• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.1
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• pp.40-45
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• 2007

Modal analysis or modal test is a routine process to get the modal parameters of a dynamic system. The modal parameters include the natural frequencies, damping ratios and mode shapes. This paper presents a method that can derive the equations of motion for a dynamic system from the modal parameters obtained by the modal analysis or modal test. The present method based on the relation between the eigenvalues and eigenvectors of the state space equation derives the mass, damping and stiffness matrices of the system. The numerical verifications for the simple mass-spring-damper system and the cantilevered beam prove the efficiency and accuracy of the present method.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.6
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• pp.1391-1407
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• 1990

In this paper, optimization procedures are presented considering the static and dynamic constraints for laminated composite plate and hybrid laminated composite plate subject to concentrated load on center of the plates. Design variables for this problem are ply angle or ply thickness. Deflection, natural frequency and specific damping capacity are considered as constraints. Using a recursive linear programming method, the nonlinear optimization problems are solved. By introducing the design scaling factor, the number of iterations is reduced significantly. Composite plates could be designed optimally combined with FEM analysis under various conditions. In the optimization procedure, verification for both analysis and design of the laminated composite plates are compared with the results of the others. Various design results are presented for the laminated composite plates and hybrid laminated composite plates.

• Composites Research
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• v.12 no.2
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• pp.26-35
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• 1999

This paper presents the vibration damping characteristic of a single lap joint beam with partial dampers analyzed using the model strain energy method and the harmonic response analysis which were based on a finite element model. The two finite element analysis methods exhibited very similar results of the resonant frequency and system loss factor which were comparable to those by the theoretical analysis. Effects of the location of partial dampers and elastic moduli and thickness of their layers on the system loss factor were studied. The damping effects due to changes of modules and loss factor of the viscoelastic layer in lap joint and partial dampers were also studied. Consequently, the geometrical and material conditions at maximizing the system loss factor were suggested.

• Journal of the Earthquake Engineering Society of Korea
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• v.14 no.2
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• pp.67-74
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• 2010

To a series of vibration records obtained from experimental modal testing using a fixed hammer and roving accelerometers for greenhouse arch structures, modal parameters such as natural frequencies, damping ratios and mode shapes are extracted by applying the two most advanced system identification methods in the frequency-domain up to now, so-called PolyMAX and FDD. The former involves both input and output data, while the latter utilizes only the output data. The possibility of determining the static buckling load, detecting damages, etc., for very slender steel-pipe arches by means of a non-destructive testing method based on vibration measurements is primarily investigated. The extracted modal parameters generally correlated well with those obtained using finite element analysis, demonstrating promising results for further on-going research.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.14 no.7
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• pp.3197-3202
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• 2013

When car builder designs the large carbody structure of railway vehicles, it is necessary to optimize the damping characteristics through the analysis of structure borne noise such as sound pressure level(SPL). This paper is a study on the structure borne noise analysis by characterizing the damping of double skin aluminum extruded panels for rolling stock carbody. The normalized SPL was calculated based on the simple source theory using measured mechanical mobility parameters from vibration tests(i.e. point, transfer and modal mobility). The reduced SPL was predicted by using finite element method by applying loss factor of damping material into laminated shell elements. It was found out that the damping material coated on the panels like underframe, which part is seriously affected by vibration during train run, took effect to reduce noise level.

• Journal of the Korean Society for Railway
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• v.16 no.1
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• pp.1-6
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• 2013

In this study, the natural frequency and damping ratio of a four-harness satin woven glass/epoxy composite material are evaluated by means of modal tests and a finite element analysis. To achieve this goal, glass/epoxy beam specimens with different lengths and thicknesses were manufactured via autoclave curing. In the test, the maximum damping ratio was found to occur at the lowest test frequency. As the test frequency increased, the damping ratio decreased exponentially to a critical value. After that value, the damping ratio increased gradually to the maximum test frequency.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1995.10a
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• pp.778-781
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• 1995

In this paper, sound radiation of a rotary compressor used in room air-conditioner is investigated. structural eigenmodes are identified through modal test. Since accumulator is found important factor of sound radiation, damping material was added around accumulator to reduce sound radiation level of rotary compressor. As result, sound radiation was reduced to 3 dB.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.11
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• pp.2742-2751
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• 1993

Nonclassically damping domes from drastic variations of energy absorption rates of the materials in different parts of structures, or from the external damping sources inserted into the structures. In this study, an approximate method to calculate the frequency response of a method is superior to other approaches in respect of computational effort and accuracy. In addition, when frequency response is calculated by neglecting the off-diagonal elements of modal damping matrix, a criterion to ensure small errors is derived. In is shown that the criterion can be described as the vector sum of each modal coupling to the corresponding mode.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.15 no.3
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• pp.6-14
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• 2007

In this research, vibration reduction and aeroelastic stability of a composite hingeless rotor hub flexure with viscoelastic constrained layer damping treatment(CLDT) were investigated. The composite flexures with viscoelastic CLDT were applied to hingeless rotor system to improve the in-plane stability of the lead-lag motion causing resonance. The modal test was performed and dynamic properties(natural frequency and loss factor) were acquired. Also, complex eigenvalue analysis(SOLlO7) in the NASTRAN structural analysis module was performed and compared with results of the modal test. To insure aeroelastic stability, damping ratio analyses of the hingeless rotor system with CLDT were accomplished at hovering condition due to collective pitch angle changes. Satisfactory results of increasing structural damping and stability were obtained.

• Journal of the Earthquake Engineering Society of Korea
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• v.14 no.2
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• pp.57-65
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• 2010

To determine the static buckling loads and evaluate the structural performance of slender steel pipe-arches such as for greenhouse structures, a series of modal tests using a fixed hammer and roving sensors was carried out, by providing no load, then a range of vertical loads, on an arch rib in several steps. More attention was given to an internal arch where vertical and horizontal auxiliary members are not placed, unlike an end arch. Modal parameters such as natural frequencies, mode shapes and damping ratios were extracted using more advanced system identification methods such as PolyMAX (Polyreference Least-Squares Complex Frequency Domain), and compared with those predicted by commercial FEA (Finite Element Analysis) software ANSYS for various conditions. A good correlation between them was achieved in an overall sense, however the reduction of natural frequencies due to the existence of preaxial loads was not apparent when the vertical load level was about up to 38% of its resistance. Some difficulties related to the field testing and parameter extraction for a very slender arch, as might arise from the influences of neighboring members, are carefully discussed.