Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

The present study focuses on a novel hydrogen-improved strength-ductility balance in several practical Fe-Cr-Ni-based austenitic alloys that directly depends on the dissolved hydrogen content. The hydrogen absorption energy of Fe-Cr-Ni model alloys that have a face-centered cubic structure was examined using first-principles calculations to verify the contribution of Cr and Ni substitutions from Fe to hydrogen solubility in these alloys. Chromium substitution reduced the hydrogen absorption energy to a substantially greater degree than does Ni substitution, with increased Cr: Ni ratios resulting in higher hydrogen solubility. This pattern seen in the calculations supports the previously obtained experimental results in practical alloys with various Cr: Ni ratios. The pronounced reduction in hydrogen absorption energy that results from Cr substitution was mostly attributed to a decrement in the chemical effect derived from the electronic binding state rather than a mechanical effect caused by changed interatomic spacing due to these substitutions. We propose here a Cr-equivalent index to predict hydrogen solubility in Fe-Cr-Ni-based alloys that have varying Cr and Ni content.

DOI： 10.1016/j.commatsci.2023.112650

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1038/s41598-023-33761-4

Repository Public URL： https://hdl.handle.net/2324/7348010

Authorship：Last author   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

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.

DOI： 10.1016/j.actamat.2022.117789

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Language：English   Publishing type：Research paper (scientific journal)  

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.

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.corsci.2020.108558

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.msea.2018.10.040

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.scriptamat.2018.05.035

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1007/s11661-017-4323-3

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Solid-solution hardening caused by dissolved hydrogen (H) atoms in face-centered cubic metals is a favorable phenomenon that counteracts the H-induced degradation of mechanical performance in structural alloys, i.e., hydrogen embrittlement. In the present study, the changes of yield and flow stresses by solute H with the concentrations of 2000∼7600 at ppm were systematically investigated in a Fe–24Cr–19Ni-based austenitic stainless steel under the temperature range of 173∼423 K and two different strain rates: 5 × 10−5 and 5 × 10−3/s. Stress relaxation tests were subsidiarily employed in order to elaborate the underlying mechanisms predominating the H-related hardening at low and ambient temperatures. Four essential ingredients of the H-induced hardening were identified: (i) H atoms in the matrix lattice as dispersed obstacles; (ii) pinning of stationary dislocations by H atmosphere; (iii) dynamic pinning of dislocations resting at obstacles; (iv) drag force to moving dislocations by migratable H clouds. The hardening around 173 K was attributed to (i) and (ii), where the primary importance of interstitial-substitutional interaction between Cr and H was explicitly invoked. Meanwhile, the magnitude of hardening was maximized at around 298 K under the slow strain rate condition owing to the increasing contributions from (iii) and (iv).

DOI： 10.1016/j.msea.2023.145281

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Direct evidence of solute hydrogen-assisted deformation twinning in Fe-24Cr-19Ni-based austenitic steel was acquired via an in-situ tensile test in a scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD). Earlier activation of twinning on both primary and secondary systems by hydrogen was successfully visualized, in addition to recognizing an accelerated thickening of the existing twins. These alterations in twinning behavior were linked to an increase in the work-hardening rate at the later deformation stage, which is believed to be a root cause of recently discovered anomalous ductility improvement in the steels under the presence of solute hydrogen.

DOI： 10.1016/j.mtcomm.2023.105433

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Local 3D short fatigue crack closure behavior was investigated in Ti-6Al-4V alloy using ultra-high-resolution X-ray microtomography (XMT). The results show that the inhomogeneous distribution of plasticity-induced and roughness-induced crack closure is caused by the variation of crack path morphologies. These crack path morphologies exhibited heterogeneous plastic deformation at the crack-tips due to the anisotropic nature of α grains and varying extents of crack tilting and twisting caused by the interaction of the crack front with α/α boundary or α/α + β interface. It was revealed via surrogate-based statistical analysis that the Schmid factor has the strongest effect on crack growth rate.

DOI： 10.1016/j.ijfatigue.2022.107428

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Publisher：Materials Letters  

The impact of hydrogen on crack-tip plastic-zone development was revisited via a novel approach, utilizing the measurement of fatigue crack-growth retardation in a medium-strength martensitic steel after a single overloading in laboratory air and in 90-MPa-hydrogen gas. The plastic zone can be characterized according to the crack-propagation length for reverting from the retardation caused by plasticity-induced crack-closure ascribed to overloading (overloading-affected, crack-growth distance). Hydrogen sharpened the shape of overloaded crack-tip and suppressed the extension of the severely-deformed zone in the crack proximity. Besides, it enhanced frequent crack-tip branching, giving rise to a slower crack growth rate than the in-air situation at the initial stage of retardation. However, no change in the overloading-affected, crack-growth distance was detected between the in-air and hydrogen-gas conditions. Ultimately, hydrogen barely altered the overall plastic-zone size.

