• Steel and Composite Structures
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• v.32 no.3
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• pp.305-312
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• 2019

In this study, we investigated the effect of fatigue crack propagation of the beams which have a vital importance in engineering applications, on the natural frequency of the system. Beams which have a wide range of applications, are used as fundamental structural elements in engineering structures. Therefore, early detection of any damages in these structures is of vital importance for the prevention of possible destructive damages. One of the widely used methods of early detection of damages is the vibration analysis of the structure. Hence, it is of vital importance to detect and monitor any changes in the natural frequencies of the structure. From this standpoint, in this study we experimentally investigated the effect of fatigue crack propagation on beams produced from 4140 steel, of the natural frequency of the beam. A crack was opened on the $8{\times}16{\times}500mm$ beam using a 3 mm long and 0.25 mm wide wire erosion. The beam, then, underwent 3 point bending tests at 10 Hz with a dynamic fatigue device and its natural frequencies were measured in scheduled intervals and any changes taking place on the natural frequencies of the beam were measured. This data allowed us to identify and measure the crack occurring on the beam subjected to dynamic loading, during the propagation phase. This method produced experimental data. The experimental data showed that the natural frequency of the beam decreased with the propagation of the fatigue crack on the beam.

• Structural Engineering and Mechanics
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• v.84 no.1
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• pp.77-83
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• 2022

In the last ten years, many researchers have studied the vibrations of carbon nanotubes using different beam theories. The nano- and micro-scale systems have wavy shape and there is a demand for a powerful tool to mathematically model waviness of those systems. In accordance with the above mentioned lack for the modeling of the waviness of the curved tiny structure, a novel approach is employed by implementing the Timoshenko-beam model. Owing to the small size of the micro beam, these structures are very appropriate for designing small instruments. The vibrations of double walled carbon nanotubes (DWCNTs) are developed using the Timoshenko-beam model in conjunction with the wave propagation approach under support conditions to calculate the fundamental frequencies of DWCNTs. The frequency influence is observed with different parameters. Vibrations of the double walled carbon nanotubes are investigated in order to find their vibrational modes with frequencies. The aspect ratios and half axial wave mode with small length are investigated. It is calculated that these frequencies and ratios are dependent upon the length scale and aspect ratio.

• Journal of the Korean Institute of Gas
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• v.20 no.6
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• pp.58-64
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• 2016

Vibrational characteristics of curved pipe structures are investigated with respect to the change of inside flow velocities. Based upon the Hamilton's principle, the equations of motions are derived, and the finite element equation is constructed to solve the frequency equation for curved pipe structures. When the initial tension is neglected in cured pipes, the natural frequencies are reduced as flow velocity increases, and the rapid decreases of the natural frequencies take place. However, when the initial tension is taken into account, the natural frequencies are not changed with the change of the flow velocity. In free vibrational simulation of pipe systems, it is necessary to calculate the initial force due to the velocity and the pressure of the fluid flow from the equilibrium. The force should be included in the equation of motion of the systems to get more accurate natural frequencies. The mechanical properties like stiffness or the location of pipe support need to be changed to avoid resonance. The natural frequencies are to be isolated from the frequency range of dominant vibration modes. The angles of elbows do not affect the change of the fundamental natural frequency, but affect the change of the third or higher natural frequencies.

• Journal of KSNVE
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• v.10 no.6
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• pp.1059-1066
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• 2000

In this paper. a new indirect feedback active noise control (ANC) scheme barred on the fundamental frequency estimation is proposed for systems with a harmonic noise. When reference signals necessary for feedforward ANC configuration are difficult to obtain, the conventional ANC algorithms for multi-tonal noise do not measure the reference signals but generate them with the estimated frequencies.$^{(4)}$ However, the beating phenomena, in which certain frequency components of the noise vanish intermittently, may make the adaptive frequency estimation difficult. The confusion in the estimated frequencies due to the beating phenomena makes the generated reference signals worthless. The proposed algorithm consists of two parts. The first part is a reference generator using the fundamental frequency estimation and the second one is the conventional feedforward control. We propose the fundamental frequency estimation algorithm using decision rules. which is insensitive to the beating phenomena. In addition, the proposed fundamental frequency estimation algorithm has good tracking capability and lower variance of frequency estimation error than that of the conventional cascade ANF method.$^{(4)}$ We are also able to control all interested modes of the noise, even which cannot be estimated by the conventional frequency estimation method because of the poor S/N ratio. We verify the performance of the proposed ANC method through simulations for the measured cabin noise of a passenger ship and the measured time-varying engine booming noise of a passenger vehicle.

• Journal of Power Electronics
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• v.14 no.6
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• pp.1109-1118
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• 2014

A Fundamental Frequency Sorting Algorithm (FFSA) is proposed in this paper to balance the voltages of floating dc capacitors for Modular Multilevel Converters (MMCs). The main idea is to change the sequences of the CPS-PWM carriers according to the capacitor voltage increments during the previous fundamental period. Excessive frequent sorting is avoided and many calculating resources are saved for the controller. As a result, more sub-modules can be dealt with. Furthermore, it does not need to measure the arm currents. Therefore, the communication between the controllers can be simplified and the number of current sensors can be reduced. Moreover, the proposed balancing method guarantees that all of the switching frequencies of the sub-modules are equal to each other. This is quite beneficial for the thermal design of the sub-modules and the lifetime of the power switches. Simulation and experimental results acquired from a 9-level prototype verify the viability of the proposed balancing method.

