Showing papers on "Modal testing published in 2006"

Two methods for estimating aeroelastic damping of operational wind turbine modes from experiments

Abstract: The theory and results of two experimental methods for estimating the modal damping of a wind turbine during operation are presented. Estimations of the aeroelastic damping of the operational turbine modes (including the effects of the aerodynamic forces) give a quantitative view of the stability characteristics of the turbine. In the first method the estimation of modal damping is based on the assumption that a turbine mode can be excited by a harmonic force at its natural frequency, whereby the decaying response after the end of excitation gives an estimate of the damping. Simulations and experiments show that turbine vibrations related to the first two tower bending modes can be excited by blade pitch and generator torque variations. However, the excited turbine vibrations are not pure modal vibrations and the estimated damping is therefore not the actual modal damping. The second method is based on stochastic subspace identification, where a linear model of the turbine is estimated alone from measured response signals by assuming that the ambient excitation from turbulence is random in time and space. Although the assumption is not satisfied, this operational modal analysis method can handle the deterministic excitation, and the modal frequencies and damping of the first tower and first edgewise whirling modes are extracted. Copyright © 2006 John Wiley & Sons, Ltd.

Experimental modal analysis of an aircraft model wing by embedded fiber Bragg grating sensors

Abstract: A critical issue in practical structural health monitoring is related to the capability of proper sensing systems integrated within the host structures to detect, identify, and localize damage generation. To this aim, many techniques have been proposed involving dynamic measurements such as modal analysis, acoustic emission, and ultrasonics. This paper relies on the use of embedded fiber Bragg grating sensors for performing an experimental modal analysis on a wing of an aircraft model. Time domain response of the embedded fiber-optic sensors induced by hammer impacts were acquired and transformed into the frequency domain. Using a classical technique based on the frequency transfer function, the first displacement and strain mode shapes of the wing have been retrieved in terms of natural frequencies and amplitudes. Experimental results confirm the excellent performances of this class of sensing devices to determine the modal behavior within complex structures compared with conventional accelerometer-based detection systems.

Experimental investigation on dynamic railway sleeper/ballast interaction

Abstract: In ballasted railway tracks, one of the im- portant components that supports the rails and distrib- utes wheel/rail loading onto the ballast supporting formation is a railway sleeper (sometimes is also called a Brailway tie^). This paper presents results of an experimental modal analysis of prestressed concrete sleepers in both free-free and in-situ conditions, incor- porating the dynamic influence of sleeper/ballast inter- action. Dynamic interaction between concrete sleepers and ballast support is crucial for the development of a dynamic model of railway track capable of predicting its responses to impact loads due to wheel flats, wheel burns, irregularities of the rail, etc. In this study, four types of prestressed concrete sleepers were in-kind provided by the Australian manufacturers. The concrete sleepers were tested using an impact hammer excitation technique over the frequency range of interest, 0-1600 Hz. Frequency response functions (FRFs) were measured using PULSE modal testing system. The FRFs were processed using STAR modal analysis package to identify natural frequencies and the corresponding mode shapes for the sleepers. The conclusions are presented about the effect of the sleeper/ballast interaction on the dynamic properties of prestressed concrete sleepers and their use for predicting railway track dynamic responses.

Free vibration of non-uniformly ring stiffened cylindrical shells using analytical, experimental and numerical methods

Abstract: In this research, the free vibration analysis of cylindrical shells with circumferential stiffeners, i.e. rings with non-uniform stiffeners eccentricity and unequal stiffeners spacing is investigated using analytical, experimental and finite elements (FE) methods. Ritz method is applied in analytical solution while stiffeners treated as discrete elements. The polynomial functions are used for Ritz functions and natural frequency results for simply supported stiffened cylindrical shell with equal rings spacing and constant eccentricity is compared with other's analytical and experimental results, which showed good agreement. Also, a stiffened shell with unequal rings spacing and non-uniform eccentricity with free–free boundary condition is considered using analytical, experimental and FE methods. In experimental method, modal testing is performed to obtain modal parameters, including natural frequencies, mode shapes and damping in each mode. In FE method, two types of modeling, including shell and beam elements and solid element are used, applying ANSYS software. The analytical and the FE results are compared with the experimental one, showing good agreements. Because of insufficient experimental modal data for non-uniformly stiffeners distribution, the results of modal testing obtained in this study could be as useful reference for validating the accuracy of other analytical and numerical methods for free vibration analysis.

