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Hilbert-Huang transform (HHT) consists of two main parts: (1) empirical mode decomposition (EMD) to extract intrinsic mode functions (IMFs) and (2) Hilbert spectral analysis to obtain time-frequency characteristics of the IMFs through the Hilbert transform. Recently, a new enhanced HHT is proposed by the authors in which, two mathematical limitations that restrict the application of the Hilbert transform are circumvented and also an additional smoothing parameter is applied to decrease noise effects on the results. In this paper based on the HHT approach, a simple method for output-only identification of natural frequencies of linear structures is proposed in which HHT or enhanced HHT can be employed. In the proposed method, ambient response data measured at all degrees of freedom of the structure are used to obtain an averaged marginal spectrum. The averaged marginal spectrum is used for identifying the natural frequencies of the structure. In order to validate the effectiveness of the proposed identification method, ambient response data of an arch bridge and a 15-story building are examined. In the first case, the first six natural frequencies of the bridge in vertical direction are extracted. And in the second case, the first three natural frequencies of the building in East-West, North-South and torsional directions are identified. From the results, first, it is found that the enhanced HHT by employing the smoothing parameter is more efficient than the HHT in increasing the readability of the time-frequency-amplitude spectrum and also is capable to provide more accurate amplitude-frequency distribution; second, by comparing results of the proposed method with those obtained from other valid methods, it is concluded that the proposed identification method by using the enhanced HHT is accurately able to estimate the natural frequencies of structures. Regarding to simplicity of the proposed method, it can be applied as an efficient tool for identification of structures or employed to extract changes in frequencies due to occurrences of damages during strong ground motions.

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In this research crack size and location in pipes under fluid pressure will be detected using pipe’s natural frequencies by neural network. Neural network used in this research is multi-layer perceptron. Comparing different inputs, appropriate inputs are selected. Pipes contain water. Steel and aluminum pipes were used in this research. Pressure condition of the pipes is: 1) without water 2) water with zero pressure 3) water with 0.498 MPa 4) water with 0.981 MPa. Crack size range from 0.19043 to 0.6346. Crack location range from 0.199 to 0.403. Many researches have been done about crack detection based on natural frequencies of structures by neural network. However, as far as authors know, no work has been done for crack detection in pipes containing pressurized water. Also in this paper two structures with different materials have been used for neural network training and testing which is another innovation of this research. Comparison of the results of this method with analytic methods shows that the proposed method is always more accurate in detecting crack size but is not always better in estimating crack location.

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Conventional modal testing is known as a powerful tool for dynamic analysis of structures. However, for some engineering structures, conventional modal testing is difficult or even impossible to conduct due to the problems associated with the artificial excitation of structure. Operational Modal Analysis (OMA) is one solution to deal with these cases. In OMA the structure is excited by ambient forces and only the responses are measured and taken into account. Accelerometers are the traditional tools for measuring the responses of structure. It is well khonwn that the measured responses are contaminated by bias errors corresponding to the mass-loading effect of accelerometers. This causes the natural frequencies of structure are measured lower than the real values. In this paper a new method is proposed for eliminating the mass-loading effects of accelerometers from measured responses in OMA. A numerical model of a mass-spring-damper system is used for validation of the method. Also a steel plate is used for experimental validation of the proposed approach. The results are confirmed by those of conventional modal testing. Both numerical and experimental results show that the proposed method can effectively eliminate the mass-loading effects of accelerometers from measured responses in OMA. Also, the method has the ability to correct the measured natural frequencies from OMA accurately.

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In this paper, the fluid- solid interaction in an electrostatic microbeam by using three- dimensional aerodynamic theory has been studied. Modified couple stress theory is used to model the elasticity depends on the size of the microbeam. The proposed model can be used as a mass micro- sensor. To analyze the dynamic behavior of the microbeam a DC voltage applied to the system and then by applying an AC voltage dynamic characteristics of the system around static deformed condition is analyzed. Because of non-linear nature of the governing equations to solve them reduced order model based on Galerkin is used. Results have shown that considering the couple stress and also increase the size of the length characteristic parameter reduces the size of the fluid pressure differential created between the two sides of the microbeam. However, according to the three- dimensional aerodynamic theory for fluid-solid interaction, change of the pressure difference created does not lead to creation difference in predicting the size of the added mass between the classical and modified couple stress theories. In another part of the results has been shown that the presence of added mass to what extent can makes changes in the frequency response curves drawn for the system. Also applied the couple stress theory and increase the size of the length characteristic parameter makes the system more rigid and consequently reduce the amplitude of the vibration and frequency response curves shift to the right.

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The fluid induced vibration in fluid conveying pipes can cause fatigue and failure in the system. Therefore, controlling these unwanted vibrations and suppressing the vibrations of the fluid conveying pipe is important. In this paper by considering the passive vibration absorber for the fluid conveying pipe, the influence of the vibration absorber parameters on the dynamic behavior of the system is investigated. The governing equations of motion are obtained via the Newton’s second law, and analytical solutions for the characteristic equation and mode shapes of the system are obtained through the power series method. After verifying the obtained results, the effect of the vibration absorber parameters and the fluid flow velocity on the vibration behavior of the fluid conveying pipe have been investigated. Results show that by increasing the absorber mass, the effect of absorber on decreasing the oscillations amplitude is diminished. At different fluid velocities, the oscillation amplitude of the system can be reduced considerably by specifying proper values of the absorber parameters. At velocities near the critical velocity, where the oscillation amplitude reaches a maximum value, using a suitable vibration absorber may reduce the maximum oscillations amplitude of the system by 98%. The method presented in current study can be easily generalized to design passive vibration absorber for fluid conveying pipes with different boundary conditions.