Experimental Modal Testing Research Articles

Dynamic Characterisation and Finite Element Updating of a RC Stadium Grandstand

This paper reports on the dynamic characterisation of a Reinforced Concrete (RC) stadium grandstand module for the Sporting Stadium in Lisbon. To this aim, a three-dimensional (3D) Finite-Element (FE) numerical model, implemented according to the technical drawings of the structure, is first presented to provide preliminary estimates of the expected modal characteristics for the examined structural system. Ambient vibration tests are then carried out on the same grandstand, and used to extract the natural frequencies and vibration modes of the system, according to conventional state-of-the-art output-only modal parameter identification techniques. A sensitivity investigation and FE model updating study is hence presented for the grandstand, giving evidence of the major influencing parameters and key input data for the numerical fitting of the experimental modal testing results.

Numerical investigation of external radiation noise induced by dipole and quadrupole sources in an axial piston pump

Extensive research has been performed on the reduction of flow ripple and pressure fluctuation in axial piston pumps. However, the acoustic characteristics and sound sources of the external radiation noise induced by the fluid within an axial piston pump body have been rarely studied. In this study, a hybrid method combining computational fluid dynamics (CFD) with computational acoustics (CA) based on the Lighthill's acoustic analogy (LAA) theory was adopted to calculate the external radiation noise. First, a three-dimensional unsteady flow field was calculated by using CFD model, and the consistency between the computed results and experimental test data was verified. Then, the quadrupole and dipole sound sources were obtained based on the LAA theory. After that, the finite element method (FEM) was adopted to numerically analyze the natural modes of the axial piston pump shell, and it had been verified by the experimental modal test. Finally, the external radiation noise induced by the different fluid sound sources was calculated based on FEM/AML (automatically matched layer) vibro-acoustic coupling method. The results show that the quadrupole sound source is the main contributor of external radiation noise. Subsequently, the effects of different triangular groove structural parameters on external radiation noise were studied. This research can provide guidance for engineers in the design of low noise axial piston pumps.

Fabrication, Experimental Modal Testing, and a Numerical Analysis of Composite Sandwich Structures with a Grid-Stiffened Core

Composite sandwich structures with a grid-stiffened core (SSGSC) are one of the new structural configurations employed in advanced industries, and therefore the knowledge of their dynamic characteristics is very important. Hence, in the present study, three composite SSGSC samples of glass/epoxy and carbon/epoxy were fabricated by the hand lay-up method using a silicon rubber die. Also, two metallic samples — a monolithic one and a SSGSC, made of Aluminum 1050, were fabricated. The core of the aluminum SSGSC was produced using the wire cutting process, and the face sheets were attached using a high-grade adhesive. Modal experiments were carried out on all the samples using B&K vibration equipment. The frequency, mode shape, and damping coefficients were obtained from each experiment. Finally, a numerical modal analysis was performed, and good agreement between experimental and numerical results was obtained.

Characterization and modeling of the acoustic field generated by a curved ultrasound transducer for non-contact structural excitation

Conventional excitation techniques typically use an impact hammer, piezoelectric actuator, or mechanical shaker excitation for experimental modal testing. However, the use of these devices may be challenging if accurate high-frequency dynamic measurements on small or lightweight structural parts have to be performed. To overcome these problems, the high-frequency radiation force generated by focused ultrasonic transducers (FUTs) can be used. This approach has shown potential to be used as a non-contact method for modal excitation of small-sized or flexible structures such as MEMS devices, small turbine blades, integral blade rotors (IBR), and biological structures. However, the sound radiation in the air of these ultrasonic transducers and the resulting radiation force imparted onto a structure is not well understood and critically crucial for performing accurate modal analysis and system identification. In this research, the technical development of ultrasound radiation pressure mapping and simulation is presented. Starting from the calibrated sound pressure fields generated by the spherically FUT, driven by amplitude modulated signals, the dynamic focused ultrasound radiation force is modeled and estimated. The acoustic pressure field of a FUT operating in the air is measured and used for validating the accuracy of a new numerical boundary element method (BEM) model in predicting the direct acoustic force generated in the high-frequency range (i.e., 300–400 kHz). The results show that an excellent agreement is found regarding both the pressure profile and amplitude. Pressure fields up to 1200 Pa can be generated as the transducer is driven at 400 kHz. Experiments also prove that the FUT is capable of creating a focal spot size of nearly 3 mm in diameter. To finish, the FUT's dynamic focused ultrasound radiation force is quantified and could be used to quantify a force-response relationship for experimental modal analysis purposes.

