A T Yokobori

Tohoku University, Sendai-shi, Miyagi, Japan

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Publications (40)40.88 Total impact

  • M. Tabuchi · H. Hongo · R. Sugiura · A.T. Yokobori · M. Yatomi · K. Kobayashi
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    ABSTRACT: High Cr ferritic steels have been used in the 600°C class ultra-super critical (USC) thermal power plants. More than a decade has passed from the application of high Cr steels to USC power plants in Japan, and type-IV creep damages in the welded components become concerns. In the present paper, long-term creep tests for the welded joints of the high Cr steels (Gr.91 and Gr.122 steels) were conducted. Microstructures and creep damages in the heat affected zone (HAZ) were investigated using the thick plate specimens. Remaining life assessment methods of high Cr steel welds were discussed based on the experimental results. Further, to assist the standardization of the testing method for creep crack growth in welded joints, we have conducted the round robin tests using the high Cr steels (Gr.91 and Gr.92 steels) and their welds as part of the VAMAS TWA31 collaboration.
    No preview · Article · May 2015
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    ABSTRACT: In order to detect creep–fatigue damage before the crack initiation stage, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method for a polycrystalline nickel base superalloy has been carried out. The average misorientation within grains increases linearly up to the initiation of cracks with the increase in creep strains. Even if stress distributions exist by stress concentration, assessment in all the damage areas allowed the evaluation of creep damage regardless of geometrical influence. Furthermore, the influence of strain rates, introducing fatigue and testing temperatures are hardly observed in the misorientation analysis. Misorientation almost corresponded to inelastic strain regardless of those influences. Quite a small misorientation caused by fatigue led to an equivalent result between creep conditions and creep–fatigue conditions. It is concluded that the misorientation analysis of damaged materials based on the EBSD method allows the quantitative estimation of creep strain and the assessment of remaining creep fracture life.
    No preview · Article · Jan 2014 · Materials Science and Technology
  • A.T. Yokobori · Y. Nagumo · T. Yajima · T.Y.T. Ohmi
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    ABSTRACT: On the basis of the analytical method using the concepts of super atoms, super dislocation and self similarity, the atomic distribution from super atoms to those with small scale was found to show the self similarity and the possibility of analyzing the behaviors of actual atoms using projection method was validated and the following results were obtained. The direct method and Verlet method were found to give the same results on the atomic distribution, however calculating time is different. Using our proposed method, the dominating regions of fracture were shown to be derived and they were in good agreement with those obtained experimentally by Ritchie et al. These analyses were found to predict quantitative values of process region or end region proposed by Kanninen and Broberg.
    No preview · Article · Jan 2014 · Strength, Fracture and Complexity
  • R. Sugiura · A.T. Yokobori · K. Sato · M. Tabuchi · K. Kobayashi · M. Yatomi · K. Nikbin
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    ABSTRACT: W added high Cr ferritic heat-resistant steels have been developed as a boiler material. Most of boiler component structures are mainly fabricated by welding which are likely to the regions of crack initiation and propagation. However, the method of predicting the life of creep crack initiation and growth have not been clearly established for weldments of high Cr ferritic heat resistant material due to many factors such as the variation in micro-structures and the residual stress caused by welding and thermal cycles. In the present study, the experiments of creep crack growth using a circular notched round bar specimen with variation of notch location in HAZ were conducted and the characteristics of creep crack growth rate and creep crack initiation life were summarized in terms of Q* parameter, which has been proposed as fracture mechanics parameter to describe creep crack growth rate.
    No preview · Article · Jan 2014 · Strength, Fracture and Complexity
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    ABSTRACT: A testing method under creep-fatigue interaction conditions has been proposed as the ASTM standard E2714-09. In accordance with the ASTM standard, round bar smooth specimens have been recommended as a testing specimen. Additionally, creep-fatigue crack initiation and growth life are usually defined as the attainment of a specific rate of decrease in the maximum tensile stress or the modulus of elasticity ratios. However, these criterions of crack initiation and growth life on ASTM standard are not characterized in terms of physical crack size. In this study, the creep-fatigue crack initiation and growth tests for P92 were conducted using circular notched round bar specimens. An attempt is made to measure the crack length during the tests by the direct current potential drop method, which results in the establishment of a high accuracy crack length measurement. Additionally, the life of creep-fatigue crack growth was characterized by linear cumulative damage law combined with the Qconcept which has been proposed as fracture parameter to describe the life of creep crack growth.
    No preview · Article · Jan 2012 · Journal of ASTM International
  • S. W. Dean · Y. Nagumo · A. T. Yokobori · R. Sugiura · T. Matsuzaki

