• Proceedings of the Korean Nuclear Society Conference
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• 1997.05a
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• pp.600-605
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• 1997

Sandia National Laboratories(SNL)에서 수행된 원자로 용기의 고온, 고압 크리프 파괴 실험의 하나인 Lower Head Failure-1(LHF-1)에 대한 코드 해석을 수행하였다. 해석 코드로는 범용 유한요소 구조해석 코드인 ABACUS를 사용하였고, Idaho National Engineering Laboratory(INEL)의 크리프 데이터를 이용하였다. 크리프 해석에는 strain hardening 식을 적용하였고, 크리프 데이터를 적용하기 위해서 user subroutine을 개발하였다. 민감도 분석의 일환으로 내부 압력을 1.2배로 증가시킨 경우에 대해 수행한 해석 결과가 실험 결과와 유사하였다 해석 결과를 분석하여 현 크리프 데이터의 절대적 부족을 확인하였고, 크리프데이터 생산을 위한 크리프 시험을 계획하였다.

• Magazine of the Korea Concrete Institute
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• v.11 no.1
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• pp.279-288
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• 1999

The creep of concrete structures caused by variable stresses is generally calculated by step-by-step method based on the superposition of creep function. Although most practical application is carried out by this linear assumption. significant deviations between predictions and experiments have been observed when unloading takes place, that is. stress is reduced. This shows that the superposition of creep function does not describe accurately the effect of sustained compressive preload. The main purpose of this study is to propose a creep analysis model which is expressed with both creep function and creep recovery function where increase or decrease of stress is repeated. In these two function method, the creep behavior is modelled by using linear creep law for loading and creep recovery law for unloading. To apply two function method to time analysis of concrete structures, the calculation method of creep strain increment under varying stress is proposed. The calculation results based on the present method correlates very well with test data, but the conventional superposition method exhibits large deviation from test results. This paper provides a more accurate method for the time dependent analysis of concrete structures subjected to varying stress, i.e. increasing or decreasing stress. The present method may be efficiently employed in the revision of future concrete codes.

• Journal of the Korea institute for structural maintenance and inspection
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• v.27 no.5
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• pp.75-83
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• 2023

As part of a study to analyze the excessive camber occurring in prestressed concrete railway bridges, this paper presents a calculation method and analysis results for the creep coefficient which defines the increase in camber of a concrete structure over time. Using the creep coefficient formula of the design code, the coefficient is obtained by applying the climatic conditions (relative humidity and temperature) of 12 regions in Korea. The effects of differences in climatic conditions by region and starting time of load on the creep coefficient are analyzed. In order to properly calculate the creep, most of which occurs in the early stages of loading, a detailed analysis is performed by applying a time step analysis method to consider varying climate conditions through loaded period. The creep coefficient obtained by applying the average climate conditions of the region is similar to the average of the creep coefficients obtained by time step analysis. Through time step analysis, it is shown that the offset and overlap effects of relative humidity and temperature on the creep coefficient and the climate effect at the time of initial loading can be appropriately represented.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.12 no.2
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• pp.171-184
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• 1999

이 논문은 시공단계를 고려한 콘크리트 프레임 구조물의 거동 해석을 다루고 있다. 고층건물의 경우 하루에 시공이 완료되지 않으므로 각 시공단계에 따라 콘크리트의 시간의존적 현상은 다르게 발생된다. 이를 위하여 이 논문에서는 일반적인 프레임 해석기법에 콘크리트의 시간의존적 특성을 고려하였다. 이 연구에 도입된 해석기법은 단면을 가상의 층으로 나누고 각층은 일축상태로 가정한 적층단면을 사용하였다. 요소는 평면 보요소를 사용하였으며 강성행렬은 변위법을 바탕으로 유도하였고 전체적인 구조해석은 비선형 구조해석 방법의 하나인 복합법을 사용하였다. 콘크리트의 시간의존적 특성을 고려하기 위하여 단면의 각 층에서 크리프와 건조수축에 의한 변형률을 계산하였으며 크리프는 크리프 Compliance의 전개에 기본을 둔 1차 순환적 단계 알고리즘을 사용하였다. 끝으로 이 연구에서 제안된 해석모델을 이용하여 프레임해석 및 기둥축소에 관한 예제를 해석하였다.

