• 대한치과교정학회지
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• 제22권3호
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• pp.557-578
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• 1992

Heat treatment which removes internal stress enhances the mechanical properties of the orthodontic arch wire. The main purpose of this experiment was to investigate the effects of the heat treatment on the mechanical properties of the Elgiloy wire. The Elfiloy wire, 0.016' X 0.022' and 0.018' X 0.025', were heat treated in an electric oven for 5, 10 and 15 minutes at selected temperatures between 300 and $900^{\circ}C$. Tensile strength and load deflection rate were measured to reveal the changes of mechanical property at various conditions, and each specimen was observed under metallurgic microscope. Also to trace the precipitation material due to overheat treatment, a qualitative analysis was carried out with EDS system. It was found that heat treatment at a low temperature caused an increase in the tensile strength and bending resistance, and a maintenance in the fibrous in the tensile strength and bending resistance, and a maintenance in the fibrous structure of both sizes of wire. The changes observed in properties and appearance were probably due to the relief of internal stresses incurred in the metal during cold working. In both sizes of wire the tensile strength and the bending resistance continued to decrease at high temperature, and the fibrous structure continued to disappear then was not observed at $900^{\circ}C$. The carbide precipitation founded in grain boundary at $750^{\circ}C$ probably was other elements carbide (Ni, Co) except Cr. The grain growth was observed at $1100^{\circ}C$. Optimum heat treatment for the 0.016' X 0.022' Elgiloy wire was 10 minutes at $500^{\circ}C$, and for the 0.018' X 0.025' Elgiloy wire it was 5 to 15 minutes at $500^{\circ}C$.

• 한국치위생학회지
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• 제2권1호
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• pp.115-129
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• 2002

The purpose of this study was to examine the actual dental hygiene status of workers in cement industry in an effort to serve as a basis for enhancing their dental hygiene. The subjects in this study were 420 laborers from the cities of Curi, Donghae and Samcheok who handled cement. As a result of conducting a survey, the following findings were given : l. The workers investigated found their working environment satisfactory, getting a mean score of 323. They also expressed a high job satisfaction with a mean score of 333. 2. They didn't have a big interest in oral health. By age group, those who were aged between 36 and 40, or who cared less about health management, showed a higher interest in oral health. Besides, the laborers who kept smoking for a longer period, or who drunk more, expressed greater interest. 3. The toothbrushing method was considered most important for periodontal health, by 45.0% of the workers, and the next most crucial one was regular dental examination, followed by refraining from smoking and staying away from sweet food in the order named. 4. Concerning daily mean toothbrushing frequency, 455% brushed their teeth three times a day on the average. More than half them didn't pay enough attention to toothbrushing. 5. Regarding scaling, the large number of the workers, 42.4%, had no experience to get their teeth scaled, 37.6%, the greatest percentage, didn't have their teeth scaled because it seemed to make their teeth painful or cold. 6. As fororal health education experience, 67.6%, the great number of them, had no experience to receive dental health education. The above-mentioned findings suggest that the cement-related workers generally neglected dental health management. They should be encouraged to correct their wrong oral health knowledge or habit to make their oral cavity more healthy. To make it happen, it's required to provide oral health education and promote organized dental health projects.

• 복식
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• 제65권6호
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• pp.79-98
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• 2015

This article studied women's headdresses that appeared in the paintings of Joseon Dynasty. This examined the shapes of women's headdresses painted in genre paintings, record paintings and nectar ritual paintings from Joseon Dynasty and compared them to literatures and relics in order to analyze their types and characteristics. Headdress can be categorized into three types: (1) square cloth worn on the top of the head; (2) a kind of small cap; (3) Cloth that completely covers the head. The first type of headdress resembles the shape of Garima. In the early period of Joseon Dynasty, it was worn not only by Gisaeng, but also by both upper and lower class women. The second type of headdress includes a cap made by connecting several cloths, round cap, and cone-shaped cap. These two types are worn on top of the head. However, sometimes these were worn between the head and the chignon to fix the chignon. Also, these were used as hair accessories for decorative purposes. In particular, old women wore these headdresses and braided their white hair around them to fix the chignon. This way, the headdress not only kept the head warm, but also hid the old women's scanty hair. Headdress was usually made of black fabric and it was a simple hair accessory that replaced the wig. The third type of headdress was widely worn among lower class women. The cloth completely covered the head so that the hair would not fall when working. It also provided protection from cold and hot weather. According to paintings, there were many ways of wearing the cloth around the head.

