• Journal of the Korean GEO-environmental Society
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• v.9 no.7
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• pp.53-60
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• 2008

The purpose of this study is to make case studies on estimation of shear wave velocity in core zone of some rockfill dams by MASW (Multi-channel Analysis of Surface Waves) and to compare the results of case studies with those of the empirical method. Furthermore, the purpose is to recommend the range of shear wave velocity in core zone by MASW and to supply the preliminary data for estimation of shear wave velocity in core zone which is needed for dynamic analysis. From the results of case studies and the comparison between the results of case studies and those of empirical equation, it was found that the shear wave velocities obtained by MASW were smaller than those by the empirical recommendation (Sawada & Takahashi) in the depth of more than 10 m. Also, it is recommended that using the lower bound of empirical formulation by Sawada and Takahashi be available and resonable in case that MASW is not available due to the field condition and the investigation is preliminary.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.2
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• pp.143-156
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• 2015

The stress and displacement fields at the crack tip were studied during the unsteady propagation of a mode III crack in a direction that was different from the property graduation direction in functionally graded materials (FGMs). The property graduation in FGMs was assumed based on the linearly varying shear modulus under a constant density and the exponentially varying shear modulus and density. To obtain the solution of the harmonic function, the general partial differential equation of the dynamic equilibrium equation was transformed into a Laplace equation. Based on the Laplace equation, the stress and displacement fields, which depended on the time rates of change in the crack tip speed and stress intensity factor, were obtained through an asymptotic analysis. Using the stress and displacement fields, the effects of the angled property variation on the stresses, displacements, and stress intensity factors are discussed.

• Journal of the Korean Geotechnical Society
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• v.17 no.5
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• pp.115-128
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• 2001

본 논문에서는 반복하중이나 지진하중을 받는 지반-구조물 시스템의 설계에 필수적인 변수인 전단탄성계수와 감쇠비에 대한 연구를 국내에 존재하는 비점성토 지반에 대하여 수행하였다. 국내 퇴적토지반 및 풍화토지반에서 채취된 자연시료와 입도분포를 조정하여 제작한 시료를 포함하여 총 60개의 시료에 대하여 20kPa에서 500kPa의 구속응력 범위에서 공진주 시험을 수행하고, 이를 조립질 사질토, 실트 및 실트질 모래, 풍화토의 3개의 그룹으로 나누어 결과를 정리하였다. 저변형률 영역의 변형특성인 최대전단탄성계수와 최소감쇠비에 대하여 구속응력의 영향을 확인하였다. 최대전단탄성계수를 예측하는 경험식을 3개의 그룹별로 제안하였다. 최소감쇠비는 구속응력에 따른 분포영역을 제시하였다. 세립분이 적은 조립질 사질토의 경우, 비선형 변형특성이 구속응력의 영향을 뚜렷이 받고 있어 이를 주요한 변수로 고려하여 대표곡선과 분포영역을 제안하였다. 구속응력의 영향을 적게 받는 실트 및 실트질 모래와 풍화토는 구속응력에 관계없이 대표곡선과 분포영역을 제안하였다. 제안된 각 시료의 대표곡선과 기존의 Vucetic-Dobry와 Seed-Idriss가 제안한 곡선과 비교하였다. 본 논문의 연구결과는 국내지반의 비점성토 지반에 대한 지진해석이나 동하중을 받는 시스템의 해석시 유용하게 사용될 것으로 판단된다.

• Journal of the Korean Society for Precision Engineering
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• v.13 no.2
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• pp.110-122
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• 1996

본 논문에서는 전기유동유체(Electro-Rheological Fluid : ERF)를 함유하는 지능구조물의 동적 모델링 및 진동제어를 수행하였다. 먼저 실리콘 오일을 기본용매로 하여 조성된 ERF의 복소 전단모듈러스를 전장부하와 가진 주파수의 함수로 동적 회전모드 실험을 통하여 도출한 후, 이를 샌드위치 보 이론과 연계하여 동적 모델링을 실시하였다. 도출된 6차 편미분방정식 형태의 지배 방정식을 유한요소 모델로 이산화하여 전장부하에 따른 지능구조물의 동탄성 특성값인 감쇠 고유 주파수 및 모달 손실계수를 주파수 영역에서 얻었다. 그리고 ERF를 함유한 샌드위치 형태의 지능구조물을 제작한 후 실험적으로 얻은 동탄성 특성값과 모델에 의해 예측된 동탄성 특성값을 비교 고찰하여 제시된 동적 모델에 대한 타당성을 입증하였다. 또한 모델을 통해 전장부하 함수로 예측된 주파수 응답곡선 중에서 각 주파수 대역에 대해 최소 변위가 되는 응답곡선을 요구응답으로 설정한 후, 그에 해당하는 전장부하를 선정하는 논리적인 능동 진동제어 알고리즘을 제안하였다. 제어알고리즘의 유용성을 입증하기 위해 실험적으로 수행된 능동 진동제어 결과를 주파수영역과 시간영역에서 제시하였다.

