• Journal of the Korea Concrete Institute
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• v.15 no.6
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• pp.915-923
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• 2003

This paper presents the test results of total 35 direct tensile specimens to investigate the effect of high-strength concrete on the tension stiffening effect in axially loaded reinforced concrete tensile members. Three kinds of concrete strength 25, 60, and 80 MPa were included as a major experimental parameter together with six concrete cover thickness ratios. The results showed that as higher strength concrete was employed, not only more extensive split cracking along the reinforcement was formed, but also the transverse crack space became smaller. Thereby, the effective tensile stiffness of the high-strength concrete specimens at the stabilized cracking stage was much smaller than those of normal-strength concrete specimens. This observation is contrary to the current design provisions, and the significance in reduction of tension stiffening effect by employment of high-strength concrete is much higher than that would be expected. Based on the present results, a modification factor is proposed for accounting the effect of the cover thickness and the concrete strength.

• Journal of Sensor Science and Technology
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• v.17 no.4
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• pp.273-280
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• 2008

Monolith aperture-type oxygen sensors with simple structure of YSZ(pin-hole)/Pt/ YSZ(solid electrolyte)/Pt were fabricated by co-firing technique. To enhance the yield of productivity, a couple of YSZ green sheets for diffused barrier and solid electrolyte were prepared by tape-casting and co-firing method. The limit current characteristics of the oxygen sensors were measured between 500 and $650^{\circ}C$ The heating temperature of $600^{\circ}C$ was optimum as a portable oxygen sensor in the range of oxygen concentration from 0 to 75 vol%. Linear proficiency of limit current behavior as a function of oxygen concentration was controlled by the variation of aperture dimension. The fabricated oxygen sensors showed the stable sensing output for 30 days. Gas leakage in bonding area due to warping, cracking and thermal cycling was not found in the period.

• Proceedings of the Korea Concrete Institute Conference
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• 2005.11a
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• pp.193-196
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• 2005

This study deals with the estimation of the crack width by stabilized cracking considering bond-slip relationships in reinforced concrete members. The proposed method utilizes the sameness of tension stiffening and a change of bond-slip relationships because of concrete's splitting. The validity, accuracy and efficiency of the proposed method are established by comparing the analytical results with the experimental date and the major code spcifications. The analytical results of analysis presented in this study indicate that the proposed method can be effectively estimated the crack width of the reinforced concrete members.

• Journal of Information Display
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• v.3 no.1
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• pp.27-34
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• 2002

POS446M and POS430Ti alloys were investigated for application to a Flatron rail instead of conventional 28 wt%Cr alloy in order to reduce material costs. It was found that the permeability decrement of materials could be successfully minimized by proper heat treatment, and that the beam landing drift of the Flatron was not overly affected by the permeability of materials. In addition, the Flatron manufacturing process using POS430Ti was also stabilized without glass cracking by the application of the modified process conditions, which were optimized through the design of the experiment method.

• Carbon letters
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• v.2 no.2
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• pp.99-104
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• 2001

Partial mesophase (PM) pitch precursor was prepared from fluidized catalytic cracking-decant oils (FCC-DO) by chemical reaction in the presence of $Br_2$. The PM pitch heated-treatment at $420^{\circ}C$ for 9 h exhibited the softening point of $297^{\circ}C$ with 23% yield, and 55% anisotropic content. The PM pitch precursor was melt-spun through circular nozzle by pressurized $N_2$, stabilized at $310^{\circ}C$, carbonized at $700^{\circ}C$, $1000^{\circ}C$, and $1200^{\circ}C$. The enough stabilization introduced 16.4% of the oxygen approximately. The stacking height ($L_{c002}$) and interlayer spacing ($d_{002}$) of the as-spun fibers were 4.58 nm and $3.45{\AA}$ and the value became minimum and maximum at $700^{\circ}C$ respectively in the carbonization procedure. The tensile strength increased with an increase in the heat treatment temperature exhibiting highest value of 750 MPa at $1200^{\circ}C$ carbonization.

• Proceedings of the KIEE Conference
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• 1997.07d
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• pp.1258-1260
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• 1997

The metallic interconnector has a superior property than the ceramic interconnector in the point of the cell performance, but has problems to form oxide films on the interconnector surface during cell operation and induce cracking in the cell due to the stress by the thermal expansion coefficient difference with stabilized Zr electrolyte. For solving the problems in the metallic interconnector, new and modified materials have been investigated. In this paper, $LaCrO_3$ dispersed Cr was evaluated in high temperature oxidized characteristics, sintering density, electrical conductivity and various thermochemical properties. The test results show that $LaCrO_3$ dispersed Cr has more superior properties than that of conventional alloy interconnector and $LaCrO_3$ dispersed Cr can be used as an interconnector in SOFC.

