担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

The effect of carbon monoxide (CO) contained in H2 gas as an impurity on the hydrogen-accelerated fatigue crack growth of A333 pipe steel was studied in association with loading frequency dependency. The addition of CO to H2 gas inhibited the accelerated fatigue crack growth due to the hydrogen. The inhibitory effect was affected by the CO content in the H2 gas, loading frequency, and crack growth rate. Based on these results, it was revealed that the inhibitory effect of CO was governed by both competition between the rate of fresh surface creation by the crack growth and the rate of coverage of the surface by CO and time for hydrogen diffusion in the material to the crack tip with reduced hydrogen entry by CO.

DOI： 10.1016/j.ijhydene.2019.09.146

記述言語：英語   掲載種別：研究論文（学術雑誌）

The effect of hydrogen on the tensile properties of the SUH660 and SUS316L different materials welded joints was characterized in conjunction with the joint shape and weld defects. The butt welded joint specimen without weld defect fractured at the SUS316L base material, and did not cause hydrogen embrittlement (HE). However, the failure position of the spigot-lap welded joint specimen moved from the SUS316L base material to the weld part when hydrogen charging was applied. This resulted in a significant reduction of the elongation. It was presumed that the HE was induced by the stress concentration due to the weld shape. The weld defect induced HE in both joints. The weld defect was produced by incomplete penetration. It also caused incomplete mixing of the weld metal. Consequently, filler nickel segregated around the weld defect, then HE occurred.

DOI： 10.18178/ijmerr.8.5.713-718

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

The effect of oxygen contained in hydrogen gas environment as an impurity on hydrogen environment embrittlement (HEE) of A333 pipe steel was studied through the fracture toughness tests in hydrogen gases. The oxygen contents in the hydrogen gases were 100, 10, and 0.1 vppm. A significant reduction in the J‐Δa curve was observed in the hydrogen with 0.1‐vppm oxygen. Under given loading conditions, the embrittling effect of hydrogen was completely inhibited by 100 vppm of oxygen. In the case of the hydrogen with 10‐vppm oxygen, initially the embrittling effect of hydrogen was fully inhibited, and then subsequently appeared. It was confirmed that 1‐vppm oxygen reduced the embrittling effect of hydrogen. The results can be explained by the predictive model of HEE proposed by Somerday et al.

DOI： 10.1111/ffe.12994

記述言語：英語   掲載種別：研究論文（学術雑誌）

As the result of a full-scale test of a thread joint connecting oil-well casing pipes, fretting fatigue failure at the middle of the thread engagement part was identified. The objective of this study is to understand the mechanism of the fretting fatigue failure at the middle of the contact part. A similar failure mode was reproduced by a bridge-pad type fretting fatigue test under gross slip conditions. It was found that the crack position moved from the contact edge toward the inside of the contact part with the progress of the fretting wear.

DOI： 10.1016/j.ijfatigue.2017.03.024

記述言語：英語   掲載種別：研究論文（学術雑誌）

Identification of the fatigue failure mode of the premium threaded connection for Oil Country Tubular Goods pipes was conducted via full-scale fatigue tests. A through-wall crack was found at the imperfect thread root of the male embodiment, but the crack initiation site depended on the stress level. At relatively higher stress amplitude region, the crack originated from the thread rounded corner by stress concentration. At relatively lower stress amplitude region, the crack originated at the middle of the thread root because of fretting fatigue. To investigate the fretting fatigue mechanism in the threaded connection, a fundamental fretting fatigue test was conducted. This test achieved the fretting fatigue failure at the middle of the contact surface under large gross slip condition.

DOI： 10.1016/j.triboint.2016.10.021

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

JIS SUS304 fretting fatigue test was done in high-purity hydrogen, an oxygen-hydrogen mixture and humidified hydrogen. The fretting fatigue strength in hydrogen was drastically changed depending on the oxygen level. Based on the XPS (X-ray photoelectron spectroscopy) analysis of the fretted surface, it was found that the fretting removed the original protection layer of the stainless steel, however, the addition of water vapor or ppm-level of oxygen produced an oxide layer on the fretted surface during the fretting that surpassed the removal effect of the initial oxide layer by fretting. In fact, a strong adhesion between the contacting surfaces occurred and no fretting wear particles were observed in the high-purity hydrogen. On the other hand, oxidized fretting wear particles were found in the oxygen-hydrogen mixture. Based on the geometry of contact used in this study, a severe concentration of the contact pressure arouse at the contact edge. This produces a compressive stress field in the specimen where the crack growth was suppressed. This stress concentration was relieved when fretting wear occurs. Therefore, the change in the fretting fatigue strength in hydrogen by the addition of oxygen is closely related to the change in the wear behavior.