Finite volume free vibration analysis of a new aerodynamic styling wind turbine

Due to the short life cycles of large-scale wind turbines and difficulties in getting permission for farms installations, micro-scale turbines have become interesting sustainable solutions to product energy. However, micro-size turbines have to be able to generate balanced powers while ensuring maximum life services under unpredictable environment conditions. Hence, designers must mainly investigate the dynamic characteristics of turbine tower–foundation systems in order to protect the structural security and stability of the system under vibration constraints. In this framework, this article aims to analyze the vibration behavior of modern micro-turbines. A new aerodynamic styling wind turbine, which is Rutland 504 six-bladed commercial turbine, is selected in this study in order to highlight the impacts of extra components like ring, nose, and tail on the vibrational properties. Accordingly, a three-dimensional model of the turbine tower–foundation system was created basing on finite volume method. The newest version of ANSYS academic software is used. Dynamic properties of the numerical model were determined by solving equations of motion. After identifying mode shapes and natural frequencies, experimental modal testing was applied to validate the numerical model. Then, deformations and vibrations of different components of the system were studied under free vibration conditions. The components of the system most sensitive to vibrate were determined. Moreover, the stress distributions were discussed in order to identify the components most exposed to fatigue fractures.

A multiobjective sensor placement optimization for SHM systems considering Fisher information matrix and mode shape interpolation

Sensor placement optimization plays a key role in structural health monitoring (SHM) of large mechanical structures. Given the existence of an effective damage identification procedure, the problem arises as to how the acquisition points should be placed for optimal efficiency of the detection system. The global multiobjective optimization of sensor locations for structural health monitoring systems is studied in this paper. First, a laminated composite plate is modelled using Finite Element Method (FEM) and put into modal analysis. Then, multiobjective genetic algorithms (GAs) are adopted to search for the optimal locations of sensors. Numerical issues arising in the selection of the optimal sensor configuration in structural dynamics are addressed. A method of multiobjective sensor locations optimization using the collected information by Fisher Information Matrix (FIM) and mode shape interpolation is presented in this paper. The sensor locations are prioritized according to their ability to localize structural damage based on the eigenvector sensitivity method. The proposed method presented in this paper allows to distribute the points of acquisition on a structure in the best possible way so as to obtain both data of greater modal information and data for better modal reconstruction from a minimum point interpolation. Numerical example and test results show that the proposed method is effective to distribute a reduced number of sensors on a structure and at the same time guarantee the quality of information obtained. The results still indicate that the modal configuration obtained by multiobjective optimization does not become trivial when a set of modes is used in the construction of the objective function. This strategy is an advantage in experimental modal analysis tests, since it is only necessary to acquire signals in a limited number of points, saving time and operational costs.

