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

A novel testing method was developed to evaluate the fracture toughness of ultra-high-strength (UHS) steel sheets. The study also examined the influence of specimen thickness, utilizing specimens of varying thicknesses (1.6 ∼ 10 mm). While the toughness of all specimens was determined by the threshold for planar crack propagation originating from the fatigue pre-crack-front near the mid-thickness zone, the apparent fracture toughness was dependent on specimen thickness. This phenomenon was attributed to the crack-closure effects experienced during the pre-cracking process. In-depth analyses revealed that, in the absence of crack-closure effects, the intrinsic toughness remained unaffected by changes in specimen thickness.

DOI： 10.1016/j.engfracmech.2024.110322

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Language：Japanese   Publisher：The Japan Society of Polymer Processing  

DOI： 10.4325/seikeikakou.36.307

CiNii Research

Language：Japanese   Publisher：The Society of Materials Science, Japan  

<p>The global promotion of hydrogen refueling stations for fuel cell vehicles (FCV) is driven by the vision of achieving a Hydrogen Society. In Japan, a design pressure of 70 MPa has become the standard to enhance the cruise distance of FCV. However, this higher gas pressure increases the risk of hydrogen leakage from mechanical joints. Consequently, the infrastructure industry emphasizes the need to understand the hydrogen compatibility of welding joints to ensure the safer and more cost-effective operation of these facilities. This paper presents the results of slow strain rate tensile (SSRT) tests conducted in a high-pressure gaseous hydrogen environment to assess hydrogen embrittlement in austenitic stainless joints. During specimen preparation, U-grooves were machined at the edges of the plates, which were then butted together to form filler welded joints using Gas Tungsten Arc Welding (GTAW) with commercially available AWS ER 316L filler wire (high Ni-equivalent) under two different weld heat inputs (9.9 and 36.0 kJ/cm). The welded joints exhibited a low susceptibility to hydrogen embrittlement, despite the increase in the amount of crystallized delta ferrite in the weld metal with a decrease in the weld input. Based on the results, we conclude that the Type 316L welded joint with AWS ER 316L filler is suitable for various components in hydrogen refueling stations with the applied welding conditions. This valuable insight contributes to realizing safer and more reliable hydrogen refueling facilities while also reducing maintenance costs.</p>

DOI： 10.2472/jsms.73.603

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CiNii Research

Publisher：Engineering Fracture Mechanics  

The susceptibility of pipeline steels to hydrogen embrittlement, which can significantly impair mechanical performance, especially fracture toughness, is a critical issue in infrastructure integrity. This study evaluates the fracture toughness of modern and vintage API X65 steels under cathodic polarization (CP) conditions, employing methods such as rising displacement testing, stepwise rising constant load testing, and constant load testing. Tearing crack growth was detected at crack tip opening displacements (CTODs) of 0.032 to 0.055 mm while subsequent crack arrest under constant force loading was observed with CTOD thresholds of 0.148 mm and 0.255 mm for modern and vintage X65, respectively. Electron channelling contrast imaging demonstrated that arrested cracks in both steels exhibited reduced plasticity and a higher propensity for crack branching compared to propagating cracks. These findings highlight the need for tailored strategies to mitigate the effects of hydrogen embrittlement in pipeline materials and underscore the differences in damage tolerance between steel generations.

DOI： 10.1016/j.engfracmech.2024.110026

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Engineering Failure Analysis  

Ductile cast irons have for long been widely used across diverse industries, due to their excellent sliding properties, remarkable castability and notable ductility. Nevertheless, the hydrogen embrittlement of these irons is yet to be comprehensively understood. This study aimed to uncover their potential for application in hydrogen service. The primary objective was to comprehend the effects of various factors such as temperature, oxygen impurity levels and loading frequency on the fatigue crack-growth (FCG) properties of the ductile cast iron, JIS-FCD400, in hydrogen gas environments. Notably, the material exhibited stable FCG behavior in pure hydrogen. Moreover, some intriguing insights emerged, indicating that oxygen impurity and elevated temperature tend to mitigate the detrimental effects of hydrogen. Collectively, these findings lay the groundwork for the plausible feasibility of incorporating ductile cast irons into hydrogen service.

DOI： 10.1016/j.engfailanal.2024.108012

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

Effect of the addition of Cr, Mn, or Fe to Ni on the material's hydrogen solubility are investigated to establish guidelines for designing materials authorized to use in high-pressure hydrogen environments. The exposure to 100 MPa hydrogen gas at 160 °C reveals that the addition of 30 wt%Fe to Ni decreases the hydrogen solubility by 40%, while the addition of 20 wt%Cr to Ni increases it by 65%. Surprisingly, incorporating 20 wt%Mn causes a remarkable increase of 992%. By employing a combined approach of a random alloy model based on the special quasi-random structure and first-principles calculations, we are able to semi-quantitatively predict the effects of these alloying elements on hydrogen solubility. Moreover, the deconvolution of hydrogen bonding energy into its volumetric, mechanical, and chemical components clarifies that the significant impact of Mn is primarily attributed to a substantial expansion of the lattice constant.

DOI： 10.1016/j.ijhydene.2023.09.056

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Publisher：International Journal of Hydrogen Energy  

The hydrogen compatibility of two X65 pipeline steels for transport of hydrogen gas is investigated through microstructural characterization, hydrogen permeation measurements and fracture mechanical testing. The investigated materials are a quenched and tempered pipeline steel with a fine-grained homogeneously distributed ferrite-bainite microstructure, and hot rolled pipeline steel with a ferrite-pearlite banded microstructure. All tests are performed both under electrochemical and gaseous hydrogen charging conditions. A correlation between electrochemical hydrogen charging and gaseous charging is determined. The results point to inherent differences in the interaction between hydrogen and the two material microstructures. Further research is needed to unveil the influence of material microstructure on hydrogen embrittlement.

DOI： 10.1016/j.ijhydene.2024.06.309

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Publisher：AMPP Annual Conference and Expo 2024  

This paper presents the output from a study with the aim to correlate hydrogen gas charging and electrochemical charging for a X65 pipeline steel. The hydrogen uptake and diffusivity were evaluated via the permeation technique by employing both hydrogen gas charging and electrochemical charging. The effective diffusion coefficient, Deff, was determined by partial transients. The sub-surface hydrogen concentration, C0R, was then employed to determine the equivalent hydrogen pressure, pH2eq, of electrochemical charging conditions. In parallel, a permeation cell consisting of two membranes with thickness 6 mm welded together with a narrow gap in between was built. This unit was exposed to equal cathodic polarization conditions as the electrochemical permeation cell. The equilibrium pressure in the gap was measured and compared to the calculated hydrogen pressure from the gas permeation test with good correlation. Finally, an initial evaluation through toughness tests with the same X65 material executed in i) hydrogen gas and ii) electrolytic charging at equivalent hydrogen concentrations, gave promising results concerning the correctness of the correlation between the two hydrogen sources.

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Language：Others   Publishing type：Research paper (scientific journal)   Publisher：Materials Letters  

In essence, additively-manufactured, Ni-based superalloy 718 (AM Alloy 718) possesses a strong crystallographic texture, δ-phases and Laves phases, whose effect on fatigue resistance under torsional cyclic loading is yet to be investigated. In order to elucidate the role of microstructural features in small-crack behavior, torsional fatigue testing was conducted on an AM Alloy 718. Test results revealed that crack-growth was controlled by the twist angle, but also by the tilt angle which had been reported to play a minor role in fatigue resistance under uniaxial cyclic loading. Furthermore, the impact of precipitates appeared to be insignificant, with no strong correlation made between their dispersion and the crack-growth rate.

DOI： 10.1016/j.matlet.2023.134844

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

Push-pull and torsional fatigue tests were conducted using additively-manufactured, Ni-based superalloy 718 samples with various defects and different microstructures, aiming to comprehensively examine fatigue limit determining factors such as detrimental defects and microstructural features. Test results revealed that the fatigue limit in each loading condition was governed by a shear-mode, crack-growth threshold, exhibiting a crack-size dependence similar to that of the wrought alloy. The additively-manufactured materials displayed a substantially reduced crack threshold as compared to wrought ones, potentially attributed to a high volume of low twist angles between neighboring crack-planes.

DOI： 10.1016/j.ijfatigue.2023.107801

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

The mechanism that determines the fatigue limit associated with small-crack arrest in Ni-based superalloy 718 remains under debate. In order to comprehensively assess the alloy's fatigue limit in relation to the growth threshold of a small fatigue crack, (i) a ΔKI-decreasing fatigue crack-growth test, (ii) torsional fatigue tests and (iii) tension-compression fatigue tests were conducted under fully-reversed loading conditions. In addition, elastic, crack-tip stress fields were analyzed for cracks under various loading conditions. Based on the results, a novel strategy for evaluating the alloy's fatigue limit is proposed after consideration of two crack-arresting modes (i.e., opening and shear modes).

DOI： 10.1016/j.engfracmech.2023.109500

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science  

Microstructural paths of hydrogen-assisted fatigue crack growth (HAFCG) in tempered martensitic steels were investigated relying on martensite boundary characteristics. Factors determining the HAFCG paths were tensile strength (TS)-dependent. HAFCG paths occurred preferentially along prior austenite grain boundaries with a brittle trend in 1025-MPa-TS steel. However, 811-MPa-TS steel showed HAFCG predominantly along block boundaries with large plasticity evolution, indicating that the mechanism of HAFCG in lower-TS steel required the assistance of plasticity accumulation during cyclic loadings. Graphical abstract: [Figure not available: see fulltext.].

DOI： 10.1007/s11661-023-07041-9

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

Fatigue tests of circumferentially notched Type 304 austenitic stainless steel (ASS) illuminated the unique dependences of fatigue limit on the stress concentration factor, Kt, and stress ratio, R. Fatigue limit under negative R was superior to that under positive R, a result was unexpected from the empirical behavior of smooth ASS. Moreover, switching R from negative to positive apparently diminished the effect of Kt escalation on the deterioration of the fatigue limit. Consolidation of microhardness tests, microstructural observation, finite element analysis revealed that these anomalies stem from the presence/absence of cyclic plasticity and martensitic transformation, which dynamically strengthen the notch-root.

