• Korean Journal of Optics and Photonics
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• v.16 no.3
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• pp.239-247
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• 2005

In this study, we have used newly developed dual-beam shearography which is based on laser speckle that includes various information about an object. Among the several shearing techniques, we used Michelson shearing interference technique which is the most powerful. Acrylate plate was used as a sample, which has inner defects and low thermal conductivity. Michelson shearing interferometer was used for obtaining speckle fringes. We also used phase shifting technique to get a phase map. Using single beam illumination, we could obtain mixture of deformation components of both in-plane and out-of-plane. In order to separate the two components, we have used dual-beam shearography technique. We have obtained a speckle pattern of both before and after deformation. Through LS filtering and unwrapping processes, we could find a position and a shape of the inner defects easily. Deformation of the acrylate plate due to thermal heating has occurred mainly in z-direction(out-of-plane) because it has low thermal conductivity. The acrylate plate was deformed only at the restricted area where the electrical heat applied.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.5
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• pp.511-518
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• 2010

The split Hopkinson pressure bar (SHPB) has been widely used to determine the mechanical properties of materials at high loading rates. However, to ensure test reliability, the source of measurement error must be identified and eliminated. During the experiment, specimens were placed between the incident and the transmit bar. Contact friction between the test bars and specimen may cause errors. In this study, numerical experiments were carried out to investigate the effect of friction on the test results. In the SHPB test, the stress measured by the transmitted bar is assumed to be the flow stress of the test specimen. However, performing numerical experiments, it was shown that the stress measured by the transmit bar is axial stress components. When the contact surface is frictionless, the flow stress and axial stress of the specimen are approximately equal. On the other hand, when the contact surface is not frictionless, the flow stress and axial stress are no longer equal. The effect of friction on the difference between the flow stress and axial stress was investigated.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.14 no.4
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• pp.941-951
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• 1994

The resilient behavior of subbase materials, locally used in Korea. were evaluated in this study by performing laboratory MR tests. The variations of the MR value according to LVDT's location in experiments and moisture conditions, respectively, were evaluated. And, in order to determine the relevant MR characteristics of the prototype materials, laboratory model tests containing smaller particles than those of the prototype were conducted. Based on above results, the relationship of the MR and the bulk stress (${\theta}$) was suggested. The case using internal LVDT. gave much larger $M_R$ value than that using external LVDT, since the external LVDT somewhat overestimates the resilient strain. The measured $M_R$ in damp conditions ($S_r$=40%) was larger than that in wet conditions ($S_r$=70%) by about 10%. The relationship between the $M_R$ and the void ratio was linear according to particle size effect. The relationship of the $M_R$ and the bulk stress (${\theta}$) on subbase materials in damp conditions to be used in Korea could be recommended as $M_R=3960{\cdot}{\theta}^{0.60}$ psi.

• Magazine of the Korea Concrete Institute
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• v.7 no.2
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• pp.126-135
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• 1995

Thermal cracks are occured when thermal stress due to the hydration of cement exceeds the tens~le strength of concrete. Since crackmg causes poor durability of concrete, the effect of ther ma1 cracking should be includod for the design and construction of massive concrete structures. In this study, an experiment is performed for the investigation of time dependent thermal stress history. In order to evaluate thermal stress. two methods are employed. One 1s the evaluation method of thermal stress based on the measurement from embedment stram gauge with non-stress strain gauge and the other 1s based on the measurement from concrete stress gauge. As a result of this study, the value corrected by the former shows good agreement with the latter. The validity of the proposed method for the evaluation of thermal stress 1s explored.

• Journal of the Korean Geosynthetics Society
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• v.10 no.2
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• pp.21-28
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• 2011

Several theoretical soil nonlinear models to predict damping ratio, which is one of the typical dynamic properties of soils, it is impractical to predict damping ratio. The resonant column and torsional shear test(RC-TS) is used to represent the dynamic behavior of soils from intermediate to medium shear strains. A limitation of RC-TS is measure precise shear strain in large strains and the modified equivalent radius($R_{eq}$) was obtained using both modified hyperbolic model and Ramberg-Osgood model. Bonneville clays were tested using RC-TS test to obtain rotation and torque. The measured rotation and torque were then compared with calculated rotation and torque using curve-fitting method. Then, the nonlinear soil model parameters were obtained and the equivalent radius was calculated using the model parameters.

