九州大学-研究者情報 [髙桑脩 (准教授) 工学研究院機械工学部門]

髙桑脩（たかくわおさむ）

データ更新日：2023.11.22

准教授　／　工学研究院機械工学部門

原著論文

1.	Osamu TAKAKUWA, Yuhei OGAWA, Ryunosuke MIYATA, Antagonistic fatigue crack acceleration/deceleration phenomena in Ni-based superalloy 718 under hydrogen-supply, Scientific Reports, 13, 6804, 2023.06, Mechanical properties of structural alloys, including Ni-based superalloy 718 (Alloy718), are degraded when hydrogen (H) is supplied: hydrogen embrittlement (HE). The presence of H notably deteriorates fatigue crack growth (FCG) property, which renders the growth rate much higher and shortens the lifetime of the components operating in the hydrogenating environment. Hence, the mechanisms behind such acceleration phenomenon in FCG should be understood comprehensively toward developing promising alloys resistant to hydrogen occlusion. In particular, Alloy718 has a meager resistance to HE, even regularly displaying superior mechanical and physical performances. Notwithstanding, the present study unveiled that the FCG acceleration by dissolved H in Alloy718 can be negligible. An abnormal deceleration of FCG can instead be pronounced by optimizing the metallurgical state, a hopeful prospect in Ni-based alloys applied to the hydrogenating environment..
2.	Yuhei OGAWA, Osamu TAKAKUWA, Kaneaki TSUZAKI, Solid-solution hardening by hydrogen in Fe-Cr-Ni-based austenitic steel: temperature and strain effects, Materials Science and Engineering A, 879, 145281, 2023.07.
3.	Yuhei OGAWA, Haruki NISHIDA, Osamu TAKAKUWA, Kaneaki TSUZAKI, Hydrogen-enhanced deformation twinning in Fe-Cr-Ni-based austenitic steel characterized by in-situ EBSD observatio, Materials Today Communications, 34, 105433, 2023.03.
4.	Valary TUBEI, Hiroyuki TODA, Worapol KENTANOND, Hiro FUJIHARA, Osamu TAKAKUWA, Akihisa TAKEUCHI, Masayuki UESUGI, Direct observation of three-dimensional short fatigue crack closure behavior in Ti-6Al-4V alloy using ultra-high-resolution X-ray microtomography, International Journal of Fatigue, 168, 107428, 2023.03.
5.	Yuhei OGAWA, Kohei Noguchi, Osamu TAKAKUWA, Criteria for hydrogen-assisted crack initiation in Ni-based superalloy 718, Acta Materialia, 229, 2022.05, The tensile mechanical properties of a Ni-based superalloy 718 uniformly precharged with ≈ 90 mass ppm hydrogen were investigated under a wide range of temperatures to shed light on the long-standing uncertainties surrounding the H-related embrittlement mechanisms of the material. The detrimental effect of H on ductility was found to be substantial in the near-ambient to high-temperature range, up to 300 °C, stemming from H-assisted microcrack initiations along annealing twin boundaries (ATBs) and crystallographic slip planes (SPs). Dynamic H-dislocation interaction, which has been thought to be a prerequisite for the onset of embrittlement, was found to be unimportant, as demonstrated by employing supplemental tests that incorporated prestraining and intermediate temperature changes. By combining the insights gained from the successfully designed test program, a new model for the nucleation process of H-induced fracturing was established..
6.	細田孝, 小川祐平, 髙桑脩, 本村達，細井日向, 松永久生, 析出硬化型高Mnオーステナイト鋼の水素脆化挙動に及ぼすNi含有量および時効熱処理の影響, 鉄と鋼, 108, 2, 2021.11.
7.	Yuhei OGAWA, Keiichiro IWATA, Saburo OKAZAKI, Masami NAKAMURA, Kazuki MATSUBARA, Osamu TAKAKUWA, Hisao MATSUNAGA, Fatigue crack-growth retardation after overloading in gaseous hydrogen: revisiting the effect of hydrogen on crack-tip plastic-zone development, Materials Letters, 308, 2021.10.
8.	野口耕平, 小川祐平, 髙桑脩, 松永久生, 内部・外部水素によるNi基超合金718の延性低下機構とその温度依存性, 鉄と鋼, 107, 11, 2021.07.
9.	Valary Tubei, Hiroyuki Toda, Meysam Hassanipour, Kyosuke Hirayama, Osamu Takakuwa, Akihisa Takeuchi, Masayuki Uesugi, 3D short fatigue crack closure behavior in Ti-6Al-4V alloy investigated using in-situ high resolution synchrotron X-ray tomography, Engineering Fracture Mechanics, 10.1016/j.engfracmech.2021.107755, 249, 2021.05.
10.	Yuhei Ogawa, Kensuke Umakoshi, Masami Nakamura, Osamu Takakuwa, Hisao Matsunaga, Hydrogen-assisted, intergranular, fatigue crack-growth in ferritic iron Influences of hydrogen-gas pressure and temperature variation, International Journal of Fatigue, 10.1016/j.ijfatigue.2020.105806, 140, 2020.11, [URL], Hydrogen-assisted, fatigue crack-growth in pure iron within a low stress-intensity range was ascribed to the emergence of intergranular fracture, the significance of which was emphasized by increased hydrogen-gas pressure, while conversely mitigated by an elevation of test temperature. Based on conventional thermodynamic theory, a single parameter, GB hydrogen-coverage (θ_x), was used to derive a systematic, unified evaluation of such a complex reliance on the dual environmental variables. Furthermore, post-mortem microscopic analyses of the crack-wake deformation microstructures were employed to elucidate the contribution of dislocation activity regarding the triggering of IG fracture, which also varied significantly with the alteration of θ_x..
11.	Kevinsanny, Saburo Okazaki, Osamu Takakuwa, Yuhei Ogawa, Yusuke Funakoshi, Hideto Kawashima, Saburo Matsuoka, Hisao Matsunaga, Defect tolerance and hydrogen susceptibility of the fatigue limit of an additively manufactured Ni-based superalloy 718, International Journal of Fatigue, 10.1016/j.ijfatigue.2020.105740, 139, 2020.10, [URL], The sensitivity of the fatigue limit of an additively manufactured, Ni-based superalloy 718 sample to surface finishing conditions and solute hydrogen was investigated via a series of tension–compression fatigue tests. The results revealed that neither defects nor hydrogen diminished the fatigue limit of the sample. The high defect tolerance of this material is attributed to a large unit of crack initiation in the coarse grain, which eclipses the detrimental effect of AM-process-induced defects. Also, from the results, it is inferred that hydrogen has little effect on the crack propagation rate and the crack growth threshold of the material..
