• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.1
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• pp.77-86
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• 1989

A modified $C^{*}$ integral as load parameter in creep fracture is proposed considering the effect of crack growth. It is shown that the parameter does not depend on crack velocity. By performing experiment using STS 304 stainless steel at 600.deg.C the validity of the parameter is investigated. The results show that the parameter is a good measure as a load parameter in creep fracture and the rate of crack tip opening displacement can also be a creep load parameter for STS 304 at 600.deg. C.C.

• Nuclear Engineering and Technology
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• v.37 no.3
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• pp.287-292
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• 2005

[ $\pi$ ]The effects of silica-based additives have been investigated to improve the creep property of a $UO_2$ pellet. The additive composition, $50wt\%SiO_2-47wt{\%}CaO-3wt\%Cr_2O_3$ (SCC), was selected according to the dihedral angle and the distribution of the second phase. It was observed that the creep rate of the $0.07 wt\%$ SCC-added $UO_2$ was slower than that of the pure $UO_2$. However, the creep rate of the $0.22 wt\%$ SCC-added $UO_2$ was about 3_48 times faster than that of the pure $UO_2$, depending on the applied stress in the lower stress range. In the case of the $0.35 wt\%$ SCC-added $UO_2$, the creep rate decreased in comparison with that of the $0.22 wt\%$ SCC-added $UO_2$. The observed enhancement in the creep rate might depend on a balance between the positive role of the viscous intergranular phase and the negative roles of the additives and the grain growth.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.10 s.253
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• pp.1219-1226
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• 2006

The ASTM test standard recommends the use of the compact tension specimen for creep crack growth rates measurement. In the creep crack growth rate test, the displacement rate due to creep is obtained by subtracting the contribution of elastic and plastic components from the total load line displacement rate based on displacement partitioning method fur determining $C^*-integral$, which involves Ramberg-Osgood (R-O) fitting procedures. This paper investigates the effect of the R-O fitting procedures on plastic displacement rate estimates in creep crack growth testing, via detailed two-dimensional and three-dimensional finite element analyses of the standard compact tension specimen. Four different R-O fitting procedures are considered; (i) fitting the entire true stress-strain data up to the ultimate tensile strength, (ii) fitting the true stress-strain data from 0.1% strain to 0.8 of the true ultimate strain, (iii) fitting the true stress-strain data only up to 5% strain, and (iv) fitting the engineering stress-strain data. It is found that the last two procedures provide reasonably accurate plastic displacement rates and thus should be recommended in creep crack growth testing. Moreover, several advantages of fitting the engineering stress-strain data over fitting the true stress-strain data only up to 5% strain are discussed.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.13 no.1
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• pp.61-68
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• 2017

Reactor pressure vessel (RPV) under severe accident conditions accompanied by core melting is exposed to direct high-temperature thermal loads. Understanding the creep behavior of the material is one of the most important factors for evaluating the structural integrity at these conditions. While damage evaluation studies have been conducted on critical structures of nuclear power plants through finite element (FE) analyses considering creep behavior, for accurate creep damage evaluation, constitutive equations considered in the FE analyses may have different results depending on the time hardening and strain hardening models as well as the tertiary creep consideration. The purpose of this study is to evaluate the creep damage under severe accident conditions by using FE method for a representative domestic RPV material, SA508 Gr.3. The effect of material hardening models and constitutive equations which are the main variables were also investigated.

• Journal of the Korean Society of Safety
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• v.4 no.1
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• pp.25-39
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• 1989

Cyclic temperature creep tests were carried out an AS 7075 alloy specimens were subjected to a constant load and stepwise temperature cycles in which temperature was fluctuated between 30$0^{\circ}C$ and 25$0^{\circ}C$ with three different cycle ratios. The highest frequency of cycling was 1 cycle per 10 hr and the lowest one was 1 cycle per 12 hr. From the creep experimental results with the above conditions the creep strain under cyclic temperature can be predicted easily by introd ucing the equivalent steady temperature because defined by Eq.(16), but the rupture life is 1.1 time than those of constant temperature because of effect of temperature history at tertiary creep range. Besides thlis result, the results of the creep test under cyclic temperature conditions are respectively compayiea with calculated rupture lives using the life fraction law and Eq.(18). The agreement between the obseried rupture times and calculated ones is fairly good. So creep rupture lives can be respectively predicted using life fraction law and Eq.(18).

