Showing papers on "Modal testing published in 2023"

High-Accuracy Finite Element Model Updating a Framed Structure Based on Response Surface Method and Partition Modification

Abstract: In this paper, a finite element model updating (FEMU) method is proposed based on the response surface model (RSM) and genetic algorithm (GA) to establish a high-precision finite element (FE) model of space station scientific experiment racks. First, the fine solid and mixed FE models are established, respectively, and a comparison of the modal test results is conducted. Then, an orthogonal experimental design is used to analyze the significance of the parameters, and the variables to be modified are determined. The design parameters are sampled via the Latin hyperbolic method and are substituted into the FE model to obtain the modal parameters of the scientific experiment rack. The mapping relationship between the design and modal parameters is fitted by constructing the Kriging function, and the RSM is established. The design parameters of the scientific experiment rack are optimized via GA, and the initial FE model is updated, which has the advantage of improving the computing efficiency. Finally, the updated FE model of the experiment rack is verified by frequency sweep and random vibration tests. The experimental results show that the proposed approach has high precision and computing efficiency, and compared with the test results, the modal frequency errors of the updated model are within 5%, and the vibration response errors under random excitation of the updated model are within 7%.

Modal Identification of Train Passenger Seats Based on Dynamic Tests and Output-Only Techniques

Abstract: Railways are one of the most efficient and widely used mass transportation systems for mid-range distances, also being pointed out as the best strategy to reach European Union decarbonisation goals. However, to increase railways attractiveness, it is necessary to improve the quality of the ride, namely its comfort, by decreasing the vibration at the passenger level. This article describes the experimental vibration modal identification of train seats based on a dedicated set of dynamic tests performed on Alfa Pendular and Intercity trains. This work uses two output-only modal identification techniques: the transmissibility functions and the Enhanced Frequency Domain Decomposition (EFDD) method. The last method allows us to clearly distinguish the seat structural movements, particularly the ones related to torsion and bending of the seat frame, from the local vertical foam vibrations. The natural frequencies and mode shapes are validated by matching the results derived from the transmissibility functions and EFDD method. The identified modal parameters are particularly relevant to characterise the vibration transmissibility provided by the foams (local transmissibility) and the vibration transmissibility derived from the metallic seat frame (global transmissibility).

Multimode vibration control of stay cables using pseudo negative stiffness MR damping system

Abstract: At present the vibration control of stay cable suffers from low damping effectiveness in single mode vibration caused by the limitation of damper installation position as well as the poor modal compatibility, that is, a rapid degradation performance when the actual mode deviates from the designed optimal mode. In this paper, one practical semi-active magnetorheological (MR) damper based control solution is proposed to address the problems faced in multi-modal vibration control of stay cables. The semi-active pseudo negative stiffness (PNS) control strategy takes full account of the MR damper’s mechanical characteristics and the demands of the stay cable vibration control. Compared with the linear equivalent model, the proposed time-varying model exhibits more details of the damping force and the nonlinear response of the damped stay cable, which shows its essential role in the optimal design of MR-PNS scheme. Then the optimal design method of MR-PNS multi-modal vibration control for stay cable is summarized by taking the first three modes vibration control of the J20 cable in Nanjing Baguazhou Yangtze River Bridge as simulation examples. The simulations of cross-modal, multi-modal and wind-induced vibration cases are conducted respectively, while the results show that the optimal designed multi-modal MR-PNS scheme can simultaneously exceed the passive maximum modal damping ratio within first three modes. The advantages of the proposed MR-PNS method in high damping efficiency and modal compatibility could be verified by comparing with the passive multi-modal damping solution.

Structural Modal Parameter Identification Method Based on the Delayed Transfer Rate Function under Periodic Excitations

Abstract: The dynamic response transfer rate function (TRF) is increasingly used in the field of structural modal parameter identification because it does not depend on the white noise assumption of the excitation. In this paper, the interference of periodic excitation on structural modal parameter identification using TRF is analyzed theoretically for a class of civil engineering structures with obvious periodic components in excitation, and then an identification method of structural real modal parameters is proposed. First, a delayed TRF is constructed, and the pseudo-frequency response function is further obtained to identify the periodic spurious poles of the whole system. Then, the effective identification of the real modal parameters of the structure is achieved by comparing the system poles identified via conventional TRF. Finally, the feasibility and robustness of the proposed method were verified using a calculation example with four-degrees-of-freedom system. In addition, the modal parameters of a structure under periodic excitation were effectively identified by taking a pumping station as an example, and the results show that the method accurately identified the structural modal parameters when the excitation contained periodic components, which has wider prospects for technical applications.

