• Journal of the Korean Ceramic Society
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• v.40 no.6
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• pp.539-546
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• 2003

The GaN/sapphire layered structure is a potential candidate for high frequency devices due to high acoustic velocity of sapphire. Generally, the GaN thin films are epitaxially grown on c, a, and r-plane sapphire substrates. In this study, wave equations of GaN/sapphire structure were calculated according to crystallographic relationship between GaN layer and sapphire substrate. On each plane, the shear velocity was changed by the kH of GaN layer and propagation direction on sapphire substrate. We found electromechanical coupling constant of r-plane was better than the others. As a result, elastic stiffness and electromechanical coupling constant of materials are affected by a cut and an orientation of substrate. GaN/r-plane sapphire structure is more advantageous for high frequency SAW devices.

• Journal of Advanced Marine Engineering and Technology
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• v.38 no.9
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• pp.1075-1080
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• 2014

This study aims at dedicating to relevant technology fields by suggesting design methods of structures and estimating their safety in relation to substructure for offshore wind power requiring high safety to various environment conditions. Especially, with respect to 5MW Offshore Wind Power System, this study will provide information about major wind directions and duration in combination with the developing wave climate at the test field. Therefore, connections between wind fields and approaching wave trains will be estimated and their intensity, direction and time shift will be pointed out. Furthermore, the local pressure distribution of breaking waves will be investigated by physical and numerical modeling. The currently applied structural and fatigue assessment of support structures for offshore wind energy converters is based on common design rules. Normally, constructions in structural engineering are treated as limited, single structures. This means that varying aspects of manufacturing are considered by high safety factors.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.3
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• pp.269-276
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• 2001

Ultrasonic testing of composite materials is much more difficult than that of isotropic materials, because of the beam skew phenomenon caused by their elastic anisotropy. An established analytical method exists for elastic wave propagation in anisotropic media as a result of previous research efforts. Yet, due to the complexity of the analytical method, solution of real problems must resort to the numerical method. In this work, analytical solutions have first been obtained for the wavefield due to a point source in a unidirectional fiber-reinforced composite, which may be modeled as transversely isotropic. Then, the corresponding numerical solutions have been obtained using the mass-spring lattice model(MSLM). The two solutions have agreed well with each other. Other problems such as reflection from free boundaries and scattering from cracks have also been solved numerically, and the results have been investigated from the viewpoint of wave mechanics. The numerical model whose validity has been confirmed by this work will be of great use in simulating ultrasonic testing of composite materials.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1995.10a
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• pp.221-226
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• 1995

터빈 익렬, 펌프 익차, 원형 냉각탑, 치차 등과 같이 동일한 형상이 원주 방향으로 반복되어 있는 순환 대칭 구조물의 진동특성을 유한 요소법을 사용하여 해석하는 경우에 전체구조를 모델링하는 대신에 구조물을 동일한 형상의 부분구조로 분할하여 부분구조 한개만을 모델링하고 분할된 경계에서 적절한 경계조건을 부과하여 진동해석을 수행함으로서 컴퓨터 기억용량을 절감시키고 계산시간을 단축할 수 있는 방법이 널리 사용되고 있다. Orris and Petyt[1]는 부분구조의 양쪽 분할 경계면, 즉 연결 경계상에 있는 절점변위의 상관관계를 복소파동전파식을 이용해서 구하여 부분구조의 감소된 복소강성행렬 및 질량행렬을 만들고 실수부와 허수부를 분리하여 유한요소해석을 수행하는 방법을 제안하였다. 유한요소 프로그램 ANSYS[2]에서는 이와 같은 방법을 사용하고 있다. Thomas[3]는 순회 정규모드를 이용하였고, 참고문헌[4]에서는 순회행렬을 이용하였다. 또한 유한요소 프로그램 MSC/NASTRAN[5]에서는 푸리에 급수를 이용하고 유한요소 절점의 위치 및 변위를 원통 좌표계를 표현하여 순환대칭구조물의 유한요소해석을 수행할 수 있도록 되어있다. 본 논문에서는 순환 대칭구조물의 형상의 주기성과 순환성을 고려하여 이산퓨리에 변환을 이용함으로써 순환대칭구조물의 유한요소진동해석을 체계적으로 저용량의 컴퓨터에서 신속하고 정확하게 수행할 수 있는 방법을 제안하고자 한다.

