• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2005.11a
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• pp.169-170
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• 2005

본 연구에서는 초음파 센서에 응용 가능한 $0.4Pb(Ni_{1/3}Nb_{2/3})O_3-0.6Pb(Zr_xTi_{1-x})O_3+0.5Wt%$ $MnO_2$ 세라믹스에 Zr/(Ti+Zr)비를 0.37에서 0.41로 변화시킨 조성을 1175 $\sim$ 1200$^{\circ}C$ 온도에서 소결하여 이의 결정구조 및 미세조직을 분석하였고, 압전, 유전 특성을 고찰하였다. 본조성에서 x=0.385 조성에서 최대 유전상수 값 3490 이 나타났으며, 그 이상의 첨가에서는 감소하였다. 상경계 영역인 x=0.385 조성에서 $\varepsilon$r, $K_p$, $d_{33}$ 값이 최대값을 나타내었다. $0.4Pb(Ni_{1/3}Nb_{2/3})O_3-0.6Pb(Zr_xTi_{1-x})O_3+0.5Wt%$ $MnO_2$, 세라믹스에서는 kp 와 $d_{33}$ 는 Zr/(Ti+Zr)비 0.385조성까지 증가하였다가 그 이상 조성에서 감소하였다. $1175^{\circ}C$에서 2시간 소결한 x=0.385조성에서 $\varepsilon$r=3490, kp=0.71, Qm=476의 우수한 압전 특성을 나타내었다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.34 no.8
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• pp.55-62
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• 2006

This paper presents experimental evaluation of an insect-mimicking flapping-wing device actuated by a unimorph piezoceramic actuator. Length of each rod and hinge point in the linkage/amplification system are carefully chosen such that the resulting wing motion can mimic clapping of wings in a real insect at the end of upstroke. In addition to this, a pair of corrugated wings are fabricated mimicking zig-zag cross section of a real insect wing. Thanks to the two additional implementation, the improved flapping wing device can generate a larger lift force than the previous model even though area of the new wing is about 50% less than that of the previous wing. In this work, effects of the wing clapping, the wing corrugation, and the input wave form on the lift force generation have been also experimentally investigated. Finally, the vortex generated by the flapping device has been captured by a high speed camera, showing that vortices are produced during up- and down-strokes.

• Journal of the Korean Society for Nondestructive Testing
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• v.23 no.1
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• pp.14-20
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• 2003

Piezoelectric ultrasonic transducers for high temperature contact measurement were developed. These high temperature ultrasonic transducers (HTUT) consisted of bismuth titanate piezoceramic element whose Curie temperature is higher than $600^{\circ}C$, a backing material of the mixture of tungsten powder and inorganic binder, an inner alumina tube, a wear Plate and a housing. The operational frequencies or the HTUT were 1.04 and 2.08 MHz, respectively. Various commercially available couplants for high temperature were evaluated and compared. As a couplant for high temperature ultrasonic testing between HTUT and test specimen, gold epoxy was selected. The peak amplitude of pulse-echo signals from steel test specimen decreased with increasing temperature. The operational temperature of the HTUT reached up to $500^{\circ}C$ at which the continuous contact measurement was possible.

• Composites Research
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• v.16 no.3
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• pp.58-67
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• 2003

To perform the realtime strain and fracture monitoring of the smart composite structures, two optical fiber sensor systems are proposed. The two types of the coherent sources were used for fracture signal detection - EDFA with FBG and EDFA with Fabry-Perot filter. These sources were coupled to EFPI sensors imbedded in composite specimens. To understand the characteristics of matrix crack signals, at first, we performed tensile tests using surface attached PZT sensors by changing the thickness and width of the specimens. This paper describes the implementation of time-frequency analysis such as short time Fourier transform (STFT) and wavelet transform (WT) for the quantitative evaluation of fracture signals. The experimental result shows the distinctive signal features in frequency domain due to the different specimen shapes. And, from the test of tensile load monitoring using optical fiber sensor systems, measured strain agreed with the value of electric strain gage and the fracture detection system could detect the moment of damage with high sensitivity to recognize the onset of micro-crack fracture signal.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.50 no.2
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• pp.75-82
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• 2022