On crack detection in curved beams using change of natural frequency

In this study, an inverse method for the purpose of identifying the parameters of a single-edge crack in a curved beam is investigated. This method requires both numerical and experimental natural frequencies. The differential quadrature element method (DQEM) is introduced to get the numerical ones. For the purpose of implementing the DQEM the beam is divided into some elements and the governing equations, the continuity and the boundary conditions, are exerted on these elements to create an eigenvalue problem which is solved to obtain the required frequencies. A rotational spring whose stiffness is a function of crack depth is used to model the crack. An objective function with the variables of crack location and depth which is the weighted mean squared error between experimental and numerical frequencies is defined. The artificial bee colony algorithm (ABC) an optimization algorithm inspired by the natural foraging behavior of honeybees is used to minimize the aforementioned objective function and obtain crack parameters. To investigate the accuracy of this non-destructive, fast and easy method, an experimental modal analysis test is conducted on a cracked beam. The results indicate that the position and depth of the crack can be determined successfully using this method. This test is done five times with a different crack depth each time, to show the efficiency of the method for all cracks even small ones. A brief investigation is also done on the effect of crack location and depth on the natural frequencies.

Free in-plane vibration of cracked curved beams: Experimental, analytical, and numerical analyses

In this study, free vibration of a cracked curved beam utilizing analytical, numerical, and experimental methods is investigated. The differential quadrature element method is used to solve the equations of motion numerically. The governing equations are also solved analytically. The crack, which is considered to be open, is modeled as a rotational spring. Furthermore, the effect of curvature on mode shapes is studied. To verify the validity of the proposed methods of determining frequencies and mode shapes, an experimental modal analysis test is conducted on a sample beam having crack with some different depths. This study revealed that the behavior of curved beams toward the mode transition phenomenon depends greatly on the boundary conditions of the beam. Also, both the location and depth of crack have considerable effects on natural frequencies.

Prediction of Firmness and pH for "Golden Delicious" Apple Based on Elasticity Index from Modal Analysis.

A nondestructive and portable device was provided for fruit quality detection during storage, transport and even growth based on experimental modal analysis. A systematic statistical analysis method about outlier detection, data set partitioning, parameter optimization, and multiple regression models were provided.

Feasibility of Detecting Natural Frequencies of Hydraulic Turbines While in Operation, Using Strain Gauges.

Nowadays, hydropower plays an essential role in the energy market. Due to their fast response and regulation capacity, hydraulic turbines operate at off-design conditions with a high number of starts and stops. In this situation, dynamic loads and stresses over the structure are high, registering some failures over time, especially in the runner. Therefore, it is important to know the dynamic response of the runner while in operation, i.e., the natural frequencies, damping and mode shapes, in order to avoid resonance and fatigue problems. Detecting the natural frequencies of hydraulic turbine runners while in operation is challenging, because they are inaccessible structures strongly affected by their confinement in water. Strain gauges are used to measure the stresses of hydraulic turbine runners in operation during commissioning. However, in this paper, the feasibility of using them to detect the natural frequencies of hydraulic turbines runners while in operation is studied. For this purpose, a large Francis turbine runner (444 MW) was instrumented with several strain gauges at different positions. First, a complete experimental strain modal testing (SMT) of the runner in air was performed using the strain gauges and accelerometers. Then, the natural frequencies of the runner were estimated during operation by means of analyzing accurately transient events or rough operating conditions.

Novel development of dynamic behavior of carbon fiber reinforced polymer sandwich panels with stepwise graded adhesive layer

Purpose The purpose of this study is to focus on the developments of carbon fiber reinforced polymer (CFRP) panels with stepwise graded properties on adhesive layer. The various arranges of the graded properties of the adhesive layer have been checked according to experimental results of the literatures and based on applicability. Design/methodology/approach The finite element (FE) models and experimental modal tests of the manufactured CFRP sandwich panel specimens have been investigated. The core thickness, core density and orientation of the fiber direction of the sandwich panel face – sheets have been parametrically checked based on modal behavior. Two fully free and fully clamped boundary conditions (BC) have been checked in stepwise graded adhesive zone (SGAZ) cases and first five non-zero natural frequencies (NF) have been compared. Dynamic response of the SGAZ includes modal analysis and transient dynamic loading have been performed numerically with ABAQUS 6.12 well-known FE code. Findings The first non-zero NF of SGAZ Case 4 was 11.69 per cent higher than homogenous Case 2 and 7.06 per cent lower than Case 1 in fully free boundary conditions. A total of 26.38 per cent is the greatest discrepancy between fist five non-zero NFs of all cases with two BCs (Case 1 vs Case 2 in fully clamped BC). Maximum structural damping behavior and minimum stress picks have been studied during transient dynamic loading analysis of CFRP panel with SGAZ. SGAZ Case 3 (middle adhesive with lower modulus) has increased the maximum structural damping while reducing the minimum out of plain tip displacements during transient dynamic loading by 111.26 per cent in comparison with homogenous Case 2. Also, Case 3 has reduced the Mises stress picks on the adhesive region by 605.68 per cent. Practical implications Making a stepwise graded adhesive region (without any added mass) has been shown that it is a novel and useful way to achieve a wide range of stiffness on CFRP panels. Originality/value Development of the sandwich panels with various stiffness and damping properties.

