Comparison of Modal Parameters of a Square Steel Plate Using Finite Element Method and Operational Modal Analysis
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In engineering domains, modal analysis is a field that is studied extensively with the goal of better characterizingcomponents or structures. Themodal analysis produces modal parameters, which are essential properties of a system that can be employed for damage identification, vibration analysis, and structural health monitoring. Plated element’s relevance in variousapplications makes it an intriguing vibration study subject. In thisresearch, modal analysis was performed on a square steel plate in free ends condition. This work aimedto provide a complete anatomical description of this complex boundary condition and its implications on modal parameters. Before comparing modal parameters with experimental data, a primary modal analysis was performed using finite element (FE)method. Then experimental analysis was conducted under the operational modal analysis setup (OMA).As the results, six natural frequencies are retrieved and compared amongst the two methods, recording the maximum of 6% percentage difference from the readings. This implied a close values estimated between the methods used. For mode shapes, ninemodes are identified from FE method and discussed according to established free plate vibration studies, while threesimilarly identified modes successfully recognized by using OMA.Keywords:
Operational Modal Analysis
Square (algebra)
Natural frequency
Operational modal analysis is an interesting procedure that allows identification of the modal characteristic parameters of structures excited randomly by environmental loads. In this paper, an existing operational modal technique, the natural excitation technique is studied and implemented to obtain the modal parameters of the Vega launcher excited by in-flight loads during its maiden flight. The modal parameters of the Vega launcher, obtained with the operational modal technique, are compared with those from a finite element model designed for the same flight event. Because the dynamic system is not stationary due to propellant mass loss, modal identification is only possible using a series of linearized models over short periods of time. In this work, the extraction of modal parameters is performed in the time window that shows the highest excitation of mode shapes. The time domain modal analysis method, Least-Squares Complex Exponential, is used to extract the modal parameters from the correlation functions generated by the natural excitation technique. Results show good agreement with the corresponding ones obtained from the finite element model; this demonstrates that the natural excitation technique can be successfully employed for the extraction of modal parameters of launch vehicles during the flight phase.
Operational Modal Analysis
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The existence of nonlinearity is an inevitable frequent occurrence that should be considered to accurately identify the modal parameters of a vibration system using operational modal analysis. A problem is that the traditional operational modal analysis method based on the linear modal theory is not applicable to modal parameter identification of vibration systems with nonlinearity. A solution is as follows: this paper is aimed at solving the problem by proposing a new operational modal analysis method to carry out modal parameter identification for a nonlinear vibration system. The new operational modal analysis method, based on the forced response and symbolic regression method without assuming any pre-existing information and only using mathematical symbols, is introduced to solve the problem by automatically searching for the expression structure and modal parameters of a system in nonlinear normal modes. The simulation result of a three-degrees-of-freedom nonlinear system reveals the high accuracy of the proposed operational modal analysis method in extracting the modal parameters. Then, a rod fastening rotor model is considered, and the capability of the proposed operational modal analysis method to precisely extract its modal parameters is further evaluated.
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Operational modal analysis (often called output-only or ambient modal analysis) is described in this article. Modal testing is performed on a plate structure with well-defined modes, resonance frequencies and damping values. Frequency Domain Decomposition (FDD) and Enhanced Frequency Domain Decomposition (EFDD) concepts are presented and applied to a plate structure. This article details the signal processing mathematical background and presents alternative curve-fitting processes.
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Natural frequency
Damping ratio
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Operational Modal Analysis
Robustness
Harmonic
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Operational Modal Analysis consists on estimate the modal parameters of a structure (natural frequencies, damping
ratios and modal vectors) from output-only vibration measurements. The modal vectors can be only estimated where a sensor is placed, so when the number of available sensors is lower than the number of tested points, it is usual to perform several tests changing the position of the sensors from one test to the following (multiple setups of sensors): some sensors stay at the same
position from setup to setup, and the other sensors change the position until all the tested points are covered. The permanent sensors are then used to merge the mode shape estimated at each setup (or partial modal vectors) into global modal vectors. Traditionally, the partial modal vectors are estimated independently setup by setup, and the global modal vectors are obtained in
a postprocess phase. In this work we present two state space models that can be used to process all the recorded setups at the same time, and we also present how these models can be estimated using the maximum likelihood method. The result is that the global mode shape of each mode is obtained automatically, and subsequently, a single value for the natural frequency and damping ratio
of the mode is computed. Finally, both models are compared using real measured data.
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Natural frequencies obtained by modal analysis are important to engineers interested in predicting the dynamic behavior of structures. Traditional modal analysis involves impact testing or shaker testing, where response signal and input force are measured to obtain the transfer function. However, for large structures, input excitation force measurement may be difficult, if not impossible. Large structures may be subjected to ambient excitation; operational modal analysis (OMA), also known as output-only modal analysis, has been used for extracting modal parameters of these types of structures. The main advantage of operational modal analysis is that no artificial excitation is needed, and the analysis is based on measurements of only the output data of the system. Operational modal analysis tests are performed under the actual operating conditions of the system without any change of boundary conditions; the tests use the ambient loads as input and thus do not interfere with the normal functioning of the system. In this study, six aluminum beams of different configurations (beams with and without cuts of various lengths) were used for conducting experiments. Results based on impact test, shaker test, and operational modal analysis are presented.
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Operational Modal Analysis
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Structural Dynamics
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Operational Modal Analysis
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Harmonic
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Dynamic analysis of the civil structures such as bridges, towers and buildings is required for their design and maintenance. Modal analysis is a powerful tool to conduct some part of dynamic analysis in determination of the modal parameters in terms of natural frequencies, damping factors and mode shapes. However, excitation of these structures is usually difficult and sometimes impossible. As these structures are usually excited by ambient forces such as wind, this idea is suggested that the structure is modeled considering the natural forces as the inputs. However, the ambient forces are unknown and have a complicated nature to be measured. An alternative approach is using the operational modal analysis concepts in which only the responses are measured and the modal parameters are extracted. In this article Frequency Domain Decomposition (FDD) method is used for identification of modal parameters of a clamped-clamped beam and the results are compared with those of the FEM. The operational modal analysis is conducted on a type of a bridge under ambient forces in a real test and the results are compared with those of the conventional Modal testing. The results confirm that the method is suitable for engineering applications.
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