• 터널과지하공간
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• 제2권1호
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• pp.116-122
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• 1992

A comprehensive, nonsteady state, computer simulation program for the environmental conditions in advancing tunnels (the HEADSIM simulation program) is constructed and successfully validated with heat balance amongst all heat sources, and with mass conservation amongst various airflows including the leakage air from ducts, under timedependent variations of inlet air conditions. which include sudden, diurnal and seasonal changes. Heat conduction in the wall strata and face strata is simulated with most complicated boundary conditions using the finite difference method, and the climatic conditions in roadway sections which contain air ducts, booster fan, spray cooler, compressed air pipes, cold water pipes, return water pipes, machinery and broken rock are simulated taking into account the variations of face operation and the heat storage mechanism in the strata. The limitations of simulation time steps and roadway section lengths are defined according to the stability criteria satisfying the principles of thermodynamics. Variations of heat transfer coefficients, which are newly set, and those of wetness factors are taken into account according to the variations of other parameters and the stepwise advance of the face. Newly-derived formulae are used for computing the air duct leakage and the pressure inside of the duct. A new concept of an 'imaginary duct' is introduced to simulate the climatic conditions in tunnels during holiday periods, which directly affect conditions on subsequent working days under the consideration of natural convection. A subsidiary program (the WALLSIM simulation program) is made to compute the dimensionless tunnel surface temperatures and to compare the results with those from analytical approaches, and to demonstrate the stability, convergence and accuracy of the strata heat conduction simulation, adopting the finite difference method. The WALLSIM also has wide applications, including those for the computation of age coefficients.

• 한국초전도ㆍ저온공학회논문지
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• 제5권1호
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• pp.111-117
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• 2003

In the design of the split type free displacer Stilting cryocooler the motion of the displacer is very important to decide the cooling capacity, which depends upon the working gas pressure, the swept volume in the compression space and the expansion space, operating frequency, the phase shift between piston and displacer, etc. In this study, Stirling cryocooler actuated by the electric farce of the dual linear motor is designed and manufactured. Cool down characteristics of the cold end with laser displacement sensor in the expander of the Stilting cryocooler is evaluated. The charging pressure was 15kg$_{f}$/$\textrm{cm}^2$ and operating frequency was 50Hz. Input power and the lowest temperature were about 32W and 67K, respectively. And, displacement of the piston is measured by LVDTs (Linear Variable Differential Transformers), displacement of thedisplacer is measured by laser optic method, and phase shift between piston and displacer is discussed. As the peak-to-peak pressure of the compressor was increased, peak-to-peak displacement of the displacer was increased. The peak-to-peak displacement of the displacer increases in the range of 0 - 64.5Hz(resonant frequency of the displacer), but decreases steeply when the operating frequency is bigger than the resonant frequency. Finally when the phase shift between displacements of the Piston and displacer is 45。, operating frequency is optimum and is decided by resonant frequency of the expander, mass and cross section area of the displacer and constant by friction and flow resistance.e.

• 한국초전도ㆍ저온공학회논문지
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• 제22권3호
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• pp.20-24
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• 2020

The purpose of this analytic study is to design and examine an efficient hydrogen liquefaction cycle by using a pre-cooler. The liquefaction cycle is primarily comprised of a pre-cooler and a refrigerator. The fed hydrogen gas is cooled down from ambient temperature (300 K) to the pre-cooling coolant temperature (either 77 K or 120 K approximately) through the pre-cooler. There are two pre-cooling methods: a single pre-coolant pre-cooler and a cascade pre-cooler which uses two levels of pre-coolants. After heat exchanging with the pre-cooler, the hydrogen gas is further cooled and finally liquefied through the refrigerator. The working fluids of the potential pre-cooling cycle are selected as liquid nitrogen and liquefied natural gas. A commercial software Aspen HYSYS is utilized to perform the numerical simulation of the proposed liquefaction cycle. Efficiency is compared with respect to the various conditions of the heat exchanging part of the pre-cooler. The analysis results show that the cascade method is more efficient, and the heat exchanging part of the pre-coolers should have specific UA ratios to maximize both spatial and energy efficiencies. This paper presents the quantitative performance of the pre-cooler in the hydrogen liquefaction cycle in detail, which shall be useful for designing an energy-efficient liquefaction system.