Modal Analysis of Car Hood with Viscoelastic Damper

The engine hood covers the engine and protects against dust and other foreign particle entering into the engine. It also absorbs the noise and vibrations generated by the engine due to excitation. The hood should withstand the buckling load and if it is not stiffened enough to absorb the vibration at certain conditions such as rough road condition and at higher speed.The 3 – D model of the conventional car hood is generated using the CATIA software, later on it’s been meshed using Hypermesh software. At next required set of results were obtained by using ABAQUAS for free – free condition. In order to compare the result, Experimental modal test was conducted using original component. Results are compared and were agreeing with each other for both the condition

Experimental modal test of the spiral bevel gear wheel using the PolyMAX method

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.

Modal analysis and structure optimisation of vehicle seat frame based on mass distribution method

The vehicle seat is the main component of vehicle interiors that connects occupants and the car body directly. The vibration characteristics of vehicle seats severely affect the security and dynamic comfort of drivers/passengers. A vehicle seat frame developed in the cooperative company is selected as the research object in this paper. The mode of vehicle seat frame is compared by experimental modal test and finite element analysis (FEA) in order to verify the correctness of the finite element model of vehicle seat. Then, the influence of mass distribution on the vehicle seat frame modal characteristics is investigated by changing the local mass distribution of the vehicle seat through finite element modelling. By optimising the structure of the vehicle seat frame based on the analysis results, the dynamic characteristics of vehicle seat can be improved and the security and dynamic comfort can be enhanced.

Influence of the modal damping on the estimated fatigue life

The durability assessment of a component or structure is often assessed using Finite Element Analysis (FEA)-based structural simulations. Fatigue is a progressive failure mode that is related to the stress cycle’s range in a power law such that the fatigue damage increases exponentially as the stress range increases.When the frequency content of the loading gets closer to the natural frequencies of the structure, resonance effects are activated. In this case, the dynamic properties of the structure must be included in the analysis in order to better account for the increased stress response. A dynamic FEA produces therefore more realistic results compared to a static analysis, but requires knowledge of additional characteristics such as mass and damping. In a dynamic FEA, whether transient or steady state frequency response, the damping property governs indeed the magnitude of the dynamic stress response and hence the durability of the component.Unfortunately a default damping value is sometimes erroneously assumed for all modes leading to errors in the stress response, which in turn leads to significant errors in the fatigue life estimates. The modal parameters – including damping ratios - for the structure’s modes of vibration can be extracted using experimental modal analysis techniques.The purpose of this article is first, to explain the critical role that damping plays in fatigue damage; and second, to recommend best practices for determining modal damping experimentally.A simple example of an automotive component will be used to illustrate the importance of using experimental modal analysis to adjust the properties of a FE-based dynamic structural analysis. An experimental modal test followed by a modal analysis will be described. The mode shapes obtained experimentally and analytically will be compared to validate the mass, stiffness and boundary conditions used in the modelling. Then, it will be shown that using realistic values for the damping ratios will help to obtain stress results that correlate with the measured stress.Finally, this example will illustrate the very high sensitivity of the fatigue life estimate to the damping ratios.