DOI： 10.1016/j.ijfatigue.2022.107229

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

DOI： https://doi.org/10.1016/j.tafmec.2022.103586

Other Link： https://www.sciencedirect.com/science/article/pii/S0167844222003305

Publisher：Theoretical and Applied Fracture Mechanics  

A novel method for the numerical analysis of stress intensity factors (SIFs) was developed for multiple, 3-D-distributed, semi-elliptical cracks on the surface of a semi-infinite elastic body under arbitrary loading conditions. Emphasis was placed on the nature of interactions between cracks, with examination by the distributed infinitesimal dislocation loop technique, an eigenstrain method. SIFs of 100 distributed surface cracks were acquired in approximately 15 min when calculated on a standard personal computer, owing to the numerical integration approaches and meshing procedure adopted. The validity of the obtained results is verified and its applicability to some engineering problems is discussed.

DOI： 10.1016/j.tafmec.2022.103586

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

Fatigue crack-growth (FCG) tests were conducted in 90-MPa-hydrogen gas on three martensitic steels with tensile strengths of 811, 921 and 1025 MPa. Increased strength levels resulted in augmented, hydrogen-induced FCG acceleration. In the highest-strength material, the FCG rate per cycle was dependent on test frequency, i.e., the crack-growth distance was proportional to load duration. Several observations and analyses revealed that such time-dependent FCG was due to stress-driven cracking along hierarchical martensite boundaries, stemming from the hydrogen-induced degradation of their cohesive strengths as a result of competition between mechanically-determined crack-tip stress (driving stress) and statistically-distributed boundary strength (resistance stress).

DOI： 10.1016/j.ijfatigue.2022.107039

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Repository Public URL： https://hdl.handle.net/2324/7161789

Language：English   Publisher：Elsevier  

Fatigue crack-growth (FCG) tests were conducted in 90-MPa-hydrogen gas on three martensitic steels with tensile strengths of 811, 921 and 1025 MPa. Increased strength levels resulted in augmented, hydrogen-induced FCG acceleration. In the highest-strength material, the FCG rate per cycle was dependent on test frequency, i.e., the crack-growth distance was proportional to load duration. Several observations and analyses revealed that such time-dependent FCG was due to stress-driven cracking along hierarchical martensite boundaries, stemming from the hydrogen-induced degradation of their cohesive strengths as a result of competition between mechanically-determined crack-tip stress (driving stress) and statistically-distributed boundary strength (resistance stress).

CiNii Research

Language：Others   Publishing type：Research paper (scientific journal)   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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Other Link： https://www.jstage.jst.go.jp/article/tetsutohagane/108/2/108_TETSU-2021-093/_pdf

Language：Others   Publishing type：Research paper (scientific journal)   Publisher：International Journal of Fatigue  

Fatigue crack growth of two carbon steels with different pearlite volume fractions were studied in pressurized gaseous hydrogen environment. Notably, pearlite was found to mitigate hydrogen-assisted fatigue crack acceleration. This positive impact of pearlite was ascribed to ferrite/cementite lamellar aligned perpendicularly to the cracking direction, which functioned as barriers to intermittently arrest the crack propagation. Meanwhile, brittle delamination fracture ensued in the pearlite lamellar lying parallel to the crack-plane increased the crack growth rate and compromised the above positive effect to some extent. The material behavior is rationalized in light of fractographical observations and microstructural analyses of the crack-wake.

DOI： 10.1016/j.ijfatigue.2021.106561

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

Investigations were conducted on the interaction-effects of adjacent, circumferential V-notches on the fatigue limit of Alloy 718, with the aid of fully-reversed, tension–compression fatigue tests. Contrary to expectation, specimen with seven adjacent circumferential notches had lower fatigue limit than that with a single circumferential notch. This phenomenon was attributed to the increased stress intensity factor (SIF) range of shear-mode fatigue cracks at notch-roots, resulting from competition between two factors: (i) stress mitigation through the mechanistic interaction of notches (decreased SIF) and (ii) the orderly, spatial alignment of crack-planes, due to the asymmetric stress-fields in the vicinity of notch-roots (increased SIF).

DOI： 10.1016/j.engfracmech.2021.108015

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

Fatigue strength of Ni-based superalloy 718 was investigated relative to shear-mode crack-growth thresholds, to evaluate the influence of crack-opening/-closing stresses perpendicular to shear-mode crack-planes. The impact of precipitation-hardening was also examined. Using solution-treated and precipitation-hardened materials, three different types of fatigue tests were performed: (i) push–pull; (ii) pure-torsion; (iii) torsion with superposed static tension. All tests revealed non-propagation of small, shear-mode cracks, while confirming the negligible impact of precipitation-hardening on shear-mode crack-growth threshold. Additionally, crack-opening/-closing stresses were determined to only imperceptibly alter shear-mode resistance. Based on these findings, a novel strategy was proposed to quantify the alloy's fatigue strength.

DOI： 10.1016/j.ijfatigue.2021.106228

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

DOI： https://doi.org/10.1016/j.ijfatigue.2020.106138

Language：English  

DOI： 10.1016/j.ijfatigue.2020.106138

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

DOI： https://doi.org/10.1299/transjsme.21-00084

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

DOI： https://doi.org/10.1016/j.msea.2020.140580

Language：English  

DOI： 10.1016/j.msea.2020.140580

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

© 2020 Hydrogen Energy Publications LLC Four, hot-rolled, plain-carbon steels with varying carbon content were subjected to slow strain-rate tensile (SSRT) tests in a 95-MPa gaseous hydrogen environment at ambient temperature. The influence of pearlite volume fraction on the magnitude of hydrogen-induced degradation of the materials’ strength and ductility was thereby determined. Hydrogen was seen to significantly affect strain-to-failure and reduction-in-area in all four materials, wherein such a loss of tensile ductility was ascribed to the premature initiation and subsequent propagation of surface micro-cracks as revealed by the quantitative damage evolution analyses on the post-fractured specimens. The pearlite grains on sample surfaces manifestly served as the preferential origins of hydrogen-induced micro-cracks, resulting in more considerable embrittlement in materials possessing a higher percentage of pearlite, due to the rapid coalescence of discrete embryonic damage during tensile straining.

DOI： 10.1016/j.ijhydene.2020.11.137

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

DOI： https://doi.org/10.1016/j.ijhydene.2020.11.137

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

DOI： https://doi.org/10.1111/ffe.13405

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

© 2020 Elsevier Ltd 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 (θx), 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 θx.

DOI： 10.1016/j.ijfatigue.2020.105806

Language：Japanese  

DOI： 10.1299/transjsme.20-00306

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

© 2020 Elsevier Ltd 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

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© 2020 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：Others   Publishing type：Research paper (scientific journal)  

© 2020 Elsevier B.V. Fatigue strength of precipitation-hardened materials is well known to be lower than that expected from their high yield and tensile strengths. To elucidate the reasons behind such anomaly, torsional fatigue tests, in addition to uniaxial fatigue tests, were conducted on samples of the Ni-based superalloy 718, subjected to two different heat treatments (solution-annealing and precipitation-hardening). Despite the significant differences in tensile and yield strengths between the two samples, the torsional fatigue strengths were mutually equivalent. This result was interpreted in light of the resistance against fatigue crack-initiation and -propagation in shear-mode, which cannot be enhanced by precipitation-hardening due to the depletion of precipitates by persistent dislocations motion within localized glide-bands.

DOI： 10.1016/j.matlet.2020.128377

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

© 2020 John Wiley & Sons Ltd In order to study the residual stress induced by foreign object damage (FOD), the distribution of residual stress caused by the impact of a hard spherical body was measured via the sin2ψ technique, using synchrotron X-ray. A steel sphere was impacted onto a flat surface of a Ti-6Al-4V alloy from an angle of either 90° or 45°, at a velocity of 180 m/s. The same sphere was also quasi-statically pressed into the surface. In the cases of right-angled impact and quasi-static indentation, a compressive residual stress was extensively distributed inside the generated crater. No remarkable difference in residual stress distribution was noted between the dynamic case and the quasi-static case. However, at an impact angle of 45°, a tensile residual stress that is more detrimental to fatigue strength was widely distributed inside the crater. Outside of the craters, tensile stress was generally observed in all cases.

DOI： 10.1111/str.12367

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

© 2020 Elsevier Ltd 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：English  

DOI： 10.5006/3551

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

DOI： https://doi.org/10.1299/transjsme.20-00172

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

© 2020 The Authors This study investigates the effect of interacting small cracks on the fatigue limits of two types of pure iron of varying strengths and of a bearing steel. The experimental results revealed that although the fatigue limits were essentially controlled by the mechanics of interacting cracks, the non-propagating crack features and the severity of crack coalescence varied greatly among the different materials. In addition, it is demonstrated that the material-independent, analytical, interaction-criterion alone is not sufficient to estimate interaction effects in reality. This research shows that the material effect can be considered in terms of hardness and non-propagating crack size.

DOI： 10.1016/j.ijfatigue.2020.105560

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

© 2020 Elsevier Ltd 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)  

DOI： https://doi.org/10.1016/j.ijhydene.2020.01.117

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

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

© 2020 Hydrogen Energy Publications LLC The hydrogen compatibility of metallic materials is often evaluated by conducting tensile tests of H-charged specimens in air or inert gas at ambient temperature; however, it is not clear whether the H distribution calculated with hydrogen diffusivity under a diffusion-controlled process is consistent with the actual distribution. This study estimated the hydrogen distribution in a H-charged nickel maintained in air at ambient temperature for a few months after exposure to hydrogen gas by using the Vickers hardness test and secondary ion mass spectrometry. Both methods provided similar H distributions, which were fitted by the solution of a diffusion equation under a diffusion-controlled process, and the hydrogen diffusivity was also determined. The estimated H distributions were successfully fitted by the solution of the diffusion equation, and the determined hydrogen diffusivity of nickel was consistent with literature data, indicating that the calculated H distribution reproduced the actual one.

