• Journal of Theoretical and Applied Mechanics
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• 제2권1호
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• pp.1-14
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• 1996

In this paper, breaking waves are generated in a 2-D wave tank and simulated by using a higher-order boundary element method. A piston-type wavemaker is operated by signals composed of elementary waves. The phase of elementary waves is determined by the linear theory such that they are focused to a prescribed position. Calculated plunging waves coincide well with experiment. A steel box with different plate thicknesses is installed at a predetermined position in the tank. Measured impulsive pressures due to breaking waves are found to be 0.8-1.2$\rho$C2, where $\rho$ corresponds to water density and C to wave celerity. The transverse displacement of the plate is described in terms of modal eigenfunctions. The natural frequencies measured by impact tests in air for thin plate coincide with the computational and theoretical values. The radiationpotential due to plate vibration is derived and the radiation force is expressed in terms of hydroelastic added mass and damping forces. Comparison of natural frequencies of plate in water proves that hydroelastic added mass and damping are properly considered. The measured strain due to regular waves supports the calculated one, but there are apparent discrepancies between theory and experiment in the impulsive case.

• 한국공간구조학회:학술대회논문집
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• 한국공간구조학회 2006년도 춘계 학술발표회 논문집 제3권1호(통권3호)
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• pp.233-238
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• 2006

이 논문은 여러 개의 진동수를 동시에 고려할 수 있는 위상최적화기법을 제공하고 이를 평면구조물에 적용하여 최적위상을 도출하고 그 결과를 기술하였다. 본 연구에서는 모드변형에너지를 최소화하고자하는 목적함수로 가정하고 구조물의 초기 부피를 제약함수로 사용하였다. 물질내부에 존재하는 구멍의 크기를 조절하기 위하여 최적정기준법을 바탕으로 한 크기조절 알고리듬을 도입하였다. 수치해석결과로 부터 제시된 위상최적화기법은 구조물의 기본고유진동수를 효과적으로 최대화하면서 동시에 고차의 진동수에 대한 영향도 최적위상에 고려할 수 있는 것으로 나타났다.

• Structural Engineering and Mechanics
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• 제40권5호
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• pp.719-743
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• 2011

A hybrid damage monitoring scheme using parallel acceleration-impedance approaches is proposed to detect girder damage and support damage in steel plate-girder bridges which are under ambient train-induced excitations. The hybrid scheme consists of three phases: global and local damage monitoring in parallel manner, damage occurrence alarming and local damage identification, and detailed damage estimation. In the first phase, damage occurrence in a structure is globally monitored by changes in vibration features and, at the same moment, damage occurrence in local critical members is monitored by changes in impedance features. In the second phase, the occurrence of damage is alarmed and the type of damage is locally identified by recognizing patterns of vibration and impedance features. In the final phase, the location and severity of the locally identified damage are estimated by using modal strain energy-based damage index methods. The feasibility of the proposed scheme is evaluated on a steel plate-girder bridge model which was experimentally tested under model train-induced excitations. Acceleration responses and electro-mechanical impedance signatures were measured for several damage scenarios of girder damage and support damage.

• Smart Structures and Systems
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• 제24권1호
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• pp.27-35
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• 2019

This paper proposes a quantitative criterion for optimization of actuator placement for the Prestress-Accumulation Release (PAR) strategy of mitigation of vibrations. The PAR strategy is a recently developed semi-active control approach that relies on controlled redistribution of vibration energy into high-order modes, which are high-frequency and thus effectively dissipated by means of the natural mechanisms of material damping. The energy transfer is achieved by a controlled temporary removal of selected structural constraints. This paper considers a short-time decoupling of rotational degrees of freedom in a frame node so that the bending moments temporarily cease to be transferred between the involved beams. We propose and test a quantitative criterion for placement of such actuators. The criterion is based on local modal strain energy that can be released into high-order modes. The numerical time complexity is linear with respect to the number of actuators and potential placements, which facilitates quick analysis in case of large structures.

