• Composites Research
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• v.37 no.4
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• pp.291-295
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• 2024

Vibration occurs not only in daily life but also in various fields such as semiconductors, aerospace, vehicles, and ships. Unexpected vibrations can cause fatigue damage to structures and degrade the performance of the entire system, having very detrimental effects. Particularly, low-frequency vibrations can be very harmful to precision equipment, human bodies, and buildings. Therefore, mitigating low-frequency vibrations is essential for effective vibration reduction. In this study, a kirigami-inspired composite meta-structure is proposed for low-frequency vibration reduction. Inspired by kirigami, the meta-structure is designed to transform from a three-dimensional to a two-dimensional form upon compression, leveraging structural advantages. Additionally, it is designed to have quasizero stiffness characteristics, providing excellent vibration reduction performance even at low frequencies. The kirigami composite meta-structure was fabricated using carbon fiber reinforced TPU through 3D printing. Its structural and vibrational characteristics were evaluated and analyzed through compression and vibration tests.

• Journal of the Korean Chemical Society
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• v.59 no.3
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• pp.209-214
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• 2015

The formation of complexes between SO₂ and acetic acid was studied theoretically. The ab initio and DFT calculations were performed with MP2 and B3LYP methods using 6-311++G(d,p), aug-cc-pVDZ and aug-cc-pVTZ basis sets. Six stable complexes were identified, and three stable bidentate complexes, C1, C2 and C3, were formed between SO₂ and syn-acetic acid, which is more stable form of acetic acid. Anti-acetic acid also form three complexes, C4, C5 and C6, with SO₂. C4 is bidentate and C5, C6 are monodentate complexes, which are less stable. The most stable complex, C1 has S⋯O=C and O⋯H-O interactions, and the S⋯O and O⋯H distances are less than the sum of van der Waals radii. The vibrational frequencies of complexes were calculated and were compared with those of monomers. The frequency shifts after formation of complex were found, and the overall pattern of frequency shifts relative to monomers is similar among the six complexes.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.3
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• pp.84-92
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• 2014

In this study, vibrational tendon tension measurement methods are applied to estimate tension of external tendons used in segmental post-tensioned bridges. The acceleration of various length type of tendons is measured and natural frequencies are obtained using FFT (Fast Fourier Transform). The obtained natural frequencies are within 1% error regardless of sensor direction and location. On the basis of natural frequency of tendon, estimation of the tendon tension is performed by using many types of solutions such as string theory equation, multi mode estimation, practical formula estimation and stiff string with clamped-clamped boundary conditions. The results are compared with each other and have shown that the flexural stiffness is not negligible in tendons of this type causing the vibration mode to be inharmonically related. The results have shown that the method using stiff string equation with clamped-clamped boundary conditions is more accurate than the other methods. Application example of in-service bridges has shown that force distribution effects from friction at deviation blocks can be effectively detected.

• Smart Structures and Systems
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• v.20 no.4
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• pp.427-440
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• 2017

One of the suitable structural damage detection methods using vibrational characteristics are damage-index-based methods. In this study, a damage index for identifying damages in plate structures using frequency response function (FRF) data has been provided. One of the significant challenges of identifying the damages in plate structures is high number of degrees of freedom resulting in decreased damage identifying accuracy. On the other hand, FRF data are of high volume and this dramatically decreases the computing speed and increases the memory necessary to store the data, which makes the use of this method difficult. In this study, FRF data are compressed using two-dimensional principal component analysis (2D-PCA), and then converted into damage index vectors. The damage indices, each of which represents a specific condition of intact or damaged structures are stored in a database. After computing damage index of structure with unknown damage and using algorithm of lookup tables, the structural damage including the severity and location of the damage will be identified. In this study, damage detection accuracy using the proposed damage index in square-shaped structural plates with dimensions of 3, 7 and 10 meters and with boundary conditions of four simply supported edges (4S), three clamped edges (3C), and four clamped edges (4C) under various single and multiple-element damage scenarios have been studied. Furthermore, in order to model uncertainties of measurement, insensitivity of this method to noises in the data measured by applying values of 5, 10, 15 and 20 percent of normal Gaussian noise to FRF values is discussed.

• Advances in nano research
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• v.8 no.3
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• pp.215-228
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• 2020

In this paper, a new method based on the Sander theory is developed for SWCNTs to predict the vibrational behavior of length and ratio of thickness-to-radius according to various end conditions. The motion equation for this system is developed using Rayleigh-Ritz's method. The proposed model shows the vibration frequencies of armchair (5, 5), (7, 7), (9, 9), zigzag (12, 0), (14, 0), (19, 0) and chiral (8, 3), (10, 2), (14, 5) under different support conditions namely; SS-SS, C-F, C-C, and C-SS. The solutions of frequency equations have been given for different boundary condition, which have been given in several graphs. Several parameters of nanotubes with characteristic frequencies are given and vary continuously in length and ratio of thickness-to-radius. It has been illustrated that an enhancing the length of SWCNTs results in decreasing of the frequency range. It was demonstrated by increasing of the height-to-radius ratio of CNTs, the fundamental natural frequency would increase. Moreover, effects of length and ratio of height-to-radius with different boundary conditions have been investigated in detail. It was found that the fundamental frequencies of C-F are always lower than that of other conditions, respectively. In addition, the existence of boundary conditions has a significant impact on the vibration of SWCNTs. To generate the fundamental natural frequencies of SWCNTs, computer software MATLAB engaged. The numerical results are validated with existing open text. Since the percentage of error is negligible, the model has been concluded as valid.

