• Transactions of the Korean Society of Automotive Engineers
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• v.12 no.2
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• pp.160-167
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• 2004

A two degree-of-freedom mathematical model is presented to investigate the friction mechanism of a disc brake system. A contact parameter is introduced to describe the coupling between the in-plane and the out-of-plane motions. The model with the contact parameter is considered under the assumption that the out-of-plane motion depends on the friction force along the in-plane motion. In order to describe the relationship between the friction force and the out-of plane motion, the dynamic friction coefficient is considered as a function of both relative velocity and normal farce. Using this friction law, a contact stiffness matrix along the normal direction can be obtained. The out-of-plane motion is then investigated by both the stability analysis and the numerical analysis for various parametric conditions. The results show that the stiffness parameters of the pad and the disc must be controlled at the same time. Also, the numerical analysis shows the existence of limit cycle caused by the effect of intermittent contact stiffness.

• Tribology and Lubricants
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• v.17 no.5
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• pp.351-357
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• 2001

Tribological behavior of NAO type brake linings containing different volume ratios of graphite and Sb$_2$S$_3$ was investigated using a scale dynamometer. Three different test modes consisting of stop, drag, and fade tests were employed to elucidate the effect of the solid lubricants on brake performance. Results of this work showed that the two solid lubricants, graphite and Sb$_2$S$_3$, significantly affected friction characteristics at various braking situations. Compared with the brake linings containing a single lubricant, the brake linings containing both solid lubricants showed better friction stability due to the synergistic effect of the two disparate lubricants during the stop and the drag test. In particular, in the case of containing two solid lubricants, the brake lining with higher contents of graphite showed better fade resistance than others.

• Korean Chemical Engineering Research
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• v.53 no.5
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• pp.609-613
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• 2015

The onset of instability induced by impulsive spin-down of the rigid-body flow placed in the gap between two coaxial cylinders is analyzed by using the energy method. In the present stability analysis the growth rate of the kinetic energy of the base state and also that of disturbances are taken into consideration. In the present system the primary flow is a transient, laminar one. But for the Reynolds number equal or larger than a certain one, i.e. $Re{\geq}Re_G$ secondary motion sets in, starting at a certain time. For $Re{\geq}Re_G$ the dimensionless critical time to mark the onset of vortex instabilities, ${\tau}_c$, is here presented as a function of the Reynolds number Re and the radius ratio ${\eta}$. For the wide gap case of small ${\eta}$, the transient instability is possible in the range of $Re_G{\leq}Re{\leq}Re_S$. It is found that the predicted ${\tau}_c$-value is much smaller than experimental detection time of first observable secondary motion. It seems evident that small disturbances initiated at ${\tau}_c$ require some growth period until they are detected experimentally.

• Geomechanics and Engineering
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• v.16 no.4
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• pp.415-421
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• 2018

A location determining method is proposed for critical sliding surface in the stability analysis of the filling materials in karst caves. First, a preliminary location of the sliding surface is determined based on simulation results which includes displacement contour and plastic zone. The sliding surface will locate on the bottom contact interface when the friction angle is relative small. However, a weakened contact interface always becomes the critical sliding surface no matter what the friction angle is. Then when the friction angle becomes larger, the critical sliding surface inside fillings can be determined by a parabola, the coefficient of which increases linearly with the friction angle under the same cohesion. Finally, the critical sliding surface approximately remains unchanged with friction angle. The influence of cohesion is similar to that of friction angle. Although affected by shape, size or position of the karst cave, the critical sliding surface mainly depends on both friction angle and cohesion. Thus, this method is always useful in determining the critical sliding surface.

• Physical Therapy Korea
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• v.18 no.3
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• pp.59-66
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• 2011

The purpose of this study is to examine the activity ratios of global trunk muscles and local trunk muscles in relation to adjustments in the level of task difficulty while performing stability exercises in easily applied bridging lumbar stabilization exercise. Twenty healthy subjects performed bridging lumbar stabilization exercise while the level of task difficulty was plate was used in the same posture for all the exercises. EMG was used to examine the activity ratios of the global muscles and multifidus in relation to the level of task difficulty. Moreover, the activity ratios of the multifidus muscle, the erector spinae and the gluteus maximus muscle were measured. A one-way ANOVA with repeated measures was used, and a Bonferroni correction was conducted (${\alpha}$=.05). When the bridging lumbar stabilization exercise were performed at different difficulty levels, the activity of the multifidus muscle, which is a local muscle, was high in all three exercises. Also, compared to low intensity and intermediate intensity exercises, high intensity exercises showed more significant differences (${\alpha}$=.05). Among all the muscles, the multifidus showed the highest activity at intermediate intensity. Based on these results, we suggest that in the case of bridging lumbar stabilization exercise, low intensity or intermediate intensity exercises are more suitable and efficient for local muscle stabilization.