A statistical algorithm for comparing mode shapes of vibration testing before and after damage in timbers

Abstract: Instances of local damage in timber such as knots, decay, and cracks can be translated into a reduction of service life due to mechanical and environmental loadings. In wood construction, it is very important to evaluate the weakest location and to detect damage at the earliest possible stage to avoid future catastrophic failure. In this study, modal testing was used on wood beams to generate the first two mode shapes. A novel statistical algorithm was proposed to extract a damage indicator by computing mode shapes of vibration testing before and after damage in timbers. The different damage severities, damage locations, and damage counts were simulated by removing mass from intact beams to verify the algorithm. The results showed that the proposed statistical algorithm is effective and suitable for the designed damage scenarios. It is reliable for the detection and location of local damage of different severities, location, and number. The peak values of the damage indicators computed from the first two mode shapes were sensitive to different damage severities and locations. They were also reliable for the detection of multiple cases of damage.

Vibration Performance of a Large Cantilever Grandstand during an International Football Match

Abstract: Vibration problems in stadia are becoming more common due to increased structural slenderness and more lively dynamic crowd excitation. Unfortunately, there is very little guidance available to design engineers dealing with the assessment and design of stadia structures. This paper presents unique data from a program of modal testing and in-service monitoring of a large contemporary cantilever grandstand in the United Kingdom. The in-service monitoring was carried out during an international football match, during which the stadium was full to capacity. Modal properties obtained from the testing on the empty structure are presented and the results from in-service monitoring are described. It is found that crowd occupation can significantly alter the modal properties of a stadium, and that the changes can vary according to the crowd configuration. Additionally, previously proposed methods for assessment of vibration serviceability have been applied and it has been shown that they can lead to inconsistent results, which is a result of their sensitivity to the data acquisition and analysis techniques used. It is concluded that it is very important that consistent methods of data acquisition, analysis, and vibration serviceability assessment are utilized by future researchers and practitioners. Also, further research is required to define vibration serviceability limits using the state-of-the-art vibration dose approach.

Modal and structural identification of a R.C. arch bridge

Abstract: The paper summarizes the dynamic-based assessment of a reinforced concrete arch bridge, dating back to the 50`s. The outlined approach is based on ambient vibration testing, output-only modal identification and updating of the uncertain structural parameters of a finite element model. The Peak Picking and the Enhanced Frequency Domain Decomposition techniques were used to extract the modal parameters from ambient vibration data and a very good agreement in both identified frequencies and mode shapes has been found between the two techniques. In the theoretical study, vibration modes were determined using a 3D Finite Element model of the bridge and the information obtained from the field tests combined with a classic system identification technique provided a linear elastic updated model, accurately fitting the modal parameters of the bridge in its present condition. Hence, the use of output-only modal identification techniques and updating procedures provided a model that could be used to evaluate the overall safety of the tested bridge under the service loads.

Cable Vibration Reduction with a Hung-on TMD System, Part II: Parametric Study

Abstract: Based on the theoretical derivations of the free vibration problem for a cable-TMD system in the companion paper, a parametric study is carried out in the present paper to investigate the influence on the solutions, and thus the system modal damping and frequency, of the cable geometry-elasticity parameter, cable inclination, damper position, mass ratio, frequency ratio, damper damping ratio, and tuning mode. Emphasis is put on the system modal damping, as this is an important index of the performance of the TMD. The features and mechanisms of energy transfer and damping redistribution from the TMD to each mode of the cable-TMD system are also investigated and discussed.

An experimental and analytical investigation of the dynamic characteristics of a flexible sandwich plate filled with electrorheological fluid

Abstract: This paper investigates the dynamic characteristics of a sandwich plate embedded with an electrorheological (ER) fluid. A laser holographic interference experiment and modal testing were conducted to identify natural frequencies, modal damping and shapes of the composite structure, under different electric fields applied to the fluid domain. Moreover, the influence of the ER effect on the structural dynamic responses were recorded. It was found that both of the damping and natural frequencies of the sandwich plate increase monotonously with an increasing electric field; while, at the same time, the resonant peaks of the frequency response and the amplitudes of dynamic responses decrease. Furthermore, based on the special properties of the ER fluid, a discrete dynamic model of the sandwich plate containing ER fluids was developed and validated. The numerical simulation verifies the effect of the ER material on the structure, and the calculated dynamic parameters show the coincident changes with the experimental results.

