• Structural Engineering and Mechanics
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• v.74 no.2
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• pp.175-187
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• 2020

The theories having been developed thus far account for higher-order variation of transverse shear strain through the depth of the beam and satisfy the stress-free boundary conditions on the top and bottom surfaces of the beam. A shear correction factor, therefore, is not required. In this paper, the effect of surface on the axial buckling and free vibration of nanobeams is studied using various refined higher-order shear deformation beam theories. Furthermore, these theories have strong similarities with Euler-Bernoulli beam theory in aspects such as equations of motion, boundary conditions, and expressions of the resultant stress. The equations of motion and boundary conditions were derived from Hamilton's principle. The resultant system of ordinary differential equations was solved analytically. The effects of the nanobeam length-to-thickness ratio, thickness, and modes on the buckling and free vibration of the nanobeams were also investigated. Finally, it was found that the buckling and free vibration behavior of a nanobeam is size-dependent and that surface effects and surface energy produce significant effects by increasing the ratio of surface area to bulk at nano-scale. The results indicated that surface effects influence the buckling and free vibration performance of nanobeams and that increasing the length-to-thickness increases the buckling and free vibration in various higher-order shear deformation beam theories. This study can assist in measuring the mechanical properties of nanobeams accurately and designing nanobeam-based devices and systems.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.26 no.6_spc
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• pp.714-720
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• 2016

Rubbing and misalignment are the most usual faults that occurs in rotating machinery and with them severe effect on power plant availability. Especially blade rubbing is hard to detect on FFT spectrum using the vibration signal. In this paper, the possibility of feature analysis of vibration signal is confirmed under blade rubbing and misalignment condition. And the lab-scale rotor test device provides the blade rubbing and shaft misalignment modes. Feature selection based on GA (genetic algorithm) is processed by the extracted feature of the time domain. Then, classification of the features is analyzed by using SVM (support vector machine) which is one of the machine learning algorithm. The results of features selection based on GA compared with those based on PCA (principal component analysis). According to the results, the possibility of feature analysis is confirmed. Therefore, blade rubbing and shaft misalignment can be diagnosed by feature of vibration signal.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2004.11a
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• pp.699-704
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• 2004

Empirical mode decomposition(EMD) method has been recently proposed to analyze non-linear and non-stationary data. This method allows the decomposition of one-dimensional signals into intrinsic mode functions(IMFs) and is used to calculate a meaningful multi-component instantaneous frequency. In this paper, it is assumed that each mode of damped vibration signal could be well separated in the form of IMF by EMD. In this case, we can have a new powerful method to calculate natural frequencies and dampings from damped vibration signal which usually has multiple modes. This proposed method has been verified by both simulation and experiment. The result by EMD method which has used only output vibration data is almost identical to the result by FRF method which has used both input and output data, thereby proving usefulness and accuracy of the proposed method.

• Special Issue of the Society of Naval Architects of Korea
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• 2015.09a
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• pp.56-62
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• 2015

In ship structure, many parts are in contact with inner or outer fluid as stern, ballast and oil tanks. Fatigue damages are sometimes observed in these tanks which seem to be caused by resonance with exciting force of engine and propeller. Vibration characteristics of these tanks in contact with fluid are significantly affected by fluid coupling effect. Therefore it is important to exactly predict vibration characteristics of tank structure. In order to estimate the vibration characteristics, the fluid-structure interaction(FSI) problem should be solved precisely. But it is difficult to estimate exactly the magnitude of the fluid coupling effect because it has some problems such as a fluid-structure interaction, influence by the free surface, vibration modes of structural panels and depth of water. In this paper, with fluid coupling effect, the effect of structural constraint between panels on the vibration characteristics are investigated numerically and discussed.

• Advances in nano research
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• v.14 no.6
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• pp.521-525
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• 2023

This paper concerned with the vibration of double walled carbon nanotubes (CNTs) as continuum model based on Timoshenko-beam theory. The vibration solution obtained from Timoshenko-beam theory provides a better presentation of vibration structure of carbon nanotubes. The natural frequencies of double-walled CNTs against half axial wave mode are investigated. The frequency decreases on decreasing the half axial wave mode. The shape of frequency arcs is different for various lengths. It is observed that model has produced lowest results for C-F and highest for C-C. A large parametric study is performed to see the effect of half axial wave mode on frequencies of CNTs. This numerically vibration solution delivers a benchmark results for other techniques. The comparison of present model is exhibited with previous studies and good agreement is found.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2023.11a
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• pp.227-228
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• 2023

