• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.18 no.12
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• pp.1106-1110
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• 2005

We fabricated (110) silicon hard master by using anisotropic wet etching for embossing. The etching chemical for the silicon wafer was a TMAH $25\%$ solution. The anisotropic wet etching produces a smooth sidewall surface and the surface roughness of the fabricated master is about 3 nm. After spin coating an organic-inorganic sol-gel hybrid material on a silicon substrate, we employed hot embossing technique operated at a low pressure and temperature to form patterns on the silicon substrate by using the fabricated master. We successfully fabricated the multi-mode planar optical waveguides showing low propagation loss of 0.4 dB/cm. The surface roughness of embossed patterns was uniform for more than 10 times of the embossing processes with a single hydrophobic surface treatment of the silicon hard master.

• Carbon letters
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• v.6 no.1
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• pp.6-14
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• 2005

The study of mechanical properties and fracture behaviour of carbon/carbon composites is significant to its application and development. These are dependent on microstructure and properties of reinforcing fibers and matrix, fiber/matrix interface and porosity/cracks present in the composites. In the present studies high-density carbon/carbon composites have been prepared using PAN and various pitch based carbon fibers as reinforcements and pitch as matrix with repeated densification cycles using high-pressure impregnation and carbonization technique. Scanning electron microscopy has been used to study the fracture behaviour of the highly dense composites and correlated with structure of the composites. The geometry of reinforcement and presence of unfilled voids/cracks was found to influence the path of crack propagation and thereby the strength of composites. The type of stresses (tensile or compressive) accumulated also plays an important role in fracture of composites.

• The Korean Journal of Ceramics
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• v.5 no.1
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• pp.19-24
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• 1999

The influences of TiB2 additions to the β-sialon on mechanical propeties and electrical discharge machinability were investigated. Samples were prepared by adding 15, 30 and 45 vol.% TiB2 particles as a second phase to a β-sialon matrix. The β-sialon-TiB2 composites were sintered by hot pressing in a nitrogen atmosphere at 1800℃ with pressure of 30 MPa. The fracture toughness of the composites was increased with TiB2 content except 45 vol.% TiB2 composite. The crack propagation and crack deflection were observed with a SEM for etched samples after vicker's indentation. The composites containing more than 30 vol.% TiB3 had resistivity lower than 10-3 Ω㎝. The electrical discharge machining (EDM) of composited was conducted with two kind of machines such as die-sinker and wire cutter. The machinability was evaluated with the cutting rate surface roughness after machining.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2006.06a
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• pp.205-206
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• 2006

The surges caused by the typhoon of Korea are analysed in Kwangyang Bay. The deviations of the high water level were $74{\sim}185cm$ and the maximum deviations of the water level (maximum surges) were $151{\sim}240cm$ in Kwangyang Bay during the typhoon. The major parameters of the maximum deviations of the water level are as follows : Analysis shows that the pressure drop increased the sea level by $43{\sim}59cm$, the flood of the Sumjin River by $4{\sim}5cm$ and the external surge propagation and wind setup by $97{\sim}192cm$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.32 no.9
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• pp.796-803
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• 2008

Ultrasonic testing uses the directivity of the ultrasonic wave, which propagates in on direction. The directivity is expressed as the relationship between the propagate direction and its sound pressure. The directivity of an ultrasonic wave is related to the choice of probe arrangement, testing sensitivity and scanning pitch and correct measurement of defect size and location. This paper describes on the directivity measurement of ultrasonic wave using the visualization method. The directivity of shear wave emitted from the angle beam transducer were constant during propagation. The difference of directivity was existed between 2 MHz and 4 MHz angle beam transducers. When these experimental results were compared with the theory which was based on the continuous wave, it showed good agreement with theoretical directivity on the principal lobe.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.05a
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• pp.718-723
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• 2004

The insertion loss of a noise barrier comes from the effects of diffraction, transmission loss, absorption coefficient and attenuation by direct propagation. The noise level after the noise barrier, differs reatly from the diffraction on the upper part of the barrier. Maekawa, furze and Anderson presented a empirical formula for calculating the diffraction of a semi infinte screen shaped noise barrier. In this syudy, Noise reduction performance software was developed for the proper design and assessment of new plastic barrier . Predicted sound pressure level from using the software is compared with the site-measurement results to verify the noise reduction performance and feasibility of prediction software for insertion loss of noise barrier.

• Journal of the Korean Society for Precision Engineering
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• v.13 no.5
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• pp.164-172
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• 1996

In hard and brittle materials as advanced ceramics indented by a hard indenter, the indenter's transition radius, was defined as critical radius which distinguishes the occurrence of the first plastic deformation from the elastic cracking as the first damaging event, is analytically and experimentally investigated. The analytical result is shown that the critical load, which not enlarge pre-existing cracks as the form of median crack beneath a indenter, is constant, and is determined by the order of $k_{IC}$$^{4}$ $P_{Y}$$^{3}$(where, $K_{IC}$ , $P_{Y}$are the fracture toughness of materials and the applied pressure by indenting, respectively). And the size of transiton radii were experimentally obtained with the similar values to the analytical results.lts..

• Journal of the Korean Society of Safety
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• v.12 no.4
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• pp.102-107
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• 1997

Ground vibrations are an integral part of the process of rock blasting. The sudden acceleration of the rock by the detonation gas pressure acting on the drillhole walls induces dynamic stresses in the surrounding rock mass. This sets up a wave motion in the ground much like the motion in a bowl of jelly when disturbed by the action of a spoon. The wave motion spreads concentrically from the blasting site, particularly along the ground surface, and is therefore attenuated, since its fixed energy is spread over a greater and greater mass of material as it moves away from its origin. Some theoretical aspects of the generation and propagation of vibrations produced in rock blasting are analyzed; although it must be indicated that this is just a mere approximation to the problem, as the actual phenomena are much more complex owing to the interaction of different types of waves and their modifying mechanics.

• Advances in aircraft and spacecraft science
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• v.6 no.6
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• pp.463-478
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• 2019

The present work investigates the effect on the flow-induced vibrations of the lay-up sequence of composite laminated axisymmetric structures, using an hybrid approach based on a wave finite element and a transfer matrix method. The structural vibrations, under deterministic distributed pressure loads, diffuse acoustic field and turbulent boundary layer excitations, are analysed and compared. A multi-scale approach is used for the dynamic analysis of finite structures, using an elementary periodic subsystem. Different flow regimes and shell curvatures are analysed and the computational efficiency is also discussed.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.622-625
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• 2008

While the research for flow over a circular cylinder has been actively carried out up to the present, it has been known that the flow has not been clarified even now. Various complex flow and aero-acoustic characteristics exist around a circular cylinder such as flow separation, wake and pressure wave propagation. In this paper, research was carried out for wake flow and aeroacoustics over a circular cylinders by using high order, high resolution techniques that are used in two dimensional aero- acoustic analysis. OpenMP parallel processing method was used. For the numerical result, the periodic characteristic of Strouhal Number due to vortex shedding was comparatively analyzed with other experiment values and two dimensional numerical results.