• Structural Engineering and Mechanics
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• v.77 no.3
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• pp.343-354
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• 2021

The frequencies formulas of the bridge are of great importance in the design process since these formulas provide insight dynamic characteristics of the structure, which guides the designers to parametric analyses and the layout of the bridge in conceptual or preliminary design. Continuous rigid frame bridge is popular in the mountainous area. Mostly, this type of bridge was simplified either as a girder or cantilever when calculating the frequency, however, studies showed that the different configuration of the bridge made the problem more complex, and there is no unified fundamental calculation pattern for this kind of bridge. In this study, an empirical frequency equation is proposed as a function of pier's height, stiffness of pier and the weight of the structure. A unified fundamental frequency formula is presented based on the energy principle, then the typical continuous rigid frame bridge is investigated by finite element method (FEM) to study the dynamic characteristics of the structure, and then several key parameters are investigated on the effect of structural frequency. These parameters include the number, position and stiffness of the tie beam. Nonlinear regression analyses are conducted with a comprehensive statistical study from plenty of engineering structures. Finally, the proposed frequency equation is validated by field test results. The results show that the fundamental frequency of the continuous rigid frame bridge increases more than 15% when the tie beams are set, and it increases with the stiffness ratio of tie beam to pier. The results also show that the presented unified fundamental frequency has an error of 4.6% compared with the measured results. The investigation can predicate the approximate longitudinal fundamental frequency of continuous ridged frame bridge, which can provide reference for the seismic response and dynamic impact factor design of the pier.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.263-268
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• 2000

In this paper, a new indirect feedback active noise control (ANC) scheme based on the fundamental frequency estimation is proposed for systems with a harmonic noise. When reference signals necessary for feedforward ANC configuration is difficult to obtain, the conventional ANC algorithms for multi-tonal noise do not measure the reference signals but generate them with the estimated frequencies. However, the beating phenomena, in which certain frequency components of the noise vanish intermittently, may make the adaptive frequency estimation difficult. The confusion in the estimated frequencies due to the beating phenomena makes the generated reference signals worthless. The proposed algorithm consists of two parts. The first part is a reference generator using the fundamental frequency estimation and the second one is the conventional feedforward control. We propose the fundamental frequency estimation algorithm using decision rules, which is insensitive to the beating phenomena. In addition, the proposed fundamental frequency estimation algorithm has good tracking capability and lower variance of frequency estimation error than that of the conventional cascade ANF method. We are also able to control all interested modes of the noise, even which cannot be estimated by the conventional frequency estimation method because of the poor SIN ratio. We verify the performance of the proposed ANC method through simulations for the measured cabin noise of a passenger ship and the measured time-varying engine booming noise of a passenger vehicle.

• Proceedings of the KIEE Conference
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• 1995.07c
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• pp.1140-1142
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• 1995

The transformer is fabricated with two piezoelectric vibrator with the divided electrodes and adhesive insulator. We applied the electric input to the driving vibrator in parallel and connect the output voltage to the generating vibrator in series to the resistor load near its fundamental resonance frequency. Then we investigate output voltage in series twice as large as in parallal. Moreover we investigate the load characteristics at resonance frequencies under various resistor and the frequency characteristics near the resonance frequency under no load. Its equvalent circuit is derived from the Mason's model of a thickness-driven piezelectric vibrator. By its equevalent circuit, symbolic expressions for input impedances, voltage ratios, resonance frequencies, and bandwidths have been derived. The values calculated from those symbolic exprssions are shown to agree well with the measurement values.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.13 no.3
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• pp.155-163
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• 2003

This paper presents the characteristics of the radial nitration of cylindrical piezoelectric transducers. The differential equations of piezoelectric radial motion have been derived in terms of the radial displacement and electric potential, which are functions of the radial and axial coordinates. Applying mechanical and electrical boundary conditions has yielded the characteristic equation of radial vibration. Numerical results of the natural frequencies have been compared with the experimental observations reported earlier for the transducers of several sizes, and have shown a good agreement for the fundamental mode. The paper discusses the dependence of the natural frequencies on the radius and thickness of the piezoelectric cylinders and the difference between Piezoelectric and elastic resonances.

• Proceedings of the KIEE Conference
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• 1991.07a
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• pp.600-604
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• 1991

In this paper, an advanced PWM modulation strategy for driving a variable-speed induction motor is introduced. According to this method, the technique of efficiently eliminating harmonics component is achived. A switching pattern was computed making use of the near-proportionality of voltage and frequency in AC machines operating with constant flux. At low magnitudes and low frequencies of the foundamental, many more harmonics are eliminated than at high magnitudes and frequencies. In order to keep the inverter switching frequency constant over the output frequency range, chopping times diminishes as the frequency of the fundamental increases.