    No preview · Article · Jan 2011 · Journal of ASTM International
  • Shinya Taketomi · A. Toshimitsu Yokobori
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    ABSTRACT: Based on a two-dimensional physical model of a mode I crack with a slip band, the analysis of mechanical interaction between hydrogen and dynamic dislocations with non-steady emission from a stressed source around a crack tip is conducted. To analyze the mechanical interaction between a crack and dynamic dislocations on a neighboring slip band, a crack is replaced by static discrete dislocations mechanically equivalent with the crack and a physical model of analyzing the mechanical interaction between static, dynamic dislocations and hydrogen is constructed. The occurrence conditions of the mechanisms of corrosive anodic chemical reaction, inter-granular cracking due to hydrogen concentration and facet like fracture due to the interaction between hydrogen and dynamic dislocations were clarified. These results were found to be in good agreement with previous results based on a convenient one-dimensional model. Furthermore, the competitive interaction between hydrogen and dynamic dislocations which causes the facet like fracture is found to occur under the limited velocity range of hydrogen and dynamic dislocations which is closely related to the corresponding range of yield stress.
    No preview · Article · Jan 2010 · Journal of Solid Mechanics and Materials Engineering
  • A. T. Yokobori · R. Sugiura · S. Kimoto · D. Yoshino · T. Matsuzaki
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    ABSTRACT: W-added 9Cr ferritic heat-resistant steels, ASME grade P92, are used under the high temperature creep-fatigue condition. Concerning the maintenance of the operational safety and minimizing operational costs, it is necessary to construct a law of predicting the crack growth life by clarifying the dominant factors of the crack growth life under the condition of creep-fatigue multiplication (the effect of stress holding time on the time-dependent fracture). In this study, crack growth tests under the conditions of creep-fatigue multiplication were conducted with various conditions of stress holding time, applied stress and temperatures. As a result, a unified law of predicting the crack growth life under the conditions of creep-fatigue multiplication was derived based on the concept of Q* with the transition function of crack growth life from fatigue to creep.
    No preview · Conference Paper · Jan 2009
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    ABSTRACT: The Ni-base supperalloy IN738LC, developed as a gas turbine blade material, is used under the conditions of creep-fatigue multiplication. In this paper, using IN738LC, in situ observational tests under the conditions of creep-fatigue multiplication were conducted and the effects of cycle-dependent and time-dependent mechanisms on the fracture life tf were investigated. Furthermore, on the basis of the concept of non-equilibrium science, the multiple effects of creep and fatigue on the fracture life tf were clarified.
    No preview · Conference Paper · Jan 2009
  • S. Taketomi · A. T. Yokobori · K. Takei · Y. Wada · Y. Tanaka · T. Iwadate
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    ABSTRACT: The 2.25Cr-1Mo steel is used for pressure vessel components and has sometimes suffered from hydrogen embrittlement. Therefore, it is important to investigate the effect of hydrogen on the embrittlement for 2.25Cr-1Mo steel. Under corrosion fatigue conditions, the effect of hydrogen embrittlement (HE) on corrosion fatigue crack growth rate (CFCGR) appears in different ways depending on the applied stress waveforms. This behavior has been clarified by previous theoretical and experimental analyses. In this paper, based on the cited theoretical analyses, the effect of stress wave form on CFCGR for 2.25Cr1Mo steel was investigated and the sensitivity to hydrogen embrittlement is examined for this material.
    No preview · Article · Sep 2008 · Corrosion -Houston Tx-
  • A. T. Yokobori · R. Sugiura · D. Yoshino · M. Tabuchi · Y. Hasegawa
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    ABSTRACT: The W added 9Cr ferritic heat resistant steel ASME grade P92, developed as a boiler tube material, is used under the conditions of creep-fatigue multiplication. In this paper, using P92 steel, crack growth tests under the conditions of creep-fatigue multiplication were conducted and the effects of cycle-dependent and time-dependent mechanisms on the crack growth life tf were investigated. Furthermore, on the basis of the concept of non-equilibrium science, the multiple effects of creep and fatigue on the crack growth life tf were clarified.
    No preview · Conference Paper · Jan 2007
  • M Tabuchi · H Hongo · T Watanabe · AT Yokobori
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    ABSTRACT: The creep strength of weldment for high Cr ferritic steels decreases due to Type IV damages in the heat affected zone during long-term services at high temperatures. It is important to predict initiation and growth of creep voids and cracks in weldment. This paper clarified the Type IV crack initiation and growth properties in fine-grained HAZ of weldments for tungsten strengthened high Cr steels. On the basis of experimental results, the computational simulation for Type IV crack growth behavior was conducted. The effect of multiaxial stress condition on vacancy diffusion and crack initiation was discussed.
