• The Proceeding of the Korean Institute of Electromagnetic Engineering and Science
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• v.3 no.1
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• pp.20-27
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• 1992

A synthesized isotropic pattern antenna can be realized by arrangeing $\lambda$/ 2 dipole along each x, y, z axis. The sleeve type $\lambda$/ 2 dipole antenna is used as a basic element for a proper impedance matching at feeding point and minimum mutual coupling effect between each element at 820 MHz - 895 MHz band. The total electric field intensity radiated from the designed isotropic pattern antenna is drived from the magnetic vector potential which is produced by each sleeve $\lambda$/ 2 dipole in the far field. This total electric field intensity is inversely proportional only to distance $\gamma$from the origin of coordinates, and pattern factor variation is less than 2.1dB. The measured value of total electric field variation depending on pattern factor is less then 2.8dB, and the difference between the calculated and measured value is 0.7dB.

• 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.

• Journal of the Korean Geotechnical Society
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• v.15 no.4
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• pp.3-17
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• 1999

Torsion shear tests under various stress paths were performed to study the behavior of sand during large stress reversal. The stress paths can be classified into the clockwise and the counterclockwise according to torque applied to specimen, and the directions of plastic strain incremental on the stress paths including large stress reversal are compared with the direction of stress state and stress incremental. From test results, the isotropic hardening theory using the principle of St. Venant desirably showed that direction of plastic strain incremental coincided with stress state on primary loading part and nearby failure point, but it might result in a rough approximation on part of unloading and reloading by stress reversal.

• Journal of Korea Foundry Society
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• v.30 no.6
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• pp.210-216
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• 2010

The aim of this study is to characterize a thixoextruded 7075 Al wrought alloy bar in terms of its isotropic behavior through the optical microscope, mechanical test and electron back scattered diffraction. It is also discussed of the extrudability improvement for 7075 Al wrought alloy by thixoextrusion, with emphasis on controlling thixoextrusion parameters. Hot extrusion shows that the maximum extrusion pressure depends on their characteristics in terms of flow stress and hot workability. In the contrary, thixoextrusion demonstrates that the maximum extrusion pressure is almost uniform regardless of the experimental parameters, such as initial ram speed, die bearing length and thixoextrusion temperature. The hot extruded microstructures become elongated to extrusion direction, while the thixoextruded microstructures are isotropic and homogeneously distributed due to the existence of liquid phase between solid grains during the process. The grain refinement due to dynamic recrystallization during thixoextrusion has been also occurred. Subsequent recrystallization would lead to the strengthening of mechanical properties, as observed in the study. The important point is that the values of tensile, yield strength and elongation of the thixoextruded bar without plastic deformation are similar to those of the hot extruded bar with severe plastic deformation.

• Fibers and Polymers
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• v.1 no.2
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• pp.97-102
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• 2000

Pitch precursors were synthesized from coal tar(CT) and pyrolysis fuel oil(PFO, petroleum residue oil) at relatively low temperature of $250^{\circ}$, in the presence of horontrifluorideidiethyletherate complex(BFDE) as a catalyst and nitrobenzene(NB) as a co-catalyst. The softening point, nitrogen content and carbon yield increased with an increase of concentration of NB. The pitch precursors with good spinnability were prepared by removing the volatile components through $N_2$ blowing. The precursor pitches were spun through a circular nozzle, stabilized at $310^{\circ}$ and finally carbonized at $1000^{\circ}$. The optically anisotropic structure formed at the absence of NB was changed into isotropic structure, showing a decrease in size of the flow domain. The hollow carbon fiber could be prepared in the process of stabilization. The results proposed that the morphology of carbon materials could be controlled by changing the concentration of catalyst and/or co-catalyst and/or stabilization condition that affect on the mobility of molecules during carbonization.

