• The Journal of the Acoustical Society of Korea
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• v.26 no.2
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• pp.80-86
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• 2007

We have investigated on the development of 2-2 type piezocomposites that have better piezoelectric activity and lower acoustic impedance than those of conventional piezoceramics. In this study, we have investigated the piezoelectric and acoustic properties of 2-2 type piezocomposites sensor which was fabricated using dice-and-fill technique for the different volume fraction of PZT. The specific acoustic impedance of 2-2 type piezocomposites decreased linearly when PZT volume fraction was decreased. The resonance characteristics measured by an impedance analyzer(HP4194A) were similar to the analysis of finite element method (FEM). The resonance characteristics and the electromechanical coupling factor were the best when the volume fraction PZT was 0.6. It also showed the highest result from the standpoint of sensitivity, bandwidth and ring-down property and so on at the same condition. The specific characteristics show that the 2-2 type piezocomposites turned out to be superior to the ultrasonic sensor composed by single phase PZT.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2005.04a
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• pp.163-166
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• 2005

사이클로콥터는 회전축에 평행하게 회전하는 블레이드로 구성된 사이클로이드 블레이드 시스템으로부터 추력을 얻는 수직이착륙 무인기이다. 본 논문에서는 공기역할을 고려한 최적 설계를 통해 결정된 로터 형상을 갖는 사이클로콥터에 대해서 구조 해석을 수행하였다. 복합재료 블레이드의 적층각, 적층수 등을 MSC/NASTRAN 과 반응면 기법 등에 의해서 결정하였다. 블레이드를 포함한 로터 정적 해석을 통해 각 요소가 허용 응력 이내의 값을 가짐을 확인하였고, 동적 해석을 통해 주요 저차 모드가 로터 회전과 불일치하게 함으로써 공진의 가능성을 없앴다.

• Journal of the Korean Society for Nondestructive Testing
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• v.15 no.2
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• pp.357-363
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• 1995

Disbond size of adhesively bonded single lap and double lap joints CFRP composite specimens has been evaluated using acousto-ultrasonic(AU) technique. Frequency spectra for all specimens were obtained by measuring peak amplitude of the stress wave propagated through the bond-lines. By analyzing these frequency spectra, peak amplitude was found to be proportional to fractional bonding area and to be maxima at the fundamental and the third order higher harmonic frequencies of specimen thickness mode. The disbond size can be evaluated quantitatively and this technique can be applied to real structures if the reference specimens are prepared in advancve.

• The Journal of the Acoustical Society of Korea
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• v.14 no.2
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• pp.92-100
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• 1995

In this study, the bearing defects were modeled and the vibration of ball bearing faults was presented for the defective pattern. The vibration signal was measured for the single and multiple defected ball bearing at the various defect positions and rotation speed, and then the signal components using the HFRT(high frequency resonance technique) were analyzed by FFT. The experimental data analysis has shown that the frequencies generated in the single or multiple defected ball bearings appear with the characteristic defect frequency and harmonics of ball pass frequency peak. Signal processing by HFRT makes it possible not only to detect the presence of a defect but also to diagnose the defect part of the bearing.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.8 no.2
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• pp.174-184
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• 1996

In the present study, for fully-open rectangular harbors, sensitivity analyses are made for the major parameters which are relevant to the practical application of a hybrid element model widely used fur the analysis of harbor oscillation. The results show that it is desirable to extend the finite element region to the area in which depth change is not large and that it is appropriate to take the depth of the outer region for analytic solution as the average along the boundary between the two regions. It is expected that the number of Fourier components of the analytic solution may not be important for a constant-depth simple-shaped harbor but its significance may increase for harbors of varying depth and complex geometry. It is found that the effect of incident wave direction is not significant for the first resonance mode but its effect becomes important as the bottom slope increases, especially for the higher resonance modes.

