• 한국주조공학회지
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• 제26권2호
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• pp.85-91
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• 2006

Inclusions in Ti investment castings are generally known to have detrimental effects on the performance of the castings. However, actual inclusions are infrequent and hard to be located. As a result, it is extremely difficult to obtain sufficient amount of fatigue test specimens of titanium investment castings having inclusions in the gage section. Thus, in-depth research of the adverse influence of inclusions is also hindered. To address this problem, a new casting methodology of specimens containing hard alpha inclusions was developed in this study. To guarantee successful introduction of an inclusion and casting, a carefully designed mold with 8 legs and a special tool were employed. After solidification, castings were cut, and X-ray radiography determined that the inclusions were successfully incorporated into the castings. The castings were further prepared to obtain multiple test specimens and they were fatigue-tested consecutively. Fractography analysis confirmed that fatigue cracks initiated at the hard alpha inclusion. In a nonlinear regression model, the fatigue life can be modeled as an exponential function with a negative exponent of the cross-sectional area of an inclusion. The fatigue life of Ti specimens containing inclusions is inversely proportional to the cross-sectional area of an inclusion.

• 생물정신의학
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• 제19권1호
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• pp.65-69
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• 2012

Objectives : Much is still unknown about the neurophysiological mechanisms or dynamics of the sleep onset process. Detrended fluctuation analysis (DFA) is a new tool for the analysis of electroencephalography (EEG) that may give us additional information about electrophysiological changes. The purpose of this study is to analyze long-range correlations of electroencephalographic signals by DFA and their changes in the sleep onset process. Methods : Thirty channel EEG was recorded in 61 healthy subjects (male:female=34:27, age=$27.2{\pm}3.0$ years). The scaling exponents, alpha, were calculated by DFA and compared between four kinds of 30s sleep-wakefulness states such as wakefulness, transition period, early sleep, and late sleep (stage 1). These four states were selected by the distribution of alpha and theta waves in O1 and O2 electrodes. Results : The scaling exponents, alpha, were significantly different in the four states during sleep onset periods, and also varied with the thirty leads. The interaction between the sleep states and the leads was significant. The means (${\pm}$ standard deviation) of alphas for the states were 0.94 (${\pm}0.12$), 0.98 (${\pm}0.12$), 1.10 (${\pm}0.10$), 1.07 (${\pm}0.07$) in the wakefulness, transitional period, early sleep and late sleep state respectively. The mean alpha of anterior fifteen leads was greater than that of posterior fifteen leads, and the two regions showed the different pattern of changes of the alpha during the sleep onset periods. Conclusions : The characteristic findings in the sleep onset period were the increasing pattern of scaling exponent of DFA, and the pattern was slightly but significantly different between fronto-temporal and parieto-occipital regions. It suggests that the long-range correlations of EEG have a tendency of increasing from wakefulness to early sleep, but anterior and posterior brain regions have different dynamical process. DFA, one of the nonlinear analytical methods for time series, may be a useful tool for the investigation of the sleep onset period.

• 대기
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• 제18권4호
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• pp.287-297
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• 2008

The vertical profiles and optical properties of Asian dust are investigated using ground-based measurements from 1998 to 2002. Vertical profiles of aerosol extinction coefficient are evaluated using MPL (Micro Pulse Lidar) data. Optical parameters such as aerosol optical thickness ($\tau$), ${\AA}ngstr\ddot{o}m$ exponent ($\alpha$), single scattering albedo ($\omega$), refractive index, and volume size distribution are analyzed with sun/sky radiometer data for the same period. We can separate aerosol vertical profiles into three categories. First category named as 'Asian dust case', which aerosol extinction coefficient is larger than $0.15km^{-1}$ and dust layer exists from surface up to 3-4km. Second category named as 'Elevated aerosol case', which aerosol layer exists between 2 and 6km with 1-2.5km thickness, and extinction coefficient is smaller than $0.15km^{-1}$. Third category named as 'Clear sky case', which aerosol extinction coefficient appears smaller than $0.15km^{-1}$. and shows that diurnal variation of background aerosol in urban area. While optical parameters for first category indicate that $\tau$ and $\alpha$ are $0.63{\pm}0.14$, $0.48{\pm}0.19$, respectively. Also, aerosol volume concentration is increased for range of 1 and $4{\mu}m$, in coarse mode. Optical parameters for second category can be separated into two different types. Optical properties of first type are very close to Asian dust cases. Also, dust reports of source region and backward trajectory analyses assure that these type is much related with Asian dust event. However, optical properties of the other type are similar to those of urban aerosol. For clear sky case, $\tau$ is relatively smaller and $\alpha$ is larger compare with other cases. Each case shows distinct characteristics in aerosol optical parameters.