12.	Yuhei Ogawa, Hyuga Hosoi, Kaneaki Tsuzaki, Timothée Redarce, Osamu Takakuwa, Hisao Matsunaga, Hydrogen, as an alloying element, enables a greater strength-ductility balance in an Fe-Cr-Ni-based, stable austenitic stainless steel, Acta Materialia, 10.1016/j.actamat.2020.08.024, 199, 181-192, 2020.10, [URL], After pre-charging at the gaseous phase with a concentration of ~7,000 at. ppm, solute hydrogen was discovered to have an abnormal effect on both the strength and ductility enhancement of a commercially-available, Fe-24Cr-19Ni-based, stable austenitic stainless steel that had been subjected to tensile testing at various strain-rates. Specifically, the impact of hydrogen on material strength was accompanied by amplified yield and flow stresses, as well as tensile strength, while the improvement in ductility featured extended uniform elongation and strain-to-fracture, both of which became more pronounced as hydrogen concentration intensified. The product between tensile strength and uniform elongation served as indicators of the strength-ductility balance, at which hydrogen maximally optimized the indicator at the particular intermediate strain-rate. The yield/flow stress augmentations were interpreted in terms of solid-solution strengthening, whereas the enhanced ductility was primarily ascribed to the facilitation of mechanical twinning, whereby dynamic hydrogen-dislocation interaction exerted a critical influence as was indirectly revealed by supplemental stress-relaxation experiments..
13.	Yuhei Ogawa, Osamu Takakuwa, Saburo Okazaki, Yusuke Funakoshi, Saburo Matsuoka, Hisao Matsunaga, Hydrogen-assisted fatigue crack-propagation in a Ni-based superalloy 718, revealed via crack-path crystallography and deformation microstructures, Corrosion Science, 10.1016/j.corsci.2020.108814, 174, 2020.09, [URL], Fatigue crack-growth (FCG) of Ni-based superalloy 718 was investigated under gaseous hydrogen environment (external hydrogen) and uniformly pre-charged state (internal hydrogen). Under external hydrogen, intergranular fracture predominated, whereas dislocation slip-band or twin boundary fracture were prevalent under internal hydrogen. This failure mode divergence encompassed unique characteristics of macroscale FCG response, leading to both cycle- and time-dependent cracking. The intergranular cracking was ascribed to short-circuit diffusion of hydrogen along grain boundaries. Meanwhile, the material's inherently inhomogeneous deformation mode exerts harmfulness when hydrogen was uniformly distributed inside the specimen, causing slip-bands or twin boundaries to become the weakest links for fracture..
14.	松岡三郎, 髙桑脩,　岡崎三郎，吉田聡子，山辺純一郎，松永久生, 転位の水素トラップ占有率に基づく高圧水素ガス中における水素拡散特性と各種破壊特性の統一的解釈－オーステナイト系ステンレス鋼と低合金鋼, 日本機械学会論文集, 88, 888, 2020.07.
15.	Osamu Takakuwa, Yuhei Ogawa, Saburo Okazaki, Masami Nakamura, Hisao Matsunaga, A mechanism behind hydrogen-assisted fatigue crack growth in ferrite-pearlite steel focusing on its behavior in gaseous environment at elevated temperature, Corrosion Science, 10.1016/j.corsci.2020.108558, 168, 2020.05, [URL], Hydrogen-assisted fatigue crack growth in gaseous environment was comparatively examined at room temperature (RT) and 423 K, based on analysis of the deformation structure evolution around crack-wakes using scanning electron microscopy techniques. In hydrogen-gas at RT, the propagating crack displayed weakly-evolved dislocation arrangement, accompanied by a significant acceleration of fatigue crack growth. However, in hydrogen-gas at 423 K, the crack-wake plasticity was well-evolved and analogous to that observed in an inert environment. This apparent recovery of deformation micro structure coincided with suppressed crack growth acceleration, the rationale for which can be interpreted by the trapping/de-trapping equilibrium between hydrogen and dislocations..
16.	Kentaro Wada, Junichiro Yamabe, Yuhei Ogawa, Osamu Takakuwa, Takashi Iijima, Hisao Matsunaga, Comparative study of hydrogen-induced intergranular fracture behavior in Ni and Cu–Ni alloy at ambient and cryogenic temperatures, Materials Science and Engineering A, 10.1016/j.msea.2019.138349, 766, 2019.10, [URL], In order to clarify the contribution of dislocation‒hydrogen interaction on the hydrogen embrittlement (HE) of pure Ni and of Cu‒55 wt% Ni binary alloy, slow strain rate tensile (SSRT) tests were conducted at room temperature (RT) and at 77 K on hydrogen-precharged specimens. Regarding the SSRT test at RT, hydrogen increased the flow stress and induced intergranular fracture in both pure Ni and Cu–Ni alloy. Furthermore, based on scanning transmission electron microscopy investigations, it was suggested that the evolution of dislocation structures had been enhanced by hydrogen, but only in the case of pure Ni. At 77 K, the ductility of pure Ni was degraded by hydrogen, whereas that of Cu–Ni alloy was not. The difference in temperature dependence of the dislocation‒hydrogen interaction between pure Ni and Cu–Ni alloy was discussed, based on the previously proposed HE mechanisms..
17.	Jean-Gabriel Sezgin, Osamu Takakuwa, Hisao Matsunaga, Junichiro Yamabe, Simulation of the effect of internal pressure on the integrity of hydrogen pre-charged BCC and FCC steels in SSRT test conditions, Engineering Fracture Mechanics, 10.1016/j.engfracmech.2019.106505, 2019.07.