DOI： 10.1016/j.ijhydene.2020.01.117

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

© 2019 Acta Materialia Inc. The fatigue limit of a circumferentially-notched austenitic stainless steel exhibited peculiar improvement under fully-reversed loading (R =−1), as compared with that under tension-tension loading (R = 0.1), behavior considered to be unconventional in smooth specimens of similar materials. This was attributed to the different cyclic stress-strain responses at the notch-root, where it was almost elastic at R = 0.1, whereas an elasto-plastic response continued up to later stage of the fatigue process at R =−1. Such cyclic plasticity enhanced martensitic transformation locally and resulted in hardening, thereby restricting fatigue crack initiation from the notch-root.

DOI： 10.1016/j.scriptamat.2019.09.014

Language：Japanese  

DOI： 10.1299/transjsme.19-00350

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

© 2019 The phenomenon of hydrogen-trapping and its quantitative contribution to hydrogen-induced intergranular (IG) fracture were studied using a combination of thermal desorption analysis, secondary ion mass spectrometry and slow strain rate tensile tests. Hydrogen was trapped along grain boundaries (GBs) with a binding energy of ≈20 kJ/mol, accompanied by IG sulfur. The true fracture stress and fracture surface morphology were strongly dependent on the concentration of trapped hydrogen, leading to the conclusion that the hydrogen-induced IG fracture of pure Ni is controlled by the concentration of hydrogen trapped along GBs, and not by the concentration of lattice hydrogen.

DOI： 10.1016/j.mtla.2019.100478

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

© 2019 Elsevier Ltd 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 (scientific journal)  

DOI： https://doi.org/10.1016/j.prostr.2019.12.034

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

DOI： 10.1016/j.prostr.2019.12.035

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Comparative study of hydrogen-induced intergranular fracture behavior in Ni and Cu–Ni alloy at ambient and cryogenic temperatures
© 2019 Elsevier B.V. In order to clarify the contribution of dislocation‒hydrogen interaction on the hydrogen embrittlement (HE) of pure Ni and of Cu‒55 wt&#37; 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：Others   Publishing type：Research paper (scientific journal)  

© 2019 Elsevier Ltd Three effects of hydrogen have been reported on an austenitic stainless steel (Type-316L), a Cr-Mo steel (JIS-SCM435) and a carbon steel (JIS-SGP) during slow strain rate tensile (SSRT) tests performed in air on H-charged, smooth round-bar specimens. Two H-charging conditions were considered: exposure to 100-MPa hydrogen gas (270 °C, 200 h for Type-316L), and immersion in a NH4SCN solution (40 °C, 48 h for other steels). Modifications of the micro-void coalescence (MVC) mechanism were observed for each steel: decrease of dimple size (Type-316L), increase of dimple size (JIS-SGP), and formation of quasi-cleavage (QC) surfaces (JIS-SCM435). To clarify the contribution of the hydrogen-induced cracking (HIC) mechanism to these failures, the pressure build-up in preexisting cavities and its impact on the material strength was simulated by finite difference method (FDM) and finite element method (FEM). The failure criterion was defined based upon the elastoplastic fracture mechanics parameter: J-integral. For Type-316L, no effect of internal pressure on the fracture was expected. For JIS-SCM435 and JIS-SGP, although an effect of internal pressure exists, its relatively low value cannot lead to failure. SSRT tests were performed on Type-316L under 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. The experimental results successfully supported the simulation-based conclusions.

DOI： 10.1016/j.engfracmech.2019.106505

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© 2019 Elsevier Ltd Fatigue tests were performed using circumferentially-notched, round bar specimens with a stress concentration factor, Kt, of 6.6 for Type 304, metastable, austenitic stainless steel. The tests were carried out in ambient air and in 0.7 MPa hydrogen gas at room temperature. In a relatively higher stress amplitude regime (i.e., the amplitude resulting in Nf < 105), the hydrogen gas environment caused a marked degradation in fatigue life. In contrast, in a relatively lower stress amplitude regime (i.e., the amplitude resulting in Nf > 105), it appeared that fatigue life did not differ between air and hydrogen gas. It was also confirmed that the fatigue limit appeared not have been degraded in the hydrogen environment though there was a slight difference between the data obtained in two environments. The fatigue life curve and fatigue limit were predicted by assuming that the notch was equivalent to a circumferential crack. Consequently, there was a significant disparity between the prediction and the experimental results. As a result of microscopic observations of the fracture process in combination with elastic-plastic finite element analyses, these discrepancies were attributed to (i) complex cyclic plastic deformation behavior under large- and small-scale yielding conditions within the vicinity of the notch root, (ii) the retardation of crack initiation in the finite life regime, and (iii) the absence of non-propagating cracks at the fatigue limit, all of which are typical characteristics of metastable austenitic stainless steel.

DOI： 10.1016/j.engfracmech.2019.05.005

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© 2019 Wiley Publishing Ltd. Tension-compression fatigue tests were performed on two types of Ni-based superalloy 718 with different microstructures in which small artificial defects of various sizes and shapes were introduced. The susceptibility of the fatigue strength to the defects varied significantly with the microstructure morphology, ie, a smaller grain size made the alloy more susceptible to defects. The fatigue limit as a small-crack threshold was successfully predicted using the √area parameter model. The evaluation of the fatigue limit was classified into the following three stages depending on the defect size: (a) harmless defect regime, (b) small-crack regime, and (c) large-crack regime. Such a classification enabled comprehensive evaluation of the fatigue limit in a wide range of defect size, considering (a) defect size over a range of small crack to large crack and (b) characteristics of matrix represented by grain size and hardness.

DOI： 10.1111/ffe.12989

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Hydrogen-assisted crack propagation in α-iron during elasto-plastic fracture toughness tests
© 2019 Elsevier B.V. Elasto-plastic fracture toughness tests of a commercially pure iron were performed in air and in hydrogen gas at two different pressures. Some unique characteristics of hydrogen-enhanced cracking were exhibited at both the macroscopic and microscopic length scales, based on the observation of fracture surface, fracture plane, plasticity distribution and dislocation structure. The possible mechanisms responsible for the hydrogen-induced degradation of fracture toughness are discussed.

DOI： 10.1016/j.msea.2019.04.084

Language：Japanese  

Other Link： https://www.jstage.jst.go.jp/article/transjsme/85/872/85_18-00415/_article/-char/en

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© 2018 Elsevier Ltd Rolling contact fatigue (RCF) tests were conducted on rolling bearings with drilled holes at the mid-point of the raceway. Flaking limit was determined by the shear-mode threshold of a crack that emanated at the edge of drilled hole. To quantify the RCF strength according to fracture mechanics principles, Mode II stress intensity factor (SIF) range of the crack was derived. Taking a small crack effect on the shear-mode fatigue crack into account, RCF strength was successfully evaluated as a crack problem, irrespective of the diameters and depths of the hole. The values of the threshold SIF range obtained by RCF tests were in good agreement with those obtained in the torsional fatigue tests under static compression.

DOI： 10.1016/j.ijfatigue.2018.10.003

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© 2018 Elsevier B.V. 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 (other academic)  

Three types of strength tests, slow strain rate tensile (SSRT), fatigue life, and fatigue crack growth (FCG) tests, were performed using six types of aluminum alloys, 5083-O, 6061-T6, 6066-T6, 7N01-T5, 7N01-T6, and 7075-T6, in air and 115 MPa hydrogen gas at room temperature. All the strength properties of every material were not deteriorated in 115 MPa hydrogen gas. In all the materials, FCG rates were lower in 115 MPa hydrogen gas than in air. This was considered to be due to a lack of water-or oxygen-adsorbed film at crack tip in hydrogen gas. In 5083-O, 6061-T6 and 6066-T6, relative reduction in area (RRA) were remarkably higher in 115 MPa hydrogen gas than in air. These differences were attributed to a hydrostatic pressure produced in 115 MPa hydrogen gas. In contrast, in 7N01-T5, 7N01-T6 and 7075-T6, the values of RRA in 115 MPa hydrogen gas were nearly the same as those in air. Observation of fractured specimens inferred that the degree of such a hydrogen-induced improvement was determined by the fracture mode (e.g. cupand-cone or shear fracture), which is dominated by the microstructure morphology.

DOI： 10.1115/PVP2019-93478

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 (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 (scientific journal)  

Hydrogen energy is a possible solution for storage in the future. The resistance of packaging materials such as stainless steels has to be guaranteed for a possible use of these materials as containers for highly pressurized hydrogen. The effect of hydrogen charging on the nucleation and growth of microdamage in two different austenitic stainless steels AISI316 and AISI316L was studied using in situ tensile tests in synchrotron X-ray tomography. Information about damage nucleation, void growth and void shape were obtained. AISI316 was found to be more sensitive to hydrogen compared to AISI316L in terms of ductility loss. It was measured that void nucleation and growth are not affected by hydrogen charging. The effect of hydrogen was however found to change the morphology of nucleated voids from spherical cavities to micro-cracks being oriented perpendicular to the tensile axis.

DOI： 10.3390/ma12091426

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

© 2018 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：Japanese  

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Fatigue limit of carbon and Cr–Mo steels as a small fatigue crack threshold in high-pressure hydrogen gas
© 2018 Hydrogen Energy Publications LLC The fatigue limit properties of a carbon steel and a low-alloy Cr–Mo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the Cr–Mo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtained S–N data inferred that the fatigue limit was not negatively affected by hydrogen in either of the steels. Observation of fatigue cracks in the unbroken specimens revealed that non-propagating cracks can exist even in 115-MPa hydrogen gas, and that the crack growth threshold is not degraded by hydrogen. The experimental results provide justification for the fatigue limit design of components that are to be exposed to high-pressure hydrogen gas.