• Journal of Mechanical Science and Technology
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• 제20권1호
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• pp.13-18
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• 2006

Ti-8Ta-3Nb, as a new biomaterial, was prepared by cast and swaging process. Their deformation behavior of Ti-8Ta-3Nb alloy has been characterized on the basis of its flow stress variation obtained from the true strain rate compression testing in the temperature of $700-900^{\circ}C$ and strain rate of $0.001-10\;s^{-1}$. At the strain rates lower than $0.1\;s^{-1}$ and the all temperature ranges which consist of two phase ${\alpha}+{\beta}$ as well as single ${\beta}$ phase fields, the flow curves show a small degree of flow softening behavior. In contrast, the shapes of the flow curves at other strain rates indicate unstable behavior. The shapes of the flow curves were similar in both as-cast and swaged specimen as well as in both ${\alpha}+{\beta}$ phase and ${\beta}$ phase. The flow stress data did not obey the kinetic rate equation over the entire regime of testing but a good fit has been obtained in the intermediate range of temperatures ($750-850^{\circ}C$). In this range, a stress exponent value of about 7.7 in as-cast specimens and about 6.2 in swaged specimens with an apparent activation energy of about 300 kJ/mol and about 206 kJ/mol respectively have been evaluated.

• Wind and Structures
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• 제11권5호
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• pp.413-426
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• 2008

A wind tunnel study has been carried out to determine the influence of spacing between buildings on wind characteristics above rural and suburban type of terrain. Experiments were performed for two types of buildings, three-floor family houses and five-floor apartment buildings. The atmospheric boundary layer (ABL) models were generated by means of the Counihan method using a castellated barrier wall, vortex generators and a fetch of roughness elements. A hot wire anemometry system was applied for measurement of mean velocity and velocity fluctuations. The mean velocity profiles are in good agreement with the power law for exponent values from ${\alpha}=0.15$ to ${\alpha}=0.24$, which is acceptable for the representation of the rural and suburban ABL, respectively. Effects of the spacing density among buildings on wind characteristics range from the ground up to $0.6{\delta}$. As the spacing becomes smaller, the mean flow is slowed down, whilst, simultaneously, the turbulence intensity and absolute values of the Reynolds stress increase due to the increased friction between the surface and the air flow. This results in a higher ventilation efficiency as the increased retardation of horizontal flow simultaneously accompanies an intensified vertical transfer of momentum.

• 한국대기환경학회지
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• 제12권4호
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• pp.459-472
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• 1996

An experimental investigation on the wind flow over smooth bell-shaped two-dimensional hills with hill slopes (the ratio of height to half width) of 0.3 and 0.5 is performed in an atmospheric boundary-layer wind tunnel. Two categories of the models are used in the present investigation; six two-dimensional single-hills, and four continuous double-hills. The measurements of the flow field and surface static-pressure distribution are carried out over the Reynolds number (based on the hill height) of 1.9 $\times 10^4, 3.3 \times 10^4, and 5.6 \times 10^4$. The velocity profiles and turbulence characteristics are measured by the pitot-tube and X-type hot-wire anemometer, respectively. The undisturbed boundary-layer profile on the bottom surface of the wind tunnel is reasonably consistent with the power-law profile with $\alpha = 7.0 (1/\alpha$ is the power-law exponent) and shows good spanwise uniformities. The profiles of turbulent intensity are found to be consistent along the centerline of the wind tunnel. The measured non-dimensional speed-up profiles at the hill crest show good agreements with the predictions of Jackson and Hunt's linear theory. The flow separation occurs in the hill slope of 0.5, and the oil-ink dot method is used to find the reattachment points in the leeside of the hill. The measured reattachment points are compared with the numerical predictions. Comparisons of the mean velocity profiles and surface pressure distributions between the numerical predictions and the experimental results show good agreements.

• 한국결정성장학회지
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• 제10권4호
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• pp.337-343
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• 2000

사파이어 ($\alpha$-Al$_2$O$_3$) 단결정에 있어 prism plane slip {11$\bar{2}$0}1/3{$\bar{1}$120} 전위속도를 에치-피트 방법으로 측정하였다. 전위속도 측정시 온도범위는 $1150^{\circ}C$에서 $1400^{\circ}C$까지 였으며, 응력범위는 140 MPa에서 250 MPa까지였다. 얻어진 전위속도의 온도 및 응력 의존성에 대해 검토하였다. 전위속도의 온도의존성을 이용하여 prism plane slip 전위속도를 위한 활성화에너지를 구하였으며, 그 값은 대략 4.2$\pm$0.4 eV이었다. 또한, 전위속도의 응력의존성을 나타내는 응력지수 m은 4.5$\pm$0.8이었다. 한편, 전위속도 측정을 통해 사파이어 단결정에서 basal 면이 3-fold 대칭을 가진다는 사실을 재확인하였다.

