Experimental Modal Analysis Tests Research Articles

Non-destructive triaxial vibroacoustic modal testing with a single sound transducer: a railway sleeper case study

ABSTRACT Vibrational experimental modal analysis (EMA) is among the most widely used modal parameter estimation techniques. However, it has several drawbacks, such as requiring accelerometer(s) mounting and using local measurements. Therefore, this study investigated whether vibroacoustic modal testing (VMT), a rarely studied method, would be a suitable alternative for EMA. In this context, a comparative case study was conducted on railway sleepers. First, modal parameter determination techniques in the literature were compared (time/frequency-domain, with/without normalisation, structural/acoustic excitation, and acoustic/vibrational response). Then, VMT and EMA test setups were reorganised considering the deficiencies identified in the literature. As a result, the difference between the VMT and EMA resonance frequency and damping ratios in the proposed mode shapes is determined as 0.06% and 3.46%, respectively. Consequently, this novel VMT setup using a single microphone that can take non-destructive, non-contact, and non-local (triaxial) measurements has been proven to provide reliable results and is recommended for future studies.

Design of Experiments to determine the influence of test procedure on Experimental Modal Analysis

This study conducts a fractional factorial design to evaluate the influence of test settings on the results of Experimental Modal Analysis (EMA) which is the method often used to determine natural frequencies and damping rate of structures. The structures in this work are simple beams cut from lightweight rail-way vehicle car bodies. The original specimens have very low structural damping, whereas others have been supplemented with Constrained Layer Damping (CLD) patches to significantly increase vibration energy dissipation. It will be shown that test settings do influence Experimental Modal Analysis (EMA) results in ways that are seldom intuitive or foreseeable. The statistical significance of some factors, or combinations thereof, proves that their effects are not entirely attributable to random phenomena. The results show that a statistical analysis like the one presented in this paper should be conducted prior to extracting accurate damping data from EMA tests. The technique reduces the number of tests needed to assess the influence of settings. This technique, not only highlights the statistically significant factors, but also quantifies their influence, thus helping the researcher make an informed decision regarding test settings. • The analysis of variance determines which factors do have a significant influence on EMA analysis. • Test settings depend on beam type, modal parameters, and support configuration. • Test settings have a strong influence on modal damping. • The fractional factorial design reduces the number of tests necessary. • The proposed statistical analysis should be conducted prior to extracting accurate damping data.

Operational modal analysis of a catamaran using a limited set of accelerometers

ABSTRACT Experimental modal analysis tests were performed during the catamaran’s normal operation and using only 06 accelerometers on the main deck. This work focuses on estimating and comparing modal parameters using experimental (time and frequency domain techniques) and numerical analyses. Ten (10) experimental natural frequencies and (incomplete) mode shapes were estimated, in the range of 4–50 Hz, using Balance Realisation, Frequency-Spatial Domain Decomposition and Enhanced Frequency Domain Decomposition techniques. As only 06 accelerometers had been used for modal testing, a finite element (FE) model was used to augment the spatial resolution of the experimental mode shapes. Experimental and numerical natural frequencies showed good correlation with only the 3rd bending mode shape exhibiting 13.5% deviation. It was concluded that the experimental techniques can: (i) identify natural frequencies in the presence of harmonic components and, (ii) estimate mode shapes using a limited set of accelerometers, aided by a FE model.

Study the damping and vibrational properties of polycarbonate reinforced ZrO2

In this paper, the effect of nano zirconia particles on damping characteristics of polycarbonate composites was investigated experimentally. The nano particles were added to polycarbonate matrix with five different weight percentage (wt %). Polycarbonate Nanocomposites filled with pristine and modified ‎Zirconium dioxide were prepared by simple melt compounding. All samples ‎were prepared by an injection molding process. The samples were studied before and after damage. Damage process was done with high velocity impact. The half-power bandwidth method was used to experimentally estimate the damping ratio. The experiments were designed based on an impulse actuator signal generated by the ‎shaker, and the responses were recorded using accelerometer sensors. The results of experiments showed that the damping properties of the composite were increased by adding nanoparticles to 3 wt% and decreased gently for samples with the upper content of nanoparticles. Transmission electron microscopy (TEM) pictures were used to investigate the distribution of nano zirconia on the polycarbonate matrix and showed a homogeneous mixture of nanocomposites and strong bonding between zirconia and polycarbonate that increased damping properties and decreased vibration. Finally, the experimental modal analysis test was done after high-velocity impact damage process to investigate the effect of defection on the nano polycarbonate composites. The results showed a great decline in vibration and an increase of damping properties.