18.	Domas Birenis, Yuhei Ogawa, Hisao Matsunaga, Osamu Takakuwa, Annett Thøgersen, Øystein Prytz, Junichiro Yamabe, Hydrogen-Assisted Crack Propagation of α-iron during Elasto-Plastic Fracture Toughness Test, Materials Science & Engineering A, 10.1016/j.msea.2019.04.084, 756, 396-404, 2019.05.
19.	Eric Maire, Stanislas Grabon, Jerome Adrien, Pablo Lorenzino, Yuki Asanuma, Osamu Takakuwa, Hisao Matsunaga, Role of Hydrogen-Charging on Nucleation and Growth of Ductile Damage in Austenitic Stainless Steels, Materials, 10.3390/ma12091426, 12, 2019.01.
20.	Yuhei Ogawa, Osamu Takakuwa, Saburo Okazaki, Saburo Matsuoka, Hisao Matsunaga, Change of crack initiation and propagation modes in hydrogenrelated failure of a precipitation-strengthened NI-based superalloy 718 under internal and external hydrogen conditions, ASME 2019 Pressure Vessels and Piping Conference, PVP 2019 Materials and Fabrication, 10.1115/PVP2019-93204, 2019.01, [URL], The influences of internal and external hydrogen on the tensile ductility loss and fracture behaviors of a precipitationhardened Ni-based superalloy 718 were investigated via slow strain rate tensile (SSRT) testing under hydrogen pre-charged conditions (internal hydrogen) or in gaseous hydrogen environments (external hydrogen). Severe degradation of tensile ductility was confirmed both in internal and external hydrogen conditions, and the degree of such degradation became more significant with increasing hydrogen content or hydrogen gas pressures. Moreover, the loss of tensile ductility was more pronounced in internal hydrogen conditions than external hydrogen environments. In association with such degradation of macroscopic tensile ductility, hydrogen also altered fracture mode from ductile microvoid coalescence to some brittle appearances. Whereas typical intergranular fracture combined with a decent fraction of quasi-cleavage fracture appeared on the fracture surface formed in external hydrogen environments, several types of unique faceted characteristics were found on the fracture surfaces in internal hydrogen conditions. The detailed observation of the mid-sectioned lateral surfaces of post-mortem samples successfully revealed that the observed distinctions consisted of the fracture along grain boundaries and {111} crystallographic planes including annealing twin boundaries, besides the frequency of the cracking along twin boundaries evidently increased at higher hydrogen concentration. On the basis of the series of experimental results, the initiation and propagation mechanisms of those hydrogen-induced cracks are discussed in terms of hydrogen distribution, intrinsic deformation character of the material itself as well as the alteration of plastic deformation mode caused by dissolved hydrogen..
21.	Kentaro Wada, Junichiro Yamabe, Yuhei Ogawa, Osamu Takakuwa, Takashi Iijima, Hisao Matsunaga, Fracture and deformation behavior in slow-strain-rate tensile testing of Cu-Ni alloy with internal hydrogen, ASME 2019 Pressure Vessels and Piping Conference, PVP 2019 Materials and Fabrication, 10.1115/PVP2019-93477, 2019.01, [URL], The effect of hydrogen on the deformation and fracture behavior in pure Cu, pure Ni and Cu-Ni alloy was studied via tensile tests of H-charged, smooth and circumferentially-notched specimens at room temperature (RT) and 77 K. Hydrogendiffusion properties were determined by the desorption method. To obtain a uniform hydrogen concentration in the H-charged specimens, specimens were exposed to 100-MPa hydrogen gas at 543 K for 200 h, based on the determined hydrogen diffusivity. In tensile tests of smooth pure Ni and Cu-Ni alloy specimens at RT, common hydrogen effects were detected, namely, an increase in yield and flow stresses-a hardening effect; and a ductility loss that was accompanied by a change in fracture surface from ductile to brittle feature-an embrittling effect. With regard to the embrittling effect, the pure Ni and Cu-Ni alloy showed different fracture-surface morphologies at RT; the pure Ni showed an intergranular (IG) surface and the Cu-Ni alloy surface was flat. However, a number of IG cracks were detected beneath the fracture surfaces on the smooth Cu-Ni alloy. The tensile tests of the H-charged smooth specimens at 77 K yielded an IG surface for the pure Ni and a ductile fracture surface with dimples in the Cu-Ni alloy. In contrast, tensile tests of the Hcharged, notched specimens at RT demonstrated clear IG fractures for the pure Ni and Cu-Ni alloy. These facts indicate that IG cracking was the first step in the embrittling process for the pure Ni and Cu-Ni alloy, and IG cracking was accompanied by a large plastic deformation that formed the flat surface (unclear IG surface) for the smooth Cu-Ni alloy. Considering that the HE of both pure Ni and Cu-Ni alloy was related to IG cracking, possible mechanisms were discussed and tensile tests performed at 77 K suggested two possibilities: (I) interaction between hydrogen-moving dislocation is more important in the HE process of the Cu-Ni alloy compared to the pure Ni; (II) hydrogen transportation towards grain boundaries are required to cause the IG fracture in the Cu-Ni alloy..
22.	Y. Ogawa, O. Takakuwa, Saburo Okazaki, Koichi Okita, Yusuke Funakoshi, H. Matsunaga, Saburo Matsuoka, Pronounced transition of crack initiation and propagation modes in the hydrogen-related failure of a Ni-based superalloy 718 under internal and external hydrogen conditions, Corrosion Science, 10.1016/j.corsci.2019.108186, 2019.01, [URL], The role of hydrogen in tensile ductility loss and on the fracture behaviours of Ni-based superalloy 718 was investigated via tensile tests under hydrogen-charged conditions (internal hydrogen) or in gaseous hydrogen environments (external hydrogen), in combination with post-mortem analyses of fractured samples using electron microscopy techniques. Whereas intergranular fracture was responsible for material degradation under external hydrogen, the failure modes under internal hydrogen conditions were primarily dominated by cracking along slip planes or twin boundaries. The mechanisms of crack initiation and propagation are extensively discussed in terms of hydrogen distribution, intrinsic deformation character of the material and hydrogen-modified dislocation behavior..
