• Wind and Structures
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• v.19 no.4
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• pp.353-370
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• 2014

A new method is proposed in this study for estimating the damping ratio of a super tall building under strong wind loads with short-time measured acceleration signals. This method incorporates two main steps. Firstly, the power spectral density of wind-induced acceleration response is obtained by the wavelet transform, then the dynamic characteristics including the natural frequency and damping ratio for the first vibration mode are estimated by a nonlinear regression analysis on the power spectral density. A numerical simulation illustrated that the damping ratios identified by the wavelet spectrum are superior in precision and stability to those values obtained from Welch's periodogram spectrum. To verify the efficiency of the proposed method, wind-induced acceleration responses of the Guangzhou West Tower (GZWT) measured in the field during Typhoon Usagi, which affected this building on September 22, 2013, were used. The damping ratios identified varied from 0.38% to 0.61% in direction 1 and from 0.22% to 0.59% in direction 2. This information is expected to be of considerable interest and practical use for engineers and researchers involved in the wind-resistant design of super-tall buildings.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.21 no.6
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• pp.67-73
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• 2022

Industrial inverters are used in a variety of fields for electric power supply. They may be exposed to vibration and heat once they are installed. This study focused on a framework of accelerated life testing of an industrial inverter considering fatigue damage as the primary source of deterioration. Instead of analyzing detailed failure mechanisms and the product's vulnerability to them, the potential of fatigue failure is considered using the fatigue damage spectrum calculated from the environmental vibration signals. The acceleration and temperature data were gathered using field measurement and spectral analysis was conducted to calculate the vibration signal's power spectral density (PSD). The fatigue damage spectrum is then calculated from the input PSD data and is used to design an accelerated fatigue life testing. The PSD for the shaker table test is derived that has the equivalent fatigue damage to the original input signal. The tests were performed considering the combined effect of random vibration and elevated temperature, and the product passed all the planned tests. It was successfully demonstrated that the inverter used in this study could survive environmental vibration up to its guarantee period. The fatigue damage spectrum can effectively be used to design accelerated fatigue life testing.

• Journal of the Earthquake Engineering Society of Korea
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• v.19 no.1
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• pp.21-28
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• 2015

The nuclear accident due to the recent earthquake in Japan has triggered awareness of the importance of safety with regard to nuclear power plants (NPPs). An earthquake is one of the most important parameters which governs the safety of NPPs among external events. Application of a base isolation system for NPPs can reduce the risk for earthquakes. At present, a soil-structure interaction (SSI) analysis is essential in the seismic design of NPPs in consideration of the ground structure interaction. In the seismic analysis of the base-isolated NPP, it is restrictive to consider the nonlinear properties of seismic isolation devices due to the linear analysis of the SSI analysis programs, such as SASSI. Thus, in this study, SSI analyses are performed using an iterative approach considering the material nonlinearity of the isolators. By performing the SSI analysis using an iterative approach, the nonlinear properties of isolators can be considered. The difference between the SSI analysis results without iteration and SSI with iteration using SASSI is noticeable. The results of the SSI analysis using an effective linear (non-iterative) approach underestimate the spectral acceleration because the effective linear model cannot consider the nonlinear properties of isolators. The results of the SSI analysis show that the horizontal response of the base-isolated NPP is significantly reduced.

• Structural Engineering and Mechanics
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• v.66 no.5
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• pp.557-568
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• 2018

An investigation is made in the present work on the post-buckling and geometrically nonlinear behaviors of moderately thick perfect and imperfect rectangular plates made-up of functionally graded materials. Spectral collocation approach based on Legendre basis functions is developed to analyze the functionally graded plates while they are subjected to end-shortening strain. The material properties in this study are varied through the thickness according to the simple power law distribution. The fundamental equations for moderately thick rectangular plates are derived using first order shear deformation plate theory and taking into account both geometric nonlinearity and initial geometric imperfections. In the current study, the domain of interest is discretized with Legendre-Gauss-Lobatto nodes. The equilibrium equations will be obtained by discretizing the Von-Karman's equilibrium equations and also boundary conditions with finite Legendre basis functions that are substituted into the displacement fields. Due to effect of geometric nonlinearity, the final set of equilibrium equations is nonlinear and therefore the quadratic extrapolation technique is used to solve them. Since the number of equations in this approach will always be more than the number of unknown coefficients, the least squares technique will be used. Finally, the effects of boundary conditions, initial geometric imperfection and material properties are investigated and discussed to demonstrate the validity and capability of proposed method.

