• Tribology and Lubricants
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• v.38 no.4
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• pp.170-178
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• 2022

Mechanical surface treatment is an excellent approach widely used to modulate and improve the performance and service life of bearings, gears, and frictional joints. The main purpose of this study is to investigate and compare the effect of ultrasonic nanocrystal surface modification (UNSM) and wonder process craft (WPC) on the surface and tribological properties of SUJ2 bearing steel. The surface roughness and hardness of the untreated and treated (UNSM- and WPC-treated) specimens were measured and compared. Their tribological properties were evaluated using a micro-tribometer under grease-lubricated and dry conditions against itself. Surface hardness measurement results revealed that both the UNSM- and WPC-treated specimens had a higher hardness than that of the untreated specimen. The surface roughness of the untreated specimen was reduced after UNSM and WPC treatments. Abrasive wear mode was observed on the surface of the specimens worn under grease-lubricated conditions, while adhesive wear mode was found on the surface of the specimens worn in dry conditions. According to the tribological test results, the friction coefficient and wear rate of the untreated specimens were reduced by the application of both the UNSM and WPC treatments under grease-lubricated and dry conditions.

• Tribology and Lubricants
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• v.38 no.1
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• pp.8-14
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• 2022

Amorphous carbon (a-C) has excellent wear resistance and, therefore is used as a coating to protect numerous mechanical components to prolong their lifetimes. Among the a-C coatings, diamond-like carbon (DLC) and DLC-containing silicon (Si-DLC) receive extensive attention owing to their enhanced wear resistance and low frictional characteristics. In this study, the friction and wear characteristics of DLC and Si-DLC coatings are analyzed. For comparative analysis, DLC-coated and Si-DLC-coated vanes are utilized with the counterpart of a roller for the friction tests. Since the lubricated mechanical components are generally vulnerable to wear when a lubricant film does not form properly, friction tests are conducted under boundary lubrication conditions to promote wear. A cylinder-on-cylinder type tribometer is used to perform the friction tests with various normal load conditions. After the friction test, a 3D laser confocal microscope is used for quantifying the wear volume to calculate the wear rate of each specimen. Consequently, the DLC-coated specimen shows a lower coefficient of friction (COF) and wear rate than the specimen without the coating, while the Si-DLC coating shows a higher COF than the bare specimen. The results of this study are expected to contribute to improving the efficiency and reliability of compressors.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.36 no.2
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• pp.121-130
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• 2023

Finite element analysis is widely used to predict the structural stability and tooth contact performance of gears. This study focused on the effect of finite element modeling conditions of a spur gear on the simulation result and the model simplification. The gear body and teeth, teeth width, configuration of mesh, frictional coefficient, and simulation time interval (gear mesh cycle division) were selected for model simplification for gear analysis. The static transmission error during a single-gear mesh cycle was calculated to represent the performance of the gear, and the elapsed time was measured as a simplification factor. Contact stress distribution was also checked. The differences in maximum transmission error and elapsed time depending on the model simplification methods were analyzed. After all simplification methods were estimated, an optimal combination of the methods was defined, and the result was compared with that of the most detailed modeling methods.

• Computers and Concrete
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• v.32 no.2
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• pp.217-232
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• 2023

Numerous studies have been performed on the behavior of pile foundations in cold regions. This study first attempted to employ artificial neural networks (ANN) to predict pile-bearing capacity focusing on pile data recorded primarily on cold regions. As the ANN technique has disadvantages such as finding global minima or slower convergence rates, this study in the second phase deals with the development of an ANN-based predictive model improved with an Elephant herding optimizer (EHO), Dragonfly Algorithm (DA), Genetic Algorithm (GA), and Evolution Strategy (ES) methods for predicting the piles' bearing capacity. The network inputs included the pile geometrical features, pile area (m²), pile length (m), internal friction angle along the pile body and pile tip (Ø°), and effective vertical stress. The MLP model pile's output was the ultimate bearing capacity. A sensitivity analysis was performed to determine the optimum parameters to select the best predictive model. A trial-and-error technique was also used to find the optimum network architecture and the number of hidden nodes. According to the results, there is a good consistency between the pile-bearing DA-MLP-predicted capacities and the measured bearing capacities. Based on the R2 and determination coefficient as 0.90364 and 0.8643 for testing and training datasets, respectively, it is suggested that the DA-MLP model can be effectively implemented with higher reliability, efficiency, and practicability to predict the bearing capacity of piles.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.12 no.4
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• pp.251-261
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• 2007