23.	Osamu Takakuwa, Yuhei Ogawa, Saburo Okazaki, Hisao Matsunaga, Saburo Matsuoka, Temperature dependence of fatigue crack growth in low-carbon steel under gaseous hydrogen, ASME 2019 Pressure Vessels and Piping Conference, PVP 2019 Materials and Fabrication, 10.1115/PVP2019-93451, 2019.01, [URL], In order to elucidate the temperature dependence of hydrogen-assisted fatigue crack growth (HAFCG), the fatigue crack growth (FCG) test was performed on low-carbon steel JISSM490B according to ASTM E647 using compact tension (CT) specimen under 0.7 MPa (≈ 0.1 ksi) hydrogen-gas at room temperature (RT: 298 K (≈ 77 °F)) and 423 K (≈ 302 °F) at stress intensity factor range of ΔK = 30 MPa m^1/2 (≈ 27 ksi in^1/2). Electron backscatter diffraction (EBSD) observation was performed on the mid-thick section of CT specimen in order to investigate change in plasticity around the crack wake in gaseous hydrogen environment and how it changes due to temperature elevation. The obtained results showed the higher temperature, the lower intense of HAFCG as reported in our previous article. Plasticity around the crack wake became less in gaseous hydrogen environment, especially tested at 298 K. The propensity of the results obtained at higher temperature (423 K) can be separated into two cases: (i) intense plasticity occurs like tested in air, (ii) crack propagates straighter accompanying less plasticity like tested in gaseous hydrogen environment at 298 K. This implies macroscopic FCG rate is determined by combination of microscopic FCG rate in the case (i) and case (ii)..
24.	Kevinsanny, Saburo Okazaki, Osamu Takakuwa, Yuhei Ogawa, Koichi Okita, Yusuke Funakoshi, Junichiro Yamabe, Saburo Matsuoka, Hisao Matsunaga, Effect of defects and hydrogen on the fatigue limit of Ni-based superalloy 718, Fatigue Design 2019 - 8th edition of the International Conference on Fatigue Design Procedia Structural Integrity, 10.1016/j.prostr.2019.12.034, 19, 312-319, 2019.01, [URL], Tension-compression fatigue tests were performed on two types of Ni-based superalloy 718 with different microstructures, to which small artificial defects of various shapes and sizes were introduced. Similar tests were also conducted on hydrogen-charged specimens with defects, with a solute hydrogen content ranging from 26.3 to 91.0 mass ppm. In the non-charged specimens in particular, the fatigue strength susceptibility to defects varied significantly according to the type of microstructural morphology, i.e., a smaller grain size made the alloy more vulnerable to defects. The fatigue limit as a small-crack threshold was successfully predicted using the √area parameter model. Depending on the size of defects, the fatigue limit was calculated in relation to three phases: (i) harmless-defect regime, (ii) small-crack regime and (iii) large-crack regime. Such a classification enabled comprehensive fatigue limit evaluation in a wide array of defects, taking into consideration (a) the defect size over a range of small crack and large crack and (b) the characteristics of the matrix represented by grain size and hardness. In addition, the effect of defects and hydrogen on fatigue strength will be comprehensively discussed, based on a series of experimental results. ..
25.	Osamu Takakuwa, Yuhei Ogawa, Junichiro Yamabe, Hisao Matsunaga, Hydrogen-induced ductility loss of precipitation-strengthened Fe-Ni-Cr-based superalloy, Materials Science and Engineering A, 10.1016/j.msea.2018.10.040, 739, 335-342, 2019.01, [URL], A brittle-like faceted morphology of a precipitation-strengthened Fe-Ni-Cr-based superalloy after charging via exposure to high-pressure hydrogen gas (100 MPa) at elevated temperature (543 K) was interpreted based on multiple electron microscopy observations: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) imaging. The observation results revealed that the brittle-like facets were derived from intergranular cracking accompanied by hydrogen-assisted microvoid nucleation at the grain boundaries (GBs). Deformation twinning also played a crucial role in triggering the final grain boundary separation due to local stress concentration at its intersection with the GBs after severe strain hardening; such a process has not yet been considered to explain the hydrogen-induced ductility loss of this type of alloy..
26.	Yuhei Ogawa, Saburo Okazaki, Osamu Takakuwa, Hisao Matsunaga, The roles of internal and external hydrogen in the deformation and fracture processes at the fatigue crack tip zone of metastable austenitic stainless steels, Scripta Materialia, 10.1016/j.scriptamat.2018.08.003, 157, 95-99, 2018.12, [URL], Fatigue crack growth (FCG) tests were performed with two types of metastable austenitic stainless steels having different austenite phase stabilities under hydrogen-precharged conditions (internal hydrogen) and in gaseous hydrogen environments (external hydrogen). The materials showed a peculiarly slower FCG rate with internal hydrogen than with external hydrogen even though the hydrogen concentration was much higher under the internal hydrogen conditions. The results are interpreted in terms of hydrogen-modified plastic deformation character comprising inhibited cross-slipping or enhanced deformation twinning in combination with the sequence of hydrogen penetration and strain-induced α′ martensite formation in the local region surrounding the fatigue crack tip..
27.	Domas Birenis, Yuhei Ogawa, Hisao Matsunaga, Osamu Takakuwa, Junichiro Yamabe, Øystein Prytz, Annett Thøgersen, Interpretation of hydrogen-assisted fatigue crack propagation in BCC iron based on dislocation structure evolution around the crack wake, Acta Materialia, 10.1016/j.actamat.2018.06.041, 156, 245-253, 2018.09, [URL], A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAFCG. The FCG in gaseous hydrogen showed two distinguishing regimes corresponding to the unaccelerated regime at a relatively low stress intensity factor range, ΔK, and the accelerated regime at a relatively high ΔK. The fracture surface in the unaccelerated regime was covered by ductile transgranular and intergranular features, while mainly quasi-cleavage features were observed in the accelerated regime. The EBSD and ECCI results demonstrated considerably lower amounts of plastic deformation, i.e., less plasticity, around the crack path in the accelerated regime. The TEM results confirmed that the dislocation structure immediately beneath the crack in the accelerated regime showed significantly lower development and that the fracture surface in the quasi-cleavage regions was parallel to the {100} plane. These observations suggest that the HAFCG in pure iron may be attributed to “less plasticity” rather than “localized plasticity” around the crack tip..