DOI： 10.1016/j.ijhydene.2014.12.129

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Fretting fatigue test of SUS304 austenitic stainless steel was performed in air, in hydrogen gas, and in oxygen–hydrogen mixture. The fretting fatigue strength is more significantly reduced in hydrogen as compared to air. An increase in the fretting fatigue strength was found in the mixture. The mechanisms were investigated focusing on crack initiation. As the result, the crack initiation limit was significantly lower in hydrogen than in air, and increased in the mixture. The tangential force coefficient in the mixture is similar to that in air. The morphology of the fretting damage in the mixture was similar to that in air. These results indicated that the adhesion between contacting surfaces was prevented by addition of oxygen.

DOI： 10.1016/j.triboint.2014.02.025

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

Fretting fatigue is one of the major factors in the design of hydrogen equipment. The effect of internal hydrogen on the fretting fatigue strength of austenitic stainless steels was studied. The internal hydrogen reduced the fretting fatigue strength. The reduction in the fretting fatigue strength became more significant with an increase in the hydrogen content. The reason for this reduction is that the internal hydrogen assisted the crack initiation. When the fretting fatigue test of the hydrogen-charged material was carried out in hydrogen gas, the fretting fatigue strength was the lowest. Internal hydrogen and gaseous hydrogen synergistically induced the reduction in the fretting fatigue strength of the austenitic stainless steels. In the gaseous hydrogen, fretting creates adhesion between contacting surfaces where severe plastic deformation occurs. The internal hydrogen is activated at the adhered part by the plastic deformation which results in further reduction of the crack initiation limit.

DOI： 10.4028/www.scientific.net/AMR.891-892.891

担当区分：筆頭著者,　責任著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）

The authors have reported a significant reduction in fretting fatigue strength of austenitic stainless steels due to hydrogen. One of the causes of the reduced fretting fatigue strength in hydrogen is adhesion between contacting surfaces and following formation of small cracks which emanate from the adhered spots. The objective of this study is to understand the effect of hydrogen on the initiation of the small cracks under fretting fatigue conditions. Since the adhesion between contacting surfaces during fretting in hydrogen is very localized, a small contact length was used in this test in order to facilitate understanding by avoiding such localization. The fretting fatigue test of an austenitic stainless steel SUS304 was performed in air and 0.13MPa hydrogen. In the fretting fatigue test, hydrogen participates in the initiation of the fretting fatigue crack. It can be presumed that high strain at the contact edge activates hydrogen assisted fracture in terms of dislocation mobility. Adhesion mimic test, in which a small contact area was welded, was also performed. As the result, the crack initiation limit evaluated by the maximum range of shear stress was significantly lower in hydrogen than in air. Hydrogen assists small crack initiation under fretting fatigue conditions. This is one of the possible causes of the significant reduction of fretting fatigue strength in hydrogen.

DOI： 10.1299/kikaia.79.536

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

The objective of this study is to gain a better understanding of the hydrogen effect on the creep properties in order to establish the science enables to secure the safety of advanced high-temperature hydrogen technologies. Creep testing was carried out in argon and hydrogen gases at 873 K. The absolute gas pressure at which the creep tests were performed was 0.12 MPa. The material was JIS SUY-1 industrial pure iron. It was used as a model material having a BCC crystalline structure. A pure single-phase microstructure may be beneficial to elucidate the central role of hydrogen in the creep deformation in the presence of hydrogen. Hydrogen accelerated the creep strain rate resulting in significant reduction in the creep life. Based on the creep deformation mechanism map by Frost and Ashby, the creep in this experiment was driven by a power-law creep. The specimens formed a double cup fracture and exhibited a dimple fracture surface regardless of the environment and creep life. There was no significant difference in the fracture profile and fracture surface morphology. These observations indicated no change in the creep deformation mechanism in hydrogen. This was similar to the creep of SUS304 stainless steel in hydrogen. A similar mechanism that hydrogen may enhance the vacancy density, which led to a magnified lattice diffusion and associated dislocation climb, may be adopted.

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

The utilization of hydrogen is one of the keys to achieving sustainable development goals. During the transition toward a hydrogen society, the repurposing of existing non-hydrogen energy systems to a hydrogen one is very important in terms of costs and smooth reforming of energy and industrial structures. In this context, the mixing of hydrogen with natural gas can be one of the important technologies. In this study, fatigue crack growth testing was carried out in air, hydrogen, methane / hydrogen mixed gases in order to characterize the effect of hydrogen on the fatigue crack growth behavior in natural gas. The test material was JIS SCM435 low alloy steel. Two materials, which have different strength levels, were used. Both materials showed acceleration of the fatigue crack growth rate in only hydrogen and the mixed gases. The extent of the acceleration was less in the mixed gas compared with that in hydrogen. It is presumed that the hydrogen effect was less pronounced by the reduced partial hydrogen pressure in the mixed gas. Also, the extent of the acceleration was less in the lower strength material than in the higher strength one. It was confirmed that methane had no detrimental effect on the fatigue crack growth behavior of both materials.