DOI： 10.1016/j.ijhydene.2018.09.026

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

© 2018 Acta Materialia Inc. 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

Other Link： https://www.sciencedirect.com/science/article/pii/S1359646218303294

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© 2018 Acta Materialia Inc. 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

Other Link： https://www.sciencedirect.com/science/article/pii/S1359645418305093

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© 2018 Elsevier B.V. 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

Other Link： https://www.sciencedirect.com/science/article/pii/S0921509318309390

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

DOI： 10.2472/jsms.67.723

Other Link： https://www.jstage.jst.go.jp/article/jsms/67/7/67_723/_article/-char/ja/

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

© 2018 The Society of Materials Science, Japan Hydrogen-entry properties of torsional prestrained ferritic-pearlitic low-carbon JIS-S10C, JIS-S25C and JIS-S45C steels were investigated. A round bar specimen was prestrained by torsion test and then charged with hydrogen by cathodic charging or immersion charging. The torsion test enabled to introduce a large plastic deformation to the specimen without necking, which led to much higher level of hydrogen content, CH, than tensile test. CH was increased with an increase in torsional prestrain at the specimen outer surface, εpre,s. In the prestrain range, εpre,s < 30&#37;, there was little influence of pearlite on CH. By contrast, when εpre,s > 40&#37;, there was a marked influence of pearlite on CH in JIS-S10C and JIS-S25C, whereas, CH in JIS-S45C was lower than that of JIS-S25C despite the fact that fraction of pearlite in JIS-S45C was much higher than that in JIS-S25C. It was presumed that such a complex hydrogen-entry property was determined by the competition between (I) the increase in dislocation density near the boundaries between ferrite and pearlite and (II) the inhibition against the increase in dislocation density by a stress redistribution in pearlite grains. The torsional strain at final rapture was the largest in JIS-S10C, which accordingly conduced to the largest hydrogen content in the broken specimen. The series of experimental results manifested that the torsion test enables the investigation of strength properties as well as the hydrogen-entry properties for BCC steels under an extremely large hydrogen content (~30 mass ppm).

DOI： 10.2472/jsms.67.723

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

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.

DOI： 10.1051/matecconf/201816503010

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

Fully reversed torsional fatigue tests were conducted to elucidate the behaviour of shear-mode crack initiation and propagation in one martensitic and two bainitic steels. The relationship between the crack initiation site and microstructure was investigated by means of an electron backscatter diffraction (EBSD) technique. From the S-N diagram, two notable results were obtained: (i) the shear-mode crack was initiated on the prior austenitic grain boundary in martensitic steel, while in bainitic steels, the crack was initiated along the {110} plane
one of the slip planes of bcc metals, and (ii) the torsional fatigue limit of lower bainitic steel with finer grains was 60 MPa higher than that of upper bainitic steel with coarser grains even though the hardnesses were nearly equivalent. The mechanism determining the torsional fatigue strength in these steels is discussed from the viewpoint of microstructure morphology.

DOI： 10.1051/matecconf/201816504009

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

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.

DOI： 10.1051/matecconf/201816503011

Language：Japanese  

準安定オーステナイト系ステンレス鋼中の疲労き裂進展加速に対する内部水素と外部水素の役割

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

DOI： https://doi.org/10.1299/transjsme.17-00437

Language：Others  

Temperature dependence of fatigue crack growth in low-alloy steel under gaseous hydrogen
Copyright © 2018 ASME. 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.

Language：Others  

Hydrogen-assisted fatigue crack propagation in a commercially pure BCC iron
Copyright © 2018 ASME. 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.

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

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.

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

In Japan, with regards to the widespread commercialization of 70 MPa-class hydrogen refueling stations and fuel cell vehicles, two national projects have been promoted on both the infrastructure and the automobile sides. These projects have been promoted to establish the criteria for determining hydrogen compatibility of materials and to expand the usable materials for high-pressure hydrogen environment. For these projects, establishing test methods to evaluate the hydrogen compatibility of materials is one of the most important tasks. This paper describes the status of common standardization of testing methods. Two projects share a common database for the testing results, which is currently put to practical use.

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As is well known, low alloy steels are widely used as materials for high pressure vessels because of their high tensile strength and reasonable price, but also show severe hydrogen embrittlement. Therefore, in 2016, the authors introduced a scenario for the safe use of low alloy steels in highly pressurized hydrogen gas as a "Guideline" at ASME PVP 2016 [1]. Following discussions with stakeholders and experts in recent years, we published Technical Document (TD) as an industrial standard prior to regulation, on the safe use of ground storage vessels made of low alloy steels in Hydrogen Refueling Stations (HRSs) based on performance requirements. This article presents an outline of the TD describing the required types of testing as performance requirements for confirming the good hydrogen compatibility of low alloy steels, such as controlling tensile strength in an appropriate range, confirming leak-before-break, determining the life of ground storage vessels by fatigue testing and determining the inspection term by fatigue crack growth analysis using the fatigue crack growth rate in highly pressurized hydrogen.

DOI： 10.1115/PVP201884099

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

To investigate the influence of hydrogen on the tensile and fatigue life properties of welded joints of 304/308 austenitic stainless steels, slow strain rate tensile (SSRT) tests and fatigue life tests were conducted in laboratory air using hydrogen exposed specimens. The specimens were fabricated from welded plates, and to elucidate the role of weld structure on hydrogen-induced degradation, the welded joint was solution-treated. In the SSRT tests of the as-welded (AW) joint, a non-exposed specimen failed at the base metal (BM), whereas a hydrogen-exposed specimen failed near the weld toe. In the case of the solution-treated-welded (STW) joint, the non-exposed specimen failed at the part of solution treated weld metal, whereas an H-exposed specimen failed near the weld toe. As a result, internal hydrogen significantly degraded the elongation of the AW joint. In the fatigue test, all the specimens failed near the weld toe. Internal hydrogen degraded the fatigue life considerably. However, the pre-charging led to little, if any, reduction in the fatigue limit. Similarly to the AW joint, hydrogen gas exposure notably degraded the fatigue life of the STW joint and led to little reduction in the fatigue limit. To investigate the relationship between the hydrogen-induced degradation and strain-induced martensitic transformation during fatigue testing, the volume fraction of ferrite in the broken specimens was measured by a ferrite scope. The volume fraction of martensitic transformation increased with an increase in the stress amplitude. These experimental results implied that the hydrogen-induced fatigue life degradation in the welded joint was closely related to the martensitic transformation during the fatigue process. The mechanisms of both the degradation in fatigue life and non-degradation in fatigue limit will be discussed further.

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

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.

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Diffusion measurements and thermal desorption measurements have been performed on a X70 pipeline steel, in as received and normalized and quenched condition, after being hydrogen charged for 200 h at 100 MPa hydrogen gas pressure and 85℃. Numerical simulations based on the assumption of thermodynamic equilibrium were performed, aiming to compare the trapping energies when fitting to the TDS spectra. TDS experiments revealed a reversible trap site with activation energy of 26.2 kJ.mol
^-1
. Irreversible trap sites with an activation energy of >100 kJ.mol-
¹
were observed from both as-received and heat-treated condition. In contrast, a reversible trap site with an activation energy of 49.2 kJ.mol
^-1
was observed only from the heat-treated condition. The numerical modelling based on the assumption of equilibrium between hydrogen in traps and hydrogen in lattice is seen to provide a good fit to the experimental data.

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

Methods to qualify materials for hydrogen service are needed in the global marketplace to enable sustainable, low-carbon energy technologies, such as hydrogen fuel cell electric vehicles. Existing requirements for qualifying materials are not adequate to support growth of hydrogen technology as well as being inconsistent with the growing literature on the effects of hydrogen on fracture and fatigue. This report documents an internationally coordinated effort to develop a test method for qualifying materials for high-pressure hydrogen fuel system onboard fuel cell electric vehicles. In particular, consistency of fatigue life testing strategies is discussed. Fatigue life tests were conducted at three different institutes in high-pressure gaseous hydrogen (90 MPa) at low temperature (233K) to confirm consistency across distinct testing platforms. The testing campaign includes testing of both smooth and notched axial fatigue specimens at various combinations of pressure and temperature. Collectively these testing results provide insight to the sensitivity of fatigue life testing to important testing parameters such as pressure, temperature and the presence of stress concentrations.

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Fatigue tests were performed using three types of round-bar specimens of Type 304, meta-stable, austenitic stainless steel. The specimens had circumferential notch with stress concentration factors, Kt, of 2, 3 or 6.6. Load controlled fatigue tests were conducted at stress ratio, R, of 0.1 and −1 in ambient air at room temperature. At R of 0.1, fatigue life was decreased with an increase in the stress concentration factor. Conversely, at R of −1, the stress concentration factor had little influence on the fatigue life. To understand the mechanism of the stress ratio effect, local deformation behavior at and beneath the notch root during the fatigue test was computed by means of finite element analysis considering that the plastic constitutive model describes the cyclic stress-strain response.

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To develop safer and more cost-effective high-pressure hydrogen tanks used in fuel cell vehicles (FCVs), the metallic materials with the following three key properties, i.e. lightweight, high strength and excellent resistance to hydrogen embrittlement should be explored. In this study, the compatibility of high-strength, precipitation-hardened aluminum alloy 7075-T6 was evaluated according to the four types of mechanical testing including slow-strain rate tensile (SSRT), fatigue life, fatigue crack growth (FCG) and fracture toughness tests in high-pressure gaseous hydrogen environments (95 ~ 115 MPa) at room temperature. Even though numerous publications have previously reported significant degradation of the mechanical properties of 7075-T6 in some hydrogenating environments, such as moist atmosphere, the understanding with regards to the performance of this alloy in high-pressure gaseous hydrogen environments is still lacking. In SSRT tests, the alloy showed no degradation of tensile strength and ductility. Furthermore, fatigue life, fatigue crack growth and fracture toughness properties were also not degraded in hydrogen gas. Namely, it was first demonstrated that the material has big potential to be used for hydrogen storage tanks for FCVs, according to its excellent resistance to high-pressure gaseous hydrogen.

Language：Others  

Assessment of the contribution of internal pressure to the structural damage in a hydrogen-charged Type 316L austenitic stainless steel during slow strain rate tensile test
© 2018 The Authors. 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.