• Journal of the Korean Applied Science and Technology
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• v.14 no.1
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• pp.77-87
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• 1997

PFO(pyrolized fuel oil) and $C_{10}^{+}$ oil, which are the residual heavy oils form a NCC(naphtha cracking center), were heat-treated to produce the precursor-pitch for carbon materials. After PFO was initially distilled near $300^{\circ}C$ to separate the volatile matters recovering as high-quality fuel oil, the residuum of nonvolatile precursor-pitch was then thermally pyrolized in the temperature ranges from $350^{\circ}C$ to $450^{\circ}C$. Spinnable isotropic pitch with the softening point of $200^{\circ}C$ and the toluene insolubles of 36wt% was obtained at $365^{\circ}C$, and then was successfully spun through a spinneret(0.5mm diameter). After spinning, an isotropic carbon fiber of $25{\mu}m$ diameter was obtained via oxidation and craboniation procedures. Mesophase spherules began to be observed from the product pitch pyrolized at $400^{\circ}C$, and bulk mesophase with a flow texture was observed above $420^{\circ}C$. In the case of $C_{10}^{+}$ was the feed was polymerized in the presence $H_2SO_4$ at room temperature to increase the molecular weight and then heat-treated gradually up to $200{\sim}250^{\circ}C$. The products obtained with the softening point of $80{\sim}190^{\circ}C$ were carbonized at 500 and $1000^{\circ}C$ to examine the morphology.

• Journal of the Korean Wood Science and Technology
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• v.32 no.4
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• pp.36-45
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• 2004

The plywood for concrete form is regarded as a laminate plate composed of orthotropic materials and the flexural analysis is conducted by applying the laminate plate theory, in which the four edges of the plate is assumed to be simply supported and the concentric point lateral load is applied. The results of flexural experiment are compared with the theoretical ones. Theoretically predicted results coincide with experimental ones up to the point of deflection less than 1/4 of plate thickness. In addition, when the plywood is regarded as an isotropic plate for simple analysis, the geometric average of the elastic modulus measured in the direction parallel to the face grain (E₁₁) and perpendicular to the face grain (E₂₂) could be used for the elastic modulus of isotropic plate.

• Nuclear Engineering and Technology
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• v.52 no.6
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• pp.1271-1276
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• 2020

The geometrical efficiency of a source-to-detector configuration is considered to be necessary in the calculation of the full energy peak efficiency, especially for NaI(Tl) and HPGe gamma-ray spectroscopy detectors. The geometrical efficiency depends on the solid angle subtended by the radioactive sources and the detector surfaces. The present work is basically concerned to establish a new mathematical approach for calculating the solid angle and geometrical efficiency, based on conversion of the geometrical solid angle of a non-axial radioactive point source with respect to a circular surface of the detector to a new equivalent geometry. The equivalent geometry consists of an axial radioactive point source with respect to an arbitrary elliptical surface that lies between the radioactive point source and the circular surface of the detector. This expression was extended to include coaxial radioactive circular disk source. The results were compared with a number of published data to explain how significant this work is in the efficiency calibration procedure for the γ-ray detection systems, especially in case of using isotropic radiating γ-ray sources in the form of point and disk shapes.

• Journal of the Korea Society of Computer and Information
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• v.6 no.1
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• pp.51-59
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• 2001

In this paper, performance of the double circular array antenna was analyzed, which has sharp directivity with specific direction and is compatible to shape pencil beam. The character of each array geometry which N numbers of isotropic point source is arranged in planar with type of a square, circular and double circular was compared and analyzed. As a result, we fined that double circular geometry was good performance rather than square and circular array geometry in the point of directivity. SLL and HPBW.

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.16 no.2 s.93
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• pp.217-221
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• 2005

The E-field intensity by mobile handsets in a phantom is measured by a 3 axes miniature probe. The detected DC voltage by a Schottky diode in a probe has nonlinear characteristics by itself. If a pertinent diode compression point (DCP) is applied for the compensation specific absorption rate(SAR) as much as 200 W/kg can be measured with a good measurement accuracy.