• Smart Structures and Systems
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• 제10권3호
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• pp.229-251
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• 2012

Guided waves have shown a great potential for structural health monitoring (SHM) applications. In contrast to traditional non-destructive testing (NDT) methodologies, a key element of SHM approaches is the high process of automation. The monitoring system should decide autonomously whether the host structure is intact or not. A basic requirement for the realization of such a system is that the sensors are permanently installed on the host structure. Thus, baseline measurements become available that can be used for diagnostic purposes, i.e., damage detection, localization, etc. This paper contributes to guided wave-based inspection in anisotropic materials for SHM purposes. Therefore, computational strategies are described for both, the solution of the complex equations for wave propagation analysis in composite materials based on exact elasticity theory and the popular global matrix method, as well as the underlying equations of two active damage localization algorithms for anisotropic structures. The result of the global matrix method is an angular and frequency dependent wave velocity characteristic that is used subsequently in the localization procedures. Numerical simulations and experimental investigations through time-delay measurements are carried out in order to validate the proposed theoretical model. An exemplary case study including the calculation of dispersion curves and damage localization is conducted on an exemplary unidirectional composite structure where the ultrasonic signals processed in the localization step are simulated with the spectral element method. The proposed study demonstrates the capabilities of the proposed algorithms for accurate damage localization in anisotropic structures.

• Composites Research
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• 제13권1호
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• pp.61-68
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• 2000

이방성 적층 복합재 판의 과도적인 파동 해석을 위한 새로운 방법을 제시하였다. Gabor wavelet을 사용하는 wavelet 변환을 분산성 굽힘파의 시간-주파수 해석에 적용하였다. 시간-주파수 영역에서 wavelet 변환의 크기의 최대값은 군속도의 도달시간을 나타냄을 보였다. 음향방출원으로서 연필심 파단을 사용하여 준등방성 판과 일방향 보강 적층판에서 실험을 수행하였다. 굽힘파의 분산 예측을 위하여 Mindlin 판이론을 사용하였으며,주파수의 함수로 몇 개의 방향에 대해 측정한 군속도 실험 결과와 잘 일치하였다. 굽힘파의 주파수 의존 도달 시간과 같은 파의군속도 방향의존성을 이용하여 이방성 판에서의 파손 위치를 결정하였으며, 그 결과를 제시하였다.

• 대한기계학회논문집A
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• 제26권11호
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• pp.2322-2327
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• 2002

The corrosion degradation characteristics of the 12Cr alloy steel, which is widely used in fossil power plants as a turbine blade material, are evaluated nondestructively by use of the backward radiated Rayleigh surface wave. In order to evaluate corrosion degradation characteristics, we constructed automated system for the backward radiation, and the frequency dependency of the Rayleigh surface wave is investigated indirectly by measuring the angular dependency of the backward radiation of the incident ultrasonic wave in the specimens. The velocity of the surface wave decrease as the increase of the aging time in the backward radiation profile, which seems to result from the increase of the effective degrading layer thickness. And, amplitude of the surface wave increase as the aging time, which seems to result from the increase of the intergranular corrosion. The result observed in this study demonstrates high potential of the backward radiated ultrasound as a tool for the nondestructive evaluation of the corrosion degradation characteristics of the aged materials.

• 천문학회보
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• 제35권2호
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• pp.39.1-39.1
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• 2010

We study the effects of the cluster environment on galactic morphology by defining a dimensionless angular momentum parameter ld, to obtain a quantitative and objective measure of galaxy type. The use of this physical parameter allows us to take the study of morphological transformations in clusters beyond the measurements of merely qualitative parameters, e.g. S/E ratios, to a more physical footing. To this end, we employ an extensive SDSS sample, with galaxies associated with Abell galaxy clusters. The sample contains 93 relaxed Abell clusters and over 34,000 individual galaxies. We find that the median ld value tends to decrease as we approach the cluster center, with different dependences according to the mass of the galaxies and the hosting cluster; low and intermediate mass galaxies showing a strong dependence, while massive galaxies seems to show, at all radii, low ld values. By analysing trends in ld as functions of the nearest galactic neighbour environment, clustercentric radius and velocity dispersion of clusters, we can identify clearly the leading physical processes at work. We find that in massive clusters (s > 700 km/s), the interaction with the cluster central region dominates, whilst in smaller clusters galaxy-galaxy interactions are chiefly responsible for driving galactic morphological transformations.