• Journal of Korean Tunnelling and Underground Space Association
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• v.5 no.1
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• pp.13-21
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• 2003

To investigate the safety and stability of a concrete lining, numerous studies have been conducted over the years and several methods have been developed. Most signal processing techniques of NDT have been based on Fourier analysis. However, the application of Fourier analysis to analyze recorded vibrational signal shows results in the frequency domain only, and it is not enough to analyze transient waves precisely. In this study, Wavelet theory was employed for the analysis of non-stationary wave induced by mechanical impact on tunnel concrete lining. The Wavelet transform of transient signals provides a method for mapping the frequency spectrum as a function of time. To verify the availability of Wavelet transform as a time-frequency analysis tool, model experiments have been conducted and the thickness of the concrete lining was estimated based on the proposed theory. From this study, it was found that the contour map by Wavelet transform provides more distinct results than the power spectrum by Fourier transform and it was also found that Wavelet transform was also an effective tool for the analysis of dispersive waves in tunnel concrete linings.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.14 no.9
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• pp.4136-4141
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• 2013

One of the most significant requisite that should be considered for effective rapping of the electrostatic precipitator using electromagnetic vibration exciter is vibration acceleration and resonance frequency of collecting plates. This vibration acceleration shows its peak points when natural frequencies of the system are corresponded with excitation frequency from the power source, and effective rapping performance can be expected. In this research, extend view of single electrostatic precipitator using one electromagnetic vibration exciter, the system was remodeled by arrangement of the exciters in view of multiple modules of the electrostatic precipitator in fields. And vibration acceleration measurement experiment is performed and measured values are compared with these remodeled systems. By this experimental comparison in series and parallel arrangement, effectiveness of arrangement methods for the electromagnetic vibration exciter, expected rapping performance, and power consumption are verified.

• Aerospace Engineering and Technology
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• v.4 no.2
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• pp.203-208
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• 2005

Electronic components for space launch vehicles are exposed to a severe vibrational environment at launch and flight. The structural reliability of each component can be verified using mathematical approaches. In order to verify the structural reliability, an important parameter is the natural frequency of PCB(Printed Circuit Board) assembly mounted electronic components on and housing mounted PCB assembly in. In this paper, in order to find natural frequencies of PCB assemblies and the housing of hydrazine TCU(Thruster Control Unit), FEM(Finite Element Method) is adapted. The analytical result of FEM is verified by experimental method.

• Journal of Institute of Control, Robotics and Systems
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• v.13 no.11
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• pp.1060-1066
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• 2007

When a high-energy ultrasound propagates through a solid body that contains a crack or a delamination, the two faces of the defect do not ordinarily vibrate in unison, and dissipative phenomena such as friction, rubbing and clapping between the faces will convert some of the vibrational energy to heat. By combining this heating effect with infrared imaging, one can detect a subsurface defect in material efficiently. In this paper a detection of the welding defect of thin SUS 304 plates using the UIR (ultrasonic infrared imaging) technology is described. A low frequency (20kHz) ultrasonic transducer was used to infuse the welded thin SUS 304 plates with a short pulse of sound for 280ms. The ultrasonic source has a maximum power of 2kW. The surface temperature of the area under inspection is imaged by a thermal infrared camera that is coupled to a fast frame grabber in a computer. The hot spots, which are a small area around the defect tip and heated up highly, are observed. From the sequence of the thermosonic images, the location of defective or inhomogeneous regions in the welded thin SUS 304 plates can be detected easily.

• Bulletin of the Korean Chemical Society
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• v.32 no.11
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• pp.4011-4015
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• 2011

We explored the interpretation of the well-accepted correlation between the apparent peak maximum position shift and extent of molecular interactions, like hydrogen bonding and dipole-dipole interactions, based on the overlapped multiple band model. The simulation of two overlapped Lorentzian bands was carried out to interpret how the maximum position of a composite peak relates to the relative contributions of two species representing the different levels of molecular interactions, i.e., free (or very weekly bound) vs. strongly bound. To demonstrate the validity of our interpretation of the origin of the peak position shift, the temperaturedependent IR spectra of ethylene glycol were also analyzed. It was found through the analysis of simulated and experimental spectra that the apparent peak shift in certain case can be safely interpreted as the measure of the strength of hydrogen bonding. The result of this study gives a new insight to interpret molecular interactions probed by vibrational spectroscopy.