• 대한조선학회논문집
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• 제48권3호
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• pp.200-206
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• 2011

The mechanical properties of the most widely used cryogenic materials, i.e. austenitic stainless steel (ASS), aluminum alloy and invar steel, strongly depend on temperatures and strain rates. These phenomena show very complicated non-linear behaviors and cannot be expressed by general constitutive equation. In this study, an unified constitutive equation was proposed to represent the effect of temperature and strain rate on the materials. The proposed constitutive equation has been based on Tomita/Iwamoto and Bodner/Partom model for the expression of 2nd hardening due to martensite phase transformation of ASS. To simulate ductile fracture, modified Bodner/Chan damage model was additionally applied to the model and the model validity was verified by comparison of experimental and simulation results.

• 대한조선학회논문집
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• 제48권3호
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• pp.189-199
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• 2011

Austenite stainless steel(ASS), aluminum alloy and nickel steel alloy are the most widely used in many cryogenic applications due to superior mechanical properties at low temperature. The Face-Centered Cubic(FCC) and Hexagonal Close-Packed(HCP) materials are used for the primary and secondary insulation barrier of Liquefied Natural Gas(LNG) carrier tank and various kinds of LNG applications currently. In this study, tensile tests of ASS, aluminum alloy and nickel steel alloy were carried out for the acquisition of quantitative mechanical properties under the cryogenic environment. The range of thermal condition was room temperature to $-163^{\circ}C$ and strain rate range was 0.00016/s to 0.01/s considering the dependencies of temperatures and strain rates. The comprehensive test data were analyzed in terms of the characteristics of mechanical behavior for the development of constitutive equation and its application.

• International Journal of Air-Conditioning and Refrigeration
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• 제6권
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• pp.124-135
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• 1998

A thermodynamic cycle analysis is performed for refrigerator-precooled Linde-Hampson hydrogen liquefiers, including catalysts for the ortho-to-para conversion. Three different configurations of the liquefying system, depending upon the method of the o-p conversion, are selected for the analysis. After some simplifying and justifiable assumptions are made, a general analysis program to predict the liquid yield and the figure of merit (FOM) is developed with incorporating the commercial computer code for the thermodynamic properties of hydrogen. The discussion is focused on the effect of the two primary design parameters - the precooling temperature and the high pressure of the cycle. When the precooling temperature is in a range between 45 and 60 K, the optimal high pressure for the maximal liquid yield is found to be about 100 to 140 bar, regardless of the ortho-to-para conversion. However, the FOM can be maximized at slightly lower high pressures, 75 to 130 bar. It is concluded that the good performance of the precooling refrigerator is significant in the liquefiers, because at low precooling temperatures high values of the liquid yield and the FOM can be achieved without compression of gas to a very high pressure.

• 항공우주산업기술동향
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• 제9권1호
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• pp.158-164
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• 2011

항공우주용으로 개발되거나 활용 가능한 상업용 체크밸브사례가 조사되었다. 개발사례로 발사체용, 높은 수명이 요구되는 NASA의 SSME (Space Shuttle Main Engine) 및 지상 지원 시스템 퍼지 라인용과 국내에서 개발된 체크밸브가 있다. 상용품은 미국 주요 밸브개발업체의 제품을 상온용과 극저온용으로 분류하여 조사하였다. 또한 운용 온도, 운용 압력 및 크래킹 압력과 같은 주요 성능과 관련된 씰의 재질, 스프링 강성과 같은 설계 인자의 관계도 설명되었다. 이러한 단품 성능 이외에 채터 및 오염과 같은 운용상의 문제점과 일반적인 해결방안도 언급되었다. 또한 기타주의사항으로 필터, 피팅과 청정도 요구조건에 대하여도 조사되었다

• 대한금속재료학회지
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• 제50권6호
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• pp.427-432
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• 2012

This paper presents the material properties of carbon steels for ships, and offshore structures (ASTM A131) are tested under a series of arctic and cryogenic temperature conditions. For material tension tests, among the ASTM 131 steels, Grades A and B of mild steel and Grade AH of high tensile steel have been used. The obtained mechanical properties of the materials from the material tension tests were applied in a 13,000TEU class container ship to define the effect of low temperature on the ultimate longitudinal strength of the target structure by using the ALPS/HULL intelligent supersize finite element method. The tensile coupon test results showed increased strength and nonuniform fracture strain behaviors within different grades and temperatures. Increasing the material strength resulted in increasing the ultimate longitudinal strength of the ship.