DOI： 10.1016/j.matlet.2021.131115

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Language：Japanese   Publisher：The Iron and Steel Institute of Japan  

<p>The effects of Ni concentration and dispersed conditions of vanadium carbide (VC) nano-particles on the hydrogen embrittlement (HE) behavior of precipitation-hardened high-Mn austenitic steels were investigated under the presence of thermally pre-charged hydrogen. Slow strain-rate tensile tests revealed that HE susceptibility decreased with an addition of Ni. VC precipitates functioned as the trapping sites for dissolved hydrogen, though its effect on resisting the HE was trivial. Hydrogen enhanced intergranular (IG) fracture wherein its area fraction was increased with the hardening by VC as well as with the escalation of internal hydrogen concentration. The IG fracture was the primary rationale for the hydrogen-induced loss of ductility. Possible mechanisms of the IG-related HE as well as future strategy for mitigating the mechanical degradation is discussed based on the fractographic observation and post-mortem microstructural analyses.</p>

DOI： 10.2355/tetsutohagane.tetsu-2021-093

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.engfracmech.2021.107755

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.ijfatigue.2020.105806

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.actamat.2020.08.024

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.ijfatigue.2020.105740

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.corsci.2020.108814

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.msea.2019.138349

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.engfracmech.2019.106505

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.msea.2019.04.084

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3390/ma12091426

Language：English   Publishing type：Research paper (scientific journal)  

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. .

DOI： 10.1016/j.prostr.2019.12.034

Language：English   Publishing type：Research paper (other academic)  

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).

DOI： 10.1115/PVP2019-93451

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.corsci.2019.108186

Language：English   Publishing type：Research paper (other academic)  

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.

DOI： 10.1115/PVP2019-93477

Language：English   Publishing type：Research paper (other academic)  

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.

DOI： 10.1115/PVP2019-93204

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.scriptamat.2018.08.003

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.actamat.2018.06.041

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.msea.2018.07.014

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.scriptamat.2017.06.037

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DOI： 10.1098/rsta.2016.0412

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DOI： 10.1299/jpes.6.63

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1299/transjsme.16-00169

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DOI： 10.1016/j.ijhydene.2017.04.055

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DOI： 10.1016/j.surfcoat.2016.08.087

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1016/j.ijhydene.2016.10.083

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijfatigue.2016.07.021

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

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DOI： 10.1299/transjsme.16-00111

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DOI： 10.4236/msa.2016.74018

Language：English   Publishing type：Research paper (other academic)  

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.

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DOI： 10.1049/joe.2015.0066

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DOI： 10.1049/joe.2015.0065

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DOI： 10.1299/transjsme.14-00638

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DOI： 10.1299/transjsme.14-00426

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DOI： 10.4236/aces.2015.51007

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DOI： 10.1016/j.ijhydene.2014.01.190

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DOI： 10.1299/transjsme.2014smm0022

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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.

DOI： 10.1016/j.sse.2013.09.007

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1007/978-94-017-8539-6_12

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.4236/msa.2013.47A2004

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.1299/kikaia.79.1019

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1299/jmmp.7.357

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.4236/ampc.2013.31A002

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1299/jfst.7.405

Language：English   Publishing type：Research paper (scientific journal)  

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.

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jmatprotec.2012.05.003

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jmatprotec.2012.03.010

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.surfcoat.2012.03.034

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.surfcoat.2012.03.027

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijhydene.2011.12.035

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.surfcoat.2011.12.018

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1179/1743284711Y.0000000027

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1299/jfst.6.510

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3233/SFC-2011-0126

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.surfcoat.2010.11.031

Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.1299/kikaia.76.1367

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Event date： 2025.3

Presentation type：Oral presentation (general)  

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Event date： 2024.11

Language：English   Presentation type：Oral presentation (general)  

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Event date： 2024.11

Language：English   Presentation type：Oral presentation (general)  

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Event date： 2024.9

Presentation type：Oral presentation (general)  

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Event date： 2024.9

Presentation type：Oral presentation (general)  

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Event date： 2024.9

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Event date： 2024.9

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Event date： 2024.8

Presentation type：Oral presentation (general)  

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Event date： 2024.8

Presentation type：Oral presentation (general)  

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Event date： 2024.8

Presentation type：Oral presentation (general)  

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Event date： 2024.6

Presentation type：Public lecture, seminar, tutorial, course, or other speech  

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Event date： 2024.5

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Event date： 2024.5

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Event date： 2024.4

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Country：Japan  

Event date： 2024.4

Language：English   Presentation type：Oral presentation (general)  

Country：Portugal  

Event date： 2024.4

Language：English   Presentation type：Oral presentation (general)  

Country：Portugal  

Event date： 2024.4

Language：English   Presentation type：Oral presentation (general)  

Country：Portugal  

Event date： 2024.4

Language：English   Presentation type：Oral presentation (general)  

Country：Germany  

Event date： 2024.4

Language：English   Presentation type：Oral presentation (general)  