Modal Identification for Integrally Bladed Rotors Under Traveling Wave Excitation

Abstract: Abstract The safety of a fielded integrally bladed rotor (IBR) is often assessed through vibration testing. Responses due to various types of excitation are measured and processed and can be inputs to follow-on analyses, such as mistuning identification. One such excitation technique is the traveling wave excitation (TWE) where all blades are simultaneously excited at phase differences that attempt to replicate naturally occurring mode shapes and certain operating conditions. This test relies on noncontact exciters, e.g., magnets and speakers, that are often not directly measured. As a result, formulating the frequency response function (FRF) is difficult and the extraction of system modal data using FRF fitting techniques in the absence of FRFs is not possible. This paper presents an approach to use measured responses from TWE tests. It is shown that the fast Fourier transform (FFT) of the TWE inputs is mostly independent over the prescribed frequency range. Consequently, the output spectral density matrix can be formulated in an operational modal analysis (OMA) sense, where direct measurement of the inputs is not needed. A full spectral density matrix is then formulated from a single measurement on each blade obtained during a single test, thus reducing the number of measurement locations and testing excitation conditions. This matrix is fit by a polyreference-least squares complex frequency-domain (P-LSCF) system identification technique tailored for OMA-type measurements. The methodology is tested using simulated TWE data for an IBR model using different damping levels. Comparisons between identified modal data and those used to create the model are made and show the methodology accurately predicts underlying system information even for closely spaced modes that are common to IBRs. Finally, the method is used on experimental TWE data of an industrial IBR.

Damage Detection in Structures Using Strain Measurement

Abstract: Vibration-based Structural Health Monitoring (SHM) is a non-destructive method and is widely used in recent years. To measure the vibration response of the structure under impact load, uniaxial/triaxial accelerometers, displacement, and velocity meters are used. Many works use modal parameters extracted from vibration such as frequency, mode shape, and modal curvature to detect damage. Among these parameters, modal curvature is proved very sensitive to damage. Modal strains (or curvatures) can be derived from displacement mode shape using the central difference method, but this procedure may contain cumulative errors. In this paper, direct modal strain measurement will be used. A steel slab equipped with strain transducers is set up in the laboratory. The strain response will be recorded and analysed directly to find the modal strains. From direct modal strain measurements, the damage location will be identified. Two damage scenarios are examined to verify the method.

Ambient vibration measurement of the institute for materials and structures building in sarajevo

Abstract: Determination of dynamic properties of structures is the first step in assessing seismic response, and they can be measured in several ways. Controlling or knowing the input excitation usually applied by impact hammer or vibration shaker, typical for experimental modal analysis (EMA) that has been around for the past few decades, is for majority of structures difficult or practically impossible. Ambient vibration testing (AVT) or operational modal analysis (OMA), on the other hand, is the output-only modal analysis. It does not require knowledge of the input excitation, which is practically induced by wind, traffic or similar random source. In this paper, an investigation of ambient vibrations and numerical modelling of the building of the Institute for Materials and Structures (IMK) of the Faculty of Civil Engineering in Sarajevo was carried out. The main goal was to determine the dynamic characteristics of the IMK building using the DIGITEX SENTRY system and Artemis modal software. In addition to testing the IMK building, testing of simpler systems such as a wooden simple beam and a steel cantilever was also conducted. For each experiment, a modal analysis was performed in the Tower 8 software package. The numerical model of the building was more flexible than measured in the experiments, and the results were only comparable after inclusion of partition walls in the analysis.