• 한국재료학회지
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• 제31권1호
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• pp.43-53
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• 2021

Dynamic behavior of piezoelectric ZnO nanowires is investigated using finite element analyses (FEA) on FE models constructed based on previous experimental observations in which nanowires having aspect ratios of 1:2. 1:31, and 1:57 are obtained during a hydrothermal process. Modal analyses predict that nanowires will vibrate in lateral bending, uniaxial elongation/contraction, and twisting (torsion), respectively, for the three ratios. The natural frequency for each vibration mode varies depending on the aspect ratio, while the frequencies are in a range of 7.233 MHz to 3.393 GHz. Subsequent transient response analysis predicts that the nanowires will behave quasi-statically within the load frequency range below 10 MHz, implying that the ZnO nanowires have application potentials as structural members of electromechanical systems including nano piezoelectric generators and piezoelectric dynamic strain sensors. When an electric pulse signal is simulated, it is predicted that the nanowires will deform in accordance with the electric signal. Once the electric signal is removed, the nanowires exhibit a specific resonance-like vibration, with the frequency synchronized to the signal frequency. These predictions indicate that the nanowires have additional application potential as piezoelectric actuators and resonators.

• 한국기계가공학회지
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• 제19권10호
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• pp.87-97
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• 2020

In general, tires require various sensors and power supply devices, such as batteries, to obtain information such as pressure, temperature, acceleration, and the friction coefficient between the tire and the road in real time. However, these sensors have a size limitation because they are mounted on a tire, and their batteries have limited usability due to short replacement cycles, leading to additional replacement costs. Therefore, vibration energy harvesting technology, which converts the dynamic strain energy generated from the tire into electrical energy and then stores the energy in a power supply, is advantageous. In this study, the output voltage and power generated from piezoelectric elements are predicted through finite element analysis under static state and transient state conditions, taking into account the dynamic characteristics of tires. First, the tire and piezoelectric elements are created as a finite element model and then the natural frequency and mode shapes are identified through modal analysis. Next, in the static state, with the piezoelectric element attached to the inside of the tire, the voltage distribution at the contact surface between the tire and the road is examined. Lastly, in the transient state, with the tire rotating at the speeds of 30 km/h and 50 km/h, the output voltage and power characteristics of the piezoelectric elements attached to four locations inside the tire are evaluated.

• Advances in concrete construction
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• 제13권3호
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• pp.223-231
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• 2022

The problem is formulated by applying the Kirchhoff's conception for shell theory. The longitudinal modal displacement functions are assessed by characteristic beam ones meet clamped-clamped end conditions applied at the shell edges. The fundamental natural frequency of rotating functionally graded cylindrical shells of different parameter versus ratios of length-to-diameter and height-to-diameter for a wide range has been reported and investigated through the study with fractions laws. The frequency first increases and gain maximum value with the increase of circumferential wave mode. By increasing different value of height-to-radius ratio, the resulting backward and forward frequencies increase and frequencies decrease on increasing height-to-radius ratio. Moreover, on increasing the rotating speed, the backward frequencies increases and forward frequencies decreases. The trigonometric frequencies are lower than that of exponential and polynomial frequencies. Stability of a cylindrical shell depends highly on these aspects of material. More the shell material sustains a load due to physical situations, the more the shell is stable. Any predicted fatigue due to burden of vibrations is evaded by estimating their dynamical aspects.