28.	Saburo Matsuoka, Osamu Takakuwa, Saburo Okazaki, Michio Yoshikawa, Junichiro Yamabe, Hisao Matsunaga, Peculiar temperature dependence of hydrogen-enhanced fatigue crack growth of low-carbon steel in gaseous hydrogen, Scripta Materialia, 10.1016/j.scriptamat.2018.05.035, 154, 101-105, 2018.09, [URL], The peculiar temperature dependence of hydrogen-enhanced fatigue crack growth (HEFCG) of low-carbon steel in hydrogen gas was successfully interpreted in terms of ‘trap-site occupancy’ of hydrogen. HEFCG decreased with increasing temperature in hydrogen gas at 0.7 MPa and 298 to 423 K due to lower occupancy of trap sites at higher temperatures. In hydrogen gas at 90 MPa, HEFCG was insensitive to the temperature because most of the trap sites were occupied by hydrogen, regardless of the temperature. Trap sites with a binding energy of 47 kJ/mol, corresponding approximately to the dislocation core, dominated the temperature dependence of HEFCG..
29.	Yuhei Ogawa, Domas Birenis, Hisao Matsunaga, Osamu Takakuwa, Junichiro Yamabe, Øystein Prytz, Annett Thøgersen, The role of intergranular fracture on hydrogen-assisted fatigue crack propagation in pure iron at a low stress intensity range, Materials Science and Engineering A, 10.1016/j.msea.2018.07.014, 733, 316-328, 2018.08, [URL], Hydrogen-assisted fatigue crack growth (HAFCG) in pure iron at a relatively low stress intensity range exhibits brittle-like intergranular (IG) fracture, while the macroscopic crack acceleration is not significant. The present study focuses on the mechanism of IG fracture in terms of the microscopic deformation structures near the crack propagation paths. We found that the IG fracture is attributed to hydrogen-enhanced dislocation structure evolution and subsequent microvoid formation along the grain boundaries. The impact of such IG cracking on the macroscopic fatigue crack growth (FCG) acceleration is evaluated according to the dependency of IG fracture tendency on the hydrogen gas pressure during testing. It is demonstrated for the first time that increased hydrogen pressure results in a larger fraction of IG fracture and correspondingly faster FCG. On the other hand, the gaseous hydrogen environment also has a positive role in decelerating the FCG rate relative to air due to the absence of oxygen and water vapor. The macroscopic crack propagation rate in hydrogen gas is eventually determined by the competition between the said positive and negative influences..
30.	Osamu Takakuwa, Junichiro Yamabe, Hisao Matsunaga, Yoshiyuki Furuya, Saburo Matsuoka, Comprehensive Understanding of Ductility Loss Mechanisms in Various Steels with External and Internal Hydrogen, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 10.1007/s11661-017-4323-3, 48, 11, 5717-5732, 2017.11, [URL], Hydrogen-induced ductility loss and related fracture morphologies are comprehensively discussed in consideration of the hydrogen distribution in a specimen with external and internal hydrogen by using 300-series austenitic stainless steels (Types 304, 316, 316L), high-strength austenitic stainless steels (HP160, XM-19), precipitation-hardened iron-based super alloy (A286), low-alloy Cr-Mo steel (JIS-SCM435), and low-carbon steel (JIS-SM490B). External hydrogen is realized by a non-charged specimen tested in high-pressure gaseous hydrogen, and internal hydrogen is realized by a hydrogen-charged specimen tested in air or inert gas. Fracture morphologies obtained by slow-strain-rate tensile tests (SSRT) of the materials with external or internal hydrogen could be comprehensively categorized into five types: hydrogen-induced successive crack growth, ordinary void formation, small-sized void formation related to the void sheet, large-sized void formation, and facet formation. The mechanisms of hydrogen embrittlement are broadly classified into hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). In the HEDE model, hydrogen weakens interatomic bonds, whereas in the HELP model, hydrogen enhances localized slip deformations. Although various fracture morphologies are produced by external or internal hydrogen, these morphologies can be explained by the HELP model rather than by the HEDE model..
31.	Yuhei Ogawa, Domas Birenis, Hisao Matsunaga, Annett Thøgersen, Øystein Prytz, Osamu Takakuwa, Junichiro Yamabe, Multi-scale observation of hydrogen-induced, localized plastic deformation in fatigue-crack propagation in a pure iron, Scripta Materialia, 10.1016/j.scriptamat.2017.06.037, 140, 13-17, 2017.11, [URL], In order to study the influence of hydrogen on plastic deformation behavior in the vicinity of the fatigue crack-tip in a pure iron, a multi-scale observation technique was employed, comprising electron channeling contrast imaging (ECCI), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The analyses successfully demonstrated that hydrogen greatly reduces the dislocation structure evolution around the fracture path and localizes the plastic flow in the crack-tip region. Such clear evidence can reinforce the existing model in which this type of localized plasticity contributes to crack-growth acceleration in metals in hydrogen atmosphere, which has not yet been experimentally elucidated..
32.	Hisao Matsunaga, Osamu Takakuwa, Junichiro Yamabe, Saburo Matsuoka, Hydrogen-enhanced fatigue crack growth in steels and its frequency dependence, Philosophical Transactions of the Royal Society A, 10.1098/rsta.2016.0412, 375, 2098, 1-14, 2017.06.
33.	髙桑脩, 宮坂晃輔, 祖山均, 岩石のメカノルミネッセンスによる発光における支配因子の検討, 日本機械学会論文集, 10.1299/transjsme.16-00169, 82, 842, 2017.06.
34.	Hitoshi Soyama, Kazuya Nagasaka, Osamu Takakuwa, Akima Naito, Optimum injection pressure of a cavitating jet for introducing compressive residual stress into stainless steel, Journal of Power and Energy Systems, 10.1299/jpes.6.63, 6, 2, 63-75, 2017.06.
35.	Junichiro Yamabe, Osamu Takakuwa, Hisao Matsunaga, Hisatake Itoga, Saburo Matsuoka, Hydrogen diffusivity and tensile-ductility loss of solution-treated austenitic stainless steels with external and internal hydrogen, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2017.04.055, 42, 18, 13289-13299, 2017.05.
