20 % if uniform physical properties were used for cement matrix. It was confirmed that our simulation well corresponded to the experimental result of at the ages of 28 days. In addition, the decrease in the strength, which would be expected for further aging mortar, can be also modeled by the porous layer at glass-cement interface, representing the Interfacial Transition Zone.
• Surface modification of machine-finished magnesium alloy AZ31 using a scanning cyclic press
  Nao Fujimura; Koyo Ooga; Kosuke Takahashi; Takashi Nakamura; Tatsuki Wajima
  Mechanical Engineering Journal, 7, 4, 2020, [Peer-reviewed], [Lead author, Corresponding author]
  English, Scientific journal
• High-resolution non-destructive observation of microstructure in high strength metallic materials using synchrotron radiation micro/nano CT
  KON Yuma; NAKAMURA Takashi; FUJIMURA Nao; YOSHINAKA Fumiyoshi; TAKAHASHI Kosuke; TAKEUCHI Akihisa; UESUGI Masayuki; UESUGI Kentaro
  The Proceedings of the Materials and Mechanics Conference, 2019, PS22, The Japan Society of Mechanical Engineers, 2019
  Japanese, To investigate the initiation and propagation processes of internal fatigue crack in high strength metallic materials, high resolution and non-destructive observation technique was proposed. The opening and closing behaviors of fatigue cracks in Ti-6Al-4V, and the non-metallic inclusions in SNCM439, which are regarded as a potential crack initiation site, were observed by synchrotron radiation CT (SR-CT) at SPring-8. To ensure a sufficient X ray transmission, fatigue specimens with a small diameter were designed, and a tensile loading jig was newly developed to measure the CTOD during SR-CT. In Ti-6Al-4V, micro CT and nano CT with a resolution around 1 μm and 200 nm, respectively, were used to observe internal cracks. As a result, it was clarified that surface cracks and internal cracks showed different behaviors. The crack tip of surface cracks had a sharp shape and the CTOD was changed by the tensile load. On the other hand, the shape of crack tips of internal cracks was blunted, and the tensile load did not affect CTOD. This crack blunting was explained by the vacuum-like environment around the internal crack. In SNCM439, small non-metallic inclusions with the size around 8 μm in diameter were clearly observed. Accordingly, it is expected that we can observe the initiation and propagation of internal cracks in high strength steel by repeating fatigue tests and SR-CT alternately.
• Fatigue life estimation of subsurface cracks based on crack growth rates measured in a vacuum environment
  MODI Sourav Kumar; MEHENDIRATTA Paras; TSUKAGOSHI Takuma; NAKAMURA Takashi; FUJIMURA Nao; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2019, PS23, The Japan Society of Mechanical Engineers, 2019
  Japanese, In recent years, it has been revealed that in high strength steel, fracture occurs from non-metallic inclusions inside of the material in the very high cycle region. However, because of the difficulty in directly observing the internal fatigue process, its mechanism is still unknown. Subsequently efforts are being made for better understanding of fatigue crack initiation and propagation. In this study, we have tried to estimate life in this subsurface fatigue region using crack growth rates measured in a vacuum environment and compare it with actual life found out in subsurface fractures in fatigue test to better understand fatigue crack properties.
• Fatigue property improving effect of low carbon steel S25C by surface modification technique Scanning Cyclic Press
  TANNO Ryoko; FUJIMURA Nao; NAKAMURA Takashi; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2019, PS01, The Japan Society of Mechanical Engineers, 2019
  Japanese, In this study, we applied a new surface modification technique, scanning cyclic press (SCP) to machined specimens of low carbon steel S25C. In the SCP process, a vibrating indenter scans the metal surface and cyclically applies a variable low compressive load to modify the microstructure at the surface. During applying the SCP, the processing parameters, such as the scanning speed, the amount of compressive load and the number of cyclic loading, were changed to investigate the effects on the microstructure and the fatigue property. After applying the SCP, the surface roughness of specimens became smaller than that before. To clarify the changes in the microstructure beneath the surface of the SCP-treated specimens, the cross-section was created using focused ion beam technique. Scanning ion microscopic observations showed the microstructure refinement at the surface layer. The microstructure refinement was promoted with increasing the scanning speed, and the surface hardness also increased. Additionally, at the higher compressive load and the larger number of cyclic loading, finer microstructure was observed at the surface layer above the modified microstructure. This result suggested that the microstructure refinement is promoted by increasing the amount and the number of compressive loading. Fatigue test results showed that the fatigue lives of the SCP-treated specimens were 2 to 50 times longer than that of non-treated specimens.
• Nano-structural refinement of metallic surfaces by using low-compression cyclical loading
  N. Fujimura; T. Nakamura; M. Ueno
  Mechanical Engineering Letters, 4, Dec. 2018, [Peer-reviewed], [Lead author, Corresponding author]
  English, Scientific journal
• Changing mechanism of surface topography in low-cycle fatigue of austenitic stainless steel
  Nao FUJIMURA; Yuto NISHIZUKA; Kosuke TAKAHASHI; Takashi NAKAMURA
  Transactions of the JSME (in Japanese), 84, 864, Aug. 2018, [Peer-reviewed], [Lead author, Corresponding author]
  Japanese, Scientific journal
• Changing Mechanisms of Surface Relief and the Damage Evaluation of Low Cycle Fatigued Austenitic Stainless Steel
  Nao Fujimura; Takashi Nakamura; Kosuke Takahashi
  MATEC Web of Conferences, 165, EDP Sciences, 25 May 2018, [Peer-reviewed], [Lead author, Corresponding author]
  English, International conference proceedings
• Effects of vacuum environment on the fatigue crack growth properties of high strength steel-Behavior of small crack by ΔK-increasing test-
  ISHIHARA Risa; MEHENDIRATTA Paras; NAKAMURA Takashi; FUJIMURA Nao; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2018, OS0802, The Japan Society of Mechanical Engineers, 2018
  Japanese, In recent years, it has been revealed that in high strength steel, fracture occurs from non-metallic inclusions inside of the material in the very high cycle region. However, because of the difficulty in directly observing the internal fatigue process, its mechanism is largely unknown. In this study, we try to clarify the internal fatigue process on the basis that the environment around an internal crack is like a vacuum environment. Also, it has been reported that the effect of vacuum environment is more prominent in the initial stages of crack propagation. This suggests that targeting the crack propagation process of small cracks, like that with the same size as a small defect, would shed light on the controlling factors in the internal fatigue process. Thus, we conducted ΔK-increasing tests in air and in vacuum using specimens with a small artificial defect on its surface. As a result, fatigue crack propagation rates in vacuum were lower than that in air. Also, when compared with the results of long cracks, the crack propagation rates in both environments were much higher. However, after eliminating the effect of crack closure for the results of long crack, the crack propagation rates in both environments matched.