• Polymer(Korea)
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• v.25 no.1
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• pp.56-62
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• 2001

When polyethylene terephthalate(PET) film specimen were stepwise elongated under tension with various speeds of about 0.5~500 mm/min, the necking area, consisted of transparent and opaque bands, was formed during plastic deformation. Stress oscillation was apparently obtained in the stress-strain curve of above specimen. However, stress oscillation was not obtained in the stress-strain curve of annealing specimen. Microstructure was examined dynamically using an optical microscopy and thermal analysis was carried out in a differential scanning calorimeter at a heating rate of 10${\circ}$/min. Also, orientation and crystallization were examined using monochromatic-pinhole technique and elastic modulus was measured by a dynamic mechanical analyzer in the temperature range of -150~70 ${\circ}$ with the frequency of 1 Hz. Transparent PET products were fabricated by use of the PET pellets annealed at 83${\circ}$ for 30 min in an electric furnace.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.21 no.6
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• pp.607-613
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• 2008

A monitoring system for the subway structures using prestrained FBG sensors fixed with partially stripped fibers was developed. The sensor packages had pre-strain controllable fixtures. Tensile and compressive strain of the structure could be measured without slip. The FBG sensor system was applied to the concrete lining structure in Taegu subway. Near the structure, the narrow tunnel construction, for the electric power cables and telecommunication cables, started. We wanted to measure the deformations of the subway structures due to the construction by the FBG sensor. The applied sensors had the gauge length of 1 meter to overcome the inhomogeneity of the concrete material with enough length. In order to fix tightly to the structure, the partially stripped parts of the sensor glued to the package and slip phenomenon between fiber and acrylate jacket was prevented. Prestrain of the sensor was imposed by controlling the two fixed points with bolts and nuts in order to measure compressive strain as well as tensile strain. The behavior of subway lining structure could be monitored very well.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2016.05a
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• pp.630-632
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• 2016

In this paper, we propose a modified rhombus slot UWB(Ultra Wide Band) antenna with fork-shaped feeding structure. The proposed modified rhombus slot structure is eliminated upper and lower part of the basic rhombus slot shape to get ultra-wideband characteristics for UWB communication. Also, feeding structure is used to fork-shaped structure to get ultra-wideband characteristics. The antenna is designed on an FR-4 substrate of which the dielectric constant is 4.4, and its overall size is $34mm(W1){\times}34mm(L1){\times}1mm(t)$, and its slot antenna size is $30mm(W2){\times}16.75mm(L3+L4)$. After the optimized process, the proposed antenna is fabricated and measured. Measured result. fabricated antenna satisfied -10 dB impedance bandwidth in UWB frequency band (3.1 ~ 10.6 GHz ). And measured results of gain and radiation patterns characteristics displayed determined for operating bands.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.1C
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• pp.53-61
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• 2006

This study is aimed at to examine the stress redistribution mechanism for tunneling in an unconsolidated ground with inclined layers through model tests. To make the unconsolidated ground, two dimensional model ground is prepared with aluminum rods and blocks, which are frictional resistance free between testing apparatus walls and ground materials, by establishing the ground materials self-supporting. It is carried out to measure the ground deformation and the stress redistribution for model ground with tunneling by measuring apparatus respectively. For the ground deformation, surface settlements are measured to examine the deformation features during tunnel excavation. For the stress redistribution, the earth pressure acting on both the tunneling part and its surrounding parts is measured to examine their mutual relationship. Based on test results, precise examination is conducted on the stress redistribution mechanism in the unconsolidated ground with inclined layers during tunnel construction.

• Journal of the Korean Vacuum Society
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• v.3 no.4
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• pp.395-400
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• 1994

본 연구에서는 LRDC 트랜스듀서를 장착한 변형된 회전 진자를 제작하여 1$\times$10-2 Torr에서 1-10-5 Torr 사이의 Ar 압력을 측정하였다. 비구동 회전 진자의 진폭은 기체 분자와의 충돌에 의한 감 쇄가 일어난다. 회전 진자의 감쇄에 의한 특성 함수를 측정하여 본 연구팀은 일반적인 진공 측정 장비 로는 측정이 어려운 1-10-5 Torr에서 1-10-5 Torr 사이의 Ar 압력을 정확하게 측정할 수 있었다.