• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2010년 춘계학술대회논문집
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• pp.487-490
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• 2010

A fluid transient analysis on the pipe network of bipropellant propulsion system is conducted through numerical parametric studies in which unsteady friction results are compared with quasi-steady friction results and also show the pressure drop results during the liquid apogee engine firing. The fluid transient analysis program has verified through comparing with the original Zielke model, the full and recursive convolution model and quasi-steady model as a reference. And the pressure drop program also has verified through comparing with results of the well-known program, EPANET2. The bipropellant propulsion system has two different fluids as fuel and oxidizer, and mostly they are hypergolic combination so that the valve opening and closing of the thrusters, that cause the pressure waves, shall take place simultaneously to get proper performance. The different physical properties of the fuel and oxidizer result in the different responsive to the same valve opening and closing. The response results may be helpful to know the characteristics of the bipropellant propulsion system and design it.

• 대한기계학회논문집A
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• 제26권9호
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• pp.1842-1848
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• 2002

Although DLC coatings have good tribological properties, these are dependant on the deposition method, the property of contact surface, and test condition. Humidity, which has little influence on tribological behavior in macro scale, is an important factor of tribological behavior in small devices like MEMS. The objective of this study is to investigate the tribological behavior of DLC coatings with particular attention to their wettability at various humidities. DLC coatings were deposited on Si substrates and tested using a reciprocating friction tester against Si$_3$N$_4$balls at various humidities. The results showed that the tribological behavior of DLC coatings was dependant on relative humidity and wettablility of DLC coatings. Friction coefficient at high relative humidity was higher thar that at low relative humidity. The tungsten-containing DLC coatings had a good wear resistance at low relative humidity whereas DLC coatings derived from argon(Ar)+cesium(Cs) gases showed a good wear resistance at high relative humidity.

• Journal of Mechanical Science and Technology
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• 제17권9호
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• pp.1276-1286
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• 2003

The performance of porous bearing under different lubricants and lubricating conditions was experimentally investigated in this study. In order to carry out the experiments, a new test rig was designed to determine the tribological properties of based sintered bronze journal bearings that were manufactured by powder metallurgy (P/M) techniques. To determine the effects of lubricating conditions with and without oil supplement (OS) on the tribological characteristics of these bearings under static loading and periodic loadings, some experiments were carried out using different lubricants. In the tests, pure base oil (SAE 20W50), two fully formulated commercial engine oils (SAE20W50) and lubricating oils with commercial additive concentration ratio of 3％ were used. The worn surfaces of test bearings were examined using optical microscopy. Experimental results showed that the change in friction coefficient was more stable and in smaller magnitude under static loading than that of periodic loading. In addition, the friction coefficient and the wear rate conducted with base oil resulted in higher values than those of fully formulated oils with and without OS lubricating conditions. The experimental results obtained in this study indicated that the correct selection of lubricant and suitable running conditions were very important on the tribological characteristics of porous bearings.

• 한국정밀공학회지
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• 제28권8호
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• pp.915-928
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• 2011

Titanium for dental implant application has the superior properties of biocompatibility, specific strength, and corrosion resistance. However, it is extremely difficult to find a suitable surface treatment method for sufficient osseointegration with biological tissue/bone cell and implant surface. Surface treatment technology using laser has been researched as the way to increase surface area of implant. In this study, to develop the surface treatment process with improved adhesion between implant and bone cell at the same time for superior biocompatibility, pulsed laser beam was overlapped continuously for scribed surface morphology and determination of friction coefficient. As the results, surface area and friction coefficient was increased over 2 times by the comparison with sand blasting, which is used for the conventional method. In this time, the optimal condition for laser beam power and beam irradiation speed was 13 watt and 50 mm/sec, respectively.

• 한국산학기술학회논문지
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• 제8권3호
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• pp.432-436
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• 2007

자동차 구조용 재료에 대하여 물리적 증착 코팅방법에 의한 트라이볼로지 특성을 연구 하였다. 코팅 재료는 탄화티탄(TiC)과 질화티탄(TiN)이다. 실험은 펠렉스 마찰 마모시험기를 이용하여 하중과 온도에 다양한 조건을 적용하여 마찰과 마모 거동에 대하여 트라이볼로지 특성을 결정하고 평가하였다. 연구결과 코팅하지 않았을 때보다 코팅하였을 때가 윤활 특성이 향상 되었고, 특히 내마모성 및 극압성 그리고 열 안정성이 훌륭하였다.

