• Journal of the Computational Structural Engineering Institute of Korea
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• v.19 no.3 s.73
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• pp.313-322
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• 2006

In the genetic algoricm (GA) based damage detection methods using vibration of structures, the selection of modal properties is important to improve the accuracy of damage detection. The objective of this study is to improve the accuracy of damage detection using natural frequency and modal strain energy, The following approaches are used to achieve the goal. First, modal strain energy is formulated and a new GA-based damage detection technique using natural frequency and modal strain energy is proposed. Next, to verify the efficiency of proposed technique, damage scenarios for free-free beam are designed and vibration modal tests of the target structure are conducted. Finally, the feasibility of the proposed technique is verified in comparison with other GA-based damage detection technique using natural frequency and mode shape.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.27 no.2
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• pp.234-242
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• 2017

This study investigates vibro-acoustic characteristics of a short, thick cylinder containing a slot given a pined-free boundaries. Using the finite element analysis results, structural modes of the asymmetric cylinder (with a slot) are expressed as the linear combinations of modes of the symmetric cylinder made of same material with identical geometry except the slot. Based on synthesized modal vibrations, acoustic modes of the asymmetric cylinder are obtained with two approaches, i.e., Rayleigh integral calculation and modal expansion of the acoustic modes of the symmetric cylinder. Also, acoustic powers, max. sound pressure and directivity pattern are obtained from acoustic modes and verified with the boundary element analyses. Based on these results, the accuracy of proposed approaches in calculating the vibro-acoustic properties of a short, thick, asymmetric cylinder has been confirmed. The procedure can be applied to the similar cylinders with other boundaries or asymmetric properties. Also, attenuation of vibration and/or sound radiation of the cylinder type practical components can be studied using these approaches.

• Journal of Navigation and Port Research
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• v.32 no.3
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• pp.215-221
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• 2008

Dynamic characteristics of a structure, i.e., natural frequency and mode shape, have been widely using as an input data in the area of structural integrity or health monitoring which combined with the damage evaluation and structural system identification techniques. It is very difficult, however, to get those information by the conventional modal analysis method from large structures, such as the offshore structure or the long-span bridge, since the source of vibration is not available. In this paper, a method to obtain the frequencies and the mode shapes of a large span truss structure using only acceleration responses is studied. The calculation procedures to obtain acceleration responses and frequency response functions are provided utilizing a numerical model of the truss, and the process to extract natural frequencies and mode shapes from the modal analysis is cleary explained. The extracted mode shapes by proposed method are compared with those from eigenvalue analysis for the estimation of accuracy. The validity of the mode shapes is also demonstrated using an existing damage detection technique for the truss structure by simulated damage cases.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2010.04a
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• pp.183-186
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• 2010

교각에서의 기초세굴 단계 및 상부구조물의 영향에 따른 진동특성을 파악하기 위해 교각 시험체를 이용하여 충격진동실험과 Midas FEA ver.2.0를 이용한 유한요소해석을 수행하였다. 기초세굴 모의는 교각시험체 기초 주위의 지반을 단계별로 굴착하였으며, 상부구조물의 영향은 철근콘크리트 블록을 제작하여 교각 시험체 위에 재하하였다. 충격진동실험과 수치해석결과, 강성이 작아지거나 질량이 커질수록 1차 모드 고유진동수도 작아지는 등 유사한 경향을 나타내었다. 대체적으로 충격진동실험으로 구한 1차모드 고유진동수는 수치해석으로 구한 값보다 작은 경향을 나타내었으며, 이는 지반의 강성변화를 수치해석 모델에서 연속적으로 반영하지 못하는 한계로 인해 발생한 오차로 판단된다. 따라서 1차 모드 고유진동수의 변화를 이용한 교각 세굴 건전성 평가를 위한 유한요소해석을 위해서는 지반물성을 보다 잘 모의할 수 있는 기법의 개발이 필요하였다.

• Journal of the Korean Society for Railway
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• v.17 no.5
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• pp.321-327
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• 2014

In this paper, linear dynamic equations are derived from nonlinear dynamic equations of constrained multibody systems using the QR decomposition method. The derived linear equations are applied to a railway vehicle bogie. The vibration characteristics of the railway vehicle are investigated by calculating the natural mode and transfer function of the bogie frame in relation to rail-roughness input. The main modes of the bogie were found below 35Hz, and the local modes above 198Hz. The magnitude of the vertical transfer function varied with the forward velocity due to vertical and pitch modes, which were influenced by the forward velocity. The magnitude of the lateral transfer function was negligibly small, and the mode in the longitudinal direction was excited for longitudinal transfer function regardless of the forward velocity.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.2
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• pp.227-233
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• 2014