36.	Osamu Takakuwa, Fumio Takeo, Mitsuru Sato, Hitoshi Soyama, Using cavitation peening to enhance the fatigue strength of duralumin plate containing a hole with rounded edges, Surface & Coatings Technology, 10.1016/j.surfcoat.2016.08.087, 307, 200-205, 2016.12.
37.	Osamu Takakuwa, Takuya Fujisawa, Hitoshi Soyama, Experimental verification of the hydrogen concentration around a crack tip using spot X-ray diffraction, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2016.10.083, 41, 48, 23188-23195, 2016.12, [URL], We employed X-ray diffraction using collimated X-rays to quantitatively evaluate the local hydrogen concentration behavior in metals. Hydrogen concentrating around a crack tip significantly accelerates crack propagation, i.e., hydrogen embrittlement. In order to clarify the mechanism leading to this, the local hydrogen concentration behavior, i.e., at a crack tip, was evaluated by numerical analysis and experimental measurements. Although thermal desorption analysis can be used to evaluate the total hydrogen content in metals, it cannot be applied to local areas. Microprint methods, which use chemical reactions between hydrogen and coated elements cannot quantitatively evaluate the hydrogen content. The present study takes account of hydrogen-induced strain, and X-ray diffraction in a confined area was employed to detect variations in lattice spacing before and after hydrogen charging. Using X-ray diffraction applied to a small area, we demonstrate that the hydrogen concentrates in the vicinity of the crack, i.e., at the elastic–plastic boundary..
38.	Osamu Takakuwa, Masaaki Nakai, Kengo Narita, Mitsuo Niinomi, Kazuhiro Hasegawa, Hitoshi Soyama, Enhancing the durability of spinal implant fixture applications made of Ti-6Al-4V ELI by means of cavitation peening, International Journal of Fatigue, 10.1016/j.ijfatigue.2016.07.021, 92, 360-367, 2016.11.
39.	熊谷直輝, 髙桑脩, 祖山均, キャビテーションピーニングによるクロムモリブデン鋼製ボルトの遅れ破壊耐性の向上, 日本機械学会論文集, 10.1299/transjsme.16-00111, 82, 842, 2016.10.
40.	Osamu Takakuwa, Naoki Kumagai, Hitoshi Soyama, Improvement of the resistance to delayed fracture in high-strength bolt made of Cr-Mo steel by means of cavitation peening, Proceedings of the 6th International Conference on Laser Peening and Related Phenomena, 2016.10.
41.	Hitoshi Soyama, Osamu Takakuwa, Improvement of fatigue life of welded parts by peening employing cavitating jet and water jet, Proceedings of the 11th Pacific Rim International Conference on Water Jet Technology, 2016.10.
42.	髙桑脩, 祖山均, 水噴流を利用した各種ピーニングによるステンレス鋼溶接部の疲労強度向上, 噴流工学, 33, 1, 4-9, 2016.06.
43.	髙桑脩, 内藤暁馬, 祖山均, 水噴流加速式循環型ショットピーニングによるステンレス鋼の下限応力拡大係数範囲の増大, 噴流工学, 32, 1, 4-9, 2016.06.
44.	Mitsuru Sato, Osamu Takakuwa, Masaaki Nakai, Mitsuo Niinomi, Fumio Takeo, Hitoshi Soyama, Using cavitation peening to improve the fatigue life of titanium alloy Ti-6Al-4V manufactured by electron beam melting, Materials Sciences and Applications, 10.4236/msa.2016.74018, 7, 4, 181-191, 2016.04.
45.	O. Takakuwa, H. Soyama, Preventing hydrogen diffusion in stainless steel by cavitating jet in air, 23rd International Conference on Water Jetting 2016 23rd International Conference on Water Jetting 2016, 215-221, 2016.01, Hydrogen diffuses quickly in metals, making them brittle and accelerating crack propagation. Controlling the mobility of hydrogen in metals is one of the key factors used to prevent hydrogen embrittlement. If the hydrogen diffusion, i.e., mobility, at the surface can be suppressed, the resistance to hydrogen embrittlement will be greater. The diffusion in austenitic stainless steel can be suppressed by introducing a compressive stress near the surface and that the hydrogen diffusion coefficient is thereby reduced to one-tenth from 1.13×10^-15 s/mm² to 1.23×10^-16 s/mm², obtained by SIMS..
46.	髙桑脩, 祖山均, 機械的表面改質によるステンレス鋼の水素脆化抑止, JCOSSAR2015論文集, 128-133, 2015.10.
47.	Osamu Takakuwa, Hitoshi Soyama, Preventing hydrogen embrittlement in stainless steel by means of compressive stress induced by cavitation peening, The Journal of Engineering, 10.1049/joe.2015.0065, 4, 2015.07.
48.	Hitoshi Soyama, Naoki Kumagai, Osamu Takakuwa, Suppression of fatigue crack propagation of duralumin by cavitation peening, The Journal of Engineering, 10.1049/joe.2015.0066, 3, 2015.07.
49.	髙桑脩, 眞野優太, 祖山均, キャビテーションピーニングによるオーステナイト系ステンレス鋼への水素侵入抑止, 日本機械学会論文集, 10.1299/transjsme.14-00638, 81, 824, 2015.04.
50.	髙桑脩, 眞野優太, 祖山均, 微小圧子押込み試験によるオーステナイト系ステンレス鋼の水素侵入量評価, 日本機械学会論文集, 10.1299/transjsme.14-00426, 81, 823, 2015.03.
51.	Osamu Takakuwa, Akihiko Chiba, Hitoshi Soyama, Movement of dislocations in the sub-surface of a polycrystalline metal by cavitation peening observed by transmission electron microscopy, Materials Sciences and Applications, 6, 2, 140-144, 2015.02.
52.	Osamu Takakuwa, Hitoshi Soyama, Effect of residual stress on the corrosion behavior of austenitic stainless steel, Advances in Chemical Engineering and Science, 10.4236/aces.2015.51007, 5, 1, 62-71, 2015.01.
53.	Osamu Takakuwa, Yuta Mano, Hitoshi Soyama, The Interaction between hydrogen and surface stress in stainless steel, Proceedings of the 12th International Conference on Material Science and Engineering Technology, 1285-1289, 2014.12.
