• Steel and Composite Structures
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• v.39 no.1
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• pp.95-107
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• 2021

In this work, the dynamic response of functionally graded beams on variable elastic foundations is studied using a novel higher-order shear deformation theory (HSDT). Unlike the conventional HSDT, the present one has a new displacement field which introduces undetermined integral variables. The FG beams were assumed to be supported on Winkler-Pasternak type foundations in which the Winkler modulus is supposed to be variable in the length of the beam. The variable rigidity of the elastic foundation is assumed to be linear, parabolic and sinusoidal along the length of the beam. The material properties of the FG porous beam vary according to a power law distribution in terms of the volume fraction of the constituents. The equations of motion are determined using the virtual working principle. For the analytical solution, Navier method is used to solve the governing equations for simply supported porous FG beams. Numerical results of the present theory for the free vibration of FG beams resting on elastic foundations are presented and compared to existing solutions in the literature. A parametric study will be detailed to investigate the effects of several parameters such as gradient index, thickness ratio, porosity factor and foundation parameters on the frequency response of porous FG beams.

• Journal of Korean Association for Spatial Structures
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• v.22 no.2
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• pp.57-64
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• 2022

In this paper, the physical model and governing equations of a shallow arch with a moving boundary were studied. A model with a moving boundary can be easily found in a long span retractable roof, and it corresponds to a problem of a non-cylindrical domain in which the boundary moves with time. In particular, a motion equation of a shallow arch having a moving boundary is expressed in the form of an integral-differential equation. This is expressed by the time-varying integration interval of the integral coefficient term in the arch equation with an un-movable boundary. Also, the change in internal force due to the moving boundary is also considered. Therefore, in this study, the governing equation was derived by transforming the equation of the non-cylindrical domain into the cylindrical domain to solve this problem. A governing equation for vertical vibration was derived from the transformed equation, where a sinusoidal function was used as the orthonormal basis. Terms that consider the effect of the moving boundary over time in the original equation were added in the equation of the transformed cylindrical problem. In addition, a solution was obtained using a numerical analysis technique in a symmetric mode arch system, and the result effectively reflected the effect of the moving boundary.

• Proceedings of the KIPE Conference
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• 1998.10a
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• pp.961-965
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• 1998

The rapid development of small-scale (1-10 MJ) Superconducting Magnetic Energy Storage Systems (SMES) can be explained by real perspective of practical implementation of these devices in electro power nets. However the serious problem of all high mechanically stressed superconducting coils-problem of training and degradation (decreasing) of operating current still exists. Moreover for SMES systems this problems is more dangerous because of pulsed origin of mechanical stresses-one of the major sources of local heat disturbances in superconducting coils. We investigated acoustic emission (AE) phenomenon on model and 0.5 MJ SMES coils taking into account close correlation of AE and local heat disturbances. Two-coils 0.5 MJ SMES system was developed, manufactured and tested at Russian Research Center in the frames of cooperation with Korean Electrical Engineering Company (KEPCO) [1]. The two-coil SMES operates with the stored energy transmitted between coils in the course of a single cycle with 2 seconds energy transfer time. Maximum operating current 1.55 kA corresponds to 0.5 MF in each coil. The Nb-Ti-based conductor was designed and used for SMES manufacturing. It represents transposed cable made of Nb-Ti strands in copper matrix, several cooper strands and several stainless steel strands. The coils are wound onto fiberglass cylindrical bobbins. To make AE event information more useful a real time instrumentation system was used. Two main measured and computer processed AE parameters were considered: the energy of AE events (E) and the accumulated energy of AE events (E ). Influence of current value in 0.5 MJ coils on E and E was studied. The sensors were installed onto the bobbin and the external surface of magnets. Three levels of initial current were examined: 600A, 1000A, 2450 A. An extraordinary strong dependence of the current level on E and E was observed. The specific features of AE from model coils, operated in sinusoidal vibration current changing mode were investigated. Three current frequency modes were examined: 0.012 Hz, 0.03 Hz and 0.12 Hz. In all modes maximum amplitude 1200 A was realized.