DOI： 10.1016/j.prostr.2018.12.340

Language：Others  

Hydrogen trapping in X70 structural pipeline steel and weldments
Copyright © 2018 by the International Society of Offshore and Polar Engineers (ISOPE) Diffusion measurements and thermal desorption measurements have been performed on a X70 pipeline steel, in as received and normalized and quenched condition, after being hydrogen charged for 200 h at 100 MPa hydrogen gas pressure and 85℃. Numerical simulations based on the assumption of thermodynamic equilibrium were performed, aiming to compare the trapping energies when fitting to the TDS spectra. TDS experiments revealed a reversible trap site with activation energy of 26.2 kJ.mol-1. Irreversible trap sites with an activation energy of >100 kJ.mol-1were observed from both as-received and heat-treated condition. In contrast, a reversible trap site with an activation energy of 49.2 kJ.mol-1was observed only from the heat-treated condition. The numerical modelling based on the assumption of equilibrium between hydrogen in traps and hydrogen in lattice is seen to provide a good fit to the experimental data.

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

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

DOI： https://doi.org/10.1016/j.prostr.2017.11.104

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

© 2016 Elsevier Ltd 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

Other Link： http://www.sciencedirect.com/science/article/pii/S1359646217303500

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

© 2017, The Minerals, Metals & Materials Society and ASM International. 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

Other Link： https://link.springer.com/article/10.1007%2Fs11661-017-4323-3

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

© 2017 Elsevier Ltd To investigate the effect of hydrogen on fatigue life characteristics and crack growth behaviors through the entire fatigue life of a carbon steel, tension-compression fatigue tests and elasto-plastic fracture toughness tests were conducted in a hydrogen gas environment under the pressures of 0.7 and 115 MPa. The fatigue tests revealed that the fatigue life and fracture morphology vary drastically with the hydrogen gas pressure. This study demonstrates that such differences can be explained by the combination of fatigue crack growth properties and fracture toughness properties in hydrogen gas at each pressure.

DOI： 10.1016/j.ijfatigue.2017.06.006

Other Link： http://www.sciencedirect.com/science/article/pii/S0142112317302566

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

© 2017 Elsevier Ltd Fatigue crack growth (FCG) tests for compact-tension specimens of low-carbon steel were performed under various combinations of hydrogen pressures (0.1–90 MPa), test frequencies (0.001–10 Hz), and test temperatures (room temperature, 363 K, and 423 K). For quantifying the FCG acceleration, the hydrogen trapping and diffusivity of prestrained specimens were determined. Depending on the test conditions, the FCG was accelerated. The hydrogen-assisted FCG acceleration always accompanied localized plastic deformations near the crack tip. The onset of the FCG acceleration was roughly quantified by using a simplified parameter representing the gradient of the hydrogen concentration at the crack tip.

DOI： 10.1016/j.ijfatigue.2017.04.010

Other Link： http://www.sciencedirect.com/science/article/pii/S0142112317301810

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

DOI： 10.1299/transjsme.17-00264

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

DOI： 10.1299/transjsme

Language：Japanese  

高圧水素ガス曝露を施したオーステナイト系ステンレス鋼板突合せ溶接継手の疲労寿命特性

Language：English  

Language：English  

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

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

DOI： 10.1098/rsta.2016.0412

Other Link： http://rsta.royalsocietypublishing.org/content/375/2098/20160412

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

© 2017 The Author(s) Published by the Royal Society. All rights reserved. In the context of the fatigue life design of components, particularly those destined for use in hydrogen refuelling stations and fuel cell vehicles, it is important to understand the hydrogen-induced, fatigue crack growth (FCG) acceleration in steels. As such, the mechanisms for acceleration and its influencing factors are reviewed and discussed in this paper, with a special focus on the peculiar frequency dependence of the hydrogen-induced FCG acceleration. Further, this frequency dependence is debated by introducing some potentially responsible elements, along with new experimental data obtained by the authors.

DOI： 10.1098/rsta.2016.0412

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

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

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

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

DOI： 10.1115/PVP2017-65671

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

Material performance of age-hardened beryllium–copper alloy, CDA-C17200, in a high-pressure, gaseous hydrogen environment
© 2017 Hydrogen Energy Publications LLC In order to develop safer and more energy-efficient, hydrogen pre-cooling systems for use in hydrogen refueling stations, it is necessary to identify a high-strength metallic material with greater thermal conductivity and lower susceptibility to hydrogen embrittlement, as compared with ordinary, stable austenitic stainless steels. To accomplish this task, the hydrogen compatibility of a precipitation-hardened, high-strength, copper-based alloy was investigated by slow-strain-rate tensile (SSRT), fatigue-life, fatigue-crack-growth (FCG) and fracture toughness tests in 115-MPa hydrogen gas at room temperature. The hydrogen solubility and diffusivity of the alloy were also determined. The hydrogen solubility of the alloy was two or three orders of magnitude lower than that of austenitic stainless steels. The alloy also demonstrated absolutely no hydrogen-induced degradation of its strength properties, a factor which could contribute to the reduction of costs related to the construction and maintenance of hydrogen refueling stations, owing to the downsizing and improved cooling performance of the pre-cooling systems.

DOI： 10.1016/j.ijhydene.2017.04.270

Other Link： http://www.sciencedirect.com/science/article/pii/S0360319917317561

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

© 2017 Hydrogen Energy Publications LLC The effects of external and internal hydrogen on the slow-strain-rate tensile (SSRT) properties at room temperature were studied for ten types of solution-treated austenitic stainless steels containing a small amount of additive elements. The hydrogen diffusivity and solubility of the steels were measured with high-pressure hydrogen gas. The remarkable tensile-ductility loss observed in the SSRT tests was attributed to hydrogen-induced successive crack growth (HISCG) and was successfully quantified according to the nickel-equivalent content (Nieq), which represents the stability of the austenitic phase. The relative reduction in area (RRA) of the steels with a larger Nieq was influenced by the hydrogen distribution, whereas that of the steels with a smaller Nieq was not. This unique trend was interpreted with regard to the hydrogen distribution and fracture morphology (HISCG or microvoid coalescence).

DOI： 10.1016/j.ijhydene.2017.04.055

Other Link： http://www.sciencedirect.com/science/article/pii/S0360319917313757

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

© 2016 Elsevier Ltd An investigation was conducted into the influence of statically-applied, mode I, crack-opening load on the threshold condition for propagation of a shear-mode fatigue crack in a bearing steel. Torsional fatigue test was carried out at an R of −1 using a hollow cylindrical specimen into which a semi-elliptical, small slit was axially introduced. A static axial compressive stress was simultaneously applied to suppress crack branching. A coplanar, shear-mode, non-propagating fatigue crack emanating from the slit was attained by appropriate control of shear stress amplitude. Internal pressure was then applied to generate a hoop stress as a static crack-opening stress, σθ static. Consequently, the threshold shear-mode stress intensity factor range, ΔKτth, was significantly decreased with increase of the static mode I stress intensity factor. To further understand the contribution of σθ static to the reduction in ΔKτth, microstructural observations for the cross-sections of a non-propagating crack were conducted using a scanning electron microscope in conjunction with the electron backscatter diffraction analysis. The results revealed that the excess loading of σθ static accounts for the change in the crack-path, resulting in a further reduction in ΔKτth.

DOI： 10.1016/j.engfracmech.2016.12.007

Other Link： http://www.sciencedirect.com/science/article/pii/S0013794416307445

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

Other Link： http://ci.nii.ac.jp/naid/40021121737

Language：Japanese  

オーステナイト系ステンレス鋼板突合せ溶接継手の疲労寿命特性に及ぼす水素の影響

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

DOI： 10.1299/transjsme

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

© 2016 Wiley Publishing Ltd. Structural steels contain various material irregularities and natural defects which cause local stress concentrations from which fatigue cracks tend to initiate. Two defects in close proximity to each other may affect local stress distributions, and thus, begin to interact. In this paper, the effect of interacting small cracks on the fatigue limit is systematically investigated in a medium carbon steel. The growth of interacting cracks, as well as the characteristics of non-propagating cracks and microstructural aspects, was closely examined via the plastic replica method. It was found that although the fatigue limit is essentially controlled by the mechanics of interacting cracks, based on their configuration, the local microstructure comprised ferrite and pearlite has a statistical scatter effect on the behaviour of interacting cracks and non-propagating thresholds. With respect to the fatigue limit, when two defects were in close proximity, they behaved as a larger single defect. However, with greater spacing between defects, rather than mechanical factors, it is the local microstructure which determines the location and characteristics of non-propagating cracks.

DOI： 10.1111/ffe.12482

Other Link： http://onlinelibrary.wiley.com/doi/10.1111/ffe.12482/full

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

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

© 2016 Hydrogen Energy Publications LLC The nickel-equivalent equation for the materials selection of solution-treated austenitic stainless steels in present Japanese regulations does not consider nitrogen, which can improve resistance to hydrogen embrittlement. For the authorization of various austenitic stainless steels for use in high-pressure hydrogen gas and based on investigations of eight types of solution-treated austenitic stainless steels, this paper presented a newly proposed nickel-equivalent equation considering nitrogen. After tensile testing to the true strain ε of 0.3 in air at both room temperature (RT) and 228 K, the strain-induced martensite content VM was measured by a saturation magnetization technique; the VM correlated well with the proposed nickel-equivalent equation. For slow strain rate tensile (SSRT) testing in hydrogen gas at RT, the relative reduction in area (RRA) was consistently lower with increased VM for ε = 0.3 at 228 K. This suggests that the austenitic phase stability is crucial in determining the RRA of the present austenitic stainless steels and the RRA was successfully quantified by the proposed equation.

DOI： 10.1016/j.ijhydene.2016.06.193

Other Link： http://www.sciencedirect.com/science/article/pii/S0360319916318997

Language：Japanese  

水素ステーション実証試験で使用されたプレクーラー用冷却パイプの事例解析

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

© 2016 Hydrogen Energy Publications LLC High-pressure components are generally designed with safety factors based on the tensile strength (TS) of the material; accordingly, materials with higher TS permit designed components with thinner walls, which reduce the weight and cost of the parts. However, many high-strength metals are severely degraded by hydrogen. To this point, efforts to develop a high-strength metal with a TS far beyond 1000 MPa and excellent resistance to hydrogen embrittlement (HE) have failed. This study introduces a high-strength metal with an excellent HE resistance, composed of a precipitation-hardened copper-beryllium alloy with the TS of 1400 MPa. Slow strain rate tensile (SSRT) tests of both smooth and notched specimens were performed in 115-MPa hydrogen gas at room temperature (RT). The alloy had a relative reduction in area RRA ≈ 1 and a relative notch tensile strength RNTS ≈ 1, without degradation in either characteristic.

