Papers by Keyword: Frequency Response Function (FRF)

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Authors: Mehrisadat Makki Alamdari, Jian Chun Li, Bijan Samali
Abstract: This paper presents a novel damage localization method based on the measured Frequency Response Functions (FRFs) without demanding any previous data records of the structure in its healthy state. The main innovation of this study starts with reconstruction of FRFs curvature to develop spatial shape functions. It is demonstrated that reconstructed data significantly magnifies the influence of low-frequency spectra in damage detection procedure which is considered the milestone of this approach as excitation of the higher frequencies is not easy to obtain in most practical applications. The modified curvature data in all measured frequencies and locations is interpreted as a two dimensional image and then processed by employing 2-D discrete wavelet transform to detect any abrupt variation at damage site. Level one wavelet decomposition is utilised to provide the finest detail coefficients. It is illustrated that this approach presents a more recognizable pattern at damage site in all measured frequencies. The pattern can be described by a horizontal line parallel to the frequency spectra in 2-D image. Hence, the horizontal detail coefficients are utilised to detect this pattern as they are more sensitive to perturbation with orientation parallel to horizontal axis in the image. The main contribution of this approach lies in the fact that the proposed technique is able to detect the structural damage in all measured frequencies and the effectiveness of the method is independent of the excitation location. Moreover, the results provide a better visualisation at damage site which other FRF-based damage detection methods could not obtain. Applying broadband FRF data in this approach and the fact that there is no need for data from the healthy state of the structure are other advantages accompanying this method. The robustness of the proposed damage identification method was examined with various damage conditions in both single and multiple states. Moreover, the feasibility of the method was verified in presence of practical uncertainties such as noise using extensive numerical simulations. It was demonstrated that the proposed method is particularly attractive for practical applications as it opens an opportunity for online monitoring of the structural integrity without demanding any previous data records of the structure.
Authors: Young Shin Lee, Hyun Soo Kim, C.H. Han
Abstract: An experimental modal analysis is the process to identify structure's dynamic characteristics. For investigating vibrational characteristics of cylindrical shell with multiple supports, modal testing is performed using impact exciting method. The frequency response function(FRF) measurements are also made on the experimental model within the frequency range from 0 to 4kHz. Modal parameters are identified from resonant peaks in the FRF’s and animated deformation patterns associated with each of the resonances are shown on a computer screen. The experimental results are compared with analytical and FEA results.
Authors: Shan Shan Sun, Wei Xiao Tang, Xi Qing Xu
Abstract: Chatter problems occurring during high speed milling affect the quality of the finished workpiece and, to a lesser extent, the tool life and the spindle life. Therefore, the prediction of stable milling regions is a critical requirement for high speed milling operations. In this paper, a dynamic model of a high speed spindle system considering the multi-mode dynamics is elaborated for the purposed of stability prediction. A stability lobes diagram (SLD) shows the boundary between chatter-free machining operations and unstable processes, in terms of axial depth of cut as a function of spindle speed. These diagrams are used to select chatter-free combinations of machining parameters. The proposed method enables a new stability lobes diagram to be established that takes into account the effect of spindle speed on multi-mode dynamic behavior.
Authors: Y.Q. Huang, Yan Shen Xu
Authors: Yan Gao, Jia Lu Li
Abstract: The work of vibration test has significant meanings for the researches and applications of 3-dimension and 5-direction braided composites. This article discusses the effects of added mass with different weight on the modal test of 3-dimension and 5-direction braided composites. The comparison of the modal parameters of 3-dimension and 5-direction braided composites tested by different weight of mass reveals that the additional mass is a mostly influence factor for vibration property of 3-dimension and 5-direction braided composites. The results of frequency response and force response curves show that smaller mass accelerometer is more effective for a wider range of frequencies around the resonance frequency, a higher natural frequency and a larger peak in these points. Force-response curves show that force response amplitude increases with the increase of additional mass weight, and the larger additional mass, the shorter time taken for reaching stationary state. The errors of natural frequency and damping ratio increase when the weight of additional mass increases. With the increase of modal orders, relative errors of modal characteristics have slighter decreasing degrees. The results derived from this article will provide a useful reference for precise modal analysis of 3-dimension and 5-direction braided composites.
Authors: Joo Yong Cho, Usik Lee
Abstract: In this paper, experiments are conducted to determine the non-ideal boundary conditions (BCs) of example beam structures. The spectral element (SE)-model is used for the beam structures, and the non-ideal BCs are represented by the frequency-dependent effective boundary springs. The boundary spring constants are then determined from the measured FRF-data. It is shown that the vibration responses analytically predicted by using experimentally identified BCs are very close to the measurements.
Authors: L. Castro, P. Viéville, Paul Lipiński
Abstract: This paper proposes an experimental method for obtaining the Frequency Response Function (FRF) between a dynamic force and the signals emitted by a piezoelectric dynamometer. This function is known as Transmissibility. In the FRF obtaining stage, different configurations of mounting and excitation have been compared to improve the function quality. The method has been developed with a three components dynamometer fixed on a milling machine. The FRF has two principal applications: it is used to evaluate the measurement system accuracy and to correct the measurements, if necessary. The method has been developed with the purpose of studying the cutting forces in machining process. Furthermore, it has been identified the influence of the parts of the measurement chain in the measuring system response.
Authors: Norlida Jamil, Ahmad Razlan Yusoff, Muhammad Hatifi Mansor
Abstract: Machine tool vibrations have great impact on machining process. Modal testing is a form of vibration testing which is able to determine the Frequency Response Function (FRF) of the mechanical test structures. In this paper, the main focus is to identify a procedure to obtain natural frequency values for machine tool components in order to establish better conditions in the cutting process on the machine tool. For this purpose, a 3D model of the machine tools structure is made using design software and exported to analysis software. Later on, the Finite Element Method (FEM) modal analysis was used to obtain the natural frequencies. The model is evaluated and corrected through an experimental modal test. In the experiment, the machine tool vibration is excited by impact hammer and the response of excited vibration is recorded. In the end, the result of both FEM and experimental shows a good consistency in comparison.
Authors: O.S. David-West, J. Wang, R. Cooper
Abstract: Simulation result of a structural dynamics problem is dependent on the techniques used in the finite element model and the major task in model updating is determination of the changes to be made to the numerical model so that dynamic properties are comparable to the experimental result. In this paper, the dynamic analysis of a thin wall structure ( approx. 1.5±0.1 mm thick) was realized using the Lanczos tool to extract the modes between 0 and 200 Hz, but the interest was to achieve a good aggreement between the first ten natural frequencies. A shell element with mid size nodes was used to improve the finite element result and the model was tunned using the damping constant, material properties and discretization. The correlation of the results from the impact excitation response test and the finite element was significantly improved. A correlation coefficient of 0.99 was achieved after tunning the model.
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