• Proceedings of the KWS Conference
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• 2007.11a
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• pp.272-275
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• 2007

• Journal of Korean Tunnelling and Underground Space Association
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• v.15 no.3
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• pp.333-344
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• 2013

This paper discussed about the effect of permeability reduction of the jointed rock mass in the vicinity of a tunnel which is one of the reasons making large difference between the estimated ground-water inflow rate and the measured value. Current practice assumes that the jointed rock mass around a tunnel is a homogeneous, isotropic porous medium with constant permeability. However, in actual condition the permeability of a jointed rock mass varies with the change of effective stress condition around a tunnel, and in turn effective stress condition is affected by the ground water flow in the jointed rock mass around the tunnel. In short time after tunnel excavation, large increase of effective tangential stress around a tunnel due to stress concentration and pore-water pressure drop, and consequently large joint closure followed by significant permeability reduction of jointed rock mass in the vicinity of a tunnel takes place. A significant pore-water pressure drop takes place across this ring zone in the vicinity of a tunnel, and the actual pore-water pressure distribution around a tunnel shows large difference from the value estimated by an analytical solution assuming the jointed rock mass around the tunnel as a homogeneous, isotropic medium. This paper presents the analytical solution estimating pore-water pressure distribution and ground-water inflow rate into a tunnel based on the concept of hydro-mechanically coupled behavior of a jointed rock mass and the solution is verified by numerical analysis.

• Journal of Korean Tunnelling and Underground Space Association
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• v.9 no.2
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• pp.121-131
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• 2007

Recently, the probability of rock joints being exposed to free faces is getting higher for the scale of rock mass structures gets larger. Also, the frequency of occurring dynamic events such as earthquakes and blasting has been increasing. Thus, the shear behavior of rock joints under different conditions needs to be investigated. In this study, a series of direct shear tests were carried out under various conditions to examine the velocity-dependent shear behavior of saw-cut rock joints. Two types of direct shear test were carried out. The first was to examine the velocity-dependent shear behavior of saw-cut rock joints at seven different shear velocities, each with three different normal stresses. The second was to examine the shear behavior of saw-cut rock joints when three different instantaneous shear velocities changed. As a result, the coefficient of friction was affected by normal stress. The breakpoint velocity, the point when the change of shear velocity starts to affect the frictional behavior, became lower as normal stress increased. Also, as the shear velocity became lower, the degree of stress-drop on stick-slip behavior became larger. As a result of examining the changes of friction coefficient, velocity weakening (decrease of friction coefficient) was observed. The decrement of friction coefficient due to the changes of shear velocity under slow shear velocity was larger than that under fast shear velocity.

• Tunnel and Underground Space
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• v.10 no.2
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• pp.237-248
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• 2000

Based on the preliminary results from the therm analysis, which is currently carrying, three-dimensional computer simulations using a finite element code, ABAQUS Ver. 5.8, were designed to determine the mechanically stable cavern and deposition hole spacing. Linear elastic modeling for the cases with different cavern and deposition hole spacing were carried out under three different in situ stress conditions. From the simulations, the response of the rock to the stress redistribution after the excavation of the openings could be investigated. Also the optimum cavern and deposition hole spacing could be estimated based on the factor of safety. When the in situ stress determined from the actual stress measurements in Korea were used, the case with cavern spacing of 40m and deposition hole spacing of 3m was in very stable condition, because the factor of safety was calculated as 3.42., When the in situ stress conditions for Sweden and Canada were used, the previous case, they seem to be in stable condition, since the factors of safety are still higher than 1.0. From these results, it was concluded that the rock will not fail even after the stress redistribution.

• Journal of Korean Tunnelling and Underground Space Association
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• v.6 no.4
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• pp.315-326
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• 2004

This study performed model tunnel tests in order to investigate the influence of discontinuity condition of rock mass to the stress and deformation of tunnel lining. Tests were carried out changing the direction of main joint and lateral earth pressure condition of rock mass. Test results revealed that the axial force in tunnel lining showed a tendency of decrease with the presence of joints. It decreased much with the increase of lateral earth pressure coefficient. And, it also showed that the location or maximum displacement and maximum stress in lining were changed by the direction of main joint of rock mass. The tangential stress and normal stress showed the difference above the maximum twenty times as lateral earth pressure coefficient due to effect of joints increased. Also, these tendencies of concentration of tensile stress in tunnel lining were confirmed by elastic theory.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.2
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• pp.331-341
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• 1993

The propagating crack problems under dynamic antiplane mode in orthotropic material is studied in this paper. To analyze the dynamic fracture problems by theoretical method or experimental method in orthotropic material, it is important to know the dynamic stress intensity factor in the vicinity of crack tip. Therefore the dynamic stress field and dynamic displacement field with dynamic stress intensity factor of orthotropic material in mode III were derived. When the crack propagation speed approachs to zero, the dynamic stress components and dynamic displacement components derived in this paper are identical to the those of static state. In addition, the relationships between dynamic stress intensity factor and dynamic energy release rate are determined by using the concept of crack closure energy with the dynamic stresses and dynamic displacements derived in this paper. Finally, the characteristics of crack propagation are studied with the properties of orthotropic material and crack speed. The variation of angle .alpha. between fiber direction and crack propagating direction and crack propagation speed fairly effect on stress component and displacement component in crack tip. The influence of crack propagation speed on the speed on the stress and displacement is greater in the case of .alpha.=90.deg. than in the case of .alpha.=0.deg. and the faster the crack propagation speed, the greater the stress value and displacement value.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.1
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• pp.34-42
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• 1986

An interface of a circular arc formed by two isotropic, homogeneous elastic materials is investigated. It is shown that L integral satisfies the conservation law for the interface if it is perfectly bonded, in frictionless contact or separated such as in a crack with the origin of the coordinate system being located at the center of the circular arc. The property of path independence of the L integral is applied to an interfacial crack problem, to obtain the stress intensity factors, where the interfacial crack is located along the arc of the circular inclusion embedded in infinite matrix. It is assumed here that the contact zone exist as in the model proposed by Comninou, thus removing the overlapping of the materials along the interface. Another example is shown for case of a circular interfacial crack in the matrix of finite size, where the stress intensity factors are determined by computing a value of the L integral numerically along the path far from the crack tip.