    No preview · Article · Jan 2006 · Journal of ASTM International
  • A.T. Yokobori
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    ABSTRACT: Mechanisms of dissolvent anodic chemical reaction and hydrogen embrittlement have been proposed as stress corrosion cracking (SCC) mechanisms. The former is feasible for the case of plastic deformation dominant metals (low-yield stress), and the latter is for high-strength metals such as high-strength steels. However, in spite of low-yield stress, a discontinuous cleavage-like fracture is sometimes observed during SCC for ductile fcc alloys, which concerns the interaction between dislocations and the hydrogen cluster. The problem of when these mechanisms will be dominant remains. In this paper, the stress corrosion cracking model on the basis of hydrogen diffusion and concentration toward the elastic-plastic stress field around a crack and the interaction of dislocations and hydrogen around a crack tip are proposed to clarify the mechanism of stress corrosion cracking for ductile and brittle materials. We conducted numerical analyses using these proposed models. Dislocation motion–HE parameter–hydrogen embrittlement–hydrogen diffusion–stress corrosion cracking–cleavage fracture.
    No preview · Article · Jul 2004 · International Journal of Fracture
  • A T Yokobori · M Ichiki · H Ohuchi · T Kobayashi · T Satoh · Y Kinoshita
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    ABSTRACT: The proposal of noninvasive diagnostic method of mechanical degradation of vascular wall is clinically useful and it will be correlated with noninvasive diagnostic method of atherosclerosis. Supersonic Doppler effect sensor has been used to measure blood flow velocity as a noninvasive measuring method. However, it is remain problem whether the output from the Doppler effect sensor really detects the pure blood flow velocity. Theoretically, when the Doppler effect sensor is set perpendicular to the blood flow direction, that is, perpendicular to the blood vessel, the output will correspond to the expansion velocity of blood vessel wall, because it detect the frequency of Doppler shifted supersonic scattered from vascular wall. Previously, on the basis of this concept, using Doppler effect sensor, we showed this method can really detect the deformation velocity of blood vessel wall and it correlates the degradation of elastic property of blood vessel. Furthermore, using this proposed measuring method, atherosclerosis is found to progress correspondingly with the visco-elastic degradation of vascular wall. In this paper, on the basis of our proposed method, the quantitative noninvasive estimation method of the degradation of vascular wall and the progressive degree of atherosclerosis by unique parameter has been proposed. Using this method, the degradation of vascular wall is correlated to the oxygen metabolic function of blood vessel corresponding to the function of oxygen transportation and progression of atherosclerosis. Furthermore, the organ correlation on the atherosclerosis between lower limb and carotid is investigated by this proposed method.
    No preview · Article · Feb 2004 · Bio-medical materials and engineering
  • CY Jian · A T Yokobori
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    ABSTRACT: It is necessary to maintain mechanical compatibility between a blood vessel and a vascular substitute to promote encapsulation around the anastomosed part. From this point of view, using linear elastic theory, we had previously performed stress analyses at the part anastomosed by tissue adhesion, in order to propose some methods of preventing stress concentration at this junction. In this study, based on the previous analyses, we have attempted to develop a concept that can be applied under the conditions of operation. That is, the initial diameter of a vascular substitute with high rigidity is chosen larger than that of a blood vessel. This will reduce the stress concentration around the anastomosed part, on average, during expansion of the blood vessel. We analysed the optimum diameter ratio between the vascular substitute and the blood vessel which causes the least stress concentration, on average, during this process, using linear elastic theory. Furthermore, numerical analyses of blood vessel deformation were performed using various nonlinear stress-strain laws. These results were compared to the analytical solution based on linear elastic theory.
    No preview · Article · Feb 1999 · Bio-medical materials and engineering
  • E Nordström · H Ohgushi · T Yoshikawa · A T Yokobori · T Yokobori