• 비파괴검사학회지
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• 제20권5호
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• pp.397-401
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• 2000

화력발전 플랜트의 터빈 블레이드(blade) 재료인 12Cr 합금강의 고속 열화 시편에 대한 레일리(Rayleigh) 표면탄성파의 산란과 속도의 주파수 의존특성을 액체/고체 경계 면에서 표면파의 발생기구에 의해 나타나는 후방복사 초음파 세기의 입사각 의존성으로 간접 평가하고 부식피로 특성과 비교 분석하였다. 후방복사 현상이 발생하는 입사각 폭은 부식온도와 반비례하였으며 이는 효과적 부식층의 두께 증가로 설명되었다. 이 프로파일의 폭은 피로시험에서 균열성장 관계식인 파리스(Paris) 법칙의 지수 m과 선형적으로 반비례하여 열화시편의 부식 피로 특성의 비파괴적 평가에 있어 후방복사된 초음파를 이용한 기술이 매우 유용함을 보여 보여주었다.

• Macromolecular Research
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• 제11권4호
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• pp.260-266
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• 2003

Ethylene vinyl acetate (EVA)/clay nanocomposites were synthesized by blending a solution of ethylene vinyl acetate copolymer containing 12% vinyl acetate abbreviated as EVA-12 in toluene and dispersion of dodecyl ammonium ion intercalated montmorillonite (l2Me-MMT) in N,N-dimethyl acetamide (DMAc). X-ray patterns of sodium montmorillonite ($Na^+$-MMT) and 12Me-MMT exhibited $d_{001}$ peak at $2{\theta}=7.4^{\circ}$ and $2{\theta}=5.6^{\circ}$ respectively; that is, the interlayer spacing of MMT increased by about 0.39 nm due to intercalation of dodecyl ammonium ions. The XRD trace of EVA showed no peak in the angular range of $3-10^{\circ}(2{\theta})$. In the XRD patterns of EVA/clay hybrids with clay content up to 6 wt% the basal reflection peak of 12Me-MMT was absent. leading to the formation of delaminated configuration of the composites. When the 12Me-MMT content was 8 wt% in the EVA-12 matrix, the hybrid revealed a peak at about $2{\theta}=5.6^{\circ}$, owing to the aggregation of aluminosilicate layers. Transmission electron microscopic photograph exhibited that an average size of 12-15 nm clay layers were randomly and homogeneously dispersed in the polymer matrix, which led to the formation of nanocomposite with delaminated configuration. The formation of delaminated nanocomposites was manifested through the enhancement of mechanical properties and thermal stability, e.g. tensile strength of an hybrid containing only 2 wt% 12Me-MMT was enhanced by about 36% as compared with neat EVA-12.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제44회 동계 정기학술대회 초록집
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• pp.176-177
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• 2013

Surface plasmon polaritons (SPPs) have attracted the attention of scientists and engineers involved in a wide area of research, microscopy, diagnostics and sensing. SPPs are waves that propagate along the surface of a conductor, usually metals. These are essentially light waves that are trapped on the surface because of their interaction with the free electrons of conductor. In this interaction, the free electrons respond collectively by oscillating in resonance with the light wave. The resonant interaction between the surface charge oscillation and the electromagnetic field of the light constitutes the SPPs and gives rise to its unique properties. In this papers, we studied theoretical and experimental extraordinary transmittance (T) and reflectance (R) of 2 dimensional metal hole array (2D-MHA) on GaAs in consideration of the diffraction orders. The 2d-MHAs was fabricated using ultra-violet photolithography, electron-beam evaporation and standard lift-off process with pitches ranging from 1.8 to $3.2{\mu}m$ and diameter of half of pitch, and was deposited 5-nm thick layer of titanium (Ti) as an adhesion layer and 50-nm thick layer of gold (Au) on the semiinsulating GaAs substrate. We employed both the commercial software (CST Microwave Studio: Computer Simulation Technology GmbH, Darmstadt, Germany) based on a finite integration technique (FIT) and a rigorous coupled wave analysis (RCWA) to calculate transmittance and reflectance. The transmittance was measured at a normal incident, and the reflectance was measured at variable incident angle of range between $30^{\circ}{\sim}80^{\circ}$ with a Nicolet Fourier transmission infrared (FTIR) spectrometer with a KBr beam splitter and a MCT detector. For MHAs of pitch (P), the peaks ${\lambda}$ max in the normal incidence transmittance spectra can be indentified approximately from SP dispersion relation, that is frequency-dependent SP wave vector (ksp). Shown in Fig. 1 is the transmission of P=2.2 um sample at normal incidence. We attribute the observation to be a result of FTIR system may be able to collect the transmitted light with higher diffraction order than 0th order. This is confirmed by calculations: for the MHAs, diffraction efficiency in (0, 0) diffracted orders is lower than in the (${\pm}x$, ${\pm}y$) diffracted orders. To further investigate the result, we calculated the angular dependent transmission of P=2.2 um sample (Fig. 2). The incident angle varies from 30o to 70o with a 10o increment. We also found the splitting character on reflectance measurement. The splitting effect is considered a results of SPPs assisted diffraction process by oblique incidence.