• Journal of the Korean Society of Safety
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• v.5 no.1
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• pp.19-29
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• 1990

The objective of this paper is to investigate the effect of residual stresses on the $\Delta$K$\sub$th/ and fatigue crack growth behavior of butt weldments. For this purpose, transverse butt sutmerged arc welding was performed on SM50A steel plate and CT(compact tension) specimens which loading direction is perpendicular to weld bead were selected. Welding residual stresses distribution on the specimen was determined by hole drilling method. The case of crack located parallel to weld bead, the states of as weld and PWHT, $\Delta$K$\sub$th/ of specimens(HAZ, weld zone) was higher than that of the base metal probably because of the compressive residual stresses of crack tip. In low $\Delta$K region, it is estimated that the effects of residual stresses for da/dN are great. In region II, the da/dN of weldments in as weld state was lower than that of the base metal. Though da/dN of Weldments in PWHT state was similar to that of the base metal. The constant of power law, m in two states consisted with the base metal. Therefore , it is estimated that the value of m is not affected by residual stresses. Fatigue crack growth behavior of weldments consisted with the base metal considering the effective stress intensity factor range($\Delta$K$\sub$eff/) included the effect of initial residual stress(Kres). Thus, we can predict the fatigue crack growth behavior of weldment by knowing the distribution of initial residual stress at the crack tip.

• The Journal of Engineering Geology
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• v.3 no.1
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• pp.1-20
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• 1993

In this study, laboratory direct shear tests on 37 core specimens of gneiss were performed to examine the characteristics of shear behavior on fractures by using a portable direct shear box. The multi-stage shear testing method was used and normal stress applied to specimens ranges from 5.60 to $25.67kg/\textrm{cm}^2$. On the basis of test results, the empirical equations for the shear strength on fractures were suggested. The methanical parameters that can influence the shear behavior were derived and compared between each parameter. The values of shear stiffness have a trend showing rapid increase with the increase of normal stress and joint roughness coeffident, and the average value of secant shear stiffness for all specimens is about $110.68kg/\textrm{cm}^3$ under the range of normal stress applied in this test In addition, the relationship between the length of specimen and shear stiffness is inversely correlated due to the size effect. Therefore, even the specimens with the same joint roughness coeffident show the trend of decreasing shear stiffness in case of the specimens being the longer length.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.99-99
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• 2013

나노결정질 다이아몬드(Nanocrystalline Diamond: NCD) 박막은 고경도와 낮은 마찰계수를 가지고 있어 초경합금이나 고속도강과 같은 절삭공구 위에 코팅하여 공구의 성능 향상을 도모하려는 노력이 있어 왔다. 그러나 NCD 박막의 잔류응력이 크고, 초경합금과 철계 금속에 NCD가 증착되지 않는다는 문제점이 있다. 따라서 잔류응력 완화와 다이아몬드 핵생성을 위하여 제3의 중간층 재료가 필요하다. 본 연구에서는 W과 Ti을 중간층으로 하여 초경합금(WC-Co)과 고속도강(SKH51)에 NCD 박막을 코팅하고 기계적 특성을 비교하였다. 초경합금 또는 고속도강기판 위에 W 또는 Ti 중간층을 DC magnetron sputter를 이용해 각 1 ${\mu}m$의 두께로 증착하고 그 위에 MPCVD (Microwave Plasma Chemical Vapor Deposition)를 이용해 NCD 박막을 2${\mu}m$의 두께로 코팅하였다. FESEM을 이용하여 표면과 단면의 형상을 관찰하였고, XRD와 Raman spectroscopy를 통해 NCD 박막의 결정성을 확인하였다. 그리고 tribology test를 실시하여 코팅된 박막의 내마모성을 비교하였으며, Rockwell C indentation test를 이용하여 밀착력을 비교하였다. 초경합금에 적용 시, W이 Ti보다 중간층으로서 더 우수한 것으로 나타났으며 이는 열팽창계수 차이에 의한 잔류응력의 차이에 의한 것으로 여겨진다. 중간층 두께에 따른 박막의 기계적 특성 변화를 알아보기 위해 W 중간층의 두께를 1, 2, 4 ${\mu}m$로 변화를 주었다. 중간층 두께가 2 ${\mu}m$ 이상일 때 박막의 밀착력이 증가되는 것으로 나타났다. 고속도강 위에 같은 방법으로 1 ${\mu}m$의 W 또는 Ti 중간층 위에 2 ${\mu}m$의 NCD 박막을 코팅한 시편들은 초경합금에 코팅한 것과 달리 두 시편 모두 낮은 밀착력을 나타내었다. 열팽창계수 차이에 의한 잔류응력을 완화하기 위해 고속도강에 W/Ti 복합박막을 중간층으로 Ti, W순으로 각각 1 ${\mu}m$ 두께로 증착 후 그 위에 NCD 박막을 2 ${\mu}m$ 두께로 코팅 한 후 특성을 비교하였다. Ti/W 복합 중간층 위에 코팅된 NCD 박막의 밀착력이 W 혹은 Ti 단일 중간층에 코팅된 박막에 비해 우수한 것으로 나타났다. 그러나 실제 공구에 적용하기에는 박막의 밀착력 개선이 요구되며 이를 위해서 더 연구가 필요하다.