• 한국산학기술학회논문지
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• 제18권11호
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• pp.124-129
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• 2017

본 연구에서는 철도차량의 경량 차체 소재로 주로 사용되는 알루미늄 합금 (Al-6005-T6) 압출재에 대한 마찰교반접합 적용 연구를 수행하였다. 용접속도를 300 mm/min으로 고정시킨 채 회전 공구의 회전속도를 600, 900, 1200, 1600 rpm으로 변화시킬 때 미세구조 및 기계적 특성의 변화에 대해 평가하였다. 특히, 동일 소재에 대해 전통적인 용접방법인 MIG 용접된 시편과의 비교를 통해 마찰교반접합법의 철도차량 적용성에 대해 논하였다. MIG 용접방법과는 달리, 마찰교반접합부에는 미세기공이 발견되지 않았으며, 경도 측정 결과 공구의 회전속도가 빨라질수록 높아진 입열량에 따라 heat affected zone (HAZ)의 범위가 넓어지고 결정립 조대화에 따라 HAZ의 경도 또한 낮아지는 것을 확인하였다. 인장시험 결과 HAZ지역에서 파단이 발생하였으며, 본 연구에서 설정한 공구의 회전속도 범위에서는 nugget 부의 경도 변화는 발견되지 않았으나 rpm이 올라갈수록 인장강도와 항복강도가 떨어지는 경향을 보였으며 이는 경도 시험에서와 같이 HAZ지역의 결정립 크기의 증가로 인한 현상으로 판단된다.

• 한국경영과학회:학술대회논문집
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• 대한산업공학회/한국경영과학회 1995년도 춘계공동학술대회논문집; 전남대학교; 28-29 Apr. 1995
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• pp.515-528
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• 1995

Slips and falls are the major causes of the pedestrian injuries in the industry and the general community throughout the world. With the awareness of these problems, the friction coefficients of the interface between floorings and footwear have been measured for the evaluation of slip resistant properties. During this measurement process, the surface texture has been shown to be substantially effective to the friction mechanism between shoe heels and floor surfaces under various types of walking environment. Roughness, either of the floor surface or shoe heels, provides the necessary drainage spaces. This roughness can be designed into the shoe heel but this is inadequate in some cases, especially a wear. Therefore, it is essential that the proper roughness for the floor surface coverings should be provided. The phenomena that observed at the interface between a sliding elastomer and a rigid contaminated floor surface are very diverse and combined mechanisms. Besides, the real surface geometry is quite complicate and the characteristics of both mating surfaces are continuously changing in the process of running-in so that a finite number of surface parameters can not provide a proper description of the complex and peculiar shoe - floor contact sliding mechanism. It is hypothesised that the interface topography changes are mainly occurred in the shoe heel surfaces, because the general property of the shoe is soft in the face of hardness compared with the floor materials This point can be idealized as sliding of a soft shoe heel over an array of wedge-shaped hard asperities of floor surface. Therefore, it is considered that a modelling for shoe - floor contact sliding mechanism is mainly depended upon the surface topography of the floor counterforce. With the model development, several surface parameters were measured and tested to choose the best describing surface parameters. As the result, the asperity peak density (APD) of the floor surface was developed as one of the best describing parameters to explain the ambiguous shoe - floor interface friction mechanism. It is concluded that the floor surface should be continuously monitored with the suitable surface parameters and kept the proper level of roughness to maintain the footwear slip resistance. This result can be applied to the initial stage of design for the floor coverings.

• 터널과지하공간
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• 제2권2호
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• pp.212-223
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• 1992

The distinct element method(DEM) si well suited to the kinematic analysis of blocky rock masses. Two distinctive problems, a rock avalache and tunnel in jointed rock masses, are chosen to apply the DEM which is based on perfectly rigid behaviour of blocks. Investigated for both problems are the effects of the input parameters such as contact stiffnesses, friction coefficient and damping property. Using various types of models of the avalanche and tunne, an extensive parametric study is done to gain experiences in the method, and then to alleviate difficulties in determining parameter values suitable for a given problem. The coefficient of frictio has significant effects on all aspects of avalanche motion(travel distance, velocity and travel time), while the stiffnesses affect the rebounding and jumping motions after collision. The motion predicted by the models having single and mutiple blocks agrees well to the observations reported on the actual avalache. For the tunnel problem, the behaviour of the key block in an example tunnel is compared by testing values of the input parameters. The stability of the tunnel is dependent primarily on the friction coefficient, while the stiffness and damping properties influence the block velocity. The kinematic stability of a tunnel for underground unclear waste repository is analyzed using the joint geometry data(orientation, spacing and persistence) occurred in a tailrace tunnel. Allowing a small deviation to the mean orientation results in different modes of failure of the rock blocks around the tunnel. Of all parameters tested, the most important to the stability of the tunnel in blocky rock masses are the geometry of the blocks generated by mapping the joint and tunnel surfaces in 3-dimensions and also the friction coefficient of the joints particularly for the stability of the side walls.