• Effects of vacuum environment on the fatigue crack growth properties of high strength steel- behavior of long crack by ΔK decrease test
  MEHENDIRATTA Paras; ISHIHARA Risa; NAKAMURA Takashi; FUJIMURA Nao; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2018, OS0801, The Japan Society of Mechanical Engineers, 2018
  Japanese, Sub-surface fractures are known as a characteristic fatigue mode of high strength steel in the very high cycle region. To reveal the mechanism of sub-surface fractures, ΔK-decreasing tests were conducted in high vacuum and air environments based on the idea that sub-surface cracks are exposed to a vacuum-like environment. As a result, fatigue crack growth rates in high vacuum were slower than those in air. In high vacuum, fatigue crack growth still took place at a considerably low rate around 10^-13 m/cycle, and the effective threshold stress intensity factor rage (ΔK_eff) in high vacuum lower to that in air. Unlike in air environment, negligible effects of oxide-induced crack closure in vacuum was considered as the reason for the results. The behavior of fatigue crack growth properties in high vacuum well explained the characteristics of sub-surface crack growths in very high cycle regime.
• Evaluation of healing characteristics of a mendable polymer with FEM
  HASEGAWA Ryo; TAKAHASHI Kosuke; FUJIMURA Nao; NAKAMURA Takashi
  The Proceedings of the Materials and processing conference, 2018.26, 910, The Japan Society of Mechanical Engineers, 2018
  Japanese
• Adhesively bonded joint strengthened by double-sided adhesive tape
  KIKUZAWA Takuma; TAKAHASHI Kosuke; FUJIMURA Nao; NAKAMURA Takashi
  The Proceedings of the Materials and processing conference, 2018.26, 909, The Japan Society of Mechanical Engineers, 2018
  Japanese
• Surface modification of magnesium AZ31 by using scanning cyclic press and the improvement effect on fatigue properties
  Nao Fujimura; Takashi Nakamura; Kosuke Takahashi; Tatsuki Wajima
  Zairyo/Journal of the Society of Materials Science, Japan, 66, 12, 906, 912, Society of Materials Science Japan, 01 Dec. 2017, [Peer-reviewed], [Lead author, Corresponding author]
  Japanese, Scientific journal
• Observation of fatigue crack initiation process in Ti-22V-4Al
  KASHIMA Shota; TAKANASHI Kei; YOSHINAKA Fumiyoshi; NAKAMURA Takashi; FUJIMURA Nao; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2017, PS32, The Japan Society of Mechanical Engineers, 2017
  Japanese, It is generally reported on titanium alloys that crack initiation occurs from a small flat plane called the “facet”. Thus, it is important to investigate facet formation processes in revealing the fatigue characteristics of titanium alloy. In the present work, β type titanium alloy Ti-22V-4Al was investigated in order to elucidate the fatigue crack initiation process. An axial load fatigue test was carried out with an etched specimen in order to examine the positional relationship between the microstructure and crack initiation site (in the grain or at the grain boundary). After the fatigue test, the fracture surface and the specimen surface were observed by using a scanning electron microscope. From the fracture surface observation, four crack propagation traces were observed on the fracture surface. When focusing on the crack initiation site, it was revealed that each crack initiated from a facet. Then, the facet angle was measured as the angle between the loading direction and the normal to the facet plane, which revealed that the shear stress largely contributed to crack initiation. The result of the specimen surface observation demonstrated that crack initiation occurred in β phase without α precipitate rather than at the β grain boundary. Therefore, in this sample material, crack initiated in β phase mainly by the shear stress.
• Fatigue crack initiation and propagation properties of Ti-22V-4Al
  TAKANASHI Kei; KASHIMA Shota; YOSHINAKA Fumiyoshi; NAKAMURA Takashi; FUJIMURA Nao; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2017, PS29, The Japan Society of Mechanical Engineers, 2017
  Japanese, To investigate the fatigue crack initiation and propagation properties of Ti-22V-4Al, uniaxial fatigue tests and surface observation were carried out. As a result of surface observation, the crack initiation life of each crack was clarified. The cracks initiated at N = 2000 to 8000 cycles. Based on the crack initiation life of each crack, the tendency of crack initiation was investigated using probability paper. As a result, it was shown that the crack initiation life follows the lognormal distribution. Based on the crack propagation observation results, the relationship between the crack growth rate da/dN and the stress intensity factor range ΔK was investigated. As a result, in the small ΔK region, the dispersions of each crack was large. However, with the increase in ΔK, the dispersions became smaller and showed a certain width. Comparing with the same material with different heat treatment samples, da/dN-ΔK relations in large ΔK regions show similar value. So it suggests that crack propagation resistance may not be affected by the precipitation amount of α phase in this material.
• Effect of Scanning Cyclic Press on Fatigue Properties of AZ31
  OGA Koyo; FUJIMURA Nao; NAKAMURA Takashi; TAKAHASHI Kosuke; WAJIMA Tatsuki
  The Proceedings of the Materials and Mechanics Conference, 2017, PS27, The Japan Society of Mechanical Engineers, 2017
  Japanese, Magnesium alloy is the lightest metal among all practical metals and recognized as a promising material; however, it has a limited use because of the poor fatigue properties. In this study, a new surface modification technique, scanning cyclic press (SCP), was applied to magnesium alloy AZ31 to improve its fatigue properties. SCP scans a metal surface with a vibrating indenter under precise loading control based on a servo testing machine and can apply a variable cyclically compressive load. The surface observation on AZ31 specimens showed an increase in surface roughness of SCP-treated specimens. However, the fatigue life of SCP-treated specimens clearly increased. Microstructure of a cross section of SCP-treated specimen was observed using an optical microscope. A fine mesh-patterned region was formed beneath the surface to a depth of 50 μm. The fracture surface observation showed that the fracture origin of SCP-treated specimens located in subsurface site whereas that of untreated specimens was at the surface of the specimen. The observation also showed a band-like layer just beneath surface in SCP-treated specimen which did not exist in the untreated specimen. The layer corresponded to the mesh-patterned region. The results suggested that SCP changed the surface microstructure of magnesium alloy and suppressed crack initiation from the specimen's surface.