• 한국재료학회지
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• 제11권3호
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• pp.221-226
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• 2001

사파이어 ($\alpha$-$Al_2$$O_3$) 단결정에 있어 basal slip (0001)1/3<1120>의 전위속도를 4점 곡강도를 이용하여, 측정하였다. 이 곡강도는 온도 $1200^{\circ}C$ 에서 $1400^{\circ}C$ 그리고 응력은 90MPa, 120MPa, 160MPa에서 행하여졌다. 전위속도는 4 점굽힘 시편의 굽힘변위속도에 의해 구하여졌다. 얻어진 전위속도를 이용하여 전위속도의 온도 및 응력 의존성에 대해 검토하였다. 전위속도의 온도의존성을 이용하여 basal slip 전위속도를 위한 활성화에너지를 구하였으며, 그 값은 대략 2.2$\pm$0.4eV이었다. 한편, 전위속도의 응력의존성을 나타내는 응력지수 m은 2.0$\pm$0.2이었다.

• 수면정신생리
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• 제17권1호
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• pp.41-49
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• 2010

본 연구의 목적은 소아 수면무홉증 환자와 정상 소아의 수면 구조를 비교하고 잘 드러나지 않는 차이를 뇌파의 비선형적 분석을 통해 비교 분석하는데 있다. 코골이 증상을 호소하는 소아 중 수면다원검사를 통해 폐쇄성 수면무호흡 지수가 시간당 1회 이상으로 소아기 폐쇄성 수면무호흡증으로 진단받은 12명(남아 10명, 평균 6.4${\pm}$3.4세)과 정상 대조군으로 연구에 참여한 15명(남아 8명, 평균 6.0${\pm}$2.2세)의 수면다원검사 자료를 비교하였다. 즉, 수면 구조 관련 변인과 뇌파 채널(C3/A2, C4/A1, O1/A2, O2/A1)에서 탈경향변동분석 과정을 거쳐 계산된 축적 지수를 비교하였다. 수면 구조에서는 1단계 수면의 비율이 폐쇄성 수면무호흡증 환아에서 9.3${\pm}4.3$%로 정상 소아의 5.6${\pm}4.3$2.8%보다 유의하게 높았고, 렘수면 입면잠복시간에서 폐쇄성 수면무호흡증 환아가 181.5${\pm}4.3$59.9 분으로 정상 소아의 133.5${\pm}4.3$42.0분보다 유의하게 길었다. 2, 3, 4단계 수면과 렘수면의 비율은 폐쇄성 수면무호흡증 환아와 정상 소아에서 유의한 차이가 없었다. 소아의 수면 뇌파에서 계산된 축적 지수(${\alpha}$)는 장기 시간적 연관성(long-range temporal correlation)의 특징을 갖는 것으로 1단계에서 4단계 수면으로 갈수록 ${\alpha}$ 값이 증가 하였다. 각각의 뇌파 채널에서 폐쇄성 수면무호흡증 환아의 축적 지수는 정상 소아의 축적 지수보다 유의하게 낮았다(C3/A2: 1.36${\pm}4.3$0.05 vs. 1.41${\pm}4.3$0.04, C4/A1: 1.37${\pm}4.3$0.04 vs. 1.41${\pm}4.3$0.04, O1/A2: 1.37${\pm}4.3$0.05 vs. 1.41${\pm}4.3$0.05, O2/A1: 1.36${\pm}4.3$0.07 vs. 1.41${\pm}4.3$0.05, p<0.05). 폐쇄성 수면무호흡증 환아에서 낮은 ${\alpha}$ 값을 보인 것은 자기조직적 고비성(self-organized criticality)이 줄어든 것으로 뇌 체계에서 자체 기전에 의한 에너지 축적이 감소한 것으로 해석해 볼 수 있다. 8시간에 이르는 수면 뇌파의 정량화가 수면무호흡증과 같은 질병 상태를 반영한다면 향후 연구에서 그 유효성과 범용성을 여러 질병에서 확인해 보아야 할 것이다.

• 한국세라믹학회지
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• 제25권5호
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• pp.552-560
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• 1988

ZnO low voltage varistor was obtained by varying a) the amount of seed grains, b) the size of seed grains, and c) sintering temperature. Also, the optimum condition for fabricating the ZnO seed grains was studied. Large ZnO seed grains were obtained by washing a ZnO sintered body containing 1m/of BaCO3 in boiling water. When the seed grains were added, abnormal grain growth occurred, and the varistor voltage sharply decreased. However, when more than 5w/o of seed grain content was added, the varistor voltage gradually increased. When 10w/o seed grains of 75~106${\mu}{\textrm}{m}$ were added and sintered for 2 hours at 1200 to 125$0^{\circ}C$, low voltage varistor properties with V1mA/mm of 19V/mm and nonlinear exponent ($\alpha$) of 12 occurred.