Country：Germany  

Event date： 2024.4

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：富山大学   Country：Japan  

Event date： 2024.4

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：富山大学   Country：Japan  

Event date： 2024.4

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：富山大学   Country：Japan  

Event date： 2024.4

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京理科大学   Country：Japan  

Event date： 2024.3

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Event date： 2024.3

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Event date： 2023.9

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Event date： 2023.9

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Event date： 2023.9

Presentation type：Poster presentation  

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Event date： 2023.9

Presentation type：Oral presentation (general)  

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Event date： 2023.9

Presentation type：Oral presentation (general)  

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Event date： 2023.8

Presentation type：Oral presentation (invited, special)  

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Event date： 2023.8

Presentation type：Oral presentation (general)  

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Event date： 2023.5

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：つくば国際会議場   Country：Japan  

Event date： 2023.5

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：つくば国際会議場   Country：Japan  

Event date： 2023.5

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：つくば国際会議場   Country：Japan  

Event date： 2023.5

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Event date： 2023.5

Presentation type：Poster presentation  

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Event date： 2023.5

Presentation type：Oral presentation (general)  

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Event date： 2023.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京大学   Country：Japan  

Event date： 2023.3

Language：Japanese  

Venue：東京大学   Country：Japan  

Event date： 2023.3

Presentation type：Oral presentation (invited, special)  

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Event date： 2023.3

Presentation type：Oral presentation (general)  

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Event date： 2022.12

Language：English   Presentation type：Oral presentation (general)  

Venue：Norwegian University of Science and Technology(NTNU), Trondheim, Norway   Country：Norway  

Event date： 2022.12

Language：English   Presentation type：Oral presentation (general)  

Venue：Norwegian University of Science and Technology(NTNU), Trondheim, Norway   Country：Norway  

Event date： 2022.12

Presentation type：Oral presentation (invited, special)  

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Event date： 2022.12

Presentation type：Oral presentation (invited, special)  

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Event date： 2022.9

Language：Japanese  

Venue：福岡工業大学   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.3

Presentation type：Oral presentation (general)  

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Event date： 2019.12

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：大阪大学   Country：Japan  

Event date： 2019.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：琉球大学   Country：Japan  

Event date： 2019.8

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：KKR伊豆長岡   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：愛媛大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：首都大学東京   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学   Country：Japan  

Event date： 2019.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Kyushu University   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：千葉工業大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：愛媛大学   Country：Japan  

Event date： 2019.4

Language：English   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2025.3

Presentation type：Oral presentation (invited, special)  

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Event date： 2024.9

Presentation type：Oral presentation (invited, special)  

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Event date： 2024.8

Presentation type：Oral presentation (general)  

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Event date： 2024.4

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：富山大学   Country：Japan  

Event date： 2024.4

Language：English   Presentation type：Oral presentation (general)  

Country：Germany  

Event date： 2023.9

Presentation type：Oral presentation (general)  

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Event date： 2023.8

Presentation type：Oral presentation (general)  

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Event date： 2023.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京大学   Country：Japan  

Event date： 2022.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：福岡工業大学   Country：Japan  

Event date： 2022.9

Presentation type：Oral presentation (invited, special)  

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Event date： 2022.9

Presentation type：Poster presentation  

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Event date： 2022.9

Presentation type：Oral presentation (general)  

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Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Kyushu University   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：オンライン   Country：Japan  

Event date： 2022.3

Presentation type：Poster presentation  

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Event date： 2022.3

Presentation type：Poster presentation  

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Event date： 2022.3

Presentation type：Oral presentation (general)  

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Event date： 2019.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2019.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2019.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：日本学術会議   Country：Japan  

Event date： 2019.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：岡山大学   Country：Japan  

Event date： 2019.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：岡山大学   Country：Japan  

Event date： 2019.8

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：KKR伊豆長岡   Country：Japan  

Event date： 2019.6

Language：English   Presentation type：Oral presentation (general)  

Venue：東京国際フォーラム   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：上智大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：トヨタ自動車東日本株式会社   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：上智大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：沖縄県青年会館   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：沖縄県青年会館   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：ネツレン株式会社   Country：Japan  

Event date： 2019.6

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：千葉工業大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Venue：エッサム神田   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：上智大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：愛媛大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：愛媛大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：愛媛大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：愛媛大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京電機大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京電機大学   Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2019.6

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：大阪大学   Country：Japan  

Event date： 2018.11

Language：English   Presentation type：Oral presentation (general)  

Venue：KRISS   Country：Korea, Republic of  

Event date： 2018.8

Language：English  

Venue：Belgrade   Country：Serbia  

The aim of this study is to provide a quantification of the internal pressure contribution to the SSRT properties of H-charged Type-316L steel tested in air at room temperature. Considering pre-existing penny-shaped voids, the transient pressure build-up has been simulated as well as its impact on the void growth by preforming J
_Ic
calculations. Several void distributions (size and spacing) have been considered. Simulations have concluded that there was no impact of the internal pressure on the void growth, regardless the void distribution since the effective pressure was on the order of 1 MPa during the SSRT test. Even if fast hydrogen diffusion related to dislocation pipe-diffusion has been assessed as a conservative case, the impact on void growth was barely imperceptible (or significantly low). The effect of internal pressure has been experimentally verified via the following conditions: (I) non-charged in vacuum; (II) H-charged in vacuum; (III) H-charged in 115-MPa nitrogen gas; (IV) non-charged in 115-MPa nitrogen gas. As a result, the relative reduction in area (RRA) was 0.84 for (II), 0.88 for (III), and 1.01 for (IV), respectively. The difference in void morphology of the H-charged specimens did not depend on the presence of external pressure. These experimental results demonstrate that the internal pressure had no effect on the tensile ductility and void morphology of the H-charged specimen.