The wind-wave interaction around the Kyung-gi Bay, Korea, was studied using the observed data from ocean buoy at DeuckJeuck-Do from Jan. to Dec., 2005, and from waverider data at KeuckYeulBee-Do on Mar. 19-26 and May 23-28, 2005. Wind-driven surface waves and wave-driven wind speed decrease were estimated from the ocean buoy data, and the characteristics of wave spectrum response were also investigated from the waverider data for the wave developing and calm stages of sea surface, including the time series of spectrum pattern change, frequency trend of the maximum energy level and spectrum slope for the equilibrium state range. The wind speed difference between before and after considering the wave effect was about $2ms^{-1}$ (wind stress ${\sim}0.1Nm^{-2}$) for the wind speed range $5-10ms^{-1}$ and about $3ms^{-1}$ (wind stress ${\sim}0.4Nm^{-2}$) for the wind speed range $10-15ms^{-1}$. Correlation coefficient between wind and wave height was increased from 0.71 to 0.75 after the wave effect considered on the observed wind speed. When surface waves were generated by wind, the initial waves were short waves about 4-5 sec in period and become in gradual longer period waves about 9-10 sec. For the developed wave, the frequency of maximum energy was showed a constant value taking 6-7 hours to reach at the state. The spectrum slope for the equilibrium state range varied with an amplitude in the initial stage of wave developing, however it finally became a constant value 4.11. Linear correlation between the frictional velocity and wave spectrum for each frequency showed a trend of higher correlation coefficient at the frequency of the maximum energy level. In average, the correlation coefficients were 0.80 and 0.82 for the frequencies 0.30 Hz and 0.35 Hz, respectively.

• Polymer(Korea)
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• v.25 no.1
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• pp.133-141
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• 2001

The friction and wear properties of carbon-carbon composites made with different weight percent of $MoSi_2$ as an oxidation inhibitor were investigated using a constant speed wear test apparatus in an oxidation environment. The results indicated the carbon-carbon composites undergoing an abrupt transition of friction coefficient, from low-friction behavior(${\mu}$=0.15~0.2) during normal wear regime to the high-friction behavior(${\mu}$=0.5~0.6) during dusting wear regime at the frictional temperature range of 150~180${\circ}C$. The existence of temperature-dependent friction and wear regimes implied that the performance of specimen made with carbon-carbon composites was markedly affected by the thermal properties of the composites. The carbon-carbon composites filled with MoSi2 exhibited two times lower coefficient of friction and wear rate in comparison with the composites without $MoSi_2$. Especially, the composites containing 4wt% $MoSi_2$ filler showed a significantly improved activation energy for wear due to the reduction of both the porosity and powdery debris film formation on sliding surface when compared to those without $MoSi_2$.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.12
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• pp.887-895
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• 2018

This paper presents a braking model that can be used to design the safety distance of a train control system and a train braking system to increase the volume of traffic. For the braking model, a train set (electric multiple unit composed 6 cars) was tested. The factors that can affect the braking characteristics include the friction coefficient, braking pressure, and regenerative braking. The braking pressure was classified into service and emergency braking and reflected the characteristics of the vehicle. The external force acting on the running railway car was tested in accordance with KS R 9217, and the running resistance of the train is presented in the form of a polynomial. The dynamic behavior of the train running on a straight flat line was simulated using UM 8.3. The results were validated with experimental data, and the results were reasonable. With the validated model, a stopping distance was determined according to the initial braking speed and compared with the deceleration braking model. In addition, a safety distance for the train control system could be changed according to the frictional coefficient limits. These results are expected to be useful for analyzing the dynamic behavior of trains, and for analyzing various railway environments and improving the braking performance.