• Journal of the Korean Geosynthetics Society
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• v.6 no.4
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• pp.1-6
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• 2007

New procedure for evaluation of creep reduction factor by the limit creep strain concept was introduced through the analysis of creep test results. To determine the limit creep strain of the geogrids, the Sherby-Dorm Plots were applied and the results of this procedure were compared and interpreted, respectively. From this, it is seen that the creep reduction factors were 1.45 for the geogrid samples used in this study. Through the comparison of creep reduction factors in 10% creep strain criteria, it was confirmed that the range of creep reduction factor is about 0.06~0.14 for the geogrid samples in this study.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.30 no.8
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• pp.103-111
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• 2002

Steady-state creep analysis of power generation turbine blade is carried out considering thermal loads and centrifugal forces. Creep strains and stresses of the turbine blade are calculated for 3-D finite clement model of the turbine blade. From the numerical results, creep life of the turbine blade is predicted. The results of creep analysis during about 200 hours indicate that creep strains of the turbine blade do not reach the rupture strain of GTD111. Creep stresses of the turbine blade are relaxed as time increases. Maximum creep strain occurs at the tip section of the airfoil pressure surface. The maximum creep strain of the turbine blade is expected close to the rupture strain after 50,000 hours approximately. The turbine blade may not have creep damage for the starting procedure of the turbine.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.9
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• pp.851-857
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• 2015

In this paper, we estimate the time-dependent C(t) integrals under elastic-plastic-creep conditions. Finite-element (FE) transient creep analyses have been performed for single-edge-notched-bend (SEB) specimens. We investigate the effect of the initial plasticity on the transient creep by systematically varying the magnitude of the initial step load. We consider both the same stress exponent and different stress exponents in the power-law creep and plasticity to elastic-plastic-creep behavior. To estimate the C(t) integrals, we compare the FE analysis results with those obtained using formulas. In this paper, we propose a modified equation to predict the C(t) integrals for the case of creep exponents that are different from the plastic exponent.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.10
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• pp.1001-1010
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• 2015

This paper estimates the time-dependent crack-tip stress fields under elastic-plastic-creep conditions. We perform Finite-Element (FE) transient creep analyses for a Single-Edge-notched-Bend (SEB) specimen. We investigate the effect of the initial plasticity on the transient creep by systematically varying the magnitude of the initial step-load. We consider both the same stress exponent and different stress exponent in the power-law creep and plasticity to determine the elastic-plastic-creep behaviour. To estimation of the crack-tip stress fields, we compare FE analysis results with those obtained numerically formulas. In addition, we propose a new equation to predict the crack-tip stress fields when the creep exponent is different from the plastic exponent.

• Journal of the Korea Concrete Institute
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• v.13 no.1
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• pp.38-45
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• 2001

Numerical studies were carried out to develop the moment magnifier method for long-term behavior of flat plates, subjected to combined in-plane compressive and transverse loads. Nonlinear finite element analyses were performed for the numerical studies. Through the numerical studies, the long term behavior of the flat plate subjected to uniform or nonuniform floor load was investigated, and creep effects on the degradation of strength and stiffness of the slabs were examined. As a result, the creep factor was implemented to describe the creep effect on the flat plate. The moment magnifier method using the creep factor was developed for long-term behavior of flat plates. Also, the design examples were shown for the verification of the proposed design method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.1
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• pp.81-90
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• 2012

The indentation test, which is one of the testing methods for evaluating the mechanical properties of materials, can be applied to the evaluation of creep properties. Many studies related to the indentation creep test, however, have just focused on the characteristics of the steady-state creep, so there are wide discrepancies between the uniaxial test and the indentation test. To obtain accurate creep properties, it is therefore important to consider the effects of transient creep. In the present work, the Ogbonna et al.'s work on the spherical indentation test including the transient creep was expanded and applied to the conical indentation creep test. The characteristics of the transient creep were analyzed via finite element simulations and compared with those obtained through spherical indentation. Other effects, such as elastic strain, indenter shape, contact area, and representative strain, which have not been considered properly in prior studies on the creep test, are also discussed.