• Bulletin of the Korean Chemical Society
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• v.32 no.4
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• pp.1273-1276
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• 2011

Laponite samples intercalated with $Tb^{3+}$ or $Tb^{3+},Ce^{3+}$ ions were prepared by exchange of Na+ ions in interlayers with $Tb^{3+}$ or $Ce^{3+}$ ions. Strong green and weak blue emissions under vacuum ultraviolet (VUV) excitation (${\lambda}$ = 158 nm) were observed due to the $^5D_4{\rightarrow}^7F_J$ and $^5D_3{\rightarrow}^7F_J$ emission lines, respectively. $Tb^{3+}$ ions in an interlayer of laponite mainly existed in ion pairs or clusters, as evidenced by the concentration-dependent luminescence of the $Tb^{3+}$ ions on the relative intensities of the $^5D_3{\rightarrow}^7F_J$ and the $^5D_4{\rightarrow}^7F_J$ emission lines, due to the action of a cross-relaxation process. The addition of $Ce^{3+}$ ions increased the thermal stability of $Tb^{3+}$ intercalated laponite up to $650^{\circ}C$ and quenched the $^5D_3{\rightarrow}^7F_J$ emission lines, probably by promoting the formation of $Tb^{3+}$ ion pairs at relatively low $Tb^{3+}$ concentrations.

• Journal of the Korean Society for Precision Engineering
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• v.16 no.7
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• pp.151-157
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• 1999

Vibrations in machine tools make many problems in precision, production efficiency, and machine performance. The relative vibration between a workpiece and a tool is very complicated due to many sources. In this study, the dynamic characteristics of a newly developed CNC lathe were analyzed and its chatter characteristics were predicted by a chatter analysis method using finite element analysis and 3 dimensional cutting dynamics. The simulated results showed very complex characteristics of chatter vibration and the borderline of limiting depth of cut was used as the stability limit. To check the validity of this method, cutting tests were done in the CNC lathe using a boring bar as a tool because boring process is very weak due to long overhang . The experimental results showed that the simplified borderline was to be considered as limiting depth of cut at which the chatter vibration starts and the stability limits depended on various cutting parameters such as cutting speed, feed and nose radius of tool.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.16 no.12 s.117
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• pp.1252-1261
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• 2006

This paper summarizes theoretical work on the multichannel decentralized feedback control of sound radiation from aircraft trim panels using piezoceramic actuators. The aircraft trim panels are generally honeycomb structures designed to meet the design requirement of low weight and high stiffness. They are resiliently-mounted to the fuselage for the passive reduction of noise transmission. It is motivated by the localization of reduction in vibration of single channel active trim panels. 12-channel decentralized feedback control systems are investigated in terms of the reduction of noise and vibration for three configurations of sensor actuator pairs. Local coupling of the closely-spaced sensor and actuator pairs was modeled using single degree of freedom systems. The multichannel control system is characterized using the state-space model. For the stability point of view, the relative stability or robustness is evaluated by comparing the real part of eigenvalues of the system matrix for the three configurations. The control performance is also evaluated and compared for the three configurations. It is found that the multichannel system can lead to the globalization of the reduction in vibration and radiated noise. It does not appear to yield a significant improvement in the vibration because of decreased gain margin. However, the reduction in the radiated noise is remarkably improved due to the variation of the vibration pattern with the actuation configurations.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.9
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• pp.859-866
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• 2008

A guidance scheme for controlling the relative geometry of multiple flight vehicle formation flying is proposed. Each flight vehicle in the formation takes the roles of leader and follower simultaneously except for the formation leader. In this scheme, the flight commands for a leader are shared by all the followers and this leaders to a synchronized flight of all flight vehicles comprising the formation. Lyapunov stability theorem is used to obtain the guidance law. High fidelity nonlinear simulation results are presented to show the effectiveness of the proposed guidance law using a reconnaissance and surveillance mission example.

• Polymer(Korea)
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• v.37 no.3
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• pp.294-301
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• 2013

Polymersomes made of biodegradable triblock copolymers based on poly(fumaric acid-co-sebacoyl chloride)/PEG (PEG-co-P(FA/SC)-co-PEG) were prepared and studied in aqueous solutions. TEM confirmed the formation of vesicles in aqueous media. Aggregation behavior of the copolymers was studied by fluorescence spectroscopy of 8-anilino-1-naphthalenesulfonic acid, and the critical aggregation concentration (c.a.c.) of the copolymer was found to be ${\sim}26.2{\mu}M$ indicating desirable stability of the vesicles. Dynamic light scattering revealed that the size of the vesicles was distributed within the range of 170-270 nm. Turbidity measurements confirmed the relative short-term stability of the polymersomes. Carboxyfluorescein, a hydrophilic compound, was simply encapsulated in the vesicles during polymersome preparation. The release of encapsulant from the polymersomes at 25 and $37^{\circ}C$ lasted about 3 weeks, and the rate of release followed a first-order kinetics. The release is speculated to be primarily carried out through diffusion. These results confirm that these polymersomes are promising as controlled-release carriers of various drugs.