• Korean Journal of Materials Research
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• v.8 no.10
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• pp.905-911
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• 1998

Damping is a property for materials and systems to dissipate energy during periodic deformations. Generally, damping causes stiff decrease in amplitudes and shifts in phases. Also, even at resonance, amplitudes are substantially attenuated. This phenomenon of damping helps in reducing stresses developed during vibrations and consequently improves fatigue lives of materials. In this work internal damping and complex moduli are experimentally determined. An impulse technique is utilized in experiments and cantilever beams are selected as test subjects for the measurements of flextural vibrations since the beams lend themselves easily as simplistic ideal models. A resonance method is employed to determine resonance frequencies which are utilized to compute storage moduli. Also, loss moduli are evaluated from damping capacities and storage moduli. The storage and loss moduli combined yield complex moduli. Finally internal damping is evaluated from bandwidth technique, the real component of the transfer function.

• Composites Research
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• v.14 no.3
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• pp.77-89
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• 2001

Since the critical whirling vibration frequency of high speed built-in type motor spindle systems is dependent on the rotor mass of the built-in motor and the spindle specific bending modulus, the rotor and the shaft were designed using magnetic powder containing epoxy and high modulus carbon fiber epoxy composite, respectively. In order to increase the amount of the magnetic flux of the composite squirrel cage rotor of an AC induction motor, a steel core was inserted into the composite rotor. From the magnetic analysis, the optimal configurations of steel core and conductor bars for the dynamic characteristics of the rotor system were determined and proposed. The temperature dependence of composite squirrel cage rotor materials was investigated by various experiments such as TMA, DMA and VSM.

• Composites Research
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• v.18 no.3
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• pp.1-6
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• 2005

Carbon nanofiber exhibits superior and of ien unique characteristics of mechanical, electrical, chemical and thermal properties. Despite of the excellent properties of carbon nanofiber, the properties of carbon nanofiber filled polymer composites were not increased largely. The reason is that it is still difficult to ensure the uniform dispersion of carbon nanofiber in a polymer matrix. In this study, for improvement of the mechanical properties of composites, carbon nanofiber reinforced hybrid composites was investigated. For the dispersion of carbon nanofiber. solution blending method using ultrasonic was used. Dispersion of carbon nanoifiber was observed by scanning electron microscope (SEH). Mechanical properties were measured by universal testing machine(UTM).

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.25 no.5
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• pp.515-525
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• 2014

In this paper, we present a design and implementation of a Voltage-tuned Planar Composite Resonator Oscillator(Vt-PCRO) with a low phase noise. The designed Vt-PCRO is composed of a resonator, two phase shifters, and an amplifier. The resonator is designed using a dual mode SIW(Substrate Integrated Waveguide) resonator and has a group delay of about 40 nsec. Of the two phase shifters (PS1 and PS2), PS1 with a phase shift of $360^{\circ}$ is used for the open loop gain to satisfy oscillation condition without regard to the electrical lengths of the employed microstrip lines in the loop. PS2 with a phase shift of about $70^{\circ}$ is used to tune oscillation frequency. The amplifier is constructed using two stages to compensate for the loss of the open loop. Through the measurement of the open loop gain, the tune voltage of the PS1 can be set to satisfy the oscillation condition and the loop is then closed to form the oscillator. The oscillator with a oscillation frequency of 5.345 GHz shows a phase noise of -130.5 dBc/Hz at 100 kHz frequency offset. The oscillation power and the electrical frequency tuning range is about 3.5 dBm and about 4.2 MHz for a tuning voltage of 0~10 V, respectively.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2001.05a
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• pp.942-947
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• 2001

This paper provides a stability analysis of the transverse vibration of a spinning disk under the torque fluctuation from a permanent magnetic motor. An analytical model has been formulated for a flexible annular disk with its spinning velocity varying harmonically with the same frequency as the cogging torque. A perturbation method based on multiple time scales is applied to perform the stability analysis. Based on expressions for the amplitude and frequency of the parametric excitation, stability boundaries are determined in terms of a nominal spindle velocity, the least common multiple of poles and slots, the magnitude of torque fluctuation and the modal characteristics of. the disk. The stability diagrams predicted by perturbation have been verified numerically using the Floquet theory, which is in good agreement. In conclusion, the fluctuation in spinning velocity is found to affect the stability of the transverse vibration of a rotating disks. The results of this work can be applied to high precision spindle systems such as computer storage systems.