• Changing Mechanisms of Surface Topography and the Damage Assessment in Low Cycle Fatigued SUS316NG
  FUJIMURA Nao; NISHIZUKA Yuto; NAKAMURA Takashi; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2017, OS0527, The Japan Society of Mechanical Engineers, 2017
  Japanese, 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 slip bands and grain deformations was conducted on SUS316NG. During the fatigue tests, the specimen's surface topography was regularly measured using a laser 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 height caused by PSBs and that by crystal grain deformation were measured respectively. As a result, the both heights developed 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 crystal 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 crystal grain deformation.
• Textural control of the surface layer of metallic materials by using scanning cyclic press
  NAKAMURA Takashi; FUJIMURA Nao; MIZUTANI Kazuhiro; TAKAYANAGI Ibuki; WAJIMA Tatsuki
  The Proceedings of Mechanical Engineering Congress, Japan, 2017, J1310106, The Japan Society of Mechanical Engineers, 2017
  Japanese
• Measurement and evaluation of fracture toughness in the thickness direction of polyimide film
  NAGUMO Yoshinori; SAKAMOTO Yusuke; NAKAMURA Takashi; FUZIMURA Nao; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2016, OS05-10, The Japan Society of Mechanical Engineers, 2016
  Japanese, Polyimide (PI) film with a surface small defect subjected to tensile load is fractured by the crack propagation from the defect in the film thickness direction. However, the evaluation method of fracture toughness in this type of fracture hasn’t yet been established. In this paper, SENT tests were carried out using 125 μm-thick PI films with a surface pre-crack having different depths from 10 μm to 60 μm, and fracture processes focusing on crack opening and crack extension were observed with a color 3D laser scanning microscope. Based on the results, fracture toughnesses J_in in the direction of film thickness were calculated by using two different equations for SENT specimen. As a result, J_in values were clarified to have a slight dependency on pre-crack depth. The differences between J_in values calculated by the two equations were considered small, and fracture toughness was in the range from 0.4 to 1 kJ/m².
• Nanostructural Surface Refinement of Low Carbon Steel by Using Cyclic Press
  NAKAMURA Takashi; NAKATANI Kentarou; MIYAZAKI Kazunori; FUJIMURA Nao; SHIBAYAMA Tamaki; WAJIMA Tatsuki
  The Proceedings of Mechanical Engineering Congress, Japan, 2016, J1610202, The Japan Society of Mechanical Engineers, 2016
  Japanese
• Changes in surface topography and the mechanisms of low cycle fatigued SUS316NG
  NISHIZUKA Yuto; FUJIMURA Nao; NAKATA Yasutaka; KATOUNO Yutaka; NAKAMURA Takashi
  The Proceedings of Mechanical Engineering Congress, Japan, 2016, J0310101, The Japan Society of Mechanical Engineers, 2016
  Japanese, To investigate the cause of the change of arithmetic mean roughness R_a and arithmetic mean waviness W_a of austenitic stainless steel under low-cycle fatigue loading, precise observation focusing on slip bands and grain deformations was conducted on SUS316NG. During the fatigue tests, the specimen's surface topography was regularly measured using a laser 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 height caused by persistent slip bands and that by crystal grain deformation were measured respectively. As a result, the both heights in initial stage of usage factor (UF) more developed compared with those in later stage of 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 Development of Scanning Cyclic Press Aiming for Nanostructural Surface Refinement and Its Application to Low Carbon Steel
  MIYAZAKI Kazunori; NAKAMURA Takashi; FUJIMURA Nao; SHIBAYAMA Tamaki; WAJIMA Tatsuki
  The Proceedings of Mechanical Engineering Congress, Japan, 2016, J1610203, The Japan Society of Mechanical Engineers, 2016
  Japanese, A surface modification technique named as scanning cyclic press (SCP) was newly developed, which refines the surface layer of metallic materials by scanning metal surface with a vibrating indenter. This study applied SCP to the surface of low carbon steel S25C and conducted surface observation, cross-sectional observation, and push-pull fatigue tests followed by fracture surface observation. As a result, SCP modification improved surface roughness and generated gradient structure consisting of nanocrystalline and refined layers. This technique also improved fatigue life compared with that of untreated specimens. Fracture surface observation indicated that nano-structural layer formed in the outermost surface of the specimen suppressed initiation of fatigue crack and improved fatigue properties of refined material.
• Quantitative characterization of the changes in surface topography of austenitic stainless steel under low cycle fatigue loading
  Tomoki Shinko; Takashi Nakamura; Nao Fujimura; Yasutaka Nakata
  Zairyo/Journal of the Society of Materials Science, Japan, 64, 11, 910, 917, Society of Materials Science Japan, 2015, [Peer-reviewed]
  English, Scientific journal
• APPLICATION OF SURFACE ROUGHNESS PARAMETERS TO THE EVALUATION OF LOW CYCLE FATIGUE DAMAGE IN AUSTENITIC STAINLESS STEEL
  Nao Fujimura; Takashi Nakamura; Hiroyuki Oguma
  PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE - 2013, VOL 5, 2014, [Peer-reviewed], [Lead author]
  English, International conference proceedings
• Fatigue damage assessment of SUS316NG based on surface roughness measurement and the applicable scope
  Nao Fujimura; Takashi Nakamura; Hiroyuki Oguma; Tomohiro Nakajima; Tomoki Shinko
  Zairyo/Journal of the Society of Materials Science, Japan, 62, 12, 750, 755, The Society of Materials Science, Japan, Dec. 2013, [Peer-reviewed], [Lead author]
  Japanese, Scientific journal
• Changes in Surface Roughness during Low Cycle Fatigue Process of Austenitic Stainless Steel
  Nao FUJIMURA; Takashi NAKAMURA; Hiroyuki OGUMA
  Journal of Solid Mechanics and Materials Engineering, 7, 3, 372, 380, The Japan Society of Mechanical Engineers, 2013, [Peer-reviewed], [Lead author]
  English, 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 UF_pre. As a result, the surface roughness was found to increase roughly linearly until approximately UF_pre = 0.4 regardless of the strain range conditions. The rate at which surface roughness increased with UF_pre 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 UF_pre = 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.