54.	Hitoshi Soyama, Osamu Takakuwa, Reduction of hydrogen embrittlement cracking of stainless steel SUS316L by cavitation peening, Proceedings of the International Congress (APCF/SIF-2014), 401-405, 2014.12.
55.	Osamu Takakuwa, Yuta Mano, Hitoshi Soyama, Effect of indentation load on Vickers hardness of austenitic stainless steel after hydrogen charging, Proceedings of the ASME2014 Pressure Vessels & Piping Conference, 2014.07.
56.	髙桑脩, 祖山均, 脊椎固定用インプラントロッドへの水噴流加速式循環型ショットピーニングの適用, 噴流工学, 31, 1, 20-27, 2014.06.
57.	Osamu Takakuwa, Yuta Mano, Hitoshi Soyama, Effect of hydrogen on the micro- and macro-strain near the surface of austenitic stainless steel, Advanced Materials Research, 936, 1298-1302, 2014.06.
58.	Osamu Takakuwa, Yuta Mano, Hitoshi Soyama, Increase in the local yield stress near the surface of austenitic stainless steel due to invasion by hydrogen, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2014.01.190, 39, 11, 6095-6103, 2014.04.
59.	髙桑脩, 左奈田一将, 祖山均, 荷重制御型平面曲げ式疲労試験機による表面改質層における疲労亀裂進展挙動の定量的評価, 日本機械学会論文集, 10.1299/transjsme.2014smm0022, 80, 810, 2014.02.
60.	K. Hasuda, O. Takakuwa, H. Soyama, Effect of load current density during the production of Cu₂O/Cu solar cells by anodic oxidation on film quality and output power, Solid-State Electronics, 10.1016/j.sse.2013.09.007, 91, 130-136, 2014.01, [URL], In this paper, we demonstrate that improvements in the Cu₂O film quality and consequently the output power of Cu₂O/Cu solar cells can be achieved by decreasing the load current density used in production of the film by anodic oxidation. Cu₂O films were fabricated under various oxidizing condition in an aqueous solution of CuSO₄, NaCl and LiCl at a temperature of 86 C. The load current density and loading time were varied. The variations in the output power of the solar cells with film thickness and quality, determined by the electrical resistance, the amount of CuCl in the Cu₂O film, and the crystal quality and crystal grain size of the Cu₂O, were evaluated. From the maximum value of the output power, the best film thickness of the Cu₂O film was found to be about 8-10 μm. Moreover, Cu₂O films with lower electrical resistance, less CuCl, greater crystal quality and larger crystal grain size led to more powerful solar cells, i.e., higher output power. These Cu₂O films were obtained by decreasing the load current density used in fabrication. The highest output power achieved (with load current density = 1.25 mA/cm², loading time = 8 h) was 702 nW..
61.	Satoshi Nishimura, Osamu Takakuwa, Hitoshi Soyama, Effect of nozzle geometry on aggressivity of cavitating jet for cavitation erosion test and applications, Fluid Mechanics and its Applications, 10.1007/978-94-017-8539-6_12, 106, 283-302, 2014.01, [URL], In order to estimate cavitation erosion rate in hydraulic machinery the erosion resistance of materials can be investigated using a cavitating jet apparatus, standardized by ASTM G134. As the jet aggressivity is affected by the nozzle outlet geometry, this should also be considered to obtain reliable erosion tests. In this paper, we investigated the effects of the nozzle outlet geometry on the aggressivity of the cavitating jet by erosion tests, impact force measurements, and high-speed movie observations. The effect of Strouhal number, defined by the shedding frequency of the cavitation cloud, the width of the cavitating region, and the jet velocity, was also studied. The aggressivity of the cavitating jet peaked at a certain nozzle outlet bore, D, and outlet length, L. It was found that the Strouhal number, St, is 0.17 at the optimum D and L, even though the nozzle throat diameter d and injection pressure p1 were different. It was also revealed that the frequency of the large cavitation impact is closely related to the shedding frequency of the cavitation cloud..
62.	Osamu Takakuwa, Yusuke Kawaragi, Hitoshi Soyama, Estimation of the yield stress of stainless steel from the Vickers hardness taking account of the residual stress, Journal of Surface Engineered Materials and Technology, 3, 4, 262-268, 2013.10.
63.	Osamu Takakuwa, Amrinder S. Gill, Gokul Ramakrishnan, Seetha R. Mannava, Vijay K. Vasudevan, Hitoshi Soyama, Introduction of compressive residual stress by means of cavitation peening into a titanium alloy rod used for spinal implants, Materials Sciences and Applications, 10.4236/msa.2013.47A2004, 4, 7A, 23-28, 2013.07.
64.	青柳吉輝, 河原木雄介, 髙桑脩, 徳江藍, 大久保力廣, 祖山均, ショットピーニングによる歯科用材料の機械的特性の向上, 日本機械学会論文集A編, 10.1299/kikaia.79.1019, 79, 803, 1019-1029, 2013.07.
65.	Osamu Takakuwa, Kouhei Yamamiya, Hitoshi Soyama, An indicator for the suppression of fatigue crack growth by hybrid peening, Journal of Solid Mechanics and Materials Engineering, 10.1299/jmmp.7.357, 7, 3, 357-371, 2013.06.
66.	Osamu Takakuwa, Hitoshi Soyama, Optimizing the conditions for residual stress measurement using a two-dimensional XRD method with specimen oscillation, Advances in Materials Physics and Chemistry, 10.4236/ampc.2013.31A002, 3, 1A, 8-18, 2013.04.
67.	Satoshi Nishimura, Osamu Takakuwa, Hitoshi Soyama, Similarity law on shedding frequency of cavitation cloud induced by a cavitating jet , Journal of Fluid Science and Technology, 10.1299/jfst.7.405, 7, 3, 405-420, 2012.12.
68.	Osamu Takakuwa, M. Nishikawa, H. Soyama, Technique for partially strengthening electrical steel sheet of IPM motor using cavitation peening, Energy Materials: Materials Science and Engineering for Energy Systems, 6, 1, 1422-1426, 2012.12, Cavitation peening (CP), which utilises the impact caused by cavitation bubbles collapsing, can improve the fatigue strength of metallic materials as well as the yield stress and tensile strength. The rotor cores in interior permanent magnet motors use electrical steel sheet. In order to increase the permissible rotating speed of the motor and achieve high efficiency, the mechanical strength of the stress concentration area in the steel sheet where the magnet is pressed needs to be enhanced. In the present paper, to improve the yield stress and tensile strength, the steel sheet was peened by CP. The peening effect on these mechanical properties was evaluated by tensile and indentation tests. The experimental results from the tensile tests revealed that CP can increase the yield stress by 22% compared to that before CP. In addition, the yield stress and tensile strength increased along with the CP processing time..