• Structural Engineering and Mechanics
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• 제72권6호
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• pp.785-796
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• 2019

Power transmission substations are susceptible to potential damage under seismic excitations. Two of the major seismic failure modes in substation supplies are: the breakage of brittle insulator, and conductor end fittings. This paper presents efficient isolation strategies for seismically strengthening of a two-item set of equipment including capacitive voltage transformer (CVT) adjacent to a Lightning Arrester (LA). Two different strategies are proposed, Case A: implementation of base isolation at the base of the CVT, while the LA is kept fixed-base, and Case B: implementation of base isolation at the base of the LA, while the CVT is kept fixed-base. Both CVT and LA are connected to each other using a cable during the dynamic excitation. The probabilistic seismic behavior is measured by Incremental Dynamic Analysis (IDA), and a series of appropriate damage states are proposed. Finally, the fragility curves are derived for both the systems. It is found that Friction Pendulum System (FPS) isolator has the potential of decreasing flexural stresses caused by intense ground motions. The research has shown that when the FPS is placed under LA, i.e. Case B (as oppose to Case A), the efficiency of the system is improved in terms of reducing the forces and stresses at the bottom of the porcelain. Several parametric studies are also performed to determine the optimum physical properties of the FPS.

• Tribology and Lubricants
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• 제24권6호
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• pp.374-377
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• 2008

Due to the environmental concerns, especially the greenhouse effect and GWP (Global Warming Potential), the carbon dioxide was investigated as an alternative natural refrigerant to replace HFCs (HydroFluoroCarbons) in refrigerator or air conditioning systems. Because new compressor with carbon dioxide is going to be operated under the high pressure, the tribology of sliding surfaces in the compressor becomes very important. To improve of wear resistance in compressor parts, especially rotary type, the friction and wear characteristics of improved sliding surfaces between vane and flange were evaluated in this paper. The method of reformed sliding surface, such as micro-dimple processes, was applied on surfaces in order to improve the tribological characteristics, and their performances were evaluated experimentally. The vane-on-flange type lubricated sliding tests were performed with a high pressure wear tester using carbon dioxide. Test results showed that the reformed surfaces were very effective to reduce the friction and the wear amounts of vane surfaces. The method of improved surfaces showed good tribological properties at vane and flange.

• 대한기계학회논문집A
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• 제35권10호
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• pp.1347-1353
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• 2011

마찰교반용접은 일반적인 용접법에 비하여 기계적 성질, 용접결함의 감소, 재료 절감 그리고 짧은 생산시간과 같은 장점 때문에 제작공정에 광범위하게 사용되고 있다. 본 연구의 목적은 이전의 실험 결과를 이용하여 마찰교반용접의 최적 조건을 검토하고 최적의 마찰용접조건에서 접합된 7075-T651 알루미늄 판재의 3가지 다른 영역, 즉 용접재, 열영향부재 그리고 모재에 대한 피로균열전파의 거동을 고찰하는 것이다. 최적의 마찰교반용접조건은 회전속도 800rpm, 이송속도 0.5mm/sec로 결정되었으며, 최적의 마찰교반용접에서 용접된 시험편에 대한 피로균열전파율은 3가지 영역의 재질, 즉 용접재, 열영향부재, 모재와 균열의 구동력에 크게 의존함을 보였다.