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    ABSTRACT: In order to investigate the significance of hydroxyapatite based microporous composite (HA/mica composite) surfaces and a macroporous synthetic hydroxyapatite, rat marrow cell culture, which shows osteogenic differentiation, was carried out on six different culture substrata (two control culture dishes, two identical HA/mica composites, and two identical macroporous synthetic hydroxyapatites). A culture period of two weeks in the presence of beta-glycerophosphate (BGP), ascorbic acid, and dexamethasone resulted in abundant mineralized nodule formations that were positive for alkaline phosphatase (ALP) stain. The stain on the macroporous synthetic hydroxyapatite and the HA/mica composites were intense, the enzyme activity being about double that of control culture dishes. These data indicate that the synthetic macroporous hydroxyapatite surface and the HA/mica composite surface promotes osteoblastic differentiation.
    No preview · Article · Feb 1999 · Bio-medical materials and engineering
  • M. Tabuchi · K. Kubo · K. Yagi · A.T. Yokobori · A. Fuji
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    ABSTRACT: Creep crack growth (CCG) tests on Ni-base superalloys were carried out in the Japanese VAMAS group as part of a round-robin program in order to assist the standardization of the CCG test method for creep-brittle alloys. The effect of temperature, load, specimen thickness and material microstructure on CCG behavior was investigated. The applicable range of fracture mechanical parameters to evaluate the CCG rate was evaluated. The CCG rate was characterized by the C∗ parameter independent of testing conditions in the range where the CCG rate accelerated. However, the acceleration stage occupied only a small portion of life time for the creep-brittle superalloys. In the range where the CCG rate was constant, the CCG rate and the fracture life could be predicted approximately by the Q∗ method based on the thermally activated process.
    No preview · Article · Jan 1999 · Engineering Fracture Mechanics
  • J. C. Ha · A. T. Yokobori · H. Takeda
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    ABSTRACT: Fatigue fracture of fiber-reinforced polymer composites (FRP) occurs when microcracks are induced by debonding, pull-out and delamination at the interface between the matrix and fiber. This microcrack area increases with increase in fatigue cycles and a damage region is formed. In our previous paper, fatigue life of a short fiber-reinforced polymer composite consisting of glass fibers and polycarbonate matrix was found to be related not to the main crack growth behavior but to the progression behavior of the damage region. In this paper, using our proposed real time observational system, we performed detailed observations on the behavior of fatigue damage and clarified the mechanism of damage progression. Furthermore, mechanical considerations were performed by finite-element elastic-plastic stress analysis. The results mentioned above indicate that control of short fiber alignment makes it possible to release the stress concentration caused in the matrix, and disperse fatigue damage. This results in an enormous improvement in fracture toughness.
    No preview · Article · Jan 1999 · Journal of Materials Science
  • E G Nordström · A T Yokobori · T Yokobori · Y Aizawa
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    ABSTRACT: By using the fracture toughness estimation method based on two-dimensional map, it was found that the ductility of the high porosity hydroxyapatite/mice composite was comparable with silicon carbide. It was measured to be higher than that of packed hydroxyapatite. Alumina ceramics with more than 96% aluminium oxide showed a higher fracture toughness than the composite material. When bending strength was compared, the strength of the composite was two or three times lower than that of packed hydroxyapatite and much lower than the other studied materials. The composite material showed high porosity, which in turn gives it a lower bending strength. However, the high porosity is more favourable for biocompatibility.
    No preview · Article · Feb 1998 · Bio-medical materials and engineering
  • A T Yokobori
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    ABSTRACT: In our earlier studies, we considered osteogenesis to be a nucleation mechanism accompanied by preliminary diffusion and showed the optimum mechanical conditions for promoting this ionic diffusion. In this study, we performed an analysis of the nucleation mechanism of hydroxyapatite (HA) in areas with a high concentration of transported Ca and PO4 ions in the pore region along the collagen alignment. We derived the equation of the HA nucleation rate as a function of hydrostatic tensile stress, sigma P and surface energy change (gamma-lambda), where gamma is the surface energy of the HA particle and lambda is the decreasing factor of gamma during HA nucleation. lambda is thus related to the effect of chemical and electric stimulations. Finally, we formulated a unified equation of the nucleation rate of HA, which consists of ionic diffusion and HA nucleation processes.
    No preview · Article · Feb 1998 · Bio-medical materials and engineering

Publication Stats

144 Citations
40.88 Total Impact Points

Institutions

  • 1994-2015
    • Tohoku University
      • • Department of Nanomechanics
      • • Graduate School of Engineering
      • • Department of Neurosurgery
      • • Department of Orthopaedic Surgery
      Sendai-shi, Miyagi, Japan
  • 1994-1995
    • Teikyo University
      • Faculty of Science and Engineering
      Edo, Tokyo, Japan