• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.8 s.251
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• pp.931-940
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• 2006

Defect assessment of a weld zone is important in fitness-for-service evaluation of plant components. Typically a J and $C^*$ estimation method for a defective homogeneous component is extended to a mismatched component, by incorporating the effect due to the strength mismatch between the weld metal and the base material. The key element is a mismatch limit load. For instance, the R6/R5 procedure employs an equivalent material concept, defined by a mismatch limit load. A premise is that if a proper mismatch limit load solution is available, the same concept can be used for any defect location (either a weld centre defect or a heat affected zone (HAZ) defect) and for any material combination (either two-material or multi-material combinations; either similar or dissimilar joints). However, validation is still limited, and thus a more systematic investigation is needed to generalise the suggestion to any geometry, any defect location and any material combination. This paper describes the effect of structural geometry on the $C^*$ integral for defective similar welds, based on systematic elastic-creep 2-D and 3-D finite element (FE) analyses, to attempt to elucidate the questions given above. It is found that the existing 'equivalent material' concept is valid only for limited cases, although it provides conservative estimates of $C^*$ for most of cases. A modification to the existing equivalent material concept is suggested to improve accuracy.

• Proceedings of the Korea Concrete Institute Conference
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• 1992.04a
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• pp.126-130
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• 1992

Recently, fatigue problem become a critical issue in the design of prestressed concrete bridges due to the increase of traffic volumes and use of high-strength materials. Most existing studies are mainly concerned with the fatigue behavior of component materials only such as concrete, reinforcing bars, and prestressing steels and few studies exist that deals with the fatigue behavior of bridge members. An improved analytic formulation for both uncracked and cracked prestressed concrete composite section with cyclic creep effect is developed to take into account the change of neutral axis with crack propagation. The procedure also enables to investigate serviceability limit states, deflection and crack width. The present study allows more realistic analysis and design of prestressed concrete composite girder bridges under fatigue loadings.

• Transactions of the Korean Society of Automotive Engineers
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• v.24 no.5
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• pp.596-604
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• 2016

In this study, we used a stochastic approach for guaranteeing the reliability and robustness of the performance with regard to the design of polymer components, while taking into consideration the degradation properties and operating conditions in automobiles. Creep and tensile tests were performed for obtaining degradation properties. The Prony series, which described the viscoelastic models, were calculated to use the creep data by the Maxwell fluid model. We obtained the stress data from the frequency response analysis of the polymer components while considering the degradation properties. Limit state functions are generated by using these data. Reliability assessments are conducted under the variation of the degradation properties and area of frequency at peak response. For this study, the input parameters are assumed to be a normal distribution, and the reliability under the yield stress criteria is evaluated by using the Monte Carlo Simulation. As a result, the reliabilities, according to the three types of polymer materials in automotive components, are compared to each other and suggested the applicable possibility of polymeric materials in automobiles.

• Journal of the Korea Concrete Institute
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• v.20 no.6
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• pp.765-772
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• 2008

For RC structures, long-term deformation occurs due to the inherent characteristics, which are creep and shrinkage. In terms of serviceability, it is important to limit deflection caused by the deformation to the allowable deflection. In the recent years, various repair and strengthening methods have been used to improve performance of the existing RC structures. One of the typical methods is FRP externally bonded method (EBR). Fiber reinforced polymer (FRP) has been used worldwide as repair and strengthening materials due to its superior properties. Besides, it has to offer improved strengthening performance not only under instantaneous load but sustained load. Therefore, accurate prediction method of deflection for the RC members externally bonded with FRP under sustained load is required. In this paper, three beams were fabricated. Two beams were externally strengthened with one of CFRP plate and GFRP plate respectively. Total three beams were superimposed under sustained load of 25 kN. During 470 days, deflections at midspan were obtained. Moreover, creep coefficients and shrinkage strains were calculated by using ACI-209 code and CEB-FIP code. In order to predict the deflection of the beams, EMM, AEMM, Branson's method and Mayer's method were used. Through the experiment, it was found that the specimen with CFRP plate has the most flexural capacity and Mayer's method is the most precise method to predict total long-term deflections.

• Steel and Composite Structures
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• v.20 no.2
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• pp.379-397
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• 2016

Due to creep and shrinkage of the concrete core, concrete-filled steel tubular (CFST) arches continue to deform in the long-term under sustained loads. This paper presents analytical investigations of the effects of geometric nonlinearity on the long-term in-plane structural performance and stability of three-pinned CFST circular arches under a sustained uniform radial load. Non-linear long-term analysis is conducted and compared with its linear counterpart. It is found that the linear analysis predicts long-term increases of deformations of the CFST arches, but does not predict any long-term changes of the internal actions. However, non-linear analysis predicts not only more significant long-term increases of deformations, but also significant long-term increases of internal actions under the same sustained load. As a result, a three-pinned CFST arch satisfying the serviceability limit state predicted by the linear analysis may violate the serviceability requirement when its geometric nonlinearity is considered. It is also shown that the geometric nonlinearity greatly reduces the long-term in-plane stability of three-pinned CFST arches under the sustained load. A three-pinned CFST arch satisfying the stability limit state predicted by linear analysis in the long-term may lose its stability because of its geometric nonlinearity. Hence, non-linear analysis is needed for correctly predicting the long-term structural behaviour and stability of three-pinned CFST arches under the sustained load. The non-linear long-term behaviour and stability of three-pinned CFST arches are compared with those of two-pinned counterparts. The linear and non-linear analyses for the long-term behaviour and stability are validated by the finite element method.