• 한국표면공학회지
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• 제43권5호
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• pp.238-242
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• 2010

In the present work, the influence of oxide on the Cr-Si-N coatings was investigated for the Cr-Si-O-N coatings on AISI 304 and Si wafer deposited by hybrid system, which combines the DC magnetron sputtering technique and arc ion plating (AIP) using Cr and Si target in an $Ar/N_2/O_2$ gaseous mixture. As the O content in the Cr-Si-N coatings increased, the diffraction patterns of the Cr-Si-O-N coatings showed CrN and $Cr_2O_3$ phases. However, as the O content increased to 28.8 at.%, diffraction peak of $Cr_2O_3$ was disappeared in the Cr-Si-O-N coating. The $d_{200}$ value was decreased with increasing of O content. The average grain size increased from about 40 nm to 65 nm as the O content increased. The maximum micro-hardness of the Cr-Si-O-N coating was obtained 4507 Hk at the O content of 24.8 at.%. The average friction coefficient of the Cr-Si-O-N coatings was gradually decreased by increasing the O content and the average friction coefficient decreased from 0.37 to 0.25 by increasing the O content. These results indicated that amorphous phase was increased in the Cr-Si-O-N coatings by increasing of O content.

• Geomechanics and Engineering
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• 제14권4호
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• pp.387-398
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• 2018

The present study evaluates the interface shear strength between sand and different construction materials, namely steel and concrete, using direct shear test apparatus. The influence of surface roughness, mean size of sand particles, relative density of sand and size of the direct shear box on the interface shear behavior of sand with steel and concrete has been investigated. Test results show that the surface roughness of the construction materials significantly influences the interface shear strength. The peak and residual interface friction angles increase rapidly up to a particular value of surface roughness (critical surface roughness), beyond which the effect becomes negligible. At critical surface roughness, the peak and residual friction angles of the interfaces are 85-92% of the peak and residual internal friction angles of the sand. The particle size of sand (for morphologically identical sands) significantly influences the value of critical surface roughness. For the different roughness considered in the present study, both the peak and residual interaction coefficients lie in the range of 0.3-1. Moreover, the peak and residual interaction coefficients for all the interfaces considered are nearly identical, irrespective of the size of the direct shear box. The constitutive modeling of different interfaces followed the experimental investigation and it successfully predicted the pre-peak, peak and post peak interface shear response with reasonable accuracy. Moreover, the predicted stress-displacement relationship of different interfaces is in good agreement with the experimental results. The findings of the present study may also be applicable to other non-yielding interfaces having a similar range of roughness and sand properties.

• 한국분말재료학회지
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• 제17권6호
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• pp.456-463
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• 2010

This paper describes the results of the application of Cr-Diamond-like carbon (DLC) films for efficiency improvement through surface modification of spur gear parts in the hydraulic gear pump. Cr-DLC films were successfully deposited on SCM 415 substrates by a hybrid coating process using linear ion source (LIS) and magnetron sputtering method. The characteristics of the films were systematically investigated using FE-SEM, nano-indentation, sliding tester and AFM instrument. The microstructure of Cr-DLC films turned into the dense and fine grains with relatively preferred orientation. The thickness formed in our Cr buffer layer and DLC coating layer were obtained the 487 nm and $1.14\;{\mu}m$. The average friction coefficient of Cr-DLC films considerably decreased to 0.15 for 0.50 of uncoated SCM415 material. The hardness and surface roughness of Cr-DLC films were measured 20 GPa and 10.76 nm, respectively. And then, efficiency tests were performed on the hydraulic gear pump to investigate the efficiency performance of the Cr-DLC coated spur gear. The experimental results show that the volumetric and mechanical efficiency of hydraulic gear pump using the Cr-DLC spur gear were improved up to 2~5% and better efficiency improvement could be attributed to its excellent microstructure, higher hardness, and lower friction coefficient. This conclusion proves the feasibility in the efficiency improvement of hydraulic gear pump for industrial applications.

• 한국표면공학회:학술대회논문집
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• 한국표면공학회 2007년도 추계학술대회 논문집
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• pp.61-62
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• 2007

HVOF thermal spray coating of micron size Co-alloy powder has been studied for the durability improvement of high velocity spindle (HVS). Optimal coating process of this system for the best surface properties is hydrogen flow rate 75 FMR, oxygen flow rate 38-42 FMR, feed rate 30 g/min at spray distance 5 inch. Friction coefficient (FC) and wear trace (WT) decrease increasing coating surface temperature from 25$^{\circ}$C to 538$^{\circ}$C due to the higher lubricant effects of the oxides at the higher temperature. At the study of adhesion of T800 coating on a light metal alloy Ti-6Al-4V (Ti64) tensile bond strength (TBS) and tensile fracture location (TFL) of Ti64/T800 are 8,740 psi and near middle of T800 coating respectively. This shows that adhesion of Ti64/T800 is higher than the cohesion strength (8,740 psi) of T800 coating. Therefore T800 coating is strongly advisable for the surface coating on HVS such as high speed air-bearing spindle.