• 한국운동역학회지
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• 제14권2호
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• pp.69-83
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• 2004

The purpose of this study is searching for technical merits and demerits of each weight lifting player through qualitative motion analysis system. Moreover, It is also analysis the repeating the establishment of exercise purpose and studying for the effect of the field adaptation. The subject of this study was five male weight lifting players who have been engaged in Korean Delegation Team. The institution of exercise target was made through two times qualitative analysis and the result of studying for the effect of the field adaptation was produced before offering feedback. Moreover, two time analysis added after offering feedback. All analysis was based on 2-D visual analysis. The results of this study are as follows: 1. Maximal barbell moving speed in starting phase was decreased after offering feedback. This result implies advancement of technical skills after offering feedback. 2. From starting posture to 앉아받기, forward and backward moving distance of hip joint was decreased after offering feedback in all subjects. This result represents advancement of technical skills after offering feedback. 3. In terms of pull phase, forward and backward moving distance of hip and shoulder joint was decreased after offering feedback in all subjects. This result represents advancement of technical skills after offering feedback. 4. In terms of pull phase, the difference of horizontal value of coordinates was decreased after offering feedback in all subjects. This result represents advancement of technical skills after offering feedback. 5. In terms of pull phase, the motion range of hip joint was decreased after offering feedback in three of five subjects and this represents advancement of technical skills after offering feedback. However, the rest of them were not variable or narrow decreasing. This result represents that feedback system could not brought tremendous effects. 6. From apex point of barbell to 앉아받기, the difference of barbell height was decreased after offering feedback in three of five subjects and this represents advancement of technical skills after offering feedback. However, the rest of them weren't variable or narrow increasing. This result represents that feedback system could not brought tremendous effects. 7. In terms of last-pull phase, the angular velocity of knee joint was increased after offering feedback in four of five subjects and this represents advancement of technical skills after offering feedback. However, the rest of them, only one subject, decreased. This result represents that feedback system could not brought tremendous effects. 8. In terms of last-pull, the conversional tendency of maximal extension to flextion came out all but simultaneously without offering feedback in four of five subjects. This is well-performed technique. Only one subject, however, could not use power effectively because the fact that his maximal extension came out in serial, from ankle to knee and waist means dispersion of power. In addition to, after offering feedback, only one subject made increasing the maximal extension of knee in last-pull and this result represents advancement of skills after offering feedback. However, the rest of them could not make meaningful development after offering feedback. 9. It might be assumed that searching for technical merits and demerits of each weight lifting player through qualitative motion analysis system could improve player's skill.

• 한국광학회지
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• 제32권4호
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• pp.163-171
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• 2021

조명에 사용되는 일반적인 백색 발광 다이오드는 청색 발광 다이오드 위에 황색 형광체를 입힌 구조가 보편적이나 발광 스펙트럼상 짙은 적색 성분이 부족해서 조명의 연색성이 떨어지는 문제가 있다. 본 연구에서는 백색 발광 다이오드의 연색 특성을 개선하기 위해 적색 양자점 조명을 백색 조명의 확산판에 적용한 후 광학 시뮬레이션을 이용해 광구조를 최적화하고자 하였다. 양자점의 평균 자유 행정 및 확산판 내 TiO₂ 입자의 농도를 조절해 연색지수, 휘도 등 광특성을 조사했다. 대부분의 조건에서 연색지수는 90을 넘었고 이는 적색 양자점 필름의 적용이 일반적인 백색 발광 다이오드의 연색 특성을 개선하는 데 효과적인 방법임을 보여준다. 색좌표의 각도 의존성은 확산판과 조명 하단의 반사판 사이에 형성되는 광학적 공동 구조를 활용함으로써 제거할 수 있었는데 이는 공동 내 위치한 양자점 필름을 통한 빛의 다중 투과가 시야각에 따른 광경로의 차이를 줄였기 때문으로 해석된다.