The Effect of Acceleration Signal Length on the Outputs from Modal Identification Methods

Abstract: The damping ratio is a significant factor influencing the dynamic behaviour of a structure. The safety, serviceability and habitability of a structure are all impacted by the damping ratio. Damping ratio does not relate to a unique physical phenomenon like mass or stiffness and in practice, design analysis relies on estimates of damping ratios from empirical measurements of similar structures. Overestimates of the damping ratio arising from uncertainty in estimates can lead to structures experiencing acceleration responses during wind and seismic events that could potentially cause human discomfort. Full-scale testing provides the most accurate insight into the actual damping ratio of a structure. Where full scale testing is performed, acceleration signals are recorded and assessed using modal identification techniques to identify the characteristic modal parameters such as natural frequency and damping ratio. However, this form of testing is not without errors which may arise as a result of response conditions during monitoring, the modal identification methods applied, the duration of acceleration signal processed and the sampling frequency of the acceleration signal.This paper considers real ambient acceleration response signals of different lengths and sampling frequencies obtained from a full-scale monitoring campaign. The influences of signal length and sampling frequency on the natural frequencies and the damping ratios calculated using two different modal identification methods are investigated. Outputs from signal lengths of 12 hours, 1 hour, and 10 minutes are compared as well as sampling frequencies of 20Hz, 10 Hz and 5Hz. The two modal identification methods used are the Bayesian Fast Fourier Transform (BFFT) and a composite method of Analytical Mode Decomposition (AMD) and the Random Decrement Technique (RDT) in which bootstrapping is also performed to identify error in the estimates.

Experimental and numerical modal analysis of the Turkish traditional instrument 'Bendir'

Abstract: 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.

Adaptive modal identification of honeycomb thin-walled composite structures with pit defects under thermal modal testing using variational mode decomposition technique based on digital image correlation

Abstract: The active imaging blue light high-speed three-dimensional digital image correlation (3D-DIC) system was combined with a transient aerodynamic heating system and random pulse excitation technology to establish a high-frequency thermal vibration optical test system capable of multi-temperature zone testing at 900 °C to accurately obtain the thermal modal parameters of thin-walled structures. The full-field temperature distribution of a novel honeycomb thin-walled composite structure (HTWCS) with pit defects under non-uniform temperature was accurately simulated through reinforcement learning. The first six modal frequencies of honeycomb thin-walled composite structure with pit defects at room temperature increased compared with the non-destructive honeycomb thin-walled composite structure, and the third and fourth modal shapes interchanged. An adaptive modal parameter identification method for full-field three-dimensional vibration measurements under high-temperature environments was developed by combining the successive multivariate variational mode decomposition (VMD) with the modal superposition method. This method can effectively identify close modes. The area ratio-based approach was used to evaluate the damping of the measured response. The finite element model (FEM) of the HTWCS with pit defects at different temperatures was updated using a multi-state step-by-step model updating method and the temperature-dependent material properties were established. The results showed that the introduction of a damping ratio improved the accuracy of the updated FEM of HTWCS with pit defects. The proposed technique is anticipated to provide an effective method for high-frequency thermal vibration optical measurement and modal identification of thin-walled structures under multi-temperature regions.

Modal testing on a limited budget: Analysis of instrumented hammer alternatives for impact testing

Abstract: In recent years, as the use of numerical modeling has increased, organizations have frequently scaled back their experimental capabilities. Often equipment and expertise are no longer available when modal correlation measurements are required. This presentation is part of a planned series examining a minimalist approach to modal testing. In this presentation, alternatives to a traditional instrumented hammer for impact testing are explored. Comparison testing between a dedicated instrumented hammer procured from a leading structural measurement company and commercial hammers acquired from a hardware store was performed. Two cases of commercial hammers were studied, one featuring a force gauge inserted into the impact hammer face and one with an accelerometer attached to the hammer body. A variety of hammer tips were explored for focused frequency content. Follow-on topics include rudimentary data acquisition options and methods of modal data analysis outside of expensive dedicated modal analysis software platforms.