Event date： 2018.7

Language：English  

Venue：Prague   Country：Czech Republic  

Hydrogen effect on fatigue performance of commercially pure BCC iron has been studied with a combination of various electron microscopy techniques. The fatigue crack growth (FCG) in gaseous hydrogen was found to consist of two regimes corresponding to a slightly accelerated regime at relatively low stress intensity factor range, ΔK, (Stage I) and the highly accelerated regime at relatively high ΔK (Stage II). These regimes were manifested by the intergranular and quasi-cleavage types of fractures respectively. Scanning electron microscopy (SEM) observations demonstrated an increase in plastic deformation around the crack wake in the Stage I, but considerably lower amount of plasticity around the crack path in the Stage II. Transmission electron microscopy (TEM) results identified dislocation cell structure immediately beneath the fracture surface of the Stage I sample, and dislocation tangles in the Stage II sample corresponding to fracture at high and low plastic strain amplitudes respectively.

Event date： 2018.7

Language：English  

Venue：Prague   Country：Czech Republic  

In order to elucidate the temperature dependence of hydrogen-enhanced fatigue crack growth (FCG), the FCG test was performed on low-alloy Cr-Mo steel JIS-SCM435 according to ASTM E647 using compact tension (CT) specimen under 0.1 - 95 MPa hydrogen-gas at temperature ranging from room temperature (298 K) to 423 K. The obtained results were interpreted according to trap site occupancy under thermal equilibrium state. The FCG was significantly accelerated at RT under hydrogen-gas, that its maximum acceleration rate of the FCG was 15 at the pressure of 95 MPa at the temperature of 298 K. The hydrogen-enhanced FCG was mitigated due to temperature elevation for all pressure conditions. The trap site with binding energy of 44 kJ/mol dominated the temperature dependence of hydrogen-enhanced FCG, corresponding approximately to binding energy of dislocation core. The trap site (dislocation) occupancy is decreased with the temperature elevation, resulting in the mitigation of the FCG acceleration. On the basis of the obtained results, when the occupancy becomes higher at lower temperature, e.g. 298 K, hydrogen-enhanced FCG becomes more pronounced. The lower occupancy at higher temperature does the opposite.

Event date： 2018.5 - 2018.6

Language：English  

Venue：Poitiers Futuroscope   Country：France  

Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to quasi-cleavage in hydrogen gas. Grain reference orientation deviation (GROD) analysis has shown a dramatic suppression of plastic deformation around the crack wake in samples fatigued in hydrogen. These results were verified by preparing site-specific specimens from different fracture features by using Focused Ion Beam (FIB) technique and observing them with Transmission Electron Microscope (TEM). The FIB lamella taken from the sample fatigued in air was decorated with dislocation cell structure indicating high amount of plasticity, while the lamella taken from the quasi-cleavage surface of the sample fatigued in hydrogen revealed a distribution of dislocation tangles which corresponds to smaller plastic strain amplitude involved at the point of fracture. These results show that a combination of critical hydrogen concentration and critical stress during fatigue crack growth at high ΔK values triggers cleavage-like fracture due to reduction of cohesive force between matrix atoms.

Event date： 2018.5 - 2018.6

Language：English  

Venue：Poitiers Futuroscope   Country：France  

The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subsequent microvoids formation along the grain boundaries. Additionally, the impact of such IG cracking on the macroscopic FCG rate was evaluated according to the dependency of IG fracture propensity on the hydrogen gas pressure. It was first demonstrated that the increased hydrogen pressure results in the larger area fraction of IG and corresponding faster FCG rate. Moreover, gaseous hydrogen environment also had a positive influence on the FCG rate due to the absence of oxygen and water vapor. The macroscopic crack propagation rate was controlled by the competition process of said positive and negative effects.