• DAMAGE ASSESSMENT ON LOW CYCLE FATIGUE PROPERTIES OF CYCLIC PRE-STRAINED AUSTENITIC STAINLESS STEEL
  Nao Fujimura; Hiroyuki Oguma; Takashi Nakamura
  PROCEEDINGS OF THE ASME PRESSURE VESSELS AND PIPING CONFERENCE VOL 1, 1061, 1067, 2012, [Peer-reviewed], [Lead author]
  English, International conference proceedings

• Fatigue Properties of Extruded Magnesium Alloy with Surface Modification Using Scanning Cyclic Press
  藤村奈央; 高橋航圭; 中村孝; 和島達希, 疲労シンポジウム講演論文集, 36th, 2024
• Characteristics of the Internal Fatigue Crack Propagation in Beta Titanium Alloy
  XUE Gaoge; 藤村奈央; 高橋航圭; 中村孝, 疲労シンポジウム講演論文集, 36th, 2024
• The Adhesion Behavior of Internal Fatigue Crack in (α+β) Ti-6Al-4V Under Cyclic Compressive Loading
  舩木優大; 増谷幸香; XUE Gaoge; 高橋航圭; 藤村奈央; 中村孝, 疲労シンポジウム講演論文集, 36th, 2024
• Propagation Properties of Surface- and Internally-Initiated Small Fatigue Cracks in Precipitation-Hardened Stainless Steel SUS630
  上野竜史; XUE Gaoge; 舩木優大; 高橋航圭; 藤村奈央; 中村孝, 疲労シンポジウム講演論文集, 36th, 2024
• Initiation and growth behaviors of small internal fatigue cracks in precipitation-hardened stainless steel SUS 630
  中村孝; 五十嵐元; 藤村奈央; 高橋航圭, 日本材料学会学術講演会講演論文集(CD-ROM), 73rd, 2024
• Initiation and propagation process of the internal fatigue crack in Ti-22V-4Al alloy based on fractographic analysis
  XUE Gaoge; FUJIMURA Nao; NAKAMURA Takashi; OGUMA Hiroyuki; TAKAHASHI Kosuke, The Proceedings of the Materials and Mechanics Conference, 2019, 0, OS0908, 2019
  

  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.

  , The Japan Society of Mechanical Engineers, Japanese
• Initiation process of internal fatigue cracks in Ti-6Al-4V via synchrotron radiation nano CT
  NAKAMURA Takashi; TOMODA Yuta; FUJIMURA Nao; YOSHINAKA Fumiyoshi; OGUMA Hiroyuki; TAKEUCHI Akihisa; UESUGI Masayuki; UESGI Kentaro, The Proceedings of the Materials and Mechanics Conference, 2019, 0, OS0909, 2019
  

  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.

  , The Japan Society of Mechanical Engineers, Japanese
• OS0913-454 Distribution Characteristics of Surface Roughness on Low Cycle Fatigued Austenitic Stainless Steel
  FUJIMURA Nao; NAKAMURA Takashi; SHINKO Tomoki, M&M材料力学カンファレンス, 2015, "OS0913, 454-1"-"OS0913-454-3", 21 Nov. 2015
  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., The Japan Society of Mechanical Engineers, Japanese
• G0301502 Evaluation of Surface Topography of SUS316NGin Low Cycle Fatigue under Two-Step Loading
  NAKATA Yasutaka; NISHIDZUKA Yuto; SHINKO Tomoki; FUJIMURA Nao; NAKAMURA Takashi, Mechanical Engineering Congress, Japan, 2015, "G0301502, 1"-"G0301502-5", 13 Sep. 2015
  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., The Japan Society of Mechanical Engineers, Japanese
• 142 Evaluation of Surface Topography of SUS316NG in Low Cycle Fatigue Process
  NAKATA Yasutaka; SHINKO Tomoki; FUJIMURA Nao; NAKAMURA Takashi, 北海道支部講演会講演概要集, 2014, 53, 27, 28, 27 Sep. 2014
  The Japan Society of Mechanical Engineers, Japanese
• OS2126 Effect of surface roughness on eddy current signals of austenite stainless steel
  HORIKAWA Noritaka; HAMAGUCHI Moe; FUJIMURA Nao; NAKAMURA Takashi, M&M材料力学カンファレンス, 2014, "OS2126, 1"-"OS2126-3", 19 Jul. 2014
  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., The Japan Society of Mechanical Engineers, Japanese
• OS0820 Changes in Surface Roughness of Low Cycle Fatigued SUS316NG under Two-step Variable Strain Ranges
  FUJIMURA Nao; NAKAMURA Takashi; OGUMA Hiroyuki; NIINUMA Tadaki, M&M材料力学カンファレンス, 2014, "OS0820, 1"-"OS0820-3", 19 Jul. 2014
  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., The Japan Society of Mechanical Engineers, Japanese
• OS0819 Quantitative Evaluation of Surface Relief of Low Cycle Fatigued Austenitic Stainless Steel
  SHINKO Tomoki; FUJIMURA Nao; NAKATA Yasutaka; OGUMA Hiroyuki; NAKAMURA Takashi, M&M材料力学カンファレンス, 2014, "OS0819, 1"-"OS0819-3", 19 Jul. 2014
  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., The Japan Society of Mechanical Engineers, Japanese
• PS15 Changes in Surface Features of SUS316NG in Low Cycle Fatigue
  NIINUMA Tadaki; FUJIMURA Nao; SINKOU Tomoki; NAKAMURA Takashi; OGUMA Hiroyuki, M&M材料力学カンファレンス, 2013, "PS15, 1"-"PS15-3", 12 Oct. 2013
  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., The Japan Society of Mechanical Engineers, Japanese
• 108 Low Cycle Fatigue Damage Evaluation of SUS316NG based on Surface Roughness
  SHINKO Tomoki; FUJIMURA Nao; NIINUMA Tadaki; OGUMA Hiroyuki; NAKAMURA Takashi, 北海道支部講演会講演概要集, 2013, 52, 15, 16, 28 Sep. 2013
  The Japan Society of Mechanical Engineers, Japanese
• 214 Low Cycle Fatigue Properties and Damage Evaluation of SUS3 16NG
  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, 20 Oct. 2012
  The Japan Society of Mechanical Engineers, Japanese
