• Smart Structures and Systems
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• v.22 no.2
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• pp.151-159
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• 2018

Maintenance activities are regarded as a key part of the repairable deteriorating system because they maintain the equipment in good condition. In practice, many maintenance policies are used in engineering fields to reduce unexpected failures and slow down the deterioration of the system. However, in traditional maintenance policies, maintenance activities have often been assumed to be performed at the same time interval, which may result in higher operational costs and more system failures. Thus, this study presents two non-periodic preventive maintenance (PM) policies for repairable deteriorating systems, employing the failure rate of the system as a conditional variable. In the proposed PM models, the failure rate of the system was restored via the failure rate reduction factors after imperfect PM activities. Operational costs were also considered, which increased along with the operating time of the system and the frequency of PM activities to reflect the deterioration process of the system. A numerical example was provided to illustrate the proposed PM policy. The results showed that PM activities performed at a low failure rate threshold slowed down the degradation of the system and thus extended the system lifetime. Moreover, when the operational cost was considered in the proposed maintenance scheme, the system replacement was more cost-effective than frequent PM activities in the severely degraded system.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.37 no.9
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• pp.38-48
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• 2000

This paper presents the photonic bandgap structure that has a defect mode within a broad stopband. In order to create a broad stopband, we eliminated one of periodic stopbands of PBG structure by using a quarter-wavelength transformer and cascaded another PBG structure having a center frequency corresponding to the eliminated stopband. We have demonstrated that it is a simple and effective method that can solve an overlapping problem of periodic stopband in two cascaded PBG structures.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.20 no.8
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• pp.726-730
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• 2007

Two dimensional photonic crystals (2D PCs) have been fabricated by a double exposure holographic method using a He-Cd laser with a wavelength of 442nm. The arrays of the 2D PCs exhibit variable lattice structures from square to triangle according to a change of rotating angle $({\gamma})$ for double exposure beams. In addition, the period and filling factor of PCs as well as the forms (dot or antidot) could be controlled by experimental conditions. $A l.18-{\mu}m-thick$ resist was spin-coated on Si substrate and the 1st holographic exposure was carried out at incident angle $({\theta})$ of $11^{\circ}$. Then the sample was rotated to ${\gamma}=45^{\circ}{\sim}90^{\circ}$ and the 2nd holographic process was performed at ${\theta}=11^{\circ}$. The variation of diffraction efficiency during the exposure process was observed using a He-Ne laser in real time. The images of 2D PCs prepared were analyzed by SEM and AFM. We believe that the double holographic method is a tool suitable to realize the 2D PCs with a periodic array of large area.

• Journal of the Society of Naval Architects of Korea
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• v.28 no.2
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• pp.228-240
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• 1991

Many experimental data of fatigue crack propagation show that the fatigue crack propagation process is stochastic. Therefore, the study on the crack propagation must be based on the probabilistic approach. In the present paper, fatigue crack propagation process is assumed to be a discrete Markov process and the method is developed, which can evaluate the reliability of the structural component by using Markov chain model(Unit step B-model) suggested by Bogdanoff. In this method, leak failure, plastic collapse and brittle fracture of the critical component are taken as failure modes, and the effects of initial crack distribution, periodic and non-periodic inspection on the probability of failure are considered. In this method, an equivalent load value for random loading such as wave load is used to facilitate the analysis. Finally some calculations are carried out in order to show the usefulness and the applicability of this method. And then some remarks on this method are mentioned.

• Journal of the Korean Society of Industry Convergence
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• v.10 no.4
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• pp.201-206
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• 2007

Most of the fatigue fractures of various machine structures start at discontinuities or small defects. In this study property of crack growth of titanium alloy was also analyzed to investigate the difference compared with the carbon steel. Titanium alloy has very high specific strength, and the material is widely utilized in advanced engineering fields such as aerospace, atomic energy and ocean development because of its excellence in corrosion and heat resistance. Generally the machine structures experience irregular loadings rather than periodic forces. The prediction of the fatigue life therefore has been analyzed to provide fundamentals of the design and estimation of the machine structures under irregular loading conditions.

