Daejeon, South Korea
Daejeon, South Korea

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Li W.,Pusan National University | Cho Y.,Pusan National University | Ju T.,Pusan National University | Choi H.S.,Korean Air R and nter | And 2 more authors.
RILEM Bookseries | Year: 2012

This study introduces a new approach to evaluate material degradation caused by thermal fatigue in composite laminates by using nonlinear Lamb waves. The correlation between normalized nonlinear parameters and degradation states is studied. S1 Lamb mode which has phase and group matching features is chosen for the practical generation of nonlinear Lamb wave. The measured acoustics normalized nonlinearity is directly related to material degradation. It could be concluded that it is possible to use nonlinear Lamb waves to evaluate composite laminates degradation. © RILEM 2013.


Kim D.-H.,Korean Air R and nter | Chang J.-W.,Korea Aerospace University | Kim H.-B.,Korea Aerospace University
Journal of Aircraft | Year: 2013

The lift and pressure-drag coefficients integrated from unsteady-pressure coefficients are investigated for a pitching airfoil at a low Reynolds number. The unsteady pressure on the pitching-airfoil surface is measured through pressure-distortion correction in the pneumatic tubing, and the pressure-distortion characteristics are estimated. The unsteady pressure on the pitching airfoil is significantly delayed with reduced frequency, and the nondimensional time delay is evaluated with respect to unsteady pressure. The hysteresis loops of the lift and pressure-drag coefficients rotate in counterclockwise and clockwise directions, respectively, and this is due to the reattachment during downstroke. Moreover, the angle of attack range, in which aerodynamic performance is nonlinearly varied, is generated due to the nonlinear boundary-layer behavior caused by the low-Reynolds-number effect. As a result, the unsteady aerodynamic characteristics at Rec = 4.8 × 104 are dominantly influenced by the reattachment and the low-Reynolds-number effect.


Choi H.S.,Korean Air R and nter | Jang Y.H.,Yonsei University
Applied Composite Materials | Year: 2011

The effects of interphase between fibers and matrix on the micro-And macromechanical behaviors of fiber-reinforced composite lamina subjected to transverse shear load at remote distance have been studied. The interphase has been modeled by the compliant spring-layers that are linearly related to the normal and tangential tractions. Numerical analyses on composite basic cells have been carried out using the boundary element method. For undamaged composites the micro-level stresses at the matrix side of the interphase and effective shear modulus have been calculated as a function of the fiber volume fraction and the interphase stiffness. Results are presented for various interphase stiffnesses from perfect bonding to total debonding. For a square array composite results show that for a high interphase stiffness k>10, an increase in a fiber volume fraction results in a higher effective transverse shear modulus. For a relatively low interphase stiffness k<1, it is shown that an increase in the fiber volume fraction causes a decrease in the effective transverse shear modulus. For perfect bonding, the effective shear modulus for a hexagonal array composite is slightly larger than that for a square array composite. Also for the damaged composite with partially debonded interphase, local stress fields and effective shear moduli are calculated and a decrease in the effective shear modulus has been observed. © Springer Science+Business Media B.V. 2010.


Shin C.,Korea Advanced Institute of Science and Technology | Park H.,Korea Advanced Institute of Science and Technology | Kang J.,Korea Advanced Institute of Science and Technology | Nam E.,Korean Air R and nter
2010 International ITG Workshop on Smart Antennas, WSA 2010 | Year: 2010

This paper deals with bit-loading techniques for a multiple-input multiple-output bit-interleaved coded orthogonal frequency division multiplexing (MIMO BIC-OFDM) system. The per-stream coded bit loading algorithm is proposed based on a spatial domain power adaptation. For each spatial stream, a different power constraint is assigned using an initial power allocation (IPA) combined with the residual power diversion (RPD). Simulation results verify that the proposed bit loading scheme yields significant capacity gain compared to the existing bit loading algorithms. We also discuss a channel decomposition technique to reduce the feedback overhead. ©2010 IEEE.


Park S.Y.,Korea Aerospace University | Choi W.J.,Korea Aerospace University | Choi H.S.,Korean Air R and nter | Kwon H.,Korea Aerospace University
Journal of Materials Processing Technology | Year: 2010

In this paper the effects of surface morphology and voids formed in the metal sheet-prepreg interface of GLARE laminates were evaluated. The experiments were carried out with aluminum sheets on which the surface morphology was systematically varied by different roughness levels of surface textures (sanding and nylon-pad abrasion) and chemical etches. The surface morphology of the substrates was characterized with profilometry and the static contact angle technique. Void contents at the metal sheet-prepreg interfaces were determined by scanning electron microscopy. The changes in surface roughness and surface energy effectively enhance the bonding strength of GLARE laminates. Experimental results also indicate that the autoclave pressure enhances GLARE interfacial bonding, to a certain extent, with low void content. © 2010 Elsevier B.V. All rights reserved.


Park S.Y.,Korea Aerospace University | Choi H.S.,Korean Air R and nter | Choi W.J.,Korea Aerospace University | Kwon H.,Korea Aerospace University
Composites Part B: Engineering | Year: 2012

The effects of the low earth orbit environment on three types of unidirectional high-modulus carbon fiber (M40 J, M55 J and M60 J)-reinforced composites were determined in detail. The synergistic environmental factors were the vacuum environment and thermal cycling. Cyclic thermal loading was performed in the temperature range between 120 °C and -175 °C for up to 2000 cycles under the high-vacuum state of 1.3 -3 Pa. The material responses were characterized through an assessment of the physical, thermal and mechanical property changes. It follows from the experimental results presented that the synergistic actions of the vacuum and the thermal cycling on the composite property degradation can be attributed to the formation of microvoids and interfacial sliding at the fiber-matrix interface in the early stages of cycling. The implications of these degradation processes based on the dependence of composite properties on vacuum thermal cycling are also discussed. © 2011 Elsevier Ltd. All rights reserved.


Hwang J.H.,Sejong University | Hwang S.,Sejong University | Hong S.K.,Sejong University | Yoo M.G.,Korean Air R and nter
Journal of Institute of Control, Robotics and Systems | Year: 2012

This paper focuses on attitude stabilization performance improvement of the quadrotor flying robot. First, the dynamic model of quadrotor flying robot was estimated through PEM (Prediction Error Method) using experimental input/output data. And attitude stabilization performance was improved by increasing the generation frequency of PWM signal from 50 Hz to 500 Hz. Also, the controller is implemented using a standard PID (Proportional-Integral-Derivative) controller augmented with feedback on angular acceleration, allowed the gains to be significantly increased, yielding higher bandwidth. Improved attitude stabilization performance is verified by experiment. © ICROS 2012.

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