• Steel and Composite Structures
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• 제46권2호
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• pp.211-220
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• 2023

Welded hollow spherical joints are commonly used joints in space grid structures. An internal stiffener is generally adopted to strengthen the joints when large hollow spheres are used. To further strengthen it, external stiffeners can be used at the same time. In this study, axial compression tests are conducted on four full-scale 550 mm spherical joints. The failure modes and strengths of the tested joints are investigated. It shows that the external stiffeners are able to increase the strength of the joint up to 25%. A numerical model for large spherical joints with stiffeners is established and verified against the experimental results. Parametric studies are executed considering six main design factors using the verified model. It is found that the strength of the spherical joint increases as the thickness, height and number of the external stiffeners increase, and the hollow sphere's diameter has a neglectable effect on the enhancement caused by the external stiffeners. Based on the experimental and numerical results, a practical formula for the compressive bearing capacity of large welded hollow spherical joints with both internal and external stiffeners is proposed. The proposed formula gives a conservative prediction on the compressive capacity of large welded hollow spherical joints with both internal and external stiffeners.

• Nuclear Engineering and Technology
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• 제32권4호
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• pp.410-423
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• 2000

A two-dimensional numerical model is developed and applied to the LAVA-EXV tests performed at the Korea Atomic Energy Research Institute (KAERI) to investigate the external cooling effect on the thermal margin to failure of a reactor pressure vessel (RPV) during a severe accident. The computational program was written to predict the temperature profile of a two-dimensional spherical vessel segment accounting for the conjugate heat transfer mechanisms of conduction through the debris and the vessel, natural convection within the molten debris pool, and the possible ablation of the vessel wall in contact with the high temperature melt. Results of the sensitivity analysis and comparison with the LAVA-EXV test data indicated that the developed computational tool carries a high potential for simulating the thermal behavior of the RPV during a core melt relocation accident. It is concluded that the main factors affecting the RPV failure are the natural convection within the debris pool and the ablation of the metal vessel, The simplistic natural convection model adopted in the computational program partly made up for the absence of the mechanistic momentum consideration in this study. Uncertainties in the prediction will be reduced when the natural convection and ablation phenomena are more rigorously dealt with in the code, and if more accurate initial and time-dependent conditions are supplied from the test in terms of material composition and its associated thermophysical properties.

• 한국수자원학회논문집
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• 제42권7호
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• pp.525-535
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• 2009

본 연구에서는 상수관망에서 개별적으로 노후도가 심하여 개량이 필요한 구간을 보다 정확하게 구분하기 위해 새로운 개별관로 정의 방법이 개발되었다. 적절한 관로 최소구성성분 길이를 결정하기 위하여 여러 가지 관로 최소구성성분 길이에 대한 평균 누적파손횟수경사선의 분산값을 비교하여 가장 큰 분산값을 나타내는 관로 최소구성성분 길이인 4 m 를 연구대상 지역의 상수관망에 적용하였으며 관로 ID는 39개로 구분되어졌다. 관로의 경제적 최적교체 시기는 한계파손율과 관로의 파손경향모형을 이용하여 결정되었는데, 각 관로 ID에 대하여 관로의 선형적 파손경향, 지수적 파손경향 또는 선형과 지수형 사이에 있는 파손경향 모두에 적용될 수 있는 General Pipe Break Prediction Model(Park and Loganathan, 2002)과 수정된 시간척도를 이용한 ROCOF(Park et al., 2007)를 적용하여 연구대상 상수관망의 최적교체시기를 산정 및 분석하였다. ROCOF 모형화 과정에서 대수-선형과 와이블 ROCOF를 적용 후 최대로그우도 추정값을 비교하여 최대로그우도가 큰 값을 가지는 ROCOF를 각 관로 ID의 ROCOF로 사용하였다. 관로파손으로 인한 사회적 비용이 관로의 최적교체시기에 미치는 영향도 분석되었다.

