• 한국지반공학회논문집
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• 제28권3호
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• pp.15-23
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• 2012

사질토의 전단강도정수 중의 하나인 내부마찰각은 일반적으로 직접전단시험 또는 삼축압축시험으로 구할 수 있다. 한편 삼축압축시험 결과는 구속압, 전단속도, 공시체의 직경이나 높이, 단부조건 등과 같은 다양한 공시체 및 실험조건에 의해 영향을 받는다. 본 연구에서는 낙동강모래로 직경이 5, 7, 10cm인 느슨한(상대밀도 40%) 및 조밀한(상대밀도 80%) 공시체를 만든 다음 배수 및 비배수 전단시험을 실시하여 공시체의 직경이 모래의 배수 및 비배수 전단강도에 미치는 영향에 대하여 연구하였다. Mohr응력원으로부터 구한 마찰각은 상대밀도에 관계없이 비배수 전응력마찰각, 비배수 유효응력마찰각 및 배수마찰각 모두 공시체의 직경이 5, 7, 10cm로 증가함에 따라 약간 증가하다가 다시 감소하는 경향을 보였다. 앞서 언급한 세 종류 마찰각에서 공시체 직경에 따른 각각의 마찰각 차이는 비배수시험에서 얻은 조밀한 공시체의 전응력마찰각이 4.5도로 가장 큰 차이를 보였으나, 나머지는 2도 이내의 미미한 차이를 나타내었다. 한편 비배수시험에서 얻은 유효응력마찰각과 배수시험에서 얻은 배수마찰각을 비교하였을 때 느슨한 공시체의 경우에는 최대 7도 정도 차이가 났으나 조밀한 공시체의 경우에는 거의 차이가 나지 않았다.

• Geomechanics and Engineering
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• 제7권3호
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• pp.297-316
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• 2014

This paper presents a detailed study focused on investigating the effects of silt content on the static and dynamic properties of sand-silt mixtures. Specimens with a low-plastic silt content of 0, 15, 30 and 50% by weight were tested in static triaxial, cyclic triaxial, and resonant columns in addition to consolidation tests to determine such parameters as compression index, internal friction angle, cohesion, cyclic stress ratio, maximum shear modulus, normalized shear modulus and damping ratio. The test procedures were performed on specimens of three cases: constant void ratio index, e = 0.582; same peak deviator stress of 290 kPa; and constant relative density, $D_r$ = 30%. The test results obtained for both the constant-void-ratio-index and constant-relative-density specimens showed that as silt content increased, the internal friction angle, cyclic stress ratio and maximum shear modulus decreased, but cohesion increased. In testing of the same deviator stress specimens, both cohesion and internal friction angle were insignificantly altered with the increase in silt content. In addition, as silt content increased, the maximum shear modulus increased. The cyclic stress ratio first decreased as silt content increased to reach the threshold silt content and increased thereafter with further increases in silt content. Furthermore, the damping ratio was investigated based on different silt contents in three types of specimens.

• 한국해양공학회지
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• 제22권6호
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• pp.58-63
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• 2008

The short-term high temperature creep rupture behavior of Ni-based Alloy718 steels jointed by friction welding wasinvestigated at the elevated temperatures of 550 to $700^{\circ}C$ under constant stress conditions. The creep rupture characteristics such as creep stress, rupture time, steady state creep rate, and initial strain were evaluated. Creep stress has a quantitative correlation between creep rupture time and steady state creep rate. The stress exponents (n, m) of the experimental data at 550, 600, 650 and $700^{\circ}C$ were derived as 26.1, -22.4, 22.5, -18.5, 17.4, -14.3 and 6.9, -8.1, respectively. The stress exponents decreased with increasing creep temperature. The creep life prediction was derived by the Larson-Miller parameter (LMP) method and the result equation obtained is as follows: T(logtr+20)=-0.00148${\sigma}^2$-3.089${\sigma}$+23232. Finally, the results were compared with those of the base metal for Alloy718.

