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Du‑Youl Choi,Ashutosh Sharma,Sang‑Ho Uhm,Jae Pil Jung 대한금속·재료학회 2019 METALS AND MATERIALS International Vol.25 No.1
Liquid metal embrittlement (LME) caused cracking of Galvanized transformation induced plasticity steels was investigatedduring resistance spot welding (RSW). Effect of electrode force (3–5 kN) for a weld time of 400, and 800 ms on crack resistanceof TRIP steel was examined in relation to LME phenomenon. The microstructural characteristics of spot weld jointsand LME cracking tendency were investigated using dye penetration tests, optical microscopy, scanning electron microscopytogether with energy dispersive X-ray spectroscopy. It was found that as the electrode force increases, the crack size decreasesdue to a fast increase in crack tip temperature which rises further with more holding time up to 800 ms in high temperaturezone. Least amount of crack size was observed at 5 kN electrode force and 400 ms of welding time. Finally, the experimentalresults have also been simulated by finite element modeling (FEM) to find suitable mechanism of crack formation, and acombination of 4 kN and 400 ms was suggested for the crack free and less thermal deformation in the spot welded TRIP steel.
고Si DP980강 스폿 용접 특성에 미치는 Phosphorus (P) 및 in-situ 후열처리 펄스 조건의 영향
최두열(Du-Youl Choi) 대한용접·접합학회 2015 대한용접·접합학회지 Vol.33 No.6
Recently, application of UHSS(Ultra High Strength Steels) whose tensile strength is over 1000MPa to car body structure are growing due to great needs for light weighting and improved crash worthiness. However, their poor weldability is one of obstacles to expand selecting to car body. In this study, effect of Phosphorus contents on resistance spot weldability of high elongation DP980 steel whose Si content is over 1% was investigated. The cross tension strength (CTS) was decreased showing partial interface fracture as Phosphorus content increase because of solidification segregation of Phosphorus. In order to improve resistance spot weldability by modification of welding condition, in-situ post-weld heating pulse was introduced after main pulse. The optimum cooling time between main and post pulse and post-pulse current condtion were determined through FEM welding simulation and DOE tests. The CTS was increased about 1.5 time showing plug fracture. The decrease of Phosphorus segregation was found to be a major reason for weld ductility and CTS improvement.
다중효용 담수설비용 열압축기의 디퓨져 형상에 관한 연구
최두열(Du-youl Choi),정한식(Han-shik Chung),정효민(Hyo-min Jeong),김진화(Zhen-hua Jin),김필환(Pil-hwan Kim) 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.10
A thermocompressor is the equipment which compresses a vapor to a desired discharge pressure. Since it was first used as the evacuation pump for a surface condenser, it has been widely adopted for energy saving systems due to its high working confidence. In the present study, the geometrical analysis of the shape between the jet nozzle and the diffuser inlet, the drag force was calculated by means of the integrated equation of motion and the computational fluid dynamic (CFD) package called FLUENT. The computer simulations were performed to investigate the effects by the various suction flow rates, the distance from jet nozzle outlet to the diffuser inlet and the dimensions of the diffuser inlet section through the iterative calculation. In addition, the results from the CFD analysis on the thermocompressor and the experiments were compared for the verification of the CFD results. Furthermore, in this study, a special attention was paid on the performance of the thermocompressor by varying the diffuser convergence angle of 0.0°, 0.5°, 1.0°, 2.0°, 3.5° and 4.5°. With the increase of the diffuser convergence angle, the suction capacity was improved up to the degree of 2.0° while it was decreased over the degree of 2.0°.
최근열(Keun Youl Choi),장영두(Yeong Du Chang) 한국정부학회 2009 한국행정논집 Vol.21 No.3
지방의회는 우리나라 지방자치제 성공의 하나의 중심축이므로 1991년 부활된 이후 지역주민의 중요한 관심사가 되고 있다. 이에 지방의회의 역할과 기능이 제대로 수행되고 있는 지에 대한 객관적인 평가가 주기적으로 이루어지도록 하는 것은 지방의회의 발전을 위하여 필수적인 일이라 할 것이다. 이러한 맥락에서 본 연구에서는 먼저 지방의회 의정활동 성과를 객관적으로 평가하기 위해 선행연구 검토를 거쳐 성과평가 분석틀을 제시하고 이를 경상북도의회를 사례로 제8대 도의회 전반기 의정활동의 성과를 성과평가분야별(전반적인 성과평가, 기능중심 세부적인 성과평가, 지원제도에 관한 성과평가) 성과평가 항목에 대해 문헌자료 및 실적분석, 설문조사, 언론보도 내용분석 방법으로 평가하였다. 이러한 의정활동의 성과평가 결과를 바탕으로 본 연구에서는 지방의회의 발전방안으로 입법정책능력의 제고. 의정활동의 기능별 전문성 확대, 의정활동에 주민참여기회의 확대, 의회운영의 혁신과 의정지원체제의 개선, 의정활동 행태의 쇄신 등을 제시하였다. 그러나 이 연구도 경상북도의회라는 하나의 사례분석을 통하여 지방의회의 발전방안을 제시하였다는 점에서 일반화에는 한계가 따른다고 본다. Since local autonomy has resumed in 1991 in Korea, as a pivotal role for the success of local autonomy, local council has been a central arena to local electorates` interests. Thus, it is urgent that objective and regular evaluations to council activities of its role and function in sessions are a precondition for the development of local councils. In this context, this study developed an evaluation model to apply local council`s legislative activities by critical reviews of the existing literatures. With this analytical model, this paper conducted a case-study approach to evaluate the first part of the eighth session at Gyeongsangbuk Do Provincial Council. To evaluate the outcomes of this local council`s performance, the local council`s legislative performance activities were divided into three parts of general performance evaluation, specific functional activity, and supporting system by using the analysis of content data in documents issued by the council office, questionnaires to the council members, and the content analysis of local newspapers reported the local council activities. After those evaluations, this study suggested some developmental perspectives to the local council as such; the progress of legislative capabilities, the professional improvement of the local council`s functional missions, the increase of the resident`s participations to the council, the betterment of supporting system of the council activity and the innovation of the council`s administration, and the change for the better attitude to the council`s legislative activities.