• Journal of the Korean Society for Precision Engineering
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• v.29 no.6
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• pp.632-639
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• 2012

A fully coupled thermo-mechanical model is adopted to study the temperature distribution and the material deformation in friction stir welding(FSW) process. Rotational speed is most important parameters in this research. Three dimension results under different process parameters were presented. Result indicate that the maximum temperature is lower than the melting point of the welding material. The higher temperature gradient occurs in the leading side of the workpiece. The maximum temperature can be increased with increasing the tool angular velocity, rpm in the current numerical modeling. In this research ABAQUS Ver.6.7 is to analyze a fully coupled thermo-mechanical model. ALE(Arbitrary Lagrangian-Eulerian) finite element formulation is used for the large deformation in FSW process and using the Mass scaling for the analysis time efficiency.

• Journal of Welding and Joining
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• v.18 no.5
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• pp.84-89
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• 2000

Friction welding of titanium and aluminium is numerically modeled by the axisymmetric thermal elastic-plastic analysis. In titanium/aluminium friction welding, heat transfers into the titanium substrate to a distance of z=10(mm) on the side of the bondline and into the whole region of the aluminium substrate having the large thermal conductivity. Adjacent to the bondline, $^{\sigma}r\;and\;^{\sigma\theta}$ are tensile in the substrate whose thermal shrinkage is large, and are compressive in the substrate whose thermal shrinkage is small. $\sigma_z$ along the radial direction is large tensile at the periphery of the component. Plastic strain occurs only close to the bondline in the aluminium substrate. In the components of plastic strain, $\varepsilon^p_r\;and\;\varepsilon^p_{\theta}$ have positive values and $\varepsilon^p_r$ has large negative value. However, $\varepsilon^p_r$ is produced not because of the severity of the mechanical restraint condition, but on purpose to satisfy the condition of the volume constant. A plastic work is proposed as a measure to evaluate the mechanical severity. The plastic work is larger in the aluminium substrate than that in the titanium substrate. The mechanical condition is severer in the aluminium substrate.

• Structural Engineering and Mechanics
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• v.10 no.6
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• pp.621-633
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• 2000

Dynamic responses of a bridge system with several simple spans under longitudinal seismic excitations are examined. The bridge system is modeled as the multiple oscillators and each oscillator consists of four degrees-of-freedom system to implement the poundings between the adjacent oscillators and the friction at movable supports. Pounding effects are considered by introducing the impact elements and a bi-linear model is adopted for the friction force. From the parametric studies, the pounding is found to induce complicated seismic responses and to restrain significantly the relative displacements between the adjacent units. The smaller gap size also restricts more strictly the relative displacement. It is found that the relative displacements between the abutment and adjacent pier unit became much larger than the responses between the inner pier units. Consequently, the unseating failure could take a place between the abutment and nearby pier units. It is also found that the relative displacements of an abutment unit to the adjacent pier unit are governed by the pounding at the opposite side abutment.

• International Journal of Highway Engineering
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• v.12 no.2
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• pp.25-31
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• 2010

Road design guideline provides the directions on how to design a road alignment based on design speed, and this guideline has a design expectation in that design speed is supposed to be equal to the operating speed of drivers. Horizontal curve design is also based on design speed, and minimum radius is derived based on the drivers comfort while negotiating the curve. However, side friction reflecting drivers comfort is lower than a physical friction measured on wet road surface, therefore, it is reasonable to regard the criterion on minimum radius has a safety margin. Futhermore, the practical preference of choosing the larger radius than minimum leads to a noticeable gap between design speed and operating speed, so links to the violation of design expectation implicated in the guideline. In order to review this assumption, friction and operating speed at rural roads was measured and observed. As the results, a safety margin brought out by the gap between comfort-based friction and measured friction is qualitatively derived. Also, the gap between design speed and operating speed presumably caused by the safety margin and practical preference is analysed. By this, it is suggested that current design guideline should provides not only the minimum radius but also the management of road alignment design to minimize the gap between the design speed and operating speed.

• Steel and Composite Structures
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• v.28 no.6
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• pp.759-766
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• 2018

Friction Stir Welding (FSW) is a solid-state process, where the objects are joined together without reaching their melting point. It has been shown that this method is a suitable way to join dissimilar aluminium alloys. The current article employed hole drilling technique to measure the residual stress distribution experimentally in different zones of dissimilar aluminium alloys AA6061-T6 and AA7075-T6 Butt welded using FSW. Results are compared with those of similar AA6061-T6 plates joined using a conventional fusion welding method called tungsten inert gas (TIG). Also, the evolution of the residual stresses in the thickness direction was investigated, and it was found that the maximum residual stresses are below the yield strength of the material in the shoulder region. It was also revealed that the longitudinal residual stresses in the joint were much larger than the transverse residual stresses. Meanwhile, Vickers micro hardness measurements were performed in the cross-section of the samples. The largest hardness values were observed in the stir zone (SZ) adjacent to the advancing side whereas low hardness values were measured at the HAZ of both alloys and the SZ adjacent to the retreating side.