Improved Numerical Modelling of Fiber Reinforced Plastics I-Beam from Experimental Modal Testing and Finite Element Model Updating

Improved Numerical Modelling of Fiber Reinforced Plastics I-Beam from Experimental Modal Testing and Finite Element Model Updating

The use of modal parameters in structural health monitoring

The concrete is liable to damage due to various stresses which compensate its adequacy and safety. The estimation of remaining strength in reinforced concrete beams when subjected to increased loading action utilizing vibration parameters is investigated. For this reason, three beams are loaded statically close to failure in various increasing load steps and then repaired. The beams are all of same dimensions, but are different in strength and range of defects introduced to each sample. Following each loading step, the experimental modal testing is utilized to collect the vibration parameters (natural frequency, damping ratio and mode shapes) of each beam when tested under free support boundary conditions. The use of vibration parameters for the purpose of damage identification are known to be an elaborate and lengthy process. On the other hand, they are successful for the structural health monitoring given that they are able to provide global on-site automated continuous monitoring. The paper features post analysis procedures for experimental modal measurements of three concrete samples to obtain and correlate the basic modal parameters (natural frequency, modal damping and mode shapes). The results of the extracted modal parameters and their combination are exploited in this research as quantified identification parameters. This paper concludes that modal parameters are successful in determining the location and quantity of structural degradation, when holistic approach considered through a system.

Modal analysis and structure optimisation of vehicle seat frame based on mass distribution method

The vehicle seat is the main component of vehicle interiors that connects occupants and the car body directly. The vibration characteristics of vehicle seats severely affect the security and dynamic comfort of drivers/passengers. A vehicle seat frame developed in the cooperative company is selected as the research object in this paper. The mode of vehicle seat frame is compared by experimental modal test and finite element analysis (FEA) in order to verify the correctness of the finite element model of vehicle seat. Then, the influence of mass distribution on the vehicle seat frame modal characteristics is investigated by changing the local mass distribution of the vehicle seat through finite element modelling. By optimising the structure of the vehicle seat frame based on the analysis results, the dynamic characteristics of vehicle seat can be improved and the security and dynamic comfort can be enhanced.

Modal analysis of isotropic beams in peridynamics

Modal analysis in continuum mechanics is widely used in damage detection, quality control and to validate numerical simulations. Peridynamics is a reformulation of continuum mechanics that allow discontinuities, such as cracks and voids, to arise and propagate without any special techniques, however, modal analysis in peridynamics has not been explored in detail. In this study we simulate the modal response of an aluminium plate with clamped-free boundary conditions in peridynamics, and compare results to a FE simulation and an experimental modal test. The natural frequencies from peridynamics agree better with the experimental results than frequencies from finite-elements for all modes except mode 1. Differences between peridynamic and experimental results ranged (in magnitude) from 0.13% to 10.38%, and between peridynamic and finite-element results from 0.23% to 9.72%. All mode shapes calculated in peridynamics corresponded to those from finite-elements and experimental measurements.

Structure and modal analysis of carbon fiber reinforced polymer raft frame

This paper mainly studies the structure of a carbon fiber reinforced polymer floating raft frame and analyzes its vibration mode. A frame-based floating raft frame is designed, and two floating rafts which are made of carbon fiber reinforced polymer and metal materials, respectively, and have the same size and structure are manufactured. Based on the modal theory of vibration, the Abaqus software is used to carry out the finite element modal analysis of the frames which are made of different materials, and compare the low-order flexible frequency and vibration mode. Later, the experimental modal test results are verified with modal assurance criterion, and the modal experiment is conducted to verify the accuracy of finite element modal calculations. The results show that under the circumstance of the same frame vibration mode, the damping ratio of the carbon fiber reinforced polymer frame is much higher than that of the metal frame. In addition, the carbon fiber reinforced polymer frame presents good vibration absorption performance. The data analysis results provide some reference value for floating raft vibration designers.