• Journal of the Korean Society of Manufacturing Process Engineers
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• v.19 no.3
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• pp.36-41
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• 2020

Mechanical systems installed in transport devices, such as vehicles, airplanes, and ships, are mostly subject to translational accelerations at the joints during operations. This base acceleration excitation has a large influence on the performance of the system, therefore, its response must be well analyzed. However, the existing methods for dynamic analysis of structures have some limitations in use. This study presents a new numerical method using relative acceleration to solve these limitations. If the governing equation of motion is linear and the mass matrix, the damping matrix, and the stiffness matrix are constant over time in the finite element analysis, the proposed method can be applied to the transient behavior analysis and the harmonic response analysis of the structure. Because it is not necessary to introduce a virtual mass and the rigid body motions are removed from the analysis, it is possible to use not only the direct integration method in the time domain but also the mode superposition method to obtain the dynamic responses. This paper demonstrates with three examples how the present method is suitable for the dynamic analysis of a structure with multi-degree of freedom.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2005.11a
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• pp.690-695
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• 2005

Flexible circular cantilever beams when excited externally introduce a lot of dynamic characteristics. The non-linear elements that these flexible beams develop include non-linearity due to inertia terms, spring, and damping. They show different characteristics of motion from each other. In the modes of lower order, the non-linearity due to spring is prevalent, while the non-linearity due to inertia Is prevalent in the modes of higher order. To analyze these effects the non-linear phenomena are analyzed experimentally. When the response characteristics of non-linear vibration are analyzed using autospectrum, it is possible to analyze the subharmonic and superharmonic mot ion by comparison. The phase change is analyzed using the method of phase portrait and the non-linear characteristics of response characteristics that are developed in flexible structures can be predicted and applied to the stage of design.

• Steel and Composite Structures
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• v.9 no.2
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• pp.119-130
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• 2009

This paper reports results of the structural response of empty steel tanks under vertical ground motions. The tanks are modeled using a finite element discretization using shell elements, and the vertical motion is applied and analyzed using nonlinear dynamics. Several excitation frequencies are considered, with emphasis on those that may lead to resonance of the roof. The computational results illustrate that as the base motion frequency is tuned with the frequency of the first roof-mode of the tank, the system displays large-amplitude displacements. For frequencies away from such mode, small amplitude displacements are obtained. The effect of the height of the cylinder on the dynamic response of the tank to vertical ground motion has also been investigated. The vertical acceleration of the ground motion that induces significant changes in the stiffness of the tank was found to be almost constant regardless of the height of the cylinder.

• Journal of the Korean Society for Precision Engineering
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• v.29 no.5
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• pp.545-553
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• 2012

A Gimbal structure system in observation reconnaissance aircraft is made up of camera module and stabilization drive device supporting camera module. During flight for image recording, the aircraft undergoes serious accelerations with wide frequencies due to several factors. Though base excitation of stabilization drive device induces vibration of camera module, it must get the stable and clean images. To achieve this aim, acceleration of camera module must be reduced. Hence, vibration isolators were installed to stabilization drive device. Considering isolators and bearings in the stabilization drive device, vibration characteristics of gimbal structure system were analyzed by finite element method. For three translational direction, acceleration transmissibility of camera module was calculated by harmonic responses analysis in the frequency range of 5 ~ 500 Hz. In addition to, sine-sweep experiment were performed to prove correctness of present analysis.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2006.05a
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• pp.62-65
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• 2006

In this paper, nonlinear nonplanar vibration of a flexible rectangular cantilever beam is analyzed when one-to-one resonance occurs to the beam. The planar and nonplanar motions of the beam are analyzed in time and frequency domains. In frequency domain, FFT analyzer is used to perform autospectrum and cepstrum analyses for nonlinear response of the beam. In time domain, an oscilloscope is used to investigate the phase difference between the planar and nonplanar motions and to perform Torus analysis in the phase space. Through those analyzing process, the main frequencies of superharmonic, subharmonic, and super-subharmonic motions are investigated in the nonplanar motion due to one-to-one resonance. Analyzing the phase difference between the planar and nonplanar motions, it is observed that the phase difference varies in time.

