• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2001년도 봄 학술발표회 논문집
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• pp.533-540
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• 2001

Material damping is an important parameter to evaluate the site response by a dynamic loading. Currently the material damping of the subgrade is mainly determined by a resonant column testing. Typical methods to evaluate material damping include half-power bandwidth method and free-vibration decay method. In the large strain range, the half-power bandwidth method gives an erratic damping factor, because the method is based on the assumption of the linear behavior of a specimen. The free-vibration decay method has also limitations in that the damping factors vary with the range of cycles in calculation, and also in that the specific shear strain can not be designated for the free vibration. In this study, the frequency-phase method, which was developed to evaluate material damping of a beam simply supported, is introduced to evaluate the material damping by the resonant column testing. Also, the comparison among half-power method, free-vibration decay method and the frequency-phase method is provided for a remolded sand.

• Coupled systems mechanics
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• 제9권5호
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• pp.455-471
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• 2020

This paper presents the results of the study of vertically natural frequency of tractor tires are effected by changing different ground contacts and inflation pressures using the Free Decay Method. The results show that the natural frequencies of the tire are not affected while the vertical acceleration increased strongly due to the increase of inflation pressure when the tire performs free decay vibration on rigid ground. In addition, the number of natural frequency peaks of the tire also increases with increasing tire inflation pressure. On the other hand, the natural frequencies of the tractor tire increases strongly while the vertical acceleration decreases slightly with the increase of tire inflation pressure as the tire performs free decay vibration on soft soil. Further, the natural frequencies of tire-soil system are always higher than that of tire only, and it changed with changing the soil depth. Results also show the natural frequency of tire and tire-soil system is in the range of 3.0 to 10.0 Hz that lie within the most critical natural frequency range of the human body. These findings have to be mentioned and used as design parameters of the tractor suspension system.

• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 1996년도 가을 학술발표회 논문집
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• pp.159-167
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• 1996

This paper describes the theory and verification of a method which utilizes the free decay response of a structure to determine its vibration parameters. The theory of the method is base on the formulation of a system matrix, contains information characterizing the complete set of modal parameters of the system, and its eigen-solution problem. The applicability of the method is verified by simulated free decay response data of a cantilever bean The method described was used to determined the parameters related to the first five generated modes of vibration of a cantilever beam. It involves two very close natural frequencies which could not be identified using a frequency sweep test(peak amplitude) because of interference between modes.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 춘계학술대회논문집
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• pp.1939-1944
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• 2000

Different test methods were used to measure and compare the loss factor of a steel beam. They are free vibration logarithmic decay method, half-power bandwidth method using narrow-band analysis, wide-band analysis method with inverse FFT. In these tests, specimens are clamped at one end. Free vibration method and half-power bandwidth method gave good results. Effect of vibration pick-ups were tested. We also tried center excitation method but could not obtain loss factor.

• International Journal of Naval Architecture and Ocean Engineering
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• 제6권2호
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• pp.206-218
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• 2014

The wave attenuation by floating breakwaters in high amplitude waves, which can lead to wave overtopping and breaking, is examined by numerical simulations. The governing equations, the Navier-Stokes equations and the continuity equation, are calculated in a fixed Cartesian grid system. The body boundaries are defined by the line segment connecting the points where the grid line and body surface meet. No-slip and divergence free conditions are satisfied at the body boundary cell. The nonlinear waves near the moving body is defined using the modified marker-density method. To verify the present numerical method, vortex induced vibration on an elastically mounted cylinder and free roll decay are numerically simulated and the results are compared with those reported in the literature. Using the present numerical method, the wave attenuations by three kinds of floating breakwaters are simulated numerically in a regular wave to compare the performance.

• 전기의세계
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• 제32권3호
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• pp.142-149
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• 1983

지금까지 알려진 내부마찰측정방법으로는 자유감쇄법(free decay method), 강제진동법(forced vibration method), 비공진강제진동법(non-resonance forced vibration method) 및 초음파법(ultrasoni method) 등이 있다. 이상의 모든 내부마찰실험방법에 관한 실험장치는 그 기계의 동작특성에 따라 실험주파수가 극히 한정되어 있어 연속적인 변화를 시킬 수 없으므로 완화효과를 연속적으로 추적하기 위하여 온도를 변화시키는 방법이 널리 사용되고 있다. 이상에서 소개한 방법들중 본인들이 국산제작실험에 성공한 바 있는 장치는 자유감쇄법에 속하는 염회진자장치(torsion pendulum device) 이므로 이 장치를 중심으로 내부마찰측정원리와 그의 장치에 관하여 소개하고저 한다.