DOI： 10.1016/j.ijhydene.2016.05.156

Other Link： http://www.sciencedirect.com/science/article/pii/S0360319916316184

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

DOI： http://doi.org/10.1299/transjsme.16-00109

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

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

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

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

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

© 2016, The Society for Experimental Mechanics, Inc. The investigation of the behavior of small shear-mode fatigue cracks in the high-cycle fatigue regime is essential to understand the mechanism of rolling-contact fatigue failures, such as flaking in bearings and shelling in rails, from the fracture mechanics point of view. The stable growth of a shear-mode fatigue crack was achieved by applying static compression to a specimen in a cyclic torsion fatigue test. This loading condition is usually obtained by a combined tension-torsion testing machine with a servo-hydraulic control system. In this study, a new testing machine was developed and found to be superior to the servo-hydraulic testing machine in terms of price, operation/maintenance costs, operating speed, and installation volume. For substantiation and demonstration purposes, a shear-mode fatigue crack growth test with a bearing steel was also carried out using both the new and the conventional servo-hydraulic testing machines. The experiments revealed that under the same loading conditions, nonpropagating shear-mode cracks of similar size and geometry could be obtained by the respective testing machines. Thus, it was concluded that the new testing machine has equivalent capabilities to the servo-hydraulic testing machine in performing shear-mode fatigue crack growth tests.

DOI： 10.1007/s40799-016-0102-0

Other Link： http://onlinelibrary.wiley.com/doi/10.1111/ext.12140/abstract

Language：Japanese  

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

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

© 2015 Elsevier Ltd. To establish novel criteria for determining the hydrogen compatibility of austenitic stainless steels, as well as to elucidate the mechanisms for hydrogen-assisted surface crack growth (HASCG), slow strain rate tensile (SSRT), elasto-plastic fracture toughness (JIC), fatigue crack growth and fatigue life tests were performed on Types 304, 316 and 316L steels in high-pressure hydrogen gas. As a criterion for the use of austenitic stainless steels with lower austenitic stability in hydrogen gas, a reduction in area (RA) in hydrogen gas, φH≥57&#37;, or a relative reduction in area, RRA≥0.68, is proposed to ensure that there is no degradation in tensile strength by hydrogen. Observation of fracture surface morphologies and crack growth behaviours demonstrated that, in high-pressure hydrogen gas, SSRT surface crack grew via the same mechanism as for JIC crack and fatigue crack, i.e., these cracks successively grew with a sharp shape under the loading process, due to a localized slip deformation near the crack tip. Based on the elucidated HASCG mechanism, total elongation in hydrogen gas, δH≥10&#37;, or, φH≥10&#37;, is also introduced as another criterion.

DOI： 10.1016/j.engfracmech.2015.12.023

Other Link： http://www.sciencedirect.com/science/article/pii/S0013794415006992

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

DOI： 1016/j.scriptamat.2015.10.016

Other Link： http://www.sciencedirect.com/science/article/pii/S1359646215300166

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

© 2016 by ASME. Pressure cycle tests were performed on two types of Cr-Mo steel pressure vessels with notches machined on their inside under hydrogen-gas pressures, between 0.6 and 45 MPa at room temperature. Fatigue crack growth (FCG) and fracture toughness tests of the Cr-Mo steels samples from the vessels were also carried out in gaseous hydrogen. The Cr-Mo steels showed accelerated FCG rates in gaseous hydrogen compared to ambient air. The fracture toughness of the Cr-Mo steels in gaseous hydrogen was significantly smaller than that in ambient air. Four pressure vessels were tested with gaseous hydrogen. All pressure vessels failed by leak-before-break (LBB). The LBB failure of one pressure vessel could not be estimated by using the fracture toughness in gaseous hydrogen K<inf>IC,H</inf>; accordingly, the LBB assessment based on K<inf>IC,H</inf> is conservative and there is a possibility that K<inf>IC,H</inf> does not provide a reasonable assessment of LBB. In contrast, the fatigue lives of all pressure vessels could be estimated by using the accelerated FCG rates in gaseous hydrogen.

DOI： 10.1115/1.4030086

Other Link： http://pressurevesseltech.asmedigitalcollection.asme.org/article.aspx?articleid=2210669

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

DOI： http://doi.org/10.1380/jsssj.36.562

Other Link： https://www.jstage.jst.go.jp/article/jsssj/36/11/36_562/_article

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

DOI： 10.1111/ffe.12331

Other Link： http://onlinelibrary.wiley.com/doi/10.1111/ffe.12331/abstract

Language：Japanese  

Language：Japanese  

Language：Japanese  

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

© 2015 Wiley Publishing Ltd. In order to clarify the effect of defect orientation on the fatigue limit of two types of steels, JIS-S15C and JIS-S45C, a small semi-circular slit was introduced onto the surface of a round specimen. The slit was tilted at 0 , 30 or 60 with respect to the plane normal to the loading axis, but all of them had the same defect size, area?=?188 μm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. In all the combinations of the materials and tilt angles, a non-propagating crack was found at or just below the fatigue limit, that is, the fatigue limit was determined by the non-propagation condition of a crack initiated from the defect. In both steels, the fatigue limit was found to be nearly independent of the tilt angle for the same value of projected size area, which was in good agreement with the prediction by the area parameter model. In this paper, a mechanistic explanation for the insensitivity of the fatigue limit to the tilt angle is proposed.

DOI： 10.1111/ffe.12321

Other Link： http://onlinelibrary.wiley.com/doi/10.1111/ffe.12321/abstract

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

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

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

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

© 2015, Gruppo Italiano Frattura. All rights reserved. High resolution synchrotron X-ray tomography has been used to obtain 3D images of arrested cracks initiated at small artificial defects located on the surface of cylindrical steel specimens subjected to mode I fatigue loading. These defects consist in small semi-circular slits tilted at 0°, 30° or 60° with respect to the plane normal to the loading axis; all of them had the same defect size, √area = 188 μm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. Arrested cracks initiated from the notch were observed for all tilt angles at the surface of samples cycled at the fatigue limit (stress amplitude at which the specimen does not fail after 1×10⁷ cycles). High resolution synchrotron X-ray tomography has been used to obtain 3D images of those small defects and non-propagating cracks (NPC). Despite the fact that steel is a highly attenuating material for X rays, high resolution 3D images of the cracks and of the initiating defects were obtained (0.65 μm voxel size). The values of surface crack length measured by tomography are the same as those obtained by optical microscope measurements. The √area values present the same tendency as the surface length of NPC, i.e. larger non-propagating cracks areas were observed in the softer steel. In the extreme case of 60° tilted defect, the crack fronts appear much more discontinuous with several cracks propagating in mode I until arrest.

DOI： 10.3221/IGF-ESIS.33.27

Other Link： http://www.fracturae.com/index.php/fis/article/view/IGF-ESIS.33.27

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

Copyright © 2015, Hydrogen Energy Publications, LLC. Abstract Slow strain rate tensile (SSRT) tests were performed using smooth specimens of two types of steels, the Cr-Mo steel, JIS-SCM435, which has a tempered, martensitic microstructure, and the carbon steel, JIS-SM490B, which has a ferrite/pearlite microstructure. The tests were carried out in nitrogen gas and hydrogen gas, under a pressure of 115 MPa at three different temperatures: 233 K, room temperature and 393 K. In nitrogen gas, these steels exhibited the so-called cup-and-cone fracture at every temperature. In contrast, surface cracking led to a marked reduction in ductility in both steels in hydrogen gas. Nonetheless, even in hydrogen gas, JIS-SCM435 exhibited some reduction of area after the stress-displacement curve reached the tensile strength (TS), whereas JIS-SM490B demonstrated little, if any, necking in hydrogen gas. In addition, tension-compression fatigue testing at room temperature revealed that these steels show no noticeable degradation in fatigue strengths in hydrogen gas, especially in the relatively long-life regime. Considering that there was little or no hydrogen-induced degradation in either the TS or the fatigue strength in JIS-SCM435, it is suggested that the JIS-SCM435 is eligible for safety factor-based fatigue limit design for hydrogen service under pressures up to 115 MPa.

DOI： 10.1016/j.ijhydene.2015.02.098

Other Link： http://www.sciencedirect.com/science/article/pii/S0360319915004784

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

© 2014 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. To clarify the usefulness of the safety factor multiplier method for Cr-Mo steel in the CHMC1-2014 standard for hydrogen-service components, slow-strain-rate tests of smooth and notched specimens and fatigue tests of notched specimens were performed in air and in 115 MPa hydrogen gas. The series of tests determined a safety factor multiplier of ≈3.0, which allowed only quite small design stresses for Cr-Mo steel. Furthermore, the safety factor multiplier can be predicted by fatigue crack growth (FCG) analysis for a notched specimen, i.e., in this respect, the fatigue test of notched specimen is a kind of FCG test. On the basis of this idea, safety factor multipliers for types 304, 316, and 316L austenitic stainless steels were predicted from FCG data to be 2.0, 1.6, and 1.3, respectively. Because the results demonstrate that the safety factor multiplier method provides overly conservative safety factors for some steels, design methods for specific component applications by testing based on design by rule and design by analysis are newly proposed.

