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서기완,김재민,김윤재 한국압력기기공학회 2021 한국압력기기공학회 논문집 Vol.17 No.2
The thickness of pipe can be locally reduced during operation due to wall thinning. Due to its significance on structural integrity, many non-destructive detecting techniques and assessment methods are available. In this study, the elastic bending compliance of local wall-thinned pipe is presented in terms of the wall thinning geometry: wall thinning depth, circumferential angle and longitudinal length. Elastic finite element (FE) analysis further shows that the presented equation can be used for any wall thinning shape. The proposed solution differs from FE results by less than 6% for all cases analyzed. The bending compliance increases linearly with increasing longitudinal thinning length and non-linearly with increasing thinning angle and depth.
서기완,김봉준,최정용,김성현,김현탁,Seo, G.,Kim, B.J.,Choi, J.,Kim, S.H.,Kim, H.T. 한국전자통신연구원 2012 전자통신동향분석 Vol.27 No.5
MIT(Metal-Insulator Transition) 물질은 온도와 전기장과 같은 외부자극에 의해 절연체에서 금속으로 전이할 때 전기적 저항이 급격하게 감소하는 물질을 말한다. 그 감소폭은 약 $10^4{\sim}10^5$배 정도로 이전에 볼 수 없었던 아주 큰 값이다. 또한 이러한 급격한 감소로 인하여 NDR(Negative Difference Resistance) 같은 현상이 발생하며, 외부에서 주어지는 광학적 에너지에 의하여 전이가 일어나기도 한다. 이러한 여러 현상들을 이용하여 전자소자가 개발되고 그에 따른 응용 분야도 활발하게 연구가 진행되고 있다. 이러한 시도는 MIT 물질의 단독으로 제조된 소자뿐만 아니라 기존의 전자소자와 병행하여 더욱 시너지를 발휘할 것으로 예측된다. 본고에서는 MIT 현상의 간략한 설명과 현재 기술의 발전 방향, 간단한 응용소자에 관하여 개괄적으로 기술하고자 한다.
Thermally- or optically-biased memristive switching in two-terminal VO2 devices
서기완,김봉준,김현탁,이용욱 한국물리학회 2014 Current Applied Physics Vol.14 No.9
We investigated thermally- or optically-biased memristive switching in two-terminal micro devices based on vanadium dioxide (VO2) thin films. For the preparation of multi-level resistance switching, the device was kept at a specific temperature or an optical illumination power so that it fell into the thermal or optical hysteresis region of the device resistance during the switching. With the application of external current pulses, the device resistance decreased in a discrete manner showing multiple resistance levels, each of which was maintained as long as the temperature (or optical) bias excited the device. In particular, in the optically-biased case, the effect of the pulse-free interval between current pulses on the device resistance was also examined with respect to three intervals including 10, 15, and 30 s. It was observed that a longer pulse-free interval and higher optical bias reduced the rate of current-induced change in the device resistance. Finally, in order to explore a trend of grain resistance change in the VO2-based device, we carefully suggested a grain network model explaining a percolative transition in inhomogeneous VO2 film.
소형펀치 시험을 이용한 API X52 저온 수소환경 파괴인성 예측
김재윤,서기완,김윤재,김기석 한국압력기기공학회 2023 한국압력기기공학회 논문집 Vol.19 No.2
Hydrogen embrittlement of a pipe is an important factor in hydrogen transport. To characterize hydrogen embrittlement, tensile and fracture toughness tests should be conducted. However, in the case of hydrogen-embrittled materials, it is difficult to perform tests in hydrogen environment, particularly at low temperatures. It would be useful to develop a methodology to predict the fracture toughness of hydrogen-embrittled materials at low temperatures using more efficient tests. In this study, the fracture toughness of API X52 steels in hydrogen at low temperatures is predicted from numerical simulation using coupled finite element (FE) damage analyses with FE diffusion analysis, calibrated by analyzing small punch test data.
3차원 노즐로드 보수적 하중 조건 결정을 위한 하중 부호 결정 방법론
유경찬,서기완,송현석,김윤재 한국압력기기공학회 2023 한국압력기기공학회 논문집 Vol.19 No.2
When performing stress analysis for a nozzle in nuclear power plants, the nozzle loads should be determined conservatively. Existing stress analysis report of 3-D nozzle loads in nuclear power plants often provide only load magnitude not the sign (direction). Since calculated stress distribution depends on load direction, determining critical load directions for conservative stress analysis is crucial. In this study, an efficient method for determining critical load directions in nozzle loads is proposed. In the proposed method, stresses are firstly calculated using elastic finite element (FE) analysis for the uni-axial load in each direction. Then stress distributions for the multi-axial loads are analytically calculated using the principle of superposition. The calculated stress values are verified by comparing with FE analysis results under multi-axial loading. By using this method, the complex task of determining conservative load directions can be simplified.