• Tribology and Lubricants
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• 제37권5호
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• pp.195-202
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• 2021

This paper presents a numerical analysis of the lubrication characteristics of condensed water molecules on a solid surface by conducting molecular dynamics simulations. We examine two models consisting of a simple hexahedral substrate with and without water molecules to reveal the lubrication mechanism of mono-layered water molecules. We perform a sliding simulation by contacting and translating a single asperity on the substrate under various normal loads. During the simulation, we measure the friction coefficient and atomic stress. When water molecules were interleaved between solid surfaces, atomic stress exerted on individual atom and friction coefficient were smaller than those of model without water molecule. Particularly, at a low load, the efficacy of water molecules in the reduction of atomic stress and friction is remarkable. Conversely, at high loads, water molecules rarely lubricate solid surfaces and fail to effectively distribute the contact stress. We found a critical condition in which the lubrication regime changes and beyond the condition, significant plastic deformation was created. Consequently, we deduce that water molecules can distribute and reduce contact stress within a certain condition. The reduced contact stress prevents plastic deformation of the substrate and thus diminishes the mechanical interlocking between the asperity and the substrate.

• Geomechanics and Engineering
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• 제23권1호
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• pp.39-50
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• 2020

In order to understand the shear mechanical properties of the interface between clay and structure and better serve the practical engineering projects, it is critical to conduct shear tests on the clay-structure interface. In this work, the direct shear test of clay-concrete slab with different joint roughness coefficient (JRC) of the interface and different normal stress is performed in the laboratory. Our experimental results show that (1) shear strength of the interface between clay and structure is greatly affected by the change of normal stress under the same condition of JRC and shear stress of the interface gradually increases with increasing normal stress; (2) there is a critical value JRC_cr in the roughness coefficient of the interface; (3) the relationship between shear strength and normal stress can be described by the Mohr Coulomb failure criterion, and the cohesion and friction angle of the interface under different roughness conditions can be calculated accordingly. We find that there also exists a critical value JRC_cr for cohesion and the cohesion of the interface increases first and then decreases as JRC increases. Moreover, the friction angle of the interface fluctuates with the change of JRC and it is always smaller than the internal friction angle of clay used in this experiment; (4) the failure type of the interface of the clay-concrete slab is type I sliding failure and does not change with varying JRC when the normal stress is small enough. When the normal stress increases to a certain extent, the failure type of the interface will gradually change from shear failure to type II sliding failure with the increment of JRC.

• Tribology and Lubricants
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• 제5권2호
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• pp.60-67
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• 1989

For the life prediction and fatigue failure prevention of the constant velocity joint, the maximum equivalent stress and its location in depth from the contact area are essential. These values give the fundamental information to determine the depth of the surface hardening treatment at the contact area. Contact stresses are evaluated at the surface and subsurface of the ball groove of the Weiss type constant velocity joint. The maximum contact pressure and the maximum equivalent stress are obtained. The effects of various parameters such as the radius of ball groove, friction coefficient, and residual stress are studied. The maximum equivalent stress and the maximum contact pressure increase as the radius of the ball grove increases. The location of the maximum equivalent stress moves toward surface as the friction coefficient increases. It was also found that the maximum equivalent stress becomes minimum when the compressire residual stress is about 0.16 times of the maximum contact pressure.