DOI： 10.1016/j.ijhydene.2014.10.114

Other Link： http://www.sciencedirect.com/science/article/pii/S0360319914029905

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

The ranges of the threshold stress intensity factor (SIF), Delta K-IIth and Delta K-IIIth, for small shear-mode cracks in bearing steel are measured using torsional fatigue testing under static compression. The threshold values are described as a function of the crack size and the crack-face interference. In addition, to simplify the evaluation of the shear-mode threshold, a single parameter, K-tau, is introduced to represent the SIF of shear-mode cracks. Moreover, a practical expression for approximating K-tau is proposed on the basis of the root area parameter model. Finally, the threshold SIF range Delta K-tau th is found to exhibit a dependence on the crack size similar to that of small mode I cracks, i.e., Delta K-tau th proportional to (root area)(1/3), for the regime of small shear-mode cracks with root area values ranging from 0.01 to 1 mm. (C) 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.tafmec.2014.07.016

Other Link： http://www.sciencedirect.com/science/article/pii/S0167844214001207#

Language：English  

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

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

DOI： 10.1299/transjsme.2014smm0254

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

DOI： 10.1115/PVP2014-28511

Other Link： http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1938229

Language：Japanese  

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

DOI： 10.1115/PVP2014-28640

Other Link： http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1938234

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

DOI： 10.1115/PVP2014-28604

Other Link： http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=1938233

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

The effect of an occasional mode II loading on the subsequent mode I fatigue crack growth was investigated in a thin-walled 7075-T6511 aluminum alloy tube. Careful observation of crack growth behavior revealed that an occasional mode II loading has two contrasting effects on the subsequent crack growth. First is a retardation effect that is associated with a crack closure development due to the mode II loading. However, this effect was insignificant with respect to the crack growth life as a whole. Second is an acceleration effect that is associated with an accelerated crack growth rate in mode II. It was found that, in a relatively high Delta K regime, mode II crack growth was from one to two orders of magnitude faster than mode I crack growth. This study shows that mode II crack growth should be considered as a predominant factor in evaluating the effect of an occasional mode II loading for a mode I crack in 7075 aluminum alloy. (C) 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.engfracmech.2014.04.015

Other Link： http://www.sciencedirect.com/science/article/pii/S0013794414001192

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

DOI： 10.1051/matecconf/20141207001

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

DOI： 10.1016/j.mspro.2014.06.268

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

Hydrogen-induced ductility loss in ductile cast iron (DCI) was studied by conducting a series of tensile tests with three different crosshead speeds. By utilizing the thermal desorption spectroscopy and the hydrogen microprint technique, it was found that most of the solute hydrogen was diffusive and mainly segregated at the graphite, graphite/matrix interface zone, and the cementite of pearlite in the matrix. The fracture process of the non-charged specimen was dominated by the ductile dimple fracture, whereas that of the hydrogen-charged specimen became less ductile because of the accompanying interconnecting cracks between the adjacent graphite nodules. Inside the hydrogen-charged specimen, the interspaces generated by the interfacial debonding between graphite and matrix are filled with hydrogen gas in the early stage of the fracture process. In the subsequent fracture process, such a local hydrogen gas atmosphere coupled with a stress-induced diffusion attracts hydrogen to the crack tip, which results in a time-dependent ductility loss. © 2013 The Minerals, Metals & Materials Society and ASM International.

DOI： 10.1007/s11661-013-2109-9

Other Link： http://link.springer.com/article/10.1007%2Fs11661-013-2109-9

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

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

Language：Japanese  

Other Link： http://ci.nii.ac.jp/naid/130003386282

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

Plasticity-induced fatigue crack closure associated with occasional mode II loading was simulated for two-dimensional middle-crack tension geometry by using the finite element method. When a single mode II cycle was superposed onto the steady-state mode I crack growth, the range of crack tip opening displacement, Delta CTOD, exhibited the initial drop and subsequent recovery under plane strain condition. On the other hand, under plane stress condition, Delta CTOD exhibited the initial jump and drop right after the single mode II loading, and then it showed the recovery. The present results indicate that occasional mode II loading can cause a small retardation for mode I crack growth due to the enhanced plasticity-induced closure. (C) 2013 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.engfracmech.2013.09.001

Other Link： http://www.sciencedirect.com/science/article/pii/S0013794413003020

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

A series of tensile tests were carried out using hydrogen-precharged specimens of SAE52100. Two types of tensile specimens were used: (i) smooth specimens and (ii) specimens having various shapes of artificial defects with sizes of about 35-500 mu m. In the smooth specimens, fracture origins were non-metallic inclusions with the size of 10-30 mu m (e.g. Al2O3.(CaO)(x), TiN and TiC). In both types of specimens, hydrogen charging drastically decreased the tensile strength. The fracture toughness determined for the small defects at the fracture origin, that is, the threshold stress intensity K-TH calculated from the defect size <mml:msqrt>area</mml:msqrt> and the applied tensile stress at the failure of the specimen sigma(f), showed a defect size dependence, where area denotes the area of the domain defined by projecting the defect on a plane normal to the cylindrical axis of the specimen. Namely, the threshold value was reduced with a decrease in the defect size, which is similar to the crack size dependence in the K-th for small fatigue crack. The values of K-TH for both the non-metallic inclusions and artificial defects were much smaller than those for large cracks measured by the standard WOL (Wedge Opening Load) and CT (Compact Tension) specimens.

DOI： 10.1111/ffe.12077

Other Link： http://onlinelibrary.wiley.com/doi/10.1111/ffe.12077/abstract

Language：Japanese  

Other Link： https://www.jstage.jst.go.jp/article/kikaia/79/803/79_961/_article/-char/ja/

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

DOI： 10.4028/www.scientific.net/MSF.750.264

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

DOI： 10.4028/www.scientific.net/MSF.750.264

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

Effect of occasional mode II loading on subsequent mode I fatigue crack growth behavior was investigated by using a thin-walled tube made of 7075-T6511 aluminum alloy. Careful observation of crack growth behavior revealed that the occasional mode II loading has two contradictory effects for crack growth behavior. The first is a retardation effect that is associated with the plastic deformation near crack tip. However, this effect is negligibly small for the crack growth life as a whole. The second is an acceleration effect caused by mode II fatigue crack growth itself. It was found that under relatively high ΔK level, the mode II crack growth was about an order magnitude faster than mode I crack growth. Therefore, to properly evaluate the effect of occasional shear loading in the 7075 alloy, the mode II crack growth should be taken into account.

DOI： 10.4028/www.scientific.net/MSF.750.264

Language：Others  

Ductility loss in Hydrogen-charged Ductile Cast Iron
Hydrogen-induced ductility loss in ductile cast iron (DCI) was studied by conducting a series of tensile tests with three different crosshead speeds. By utilizing the thermal desorption spectroscopy and the hydrogen microprint technique, it was found that most of the solute hydrogen was diffusive and mainly segregated at the graphite, graphite/matrix interface zone and the cementite of pearlite in the matrix. The fracture process of the non-charged specimen was dominated by the ductile dimple fracture, whereas that of the hydrogen-charged specimen became less ductile by accompanying the interconnecting cracks between the adjacent graphite nodules. Inside of the hydrogen-charged specimen, the interspaces generated by the interfacial debonding between graphite and matrix are filled with hydrogen gas in the early stage of the fracture process. In the subsequent fracture process, such a local hydrogen gas atmosphere coupled with a stress-induced diffusion attracts hydrogen to the crack tip, which results in a time-dependent ductility loss.

Language：Others  

Small crack effect on threshold stress intensity K<inf>TH</inf> for high strength steel with internal hydrogen
The size effect of the threshold stress intensity KTH for hydrogen-precharged specimens of SAE52100 were investigated. Four types of tensile specimens were used: (i) smooth specimens and (ii) specimens having various shapes of artificial defects with sizes of about 35 ∼ 500 um. In the smooth specimens, fracture origins were nonmetallic inclusions with the size of 10 ∼ 30 um (e.g. Al2O3·(CaO)x, TiN and TiC). The fracture toughness determined for the small defects at fracture origin, i.e. the KTH calculated from the defect size √area and the fracture tensile stress σf, showed a defect size dependence, where, area denotes the area of the domain defined by projecting the defect on a plane normal to the cylindrical axis of the specimen. The values of KTH for both the nonmetallic inclusions and artificial defects were much smaller than those for large cracks measured by the standard WOL and CT specimens.

Language：English  

DOI： 10.1142/S2010194512003522

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

To study the influence of hydrogen on the fatigue strength of AISI type 304 metastable austenitic stainless steel, specimens were cathodically charged with hydrogen. Using tension-compression fatigue tests, the behavior of fatigue crack growth from a small drill hole in the hydrogen-charged specimen was compared with that of noncharged specimen. Hydrogen charging led to a marked increase in the crack growth rate. Typical characteristics of hydrogen effect were observed in the slip band morphology and fatigue striation. To elucidate the behavior of hydrogen diffusion microscopically in the fatigue process, the hydrogen emission from the specimens was visualized using the hydrogen microprint technique (HMT). In the hydrogen-charged specimen, hydrogen emissions were mainly observed in the vicinity of the fatigue crack. Comparison between the HMT image and the etched microstructure image revealed that the slip bands worked as a pathway for hydrogen to move preferentially. Hydrogen-charging resulted in a significant change in the phase transformation behavior in the fatigue process. In the noncharged specimen, a massive type alpha' martensite was observed in the vicinity of the fatigue crack. On the other hand, in the hydrogen-charged specimen, large amounts of epsilon martensite and a smaller amount of alpha' martensite were observed along the slip bands. The results indicated that solute hydrogen facilitated the epsilon martensitic transformation in the fatigue process. Comparison between the results of HMT and EBSD inferred that martensitic transformations as well as plastic deformation itself can enhance the mobility of hydrogen.

DOI： 10.1007/s11661-011-0661-8

Other Link： http://link.springer.com/article/10.1007%2Fs11661-011-0661-8

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

DOI： 10.4028/www.scientific.net/AMR.275.15

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

DOI： 10.4028/www.scientific.net/AMR.275.43

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

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

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

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

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

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

DOI： 10.1111/j.1460-2695.2010.01495.x

Other Link： http://onlinelibrary.wiley.com/doi/10.1111/j.1460-2695.2010.01495.x/abstract

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

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

DOI： doi:10.1117/12.852121

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

Shear mode growth and threshold of small fatigue cracks in SUJ2 bearing steel
The shear mode propagation and threshold behaviors of small, semi-elliptical fatigue cracks were investigated in the JIS-SUJ2 bearing steel. Fully-reversed torsional loading was combined with a static axial compressive load to obtain crack growth in the longitudinal direction of cylindrical specimens. Non-propagating cracks smaller than ≅1 mm in size were obtained (i) by decreasing the stress amplitude in tests using notched specimens, and (ii) by using smooth specimens in constant stress amplitude tests. The Mode II and Mode III threshold stress intensity factor ranges, δKIIth and δKIIIth, were estimated from the shapes and dimensions of non-propagating cracks. The threshold values decreased with a decrease in crack size, and there was no significant difference between δKIIth and δKIIIth- Wear on the crack faces was inferred by a powder and also by changes in microstructure in the wake of a crack. The Aftnth in the absence of interference of crack faces was estimated to be approximately 13MPa- m1/2 for a 1mm long crack. © 2009 The Society of Materials Science.