69.	Shinya Kanou, Osamu Takakuwa, Seetha R. Mannava, Dong Qian, Vijay K. Vasudevan, Hitoshi Soyama, Effect of the impact energy of various peening techniques on the induced plastic deformation region, Journal of Materials Processing Technology, 10.1016/j.jmatprotec.2012.05.003, 212, 10, 1998-2006, 2012.10.
70.	Hitoshi Soyama, Osamu Takakuwa, Akima Naito, Effect of nozzle shape for high injection pressure on aggressivity of cavitating jet, Proceedings of the 21th International Conference on Water Jetting, 367, 378, 2012.09.
71.	Osamu Takakuwa, Masaaki Nishikawa, Hitoshi Soyama, Estimation of the depth of surface modification layer induced by cavitation peening, Journal of Materials Processing Technology, 10.1016/j.jmatprotec.2012.03.010, 212, 8, 1716-1722, 2012.08.
72.	Osamu Takakuwa, Hitoshi Soyama, The effect of scanning pitch of nozzle for a cavitating jet during overlapping peening treatment, Surface & Coatings Technology, 10.1016/j.surfcoat.2012.03.034, 206, 23, 4756-4762, 2012.07.
73.	Osamu Takakuwa, Hitoshi Soyama, Using an indentation test to evaluate the effect of cavitation peening on the invasion of the surface of austenitic stainless steel by hydrogen, Surface & Coatings Technology, 10.1016/j.surfcoat.2012.03.027, 206, 18, 3747-3750, 2012.05.
74.	Osamu Takakuwa, Hitoshi Soyama, Suppression of hydrogen-assisted fatigue crack growth in austenitic stainless steel by cavitation peening, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2011.12.035, 37, 6, 5268-5276, 2012.03.
75.	Osamu Takakuwa, Masaaki Nishikawa, Hitoshi Soyama, Numerical simulation of the effects of residual stress on the concentration of hydrogen around a crack tip, Surface & Coatings Technology, 10.1016/j.surfcoat.2011.12.018, 206, 11-12, 2892-2898, 2012.02.
76.	Akima Naito, Osamu Takakuwa, Hitoshi Soyama, Development of peening technique using recirculating shot accelerated by water jet, Materials Science and Technology, 10.1179/1743284711Y.0000000027, 28, 2, 234-239, 2012.02.
77.	Hitoshi Soyama, Osamu Takakuwa, Akima Naito, Effect of injection pressure and size of nozzle throat of a cavitating jet on cavitation peening, Proceedings of the 11th International Conference on Shot Peening, 129-134, 2011.09.
78.	内藤暁馬, 髙桑脩, 祖山均, 水噴流加速式循環型ショットピーニングによるステンレス鋼の水素脆化抑止, 噴流工学, 28, 2, 12-17, 2011.06.
79.	Hitoshi Soyama, Osamu Takakuwa, Enhancing the aggressive strength of a cavitating jet and its practical application, Journal of Fluid Science and Technology, 10.1299/jfst.6.510, 6, 4, 510-521, 2011.06.
80.	Osamu Takakuwa, Masaaki Nishikawa, Hitoshi Soyama, Suppression of fatigue crack growth in austenite stainless steel by cavitation peening, Key Engineering Materials, 452-453, 641-644, 2011.06.
81.	Osamu Takakuwa, Toshihito Ohmi, Masaaki Nishikawa, A. Toshimitsu Yokobori, Jr., Hitoshi Soyama, Suppression of fatigue crack propagation with hydrogen embrittlement in stainless steel by cavitation peening, Strength, Fracture and Complexity, 10.3233/SFC-2011-0126, 7, 1, 79-85, 2011.02.
82.	Hitoshi Soyama, Tsutomu Kikuchi, Masaaki Nishikawa, Osamu Takakuwa, Introduction of compressive residual stress into stainless steel by employing a cavitating jet in air, Surface & Coatings Technology, 10.1016/j.surfcoat.2010.11.031, 205, 10, 3167-3174, 2011.02.
83.	髙桑脩, 西川雅章, 祖山均, キャビテーションピーニングを受けたステンレス鋼の表面改質層の降伏応力分布と疲労特性の評価, 日本機械学会論文集A編, 76, 770, 1367-1372, 2010.10.
84.	Masaaki Nishikawa, Osamu Takakuwa, Hitoshi Soyama, Evaluation of yield stress distribution in the surface layer and fatigue properties of the stainless steel modified by cavitation peening, Nihon Kikai Gakkai Ronbunshu, A Hen/Transactions of the Japan Society of Mechanical Engineers, Part A, 10.1299/kikaia.76.1367, 76, 770, 1367-1372, 2010.10, [URL], The present study evaluated the yield stress distribution in the surface layer and the fatigue properties of a steel SUS316L modified by cavitation peening (CP). The yield stress was evaluated based on micro-indentation tests using a spherical indenter. Especially, a technique of inverse analysis using elastic-plastic finite element analyses was utilized to determine the yield stress from the results of indentation tests, at which the effect of compressive residual stress introduced by CP was excluded. The identified results revealed that the surface with 50 μm in thickness was remarkably work-hardened by CP, and that the yield stress was increased within 500 urn in depth from the surface. Moreover, the plane bending fatigue tests demonstrated that the fatigue limit of the specimen was increased from 277 MPa to 362 MPa by C.P. Therefore, we can conclude that CP is effective for introducing a high yield stress on the metal surface, which suppresses the initiation of surface crack during fatigue cycles..
85.	Hitoshi Soyama, Nao Yamada, Osamu Takakuwa, Yuichi Sekine, Mitsuhiro Mikami, Release of micro strain in tool alloy steel by a cavitating jet in air, Proceedings of the 19th International Conference on Water Jetting, 219-228, 2008.10.

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