• OS2109 Effects of Surface Roughness on Low Cycle Fatigue Properties of Austenitic Stainless Steel
  FUJIMURA Nao; NAKAMURA Takashi; OGUMA Hiroyuki, M&M材料力学カンファレンス, 2012, "OS2109, 1"-"OS2109-3", 22 Sep. 2012
  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., The Japan Society of Mechanical Engineers, Japanese
• 704 Damage Assessment on Low Cycle Fatigue Properties of Cyclic Pre-strained SUS316NG
  FUJIMURA Nao; OGUMA Hiroyuki; NAKAMURA Takashi, 学術講演会講演論文集, 61, 181, 182, 2012
  日本材料学会, Japanese
• 220 Effects of Cyclic Pre-strain on Low Cycle Fatigue Properties of SUS316NG
  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, 01 Oct. 2011
  The Japan Society of Mechanical Engineers, Japanese
• 604 Effects of Cyclic Pre-strain on Low Cycle Fatigue Properties of Structural Steel for Nuclear Power Plants
  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, 07 Nov. 2010
  The Japan Society of Mechanical Engineers, Japanese
• 534 Effects of Mean Strain on Low Cycle Fatigue Properties of Austenitic Stainless Steel
  FUJIMURA Nao; SUIMON Eri; NAKAMURA Takashi; OGUMA Hiroyuki, 学術講演会講演論文集, 59, 405, 406, 2010
  日本材料学会, Japanese

• Size Effect on Internal Fatigue Fractures of Ti-22V-4Al
  PoYu Chen; Gaoge XUE; Takashi NAKAMURA; Nao FUJIMURA; Kosuke TAKAHASHI
  日本機械学会 M&M2021材料力学カンファレンス, 15 Sep. 2021, English, Oral presentation
  15 Sep. 2021 - 16 Sep. 2021
• The Formation Process of Multiple Facets in Internal Fatigue Fractures of (α+β) Ti-6Al-4V
  Takuya YAMAZAKI; Hironori MORISHITA; Yuta TOMODA; Takashi NAKAMURA; Nao FUJIMURA; Kosuke TAKAHASHI
  日本機械学会 M&M2021材料力学カンファレンス, 15 Sep. 2021, Japanese, Oral presentation
  15 Sep. 2021 - 16 Sep. 2021
• Analysis of fatigue life distribution of Ti-6Al-4V based on the initiation and propagation behaviors of small internal cracks
  Hironori Morishita; Takashi Nakamura; Nao Fujimura; Kosuke Takahashi
  Eighth International Conference on Very High Cycle Fatigue (VHCF8), 09 Jul. 2021, English, Oral presentation
  05 Jul. 2021 - 09 Jul. 2021
• Classification of internal fatigue crack growth stages of (α+β) Ti-6Al-4V by fractographic analysis focusing on multiple facet formation
  Takuya Yamazaki; Hironori Morishita; Yuta Tomoda; Takashi Nakamura; Nao Fujimura; Kosuke Takahashi
  Eighth International Conference on Very High Cycle Fatigue (VHCF8), 09 Jul. 2021, English, Oral presentation
  05 Jul. 2021 - 09 Jul. 2021
• Improvement of the fatigue property of magnesium alloy AZ31 through a new surface modification technique: scanning cyclic press
  Nao Fujimura; Suzuka Abe; Koyo Ooga; Takashi Nakamura; Kosuke Takahashi; Tatsuki Wajima
  Eighth International Conference on Very High Cycle Fatigue (VHCF8), 09 Jul. 2021, English, Oral presentation
  05 Jul. 2021 - 09 Jul. 2021
• Initiation and propagation process of small fatigue crack in beta titanium alloy via multiscale synchrotron radiation computed tomography
  Gaoge Xue; Takashi Nakamura; Nao Fujimura; Hiroyuki Oguma; Akihisa Takeuchi; Masayuki Uesugi; Kentaro Uesugi
  Eighth International Conference on Very High Cycle Fatigue (VHCF8), 06 Jul. 2021, English, Oral presentation
  05 Jul. 2021 - 09 Jul. 2021
• Nondestructive observation of internal fatigue crack initiation in Ti-6Al-4V via synchrotron radiation X-ray CT
  Fumiyoshi Yoshinaka; Takashi Nakamura; Hiroyuki Oguma; Nao Fujimura; Akihisa Takeuchi; Masayuki Uesugi; Kentaro Uesugi
  Eighth International Conference on Very High Cycle Fatigue (VHCF8), 06 Jul. 2021, English, Oral presentation
  05 Jul. 2021 - 09 Jul. 2021
• Sanning Cyclic Pressを用いたマグネシウム合金の表面改質に及ぼす負荷繰返し数の影響
  阿部すずか; 藤村奈央; 大賀向陽; 髙橋航圭; 中村孝; 和島達希
  日本機械学会 2020年度年次大会, Sep. 2020
• 微小内部き裂の発生・進展挙動に基づくα+β型チタン合金の疲労寿命分布解析
  森下弘法; 中村孝; 藤村奈央; 髙橋航圭
  日本材料学会第69期学術講演会, May 2020
• 表面改質技術Scanning Cyclic Pressを施した低炭素鋼S25Cの疲労特性
  藤村奈央; 丹野椋子; 宮﨑数則; 中村孝
  日本材料学会 第69期学術講演会, May 2020
• Scanning Cyclic Pressを施したマグネシウム合金の疲労特性と表層組織観察
  藤村奈央; 大賀向陽; 大久保賢二; 中村孝; 髙橋航圭; 和島達希
  日本材料学会第68期学術講演会, 24 May 2019, Japanese, Oral presentation
  [Domestic Conference]
• Ti-6Al-4Vにおける表面および内部微小疲労き裂の開閉口挙動
  友田悠太; 中村孝; 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
  日本材料学会第68期学術講演会, May 2019, Oral presentation
  May 2019 - May 2019
• 接着継手強度に及ぼす両面テープ併用の効果
  髙橋航圭; 菊澤卓馬; 藤村奈央; 中村孝
  日本材料学会第68期学術講演会, May 2019, Oral presentation
• 大気および真空環境における微小疲労き裂の進展・停留挙動
  塚越拓摩; 石原梨沙; Paras MEHENDIRATTA; 中村孝; 藤村奈央; 髙橋航圭
  日本材料学会第68期学術講演会, May 2019, Oral presentation
• Initiation process of internal fatigue cracks in Ti-6Al-4V via synchrotron radiation nano CT
  NAKAMURA Takashi; TOMODA Yuta; FUJIMURA Nao; YOSHINAKA Fumiyoshi; OGUMA Hiroyuki; TAKEUCHI Akihisa; UESUGI Masayuki; UESGI Kentaro
  The Proceedings of the Materials and Mechanics Conference, 2019, The Japan Society of Mechanical Engineers, Japanese
  2019 - 2019,

  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.