A floating floor generally consists of mortar bed separated from the structural RC slab by a continuous resilient layer. It is known that the floating floors are a type of vibration-isolation system to improve the impact sound insulation performance. However, some researchers have demonstrated that the amplification of vibration response at a specific range of frequencies results in an increase in the impact sound level. This study carried out the forced vibration tests to obtain the frequency response function (FRF) of a floating floor compared with a bare RC slab. Test results shows that the additional peak occur in vibrational spectrum of the floating floor except natural vibration modes of the bare RC slab. This is because the relatively flexible resilient material and mass of the mortar bed offer an additional degree of freedom in the structural system. Therefore, it could be efficient for reduction of floor impact vibration and noise to control the additional mode frequency and response of floating floors.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1996.04a
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• pp.361-369
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• 1996

The accurate prediction of modal parameters of a rotating blade is an important requirement in the assessment of the dynamics of a helicopter rotor. Indeed, predictions of flight loads and stability are normally dependent on initially predicting the undamped mode shapes. A measuring technique, known as Strain Pattern Analysis (SPA), appears to be the most successful technique for measuring the mode shapes of rotating blades. This method was developed to be used on actual aircraft so no attempt was made to measure rotating mode shapes directly in order to validate the SPA method. This report summarizes results from experimental investigations which were carried out to validate the SPA method for the prediction of aerodynamically damped modes of a rotating blade. A series of modal tests were carried out on two rotor models in which the non-rotating, undamped and aerodynamically damped rotating modes were measured directly (strain and displacement patterns). It is shown that the SPA method to be very successful in itself but there are a number of limitations in validating this technique. To provide data which could be used to confidently validate theoretical prediction codes, existing limitations should be addressed.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.12 no.4
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• pp.302-309
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• 2002

A FRF-based substructuring method attempts to predict the dynamic characteristics of a complex structure from predetermined FRFs of the comprising uncoupled substructures. Although this method has the advantage of being able to incorporate experimental component FRFs directly, it is prone to errors : measurement errors, coordinate incompleteness, modal incompleteness, etc. Among the various sources of errors, this paper deals with the problem of modal incompleteness (or residual problem) of which importance is underestimated compared to others. It is a well-known rule of thumb that such a problem can be overcome by including modes up to 2 or 3 times the upper frequency of interest. Using a simulated case study, it is demonstrated that even including modes up to 20 times the upper frequency of interest does not guarantee a satisfactory result. A method to compensate the residual errors is introduced. This method requires the whole FRF matrices of substructures which is practically impossible for a complex structure. An applicable alternative is suggested and applied successfully to the case study. Finally, the effects of measurement errors on the residual compensation are also discussed.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1998.04a
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• pp.444-449
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• 1998

A modal transducer in two-dimensional structure can be implemented by varying the distributed transducer's gain spatially. In this paper, a method based on finite element method is developed for optimizing spatial gain distribution of PVDF transducer to create the modal transducer for specific modes. Using this concept, one can design the modal transducer in two-dimensional structure having arbitrary geometry and boundary conditions. As a practical means for implementing this continuous gain distribution without repoling die PVDF film, the gain distribution is approximated by optimizing gain-weights of interface circuit. The whole spatial area of the PVDF film is divided into several electrode segments and the signals from each segment are properly weighted and summed by interface circuit. This corresponds to the approximation of a continuous function using discrete values. The electrode partition is optimized using the genetic algorithm. Gain-weights are optimized using the simplex search method. A modal sensor for first to fourth modes of aluminum plate is designed using PVDF film with gain-weighted interface circuit. Various lamination angles of PVDF film are taken into consideration to utilize the anisotropy of the PVDF film. Performance of the optimized' PVDF sensor is demonstrated by numerical simulations..

• Geomechanics and Engineering
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• v.11 no.3
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• pp.421-438
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• 2016

Explosions inside transportation tunnels might result in failure of tunnel structures. This study investigated the failure mechanisms of circular cast-iron tunnels in saturated soil subjected to medium internal blast loading. This issue is crucial to tunnel safety as many transportation tunnels run through saturated soils. At the same time blast loading on saturated soils may induce residual excess pore pressure, which may result in soil liquefaction. A series of numerical simulations were carried out using Finite Element program LS-DYNA. The effect of soil liquefaction was simulated by the Federal Highway soil model. It was found that the failure modes of tunnel lining were differed with different levels of blast loading. The damage and failure of the tunnel lining was progressive in nature and they occurred mainly during lining vibration when the main event of blast loading was over. Soil liquefaction may lead to more severe failure of tunnel lining. Soil deformation and soil liquefaction were determined by the coupling effects of lining damage, lining vibration, and blast loading. The damage of tunnel lining was a result of internal blast loading as well as dynamic interaction between tunnel lining and saturated soil, and stress concentration induced by a ventilation shaft connected to the tunnel might result in more severe lining damage.