Papers by Keyword: Modal Parameters

Abstract: Structural health monitoring (SHM) is a modern technique for damage identification in the existing structure. The structural stiffness, frequency, damping, and dominant mode shapes represent the actual operating conditions of the structure. The main principle of structural health monitoring is to identify the modal parameters from experimental results both damaged and undamaged conditions. Damage is much effective to decrease stiffness and strength of structural components and it changes dynamic behaviour and damping ratio of whole structures. Bruel & Kjaer experimental modal analysis technique is recently used for civil engineering structures for modal parameters estimation. The paper describes the initial structural health monitoring of a steel frame. The modal parameters were estimated for undamaged conditions and these results are verified and updated by the numerical FEM tool SAP2000. For the undamaged structure, mode shapes and frequencies were calibrated properly. In the second step, damaged was initiated by dismantling one element from the lower part of the frame. The estimated modal parameters were compared to the initial one. The mode shapes and frequencies are quite different for some specific mode due to damage initiation. One extra mode was created for the damaged frame due to damage initiation. The 4^th mode was not found for the initial measurement because of presence of lower beam. Lower beam restraints the 4^th mode and the frame behaves more flexible. Keywords: SHM, Modal parameters, FEM modelling, Damage characterization, Experimental modal analysis (EMA).

Abstract: In this paper, there are built new composite bars with different core: with the core made of polypropylene honeycomb and polystyrene reinforced with fiber-glass. Starting from the theoretical background the modal parameters identification, there will be established an experimental method used to determine the eigenparameters of the sandwich bars. In the end, some comparisons between the eigenparameters will be made.

Abstract: The paper deals with measurements of modal parameters using high-speed digital image correlation and assesses how the size of image elements (facets) influences the accuracy of modal parameters of the analyzed structure. The facet size is an important parameter that significantly affects the correlation process and its results. In the paper the experimental modal analysis of steel circular plate that has been excited by the impact hammer is described. Correlation system Q-450 Dantec Dynamics was used to measure the responses of the analyzed structure.

Abstract: In any flutter prediction analysis, modal testing is necessary because flutter, a resonant like vibration occurs at a flutter frequency and adopts a mode shape akin to its structural natural modes. Modal testing can be performed computationally with knowledge of the mechanical properties of the structure. In the present work, computational modal analysis is first performed on a cantilevered hybrid composite thin plate and validated experimentally. Then, the computational procedure is demonstrated on a composite hollow wing model of same material. The concept of hollow wing is explored due to the superior mechanical properties of carbon/kevlar composite plate. It is observed that the natural frequencies of the hollow wing model are higher than thin plate due to stiffer configuration. A breathing mode was also observed at mode 4 for the hollow wing.

Abstract: Identification of modal parameters is crucial especially in aerospace applications whereby the interactions of airflow with aircraft structures can result in undesirable structural deformations. This structural deformation can be predicted with knowledge of the modal parameters. This can be achieved through conventional modal testing that requires a known excitation force in order to extract these dynamic properties. This technique can be experimentally complex because of the need for artificial excitation and it also does not represent actual operational condition. The current work presents part of research work that address the practical implementation of operational modal analysis (OMA) applied to a cantilevered hybrid composite plate exposed to low speed airflow in a wind tunnel. A single contactless sensing system via a laser vibrometer is employed to measure the response. OMA technique applied in a wind-on condition succeeded in extracting the modal parameters of the hybrid composite plate which correlate well with modal testing using impact hammer excitation.

Abstract: Research on the dynamic characteristics of gravity dam was carried out by adopting ARX model using the seismic response data of concrete gravity dam at Shui-Kou hydropower station. The applicability and effectiveness of single-output-multiple-input ARX model were deduced and verified. A corresponding computer program was developed, and performed to identify the modal parameters of the system. The identified natural frequencies and damping ratios were basically same with the results by traditional peak point pick-up method, and also close to the finite element method (FEM) results. It is indicated that the structure natural frequencies and damping ratios are determined by the characteristics of the structure, the dynamic characteristics identified by ARX model are correct, and ARX model can avoid frequency leakiness when smoothing processing and Fourier transform are conducted in solving process of the peak point pick-up method. The modal identification can be applied to other structures.

Abstract: To reduce the noise and vibration of mobile wastewater treatment plant, this paper presents testing and analysis of 50 measuring points in the working state of mobile wastewater treatment plant based on TST test analysis system. The structure’s natural frequency, damping and modal vibration modes are extracted within the frequency band of 0-1000HZ. By comparing the operating modal and the finite element modal, it is believed that the operating modal can accurately recognize modal parameters of the container.

Abstract: The second vibration isolation support (SVIS) is very important parts of the vibration engineering equipment, and its dynamic performance will affect the operating condition of the entire device. In the design process, the sensitivity of design parameters provides a very effective way for design of SVIS. By analyzing the sensitivity of the amplitude of SVIS relative to the mass of SVIS, the spring stiffness and the operating frequency, as well as the sensitivity of the modal natural frequency relative to the plate thickness, the influence degree of these design parameters on the dynamic characteristics of SVIS were obtained and the best optimization parameters the structure of SVIS were obtained.

Abstract: Structural modal parameters can be identified using both output responses and input excitations or only using output responses of structures. When the input excitations are known, some frequency response functions (FRF) can be obtained and further used to accurately identify the modal frequencies and mode shapes of structures, but the identified modal damping ratio is often lack of reliability. Under ambient excitations, the peak picking method (PP) is often used to quickly identify the modal parameters of structures, but some vibration modes are missed sometimes, particularly when the dense modes or subjective errors may appear around the peaks picked. Modal parameters can be identified using the stochastic subspace identification method (SSI) based on statistical data, however, the SSI method easily produces false modes in practice. In order to compensate the deficiencies of SSI method, the PP and the SSI methods are combined to identify the modal parameters of the benchmark structure which proposed by the International Association for Structure Control and the task group of American Society Civil Engineers (IASC-ASCE) in this paper. The illustrated results show that this combination method can accurately identify the modal parameters of the benchmark structure with some application prospects.

Abstract: In the condition of environmental excitation and the modal parameters can be identified to only use response data. Based on the improved stochastic subspace identification method, can avoid the fitting error from asynchronous samples, which can integrate asynchronous multi-groups response samples into synchronous pulsing response data, Finally, the identification results of different system order number was utilized to construct stable map to acquire more accurate identification and avoid lost root or duplicate root than the conventional method.