Papers by Keyword: Experimental Modal Analysis

Abstract: An analytical method is presented for free vibration of a symmetrically laminated rectangular plate with point masses, and experimental modal analysis is conducted to compare both sets of the frequency data. The problem is solved by an extending Ritz method to include kinetic energy caused by added point masses under any sets of edge conditions, and a frequency equation is derived by minimizing the energy functional. In numerical computation, the accuracy of the solution is studied by convergence test and comparison with the existing result in the specific case. Then, the experimental modal analysis is applied to measure the natural frequencies and mode shapes. The two sets of results are compared, and the validity of both theoretical and experimental approaches is established.

Abstract: Structural dynamics in structural engineering analysis involves the modal parameters (natural frequency, mode shape and damping ratio). The modal parameters of engineering structures is mainly influenced by the damping and stiffness properties. This research paper presents the reliability of CELAS element in the finite element modelling to represent the stiffness parameter. The simplified engineering structure considered in this study is a mass-spring system with multi-degree of freedom. Experimental modal testing is performed using an electro-magnetic vibration shaker as an exciter and an accelerometer to record the natural frequency of the system. HyperMesh normal mode analysis is used to compute the natural frequency of the mass-spring system. The comparative evaluation is performed in order to identify the accuracy of the natural frequencies obtained from the modelling analysis and the measured counterparts. Consequently, it is found that the element CELAS has a good capability to represent as the stiffness parameter in the finite element modelling.

Abstract: This paper deals with a natural frequency distribution of a six-axis industrial robot in order to analyze chatter vibrations in upcoming milling processes. Since the dynamic vibration behavior of the robotic system can be manipulated by changing the robot’s joint configuration, experimental modal analysis is performed to determine the natural frequencies in the entire workspace. In this study, methods of design of experiments are used to derivate a mathematical model that predicts the natural frequencies of the robotic structure for any joint configuration within the considered workspace.

Abstract: The paper deals with mode separation process in experimental modal analysis. The main role in this process plays a technique known as Enhanced Frequency Response Function (EFRF), which allows separate the different modes of a structure. The results of the separation are responses of isolated systems with one degree of freedom by means of which complex poles can be estimated with greater accuracy. The paper describes the basic principle of EFRF technique and also presents its application in experimental modal analysis.

Abstract: The paper deals with the estimation of modal parameters and its main purpose is to compare differences in the values of natural frequencies and damping ratios, which were estimated using three different extraction methods: Rational Fraction Polynomial method, Complex Mode Indicator Function and Polyreference Time Domain Technique. These methods are well suited to the more general application to multi-FRF data, both of the SIMO and the MIMO types. The object of measurement was a freely suspended steel rod of circular cross section. The responses of the analyzed structure were measured by accelerometer and laser vibrometer. The results of these measurements are also discussed in the paper.

Abstract: Paper deals with the identification of coupled mode shapes by experimental modal analysis. Main attention is focused on the using of Complex Mode Indicator Function that is based on singular value decomposition of frequency response function matrix and allows to separate coupled and also closed modes. In the paper there is described experimental modal analysis at which digital image correlation method is used to measure responses of a circular plate. The measurement was evaluated in program Modan 3D that is being developed by the authors.

Abstract: Dynamic behavior of mechanical structures results from complex interactions between applied forces and the stiffness properties of the structure. Currently, many problems of structural dynamic analysis are solved using Finite Element Method (FEM). However, in recent years, the implementation of the Fast Fourier Transform (FFT) in low cost computer-based signal analyzers has provided a powerful tool for acquisition and analysis of vibration data. This article discusses combination of two approaches to structural dynamics testing; the experimental part which is referred to as Experimental Modal Analysis (EMA), respectively the analytical part, which is realized by Finite Element Analysis (FEA). Main goal of the paper is calculation of material properties from experimentally determined modal frequencies.

Abstract: Failures of turbine blades are one of the most critical problems in power generating industry. Among the different failure mechanisms, resonant vibration of blades has a major role and therefore is the subject of many recent research works. Therefore, in this paper, modal analysis of a first stage blade in ALSTOM gas turbine is investigated and natural frequencies and vibration modes of blade are found in various conditions. For this purpose, a cloud-point model of a gas turbine blade has been created using 3D Laser Digitizer. Then the numerical calculation by finite element method using ANSYS software based on experimental test conditions is utilized and Experimental natural frequencies have been obtained. The results show acceptable agreement between the experimental and FEM results.

Abstract: To determine modulus of elasticity (MOE) of the whole full-sized medium density fiberboard (MDF) by using vibration method in the future, this paper studies MDF vibration characteristics. To solve modal parameters of full-sized MDF in the condition of free vibration, the writers conducted calculation modal analysis and experimental modal analysis of the full-sized MDF with three different thicknesses respectively, compared and analyzed the first three order modal shapes and frequencies. It is found that the full-sized MDF with three different thicknesses showed the same vibration modal forms: the first and second vibration modes had bending vibration along the length direction, while the third one had bending vibration along the width direction; the frequency obtained through calculation modal analysis and experimental modal analysis had a certain difference—the first calculation modal frequency was slightly lower than the first experimental modal frequency, and the second and third calculation modal frequencies higher than the corresponding experimental modal frequencies. However, there is a good correlation between calculation modal frequency and test experimental modal frequency with the determination coefficient reaching 0.9816.

Abstract: The investigated footbridge across Vltava River in Prague was built in 1984. Almost the entire footbridge structure was overflowed by the extreme flood in August 2002 and the substantial part of the footbridge deck was overflowed afterwards by the great flood in June 2013 again. A significant damage on the footbridge deck was found out by a visual inspection of the load-bearing structure of the footbridge after the second flood. The basic objective of the performed experimental modal analysis was the initial experimental reliability verification of the obviously damaged footbridge structure. The measured natural frequencies and modes were compared with results of the several dynamic tests which were performed regularly during the lifetime of the footbridge. The localization of potential invisible damages was carried out. The recommendations for the range and the arrangement of the static load test, which was performed afterwards for fundamental reliability assessment of a footbridge structure, were made on the basis of the evaluated results of the experimental modal analysis.