• Initiation and propagation process of the internal fatigue crack in Ti-22V-4Al alloy based on fractographic analysis
  XUE Gaoge; FUJIMURA Nao; NAKAMURA Takashi; OGUMA Hiroyuki; TAKAHASHI Kosuke
  The Proceedings of the Materials and Mechanics Conference, 2019, The Japan Society of Mechanical Engineers, Japanese
  2019 - 2019,

  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.

• Changing mechanisms of surface relief and the damage evaluation of low cycle fatigued austenitic stainless steel
  N. Fujimura; T. Nakamura; K. Takahashi
  12th international fatigue congress Fatigue 2018, May 2018, English, Oral presentation
  [International presentation]
• 加熱修復機能を有する樹脂材料の硬化成形温度の検討
  岸本繁人; 高橋航圭; 中村孝; 藤村奈央
  日本材料学会 第3回材料WEEK 若手学生研究発表会, 11 Oct. 2017, Japanese, Poster presentation
  [Domestic Conference]
• Scanning Cyclic Pressによるアルミニウム合金のナノ微細化表面改質
  髙栁伊吹; 中村孝; 藤村奈央; 高橋航圭; 和島達希
  11 Oct. 2017, Japanese, Poster presentation
  [Domestic Conference]
• 走査型 Cyclic Press “SCP”の開発と低炭素鋼の表面改質
  水谷一皓; 中村孝; 藤村奈央; 高橋航圭; 和島達希
  日本材料学会 第3回材料WEEK 若手学生研究発表会, 11 Oct. 2017, Japanese, Poster presentation
  [Domestic Conference]
• 振動圧縮荷重を用いた表面改質技術“Cyclic Press”の提案とチタン合金への適用
  上遠野寛; 中村孝; 吉中奎貴; 藤村奈央; 高橋航圭; 和島達希
  日本材料学会 第3回材料WEEK 若手学生研究発表会, 11 Oct. 2017, Japanese, Poster presentation
  [Domestic Conference]
• OS0527 低サイクル疲労を受けたSUS316NGの表面性状変化機構と損傷評価
  藤村奈央; 西塚祐斗; 中村孝; 髙橋航圭
  M&M2017材料力学カンファレンス, 08 Oct. 2017, Japanese, Oral presentation
  [Domestic Conference]
• PS32 Ti-22V-4Al における疲労き裂発生過程の観察
  鹿島彰太; 髙梨圭; 吉中奎貴; 中村孝; 藤村奈央; 髙橋航圭
  M&M2017材料力学カンファレンス, 07 Oct. 2017, Japanese, Poster presentation
  [Domestic Conference]
• PS29 Ti-22V-4Alにおける疲労き裂発生・進展特性
  髙梨圭; 鹿島彰太; 吉中奎貴; 中村孝; 藤村奈央; 髙橋航圭
  M&M2017材料力学カンファレンス, 07 Oct. 2017, Japanese, Poster presentation
  [Domestic Conference]
• PS27 AZ31の疲労特性に及ぼすScanning cyclic pressの影響
  大賀向陽; 藤村奈央; 中村孝; 髙橋航圭; 和島達希
  M&M2017材料力学カンファレンス, 07 Oct. 2017, Japanese, Poster presentation
  [Domestic Conference]
• J1310106 Scanning cyclic pressを用いた金属表層の組織制御
  中村孝; 藤村奈央; 水谷一皓; 髙栁伊吹; 和島達希
  日本機械学会年次大会, 04 Sep. 2017, Japanese, Oral presentation
  [Domestic Conference]
• Development of a new surface modification technique: scanning cyclic press and its application for the improvement of fatigue life of magnesium alloy
  Nao FUJIMURA; Takashi NAKAMURA; Tatsuki WAJIMA
  Seventh International Conference on Very High Cycle Fatigue, 05 Jul. 2017, English, Oral presentation
  [International presentation]
• Scanning Cyclic Pressを用いたマグネシウム合金AZ31の表面改質と疲労特性改善効果
  藤村奈央; 中村孝; 上遠野寛; 宮崎数則; 柴山環樹; 和島達希
  第33回疲労シンポジウム・第1回生体・医療材料シンポジウム, 12 Nov. 2016, Japanese, Oral presentation
  [Domestic Conference]
• OS0510 ポリイミドフィルムにおける膜厚方向破壊靱性の計測と評価
  南雲慶憲; 坂本悠介; 中村孝; 藤村奈央; 髙橋航圭
  M&M2016材料力学カンファレンス, 08 Oct. 2016, Japanese, Oral presentation
  [Domestic Conference]
• J1610203 ナノ微細化表面改質を目的とするScanning Cyclic Pressの開発と低炭素鋼への適用
  宮崎数則; 中村孝; 藤村奈央; 柴山環樹; 和島達希
  日本機械学会年次大会, 14 Sep. 2016, Japanese, Oral presentation
  [Domestic Conference]
• J1610202 Cyclic Press による低炭素鋼のナノ微細化表面改質
  中村孝; 中谷建太朗; 宮崎数則; 藤村奈央; 柴山環樹; 和島達希
  日本機械学会年次大会, 14 Sep. 2016, Japanese, Oral presentation
  [Domestic Conference]
• J0310101 SUS316NGの低サイクル疲労における表面性状変化とその機構
  西塚祐斗; 藤村奈央; 中田康敬; 上遠野寛; 中村孝
  日本機械学会年次大会, 14 Sep. 2016, Japanese, Oral presentation
  [Domestic Conference]
• OS0913-454 Distribution Characteristics of Surface Roughness on Low Cycle Fatigued Austenitic Stainless Steel
  FUJIMURA Nao; NAKAMURA Takashi; SHINKO Tomoki
  M&M材料力学カンファレンス, 21 Nov. 2015, Japanese
  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.