• The Journal of the Acoustical Society of Korea
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• v.11 no.4
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• pp.22-30
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• 1992

The propagation of coherent time-harmonic elastic SH waves in a medium with random distribution of cylindrical inclusions is studied for characterizing the dynamic elastic modulus and the attenuation property of fiber-reinforced composite materials. A multiple scattering theory using the single scattering coefficients in conjunction with the Lax's quasicrystalline approximation is derived and from which the dispersion relation for such medium is obtained. The pair-correlation functions between the cylinders which are needed to formulate the multiple scattering interaction between the cylinders are obtained by Monte Carlo simulation method.From the numerically calculated complex wavenumbers, the propagation speed of the average wave, the coherent attenuation coefficient and the effective shear modulus are presented as functions of frequency and area density.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2002.05a
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• pp.554-558
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• 2002

In this paper, we analysed the technical characteristics of a universal shipborne automatic identification system using self-organizing time division multiple access in the VHF marine mobile band. IMO's Marine Safety Committee approved revision of chapter V of the Safety of Life at Sea(SOLAS) Convention in 73rd meeting. According to this revision, AIS will become a nandatary carriage requirement by 01 July 2002. AIS is a broadcast system, operating in the VHF marine band. It is capable of sending infomation such as would be beneficial to the safety of navigation and the identification and monitoring of maritime traffic. It is absolutely necessary to analyse the related international and domestic specifications for the AIS implementation and installation.

• Composites Research
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• v.20 no.1
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• pp.15-22
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• 2007

As a fundamental approach to studying elastic wave behaviors in fiber-reinforced composites, this paper introduces the analytical method to predict the modes, directions, and amplitudes of all reflected waves that are generated by free-surface reflection in fiber-reinforced composites. The paper also explores a new phenomenon where a reflected wave that is predicted to exist in accordance with the slowness surface may disappear. This may occur when the angle of incidence of a quasi-shear wave exceeds a newly defined critical angle, named the third critical angle. It is hoped that the analytical approach introduced in this paper will provide an easy-to-follow guideline for researchers in the relevant area such as ultrasonic nondestructive testing.

• KDI Journal of Economic Policy
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• v.15 no.3
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• pp.53-93
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• 1993