• Composites Research
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• 제36권4호
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• pp.246-252
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• 2023

멤브레인 형 LNG 화물창시스템(Cargo containment system, CCS) 내 유리섬유 강화 복합재료 기반 2차 방벽 설치 시 본딩 자동화 머신(automatic bonding machine, ABM) 활용한 prolonged 구조임과 동시에 양단 접착 고정 사이 비 접착(non-bonding, N-B) 영역의 복합재에서 극저온 열 수축 기인한 상당한 열 응력이 발생하기 때문에 이를 고려한 구조 안전성평가 관점에서 수행된 연구가 있으나 실제로 무한히 긴 길이 고려한 기계적 물성 평가를 수행한 연구는 찾아볼 수가 없었다. 해당 복합재는 파단강도 분산이 큰 세라믹 재료 취성 파괴임을 고려하여 2-파라미터 Weibull 통계분석을 통해 실제 길이 대상 신뢰도 있는 기계적 물성치 값을 표준화 하였으며 LNG 운반 환경을 고려한 극저온 환경까지의 특정 온도별 단축 인장실험을 수행하였다. 실험결과, -70℃에서 기계적 강도가 -20℃에 비해 약 1.5배 급증하고 초기 권축 섬유 신장의 비선형 거동이 억제되었다. 또한, 극저온 환경까지 온도가 낮아질수록 기계적 강도는 계속해서 증가하였으나 반대로 연신은 줄어드는 저온 취성의 현상이 확인되었다. 본 연구에서 제시하는 기계적 물성치 데이터는 멤브레인 형 LNG 화물창 구조 안전성 평가 시 신뢰도 높은 해석 지원 물성치 확보 측면에서 유용하게 적용되어지리라 사료된다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제5권4호
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• pp.580-597
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• 2013

Three materials SUS304, 9% Ni steel and Al 5083-O alloy, which are considered possible candidate for International Maritime Organization (IMO) type B Cargo Containment System, were studied. Monotonic tensile, fatigue, fatigue crack growth rate and Crack Tip Opening Displacement tests were carried out at room, intermediate low ($-100^{\circ}C$) and cryogenic ($-163^{\circ}C$) temperatures. The initial yield and tensile strengths of all materials tended to increase with decreasing temperature, whereas the change in elastic modulus was not as remarkable. The largest and smallest improvement ratio of the initial yield strengths due to a temperature reduction were observed in the SUS304 and Al 5083-O alloy, respectively. The fatigue strengths of the three materials increased with decreasing temperature. The largest increase in fatigue strength was observed in the Al 5083-O alloy, whereas the 9% Ni steel sample showed the smallest increase. In the fatigue crack growth rate test, SUS304 and Al 5083-O alloy showed a decrease in the crack propagation rate, due to decrease in temperature, but no visible improvement in da/dN was observed in the case of 9% Ni steel. In the Crack Tip Opening Displacement (CTOD) test, CTOD values were converted to critical crack length for the comparison with different thickness specimens. The critical crack length tended to decrease in the case of SUS304 and increase for the Al 5083-O alloy with decreasing temperature. In case of 9% Ni steel, change of critical crack length was not observed due to temperature decrease. In addition, the changing material properties according to the temperature of the LNG tank were analyzed according to the international code for the construction and equipment of ships carrying liquefied gases in bulk (IGC code) and the rules of classifications.