• Structural Engineering and Mechanics
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• v.60 no.4
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• pp.633-653
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• 2016

One of the major threats to the stability of classical columns and colonnades are earthquakes. The behavior of columns under high seismic excitation loads is non-linear and complex since rocking, wobbling and sliding failure modes can occur. Therefore, three dimensional simulation approaches are essential to investigate the in-plane and out-of-plane response of such structures during harmonic and seismic loading excitations. Using a software based on the Distinct Element Method (DEM) of analysis, a three dimensional numerical study has been performed to investigate the parameters affecting the seismic behaviour of colonnades' structural systems. A typical section of the two-storey colonnade of the Forum in Pompeii has been modelled and studied parametrically, in order to identify the main factors affecting the stability and to improve our understanding of the earthquake behaviour of such structures. The model is then used to compare the results between 2D and 3D simulations emphasizing the different response for the selected earthquake records. From the results analysis, it was found that the high-frequency motion requires large base acceleration amplitude to lead to the collapse of the colonnade in a shear-slip mode between the drums. However, low-frequency harmonic excitations are more prominent to cause structural collapse of the two-storey colonnade than the high-frequency ones with predominant rocking failure mode. Finally, the 2D analysis found to be unconservative since underestimates the displacement demands of the colonnade system when compared with the 3D analysis.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.4
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• pp.409-415
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• 2011

A gimbal system in observation reconnaissance aircraft was fabricated by assembling many parts and bearings. This system consists of a camera module and a stabilization gimbal that supports the camera module. During the flight for recording images, the gimbal system experiences various accelerations with wide frequencies. Although base excitation of stabilization gimbal results in vibration of the camera module, the camera module must be able to capture the correct and clear image even while vibrating. Hence, it is important to know the natural frequencies and vibration modes of the gimbal system with the camera module. Considering bearings as spring elements, the vibration characteristic of the gimbal system was analyzed by finite element method. In addition, harmonic response analysis was performed to determine the correct transmissibility of acceleration for the camera module in the frequency range of 0-500 Hz.

• Steel and Composite Structures
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• v.28 no.5
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• pp.629-639
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• 2018

In this paper, the vibroacoustic responses of baffled laminated composite sandwich flat panel structure under the influence of harmonic excitation are studied numerically using a novel higher-order coupled finite-boundary element model. A numerical scheme for the vibrating plate has been developed in the frame work of the higher-order mid-plane kinematics and the eigen frequencies are obtained by employing suitable finite element steps. The acoustic responses are then computed by solving the Helmholtz wave equation using boundary element method coupled with the structural finite elements. The proposed scheme has been implemented via an own MATLAB base code to compute the desired responses. The validity of the present model is established from the conformance of the current natural frequencies and the radiated sound power with the available benchmark solutions. The model is further utilized to scrutinize the influence of core-to-face thickness ratio, modular ratio, lamination scheme and the support condition on the sound radiation characteristics of the vibrating sandwich flats panel. It can be concluded that the present scheme is not only accurate but also efficient and simple in providing solutions of the coupled vibroacoustic response of laminated composite sandwich plates.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1995.10a
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• pp.185-190
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• 1995

In recent years, many researcher have been interested in the stability of a thin beam. Among them, Pai and Nayfeh[1] had investigated the nonplanar motion of the cantilever beam under lateral base excitation and chaotic motion, but this study is associated with internal resonance, i.e. one to one resonance. Also Cusumano[2] had made an experiment on a thin beam, called Elastica, under bending loads. In this experiment, he had shown that there exists out-of-plane motion, involving the bending and the torsional mode. Pak et al.[3] verified the validity of Cusumano's experimental works theoretically and defined the existence of Non-Local Mode(NLM), which is came out due to the instability of torsional mode and the corresponding aspect of motions by using the Normal Modes. Lee[4] studied on a thin beam under bending loads and investigated the routes to chaos by using forcing amplitude as a control parameter. In this paper, we are interested in the motion of a thin beam under torsional loads. Here the form of force based on the natural forcing function is used. Consequently, it is found that small torsional loads result in instability and in case that the forcing amplitude is increasing gradually, the motion appears in the form of dynamic double potential well, finally leads to complex motion. This phenomenon is investigated through the poincare map and time response. We also check that Harmonic Balance Method(H.B.M.) is a suitable tool to calculate the bifurcated modes.

• Advances in aircraft and spacecraft science
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• v.3 no.2
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• pp.197-209
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• 2016

The Spacecraft (S/C) numerical sine test-predictions are usually performed through Finite Element Method (FEM) Frequency Response Analysis (FRA), that is the hypothesis of steady-state responses to harmonic excitation to the S/C base is made. In the test practice, the responses are transient and may be significantly different from those predicted through FRA. One of the most significant causes of discrepancy between prediction and test consists in the beating phenomena. After a brief overview of the topic, the typical causes of beating are described in the first part of the paper. Subsequently, focus is made on the sine sweep rate effect, which often leads to have beatings after the resonance of weakly damped modes. In this work, the approach illustrated in the literature for calculating the sine sweep rate effect in the case of Single-Degree-Of-Freedom (SDOF) oscillators is extended to Multi-Degrees-Of-Freedom (MDOF) systems, with the aim of increasing the accuracy of the numerical sine test-predictions. Assumptions and limitations of the proposed methodology are detailed along the paper. Several assessments with test results are discussed and commented.