• G0301502 Evaluation of Surface Topography of SUS316NGin Low Cycle Fatigue under Two-Step Loading
  NAKATA Yasutaka; NISHIDZUKA Yuto; SHINKO Tomoki; FUJIMURA Nao; NAKAMURA Takashi
  Mechanical Engineering Congress, Japan, 13 Sep. 2015, Japanese
  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.
• 142 Evaluation of Surface Topography of SUS316NG in Low Cycle Fatigue Process
  NAKATA Yasutaka; SHINKO Tomoki; FUJIMURA Nao; NAKAMURA Takashi
  北海道支部講演会講演概要集, 27 Sep. 2014, Japanese
• OS2126 Effect of surface roughness on eddy current signals of austenite stainless steel
  HORIKAWA Noritaka; HAMAGUCHI Moe; FUJIMURA Nao; NAKAMURA Takashi
  M&M材料力学カンファレンス, 19 Jul. 2014, Japanese
  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.
• OS0820 Changes in Surface Roughness of Low Cycle Fatigued SUS316NG under Two-step Variable Strain Ranges
  FUJIMURA Nao; NAKAMURA Takashi; OGUMA Hiroyuki; NIINUMA Tadaki
  M&M材料力学カンファレンス, 19 Jul. 2014, Japanese
  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.
• OS0819 Quantitative Evaluation of Surface Relief of Low Cycle Fatigued Austenitic Stainless Steel
  SHINKO Tomoki; FUJIMURA Nao; NAKATA Yasutaka; OGUMA Hiroyuki; NAKAMURA Takashi
  M&M材料力学カンファレンス, 19 Jul. 2014, Japanese
  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.
• 繰返し負荷を受けたSUS316NGの表面粗さ測定に基づく損傷評価
  FUJIMURA NAO; SHINKO TOMOKI; NAKAMURA TAKASHI
  疲労シンポジウム講演論文集, 2014, Japanese
• Application of surface roughness parameters to the evaluation of low cycle fatigue damage in austenitic stainless steel
  Nao Fujimura; Takashi Nakamura; Hiroyuki Oguma
  American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP, 01 Dec. 2013
  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 Ra, surface profile maximum height R max, and maximum valley depth Rv. 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 Rmax and Rv are suitable for damage assessment. Copyright © 2013 by ASME.
• PS15 Changes in Surface Features of SUS316NG in Low Cycle Fatigue
  NIINUMA Tadaki; FUJIMURA Nao; SINKOU Tomoki; NAKAMURA Takashi; OGUMA Hiroyuki
  M&M材料力学カンファレンス, 12 Oct. 2013, Japanese
  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.
• SUS316NGの低サイクル疲労における表面性状の変化
  NIINUMA TADAKI; FUJIMURA NAO; SHINKO TOMOKI; NAKAMURA TAKASHI; OGUMA HIROYUKI
  日本機械学会M&2013M材料力学カンファレンス(CD-ROM), 10 Oct. 2013, Japanese
• 108 Low Cycle Fatigue Damage Evaluation of SUS316NG based on Surface Roughness
  SHINKO Tomoki; FUJIMURA Nao; NIINUMA Tadaki; OGUMA Hiroyuki; NAKAMURA Takashi
  北海道支部講演会講演概要集, 28 Sep. 2013, Japanese
• 表面粗さに基づいたSUS316NGの低サイクル疲労損傷評価
  SHINKO TOMOKI; FUJIMURA NAO; NIINUMA TADAKI; OGUMA HIROYUKI; NAKAMURA TAKASHI
  日本機械学会北海道支部講演会講演概要集, 28 Sep. 2013, Japanese
• 表面粗さ計測に基づくSUS316NGの疲労損傷評価と適用範囲
  FUJIMURA NAO; NAKAMURA TAKASHI; OGUMA HIROYUKI; NAKAJIMA TOMOHIRO; SHINKO TOMOKI
  日本材料学会学術講演会講演論文集, 17 May 2013, Japanese
• 214 Low Cycle Fatigue Properties and Damage Evaluation of SUS3 16NG
  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, 20 Oct. 2012, Japanese
• オーステナイト系ステンレス鋼の低サイクル疲労特性と損傷評価
  NAKASHIMA TOMOHIRO; FUJIMURA NAO; NIINUMA TADAKI; OGUMA HIROYUKI; NAKAMURA TAKASHI
  日本機械学会北海道支部講演会講演概要集, 20 Oct. 2012, Japanese
• OS2109 Effects of Surface Roughness on Low Cycle Fatigue Properties of Austenitic Stainless Steel
  FUJIMURA Nao; NAKAMURA Takashi; OGUMA Hiroyuki
  M&M材料力学カンファレンス, 22 Sep. 2012, Japanese
  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.
• オーステナイト系ステンレス鋼の低サイクル疲労特性に及ぼす繰返し予ひずみの影響と損傷評価
  FUJIMURA NAO; OGUMA HIROYUKI; NAKAMURA TAKASHI
  日本材料学会学術講演会講演論文集, 25 May 2012, Japanese
• 704 Damage Assessment on Low Cycle Fatigue Properties of Cyclic Pre-strained SUS316NG
  FUJIMURA Nao; OGUMA Hiroyuki; NAKAMURA Takashi
  学術講演会講演論文集, 25 May 2012, Japanese
• オーステナイト系ステンレス鋼の低サイクル疲労における表面粗さの影響
  FUJIMURA NAO; NAKAMURA TAKASHI; OGUMA HIROYUKI
  日本機械学会M&M2012材料力学カンファレンス(CD-ROM), 2012, Japanese
• Damage assessment on low cycle fatigue properties of cyclic pre-strained austenitic stainless steel
  Nao Fujimura; Hiroyuki Oguma; Takashi Nakamura
  American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP, 01 Dec. 2011
  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 straincontrolled tests were conducted under strain ratio Rε = -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: UFpre + 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 UF pre, 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. © 2011 by ASME.
