Structural Complexity in Structural Health Monitoring: Preliminary Experimental Modal Testing and Analysis

In this study, 4 different pieces of automotive rear lamp lenses which are made of Polymethylmethacrylate (PMMA) and have different geometries have been inspected. Free-free natural frequencies and mode shapes of lenses have been estimated by the help of ANSYS WB® Finite Element Analysis (FEA) software. Meanwhile, by the help of Experimental Modal Analysis (EMA) methods; natural frequencies, damping ratios and mode shapes of the lenses have been determined. Finally, mode shapes calculated by FE and measured by EMA were compared and Modal Assurance Criteria matrix (MAC) was determined. After experimental modal tests, FRF’s were calculated by DEWESoft® X3 software then natural frequencies, damping ratios and mode shapes were calculated by the use of ME’scope® VESVT-570 Visual Modal software. In addition, MAC was calculated by the help of MEScope® software. The main reason for the use of such experimental methods was to understand if the assumptions made before theoretical analysis were satisfied in reality or not.

Modal analysis based structural health monitoring (SHM) is required now more than ever due to many factors such as aging infrastructure, loading incidents likely earthquakes, over loading etc. Although the continuous monitoring of structures is a full-fledged commercial sector, its high cost makes it infeasible for large structures as the number of sensors required is large. Accelerometers, widely used as sensors in SHM, in addition to high cost, possess some other challenges such as damage vulnerability in environmental and operational conditions. In this research, low cost lead-zirconate-zitanate (PZT) sensors are proposed as replacement of costly and fragile accelerometers as sensors for SHM. PZT patches, which cost less than one-tenth of the accelerometers, can also be embedded in the reinforced concrete structures to protect from the harsh operational conditions. In this paper, PZT patches are studied by technique of experimental modal analysis (EMA) on a steel beam specimen and the results are compared with the traditionally used accelerometers. Single input single output (SISO) approach is adopted for EMA of the rectangular steel beam. PZT sensors are able to capture modal data in terms of natural frequencies and mode shapes in good agreement with accelerometer. The fundamental natural frequency of the beam is obtained with the error of less than 1 % as compared to the accelerometer. The signal to noise ratio is of same order as accelerometer. The strain mode shape from the PZT sensors is well correlated to displacement mode shapes from the accelerometer by a modal assurance criteria (MAC) values greater than 0.9 for observed experimentally.

The paper deals with an experimental and a numerical investigation of dynamic properties of the structure. Object of interest is a steel rod with circle section. The modal parameters, such as mode shapes and natural frequencies, are obtained by two different ways: experimentally by using experimental modal analysis and numerically by finite element analysis which were computed in NX Nastran solver. The results from both methods are compared in NX through the correlation function. A relation between measured mode shapes and computed mode shapes is expressed by Modal Assurance Criterion. This process of comparing is appropriate for verification of experimental approaches.

This paper presents an experimental and numerical modal analysis for an external gear pump considering its mounting on a test rig in a laboratory setting. Most of the previous studies on experimental modal analysis (EMA) of hydraulic pumps focused on the modal frequencies to allow model validation. However, the mode shapes of pump bodies have not extensively been discussed. Furthermore, the nature of the pump components assembly and mounting poses some modeling challenges, such as the uncertain material properties of each component, the behavior of the bolted joints, and some critical modeling boundary conditions related to pump mounting. In this regard, the experimentally obtained vibration modes of a reference pump using the least-square complex exponential (LSCE) method are analyzed with an emphasis on the characteristics of the mode shapes. Then, simple modeling strategies are proposed and validated by performing the analysis from the component level to the full assembly. As a result, the mode shapes are categorized depending on the type of motions that the modes exhibit. It is observed that the pump casing does not show any substantial deformation but is close to the rigid body motion. Moreover, without considerably increasing model complexities, the proposed numerical approach provides reasonable accuracy with average errors in modal frequencies of 6%, as well as good agreement in terms of mode shapes. The vibration reduction strategy is briefly discussed based on the measured mode shapes, and the proposed modeling approaches can be useful to study external gear pumps with minimal model complexities yet allowing reasonable result accuracy.

Structural Health Monitoring (SHM) is nowadays common in many branches of engineering since it allows to have a continuous or periodic report of the structural conditions and therefore to promptly intervene if there are incipient damages. The first step to perform a SHM is the identification of the dynamic parameters, i.e. natural frequencies, damping ratios and modal shapes, and it is a crucial step since a modification of the structural parameters can be a direct consequence of structural damages. Among the structural identification methods, Operational Modal Analysis (OMA) methods have received increasing attention from the researchers since they do not require the knowledge of the structural excitation that is due to ambient vibrations and that is usually modeled as a white noise. This aspect makes this kind of methods cheaper and simpler than the classical Experimental Modal Analysis (EMA) methods. In this paper an innovative OMA method is proposed. It is a semi - automated method that allows to identify natural frequencies, damping ratios and modal shapes of a structural system and that can be used also from users that have not knowledge in stochastic dynamics and signal analysis. First of all, the modal shapes are estimated through the use of signal filtering techniques applied on the stochastic properties of the output process and then natural frequencies and damping ratios can be estimated from the mono - component analytical signals obtained by performing a decomposition of the analytical signals matrix. The proposed method has been used to perform the dynamic identification of a real historic building situated in Palermo, i.e. Chiaramonte palace, and the results obtained have been compared with those obtained by using other OMA methods.