• Journal of Korean Tunnelling and Underground Space Association
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• v.12 no.4
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• pp.295-306
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• 2010

Contrary to an intact rock, the jointed rock mass shows strain-dependent deformation characteristics (elastic modulus and damping ratio). The maximum elastic modulus of a rock mass can be obtained from an elastic wave-based exploration in a small strain level and applied to seismic analyses. However, the assessment and application of the non-linear characteristics of rock masses in a small to medium strain level ($10^{-4}{\sim}0.5%$) have not been carried out yet. A non-linear dynamic analysis module is newly developed for FLAC3D to simulate strain-dependent shear modulus degradation and damping ratio amplification characteristics. The developed module is verified by analyzing the change of the Ricker wave propagation. Strain-dependent non-linear characteristics are obtained from disks of cored samples using a rock mass dynamic testing apparatus which can evaluate wave propagation characteristics in a jointed rock column. Using the experimental results and the developed non-linear dynamic module, seismic analyses are performed for the intersection of a shaft and an inclined tunnel. The numerical results show that vertical and horizontal displacements of non-linear analyses are larger than those of linear analyses. Also, non-linear analyses induce bigger bending compressive stresses acting on the lining. The bending compressive stress concentrates at the intersection part. The fundamental understanding of a strain-dependent jointed rock mass behavior is achieved in this study and the analytical procedure suggested can be effectively applied to field designs and analyses.

• Composites Research
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• v.20 no.3
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• pp.43-49
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• 2007

Dynamic instability of rotating disks is the most significant factor to limit its rotating speed. Application of composite materials to rotating disks may enhance the dynamic stability leading to a possible design of rotating disks with lightweight and high speed. Whereas much work has been done on the effect of transverse shear and rotary inertia, called Timoshenko effect, on the dynamic behavior of plates, there is little work on the correlation between the effect and the rotation of disk, especially nothing in case of composite disks. The dynamic equations of a rotating composite disk are formulated with the Timoshenko effect and the vibrational analysis is performed by using a commercial package MSC/NASTRAN. According to the results, the Timoshenko effect goes seesaw in some modes, unlike the well-known fact that the effect decreases as the rotating speed increases. And it can be concluded, based only on the present results, that decrement of the Timoshenko effect by disk rotation grows larger as the thickness ratio decreases, the diameter ratio increases, the modulus ratio increases, and the mode number increases.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.9
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• pp.2252-2263
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• 1993

The purpose of this study is to verify the vibration and damping characteristics of elastic-viscoelastic-elastic structures, theoretically and experimentally. The forth-order differential equations of motion are derived for the transverse vibration of three-layered plates with viscoelastic core layer. The equations consider both transverse displacements of the constraining layer and the bare base plate as variable and account for the effect of the transverse normal strain and the shear strain of viscoelastic core layer on the vibration of the plates. Finite difference analysis of the equations and experimental measurements are performed on the three-layered plates of completely free boundary condition. Comparative investigations on the theory and the results of direct frequency analysis of NASTRAN are carried out on the same structures.

• Journal of the Korean Geotechnical Society
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• v.33 no.3
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• pp.37-47
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• 2017

The geotechnical characteristics of frozen ground is one of the key design issues for the construction of infrastructure in cold region. In this study, the dynamic properties (shear modulus and damping ratio) of frozen and unfrozen soils sampled from Terra Nova Bay located in eastern Antarctica, where Jang Bogo station was built, were investigated using Stokoe-type resonant column test (RC). In order to freeze the reconstituted soil specimen, the RC testing equipment was modified by adding a cooling system. A series of resonant column tests were performed in frozen and unfrozen soils with various soil densities and temperatures. The shear modulus (G) and damping ratio (D) of soil frozen at $-7^{\circ}C$ were compared with those of unfrozen soil. In addition, the effect of temperature rise on the maximum shear modulus ($G_{max}$) and damping ratio was experimentally investigated. This study has significance in that the difference of dynamic soil properties between frozen and unfrozen soils and the effect of temperature rise on frozen soil were identified.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.7
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• pp.851-858
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• 2010

A generalized elastic solution for a transient mode III crack propagating along the gradient in functionally graded materials (FGMs) is obtained through an asymptotic analysis. The shear modulus and density of the FGMs are assumed to vary exponentially along the gradient. The stress and displacement fields near the crack tip are obtained in terms of powers of radial coordinates, and the coefficients depend on the time rates of the change of the crack tip speed and stress intensity factors. The influence of nonhomogeneity and transients on the higher order terms of the stress and displacement fields is discussed.

• Journal of the Korean Geotechnical Society
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• v.17 no.6
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• pp.207-213
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• 2001

Shaking table tests to investigate the dynamic friction behavior of interfaces between dense dry granular soils and construction material(concrete) were performed and the results are reported. The results show the variation of dynamic interface friction coefficients between dense dry granular soils and construction material was small in the sliding velocity range employed in this study. It was also observed that dynamic interface friction coefficients decreased as mean grain sizes of granular soils increased. These coefficients were compared with the friction coefficients obtained from the peak internal friction angles of the same granular soils by plane strain compression tests.