• 한국전산구조공학회논문집
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• 제20권3호
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• pp.273-279
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• 2007

복잡한 구조물의 유한요소해석 응답값으로부터 계산된 요소신뢰성 지수와 파괴확률은 음함수의 미분 및 극히 작은 파괴확률의 계산상 1계2차 모멘트법 (FOSM)과 몬테카를로 시뮬레이션(Monte-Carlo Simulation)의 적용이 어려우므로, 선택된 확률변수만의 함수인 한계상태방정식을 음함수로 구성하는 응답면기법을 적용하여, 기본설계단계의 아치교량의 설계값으로 구성된 확률변수를 고려하여 교량시스템의 완성단계에 대한 위험성을 해석하였다. 체계신뢰성 해석에서는 모든 붕괴모드의 발생경로와 단면에 대한 해석에 많은 시간이 소요되어 유한요소 구조해석 결과와 기존의 아치교 사고사례 등을 통해서 밝혀진 중요한 위험경로 및 위험단면의 파괴를 일으키는 사건에 대하여만 고려하였다. 교량시스템의 체계신뢰성을 평가하여 상관관계의 변동에 따른 시스템붕괴에 대한 발생확률의 상하한계를 결정하였다. 시스템의 요소간 저항연결구조는 요소파괴의 순서와 전체강성의 감소정도에 따라서 결정하였다. 체계신뢰성에서 검토되지 않는 다른 요소간의 상관관계를 검토하기 위해서 가정된 붕괴순서와 다른 거동을 보일 경우에 대해서는 통계적으로 독립된 요소들의 파괴와 그 모든 붕괴모드 조합경우에 대해서 검토하였다. 붕괴모드조합의 검토 결과 거더와의 파괴조합과 아치리브와의 파괴조합에서 체계신뢰성보다 위험한 상한계 시스템 파괴확률과 안전한 하한계 파괴확률이 발견되었다.

• 대한금속재료학회지
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• 제50권8호
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• pp.583-589
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• 2012

This work is concerned with a statistical approach to the life prediction on irradiation-assisted stress corrosion cracking (IASCC) of stainless steel (SS) for core internals of a pressurized water reactor (PWR). The previous results of the time-to-failure of IASCC measured on neutron-irradiated stainless steel components were statistically analyzed in terms of stress and irradiation. The accelerating life testing model of IASCC of cold worked Type 316 SS was established based on an inverse power model with two stress-variables, the applied stress and irradiation dose. Considering the variation of the yield strength and applied stress with the irradiation dose in the model, the remaining life of the baffle former bolt was statistically predicted during operation under complex environments of stress and irradiation.

• Nuclear Engineering and Technology
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• 제52권9호
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• pp.1998-2008
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• 2020

Solenoid operated valves (SOV) play important roles in industrial process to control the flow of fluids. Solenoid valves can be found in so many industries as well as the nuclear plant. The ability to be able to detect the presence of faults and predicting the remaining useful life (RUL) of the SOV is important in maintenance planning and also prevent unexpected interruptions in the flow of process fluids. This paper proposes a fault diagnosis method for the alternating current (AC) powered SOV. Previous research work have been focused on direct current (DC) powered SOV where the current waveform or vibrations are monitored. There are many features hidden in the AC waveform that require further signal analysis. The analysis of the AC powered SOV waveform was done in the time and frequency domain. A total of sixteen features were obtained and these were used to classify the different operating modes of the SOV by applying a machine learning technique for classification. Also, a deep neural network (DNN) was developed for the prediction of RUL based on the failure modes of the SOV. The results of this paper can be used to improve on the condition based monitoring of the SOV.

• Advances in aircraft and spacecraft science
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• 제9권3호
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• pp.263-278
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• 2022

The time-varying structural reliability of an aeroelastic launch vehicle subjected to stochastic parameters is investigated. The launch vehicle structure is under the combined action of several stochastic loads that include aerodynamics, thrust as well as internal combustion pressure. The launch vehicle's main body structural flexibility is modeled via the normal mode shapes of a free-free Euler beam, where the aerodynamic loadings on the vehicle are due to force on each incremental section of the vehicle. The rigid and elastic coupled nonlinear equations of motion are derived following the Lagrangian approach that results in a complete aeroelastic simulation for the prediction of the instantaneous launch vehicle rigid-body motion as well as the body elastic deformations. Reliability analysis has been performed based on two distinct limit state functions, defined as the maximum launch vehicle tip elastic deformation and also the maximum allowable stress occurring along the launch vehicle total length. In this fashion, the time-dependent reliability problem can be converted into an equivalent time-invariant reliability problem. Subsequently, the first-order reliability method, as well as the Monte Carlo simulation schemes, are employed to determine and verify the aeroelastic launch vehicle dynamic failure probability for a given flight time.