• 한국재료학회지
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• 제10권5호
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• pp.335-342
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• 2000

리늄판의 미세경도를 압흔 하중 및 온도의 함수로 구하였다. 미세경도의 온도의존성은 상온에서 100$0^{\circ}C$까지의 범위에서 Vickers 압흔기가 내장된 고온 미세압흔기를 이용하여 연구되었다. 미세경도의 하중의존성은 Vickers와 Knoop 압흔기를 이용하여 검토되었다. 압흔 크기 영향은 표준화된 Meyers법칙에 위해 충분히 설명 되었다. 압흔도중 압흔기 아래에서의 가공경화 때문에 어닐링된 리늄판 경도값은 높은 압흔 하중에서 압련되 리늄판의 경도값에 접근하였다. 경도의 하중의존성으로부터 외삽하여 얻어진 하중 영에서의 경도값은 경도값이 다른 열활성을 나타내는 두 개의 다른 기구에 의해 제어됨을 제시하였다. 낮은 온도에서 활성화에너지는 0.02eV 였으며, 한편 높은 온도에서 활성화에너지는 0.15eV 였다. 이때 전이온도는 대략 $250^{\circ}C$ 이었다.

• 한국산학기술학회논문지
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• 제14권2호
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• pp.681-687
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• 2013

본 연구는 학습 환경의 실내 온도와 학습재료 색채와의 조합이 학습수행에 어떠한 영향을 미치는 지를 살펴보고자 하였다. 이를 위해 학습활동 적정온도($22.5{\sim}24^{\circ}C$)를 중심으로(중립 실내 온도 조건), 그 이상인 조건(고온 실내온도 조건), 그리고 그 이하인 조건(저온 실내 온도 조건)으로 각각 실내 온도 조건을 구분하였으며, 난색계열인 빨간색과 한색계열인 파란색, 그리고 중성인 검은색과 연두색으로 각각 색채 조건을 구분하였다. 학습과 관련된 과제로는 음운 작업기억 과제를 사용하여 집단 간 실내 온도 조건에 따른 색채 조건에서의 과제 수행을 살펴보았다. 그 결과, 학습과제의 반응시간에서는 각 독립변수들에 의한 차이가 유의하지 않은 반면, 정확률에서는 색채 조건 중 빨간색과 검은색 조건에서 보다 정확한 수행이 나타났다. 이는 빨간색이 가진 현저성과 색채 온도감 및 검정색이 가진 친숙성과 다른 색에 비해 유일하게 현저성을 가지지 않는 특이성이 존재하기 때문에 나타난 결과로 해석할 수 있다.

• Smart Structures and Systems
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• 제13권3호
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• pp.353-374
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• 2014

Recent studies were dedicated to the realization of measurements on stay-cable samples of different geometry and static conditions as available at several facilities. The elaboration of the acquired data showed a a satisfactory efficacy of the dampers made of NiTi wires in smoothing the cable oscillations. A further attempt to investigate the applicability of the achieved results beyond the specific case-studies represented by the tested cable-stayed samples is herein pursued. Comparative studies are carried out by varying the diameter of the NiTi wire so that similar measurements can be taken also from laboratory steel cables of reduced size. Details of the preparation of the Ni-Ti wires are discussed with particular attention being paid to the suppression of the creep phenomenon. The resulting shape of the hysteretic cycle differs according to the wire diameter, which affects the order of the fitting polynomial to be used when trying to retrieve the experimental results by numerical analyses. For a NiTi wire of given diameter, an estimate of the amount of dissipated energy per cycle is given at low levels of maximum strain, which correspond to a fatigue fracture life of the order of millions of cycles. The dissipative capability is affected by both the temperature and the cycling frequency at which the tests are performed. Such effects are quantified and an ageing process is proposed in order to extend the working temperature range of the damper to cold weathers typical of the winter season in Northern Europe and Canada. A procedure for the simulation of the shape memory alloy behavior in lengthy cables by finite element analysis is eventually outlined.

• 설비공학논문집
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• 제30권4호
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• pp.175-185
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• 2018

This research investigates the internal flow and heat transfer in a plate heat exchanger with chevron shape by utilizing the computational fluid dynamics (CFD) software. The basic unit of the plate heat exchanger is generally composed of a hot channel, an intermediate chevron plate, and a cold channel. Several studies have reported experimental and numerical simulation of heat transfer and pressure drop. This study focused on the detailed numerical simulation of flow and heat transfer in the complicated chevron shape channel. The long chevron plate was designed to include 16 chevron patterns. For proper mesh resolution, the number of cells was determined after the grid sensitivity test. The working fluid is water, and its properties are defined as a function of temperature. The Reynolds number ranges from 900 to 9,000 in the simulation. A realizable $k-{\varepsilon}$ model and non-equilibrium wall function are properly considered for the turbulent flow. The friction factors and heat transfer coefficient are validated by comparing them with existing empirical correlations, and other patterned flow phenomena are also investigated.