Laminated rubber bearing is a significant device found in structures such as bridges and buildings. It is used to isolate the foundation and the superstructure from seismic loads. Since it is made up from a combination of rubbers and steel plates in alternate layers which make the laminated rubber bearing a complex material, the measurements of damping of the laminated rubber bearing is difficult in practice. Damping is a dissipation of energy or energy losses in the vibration of the structure when the structures are excited with external dynamic loading. An accurate value of damping is very important as damping plays a crucial role in fixing the borderline between stability and instability in many structural systems. Therefore, it is essential to determine the accurate value of damping in structural analysis. Finite element and experimental modal analysis is one of the methods that can be used to determine dynamic properties including damping in any structures. Hence, the main objectives of this research are to determine the natural frequencies, mode shapes, Rayleigh’s damping coefficients, α and β and to evaluate the performance of the laminated rubber bearing components i.e. steel and rubber plates using finite element and experimental modal analysis. Based on the finding, the finite element modal analysis with the added Rayleigh’s damping coefficients α and β shows a good agreement with the experimental modal analysis in term of natural frequencies and mode shapes. The value of natural frequencies reduces after the Rayleigh’s damping coefficients are added into the finite element modal analysis and the minimum and maximum displacement are found to be unaffected by the Rayleigh’s damping coefficients for rubber plate. It can be concluded that modal analysis method can be used to estimate the accurate values of damping ratio and to determine the Rayleigh’s damping coefficients α and β as well.

To verify the effectiveness and correctness of free modal analysis results from a Spiral bevel gear (SBG) wheel by using Finite element method (FEM), an experimental platform was constructed through the free-hanging support of the SBG wheel. The experiment used the hammer knock percussion for excitation and a three-directional acceleration sensor as signal acquisition equipment and utilized the LMS modal analysis module. The geometric model of the SBG wheel was constructed using an eight-node octagon instead of the SBG wheel outer contour. The experiment then extracted the modal parameters of the wheel using the PolyMAX method and obtained the first- and second-order natural frequencies, damping ratios, and mode shapes of the SBG wheel at 0-7 kHz during the experimental modal test. The results of the experimental test were compared with those of the FEM free modal analysis. The first- and second-order natural frequency error rates by FEM were 0.25 % and 0.45 %, respectively. The experimental modal test result verified the rationality of the model by FEM, thus showing that the result of modal analysis by FEM is reliable and providing a basis for the dynamic characteristic analysis of SBG.

An experimental and numerical modal analysis was performed on an 8 m2 T-shaped heliostat structure at different elevation angles. The experimental results were used to validate a finite element (FE) model by comparing natural frequencies and mode shapes. The agreement between experiments and simulations is good in all operating points investigated. In addition, damping coefficients were determined experimentally for each mode, in order to provide all necessary information for the development of a dynamic model. Furthermore, potentially critical operating conditions caused by vortex shedding were identified using a simple approach.

Damage Detection in Structural Beams Using Model Strain Energy Method and Wavelet Transform Approach

Operational modal analysis based dynamic parameters identification in milling of thin-walled workpiece

The paper studies methods for obtaining dynamic characteristics (dynamic modulus of elasticity, loss coefficient) of structural materials, such as concretes and cement mortars, based on the results of experimental modal analysis. Closed expressions for determining the loss coefficient of mechanical vibrations are presented, obtained from solving differential equations of vibrations of single-mass systems, which are caused by vibrations of extended structures in the first forms. The procedure of modal analysis of short beams made of test samples of structural materials is presented. The procedure for calculating the natural frequencies of the first forms of vibration of beams is presented. Based on the solution of the equation, a formula is presented for determining the damping coefficients and the dynamic modulus of elasticity of the beam material.

Based on the principle of axial active magnetic bearing system with single degree of freedom, the difference between magnetic system vibration mode and the traditional bearing system vibration mode were described. Initial four order natural frequencies and vibration mode were calculated by the module of modal analysis of ANSYS11. Axial displacement and control current influence on natural frequencies and vibration mode were also analyzed. At last they were compared with the results of experimental modal analysis. Analysis shows that there was a certain difference between the results of finite element method and experimental model analysis, but the difference was small. It provided theoretical basis for flexible system design and dynamic design.

Modal analysis describes the properties of an elastic structure in terms of its modes of vibration. Videotape is a convenient medium for presenting the results of experimental modal analysis of structural vibrations, both in a lecture and in the laboratory. Some modes of vibration of several musical instruments, including a guitar, a handbell, a Caribbean steel drum, a snare drum, and a plane soundboard, will be demonstrated on videotape.

Experimental Modal Analysis (EMA) is very helpful in engineering design and manufacturing of machine components. In this paper, modal parameters which are natural frequencies, mode shapes and damping ratios are extracted for free-free boundary conditions circular shaft, using EMA, then two disks are added to this shaft as second case study. EMA has been verified as effective and accurate tool, despite of increasing geometrical complexity and nonlinear behaviour of the structure. The two well-known excitation techniques (i.e. impact hammer and shaker) are used for this purpose. For validation, natural frequencies and mode shapes are determined analytically and numerically by Finite Element Modelling (FEM) ANSYS 15 workbench software, and then compared with results obtained experimentally. Noticeable remarks about these approaches are listed. Coherence plots are compared between impact and shaker tests. Digital Signal Processing (DSP) settings like windows type and sample (record) time are compared. The recommended DSP settings mentioned in this work can be used as a general guidance for researchers and people who are working in modal analysis.

This study carried out an experimental and numerical modal analysis of the Turkish traditional instrument 'Bendir' by an acoustical excitation source. In experimental modal analysis, Bendir was excited acoustically for specified frequencies and elastic deformations of different areas of Bendir due to the excitation were observed with a laser vibrometer. Using the Fourier series handled gained data, modal analysis was done. The first six natural frequencies obtained experimentally and numerically were compared. In addition, other hits in Bendir, symbolizing three fundamental voices such as 'dum', 'te', 'ke' were used to excite the instrument. As a result, time dependent deformation & velocity data were determined and different responses of Bendir were compared.