• Physical Therapy Korea
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• v.6 no.2
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• pp.16-31
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• 1999

The purpose of this study was to assess the fatigue in lumbar and abdominal muscles in patients with chronic low back pain compared with normal subjects using spectral analysis with mean power frequency and median power frequency. The experimental group consisted of twenty subjects who had experienced chronic low back pain for over one year after the onset day. A control group consisted of twenty normal subjects with no history of low back pain. All subjects stood in an apparatus to perform sustained contraction in the lumbar and abdominal muscles for 30 seconds with 60% maximal voluntary isometric contraction (MVIC). The resulting electromyographic (EMG) recorded time serial data were transformed into frequency serial data by Fast Fourier Transformation (FFT). The results were as follows: 1) lumbar muscles measured, the frequency change ratio of both median power frequency and mean power frequency was significantly greater for experimental group compared with control group group (p<0.05). In measured two abdominal muscles (inferior rectus abdominis, obliquus externus abdominis) except superior rectus abdominis, the frequency change ratio of both median power frequency and mean power frequency was significantly greater for experimental group compared with control group (p<0.05). 2) In all three (longissimus thoracis, iliocostalis lumborum, multifidus) lumbar muscles measured, the initial frequency value of both median power frequency and mean power frequency was significantly lower for the experimental group compared with the control group (p<0.05). In the two (inferior rectus abdominis, obliquus externus abdominis) abdominal muscles measured (superior rectus abdominis not included), the initial frequency value of both median power frequency and mean power frequency was significantly lower for the experimental group compared with the control group (p<0.05). These results suggest that in patients with chronic low back pain there is a trend for more fatigue to occur in both lumbar and abdominal muscles than in the normal control group. This would seem to suggest that in treatment programs for patients with chronic low back pain, improvement of endurance in all trunk muscles should be considered.

• Journal of Astronomy and Space Sciences
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• v.30 no.2
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• pp.79-82
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• 2013

We present the Hilbert-Huang transform (HHT) analysis on the quasi-periodic modulation of SMC X-1. SMC X-1, consisting of a neutron star and a massive companion, exhibits superorbital modulation with a period varying between ~40 d and ~65 d. We applied the HHT on the light curve observed by the All-Sky Monitor onboard Rossi X-ray Timing Explorer (RXTE) to obtain the instantaneous frequency of the superorbital modulation of SMC X-1. The resultant Hilbert spectrum is consistent with the dynamic power spectrum while it shows more detailed information in both the time and frequency domains. According to the instantaneous frequency, we found a correlation between the superorbital period and the modulation amplitude. Combining the spectral observation made by the Proportional Counter Array onboard RXTE and the superorbital phase derived in the HHT, we performed a superorbital phase-resolved spectral analysis of SMC X-1. An analysis of the spectral parameters versus the orbital phase for different superorbital states revealed that the diversity of $n_H$ has an orbital dependence. Furthermore, we obtained the variation in the eclipse profiles by folding the All Sky Monitor light curve with orbital period for different superorbital states. A dip feature, similar to the pre-eclipse dip of Her X-1, can be observed only in the superorbital ascending and descending states, while the width is anti-correlated with the X-ray flux.

• ETRI Journal
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• v.35 no.2
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• pp.188-192
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• 2013

Previous work proposed combining multipulse pulse position modulation (MPPM) with pulse amplitude modulation to form multipulse amplitude and position modulation (MPAPM), which is a hybrid modulation that results in an improvement in bandwidth efficiency but a degradation in power efficiency. In this paper, to achieve greater power efficiency and a better data rate, we propose multipulse dual amplitude-width modulation, based on MPAPM and pulse width modulation. The proposed scheme shows a remarkable improvement in data rate and a 1.5-dB improvement in power efficiency over MPAPM, while sustaining the bandwidth efficiency. After introducing symbol structure, we present the theoretical expressions of spectral efficiency, the power requirements, and the normalized data rate, as well as the results of comparing the proposed modulation to MPPM and MPAPM.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.31 no.10
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• pp.109-115
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• 1982

Reactor dynamics were studied by reactor noise at TRIGA MARK Il reactor whose rated power is 250KW thermal. The power spectral densities(PSD) of the noise were measured by stochastic method with high resolution digital filters and Fast Fourier Transformers. The transfer function of the reactor at zero power was identical to the theoretical characteristics. When the power was increasec above 1KW, reactor showed its poswer resonances at 3Hz and 10 Hz. It was analyzed that 3Hz peak was generated by heat transfer and coolant flow effects and 10Hz peak by nuclear reaction effects.

• Journal of KSNVE
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• v.10 no.1
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• pp.129-137
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• 2000

This paper presents the vibration characteristics of reactor internals of Ulchin-1 nuclear power plant, which are identified by using the conventional and the phase separated spectral analysis of the pressure vessel acceleration and ex-core neutron signals. These identified vibration characteristics show excellent agreement with those of Tricastin-1 nuclear power plant that is the prototype of Ulchin-1. And the trend of ex-core neutron signals has been observed during one reactor cycle. These results can be used as basic data for fault diagnosis of reactor internals.

• Structural Engineering and Mechanics
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• v.2 no.3
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• pp.285-294
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• 1994

Based on a fast and accurate method for the stationary random seismic response analysis for discretized structures(Lin 1992, Lin et al. 1992), a Ritz method for dealing with such responses of continuous systems in developed. This method is studied quantitatively, using cantilever shear beams for simplicity and clarity. The process can be naturally extended to deal with various boundary conditions as well as non-uniform Bernoulli-Euler beams, or even Timoshenko beams. Algorithms for both proportionally and non-proportionally damped responses are described. For all of such damping cases, it is not necessary to solve for the natural vibrations of the beams. The solution procedure is very simple, and equally efficient for a white or a non-white ground excitation spectrum. Two examples are given where various power spectral density functions, variances, covariances and second spectral moments of displacement, internal force response, and their derivatives are calculated and analyses. Some Ritz solutions are compared with "exact" CQC solutions.