DOI： 10.2472/jsms.58.773

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

On the mechanism of very high cycle fatigue in Ti-6Al-4V
A mechanism of fatigue crack advance involving the erosion of crack closure in vacuum is proposed to account for the very high cycle fatigue of Ti-6Al-4V. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.scriptamat.2008.08.012

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

It has long been known that the fatigue notch factor K-F, given by sigma(w0)/sigma(w) where sigma(w0) is smooth specimen fatigue strength (R = -1) and sigma(w), is the fatigue strength of a notch-containing specimen, is less than the theoretical stress concentration factor, K-T. Empirical rules have been proposed by Peterson and by Kuhn and Hardrath for the estimation of K-F which involve the notch radius and a material constant. El Haddad, Topper and Smith provided one means for understanding the factors determining K-F. The present paper provides an alternative approach based upon a consideration of the development of fatigue crack closure. The effect of crack closure on the formation of non-propagating cracks is also discussed. (C) 2008 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2008.07.001

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

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

DOI： 10.4028/www.scientific.net/KEM.385-387.449

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

Recent data on short fatigue crack growth in two cast and hot isostatically pressed (hipped) aluminum alloys obtained by Shyam, Allison and Jones have been analyzed in terms of a previously proposed one-parameter short crack model which includes consideration of elastic-plastic effects, the Kitagawa effect and the development of crack closure in the wake of a newly formed crack. The material constants obtained in a prior investigation of short crack growth behavior in a cast aluminum alloy tested under fully reversed loading were used as a basis for the present analysis. The predicted rates of fatigue crack propagation are in accord with the experimental results. In the discussion, aspects of the two-parameter approach presented by Shyam et al. are compared with those of the one-parameter method of analysis used herein. (C) 2007 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2006.11.012

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

A study has been made of the striations and fissures developed in the aluminum alloy 2024-T3 during fatigue crack growth. Fissures were found to form on inclined facets. They were uniformly spaced as the result of a shielding process. Striation spacings were in accord with da/dN values at the higher levels of K investigated, but at low K levels striation spacings were larger than the corresponding da/dN values. The percentage of the fracture surface containing striations varied with the K level, ranging from less than 1 % at low K levels to 80 % at higher K levels. The reason for the discrepancy between the spacing of striations and the corresponding da/dN values is discussed.

DOI： 10.4028/www.scientific.net/MSF.567-568.397

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

DOI： 10.4028/www.scientific.net/KEM.353-358.287

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

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

DOI： 10.4028/www.scientific.net/KEM.345-346.319

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

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

The effect of hydrogen on the fatigue properties of alloys which are used in fuel cell (FC) systems has been investigated. In a typical FC system, various alloys are used in hydrogen environments and are subjected to cyclic loading due to pressurization, mechanical vibrations, etc. The materials investigated were three austenitic stainless steels (SUS304, SUS316 and SUS316L), one ferritic stainless steel (SUS405), one martensitic stainless steel (0.7C-13Cr), a Cr-Mo martensitic steel (SCM435) and an annealed medium-carbon steel (0.47C). In order to simulate the pick-up of hydrogen in service, the specimens were charged with hydrogen. The fatigue crack growth behavior of charged specimens of SUS304, SUS316, SUS316L and SUS405 was compared with that of specimens which had not been hydrogen-charged. The comparison showed that there was a degradation in fatigue crack growth resistance due to hydrogen in the cases of SUS304 and SUS316 austenitic stainless steels. However, SUS316L and SUS405 showed little, if any, degradation due to hydrogen. The results of S-N testing showed that in the cases of the 0.7C-13Cr stainless steel and the Cr-Mo steel a marked decrease in fatigue resistance due to hydrogen occurred. In the case of the medium carbon steel hydrogen did not cause a reduction in fatigue behavior. Examination of the slip band characteristics of a number of the alloys showed that slip was more localized in the case of hydrogen-charged specimens. (c) 2006 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ijfatigue.2005.06.059

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

Language：Japanese  

10-104 Practice of Engineering Drawing Education in Mechanical Engineering

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

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

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

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

The effects of small defects on the fatigue strength of Ti-6Al-4V were investigated in tension-compression fatigue tests. To simulate the defects, holes having 50, 200, 400, 760 and 1000mum diameters were introduced onto the surface of a series of specimens (Series A). Another series of specimens (Series B) were prepared to investigate the effect of a burr or a p re-crack which was introduced at the edge of the artificial hole. For Series A specimens, the fatigue limit was defined as the threshold for crack initiation from the edge of the hole. For Series B specimens, the fatigue limit was defined as the threshold for crack propagation from the burr or the tip of the pre-crack.
The results of the fatigue tests for R=-1 indicated that the fatigue limits of Series B specimens were 20similar to60MPa lower than those of Series A specimens. In contrast to steels, non-propagating cracks did not form at the holes, a fact associated with the relatively high stress level for crack intiation from hole in Ti-6Al-4V. It was also found that the rootarea parameter model underestimated the fatigue limit of Series A specimens, but the model accurately predicted the fatigue limit defined by crack-growth threshold for Series B specimens. In addition it was noted that the presence of a burr resulted in a decrease in fatigue life, since the burr facilitated the crack initiation process.

DOI： 10.2472/jsms.52.12Appendix_263

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

Effect of FOD on Fatigue Strength of Ti-6Al-4V Alloy

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

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

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

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

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

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

Language：Japanese   Book type：Scholarly book

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Language：English   Book type：Scholarly book

Other Link： http://www.springer.com/us/book/9784431560401

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Event date： 2021.7

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Language：English   Presentation type：Oral presentation (general)  

Venue：Futuroscope Convention Center, Poitiers   Country：Japan  

Other Link： http://s550682939.onlinehome.fr/Fatigue2018/default.htm

Event date： 2018.5 - 2018.6

Language：English  

Country：France  

Event date： 2018.5 - 2018.6

Language：English  

Venue：Poitiers Futuroscope, France   Country：Japan  

Event date： 2018.5 - 2018.6

Language：English  

Country：Japan  

Event date： 2018.5 - 2018.6

Language：English   Presentation type：Oral presentation (general)  

Country：France  

Event date： 2018.5 - 2018.6

Language：English  

Venue：Poitiers Futuroscope, France   Country：Japan  

Event date： 2018.5 - 2018.6

Language：English   Presentation type：Oral presentation (general)  

Country：Japan  

Event date： 2018.5 - 2018.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Futuroscope Convention Center, Poitiers   Country：Japan  

Other Link： http://s550682939.onlinehome.fr/Fatigue2018/default.htm

Event date： 2018.5 - 2018.6

Language：English   Presentation type：Oral presentation (general)  

Venue：Futuroscope Convention Center, Poitiers   Country：Japan  

Other Link： http://s550682939.onlinehome.fr/Fatigue2018/default.htm

Event date： 2018.3

Language：Japanese  

Venue：千葉工業大学   Country：Japan  

Event date： 2018.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2018.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2018.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2018.2 - 2018.5

Language：English  

Venue：九州大学   Country：Japan  

Event date： 2018.2

Language：English  

Venue：九州大学   Country：Japan  

Event date： 2018.2

Language：English  

Venue：九州大学   Country：Japan  

Event date： 2018.2

Language：English  

Venue：九州大学   Country：Japan  

Event date： 2018.2

Language：English  

Venue：九州大学   Country：Japan  

Event date： 2018.2

Language：English   Presentation type：Oral presentation (general)  

Venue：九州大学   Country：Japan  

Event date： 2018.2

Language：English  

Venue：九州大学   Country：Japan  

Event date： 2017.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：沖縄県青年会館   Country：Japan  

Event date： 2017.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：沖縄県青年会館   Country：Japan  

Event date： 2017.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：沖縄県青年会館   Country：Japan  

Event date： 2017.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：沖縄県青年会館   Country：Japan  

Event date： 2017.11

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：沖縄県青年会館   Country：Japan  

Event date： 2017.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学   Country：Japan  

Event date： 2017.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学   Country：Japan  

Event date： 2017.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学   Country：Japan  

Event date： 2017.10

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学   Country：Japan  

Event date： 2017.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Lecco   Country：Italy  

Event date： 2017.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Lecco   Country：Italy  

Event date： 2017.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Lecco   Country：Italy  

Event date： 2017.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Lecco   Country：Italy  

Event date： 2017.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学　札幌キャンパス   Country：Japan  

Event date： 2017.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学　札幌キャンパス   Country：Japan  

Event date： 2017.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学　札幌キャンパス   Country：Japan  

Event date： 2017.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学　札幌キャンパス   Country：Japan  

Event date： 2017.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：北海道大学　札幌キャンパス   Country：Japan  

Event date： 2017.8

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：上智大学   Country：Japan  

Event date： 2017.8 - 2017.9

Language：English   Presentation type：Oral presentation (general)  

Venue：KINTEX (Korea International Exhibition Center), Seoul   Country：Korea, Republic of  

Event date： 2017.8 - 2017.9

Language：English  

Venue：KINTEX (Korea International Exhibition Center), Seoul   Country：Korea, Republic of  

Event date： 2017.8 - 2017.9

Language：English   Presentation type：Oral presentation (general)  

Venue：KINTEX (Korea International Exhibition Center), Seoul   Country：Korea, Republic of  

Event date： 2017.7

Language：English   Presentation type：Oral presentation (general)  

Venue：Hilton Waikoloa, Hawaii   Country：United States  

Event date： 2017.7

Language：English   Presentation type：Oral presentation (general)