Non‐linear random response of laminated composite shallow shells using finite element modal method

Abstract: This paper investigates the large-amplitude multi-mode random response of thin shallow shells with rectangular planform at elevated temperatures using a finite element non-linear modal formulation. A thin laminated composite shallow shell element and the system equations of motion are developed. The system equations in structural node degrees-of-freedom (DOF) are transformed into modal co-ordinates, and the non-linear stiffness matrices are transformed into non-linear modal stiffness matrices. The number of modal equations is much smaller than the number of equations in structural node DOF. A numerical integration is employed to determine the random response. Thermal buckling deflections are obtained to explain the intermittent snap-through phenomenon. The natural frequencies of the infinitesimal vibration about the thermally buckled equilibrium positions (BEPs) are studied, and it is found that there is great difference between the frequencies about the primary (positive) and the secondary (negative) BEPs. All three types of motion: (i) linear random vibration about the primary BEP, (ii) intermittent snap-through between the two BEPs, and (iii) non-linear large-amplitude random vibration over the two BEPs, can be predicted. Copyright © 2006 John Wiley & Sons, Ltd.

On a modal damping identification model of building structures

Abstract: The simple modal damping identification model of Hart and Vasudevan is generalized. The model works in the frequency domain and provides time-invariant modal damping ratios of building structures under seismic excitations in terms of modal participation factors and the roof-to-basement transfer function. Translational as well as torsional modes of vibration are considered. The performance of the model is assessed through a small, yet indicative, number of numerical examples involving steel plane and space frames under seismic excitations and on the basis of a number of criteria an ideal identification model should satisfy. It is concluded that the presented model, in spite of its simplicity, gives very good results for low-amplitude seismic excitations resulting in linear elastic structural behavior (with damping) even for cases of closely spaced modes, local modes, and very small or large amounts of damping. Some numerical pitfalls regarding the application of the model are mentioned and carefully treated. The limitations of the model when used in conjunction with inelastic structural behavior are determined and discussed. Experimental verification of the model is also provided.

Observations from Vibration Testing of In-Situ Structures

Abstract: Low-frequency floor and footbridge vibration serviceability problems typically arise when the structure is excited in resonance due to a walking excitation and the resulting accelerations exceed human comfort levels. The measures required to resolve an annoying vibration problem after the structure is constructed can be very difficult and expensive to implement. In most cases, the costs of fixing the problem in-situ are much greater than tackling the problem in the design phase, prior to the structure's construction, considering the potential cost to building owners from possible legal expenses, loss of rental revenue, and consultation fees. Design guidance for composite steel framed floor systems and footbridges is available in the AISC/CISC Steel Design Guide Series 11: Floor Vibrations Due to Human Acitivity . Although the current design guidance is generally acceptable, there is a need to continue characterizing the often-complicated vibration behavior of these structures in an effort to refine current design and analysis techniques, particularly as researchers gain a better understanding of behavior by collecting high-quality experimental data on in-situ floor and footbridge structures. This paper presents observations from such efforts to further characterize behavior through experimental modal testing of a large in-situ composite steel office floor and a laboratory constructed multi-span footbridge. While not entirely inclusive, some general observations are noted on the dynamic behavior, problems encountered, and the consistency/reliability of the applied testing/analysis techniques employed by the researchers.

Support Conditions for Free Boundary-Condition Modal Testing.

Abstract: When modal testing a structure for model validation, free boundary-conditions are frequently approximated in the lab to compare with free boundary-condition analyses. Free conditions are used because they are normally easy to simulate analytically and easier to approximate experimentally than boundary conditions with fixed conditions. However, the free conditions can only be approximated in the lab because the structure must be supported in some manner. This paper investigates and quantifies the effects of the support conditions on both the measured modal frequencies and damping factors. The investigation has determined that the measured modal damping is significantly more sensitive to the support system (stiffness and damping) than the measured modal frequency. Included in the paper are simple formulas which can be used to predict the effect on the measured modal parameters given the support stiffness and damping.

Ambient vibration study of Gi-Lu cable-stay bridge: application of wireless sensing units

Abstract: An extensive program of full-scale ambient vibration testing has been conducted to measure the dynamic response of a 240 meter cable-stayed bridge - Gi-Lu Bridge in Nan-Tou County, Taiwan. A MEMS-based wireless sensor system and a traditional microcomputer-based system were used to collect and analyze ambient vibration data. A total of four bridge modal frequencies and associated mode shapes were identified for cables and the deck structure within the frequency range of 0~2Hz. The experimental data clearly indicated the occurrence of many closely spaced modal frequencies. Most of the deck modes were found to be associated with the cable modes, implying a considerable interaction between the deck and cables. The results of the ambient vibration survey were compared to modal frequencies and mode shapes computed using three-dimensional finite element modeling of the bridge. For most modes, the analytical and the experimental modal frequencies and mode shapes compare quite well. Based on the findings of this study, a linear elastic finite element model for deck structures and beam element with P-Delta effect for the cables appear to be capable of capturing much of the complex dynamic behavior of the bridge with good accuracy.

