• Journal of the Korea institute for structural maintenance and inspection
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• v.1 no.1
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• pp.107-112
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• 1997

A series of reinforced concrete beams was tested to evaluate the flexural performance of the repaired RC beams. The key parameters for this study were the size and location of the patch, and the repair materials, including polymer, polymer-cementitious and cementitious materials. The repaired specimens failed by a typical flexural mode with minor interfacial bond failure. Beams repaired with polymer, polymer-cementitious and cementitious materials recover 100%, 91%, and 97% of the flexural strength respectively, while beams with cement mortar lose approximately 30% of the strength. Compared with the pressure injection techniques the specimens repaired with patching techniques show low flexural strength, with significant interfacial bond failure. Location and size of the repaired part do not affect the recovering performance. Interfacial behavior between repair and strengthening materials is the major influencing factor for the composite structures.

• Transactions of the Korean Society of Automotive Engineers
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• v.12 no.1
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• pp.83-90
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• 2004

In this study, CT specimens were prepared from spring steel(SUP9) processed shot peening which was room temperature and low temperature experiment. And we got the following characteristics from fatigue crack growth test carried out in the environment of room temperature and low temperature at $25^{\circ}C$, $-30^{\circ}C$, $-50^{\circ}C$, $-70^{\circ}C$,$-100^{\circ}C$, and $-150^{\circ}C$, in the range of stress ratio of 0.05 by means of opening mode displacement. The threshold stress intensity factor range ΔKth in the early stage of fatigue crack growth (Region I)was increased but stress intensity factor range ΔK in the stable of fatigue crack growth (Region II) was decreased in proportion to decrease temperature. It is assumed that the fatigue resistance characteristics and fracture strength at low temperature and high temperature is considerably higher than that of room temperature in the early stage and stable of fatigue crack growth region.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.17 no.4
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• pp.42-47
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• 2009

Recently, the wide range of the composite materials is used for the making airplanes, trains and automobiles body for the lightweight. Despite having complex structures, composite materials usually have well defined mechanical characteristics. However, composite materials are difficult to understand the fracture mechanism clearly by simple mechanical test. Nondestructive evaluation (NDE) combined with mechanical testing can play a more important role and especially Acoustic Emission Testing (AET) would become known to be a useful tool to assess damage and fracture behavior of composites. In this study The experiment was performed to acquire the acoustic emission signal during tensile test using unidirectional CFRP specimen and the data was analyzed the acoustic emission parameters with the waveform.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 2001.06a
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• pp.49-54
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• 2001

Monolayers such as self-assembled monolayer (SAM) have received considerable attention to reduce stiction and friction in micro-devices and microelectromechanical systems (MEMS). Various organic molecular films were investigated to obtain better understanding of their tribological behaviors and adhesion property. The organic molecular films studied in this work are: epoxysilane SAMs, octadecyltricholosilane (OST), multi-layers composed of epoxysilane SAMs, poly〔styrene-b-(ethylene-co-butylene)-b-styrene〕(SEBS) and compound of epoxy resin and poly (paraphenylene) (EP/PPP). The pull-off forces of these films were also obtained from force-distance curves measured in static mode of operation of atomic force microscope (AFM). Tribological tests were conducted with a ball-on-flat reciprocating friction tester. The OST showed the lowest pull-off force, indicating its low adhesion property. It was revealed that, the OST, EP/PPP and the multi-layer of epoxysilane SAMs, SEBS and EP/PPP exhibited good tribological properties at the lower load (0.3 N) whereas the OST showed best performance at the higher load (1.8 N).

