• Journal of the Korean Geotechnical Society
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• v.21 no.5
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• pp.207-214
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• 2005

The reflected signals from the defects of a pile and the boundaries between the pile and soils are analyzed to evaluate the integrity of drilled shafts in the impact-echo test. Signals varied according to both of the stiffness ratio of the pile to defects and that of the pile to surrounding soils. Model tests using the small size pile in the laboratory and numerical analyses have limitations in finding the characteristics of the signals due to different stress wave characteristics and unreliable modelling for the interaction between the pile and soils respectively. Full scale testing piles which have artificial defects are installed by the actual construction method and they were used to investigate the characteristics of reflected signals according to defects and the stiffness ratios of the pile to soils around.

• International Journal of Highway Engineering
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• v.3 no.4 s.10
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• pp.127-136
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• 2001

To increase the structural integrity of concrete box culvert good compaction by the dynamic compaction roller with bi9 capacity is as effective as good backfill materials. It is needed for effective compaction that a compaction roller closes to concrete structure with high frequency. However structural distress of the culvert could be occur due to the excessive earth pressure by great dynamic compaction load. To investigate the characteristics of Induced stress by compaction, a box culvert was constructed with changing cushion materials and compaction methods. Two types of cushion material such as tire rubber chip and EPS(Expanded Polystyrene) were used as cushion panels and they are set on the culverts before backfill construction. Laboratory test result of cushion material says that the value of dynamic elastic modulus of rubber is lesser than that of EPS. On the other hand, material damping of rubber material is greater than that of EPS. In most case, dynamic compaction rollers with 10.5 ton weights were used and vibration frequency was applied 30Hz for the great compaction energy. This paper presents the main results on the characteristics of dynamic earth pressures during compaction. The amounts of induced dynamic pressures$(\Delta\sigma\;h)$ by compaction are affected with construction condition such as compaction frequency, depth of pressure cell, distance between roller and the wall of culvert and roller direction. Based on the measured values dynamic lateral pressure on the culverts, it could be said that orthogonal direction of roller to the length of culvert is more effective to compaction efficiency than parallel direction.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1990-1997
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• 1992

The characteristic of supersonic flow with condensation along a wavy wall of small amplitude in channel is investigated through the direct marching method of characteristics. The very complex problem that may appear where the overlapping of the same family characteristics occurs, can be satisfactorily solved by means of the modified method suggested by Zucrow. In the present study for the case of supersonic moist air flow, the dependency of location of formation and reflection of oblique shock wave generated by the wavy wall, and the distributions of flow properties, on the relative humidity and temperature at the entrance of wavy wall is clarified by plots of streamline, ios-Mach umber and ios-flow properties. Also, it is confirmed that the wavy wall plays an important key role in the formation of oblique shock wave, and that the effect of condensation on the flow field appears apparently.

• Transactions of the Korean Society of Machine Tool Engineers
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• v.18 no.2
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• pp.162-169
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• 2009

This paper describes a study on dynamic characteristics analysis and dynamic compliance evaluation of a 5-axis multi-tasking machine tool of ram-head type. Structural dynamics analysis and evaluation are necessary to machine tool design and development to secure good machine tool performance against tough and harsh machining conditions. In this study, natural frequencies and corresponding vibration modes of the machine tool structure were analyzed by using both F.E.M. modal analysis and impulse hammer test. Furthermore, dynamic compliance of the machine tool was analyzed by using F.E.M. and also measured by using a hydraulic exciter test. Both the theoretical analysis and experimental test results showed good agreement with each other.

• Journal of Chest Surgery
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• v.31 no.4
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• pp.398-401
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• 1998

A 56-year-old female with a permanent unipolar pacing(VVI) system underwent elective coronary bypass surgery and mitral valve replacement. Because the unipolar sensing is subject to considerable nonmyocardial electrical noise, the pacing function may be disturbed by the use of electrocautery. Temporary atrial and ventricular bipolar epicardial leads and external generator were used for maintaining A-V sequential pacing during the use of electrocautery. Before aortic cross-clamp was released after cardiac operation, regular cardiac rhythm could spontaneously be resumed with an integrated cardioplegic strategy, avoiding the use of defibrillator

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1996.10a
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• pp.113-117
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• 1996

구조물의 진동에 의해 소음이 방사되는 현상은 기계에서 소음의 발생원으로 볼 수 있기 때문에 기게류의 소음을 예측하거나 저감방안을 제시하기 위해서는 구조물의 동특성과 방사특성을 이해하고 있어야 한다. 특히, 엔진블럭, 펀치프레스, 배의 갑판구조물등과 같은 대다수의 소음 발생기계는 평판의 형상을 가진 구조물로서 기계적인 충격 등에 의해 그 표면에서 소음이 발생되므로 강성을 증가시키고, 소음저감을 목적으로 빔과 같은 보강재를 통해 보강되어 있다. 그런데, 해석적인 방법으로는 평판이나 원판 또는 구와 같은 단순한 형태의 특정구조물에 대해서만 그 결과를 얻을 수 있으므로 이와 같은 불연속 평판구조물의 진동 및 방사특성은 평판에 대한 순수 이론으로는 해석이 곤란하여 따라서 본 연구에서는 수치해석적인 방법을 통해 이를 해결하고자 하였다. 수치해석적인 방법으로는 유한요소법(FEM)과 경계요소법(BEM), 및 통계적 에너지 해석기법(SEA)등이 있으며 구조물의 진동-소음연성문제의 경우에 있어서는, 진동해석을 FEM과 SEA으로, 공기 중에서의 방사현상은 BEM으로 예측하고 있다. 본 연구에서는 재질이 균일한 얇은 2차원 평판구조와 보강평판에 대해서 진동특성은 유한요소해석 프로그램을 사용하여 해석하였으며 이때의 진동특성값을 입력데이터로 사용하여 경계요소해석 프로그램으로 방사효율 등을 예측하였다. 또한 이 과정에서 2차원 평판구조의 모우드 밀도와 가진점 모빌리티의 실수값이 가지는 평균치의 물리적 특성을 분석하였으며, 추후 실험을 통해 이를 검증코자 한다.