• Journal of the Korean Geosynthetics Society
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• v.3 no.2
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• pp.39-45
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• 2004

Geocomposite mechanically bonded with woven type geotextile and nonwoven geotextile was used to examine to the long-term creep deformation behaviors by the SIM(Stepped Isothermal Method). The temperature steps were $26^{\circ}C$, $40^{\circ}C$, $54^{\circ}C$, $68^{\circ}C$, $82^{\circ}C$ and loading levels were 40%, 50%, 60% of designed strength for stepped isothermal method. Results of creep tests are showing that their strain were lower than 10% during 10,000 hours(GRI GS 10). Also, the effect of weft injection density to the creep deformation behaviors were examined. The weft densities of 0%, 50%, 100% of the original weft density showed the creep strain within 10% and the creep strain was increased with the decrease of weft injection density.

• Journal of the Korean Geosynthetics Society
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• v.4 no.4
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• pp.23-37
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• 2005

The factors affecting the long-term design strength of geogrid can be classified into factors on creep deformation, installation damage, temperature, chemical degradation and biological degradation. Especially, creep deformation and installation damage are considered as main factors to determine the long-term design strength of geogrid. Current practice in the design of a reinforced soil structures is to calculate the long-term design strength of a geosynthetic reinforcement damaged during installation by multiplying the two partial safety factors, $RF_{ID}$ and $RF_{CR}$. This method assumes that there is no evaluation of synergy effect between installation damage and creep deformation of geogrids. This paper describes the results of a series of experimental study, which are carried out to assess the combined effect of the installation damage and the creep deformation for the long-term design strength of geogrid reinforcements. A series of field tests was carried out to assess installation damage of various geogrids with respect to different fill materials, and then creep tests are conducted to evaluate the creep deformation of both undamaged and damaged geogrids. The results indicated that the tensile strength reduction factors, RF, considering the combined effect between the installation damage and the creep deformation is less than that calculated by the current design method.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2012.11a
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• pp.75-76
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• 2012

In this study, it was evaluated about creep properties of high strength concrete according to compressive strength and moisture evaporation condition. As a results, creep strain and creep coefficient was greatly affected by moisture evaporation conditions rather than compressive strength. Also, the effect of fiber mixed was not show big difference.

• Proceedings of the Korea Concrete Institute Conference
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• 2003.11a
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• pp.309-312
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• 2003

Quasi-instanteneously performed creep loading is probably one way to seperate the viscous and the plastic creep from the elastic deformation. For mature concrete no differences seem to exist depending on whether the loading is rapidly applied or not. At early age the plastic deformations probably dominate the deformation. A series of test have been done to measure the quasi-instanteneous deformation in concrete specimens according to loading durations and concrete ages. The present study indicates that even very short-term loading contains creep deformation. As concrete is younger and load duration is longer, the proportion of creep deformation in quasi-instanteneous deformation is increased.

• Journal of the Korean Ceramic Society
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• v.14 no.3
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• pp.182-186
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• 1977

Compression creep of polycrystalline magnesia at about 1$600^{\circ}C$ under 5-40kg/$\textrm{cm}^2$ was examined, and also the effects on it of minor additives such as B2O3, CaO and SiO2 were examined. The high temperature creep of high purity magnesia was primarily controlled by the Nabarro-Herring type lattice diffusion of Mg in magnesia. B2O3 was included in the molten state and showed on increasing B2O3 contents. Some of the CaO and SiO2 were also included in the molten state, promoted the grain boundary sliding, so that creep rate was increased with an increasing content of them.