• Proceedings of the Korean Vacuum Society Conference
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• 2000.02a
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• pp.140-140
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• 2000

The initial growth mode of Nb on Ag(11) in sub-monolayer regime and the influence of subsequent 520K annealing are studied using UHV Scanning Tunneling Microscopy. E-beam evaporated Nb is deposited onto the substrate at RT, and STM measurements are carried out at RT and 78 K. With Nb being immiscible in bulk Ag, 3D islands formation begins at early stage and no particular ordered structure is found. After annealing to 520K, most of islands are disappeared from terrace. There exist 2 possibilities. : (1) Diffusion of Nb into the 2nd or 3rd layer of Ag substrate or (2) agglomeration of Nb on Ag at higher temperature. A model will be given to explain the evidence. In addition, we investigated the change of STM image according to bias voltage depending on island size. Possible physical mechanism responsible for such behavior together with interaction between Nb islands and reactive gases will be also discussed.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.291.2-291.2
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• 2013

Growth behavior of b-Ga2O3 nanowires (NWs) on sapphire(0001) substrates during radio-frequency magnetron sputtering is reported. Upon fabrication, flat thin films grew initially, subsequent to which, NW bundles were formed on the surface of thin film with increasing film thickness. This transition of the growth mode occurred only at temperatures greater than ${\sim}450^{\circ}C$. The b-Ga2O3 NWs were grown through the self-catalytic vapor-liquid-solid mechanism with self-assembled Ga seeds. Secondary growth of NWs, which occurred from the sides of primary NWs resulting in branched NW structures, was also observed. Finally, the room temperature photoluminescence properties of as-grown and annealed b-Ga2O3 NW samples were investigated.

• Journal of the Korean Society for Precision Engineering
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• v.17 no.11
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• pp.81-87
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• 2000

A new resonator that is fabricated by single polysilicon layer process is presented. The resonator can move in two orthogonal direction on the plane parallel to the substrate. And the resonant frequencies of the two modes are intrinsically designed to be identical since the overall structure of the resonator is symmetric about the two directions of motion. Since the resonator ideally has two identical vibration mode, it can be applied to various micro-devices that requires multi DOF motion, especially to microgyroscopes. To investigate the feasibility of application of the resonator, dynamic model of the resonator including the nonlinear behavior of driving electrodes is derived and evaluated with the fabricated one, and the self-tuning characteristics are proved though experiments.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.4
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• pp.402-409
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• 2004

The purpose of this study is to investigate the effect of local wall thinning on the collapse of elbow subjected to internal pressure and bending moment. Thus, the nonlinear three-dimensional finite element analyses were performed to obtain the collapse moment of elbow containing various wall thinning defects located at intrados and extrados under two loading modes (closing and opening modes) with internal pressure. From the results of analysis, the effect of wall thinning defect on the global moment-rotation behavior of elbow was discussed, and the dependence of collapse moment of elbow on wall thinning depth, length, and circumferential angle was investigated under different loading mode and defect location.

• Journal of Mechanical Science and Technology
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• v.20 no.2
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• pp.197-204
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• 2006

The purpose of this study is to evaluate the AE characteristics for the basemetal, PWHT (post-weld heat treatment) and weldment specimens of SA-516 steel during fracture testing. Four-point bending and AE tests were conducted simultaneously. AE signals were emitted in the process of plastic deformation. AE signal strength and amplitude of the weldment was the strongest, followed by PWHT specimen and basemetal. More AE signals were emitted from the weldment samples because of the oxides, and discontinuous mechanical properties. AE signal strength and amplitude for the basemetal or PWHT specimen decreased remarkably compared to the weldment because of lower strength. Pre-cracked specimens emitted even lower event counts than the corresponding blunt notched specimens. Dimple fracture from void coalescence mechanism is associated with low-level AE signal strength for the basemetal or PWHT. Tearing mode and dimple formation were shown on the fracture surfaces of the weldment, but only a small fraction produced detectable AE.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.600-603
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• 2005

In this paper we studied about the effect of the double cracks on the dynamic behavior of a simply supported beam. The equation of motion is derived by using Lagrange's equation and analyzed by numerical method. The simply supported beam is modeled by the Euler-Bernoulli beam theory. The crack section is represented by a local flexibility matrix connecting three undamaged beam segments. The influences of the crack depth and position of each crack on the vibration mode and the natural frequencies of a simply supported beam are analytically clarified. The theoretical results are also validated by a comparison with experimental measurements.