Event date： 2018.4

Language：English   Presentation type：Oral presentation (general)  

Venue：KRISS   Country：Korea, Republic of  

Event date： 2017.7

Language：English  

Venue：Waikoloa   Country：United States  

The paper presents the hydrogen-entry, tensile, and fatigue properties of a precipitation-hardened martensitic stainless steel, JIS-SUS630, with a newly developed coating, whose thickness ranges from 10 to 20 μm. The newly developed coating consists of alumina, aluminum, and ferroaluminum, and has an excellent resistance to hydrogen entry in 100-MPa hydrogen gas at 270°C. The hydrogen entry in the coated specimen occurred under a diffusion-controlled process and the effective hydrogen diffusivity was approximately one thousandth of that of the base steel. Although the hydrogen diffusivity of JIS-SUS630 was two orders of magnitude larger than that of JIS-SUS304, the effective hydrogen diffusivity of the coated JIS-SUS630 was nearly equal to that of the coated SUS304. In our previous study with secondary-mass ion spectroscopy (SIMS), the coating's excellent resistance to hydrogen entry was attributed to interfacial hydrogen trapping between the aluminum and ferroaluminum layers. The experimental result obtained in this study suggested that the excellent resistance to hydrogen entry demonstrated by the developed coating can be attributed to the reduction in the permeation area induced by the interfacial trapping of hydrogen. The tensile tests of a smooth, round-bar specimen and fatigue tests of a circumferentially notched specimen with exposure to 100-MPa hydrogen gas at 270°C were performed in air at room temperature (RT). The test results showed that the tensile and fatigue properties of the coated specimens were not degraded by hydrogen exposure, whereas those for the non-coated specimens were significantly degraded. Hydrogen-pressure cycle tests of the coated, tubular specimens with an inner notch were also carried out with 95-MPa hydrogen gas at 85°C, demonstrating that the fatigue life of the tubular specimen was improved by the developed coating.

Event date： 2017.7

Language：English  

Venue：Waikoloa   Country：United States  

Slow-strain rate tensile (SSRT) tests on various metals having γ-Fe phase; Type 304 and 316L stainless steels, HP160 high strength stainless steel, and A286 Fe-based super alloy were conducted in external hydrogen and with internal hydrogen. The external hydrogen indicates non-charged specimens tested in high-pressure hydrogen-gas environment, whereas the internal hydrogen indicates hydrogen-charged specimens, with uniform distribution of hydrogen, tested in inert gas. The hydrogen distribution was calculated based on the measured hydrogen diffusivity and solubility. The fracture morphologies were observed by scanning electron microscopy (SEM). For Types 304, 316L, and HP160 the relative reduction in area (RRA of the steels was successfully reproduced by the nickel equivalent, Ni_eq, showing the higher Ni_eq, the lager RRA. Furthermore, at a low Ni_eq, the RRA of the steel with external hydrogen was nearly equal to that with internal hydrogen. In contrast, at a high Ni_eq, the RRA of the steel with internal hydrogen was slightly degraded by hydrogen, RRA ∼ 0.8, whereas that in external hydrogen was not degraded, RRA ∼ 1. For A286, despite a high Ni_eq, the RRA of the alloy with internal hydrogen was significantly degraded by hydrogen, RRA ∼ 0.5. The fracture morphologies were categorized into four types: quasi-cleavage fracture associated with hydrogen-assisted surface cracks; ordinary void formation with no hydrogen effect; small-void formation associated with void sheet enhanced by hydrogen; facet formation induced by hydrogen. These categorized morphologies could be interpreted in terms of hydrogen distribution (internal or external hydrogen), austenitic stability (a low or high Ni_eq), and microstructure (solution or precipitation-hardened treatment).

Event date： 2017

Language：Japanese  

&lt;p&gt;In order to investigate the effects of hydrogen gas pressure and temperature on fatigue crack growth characterizations of low carbon steel JIS-SM490B and low alloy steel JIS-SCM435, fatigue crack growth tests using a CT specimen were conducted in pressurized gaseous hydrogen in various temperature. In low-pressure hydrogen gas, the fatigue crack growth rate was decreased with increasing of temperature. On the other hand, in high-pressure hydrogen gas, the fatigue crack growth rate was not depending on temperature. The mechanism of fatigue crack growth under hydrogen environment with various temperature and pressure was discussed based on the theory of dislocation trapping of hydrogen.&lt;/p&gt;

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Event date： 2015.9

Language：Japanese  

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Event date： 2015.9

Language：Japanese  

Here the present paper proposes a method employing micro-area X-ray diffraction analysis to evaluate local hydrogen concentration behavior. Hydrogen concentrating around a crack tip significantly accelerates crack propagation in metals, i.e., hydrogen embrittlement. The concentration behavior of hydrogen has not been revealed yet. In order to solve mechanism of hydrogen embrittlement, hydrogen concentration behavior in local area, e.g., a crack tip, has to be cleared by not only a numerical analysis but also an experimental approach. The present study takes notice of hydrogen-induced lattice strain and X-ray diffraction was employed to detect variation of lattice spacing before and after hydrogen charging. Here we demonstrate using X-ray diffraction applied to a micro area that hydrogen concentrates around an elastic-plastic boundary at vicinity of a crack.