본고(本稿)는 6.25전쟁(戰爭)이후 한국경제(韓國經濟)가 겪은 아홉번의 경기순환(景氣循環)으로부터 순환(循環)의 특징(特徵)과 양태(樣態)를 정리한 것이다. 경기순환(景氣循環)의 특징은 경기순환(景氣循環)의 지속기간(持續期間), 순환심도(循環深度), 변동성(變動性), 공행성(共行性) 및 시차성(時差性), 해외경기순환(悔外景氣循環)과의 관계(關係) 등 여섯가지 면에서 살펴보였으며, 순환양태(循環樣態)는 순환기별(循環期別)로 정리하였다. 우리 경제를 순환기별(循環期別)로 살펴볼 때 경우에 따라서는 경기과열(景氣過熱)을 식히기 위해 실시한 정책이 세계경제여건의 악화와 맞물려 경기(景氣)를 급냉(急冷)시킨 시기도 있었으며, 반대로 경기침체기(景氣沈滯期)에 도입한 내수부양정책(內需浮揚政策)에 기대와는 달리 경기(景氣)를 급격히 과열시켜 단기간내에 물가(物價)를 급등시키고 국제수지(國際收支)를 악화시킨 경우도 있었다. 경기조절(景氣調節)을 위해 실시한 정책이 자칫하면 서다가다(stop-go)를 반복하여 국민경제에 오히려 부담을 줄 가능성이 있기 때문에 정책결정자(政策決定者)는 항상 경기조절(景氣調節)을 위한 최적(最適)의 정책(政策)을 선택해야 하는 부담을 안고 있다. 특히 석유파동(石油波動)이나 UR타결과 같은 세계경제질서(世界經濟秩序)의 변동이 발생한 경우에는 가급적이면 그 충격(衝擊)이 경제에 서서히 흡수되도록 적절한 정책(政策)을 구사해야 한다. 경기정책(景氣政策)은 장기성장(長期成長)을 제고(提高)시키는 방향으로 전전되어야 하기 때문에 정책(政策)의 선택은 매우 산중하제 결정될 수밖에 없다. 또한 1953년 이후 지금까지 겪은 아홉번의 경기순환(景氣循環)과 각종 경기대책(景氣對策)의 실시 경험으로부터 정부(政府)는 추후 경기조절정책(景氣調節政策)의 시행과정에서 불가피하게 야기되는 문제점을 극소화하고, 정책목표 달성의 효율성(效率性)을 제고(提高)하기 위해서 민간(民間)에 대한 대정부(對政府) 신뢰성(信賴性)을 쌓아 나가야 할 것이다.

• The Journal of the Petrological Society of Korea
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• v.23 no.2
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• pp.61-73
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• 2014

The dextral strike-slip Yecheon Shear Zone of (E)NE-(W)SW trend is developed in the Pukhumyeon-Pyeongeunmyeon area, Gyeongsangbukdo, in the central part of Sobaeksan Massif, Korea which mainly consists of Precambrian metasedimentary rocks and Mesozoic granite. This paper researched the relative formation time, the deformation temperature, and the distribution of ultramylonite zone of Yecheon Shear Zone from the analysis of their deformed rock structures and microstructures. There are the mylonite foliation and stretching lineation by shear deformation, and the (E)SE-vergence gentle plunging, moderately inclined tight fold defined by the gneissosity and mylonite foliation as ductilely deformed main rock structures. Flame perthites, myrmekites, microfractures, microkinks, kinkband, and undulatory extinction are recognized as characteristic microstructures in feldspars during ductile shear deformation. Patterns of c-axis fabrics of recrystallized quartz aggregate display single girdle and type I crossed girdle where the rhomb and basal slip systems are predominant. The shearing senses on sense-of-shear plane show top-to-the-NE and top-to-the-SW on the predominant NW-dipping and subordinate SE-dipping mylonite foliations, respectively. These characteristic rock structures and microstructures suggest that the Yecheon ductile shearing occurred under $350{\sim}450^{\circ}C$ deformation temperature after the tight folding. The (E)NE trending ultramylonite zone of Yecheon Shear Zone, which passes through the Pulnobong and the north part of Pakdalsan, is established from the zonal distribution of Precambrian metasedimentary rocks and the grain-size distribution of feldspars in the Precambrian metasedimentary rocks and quartzs in the Mesozoic granite.

• Journal of the Korean Society of Propulsion Engineers
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• v.23 no.6
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• pp.1-10
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• 2019

The wave motion inside the nozzle is known as one of the major damping elements of the rocket's combustion instability by it's aeroacoustic effects that result from the flow passage through the nozzle throat. These effects can be quantitatively evaluated by the nozzle admittance. In this study, one-dimensional linearized Euler equation was adopted to calculate the nozzle admittance, and trend analysis was performed depending on the nozzle's main design variables. As a result, when nozzle converging part shortens, it is verified that the frequency dependency of the nozzle admittance is decreased due to the widened frequency range with lowered longitudinal nozzle admittance. Also, admittance estimation using the short nozzle theory is not appropriate when the first tangential mode of the pressure perturbation arises.