• Journal of the Korean Society for Railway
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• v.5 no.3
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• pp.148-157
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• 2002

A health monitoring system becomes a useful tool to obtain information on long term behavior of the important railway structures such as very long span and special type bridges. The health monitoring system not only gives the direct measurement data of the railway bridges but also provides the basic data on the maintenance of the structures. Therefore, periodic calibrations of the health monitoring system will be a necessary step toward precise and accurate assessment of the railway bridges. In this study, the calibration and gauge factor readjustment process made for the health monitoring system installed in the railroad bridges is reviewed and some findings are explained in detail: specifically, the calibrators made for this purpose are illustrated and the regression processes of the calibration on long-term displacement using water level sensor, longitudinal displacement using LVDT sensor, instantaneous displacement using LVDT sensors and accelerometer are described in full length. Based on the regression results, it was found that the gauge factors need to be readjusted according to the regression equation but, since the deviation or shift is not serious so far, long-term observation on each sensor is also recommended. Future work will be concentrated on the long-term analysis of each sensor and on the database creation so that the assessment of the structures is possible.

• Current Optics and Photonics
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• v.2 no.2
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• pp.140-146
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• 2018

In this paper, mesoscale optical surface structures are found to possess both geometric and wave optics features. The study reveals that geometric optic analysis cannot correctly predict the experimental results of light transmission or reflection by mesoscale optical structures, and that, for reliable analyses, a hybrid approach incorporating both geometric and wave optic theories should be employed. By analyzing the transmission patterns generated by the mesoscale periodic pyramid prism plates, we show that the wave optic feature is mainly ascribed to the edge diffraction effect and we estimate the relative contributions of the wave optic diffraction effect and the geometric refraction effect to the total scattering field distribution with respect to the relative dimension of the structures.

• Journal of the Korean Society of Visualization
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• v.19 no.2
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• pp.26-31
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• 2021

The spanwise growth of turbulence structures in turbulent pipe flow was investigated using the direct numerical simulation data of Re_𝜏 = 544, 934 and 3008. Two-point correlations and pre-multiplied energy spectra of streamwise velocity fluctuations were examined along the spanwise direction. The arclength direction is defined as r𝛳, which is useful for an analogy with the spanwise direction for channels or boundary layers; here, r and 𝛳 are the radial distance from the core and the azimuthal angles, respectively. Both analyses showed that the arclength scales increased with increasing the wall-normal distance. It showed that the coherent structures were confined in the core region due to the crowding effect of a circular pipe geometry. The pipe flow simulation could describe a realistic geometrical flow along the azimuthal direction, unlike the simulations of turbulent channel or boundary layer flow using periodic boundary conditions along the spanwise direction. The present results provided the spanwise organization of energy-containing motions over a broad range of scales in turbulent pipe flow.

• Korean Journal of Optics and Photonics
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• v.34 no.6
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• pp.241-247
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• 2023

We examine the spectral characteristics of laser-induced periodic surface structures (LIPSSs) formed on an amorphous silicon film irradiated by a 355-nm nanosecond laser. A Gaussian beam with a diameter of 196 ㎛ is used to perform a two-dimensional raster scan. The laser's pulse number is varied from 190 to 280, and its intensity is adjusted within 100-130 mJ/cm². LIPSSs with a periodicity of approximately 330 nm form on the surface of the Si film, aligned perpendicular to the laser's polarization. Transmission spectra of the samples show dips around 700 nm for transverse electric polarization and around 500 nm for transverse magnetic polarization. The features are investigated with a one-dimensional-grating model using a rigorous coupled-wave analysis. Simulations confirm that the observed dips are due to the resonant modes, depending on the polarization.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.28 no.1
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• pp.79-83
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• 2015

This paper presents a numerical study on the alignment of ridge-like surface structures evolving on silicon-germanium thin films under localized modulation of surface diffusivity. A situation is considered in which the surface diffusion of film material is selectively promoted such that its morphology is perturbed to periodic patterns. To simulate the growth behavior, a governing equation is formulated taking the surface chemical potential into account, and its solution is numerically sought using a finite-difference method. Results show that an initially planar surface coalesces upon the diffusivity modulation, and the surface structures can be aligned by changing the frequency of modulation condition. This research suggests a bottom-up fabrication technique that can manage the regularity of surface structures for thin film devices.