Efficient support for relaxed memory models in shared-memory multiprocessors

학위논문(박사)--서울대학교 :컴퓨터공학과,1999.Docto

Study on Weldability of Stainless Steel for LNG Carrier Using High Power Fiber Laser

These days, world wide interest about global warming and environmental pollution and exhausting fossil fuel which have been main energy source in all around the world. So many country have tried to find out the solution by investing new & renewable and clean energy. Therefore LNG have been widely used as a substitution of fossil fuel and clean energy that emits less pollutant like SOx, NOx. Therefore LNG consumption has been quickly raised and LNG carriers have been getting larger for decades. Our shipbuilding industry has been the most powerful in the world. On the other hand, it's also in the middle of stiff competition between developed nations with original technique and quickly progressing chinese shipbuilding companies. In this situation, increasing productivity is one of the most important factor to lead the industry. Meanwhile, although the laser welding technique has been already used for various industry such as automobile, semiconductor, electronics and so on, not very widely applied in shipbuilding except for cruise ships. Laser is high density heat source, so it can make high speed welding with minimum heat input possible. Especially the high power fiber laser is recently commercialized and has high beam quality and the smallest system size compare with conventional laser due to it's unique oscillating mechanism. Because of these advantages it's thought that the most suitable heat source for LNG cargo tank welding precess which has to be conducted inside of ships. So we suggest fiber laser welding technique as a substitutional heat source of exiting plasma and arc welding. In this study fiber laser was used for welding of stainless steel for LNG carrier to applicate laser welding technique for shipbuilding industry. Used material was STS304L and STS316L which have low carbon less than 0.03% and their thickness was 1.2mm and 8mm respectively. First of all, we carried out bead on plate welding changing defocused distance and shield gas-related factor such as kind of shield gas, nozzle angle, distance of nozzle tip from the laser incident point and flow rate. Through this experiment, penetration characteristic was analyzed to understand characteristic of fiber laser because there's a few research about it's basic quality. And we could find out it's distinctive nature in welding process. Optimal welding condition decided from the results was shield gas : Ar(25ℓ/min), nozzle angle : 40°, nozzle distance : 20mm and defocused distance(fd) : -4mm for STS316L, 0mm for STS304L. Second, butt and lap welding was conducted changing laser power, welding speed, Then penetration characteristic was analyzed and optimal parameters for each materials and joint configuration was decided based on experimental results. Third, mechanical properties of weld such as hardness, tensile strength and toughness were measured and compared to that of base metal. As a result same or better mechanical properties were obtained in weld. Last, weldability, microstructure and mechanical properties of stainless steel welded by fiber laser were compared to those by plasma. As a result, although the laser welding has several times faster speed similar properties and smaller weld and heat affected zone were obtained. Consequently. this study proves the superiority of fiber laser welding for LNG carrier. Increasing welding speed and controling defects technique are important factor related to productivity. Therefore, the achievement in this study can be utilized to prove the superiority of laser as a substitutional heat source of exiting plasma and arc.1. 서론 - 1 1.1 연구 배경 - 1 1.2 연구 목적 및 범위 - 4 2. 이론적 배경 - 6 2.1 LNG선의 특성 및 종류 - 6 2.1.1 LNG선의 특성 - 6 2.1.2 LNG선의 종류 - 9 2.1.3 LNG선용 스테인리스강 및 기타재료의 성질 - 14 2.2 LNG선용 스테인리스강의 용접특성 - 18 2.2.1 LNG선용 스테인리스강의 용접성 - 18 2.2.2 기존의 열원에 의한 용접성 - 29 2.3 화이버 레이저의 특성 및 LNG선으로의 적용성 - 32 2.3.1 화이버 레이저의 특성 - 32 2.3.2 LNG선으로의 적용성 - 36 3. 실험 방법 - 38 3.1 실험 재료 및 장치 - 38 3.1.1 실험 재료 - 38 3.1.2 실험 장치 - 38 3.2 실험 방법 및 시편제작 - 41 3.3 경도 및 기계적 성질 시험 - 43 3.3.1 시편채취 및 에칭 - 43 3.3.2 비커스 경도시험 - 43 3.3.3 인장시험 - 46 3.3.4 극저온 샤르피 충격시험 - 46 4. 실험 결과 및 고찰 - 49 4.1 비초점거리에 따른 용입특성 - 49 4.1.1 STS316L의 용입특성 - 49 4.1.2 STS304L의 용입특성 - 54 4.2 실드가스의 종류 및 변수에 따른 용입특성 - 57 4.2.1 실드가스 종류가 용접에 미치는 영향 - 57 4.2.2 노즐 각도에 따른 용입특성 - 60 4.2.3 노즐 거리에 따른 용입특성 - 62 4.2.4 유량에 따른 용입특성 - 64 4.3 레이저 가공인자와 용접형상이 용입에 미치는 영향 - 67 4.3.1 겹치기 용접시의 용입특성 - 67 4.3.2 맞대기 용접시의 용입특성 - 76 4.4 용접조건에 따른 용접부의 기계적 성질 - 81 4.4.1 경도분포 - 81 4.4.2 인장특성 - 84 4.5 레이저와 플라즈마 아크의 용접특성 비교 - 89 4.5.1 용입특성 - 89 4.5.2 기계적 성질 - 93 4.5.3 용접부의 미세조직 - 96 5. 결론 - 102 참고문헌 - 10

彈力的 勤勞時間制

학위논문(석사)--서울대학교 :법학과 사회법 전공,1999.Maste

(구 인적자원센터)충남지역 전략산업 인재육성을 위한 Sector Council 중심의 인력양성시스템 구축방안 - 디스플레이산업을 중심으로

제1장 서론 1. 연구배경 및 목적 1)연구배경 및 목적 ○패러다임 쉬프트(paradigm shift)에 따른 경쟁 환경변화에 따라 혁신적 인적자원의 확보가 경쟁력의 핵심 원천 가운데 하나로 등장 ○기업의 입장에서 인적자원의 경쟁력을 확보한다는 것은 기업에서 필요로 하는 전문지식과 기술을 갖춘 인력을 적정비용으로 확보하고, 입무수행과정에서 이들의 생산성이 최대한 발휘되도록 한다는 것을 의미 - 이후 생략1. 서론 2. 디스플레이산업 현황 3. 충남 디스플레이산업 인적자원개발 현황 4. 산업별 인적자원개발 시스템 5. 산업별 인력양성시스템 구축방안 6. Sector Council 중심의 인력양성시스템 구축방안 7. 결론 참고문

고효율 서버용 전원 장치를 위한 하프-브리지 LLC 컨버터의 공진 커패시터 온-오프 제어

In this letter, a simple control method of the HB LLC converter with one additional switch and capacitor in the primary side is proposed for wide-input-voltage applications with the hold-up time conditions. At nominal input, since the proposed method enables the HB LLC converter to operate with large transformer magnetizing inductance, it can reduce the conduction and switch turn-off losses in the primary side, which makes a high efficiency. On the other hand, during the hold-up time, since the proposed method increases the resonant capacitance by turning on one additional switch, the HB LLC converter can obtain high voltage gain