• Journal of the Korean Geotechnical Society
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• v.20 no.6
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• pp.41-49
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• 2004

In general, open trenches or in-filled trenches have been used to isolate transmitting of vibration. Recently alternative methods, in which various materials are used for wall barriers have been proposed and performed in several sites. Although the effects of these methods are verified, resonable and qualitative evaluation methods have not yet been conducted. It may be because many factors, such as the characteristics of vibration sources, ground condition, and parameters of wall barrier are coupled complicatedly. In this study, a series of centrifugal modelings were conducted in order to examine the evaluation method of a vibration wall barrier when point load transmits to the surface. The experiments were performed with different stiffness of wall barriers and different depths of installations. Using the results obtained from tests, effects of those variables on the efficiency of vibration barrier were analyzed. Through this investigation we observed vibration transmission of under ground, and verified the applicability of ball dropping system which was developed in this study.

• The Journal of the Acoustical Society of Korea
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• v.33 no.1
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• pp.10-20
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• 2014

To study the water-entry impact noise, on-board experiment using a small launcher firing various objects was performed in the Yellow Sea. As the launcher fires a cylindrical object from the ship vertically, generated noise is measured with a hydrophone on the starboard of Chung-hae, Marine surveyor. Three types of cylindrical objects, which have noses of flat-faced, conical, and hemisphere, were used during the experiment. The measured noise exhibits a time-dependency which can be divided into three phases: (1) initial impact phase, (2) open cavity flow phase, (3) cavity collapse and bubble oscillation phase. In most cases, the waveform of bubble oscillation phase is dominant rather than that of initial impact phase. Pinch-off time, where a cavity begins to collapse, occurs at 0.18 ~ 0.2 second and the average lasting time of bubble was 0.9 ~ 1.3 second. The energy of water-entry impact noise is focused in the frequency region lower than 100 Hz, and the generated noise is influenced by the nose shapes, object mass, and launching velocity. As a result, energy spectral density on the bubble frequency is higher in the order of flat-faced, conical, hemisphere nose, and the increase of initial energy raises the energy spectral density on the bubble frequency in the cylinder body of same shape. Finally, we compare the measurements with the simulated signals and spectrum based on the bubble explosion physics, and obtain satisfactory agreements between them.

• Journal of the Korea Concrete Institute
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• v.21 no.6
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• pp.705-712
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• 2009

HPFRCCs (high-performance fiber reinforced cementitious composites), which is relatively more ductile and has the characteristic of high toughness with high fiber volume fractions, can be used in structures subjected to extreme loads and exposed to durability problems. In the case of PVA (polyvinyl alcohol) fiber, it is noted by former studies that around 2% fiber volume fractions contributes to the most effective performance at HPFRCCs. In this study, flexural tests were carried out to evaluate the flexural behavior of HPFRCCs and to optimize mix proportions. Two sets of hybrid fiber reinforced high performance specimens with total fiber volume fraction of 2 % were tested: the first set prepared by addition of short and long PVA fibers at different combination of fiber volume fractions, and the second set by addition of steel. In addition, in order to assess the performances of the HPFRCCs against to high strain rates, drop weight tests were conducted. Lastly, the sprayed FRP was applied on the bottom surface of specimens to compare their impact responses with non-reinforcing specimens. The experimental results showed that the specimen prepared with 1.6% short fibers (REC 15) and 0.4% long fiber (RF4000) outperformed the other specimens under flexure, and impact loading.

• Applied Chemistry for Engineering
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• v.10 no.3
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• pp.336-342
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• 1999

Various surface treatment techniques can be applied onto the surface of carbon fibers to increase interlaminar shear strength (ILSS). In a commerciaI treatment, first, surface of carbon fiber was oxidized, after that, a sizing agent was coated to improve handleability and adhesion to the matrix. Carbon fiber reinforced composites (CFRC) which is made of these fibers show excellent ILSS but show low vaIues of impact strength In this study, reactive and ductile interphase was introduced between fiber and matrix to increase both the ILSS and impact strength. By using electric conductivity of carbon fibers, flexible polymers which have ionizable group, i.e., MVEMA and EMA, were coated onto the surface (oxidized) of carbon fiber by the technique of electrodeposition. ILSS and impact strength of composites were evaluated according to the surface treatments, i.e., commercial sizing treatment, interphase introduction, and without sizing treatment. Izod impact strength and ILSS of CFRC were simultaneously improved in thc thickness range of $0.08{\sim}0.12{\mu}m$ of MVEMA interphase. Water resistance of the composites was decreased by introducing MVEMA interphase.