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이탁기(Tak-Kee Lee),남용윤(Yong-yun Nam),한승호(Seung-ho Han),신병천(Byung-Chun Shin) 대한기계학회 2001 대한기계학회 춘추학술대회 Vol.2001 No.8
This paper reports a new model for the fatigue strength assessment of welded joints with residual stresses. The effects of residual stresses are assumed to be equivalent to those of mean stresses However residual stresses are released during cyclic loadings. The proposed model consists of three ingredients, a hot-spot stress approach, a residual stress relaxation mechanism, and an equivalent stress. The equivalent stress is modeled by stress ranges and the ratios between mean stresses and the ultimate stress of material. Once being tuned with two specific fatigue tests, this model can be applied to arbitrary stress ratios and many kinds of welded joints. Three fatigue tests were carried out under a constant amplitude loading and two variable amplitude loadings, and the results were putted together with those of present model. The two results are going quite closely.
북극해에서 계측된 국부 빙하중에 대한 선속 및 빙두께 영향
이탁기(Tak-Kee Lee),이종현(Jong-Hyun Lee),임채환(Chae-Whan Rim),최경식(Kyungsik Choi) 한국해양공학회 2013 韓國海洋工學會誌 Vol.27 No.5
The icebreaking research vessel ARAON conducted her second ice trial in the Arctic Ocean during the summer season of 2010. During this voyage, the local ice loads acting on the bow of the port side were measured using 14 strain gauges. The measurement was carried out during icebreaking while measuring the thickness of the ice very 10 m. The obtained strain data were converted to the equivalent stress values, and the effects of the ship speed and ice thickness on the ice load were investigated. As a result, it was found that a faster speed produced a larger stress, according to the variation in the peak values below an ice thickness on the ice load was not clear.
서해 도서지역의 풍력-디젤 하이브리드 발전에 대한 경제성 분석
이탁기(Tak Kee Lee),남용윤(Yong Yun Nam),김재동(Jae Dong Kim),한정우(Jeong Woo Han) 한국항해항만학회 2011 한국항해항만학회지 Vol.35 No.5
본 연구에서는 최대 발전량이 500kW인 서해 도서의 풍력-디젤 하이브리드 발전시스템에 대한 경제성 분석을 수행하였다. 이를 위해 해당 도서에 대한 연간 전력 부하 변동치와 바람 데이터를 수집하고 분석하였으며, 풍력을 포함하여 신재생 에너지에 대한 하이브리드 최적화 모델로 미국의 NREL에서 개발한 Homer 프로그램을 이용하였다. 풍력-디젤 하이브리드 발전시스템의 경제성을 판단하기 위해 풍속과 경유 가격을 변수로 하는 민감도 해석도 수행하였다. 그 결과, 현재의 조건에서는 경제성이 낮았으나, 풍속이 초속 3미터가 넘거나 경유 가격이 리터당 2.4달러를 초과할 경우 대상지역에도 풍력-디젤 하이브리드 발전시스템이 경제성을 갖는 것으로 평가되었다. In this paper, an economic feasibility study of wind-diesel hybrid power systems for an island in the Yellow Sea, where the maximum power generation is about 500kW, was performed. For the study, annual electric load variation and wind resource data of the island were collected and analyzed. HOMER program - a typical hybrid optimization model for electric renewables including wind resource, developed by the National Renewable Energy Laboratory - was used. Wind speed and diesel price were picked out as variables for the sensitivity analysis in order to find the economic accountability for the wind-diesel hybrid power system. As the result, even though it is not feasible economically under the present condition, if mean wind speed is over 3 m/sec. or diesel price goes up to 2.4 $ per liter, the wind-diesel hybrid power system for the island becomes a prospective candidate.
북극해 계측자료에 기초한 아라온호의 국부 빙압력 계산 연구
이탁기(Tak-Kee Lee),김태욱(Tae-Wook Kim),임채환(Chae Whan Rim),김성찬(Sungchan Kim) 한국해양공학회 2013 韓國海洋工學會誌 Vol.27 No.5
The icebreaking research vessel (IBRV) ARAON had her second ice trial in the Arctic Ocean in the summer season of 2010. During the voyage, the local ice loads acting on the bow of the side were measured using 14 strain gauges. These measurements were carried out in three icebreaking performance tests. To convert the measured strains into the local ice pressures, a finite element model of the instrumented area was developed. The influence coefficient method (ICM), which uses the influence coefficient from the finite element model, and the direct method, which uses the measured strain, were selected as the conversion methods. As a result, the maximum measured pressure was 1.236MPa, and the average difference between ICM and the direct method was about 5% for an area of 0.2㎡. The pressure-area relationship of the measurement falls below the range of the existing pressure-area curve, which is due to the low ice strength of melted ice in the summer.
한승호,이탁기,신병천,Han, Seung-Ho,Lee, Tak-Kee,Shin, Byung-Chun 대한기계학회 2002 大韓機械學會論文集A Vol.26 No.10
It is well known that the strength and the fatigue life of welded steel components are affected extensively by welding residual stresses distributed around their weldments under not only monotonic but also cyclic loads. The externally applied loads are to be superimposed with the welding residual stresses, so that unexpected deformations and failures of the components might occur. These residual stresses are not kept constant, but relaxed or redistributed during in service. Under monotonic loads the relaxation takes place when the sum of external and welding residual stress exceeds locally the yield stress of material used. By the way, it is shown that under cyclic loads the welding residual stress is considerably relieved by the first or the early cycles of loads, and then gradually relaxed with increasing loading cycles. Although many investigations in this field have been carried out, the phenomenon and mechanism of the stress relaxation are still not clear, and there are few comprehensive models to predict amount of relaxed welding residual stress. In this study, the characteristics of the welding residual stress relaxation under monotonic and cyclic loads were investigated, and a model to predict quantitatively amount of welding residual stress relaxation was proposed.