Nondestructive method to predict buckling load for the propellant tank of launch vehicle should be evaluated. Vibration correlation technique can predict the global buckling load of unstiffened cylindrical structure with geometric initial imperfection using correlation of natural frequency and compressive load from compressive test below the buckling load. In this study, vibration and buckling tests of a thin metal unstiffened propellant tank model subjected to internal pressure and compressive loads were performed and the test results were used for VCT to predict global buckling load. For the vibration test of thin structure, non-contact excitation method using a speaker was used. The response was measured with piezoelectric polymer(PVDF) sensor. Prediction results of VCT were compared with the measured buckling load in the test and the nondestructive global buckling load prediction method was verified.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.23 no.12
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• pp.1066-1072
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• 2013

In this work, active control algorithm is adopted to reduce delamination effects of the damaged composite structure and control performance with MFC actuator is numerically evaluated. Finite element model for the damaged composite structure with piezoelectric actuator is established based on improved layerwise theory. In order to achieve high control performance, MFC actuator, which has increased actuating force, is considered as a piezoelectric actuator. Mode shapes and corresponding natural frequencies for the damaged smart composite structure are studied. After design and implementation of active controller, dynamic characteristics of the damaged smart composite structure are investigated.

• Journal of KSNVE
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• v.8 no.5
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• pp.832-840
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• 1998

Experimental studies on the adaptive vibration control of composite beams have been performed using a piezoelectric actuator and the neuro-controller. The variations in natural frequencies of the specimen and the actuation characteristics of the piezoelectric actuator according to the delamination in the bonding layer have been studied. In addition, the simulation of adaptive vibration control has been performed for the composite specimens with delaminated piezoelectric actuator using neuro-controller. The hardware for the adaptive vibration control experiment was prepared. A DSP(digital signal processor) has been used as a digital controller. Using neuro-controller, the adaptive vibration control experiment has been performed. The vibration control results using the neuro-controller show that the present neuro-controller has good performance and robustness with the system parameter variations.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.4 s.97
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• pp.395-403
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• 2005

The dynamic characteristics of delaminated smart composite laminates are studied using animproved layerwise laminate theory. The theory is capable of capturing interlaminar shear stresses that are critical to delamination. The presence of discrete delamination is modeled through the use of Heaviside unit step functions. Stress free boundary conditions are enforced at all free surfaces. Continuity in displacement field and transverse shear stresses are enforced at each ply level. In modeling piezoelectric composite plates, a coupled piezoelectric-mechanical formulation is used in the development of the constitutive equations. Numerical analysis is conducted to investigate the effect of nonlinearity in the transient vibration of bimodular behavior caused by the contact impact of delaminated interfaces. Composite plates with delamination, subject to external loads and voltage history from surface bonded sensors, are investigated and the results are compared with corresponding experimental results and plates without delamination.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2013.10a
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• pp.535-540
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• 2013

In this work, active control algorithm is adopted to reduce delamination effects of the damaged composite structure and control performance with MFC actuator is numerically evaluated. Finite element model for the damaged composite structure with piezoelectric actuator is established based on improved layerwise theory. In order to achieve high control performance, MFC actuator, which has increased actuating force, is considered as a piezoelectric actuator. Mode shapes and corresponding natural frequencies for the damaged smart composite structure are studied. After design and implementation of active controller, dynamic characteristics of the damaged smart composite structure are investigated.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2009.04a
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• pp.106-109
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• 2009

본 논문에서는 교량의 상태감시 테스트베드 구축을 위한 한-미 국제공동연구의 현황 및 활동 내용들을 논하였다. 이 국제공동연구는 최첨단 센서와 구조건전도 모니터링 방법의 유용성 및 통합화하는데 그 목적을 두고 있다. 테스트베드 구축을 위해 가속도계과 동적 FBG 센서, 압전 센서 등과 같은 스마트 센서를 사용하였으며, 무선 데이터수집 시스템이 도입되었다. 교량 모니터링 기법으로는 압전 센서 및 EM센서로부터 취합된 데이터를 이용하여 국부손상검색을 수행하였으며, 가속도계, 동적 FBG센서 및 이미지 프로세싱을 이용하여 진동기반 전역손상검색을 수행하였다. 테스트베드 교량으로는 PC박스 거더교, 강상자형교, 강판형교, 사장교의 4가지 형식의 교량이 사용되었다. 테스트베드 교량에 최신 이동통신 인터넷 연결기술을 이용하여 교량에 설치된 센서로부터 취합된 데이터와 모니터링 시스템으로부터 교량의 상태를 실시간 감시할 수 있는 네트워크 시스템을 구축하였다. 이러한 원거리 이동통신시스템을 통하여 구조물의 건전성 평가결과를 실시간으로 전송 및 분석할 수 있도록 하였다.