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Event date： 2015

Language：Japanese  

In order to investigate the relationship between hydrogen-induced hardening of austenitic stainless steel and the hydrogen content, the spherical micro-indentation tests and thermal desorption analysis have been conducted on hydrogen charged austenitic stainless steel JIS SUS316L. Due to the relatively low hydrogen embrittlement susceptibility, this material has been widely used in a hydrogen environment. Hydrogen has been charged by using a cathodic method. In order to change the hydrogen content, charging current density and charging time were changed from 0.01 to 1.0 mA/mm², from 0 to 48 h, respectively. The obtained results show the hardness increases along with charging current density and charging time. The maximum change in the hardness is 28 %. Hydrogen content also depends on the charging current density and charging time. These relationships result in the strong linear correlation between hydrogen-induced hardening rate and the hydrogen content. Their correlation factor is 0.98 and the probability that these are uncorrelated is less than 0.001 %. The depth of hydrogen absorption was investigated by using secondary ion mass spectroscopy (SIMS). Hydrogen-induced hardening might be due to dislocation-pinning effect of hydrogen during indentation process. This phenomenon is dependent on not so much hydrogen absorption depth as hydrogen content. By using this phenomenon, there is a possibility to apply micro-indentation tests for evaluation of local hydrogen content.

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Event date： 2014.9

Language：Japanese  

Ti-29Nb-l3Ta-4.6Zr alloy (TNTZ) is expected to be used as biomedical applications such as a spinal fixation device. For a practical application, high durability of the spinal fixation device is required in order to ensure mechanical safety. The durability of the spinal fixation device with a rod made of TNTZ subjected to a surface hardening treatment was investigated in this study. The rod was prepared through a levitation melting, a hot-forging at 1273K, a solution treatment at 1063K for 3.6 ks subsequently water quenching, machining and finally a cavitation peening (CP) treatment for surface hardening (TNTZ-CP rod). Microstructure was observed using transmission electron microscopy (TEM). The durability was evaluated by compressive fatigue test applied to the spinal fixation device. Nano-scale graines were observed at the surface of the TNTZ-CP rod. The nanocrystalization was caused by the CP treatment. The compressive fatigue strength of the spinal fixation device with the TNTZ-CP rod was higher than that of spinal fixation device with a non-CP treated TNTZ rod.

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Event date： 2014.9

Language：Japanese  

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Event date： 2014.9

Language：Japanese  

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Event date： 2014.7

Language：English  

Venue：Anaheim   Country：United States  

In order to reveal the effect of indentation load on Vickers hardness of austenitic stainless steel after hydrogen charging, the Vickers hardness measurements have been conducted with three different indentation load of 0.49, 1.96 and 9.80 N on the surface of type 316L austenitic stainless steel after hydrogen charging. Relationship between plastic deformation behavior during indentation process and hydrogen absorption behavior was revealed. In the Vickers hardness test, Vickers hardness keeps same value though the indentation load varies. Needless to say, the value did not depend on magnitude of the indentation load before hydrogen charging in the present study. However, the Vickers hardness increased along with hydrogen charging time and, interestingly, the increase in the Vickers hardness due to the presence of hydrogen depends on magnitude of the indentation load. In the load of 0.49 N and 9.80 N, the Vickers hardness has a maximum value of 3.04 and 2.04 GPa which is 1.58 and 1.15 times larger than value of 1.73 and 1.70 GPa before hydrogen charging, respectively. The hydrogen-induced hardening behavior observed by the Vickers hardness tests employing different indentation load would be evaluated by the relationship between the plastic deformation depth and the hydrogen absorption depth.

Event date： 2014.7

Language：Japanese  

In order to realize sustainable society using hydrogen for main energy source, hydrogen embrittlement is big issue to be solved. As hydrogen embrittlement is accelerated near crack tip region where tensile stress exists, improvement of residual stress, i.e., change of residual stress from tension to compression, might suppress the hydrogen embrittlement. In the present paper, the crack propagation of stainless steel exposed to hydrogen was examined considering the residual stress on the surface. The tensile residual stress was introduced by an angle grinder, then the residual stress was improved by cavitation peening. Cavitation peening was a peening method in the same way as shot peening using cavitation impacts produced by a cavitating jet, i.e., a submerged water jet. It was revealed that the crack propagation rate of the specimen in which tensile residual stress was introduced and exposed to hydrogen was suppressed by improving residual stress using cavitation peening.

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Event date： 2014.7

Language：Japanese  

In order to investigate suppression of fatigue crack growth in hydrogen-charged chrome molybdenum steel SCM 435 by cavitation peening employing a cavitating jet in water, a load controlled plate bending fatigue tests were conducted with load stress of 400 MPa after cathodic hydrogen charging with and without cavitation peening. The obtained results show crack growth rate were significantly accelerated by hydrogen charging. Number of cycle to failure from crack initiation of hydrogen-charged specimen increase 2.66 times by cavitation peening compared to an untreated one. It might be due to introducing compressive residual stress by cavitation peening.

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Event date： 2014.7

Language：Japanese  

In order to reveal the effect of surface stress on the invasion of austenitic stainless steel by hydrogen and its interaction, thermal desorption analysis and X-ray diffraction stress measurements were conducted on hydrogen charged specimens after surface finishing. The obtained results show tensile residual accelerated invasion by hydrogen by 2.7 times compared to the case of which compressive residual stress was introduced. In addition, the stress was varied due to the presence of hydrogen as a reactive stress was generated by base metal against increase in the volume. This reactive stress was defined as hydrogen-induced compressive stress. It had a close relation with hydrogen content.