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

The addition of carbon monoxide (CO) to hydrogen (H2) gas can mitigate the hydrogen environment embrittlement (HEE) of steels. The effect of gas pressure on the mitigation of the HEE of low carbon steels by the addition of CO was investigated in this study by a concerted effort of fracture toughness tests in H2 gas containing CO and molecular dynamics (MD) simulations of the surface chemical reactions of H2 and CO with the iron (Fe) surface. The degree of mitigation of the HEE depended on the gas pressure and CO concentration; the CO concentration to achieve complete HEE prevention was enhanced with an increase in the gas pressure, in other words, the CO mitigation effect was reduced by the increased gas pressure. The MD simulations revealed that the dissociation rate of dihydrogen molecules to atomic hydrogen on the Fe surface was significantly enhanced with an increase in the gas pressure, whereas the adsorption rate of CO on the Fe surface was almost independent of the gas pressure. These simulation results suggest that the hydrogen uptake into a steel is more effectively prohibited in a lower pressure H2 gas containing CO, which accounts for the gas pressure dependence of the CO mitigation effect observed in the fracture toughness tests.

担当区分：筆頭著者,　最終著者,　責任著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

The mitigation effect of carbon monoxide (CO) on hydrogen embrittlement (HE) was studied in terms of the loading rate dependence by fracture toughness tests, fatigue crack growth tests, and density functional theory (DFT) simulations. The experimental results demonstrated that the mitigation effect is reduced with a decreasing loading rate in the lower crack growth rate region. The DFT simulations revealed that this is caused by the incomplete coverage of the iron surface by CO. Oppositely, the mitigation effect increased with the decreasing loading rate in the higher crack growth rate region due to a lack of CO adsorption on the iron surface.

記述言語：英語   掲載種別：記事・総説・解説・論説等（学術雑誌）

The recently released 6th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) makes it clear that keeping global temperature rise below 1.5 degrees will require the world to achieve carbon-neutrality by 2050. Meeting this decarbonization challenge requires rapid and sustained innovations in materials science and engineering, with a focus on accelerating technology which will enable the realization of a carbon neutral energy system. This study details the broad, yet targeted research themes being pioneered within the International Institute for Carbon-Neutral Energy Research (I2CNER). These approaches include hydrogen materials compatibility, novel catalysis concepts anchored on bio-mimetic activation of small molecules, promising materials for efficient and durable fuel and solar cells, nanostructure-based thermal energy storage, nanomembranes and technology for direct air capture of CO2, geomaterials and devices for carbon storage, and next generation nonflammable refrigerants. We outline the state of the art for this suite of materials and technologies and detail the impact of their deployment on reducing the CO2 emissions of Japan by 2050. More specifically, approximately 0.46% of the total required CO2 reductions can be realized by I2CNER’s current innovations if they are applied to appropriate energy systems. This study represents a solid outcome of interdisciplinary, international collaboration.

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

Ammonia (NH3) was added to hydrogen gas as an impurity, then hydrogen embrittlement (HE) of the SCM440 low-alloy steel was investigated by fracture toughness tests. NH3 showed inhibition of the HE when the loading rate was relatively high. However, the inhibitory effect decreased with the decrease in the loading rate, as well as with an increase in the gas pressure. It was found that NH3 acted as both an inhibitor of the HE and a cause of the HE depending on the loading rate and gas pressure.

担当区分：最終著者   記述言語：英語   掲載種別：研究論文（学術雑誌）

The objective of this study is to gain a basic understanding of the effect of hydrogen on the fatigue limit. The material was a low-alloy steel modified to be sensitive to hydrogen embrittlement by heat treatment. A statistical fatigue test was carried out using smooth and deep-notched specimens at a loading frequency of 20 Hz. The environment was laboratory air and hydrogen gas. The hydrogen gas pressure was 0.1 MPa in gauge pressure. The fatigue limit of the smooth specimen was higher in the hydrogen gas than that in air, although the material showed severe hydrogen embrittlement during the SSRT (Slow Strain Rate Test). The fatigue limit of the deep-notched specimen in the hydrogen gas was the same as that in air. For the smooth specimen, the fatigue limit was determined by whether or not a crack was initiated. For the deep-notched specimen, the fatigue limit was determined by whether or not a crack propagated. The results can be interpreted as that hydrogen has no significant effect on crack initiation in the high-cycle fatigue regime and affected the threshold of the crack propagation.