Lessons learned in modal testing—part 3: transient excitation for modal testing, more than just hammer impacts

Abstract: This study demonstrated that transient inputs for modal testing include much more than just hammer impacts. All the techniques discussed are an important resource that should be maintained in an engineer’s “toolbox” for modal testing. These techniques are generally best suited for lightly damped, linear structures and are very versatile due to the variety of methods. Hammer impacts are fast, convenient, and very useful for quick diagnostics. Projectile impacts produce much higher frequency content, but generally the input cannot be measured unless the projectile can be instrumented. Explosive inputs contain incredible high-frequency content up to the MHz range. Again, no direct measurement of the explosive input is available, although the impulse can typically be calculated. Step relaxation is an underutilized yet highly capable technique, versatile for both small and very large structures, particularly for massive structures where very large forces are required for excitation. It can be used for operational structures in which it would be may be impossible to apply other kinds of excitation techniques. Electrodynamic shakers and drop tables can also be used for transient testing when base excitation is required by test specifications. In conclusion, many types of transient inputs can be applied, limited only by the application and the imagination of the engineer.

Part 4: what’s shakin’, dude? effective use of modal shakers

Abstract: There are several significant modal testing challenges to overcome when striving for the goal of an “adequate linear estimate” of the structural dynamics model. Shaker excitation often has an advantage over impact excitation in meeting this goal. In many cases, shaker testing can reduce the effects of nonlinear response, provide better signal to noise ratio, eliminate overload problems associated with out-of-band force input, provide more consistent data, and provide faster testing by utilizing MIMO acquisition. There are tradeoffs between FRF linearization, FRF distortion, signal to noise ratio, and testing time depending on the shaker forcing function. Some of the major tradeoffs have been addressed for random, pseudorandom, periodic random, burst random, chirp, and stepped sine testing. Shaker mounting and fixturing requires special considerations to mitigate undesirable frequency and damping effects of the shaker and its hardware upon the test article and to minimize side loads and moments on the force gage. Certain steps can be taken to ensure the shaker fixturing is adequate, and many diagnostics are available to ensure the goal of obtaining an “adequate linear estimate” of the structural dynamics model.

Dynamic Testing of an Inflatable, Self-Supporting, Unpressurized Thin-Film Torus

Abstract: Dynamic testing of an inflatable thin-film torus structure with regular formed convex dome surface features has been performed to evaluate structural natural frequencies, mode shapes, and modal damping ratios. The structure presented two unique challenges with respect to modal testing. First, it is extremely lightweight, flexible, and highly damped. Second, the thin film that provides the integrity to the structure is not smooth and flat, but has a pattern of hexagonal dome structures formed into it that increases the local stiffness of the thin film to the point that the structure is self-supporting in the gravity environment of Earth when the internal pressure is released. The dynamic testing was performed in this self-supporting state. In the modal test a loudspeaker provided acoustic excitation, and a laser displacement sensor was used to measure the vibration response at various points on the torus surface. The acoustic excitation and the laser displacement measuring technique were chosen because they are all noncontact methods that avoid mass loading of the structure as would happen if accelerometers and a shaker were used. Three in-plane and three out-of-plane modes were extracted using this approach. The experimental results indicate that the noncontact modal data-acquisition approach for extremely lightweight structures is suitable and effective. The modal identification procedure found modes that are analogous to elastic ring modes as well as widely spaced repeated modes.

Non-uniformly polarized piezoelectric modal transducer: fabrication method and experimental results

Abstract: The study of modal sensors and actuators is motivated by their ability to control the resonance frequency and the vibration modes of mechanical structures. In this work, a non-homogeneous poling method to obtain a radial modal sensor from a piezoelectric disc is presented. The construction method for three modal elements is described and their electrical behavior is presented and analyzed in comparison to a uniform poled disc.

Finite Element Model Verification for the Use of Piezoelectric Sensor in Structural Modal Analysis

Abstract: This paper presents the theoretical modal analysis for the use of PVDF sensor in structural modal test- ing via finite element analysis (FEA). A series of rectangular PVDF films are adhered on the surface of cantilever beam as sensors, while the point impact force is applied as the actuator for experimental modal analysis (EMA). Natural frequencies and mode shapes determined from both FEA and EMA are vali- dated. In FEA, the beam structure is modeled by 3D solid elements, and the PVDF films are modeled by 3D coupled field piezoelectric elements. Both modal analysis and harmonic response analysis are per- formed to obtain the structural modal parameters and frequency response functions, respectively. Results show that both FEA and EMA results agree well. In particular, the PVDF sensor mode shapes, propor- tional to the slope difference between the two edges of PVDF film, are numerically and experimentally validated by FEA and EMA, respectively. Therefore, the simulation of PVDF films for vibration analy- sis in FEA can be verified and easily extended to other complex structures that may contain piezoelectric materials.