This study investigates the vibration characteristics of a wire-bonding piezoelectric transducer and ultrasonic horn for high-speed and precise welding. A ring-type piezoelectric stack actuator is excited at 136 kHz to vibrate a conical-type horn and capillary system. The nodal lines and amplification ratio of the ultrasonic horn are obtained using a theoretical analysis and FEM simulation. The vibration modes and frequencies close to the driving frequency are identified to evaluate the bonding performance of the current wire-bonder system. The FEM and experimental results show that the current wire-bonder system uses the bending mode of 136 kHz as the principal motion for bonding and that the transverse vibration of the capillary causes the bonding failure. Because the major longitudinal mode exists at 119 kHz, it is recommended that the design of the current wire-bonding system be modified to use the major longitudinal mode at the excitation frequency and to minimize the transverse vibration of capillary in order to improve the bonding performance.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2000.06a
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• pp.340-345
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• 2000

진동하는 구조물의 음향 방사 예측에는 키르히호프-헬름홀쯔 적분 방정식에 근본을 둔 경계 요소 해석이 널리 사용된다. 이 경계 요소 해석은 익히 알고 있듯이 구조물의 동적 거동이 정량적으로 표현될 수 있는 경우는 매우 높은 정확도의 예측 결과를 제공한다. 그러나 실제 현상에서 접할 수 있는 복잡한 구조물의 음향 방사 예측에는 많은 변수들로 인해 예측의 정확도가 감소됨은 확실하다. 다른 방법으로는 실험을 통한 임의의 음장 예측 방법인 근음장 음향 홀로그래피(nearfield acoustical holography) 방법을 들 수 있다. 이 방법은 실제로 발생되는 음향 방사로부터 마이크로폰을 이용하여 홀로그램면의 음압 또는 속도를 측정하고 키르히호프-헬름홀쯔 적분 방정식에 적용하여 임의의 홀로그램면에 투사(mapping)시켜 음장을 예측하는 방법이다. 근음장 음향 홀로그래피는 탁월한 정확성을 갖고 있으나, 측정의 복잡성과 홀로그램면을 형성하기 위한 많은 이산점(절점)의 필요성 등의 단점을 갖고 있다. 본 논문에서는 또 다른 음장 예측 방법인 실험의 장점과 유한 요소 해석의 장정을 복합시킨 모드 확장 방법(modal expansion method)을 사용하여 단순 구조물인 평판의 진동에 의한 음장을 예측해 보았다. 모드 확장 방법은 구조물의 동적 거동은 모드의 선형 조합으로 표현될 수 있다는 것에 그 원리를 둔다. 본 논문은 단순 평판을 대상으로 유한 요소 해석으로 구한 모드 정보와 실험에 의해 얻은 입의 가진 주파수에 대한 진동 표면의 속도 분포를 조합하여 속도 경계 조건을 구성, 경계 요소 해석으로 음장 예측을 수행하였으며 모드 확장 방법을 사용함에 있어 고려해야할 몇 가지 사항에 대해 다루었다.

• Journal of the Korea institute for structural maintenance and inspection
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• v.10 no.6
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• pp.113-123
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• 2006

As Cable-stayed bridges were constructed to the long span, they have become bigger and had weaknesses to vibration induced by earthquake, wind and vehicle loads. Structural damages induced by these loads affect the characteristic of vibration modes of structure. Damage detection of cable-stayed bridges by using existing safety diagnosis is difficult to detect the characteristic change of overall structural action. Also it requires very much time and cost. So in this study, the investigation of characteristic change of structural action and the detection of structural damages is analyzed by using characteristic properties of vibration mode before and after structural damage.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 1994.10a
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• pp.282-287
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• 1994

본 연구에서는 연구대상 차량의 소음저감 방안을 마련하기 위하여 차체의 진동 및 차실의 음향 특성해석, 연성해석을 수행하였다. 차실 음향특성을 나타내는 음향모드는 유한요소 해석으로부터 결정하였다. 이때 해석결과를 확인하기 위하여 음향모드를 측정, 수치해석결과와 비교하였다. 차실소음의 가진 특성을 갖는 차체의 진동특성은 모드시험을 통해서 결정하였다. 결정된 이들 모드들의 연성해석은 연성해석 전용 컴퓨터 프로그램을 사용하여 수행하였고, 그 결과를 소음실험 결과와 비교하여 Booming 소음에 기여가 큰 차체 panel부위를 결정하였다. 기여가 큰 panel의 진동특성 변경시 소음효과를 측정하여 구조변경 방안을 검토하였다.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.16 no.4 s.109
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• pp.339-345
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• 2006

In this paper, a physically meaningful methodology which can assess the contribution of each vibration mode to various vibration response signals (displacement, velocity, acceleration) is developed. Based on these results, the problem of quantitative assessment of the relative importance of a structural system's vibrational modes is discussed. In addition, a direct method which ran assess the relative importance of each mode from uniformly sampled experimental data is also proposed.