• 繰返し予ひずみを受けたSUS316NGの低サイクル疲労特性
  NAKAJIMA TOMOHIRO; FUJIMURA NAO; OGUMA HIROYUKI; NAKAMURA TAKASHI
  日本機械学会北海道支部講演会講演概要集, 01 Oct. 2011, Japanese
• 220 Effects of Cyclic Pre-strain on Low Cycle Fatigue Properties of SUS316NG
  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, 01 Oct. 2011, Japanese
• 604 Effects of Cyclic Pre-strain on Low Cycle Fatigue Properties of Structural Steel for Nuclear Power Plants
  藤村 奈央; 小熊 博幸; 中村 孝
  北海道支部講演会講演概要集, 07 Nov. 2010, Japanese
• オーステナイト系ステンレス鋼の低サイクル疲労特性に及ぼす平均ひずみの影響
  FUJIMURA NAO; SUIMON ERI; NAKAMURA TAKASHI; OGUMA HIROYUKI
  日本材料学会学術講演会講演論文集, 21 May 2010, Japanese
• 534 Effects of Mean Strain on Low Cycle Fatigue Properties of Austenitic Stainless Steel
  FUJIMURA Nao; SUIMON Eri; NAKAMURA Takashi; OGUMA Hiroyuki
  学術講演会講演論文集, 21 May 2010, Japanese

• 機械加工学実習, 2024年, 学士課程, 工学部
• 物理学Ⅰ, 2024年, 学士課程, 全学教育

• THE SOCIETY OF MATERIALS SCIENCE, JAPAN
• THE JAPAN SOCIETY OF MECHANICAL ENGINEERS

• 表面改質技術Scanning Cyclic Pressによる金属の改質機構の解明
  科学研究費助成事業
  01 Apr. 2022 - 31 Mar. 2025
  藤村 奈央
  日本学術振興会, 基盤研究(C), 北海道大学, 22K03820
• 超高サイクル疲労における内部起点型破壊評価手法の確立
  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)
  Apr. 2021 - Mar. 2025
  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
  本研究の目的は，高強度金属材料に生じる超高サイクル疲労破壊のメカニズムを明らかにし，その評価法を構築することにある．析出硬化系ステンレス鋼（SUS630）と2種類のチタン合金（（α+β)型Ti-6Al-4Vおよびβ型Ti-22V-4Al）を対象として，材料内部に発生する微小き裂の挙動をSPring-8の放射光X線マイクロ/ナノCT（ビームライン：BL20XU）を用いて明らかにする．主な実施項目は，① 放射光X線マイクロ/ナノCT技術の高度化とin-situ疲労試験システムの開発，② 高強度鋼とチタン合金の２系統の材料における内部き裂発生・進展・停留挙動の解明，③ 超高サイクル疲労評価コードの開発，の３つである．2021年度は，このうち①に注力し，以下に示す２項目を行う計画とした．
  
  
  【放射光X線マイクロ/ナノCT技術の高度化】
  本研究で用いるマイクロCTおよびナノCTの分解能は，それぞれ約1μm，数百nmである．材料内部のき裂をマイクロCTで検出し，そのき裂をナノCTにより高倍率で非破壊観察する．ナノCTはSEMに匹敵する分解能の観察を可能とするが，従来のシステムでは一回の測定に約1時間が必要であり，疲労試験とCT撮影を多数繰返す場合には，この観察時間が研究の律速因子となっていた．そこで2021年度は，ナノCTの光学系（ゾーンプレート，位相板など）の最適化を試みた．その結果，従来の半分以下の時間でSEMレベルのイメージングが可能となった．
  【in-situ疲労試験システムの開発】
  ビームライン上に設置し，任意の負荷を試験片に与えられるin-situ疲労試験機の開発を行った．ピエゾアクチュエータ，ロードセル，アクリル製透過菅などで構成される小型サーボ疲労試験機を新たに開発し，ビームラインの試料台に直接設置して，疲労試験と観察を同時に行う実験手法を確立した．
  Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, 21H04529
• 表面改質技術Scanning Cyclic Pressによる金属の疲労特性改善
  科研費 若手研究
  Apr. 2019 - Mar. 2022
  藤村奈央
  日本学術振興会, Principal investigator, Competitive research funding
• Elucidation of the mechanism of very high cycle fatigue and the development of its evaluation method based on synchrotron radiation micro/nano- CT
  Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A)
  Apr. 2018 - Mar. 2022
  Nakamura Takashi
  The study aims to clarify the mechanism of very high cycle fatigue, which deteriorates the fatigue strength in the long-life regime due to the crack initiation inside materials. A nondestructive inspection technique of small cracks with a few micrometers was developed using synchrotron radiation X-ray computed tomography (SR-CT) at SPring-8. The SR-CT revealed that the crack growth rate of internal crack was much lower than that of surface crack and almost similar to that in a high vacuum. Fatigue fracture in the very high cycle regime can result from the extremely low internal crack growth rate, which propagates in a vacuum-like environment in materials.
  Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (A), Hokkaido University, 18H03748
• 繰返し負荷を受けた金属材料における表面性状の変化機構と疲労損傷評価
  科学研究費補助金（若手研究（B））
  Apr. 2017 - Mar. 2019
  藤村奈央
  日本学術振興会, Principal investigator, Competitive research funding
• Proposing of damage assessment method of seismic-loaded structural material based on surface roughness measurement
  Grants-in-Aid for Scientific Research
  Apr. 2014 - Mar. 2016
  FUJIMURA NAO
  In order to evaluate seismic-loaded damage based on the changes in surface roughness of material due to cyclic deformation, the effects of strain rate on the fatigue life and the changes in surface roughness of austenitic stainless steel was investigated. Low cycle fatigue tests were conducted under strain rate: 4%/sec, and the fatigue lives were compared with the fatigue lives obtained under 0.4%/sec. As a result, the fatigue lives at 4%/sec and 0.4%/sec were nearly equal. This shows that the tenfold increase in strain rate do not affect low cycle fatigue life. During the fatigue tests, specimen’s surface roughness was regularly measured using a laser scanning microscope. Surface roughness measurement showed that the changes in surface roughness at 4%/sec were little different from that at 0.4%/sec. Based on these results, the damage assessment method on the basis of surface roughness was discussed.
  Japan Society for the Promotion of Science, Grant-in-Aid for Research Activity Start-up, Hokkaido University, Principal investigator, Competitive research funding, 26889001