• Journal of the Korean institute of surface engineering
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• v.34 no.5
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• pp.486-493
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• 2001

Partially stabilized zirconia (PSZ) is an attractive material for thermal barrier coating. Zirconia exists in three crystallographic phases: cubic, tetragonal and monoclinic. Especially, the phase transformation of tetragonal phase to monoclinic phase accompanies significant volume expansion, so this transition generally results in cracking and contributes to the failure of the TBC system. Both the plasma sprayed ZrO$_2$-8Y$_2$O$_3$ (YSZ) coat and the ZrO$_2$,-25CeO$_2$,-2.5Y$_2$O$_3$ (CYSZ) coat are isothermally heat -treated at 130$0^{\circ}C$ and 150$0^{\circ}C$ for 100hr and cooled at different cooling rates. The monoclinic phase is not discovered in all the CYSZ annealed at 130$0^{\circ}C$ and 150$0^{\circ}C$. In the 150$0^{\circ}C$ heat-treated specimens, the YSZ contains some monoclinic phase while none exists in the 130$0^{\circ}C$ heat-treated YSZ coat. For the YSZ, the different phase transformation behaviors at the two temperatures are due to the stabilizer concentration of high temperature phases and grain growth. For the YSZ with 150$0^{\circ}C$-100hr annealing, the amount of monoclinic phase increased with the slower cooling rate. The extra oxygen vacancy, thermal stress, and c to t'phase transformation might suppress the t to m martensitic phase transformation.

• Journal of Powder Materials
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• v.20 no.6
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• pp.460-466
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• 2013

4 mol% Yttria-stabilized zirconia (4YSZ) coatings with $200{\mu}m$ thick are fabricated by Electron Beam Physical Vapor Deposition (EB-PVD) for thermal barrier coating (TBC). $150{\mu}m$ of NiCrAlY based bond coat is prepared by conventional APS (Air Plasma Spray) method on the NiCrCoAl alloy substrate before deposition of top coating. 4 mol% YSZ top coating shows typical tetragonal phase and columnar structure due to vapor phase deposition process. The adhesion strength of coating is measured about 40 MPa. There is no delamination or cracking of coatings after thermal cyclic fatigue and shock test at $850^{\circ}C$.

• Journal of the Korean Society of Safety
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• v.24 no.2
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• pp.17-22
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• 2009

Thermal stress and elastic creeping stress analysis was conducted by finite element method to simulate start-up process of a boiler header of 500MW standard fossil power plant. Start-up temperature and operating pressure history were simplified from the real field data and they were used for the thermal stress analysis. Two kinds of thermal stress analysis were considered. In the first case only temperature increase was considered and in the second case both of temperature and operating pressure histories were considered. In the first analysis peak stress was occurred during the temperature increase from the room temperature. Hence cracking or fracture may occur at the temperature far below the operating maximum temperature. In the results of the second analysis von Mises stress appeared to be higher after the second temperature increase. This is due to internal pressure increase not due to the thermal stress. When the stress components of radial(r), hoop($\theta$) and longitudinal(z) stress were investigated, compression hoop stress was occurred at inner surface of the stub tube when the temperature increased from room temperature to elevated temperature. Then it was changed to tension hoop stress and increased because of the operating pressure. It was expected that frequent start-up and shut-down operations could cause thermal fatigue damage and cracking at the stub tube hole in the header. Elastic-creeping analysis was also carried out to investigate the stress relaxation due to creep and stabilized stress after considerable elapsed time. The results could be used for assessing the creep damage and the residual life of the boiler header during the long-tenn service.

• Journal of the Korea Concrete Institute
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• v.23 no.6
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• pp.791-797
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• 2011

This paper presents the test results of 12 direct tensile specimens to investigate the effect of cover thickness on the tension stiffening behavior in axially loaded reinforced concrete tensile members. Six concrete cover thickness ratios are selected as a main experimental parameter. The results showed that, as cover thickness became thinner, more extensive split cracking along the reinforcement occurred and transverse crack spacing became smaller, making the effective tensile stiffness of thin specimens at the stabilized cracking stage to be much smaller than that of thick specimens. This observation is not implemented in the current design provisions, in which the significant reduction of tension stiffening effect can be achieved by applying thinner cover thickness. Based on the present results, a modified tension stiffening factor is proposed to account for the effect of the cover thickness.