• Journal of the Korean Geotechnical Society
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• v.35 no.2
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• pp.19-27
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• 2019

In this study, an analytical model is proposed to estimate the buckling responses of tapered piles. The governing differential equation of the soil-pile system considering the tapering and side friction of the pile and the soil nonhomogeneity is derived, which is numerically integrated by the Runge-Kutta method and then the eigenvalue of bucking load is determined by Regula-Falsi algorithm. For a cylindrical pile, the results obtained from this study are found to compare well with those reported in literature. Illustrative examples for buckling load and stress as well as buckled shape are provided to investigate the effects of dimensionless parameters related to the soil-pile system.

• Journal of Korean Society of Transportation
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• v.22 no.7 s.78
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• pp.25-32
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• 2004

It is implicated in the current road design manual that design speed is well representing the operating speed of drivers. On the contrary, the disparity between the design speed and operating speed means that current road design cannot guarantee the safety and comfort of driving. In this context, operating speed was observed at the two lane rural roadways to find out the relation of operating speed and design speed. In addition to this, the friction factor from supply and demand which was derived from the operating speed was compared in terms of the dynamics in driving safety factor. It was concluded that the actual speed was consistently exceeding the design speed at the horizontal curves having the radius less than or equal to 200 m. Similarly, the demanded side friction was also consistently exceeding the supplied side friction at the horizontal curves having the radius less than or equal to 200 m and the amount of difference was also inversely proportional to the radius of the horizontal curves.

• Structural Engineering and Mechanics
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• v.48 no.3
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• pp.367-382
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• 2013

Different types of long slender pile shall buckle with weak soil and liquefied stratum surrounded. Different from considering single side earth pressure, it was suggested that the lateral earth pressure can be divided into two categories while buckling: the earth pressure that prevent and promotes the lateral movement. Active and passive earth pressure calculation model was proposed supposing earth pressure changed linearly with displacement considering overlying load, shaft resistance, earth pressure at both sides of the pile. Critical buckling load calculation method was proposed based on the principle of minimum potential energy quoting the earth pressure calculation model. The calculation result was contrasted with the field test result of small diameter TC pile (Plastic Tube Cast-in-place pile). The fix form could be fixed-hinged in the actual calculation assuring the accuracy and certain safety factor. The contributions of pile fix form depend on the pile length for the same geological conditions. There exists critical friction value in specific geological conditions that the side friction has larger impact on the critical buckling load while it is less than the value and has less impact with larger value. The buckling load was not simply changed linearly with friction. The buckling load decreases with increased limit active displacement and the load tend to be constant with larger active displacement value; the critical buckling load will be the same for different fix form for the small values.

• The Journal of Korea Robotics Society
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• v.9 no.1
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• pp.20-27
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• 2014

In this paper, an exact reshaping method for the motor dynamics of a flexible-joint robot is proposed using an integral manifold approach. Obtaining the exact model for both motor-side and link-side dynamics of a flexible-joint robot is difficult due to its under-actuated nature and complex dynamics. Despite the simple structure of the motor-side dynamics, they are difficult to model accurately for a flexible-joint robot due to motor disturbances, especially when speed reducers such as harmonic drives are installed. An integral manifold feedback control (IMFC) is proposed to reshape the motor dynamics. Based on the integral manifold approach, it is theoretically proved that the IMFC reshapes motor dynamics exactly even with bounded disturbances such as motor friction. The performance of the proposed IMFC is verified experimentally using a single degree-of-freedom flexible-joint robot under gravity conditions.

• Journal of Navigation and Port Research
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• v.33 no.3
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• pp.227-233
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• 2009

This experimental study has been carried out to investigate the reduction in side friction of open caisson using air-jet method. For this study, the large size caisson having air-pockets was used. This caisson was used as substructure of bridge. By using air-jet method, settlement time was decreased and even settlement was reached to the depth which conventional methods could not be. We found that the side resistance decreases to less then half the expected value. In addition to these merits, the air-jet method can result in self-weight settlement without applying additional load. Also the open caisson damage caused by blasting can be prevented with this method.