Vibration Response of Steam Turbine Healthy and Cracked Blade under the Stress Stiffening and Spin Softening Effects

In the present paper, vibration responses of the healthy and cracked blade under the influence of stress-stiffening and spin-softening have been studied. The three-dimensional finite element-based numerical analysis is carried out to study the blade's mode shape and natural frequencies and are validated with the experimental modal analysis test results. In the case of spin softening, for healthy blade at the turbine’s speed over 1000 RPM, natural frequencies increase significantly in both first flap-wise and chordwise bending modes. In both cases of STF and SOF, for the cracked blade, the deviation from the healthy blade in the natural frequency increases with increasing crack size, maximal in a crack size of 90%. It is also observed that irrespective of crack size, deviation in natural frequencies as compared to healthy blade increases as spin speed increases in the cases of chordwise bending mode (Mode II) and higher-order overlapped mode IV, and in the case of fundamental torsion mode (Mode III), higher-order overlapped mode V and VI natural frequencies decrease in both STF and SOF cases.

Mechanical characterization of an epoxy panel reinforced by date palm petiole particle

The past years were marked by an increase in the use of wood waste in civil and mechanical constructions. Date palm waste remains also one of the most solicited renewable and recyclable natural resources in the composition of composite materials. In Algeria, a great amount of this type of plant wastes accumulates every year. In order to make use of this waste, a new wood-epoxy composite material based on date palm petiole particleboard is developed. It makes use of date palm petiole particleboard as reinforcement and epoxy resin as matrix. The size of the particles reinforcement are between 1 similar to 3 mm and proportion of reinforcement used is 37%. In this work, experimental and numerical studies are conducted in order to characterize the wood fibre-epoxy plates. Firstly, experimental modal analysis test was carried out to determine Young's modulus of the elaborated material. Then, in order to validate the results, compression test was conducted. Furthermore, additional information about the shear modulus of this material is obtained by performing an experimental modal analysis to extract the first torsional mode. Moreover, a finite element model is developed using ANSYS software to simulate the vibration behaviour of the plates. The results show a good agreement with the experimental modal analysis, which confirms the values of Young's modulus and shear modulus.

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.

Comparison of experimental and operational modal analysis on a laboratory test plate

Abstract Operational modal analysis (OMA) is widely used whenever the dynamic characteristics of structures that do not fit into a laboratory are desired. In addition, OMA offers a test of the structure under its real boundary conditions which may sometimes be preferable for validation of numerical models. Theoretically, the natural frequencies and damping ratios should be identically estimated by an OMA test and an experimental modal analysis (EMA) test. However it is still often reported that EMA tests are more reliable. The present paper presents a thorough comparison of EMA and OMA tests of a Plexiglas plate. The experiments were carefully designed, to ensure that the plate was tested under similar boundary conditions. Estimated modal parameters from the EMA test and OMA test are presented and compared, for the first ten modes of the plate. It is found that natural frequencies are deviating by less than 0.3%, damping ratios by less than 7%, whereas cross-MAC values between the mode shapes of the two tests are found to be above 0.99. The experimental test was conducted first by an EMA test, followed by the OMA test and finally another EMA test was conducted in order to catch any time-variance. It is concluded that no significant differences were found between modal parameters obtained by OMA and EMA.