• 한국항공우주학회지
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• 제40권3호
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• pp.222-229
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• 2012

점화 시의 분사기의 냉각은 분사순간의 초저온액체상태의 산화제 분무의 증기압에 영향을 미치고, 이는 연소반응에 따른 연소실 압력상승 과도단계에서 분무의 상(phase) 천이 시점을 결정하는 인자 중 하나이다. 분무의 상변화는 액체로켓 연소기의 점화특성에 큰 영향을 미치며, 액체산소/메탄 추진제를 사용하는 연료과잉 폐쇄형사이클 액체로켓엔진의 주연소기용 분사기로 사용될 수 있는 액체-기체 동축형 스월분사기에 대하여 점화초기 분사기 냉각온도에 따른 점화시험을 수행하였다. 초기 냉각온도에 따라 점화 시 산화제 분무의 액상으로의 천이시기가 달라지며, 충분한 냉각을 통해 산화제 분무의 증기압을 낮춘 경우 산화제 분무의 상 천이 시기를 나타내는 화염 quenching 현상이 일찍 나타나는 것을 확인할 수 있었다.

• 대한조선학회논문집
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• 제55권6호
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• pp.514-520
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• 2018

Polyurethane foam has excellent mechanical strength and insulation performance, and has been adopted as an insulation material for $-163^{\circ}C$ liquefied natural gas carrier. In this study, Kevlar Polyurethane Foams(K-PUF) were synthesized by adding Kevlar pulp with excellent mechanical strength for the purpose of improving the performance of existing polyurethane foam. Since polyurethane foam has mechanical performance depending on the proportions of Kevlar pulp added, the mechanical strength of the K-PUF with ratios of fiber0.2wt.%, 0.4wt.%, 0.6wt.%, 0.8wt.% and 1.0wt.%) was evaluated. The compression tests were performed on the 4 different temperatures($20^{\circ}C$, $-50^{\circ}C$, $-110^{\circ}C$ and $-163^{\circ}C$) in consideration of the environmental characteristics as a cryogenic insulation used in LNG carrier. Besides, the effects of the fiber addition on polyurethane foam with closed cell structure were evaluated in a phenomenological approach through SEM analysis. All the results were compared to Neat-polyurethane foam. As a results, 0.8wt.% K-PUF showed the improved mechanical strength, and the addition of Kevlar pulp in a certain ratio improves the mechanical performance by enhancing the compression resistance.

• Smart Structures and Systems
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• 제29권3호
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• pp.499-512
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• 2022

This work proposes a novel contactless vibration damping and thermal isolation tripod platform based on Superconducting Magnetic Levitation (SML). This prototype is suitable for cryogenic environments, where classical passive, semi active and active vibration isolation techniques may present tribological problems due to the low temperatures and/or cannot guarantee an enough thermal isolation. The levitating platform consists of a Superconducting Magnetic Levitation (SML) with inherent passive static stabilization. In addition, the use of Operational Modal Analysis (OMA) technique is proposed to characterize the transmissibility function from the baseplate to the platform. The OMA is based on the Stochastic Subspace Identification (SSI) by using the Expectation Maximization (EM) algorithm. This paper contributes to the use of SSI-EM for SML applications by proposing a step-by-step experimental methodology to process the measured data, which are obtained with different unknown excitations: ambient excitation and impulse excitation. Thus, the performance of SSI-EM for SML applications can be improved, providing a good estimation of the natural frequency and damping ratio without any controlled excitation, which is the main obstacle to use an experimental modal analysis in cryogenic environments. The dynamic response of the 510 g levitating platform has been characterized by means of OMA in a cryogenic, 77 K, and high vacuum, 1E-5 mbar, environment. The measured vertical and radial stiffness are 9872.4 N/m and 21329 N/m, respectively, whilst the measured vertical and radial damping values are 0.5278 Nm/s and 0.8938 Nm/s. The first natural frequency in vertical direction has been identified to be 27.39 Hz, whilst a value of 40.26 Hz was identified for the radial direction. The determined damping values for both modes are 0.46% and 0.53%, respectively.