• The Korean Journal of Quaternary Research
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• v.9 no.1
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• pp.43-60
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• 1995

The Kanweoldo Deposit in the Cheonsoo Bay western coast of Korean Peninsula is considered to be influenced by severe freezing condition under cold humid environment of the last glacial age. The evidence of severe freezing in the some upper part of the fine-grained Kanweoldo Deposit is characteristically irregular wavy la-mellar structure with the interval of 2∼8mm. In particular lamina show very compacted fabrics composed of rounded or spheroidal discrete aggregates covered by silt caps. Such laminar structure and associated micro-fabrics might owe to soil freezing such as ice segregation in lens form cryophoresis pressure from growing ice and disturbance by frost-creep. Furthermore pedogenesis of cold-humid type such as gleyzation or peseudo-gleyzation also might af-fect the kanweoldo Deposit in the priod of severe cold-humid cli-mate of the Wrm. The Kanweoldo sediment and organic remnant(16,708 B.P. with error limit of 250 years) affected by severe cryogenic activities sug-gest that the paleoclimate of Late Wrm in Korea might be so cold and humid as to engender the cryogenic structure in subaerial silty and sandy silt deposits.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.4
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• pp.399-406
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• 2005

Alloy 625 is used widely in industrial applications such as aeronautical aerospace, chemical, petrochemical and marine applications. Because of a good combination of yield strength. tensile strength, creep strength, excellent fabricability, weldability and good resistance to high temperature corrosion on prolonged exposure to aggressive environments. High qualify weldments for this material are readily produced by commonly used processes. But all of processes are not applicable to this material by reason of unavailability of matching, position or suitable welding filler metals and fluxes may limit the choice of welding processes. Recently, the flux cored wire is developed and applied for the better productivity in several welding position including the vortical position. In this study. the weldability and weldment characteristics of Alloy 625 are evaluated in FCAW weld associated with the several shielding gases($80\%Ar+20\%\;CO_2,\;50\%Ar+50\%\;CO_2.\;100\%\;CO_2$) in viewpoint of welding productivity. The results of the experimental study on corrosive characteristics of Alloy 625 are as follows; There is no remarkable difference among shielding gases. however they has a striking difference among corrosive solutions by results of distinguished density and time of corrosive solution. Generally, the shielding gases($80\%Ar+20\%\;CO_2$) was superior to the other gases on high temperature tensile and a low temperature impact. but all of the shield gases were making satisfactory results on corrosion test.

• Journal of the Korean Geotechnical Society
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• v.28 no.7
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• pp.55-65
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• 2012

Total reduction factor that is used when calculating allowable tensile strength of geogrids is made by multiplying the installation damage reduction factor ($RF_{ID}$), chemical degradation reduction factor ($RF_D$), and creep reduction factor ($RF_{CR}$) etc. In case of a model estimating allowable tensile strength considering reduction factor over the short-term tensile strength of geogrids, it has a limit of not considering interaction force between reduction factors. Junction strength comes to be reduced by installation damages or chemical degradation in the same way as tensile strength. Single junction test method cannot properly test damaged samples and shows large deviations as it does not consider scale effect. Besides, regarding calculating shear strength, no reasonable study on reduction factors was conducted yet. Therefore, in this study, reduction factors that may affect the long-term performance of geogrids were revaluated considering various conditions and accurate long-term allowable tensile strength was calculated considering interrelation between reduction factors. Creep results after installation damage and chemical resistance test showed lower value than calculated value according to GRI GG-4. After the installation damage test and the chemical resistance test, the reduction factor of junction strength was less than that of tensile strength. Shear strength before and after installation damage showed no change or increase.

• Corrosion Science and Technology
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• v.11 no.2
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• pp.48-55
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• 2012

Clamping force of a high strength bolt is reduced by a certain period of time after the initial set-up. In case of special treatments on faying surfaces such as protective coating, clamping force is relaxed more severely. Tests for slip critical joints subject to various faying surface parameters were conducted. Five different surface treatments were tested including mill scale surface, blast surface, rust surface and coated surfaces. Each specimen was composed of F10T M20 of high strength bolts and steel plates. Based on the result of slip coefficient test, blast treatment surface showed 0.59, rust treatment surface showed 0.54 and inorganic zinc treatment surface exhibited 0.44. Clean mill treatment surface and red lead paint treatment surface were 0.23, 0.21 respectively. It is identified that the slip coefficient in Korean structural design guide should be determined for various surface conditions. Subsequently from long term relaxation test of ASTM A 490 high strength bolts, relaxation of no-coated surfaces such as blast, clean mill, rust treatment, the loss of initial clamping load was 10.5%, 13.6% and 7.9% for 1,000 hours, while the loss of initial clamping force was reached as 15.0%, 18.7% more than the required redundancy 10% in case of inorganic zinc and red lead painted treatment. It is required that the limit of relaxation on coated faying surface should be established separately for various surfaces.

• Tunnel and Underground Space
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• v.27 no.3
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• pp.121-131
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• 2017

We studied the time-dependent behaviors of rock and concrete materials by conducting the static and dynamic long-term strength tests. In particular, acoustic emission(AE) signals generated while the tests were analyzed and used for the long-term stability evaluation. In the static subcritical crack growth test, the long-term behavior and AE characteristics of Mode I and Mode II were investigated. In the dynamic long-term strength test, the fatigue limit and characteristics of generation of AE were analyzed through cyclic four points bending test. The graph of the cumulative AE hits versus time showed a shape similar to that of the creep curve with the first, second and third stages. The possibility for evaluating the static and dynamic long-term stability of rock and concrete is presented from the log - log relationship between the slope of the secondary stage of cumulative AE hits curve and the delayed failure time.