DOI： 10.1016/j.prostr.2019.12.056

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

This study investigated effects of addition of O2 and CO to H2 gas environments on a hydrogen-accelerated fatigue crack growth in a pipe steel. Both O2 and CO inhibited the hydrogen-accelerated fatigue crack growth. The inhibitory effect of O2 was well-interpreted by the mechanism proposed by Somerday et al. (2013). On the other hand, different mechanisms, which are the reaction rate of CO with the iron surface and time for hydrogen diffusion in the material, dominated the inhibitory effect of CO.

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

Fracture toughness tests of a pipe steel were conducted in hydrogen environments. The crosshead speed was 2.0 × 10-5 mm/s. The gas pressure was 0.6 MPa. The temperature was 293 K. Two gas systems were used to investigate the effect of a small amount of oxygen on hydrogen environment-assisted cracking (HEAC). One was a closed gas system, where a hydrogen gas is stored in the gas chamber. In this system, oxygen content in the gas chamber gradually increased with time and reached 1 vppm during the test. Another was an open gas system, where a high-purity hydrogen gas is continuously supplied into the gas chamber. In this system, the oxygen content in the gas chamber can be kept at 0.1 vppm. The fracture toughness in the hydrogen with 0.1 vppm oxygen was significantly lower than that in air. The fracture toughness in the hydrogen with the maximum oxygen content of 1 vppm was higher than that in the hydrogen with 0.1 vppm oxygen. 1 vppm oxygen partially inhibited the HEAC. Owing to a detailed analysis of the test results, the critical oxygen content to appear the inhibitory effect of oxygen under the test conditions could be estimated as 0.3 vppm.

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

The effect of impurities, added to hydrogen environment, on hydrogen embrittlement (HE) was investigated in association with the effect of material strength. Addition of CO and O2 deactivated the HE. O2 prevented the HE with lower concentration than CO. Material with higher strength required larger amount of impurities to prevent the HE. Reduction of hydrogen uptake was the primary result of the addition of the impurities, but a certain amount of hydrogen uptake occurred when the HE was completely inhibited. Discussion regarding essential HE mechanisms for the fracture morphologies was required to interpret the effect of material strength.

担当区分：筆頭著者,　責任著者   記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

An optical MEMS sensor device was developed to measure the relative slip range during fretting fatigue in hydrogen. The relative slip range is an important parameter to understand the mechanism of fretting fatigue failure, but there are measurement difficulties in a hydrogen environment. The MEMS sensor we developed showed a good reproducibility, accuracy and stability in hydrogen. For further improvement of the measurement, a calibration of the elastic deformation was done by both experiments and calculations. By applying this MEMS sensor, it was found that the relative slip range in hydrogen was significantly lower than that in air. This result was consistent with the experimental fact that adhesion between contacting surfaces occurred during fretting fatigue in hydrogen.

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

To clarify the effect of O2 addition as an impurity in a H2 environment on the fretting fatigue strength of JIS SUS304 austenitic stainless steel, a control method to add ppm-level O2 to a H2 environment was established. The experiments were then carried out in H2 environments with O2 concentrations of 0.088, 5, 35 and 100 vppm. The fretting fatigue strength in the H2 was significantly reduced by the addition of a small amount of O2. The reduction was caused by the change in the stress conditions at the contact part due to the addition of O2 that changed the oxidation and fretting wear behavior on the fretted surface.

担当区分：最終著者   記述言語：日本語   掲載種別：記事・総説・解説・論説等（学術雑誌）

担当区分：最終著者   記述言語：日本語   掲載種別：研究論文（学術雑誌）

DOI： 10.18914/tribologist.60.10_651

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

Fretting fatigue, which is a composite phenomenon of metal fatigue and friction, is one of the major factors in the design of mechanical components as it significantly reduces fatigue strength. Since hydrogen can influence both fatigue and friction, fretting fatigue is one of the important concerns in designing hydrogen equipment. The authors carried out the fretting fatigue tests on austenitic stainless steels in order to characterize the effect of hydrogen and to explain the mechanism responsible for hydrogen embrittlement. In this study, the significant reduction in fretting fatigue strength due to hydrogen is shown including other factors influencing the fretting fatigue strength such as surface roughness, hydrogen content and the addition of oxygen. The cause of the reduction in the fretting fatigue strength in hydrogen is local adhesion between the contacting surfaces and subsequent formation of many small cracks. Furthermore, hydrogen enhances crack initiation under fretting fatigue conditions. Transformation of the microstructure from austenite to martensite is another possible reason. A hydrogen charge also reduces the fretting fatigue strength. The cause is the reduction in the crack growth threshold, ΔKth, due to hydrogen.

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

記述言語：英語   掲載種別：研究論文（国際会議プロシーディングス）

受賞国：日本国