• Coupled systems mechanics
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• 제7권4호
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• pp.395-406
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• 2018

This paper proposes a linear damping model of tire-soil system using semi-empirical method. A test rig was designed and developed to measure the vertical equivalent linear damping ratio of tire only and tire-soil system using Free-Vibration Logarithmic Decay Method. The test was performed with two kinds of tractor tires using a combination of five inflation pressure levels, two soil depths and four soil moisture contents in the paddy soil. The results revealed that the linear damping ratio of tires increased with decreasing tire inflation pressure; the linear damping ratio of tire-soil system also increased with decreasing tire inflation pressure and increased with the increasing soil depth (observed at 80 and 120 mm). It also increased with a relative increase of soil moisture contents (observed at 37.9%, 48.8%, 66.7% and 77.4%). The results also indicated that the damping ratio of tire-soil system was higher than that of tire only. A linear damping model of tire-soil system is proposed as a damping model in parallel which is established based on experimental results and vibration theory. This model will have a great significance in study of tractor vibration.

• Wind and Structures
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• 제10권5호
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• pp.463-479
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• 2007

A series of wind tunnel sectional model dynamic tests of a twin-deck bridge were conducted at the CLP Power Wind/Wave Tunnel Facility (WWTF) of The Hong Kong University of Science and Technology (HKUST) to investigate the effects of gap-width on the self-excited vibrations and the dynamic and aerodynamic characteristics of the bridge. Five 2.9 m long models with different gap-widths were fabricated and suspended in the wind tunnel to simulate a two-degrees-of-freedom (2DOF) bridge dynamic system, free to vibrate in both vertical and torsional directions. The mass, vertical frequency, and the torsional-to-vertical frequency ratio of the 2DOF systems were fixed to emphasize the effects of gap-width. A free-vibration test methodology was employed and the Eigensystem Realization Algorithm (ERA) was utilized to extract the eight flutter derivatives and the modal parameters from the coupled free-decay responses. The results of the zero gap-width configuration were in reasonable agreement with the theoretical values for an ideal thin flat plate in smooth flow and the published results of models with similar cross-sections, thus validating the experimental and analytical techniques utilized in this study. The methodology was further verified by the comparison between the measured and predicted free-decay responses. A comparison of results for different gap-widths revealed that variations of the gap-width mainly affect the torsional damping property, and that the configurations with greater gap-widths show a higher torsional damping ratio and hence stronger aerodynamic stability of the bridge.

• 한국소음진동공학회논문집
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• 제21권12호
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• pp.1192-1198
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• 2011

Design routines of a torsional spring-viscous damper for a 1800 kW four cycle diesel engine-generator system are described. Modal techniques for system normalization and optimal equations for damper design are used to obtain proper design parameters of the damper. A prototype damper is manufactured according to the described design process and its two design parameters, stiffness and damping, are evaluated experimentally by torsional actuator test and free decay test. Experimentally obtained values of stiffness and damping coefficients showed good agreements with the designed values of the prototype damper.

• Smart Structures and Systems
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• 제22권1호
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• pp.41-55
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• 2018

Damping ratio and frequency have influence on dynamic serviceability or instability such as vortex-induced vibration and displacement amplification due to earthquake and critical flutter velocity, and it is thus important to make determination of damping ratio and frequency accurate. As bridges are getting longer, small scale model test considering similitude law must be conducted to evaluate damping ratio and frequency. Analysis conditions modified by similitude law are applied to experimental test considering different scale ratios. Generally, Nyquist frequency condition based on natural frequency modified by similitude law has been used to determine sampling rate for different scale ratios, and total time length has been determined by users arbitrarily or by considering similitude law with respect to time for different scale ratios. However, Nyquist frequency condition is not suitable for multimode system with noisy signals. In addition, there is no specified criteria for determination of total time length. Those analysis conditions severely affect accuracy of damping ratio. The focus of this study is made on the determination of minimum analysis conditions for different scale ratios. Influence of signal to noise ratio is studied according to the level of noise level. Free initial value problem is proposed to resolve the condition that is difficult to know original initial value for free vibration. Ambient and free vibration tests were used to analyze the dynamic properties of a system using data collected from tests with a two degree-of-freedom section model and performed on full bridge 3D models of cable stayed bridges. The free decay is estimated with the stochastic subspace identification method that uses displacement data to measure damping ratios under noisy conditions, and the iterative least squares method that adopts low pass filtering and fourth order central differencing. Reasonable results were yielded in numerical and experimental tests.