Crack Assessment of Beam Using Machine Learning with Augmented Sensing

Abstract: The modal frequency of a cracked beam is known to be smaller than that of an intact beam because cracks in the beam do not change the mass and only reduce the rigidity. Most previous studies have focused on assessing the crack location and depth by using the fractional reduction of the modal frequency based on the Euler–Bernoulli (E–B) beam theory. Because the effects of rotational inertia and shear deformation are disregarded in the E–B beam theory, the high-frequency dynamic behavior cannot be predicted appropriately. Furthermore, more than three cracks cannot be identified because 2n modal frequencies are required to assess n cracks, whereas the maximum number of available modal frequencies is six at most, based on previous studies. In this study, the broadband modal frequencies of a cracked beam are accurately predicted by using the Timoshenko beam theory. The use of broadband modal frequencies alleviates the sparsity of measurement information for crack assessment. Using the closed-form solution of the modal frequency for a Timoshenko beam with multiple incipient cracks, the computational cost can be reduced significantly during data augmentation. Data augmentation allows for augmented sensing, thereby providing machine learning with a sufficiently huge database of broadband modal frequencies associated with multiple cracks of arbitrary depths and locations on the beam. The proposed method is validated through the numerical simulation in which the number of cracks is estimated in free-free cracked aluminum beams with up to four cracks.

Statistical characteristics of modal damping of long-span suspension bridge at different wind speeds

Abstract: The present paper combined the natural environment excitation technology (NExT) with the eigen system realization algorithm (ERA) to identify the modal parameters of a suspension bridge. Firstly, the critical parameters in NExt–ERA, such as record length, window length, and ranks of the Hankel matrix, and their influence on identification accuracy have been discussed. Subsequently, based on the acceleration series obtained from the Xihoumen Bridge Health Monitoring System, 3015 sets of modal frequencies and modal damping ratios of the Xihoumen Bridge were identified under 7 sets of wind speeds by the NExt–ERA. Finally, the probability distribution and confidence interval of the structural modal damping ratios were analyzed. Besides, the dependence of identified frequencies damping ratios on wind speed was also discussed. The results show that at the same wind speed, the mean value, and variance of the torsional and lateral mode damping ratios are larger than those of the vertical mode, but this difference gradually decreases with the increment of wind speed. The modal damping ratios along three directions under different wind speeds all follow the generalized extreme value distribution, but wind speed affects the tailing properties of probability distribution of the vertical and torsional mode damping ratios. The estimated damping ratios could provide references for structural design and vibration control of similar long-span bridges.

Identification and comparative analysis of vibration modal parameters of rice planter frame

Abstract: Modal parameter analysis is a common method for structural characteristic analysis. To accurately identify the modal parameters that describe the vibration characteristics of the structure, the chassis frame of the rice transplanter is taken as the research object. The least squares complex frequency domain method, the admittance circle method, and the modal peak picking method are used to process the measured vibration signal. The modal parameters, such as the natural frequency, damping ratio, and modal shape of the chassis frame, are obtained. Except for the eight-order frequency error greater than 10%, the errors of all other orders are all less than 10%. In the modal test of the frame structure of the rice transplanter, the identification results of the modal parameters are generally reliable. The recognition accuracy of the PolyLeast-Squares Complex Frequency (PolyLSCF) algorithm is lower than the recognition accuracy of the admittance circle method and modal peak picking. The values of some off-diagonal elements in the modal mass matrix calculated by the PolyLSCF algorithm and the admittance circle method are greater than 0.2. The diagonal elements of the modal mass matrix calculated by the modal peak picking method are all 1.

Identification Algorithm and Improvement of Modal Damping Ratios for Armature Assembly in a Hydraulic Servo-Valve with Magnetic Fluid

Abstract: The high-frequency vibration and resonance of armature assembly in the hydraulic servo valve are the main reasons for instability and failure. Magnetic fluid (MF) operating in the squeeze mode can be taken as an effective damper for resonance suppression in the servo valve. Due to excitation difficulty and the low signal-to-noise ratio of high-frequency vibration signals, the capability of MF to modify multiple-order modal damping ratios in a multi-degree-of-freedom system is still unclear. To reveal the mechanism of magnetic fluid for improving modal damping ratios, an algorithm for modal damping ratio identification is proposed. The modal damping ratios of the armature assembly with and without magnetic fluid are identified based on the tested resonance free decay responses. Four resonance frequencies of armature assembly are observed, and the corresponding damping ratios are identified. The equivalent modal damping ratios due to squeeze flow of MF are obtained. The results show that the proposed algorithm can identify damping ratios with an accuracy of up to 98.79%. The damping ratios are improved by double or more due to the magnetic fluid, and the maximum resonance amplitudes are significantly reduced by 65.2% (from 916.5 μm to 318.6 μm).