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Event date： 2014.7

Language：Japanese  

In order to investigate the relationship between hydrogen induced hardening of austenitic stainless steel and the hydrogen content, the spherical micro-indentation tests and thermal desorption analyses have been conducted on hydrogen charged austenitic stainless steel JIS SUS316L. Hydrogen has been charged by using cathodic method. In order to change the hydrogen content, current density and charging time are changed from 0.01 mA/mm^2 to 1.0 mA/mm^2, from 0 h to 48 h respectively. The obtained results show the hardness increase along with charging current density and charging time. Hydrogen content also depends on the charging current density and charging time. These relationships result in the linear correlation between hydrogen induced hardening of austenitic stainless steel and the hydrogen content. Hydrogen enhanced hardening might be due to dislocation-pinning effect of hydrogen. Thus there is a possibility to apply micro-indentation tests for measurements of hydrogen content.

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Event date： 2014.6

Language：English  

Venue：Shanghai   Country：China  

The objective of this study is to evaluate the effect of hydrogen on the micro- and macro-strain of austenitic stainless steel using X-ray diffraction. When hydrogen is trapped in lattice sites, it can affect both the micro- and macro-strain. The micro-strain was evaluated through fitting profiles to measured X-ray diffraction profile using a fundamental parameter method. The macro-strain, i.e., the residual stress, was evaluated by a 2D method using a two-dimensional PSPC. The experimental samples were charged with hydrogen by a cathodic charging method. The results revealed that the induced residual stress was equi-biaxial and compressive, and that the micro-strain increased. Both of these varied rapidly with increasing hydrogen charging time. Saturation occurred at a compressive stress of around 130 MPa. On reaching saturation, the hydrogen charging was terminated and desorption of hydrogen began at room temperature. Then, the strains decreased and the compressive stress reverted, ultimately, to a tensile stress of 180 MPa. Martensitic transformation occurred due to hydrogen charging and this had a significant effect on the X-ray diffraction profile.

Event date： 2014

Language：Japanese  

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Event date： 2014

Language：Japanese  

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Event date： 2013.10

Language：Japanese  

In order to reveal the effect of indentation load on Vickers hardness of austenitic stainless steel after hydrogen charging, the Vickers hardness measurements have been conducted with three different indentation load of 0.49, 1.96 and 9.80 N. The obtained results show the Vickers hardness increases along with hydrogen charging time and the increase in the Vickers hardness depends on magnitude of indentation load. In the load of 0.49 N and 9.80 N, the Vickers hardness has a maximum value of 3.04 and 2.04 GPa which is 1.58 and 1.15 times larger than value of 1.73 and 1.70 GPa before hydrogen charging, respectively. It might be due to effect of hydrogen concentration gradient in respect to depth on deformation during indentation.

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Event date： 2013.9

Language：Japanese  

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Event date： 2013.9

Language：Japanese  

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Event date： 2013.9

Language：Japanese  

In order to optimize the conditions for residual stress measurement using a two-dimensional X-ray diffraction in terms of both efficiency and accuracy, the measurements were conducted on two specimens made of stainless steel in this study. The two specimens were treated by annealing and a cavitating jet in air so as to realize large grain and introduce residual stress. The specimens were oscillated in the ω- direction, representing a right-hand rotation of the specimen about the incident X-ray beam. The range of the oscillation, Δω, was varied and optimum Δω was determined. Moreover, combinations of the tilt angle between the specimen surface normal and the diffraction vector, ψ, with the rotation angle about its surface normal, φ, have been studied with a view to find the most optimum condition. The results show that the use of ω oscillations is an effective method for improving analysis accuracy, especially for large grain metals. The standard error rapidly decreased with increasing range of the co oscillation, especially for the annealed specimen which generated strong diffraction spots due to its large grain size.

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Event date： 2013.9

Language：Japanese  

In this paper, the distribution of compressive residual stress caused by shot peening and cavitation peening was estimated by measurements of impact. It is known that the fatigue strength of metallic materials can be improved by introducing compressive residual stress induced by impact during peening process. The compressive residual stress at near surface and its distribution are important factors for improvement of the fatigue strength. In the case of shot peening, it is possible to estimate the distribution of compressive residual stress by numerical simulation using Hertz contact theory. However in the case of cavitation peening, it is hard to estimate residual stress distribution by numerical simulation. In this study, in order to estimate the distribution of compressive residual stress introduced by using shot peening and cavitation peening, plastic deformation pit and impact were measured. It is said that the compressive residual stress introduced region obtained by X-ray diffraction method can estimate by the maximum applied stress obtained by impact measurements and pit shape.