• Korean Journal of Fisheries and Aquatic Sciences
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• v.30 no.5
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• pp.700-707
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• 1997

In order to express exactly the total resistance of bottom trawl nets subjected simultaneously to the water flow and the bottom friction, the influence of frictional force was added to the formular for the flow resistance of trawl nets obtained by previous papev and the experimental data obtained by other investigators were analyzed by the formula. The analyzation produced the total resistance R (kg) expressed as $$R=4.5(\frac{S_n}{S_m})^{1.2}S\;v^{-1.8}+20(Bv)^{1.1}$$ where $S(m^2)$ was the wall area of nets, $S_m\;(m^2)$ the cross-sectional area of net mouths, $S_n\;(m^2)$ the area of nets projected to the plane perpendicular to the water flow, B (m) the made-up circumference at the fore edge of bag parts, and v(m/sec) the dragging velocity. From the viewpoint that expressing R in the form of $R=kSv^2$ was a usual practice, however, the resistant coefficient $k(kg{\cdot}sec^2/m^4)$ was compared with the factors influencing it by reusing the experimental data. The comparison gave that the coefficient k might be expressed approximately as a function of BL only and so the resistance R (kg) as $$R=18{\alpha}B^{0.5}L\;v^{1.5}$$ where L (m) was the made-up total length of nets and $\alpha=S/BL$. But the values of a in the nets did not deviate largely from their mean, 0.48, for all the nets and so the general expression of R (kg) for all the bottom trawl nets could be written as $$R=9\;B^{0.5}\;L\;v^{1.5}$$.

• The Journal of Korean Academy of Prosthodontics
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• v.56 no.2
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• pp.105-113
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• 2018

Purpose: The purpose of this study was to investigate the effects of the insertion depth of an immediately loaded implant on the stress distribution of the surrounding bone and the micromovement of the implant using the three-dimensional finite element analysis. Materials and methods: A total of five bone models were constructed such that the implant platform was positioned at the levels of 0.00 mm, 0.25 mm, 0.50 mm, 0.75 mm, and 1.00 mm depth from the crest of the cortical bone. A frictional coefficient of 0.3 and the insertion torque of 35 Ncm were simulated on the interface between the implant and surrounding bone. A static load of 178 N was applied to the provisional prosthesis with a vertical load in the axial direction and an oblique load at $30^{\circ}$ with respect to the central axis of the implant, then a finite element analysis was performed. Results: The implant insertion depth significantly affected the stress distribution on the surrounding bone. The largest micromovement value of the implant was $39.34{\mu}m$. The oblique load contributed significantly to the stress distribution and micromovement in comparison to the vertical load. Conclusion: Increasing the implant insertion depth was advantageous in dispersing the concentrated stress in the cortical bone and did not significantly affect the micromovement associated with early osseointegration failure.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.39 no.8
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• pp.765-772
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• 2015

Cast iron is widely used in fields such as the transport and heavy industries. For parts where contact damage is expected to occur, it is necessary to understand the friction and wear characteristics of cast iron. In this study, we use cast iron plates as the specimens to investigate their friction and wear characteristics. We perform various experiments using a reciprocating type tribotester. We assess the frictional characteristics by analyzing the friction coefficient values that were obtained during the sliding tests. We observe the wear surfaces of cast iron and steel balls using a scanning electron microscope, confocal microscope, and 3d profiler. We investigate the friction and wear characteristics of cast iron by injecting sand and alumina particles having various sizes. Furthermore, we estimate the effect of temperature on the friction and wear characteristics. The results obtained are expected to aid in the understanding of the tribological characteristics of cast iron in industry.