• Journal of KSNVE
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• v.8 no.5
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• pp.964-973
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• 1998

In this paper we propose a levitation and imbalance compensation controller design methodology of magnetic bearing system. In order to achieve levitation and elimination of unbalance vibartion in some operation speed we use the discrete-time Q-parameterization control. When rotor speed p = 0 there are no rotor unbalance, with frequency equals to the rotational speed. So in order to make levitatiom we choose the Q-parameterization controller free parameter Q such that the controller has poles on the unit circle at z = 1. However, when rotor speed p $\neq$ 0 there exist sinusoidal disturbance forces, with frequency equals to the rotational speed. So in order to achieve asymptotic rejection of these disturbance forces, the Q-parameterization controller free parameter Q is chosen such that the controller has poles on the unit circle at z = $exp^{ipTs}$ for a certain speed of rotation p ( $T_s$ is the sampling period). First, we introduce the experimental setup employed in this research. Second, we give a mathematical model for the magnetic bearing in difference equation form. Third, we explain the proposed discrete-time Q-parameterization controller design methodology. The controller free parameter Q is assumed to be a proper stable transfer function. Fourth, we show that the controller free parameter which satisfies the design objectives can be obtained by simply solving a set of linear equations rather than solving a complicated optimization problem. Finally, several simulation and experimental results are obtained to evaluate the proposed controller. The results obtained show the effectiveness of the proposed controller in eliminating the unbalance vibrations at the design speed of rotation.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.21 no.5
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• pp.465-474
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• 2008

The real-time hybrid testing method(RT-HYTEM) is a structural testing technique in which the numerical integration of the equation of motion for a numerical substructure and the physical testing for an experimental substructure are performed simultaneously in real-time. This study presents the quantitative evaluation of the seismic performance of a building structure installed with an passive and semi-active MR damper by using RT-HYTEM. The building model that was identified from the force-vibration testing results of a real-scaled 5-story building is used as the numerical substructure, and an MR damper corresponding to an experimental substructure is physically tested by using the universal testing machine(UTM). The RT-HYTEM implemented in this study is validated because the real-time hybrid testing results obtained by application of sinusoidal and earthquake excitations and the corresponding analytical results obtained by using the Bouc-Wen model as the control force of the MR damper respect to input currents were in good agreement. Also for preliminary study, some semi-active control algorithms were applied to the MR damper in order to control the structural responses optimally. Comparing between the test results of semi-active control using RT-HYTEM and numerical analysis results show that the RT-HYTEM is more resonable than numerical analysis to evaluate the performance of semi-active control algorithms.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.4
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• pp.489-496
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• 2019

This paper is a study on the evaluation of loss factor of damping treatment materials to reduce the noise and vibration for panels of railway vehicles and automobiles. In order to determine the modal parameters of damping materials, beam excitation tests were carried out using different type PVC coated aluminum and steel base beam specimens. The specimens were excited from 10 Hz to 1000 Hz frequency range using sinusoidal force, and transfer mobility data were measured by using an accelerometer. The loss factors were determined by using integrated program, based on theories of Half Power Method, Minimum Tangent Error Method, Minimum Angle Error Method and Phase Change Method, which enable to evaluate the parameters using modal circle fit and least squares error method. In the case of lower loss factor and data of linear characteristics, any method could be applied for evaluation of parameters, however the case of higher loss factor or data including non-linear characteristics, the minimum angle error method could reduce the loss factor evaluation. The obtained dynamic properties of the coating material could be used for application of Finite Element Method analyzing the noise control effects of complex structures such as carbody or under-floor boxes of rolling stock. The damping material will be very useful to control the structural noise, because the obtained modal loss factors of each mode show very good effect on over $2^{nd}$ mode frequency range.