• 방사성폐기물학회지
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• 제17권2호
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• pp.167-181
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• 2019

최근 국내에서 육상 및 해상을 통한 소외 정상운반 시 진동 및 충격하중에 대한 사용후핵연료의 건전성 평가 기술 개발이 수행되고 있다. 이와 관련된 국내 연구사례는 전무하여 기존에 진행된 또는 현재 수행중인 해외연구사례를 조사하여 국내 연구에 참고하고자 한다. 2000년 이전 과거 미국의 사용후핵연료의 정상운반 시 진동 및 충격하중 측정 관련 연구현황을 조사하였고 2009년부터 미국국립연구소 주관으로 실시한 단축가진시험, 콘크리트블럭 트럭운반시험, 다축가진시험에 대해서 조사하였으며 2017년 미국 SNL, 스페인의 ENSA, 한국이 공동으로 수행한 복합운반시험을 상세히 조사하였다. 시험 준비과정, 절차, 가속도 및 변형률 측정결과, 유한요소 및 다물체동역학 해석과정 등이 조사되었다. 각 시험 별로 측정된 변형률 자료를 바탕으로 사용후핵연료 피로곡선과 비교한 결과 손상을 일으키기에는 매우 미미한 정도의 변형률이 발생한다는 초기 결론을 얻었음을 확인하였다. 하지만 현재 결론은 일부 결과만을 검토한 예비 결론으로 상세한 검토가 현재 미국에서 진행 중이다. 미국에서 지금까지 수행한 사용후핵연료의 정상운반조건에서의 진동 및 충격하중 측정과 관련하여 조사된 내용은, 국내 운반환경에서 사용후핵연료의 정상운반시험을 수행할 때 참고할만한 유용한 자료라 판단된다.

• 한국항해항만학회지
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• 제39권3호
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• pp.179-184
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• 2015

이 연구의 목적은 건전성 평가 기법을 통해 해양 구조물 중 생산 라이저의 건전성 및 안전성을 확보할 수 있도록 하는 것이다. 생산 라이저의 건전성 평가 기법을 선정하기 위해서 레벨 I, II의 비파괴 손상평가(NDE) 기법을 분류 및 검토하였다. 그리고 수치해석을 통해 검토한 기법의 손상 탐지 성능 및 적용성을 확인하였다. 그 결과 모달 변형 에너지를 이용한 손상탐지 기법이 다른 기법들에 비해 가장 손상에 대해 민감한 결과를 나타냈다. 실제 구조물에서 모니터링 시스템을 적용하는 경우 환경적, 경제적 요인들에 의해 센서 개수는 제한적이다. 센서 수의 감소함에 따른 손상탐지 성능에 대한 영향을 일련의 수치해석 과정을 통해 분석하고 그 결과를 논의하였으며, 생산 라이저의 건전성 평가에 대한 최적 센서 개수를 추천하였다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2007년도 춘계학술대회논문집
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• pp.859-865
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• 2007

This paper presents a method to analyze the unbalance response of a high speed polygon mirror scanner motor supported by sintered bearing and flexible supporting structures by using the finite element method and the mode superposition method. The appropriate finite element equations for polygon mirror are described by rotating annular sector element using Kirchhoff plate theory and von Karman non-linear strain, and its rigid body motion is also considered. The rotating components except for the polygon mirror are modeled by Timoshenko beam element including the gyroscopic effect. The flexible supporting structures are modeled by using a 4-node tetrahedron element and 4-node shell element with rotational degrees of freedom. Finite element equations of each component of the polygon mirror scanner motor and the flexible supporting structures are consistently derived by satisfying the geometric compatibility in the internal boundary between each component. The rigid link constraints are also imposed at the interface area between sleeve and sintered bearing to describe the physical motion at this interface. A global matrix equation obtained by assembling the finite element equations of each substructure is transformed to a state-space matrix-vector equation, and both damped natural frequencies and modal damping ratios are calculated by solving the associated eigenvalue problem by using the restarted Arnoldi iteration method. Unbalance responses in time and frequency domain are performed by superposing the eigenvalues and eigenvectors from the free vibration analysis. The validity of the proposed method is verified by comparing the simulated unbalance response with the experimental results. This research also shows that the flexibility of supporting structures plays an important role in determining the unbalance response of the polygon mirror scanner motor.