• 한국지반신소재학회논문집
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• 제4권1호
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• pp.3-6
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• 2005

사면 또는 옹벽 등 토목구조물에 보강용으로 적용되는 지오그리드의 고장은 토목구조물의 형태변형을 유발할 수 있는 과도한 크리프 변형으로 정의할 수 있다. 본 연구에서는 지오그리드의 크리프 변형을 단시간에 시험할 수 있는 SIM(Stepped Isothermal Methods)과 통계적 수명평가기법을 적용하여, Polyester 편포형 지오그리드의 수명을 예측하였다. 그 결과 예측한 75, 100, 114년에서의 크리프 변형율은 각각, 8.74, 8.79, 8.80%이고, CV%는 2.16~2.20%로 매우 반복성 있는 결과를 보여준다. 수명 예측 결과는 $27^{\circ}C$에서 100년 사용 후 지오그리드의 크리프 변형율이 9.3%로 추정되었고, 현장에서 요구되는 수명(100년 사용 후 크리프 변형율 10% 미만)을 만족하는 것을 확인할 수 있었다. 이 연구를 통하여, SIM을 적용할 경우 기존의 시간-온도 중첩원리(Conventional Time-Temperature Superposition)를 적용한 수명평가 방법에 비하여 시료간의 편차를 최소화 시킬 수 있고 또한 99.5%의 시험시간을 단축시킬 수 있어서 매우 효과적인 것으로 판단된다.

• 대기
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• 제33권4호
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• pp.331-341
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• 2023

A numerical forecasting models usually predict future states by performing time integration considering fixed static time-steps. A time-step that is too long can cause model instability and failure of forecast simulation, and a time-step that is too short can cause unnecessary time integration calculations. Thus, in numerical models, the time-step size can be determined by the CFL (Courant-Friedrichs-Lewy)-condition, and this condition acts as a necessary condition for finding a numerical solution. A static time-step is defined as using the same fixed time-step for time integration. On the other hand, applying a different time-step for each integration while guaranteeing the stability of the solution in time advancement is called an adaptive time-step. The adaptive time-step algorithm is a method of presenting the maximum usable time-step suitable for each integration based on the CFL-condition for the adaptive time-step. In this paper, the adaptive time-step algorithm is applied for the Korean Integrated Model (KIM) to determine suitable parameters used for the adaptive time-step algorithm through the monthly verifications of 10-day simulations (during January and July 2017) at about 12 km resolution. By comparing the numerical results obtained by applying the 25 second static time-step to KIM in Supercomputer 5 (Nurion), it shows similar results in terms of forecast quality, presents the maximum available time-step for each integration, and improves the calculation efficiency by reducing the number of total time integrations by 19%.

• 한국물환경학회지
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• 제23권3호
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• pp.356-366
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• 2007

In this paper a methodology is developed to prioritize replacement of water distribution pipes according to the economical efficiency of replacement and assess the long-term effects of water main replacement policies on water distribution systems. The methodology is implemented with MATLAB to develop a computer algorithm which is used to apply the methodology to a case study water distribution system. A pipe break prediction model is used to estimate future costs of pipe repair and replacement, and the economically optimal replacement time of a pipe is estimated by obtaining the time at which the present worth of the total costs of repair and replacement is minimum. The equation for estimating the present worth of the total cost is modified to reflect the fact that a pipe can be replaced in between of failure events. The results of the analyses show that about 9.5% of the pipes in the case study system is required to be replaced within the planning horizon. Analyses of the yearly pipe replacement requirements for the case study system are provided along with the compositions of the replacement. The effects of water main replacement policies, for which yearly replacement length scenario and yearly replacement budget scenario are used, during a planning horizon are simulated in terms of the predicted number of pipe failures and the saved repair costs.