• 산업기술연구
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• 제22권A호
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• pp.153-159
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• 2002

This paper is an experimental work of estimating friction angle of very coarse grained soil such as rubble mound by performing laboratory experiments. Two crushed rocks of rubble mound were used for tests. Triaxial compression tests with drained conditions were performed to measure friction angles of soils prepared by mixing the crushed soil having an identical coefficient of uniformity with different maximum grain size distribution. Centrifuge model experiments with those soils were also performed to measure angle of repose and to estimate friction angle of soil from measuring the slope of slip line in the active stress state. Model tests were carried out by changing the G-levels of 1G and 50G. From triaxial compression tests, the measured value of friction angle of soil is in the range of $41{\sim}57^{\circ}$. The measured value of repose angle is in the range of $32{\sim}35^{\circ}$. The values of friction angle are found not so sensitive to the maximum grain size of soil as long as the coefficient of uniformity is identical. Estimated value of friction angle from measuring the slope of slip line in the active stress state is in the range of $30{\sim}46^{\circ}$. Thus, the estimated angle of friction are found to be greater in the order of the measured angle of repose, the estimated value from the slope of active state, and triaxial compression test results. On the other hand, the measured values of friction angle from triaxial tests were compared with empirical equations, based on the relation between friction angle and void ratio. Equations proposed by Helenelund(l966) and Hansen(1967) found to be relatively reliable to estimate friction angles of soil.

• 한국지반환경공학회 논문집
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• 제4권2호
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• pp.57-64
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• 2003

좁고 길게 굴착된 고랑형태의 되메움 지반의 토압에 관한 해석에 널리 사용되어져 오던 Marston(1913), Spangller(1982) 이론은 R.L. Handy(1985)가 굴착벽면과 되메움 흙과의 상호작용에 의한 최소주응력의 고리를 연결한 최소주응력 아치로서 표현함으로 보다 명확한 설명이 가능하게 되었다. 이후 C.G. Kellogg(1993)는 지금까지 주로 수직하게 대칭된 공간만을 대상으로 논의 되오던 되메움 공간형태를 경사지게 대칭된 공간형태에 대하여 연구를 확장하였다. 발표된 C.G. Kellogg(1993) 이론식은 이론식의 도출 과정에 있어 경사 벽면의 마찰저항이 아닌 경사단면 저부에서 연직한 되메움 흙 자체의 내부 마찰의 크기로서 가정하였다. 본 연구는 C.G. Kellogg가 적용한 경사단면 저부에서 연직한 면의 마찰저항은 경사진 단면에서의 마찰저항과는 다른 크기를 갖을 것이며 이 크기는 토압 산출에 영향을 미칠것으로 판단되어 이를 실내모형 토조실험, C.G Kellogg(1993) 이론식, 수치해석, 실제 경사벽면에 미치는 마찰저항을 고려한 수정된 C.G. Kellogg 이론식으로 규명하고자 하였다.

• 한국공작기계학회논문집
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• 제17권3호
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• pp.101-107
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• 2008

A composite mechanics for discontinuous fiber reinforced polymer matrix composites(PMC) is analysed in order to predict fiber axial stresses. In continuum approach. frictional slip which usually takes place between fibers and polymers is accounted to derive PMC equations. The interfacial friction stress is treated by the product of the coefficient of friction and the compressive stress norma1 to the fiber/matrix interface. The residual stress and the Poisson's contraction implemented by the rule of mixture(ROM) are considered for the compressive stress normal to the fiber/matrix interface. In addition. the effects of fiber aspect ratio and fiber volume fraction on fiber axial stresses are evaluated using the derived equations. Results are illustrated numerically using the present equations with reasonable materials data. It is found that the fiber axial stress in the center region shows no great discrepancy for different fiber aspect ratios and fiber volume fractions while some discrepancies are shown in the fiber end region.

• Tribology and Lubricants
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• 제11권5호
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• pp.172-176
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• 1995

This study deals with thermomechanical cracking between the friction surface and the interior of the brake disc. Analytical model considered in this study was a semi-infinite solid subjected to the thermal loading of an asperity moving with a high speed. The temperature field and the thermal stress state were obtained and discussed on the basis of Von Mises and Tresca Yielding Criterion. Analytical results showed that the dominant stress in cracking of friction brake is thermal stress and cracking location is dependent on the friction coefficient of contact and Peclet number. On the basis of analytical results thermomechanical cracking model is proposed.