• Tribology and Lubricants
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• 제28권6호
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• pp.333-339
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• 2012

Typical magneto-rheological (MR) elastomers consist of silicon-based material. A number of studies have been carried out to evaluate the vibration and tribological characteristics of silicon-based MR e-lastomers. However, these elastomers have quite low strength, so they have low wear resistance. In this study, polyurethane-based MR elastomers with performances better than those of MR elastomers. Experiments have been conducted on different MR elastomers (Pu MR elastomer, Pu-Si MR elastomer, and Pu-wrapped-Si MR elastomer) and different predefined magnetic directions (Non-Direction, Vertical Direction, and Horizontal Directionality) to evaluate the friction and wear performance under a magnetic field. The results show that Pu-wrapped-Si MR elastomer with a horizontal predefined magnetic field has the best performance in terms of wear.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2009년도 춘계 학술발표회
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• pp.119-122
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• 2009

The gravel compaction pile method has been used as a soft foundation improvement method because bearing capacity and discharge capacity is excellent. But the discharge capacity decreased when the clogging was generated because the clay penetrate into a void of gravel compaction pile. Accordingly, the purpose of this study is to reduce the clogging generation in gravel compaction pile constructing in the soft ground and take a step to minimize a void of gravel compaction pile. And the proper mixing ratio was determined with the large scale direct shear test performed to get strength and permeability with mixing ratio of crushed stone and sand(100:0, 90:10, 85:15, 80:20, 75:25). As a result of the test, it was showed that internal friction angle was the highest at 85:15 mixing ratio of crushed stone and sand and we can make sure a tendency of internal friction angle's decrease when the mixing ratio of crushed stone and sand passed 15%.

• International Journal of Advanced Culture Technology
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• 제3권1호
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• pp.179-187
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• 2015

This study presents the experimental results of the Friction Stir Welding (FSW) of AA6063 aluminum alloy and AISI304 stainless steel butt joint by varying the welding parameters such as the rotating speed and the welding speed. The main results are as follows. The variation of the welding parameters produced various characteristic interfaces and had distinct influences on the joint properties. Increasing the rotating speed and the welding speed decreased the joint tensile strength because it produced the defect on the joint interface. The optimum welding parameter that could produce the sound joint was a rotating speed of 750 rpm and the welding speed of 102 mm/min with the tensile strength of 71 MPa.

• 한국철도학회:학술대회논문집
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• 한국철도학회 2008년도 춘계학술대회 논문집
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• pp.980-985
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• 2008

Since its invention at TWI in 1991, Friction Stir Welding (FSW) has become a major joining process in the aerospace, railway and ship building industries especially in the fabrication of aluminium alloys. In an attempt to optimize the friction stir welding process of Al alloys for extrusion Aluminium carbody of HEMU-400X (Extrusion Aluminum 6xxx series), effects of joining parameters such as tool rotating speed, plunging depth and dwelling time on the weld joints properties were evaluated. Experimental tests were carried out for butt joined Al plates. A wide range of joining conditions could be applied to join Al alloys for Extrusion Aluminum 6xxx series without defects in the weld zone except for certain welding conditions with an insufficient heat input. The microstructures of welds have dynamic-recrystallized grain similar to stir zone in FSW weld. For sound joints without defects, at the rotation speed of 700 rpm with different welding speeds, the tensile strengths of the Stir Zone(SZ) were almost the same, 80% of those of the base metal. (JIS Z 2201)

• Journal of Welding and Joining
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• 제20권5호
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• pp.87-92
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• 2002

Weldability of Friction Stir Welded(FSW) AZ31B-H24 Mg alloy sheet with 4m thick was evaluated by changing welding speed. The sound welding conditions mainly depended on the suffiicient welding heat input during the process. The insufficient heat input resulted in the void like defect in the weld zone. Higher welding speed caused a larger inner void or lack of bonding. The defects were distributed at the stir zone or the transition region between stir zone and thermo-mechanical affected zone (UE). The size of defects slightly increased with increasing welding speed. These defects had a great effect on the joint strength of weld zone. The weld zone was composed of stir zone, TMAZ and heat affected zone. The stir zone was cosisted of fine recrystallized structure with $5-8\mu\textrm{m}$ in the mean grain size. The hardness of weld zone was near the 60HV, which was slightly lower than that of base metal. The maximum joint strength was about 219MPa that was 75% of that of base metal and the yield strength was also lower than that of base metal partly due to the existance of defects.