Identification of modal strains using sub-microstrain FBG data and a novel wavelength-shift detection algorithm

The presence of damage in a civil structure alters its stiffness and consequently its modal characteristics. The identification of these changes can provide engineers with useful information about the condition of a structure and constitutes the basic principle of the vibration-based structural health monitoring. While eigenfrequencies and mode shapes are the most commonly monitored modal characteristics, their sensitivity to structural damage may be low relative to their sensitivity to environmental influences. Modal strains or curvatures could offer an attractive alternative but current measurement techniques encounter difficulties in capturing the very small strain (sub-microstrain) levels occurring during ambient, or operational excitation, with sufficient accuracy. This paper investigates the ability to obtain sub-microstrain accuracy with standard fiber-optic Bragg gratings using a novel optical signal processing algorithm that identifies the wavelength shift with high accuracy and precision. The novel technique is validated in an extensive experimental modal analysis test on a steel I-beam which is instrumented with FBG sensors at its top and bottom flange. The raw wavelength FBG data are processed into strain values using both a novel correlation-based processing technique and a conventional peak tracking technique. Subsequently, the strain time series are used for identifying the beam's modal characteristics. Finally, the accuracy of both algorithms in identification of modal characteristics is extensively investigated.

Thermal modification of European beech at relatively mild temperatures for the use in electric bass guitars

The possibility of the use of thermally modified European beech (Fagus sylvatica L.) in necks of electric bass guitars for the substitution of Hard maple (Acer saccharum) has been studied. The heat treatments were performed at relatively mild treatment temperatures of 140 and 160 °C for 12 h. The acoustic properties were determined by means of experimental modal analysis (EMA) and the mechanical properties by means of static and impact bending tests. The results show that both the acoustic and mechanical, static properties of beech improve significantly owing to thermal modification, being similar or better compared to Hard maple. The impact bending strengths decrease owing to thermal modification. Additional tests on complete necks of electric bass guitars by means of EMA and plucking tests on total instruments show that thermally treated beech at mild temperatures can substitute Hard maple for the use as neck material in electric bass guitars.

Triple correlation for detection of damage-related nonlinearities in composite structures

Nonlinear effects in vibration responses are investigated for the undamaged composite plate and the composite plate with a delamination. The analysis is focused on higher harmonic generation in vibration responses for various excitation amplitude levels. This effect is investigated using the triple correlation technique. The dynamics of composite plate was modelled using two-dimensional finite elements and the classical lamination theory. The doubled-node approach was used to model delamination area. Mode shapes and natural frequencies were estimated based on numerical models. Next, the delamination divergence analysis was used to obtain relative displacements for delaminated plies. Experimental modal analysis test was carried out to verify the numerical models. The two strongest vibration modes as well as two vibration modes with the smallest and largest motion level of delaminated plies were selected for nonlinear vibration test. The Fisher criterion was employed to verify the effectiveness and confidence level of the proposed technique. The results show that the method can be used not only to reveal nonlinearities, but also to reliably detect impact damage in composites. These results are confirmed using the statistical analysis.

Direct multivariate subspace time identification

The paper presents a time domain method to identify structural modal parameters by fitting a discrete multivariate space-time model into noise corrupted, input–output measurement data. The subspace identification scheme proposed is an important characteristic of the method, leading to a deterministic and statistically bias free estimation of parameters. The identification model in this scheme, is kept to the minimum description order, in spite of the presence of noise in the system's input and output The method is used to estimate natural frequencies, damping factors and mode shapes in an experimental modal analysis test.

Experimental analysis of the dynamic, mechanical behaviour of a chicken egg

The dynamic, mechanical behaviour of a chicken egg is assessed. Experimental modal analysis tests revealed several spherical modes in the 2– 7 kHz frequency band. An identification of the mode shapes, the resonant frequencies and corresponding damping ratios of these modes is obtained. The study allows the interpretation of the vibration response of an egg excited at its equator with a light mechanical impact which could open the door towards an automated egg inspection system.

Damage Detection Using Dynamic Testing Techniques

The use of Experimental Modal Analysis testing as a tool for Structural System Identification of model bridge superstructures is reported in this paper. Three 40% scale models of 5-span continuous flat slab bridges were tested using this technique, before and after application of high level static loading (approaching incipient collapse). Results obtained demonstrate the utility of the technique for identification of the in-service condition of bridge superstructures and for damage-detection applications in particular.