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Event date： 2013.3

Language：Japanese  

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Event date： 2013.3

Language：Japanese  

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Event date： 2013.3

Language：Japanese  

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Event date： 2012.9

Language：Japanese  

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Event date： 2012.9

Language：Japanese  

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Event date： 2012.9

Language：Japanese  

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Event date： 2012.9

Language：English  

Venue：Ottawa, ON   Country：Canada  

A cavitating jet can be used for cutting and a material testing to evaluate cavitation erosion resistance of materials. In order to use the cavitating jet more efficiently, effect of nozzle shape should be investigated, as aggressivity of the jet was affected by the nozzle shape. In the present paper, erosion rate, intensity of luminescence and acoustic power were measured to evaluate the aggressivity of the jet from various types of the nozzle for high injection pressure. It was found that the aggressivity of the jet was drastically changed by the nozzle shape.

Event date： 2012.3

Language：Japanese  

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Event date： 2012.3

Language：Japanese  

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Event date： 2012.3

Language：Japanese  

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Event date： 2012.3

Language：Japanese  

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Event date： 2012.3

Language：Japanese  

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Event date： 2011.9

Language：English  

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Event date： 2011.7

Language：Japanese  

In this paper, the method for optimizing the pitch of cavitation peening based on Gaussian distribution is proposed. The Gaussian distribution showing the peening intensity was obtained by considering the white and black contrast of erosion ring caused in A1070 aluminum specimen by a cavitating jet in air. For evaluating the intensity of whole of the peened area, the intensity distribution was divided and summed using toroidal coordinate. The peening intensity distribution of erosion ring was summed with varying interval, and the optimum pitch of cavitation peening can be determined by the shape and height of intensity distribution.

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Event date： 2011.3

Language：Japanese  

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Event date： 2011.3

Language：Japanese  

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Event date： 2010.10

Language：Japanese  

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Event date： 2010.10

Language：Japanese  

Cavitation is well known as evil phenomenon which causes severe erosion in hydraulic machinery such as pumps, valves and propellers because of impacts produced at cavitation bubbles collapse. However, a cavitating jet which is a submerged high-speed water jet with cavitation is used for surface modification such as introduction of compressive residual stress and improvement of fatigue strength in the same way of shot peening. In order to enhance ability of cavitating jet which depend on nozzle throat diameter and injection pressure, an erosion test for aluminum and a measurement of compressive residual sterss of duralumin induced by cavitation peening were carried out.

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Event date： 2010.10

Language：Japanese  

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Event date： 2010.3

Language：Japanese  

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Event date： 2010

Language：Japanese  

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Event date： 2010

Language：Japanese  

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Event date： 2009.9

Language：English  

Venue：Nagasaki   Country：Japan  

Cavitation normally causes severe damage in hydraulic machinery such as pumps and turbines by the impact produced by cavitation bubbles collapsing. Although cavitation is known as a factor of erosion, Soyama et al. succeeded in utilizing impacts of cavitation bubble collapsing for surface modification by controlling cavitating jet in the same way as shot peening. The local plastic deformation caused by cavitation impact enhances the fatigue strength of metallic materials, and the surface modification technique utilizing cavitation impact is called "cavitation peening (CP)". It is well known that the peening improves fatigue strength by introducing compressive residual stress on the surface, but little attention has been paid to the behavior of fatigue crack growth of the material which was modified by CP. In the present study, the fatigue behavior of austenite stainless steel with and without CP was evaluated by a plate bending fatigue test, and the results revealed that the compressive residual stress introduced by CP suppresses fatigue crack growth rate by 70 % compared to that without CP.

Event date： 2009.3

Language：Japanese  

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Event date： 2008.10

Language：English  

Venue：Nottingham   Country：United Kingdom  

Cavitation impacts can be utilized to improve material properties such as fatigue strength. In case of such application, cavitation was produced by a cavitating jet. In order to investigate mechanism of surface modification, tool alloy steel was treated by a cavitating jet and the surface was analyzed by using X-ray diffraction methods, i.e., 2D method and a fundamental approach. Residual stress was measured by 2D method and micro strain, which is strain in the grain, was evaluated by a fundamental parameter approach. In the present paper, in order to enhance cavitation impacts, a cavitating jet in air was used for surface treatment of tool alloy steel. It was revealed that a cavitating jet in air can introduce compressive residual stress. It was also shown that micro strain introduced by heat treatment or mechanical finishing was released by a cavitating jet in air.

Event date： 2008.3

Language：Japanese  

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Event date： 2007.9

Language：Japanese  

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Event date： 2007

Language：Japanese  

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Type：Peer review 

Number of peer-reviewed articles in foreign language journals：12

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：15

Proceedings of International Conference Number of peer-reviewed papers：4

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：11

Number of peer-reviewed articles in Japanese journals：0

Proceedings of International Conference Number of peer-reviewed papers：4

Proceedings of domestic conference Number of peer-reviewed papers：0

Type：Peer review 

Number of peer-reviewed articles in foreign language journals：6

Number of peer-reviewed articles in Japanese journals：0

Proceedings of International Conference Number of peer-reviewed papers：4

Proceedings of domestic conference Number of peer-reviewed papers：0