Abstract: In this paper a new frequency domain technique is introduced for the modal identification of output-only systems, i.e. for the case where the modal parameters must be estimated without knowing the input exciting the system. By its user friendliness the technique is closely related to the classical approach where the modal parameters are estimated by simple peak picking. However, by introducing a decomposition of the spectral density function matrix, the response spectra can be separated into a set of single degree of freedom systems, each corresponding to an individual mode. By using this decomposition technique close modes can be identified with high accuracy even in the case of strong noise contamination of the signals. Also, the technique clearly indicates harmonic components in the response signals.

Abstract: As a first step in developing a health monitoring system, the effect of delamination on the modal frequencies of laminated composite beams has been investigated. A piezoceramic patch driven with a linear rapid frequency sweep was used to induce vibrations on the structure and its response registered via piezoelectric sensors. Modal frequencies were obtained using concepts of resonant ultrasound spectroscopy (RUS). Changes of the modal frequencies after delamination initiation, compared to those of a non-delaminated specimen, gave a good indication of the degree of damage, demonstrating the feasibility of using measured changes in the vibration characteristics to detect damage.

Abstract: In this paper a new frequency domain technique is introduced for the modal identification from ambient responses, i.e. in the case where the modal parameters must be estimated without knowing the input exciting the system. By its user friendliness the technique is closely related to the classical approach where the modal parameters are estimated by simple peak picking. However, by introducing a decomposition of the spectral density function matrix, the response can be separated into a set of single degree of freedom systems, each corresponding to an individual mode. By using this decomposition technique close modes can be identified with high accuracy even in the case of strong noise contamination of the signals.

Abstract: In this paper a technique for separation of harmonic and structural modes in output-only modal testing and identification is presented. The indicator is based on the basic differences of the statistical properties of a harmonic response and narrow-band stochastic response of a structural mode. The indicator is demonstrated on an example where a plate is loaded by an engine rotating with quasi-stationary speed. An output-only modal identification is performed using a technique based on Frequency Domain Decomposition (FDD), and what appears to be three harmonic components and five structural modes were identified.

Abstract: If a laser Doppler vibrometer is used in a continuously-scanning mode to measure the response of a vibrating structure, its output spectrum contains side-bands from which the response mode shape, as defined along the scan line, may be obtained. With impact excitation, the response is the summation of a set of exponentially-decaying sinusoids which, in the frequency domain, has peaks at the natural frequencies and at `sideband' pseudo-natural frequencies, spaced at multiples of the scan frequency. Techniques are described for deriving natural mode shapes from these, using standard modal analysis procedures. Some limitations as to the types of mode which can be analysed are described. The process is simple and speedy, even when compared with a normal point-by-point impact test survey. Information may also be derived, using a circular scan, on the direction of vibration, and angular vibration, at individual points.

Abstract: Most modern modal model estimation algorithms start from the observation that parameters such as resonance frequencies, damping ratios and modal participation factors are global for the structure under test. By means of a least squares procedure, a global model is forced on the available time or frequency domain data. In many practical cases however, these data are slightly to strongly inconsistent. Mass loading effects, temperature variations etc. make that the data measured consecutively in different patches can show slightly differing resonant frequencies. When trying to fit a global model through these data, errors can result by identifying multiple close poles instead of one single pole near single resonances. But most importantly, the mode shape extraction can be seriously affected as modal residues are extracted for global pole values that do not correspond to the actual value of the FRF or impulse response under consideration. This may lead to major problems in postprocessing the data by modal substructuring or modification analysis. Also, the columns of FRF matrices relating to multiple excitation tests are often inconsistent. Shaker or suspension constraints and small nonlinearities are often indicated as possible error sources. The relevance of these problems will be briefly reviewed by some practical case studies and pragmatic remedies are evaluated. This leads to the observation that such methods could also be useful as a smoothing pre-processor for FRF based impedance methods (substructuring, load analysis), where data inconsistencies may lead to major matrix inversion errors.

Abstract: Background: When measuring periodontal disease, various types of equipment for making objective measurements of tooth mobility have been proposed. However, these devices and methodology are insufficient in terms of reliability. An innovative method using vibrational theories to assess the periodontal attachment level of natural teeth is presented in this study. Methods: Modal testing technique, a non-destructive and time-saving method, was used for non-invasive and quantitative measurement of the natural frequencies of the upper central incisor in vivo and in vitro. A finite element model was established, and modal testing experiments were simulated to assess the relationship between bone level and teeth. Results: The first dominant natural frequency of healthy human upper central incisors ranges from 710 Hz to 3,360 Hz, with an average of 1,701 ± 679 Hz. Both in vitro experiment and finite element simulation showed that lowering of the attachment level causes a significant decrease in the natural frequen...

Abstract: In this paper, the current status of the technology of modal testing is reviewed with particular reference to the application of these methods to the task of ensuring the optimum design of mechanical structures Existing methods are summarised and new techniques which are currently under development are described Some of the current limitations and problem areas are also identified

Abstract: Many researchers have devoted their work to the development of modal analysis extraction techniques in order to obtain more reliable identification of the modal parameters. Also, as a consequence of all this work, there are some other works devoted to the evaluation and comparison of these methods in order to find which one is the most reliable method with respect to certain characteristics. In this thesis the Rational Fraction Polynomial (RFP) Method, the Prony or Complex Exponential Method (CEM), the Ibrahim Time Domain (ITD) Method, and Hilbert Envelope Method are used to evaluate how the accuracy of the damping ratio is affected with respect to various parameters and conditions. The investigation focuses in the estimation of damping ratio because among the modal parameters, it is the most difficult to model. Each method is evaluated individually in order to understand how the damping ratio estimation is affected with respect to each method when the characteristics of the FRF are changed. Also, they are compared to show that, in general, the Rational Fraction Polynomial Method is a more reliable method than the other methods. To investigate this, a simulated analytical data and an experimental data are processed to estimate the modal parameters, but focusing in the damping ratio. For the simulated analytical data the damping ratio’s percent of error were calculated. The highest damping ratio’s percent of error of the RFP was 0.0073501%. In the other hand, for the CEM, ITD, and Hilbert Envelope Method their highest damping ratio’s percent of error were 83.02%, 99.82%, and 4.077%, respectively.

Abstract: In this paper an output only modal testing and identification of a car body subject to engine excitation is presented. The response data were analysed using two different techniques: a non-parametric technique based on Frequency Domain Decomposition (FDD), and a parametric technique working on the raw data in time domain, a data driven Stochastic Subspace Identification (SSI) algorithm. Both techniques identified 16 modes under 85Hz. The results of the two techniques were validated against each other. It was concluded, that even though some of the identified modes showed an unsatisfactory agreement when comparing results of the two techniques, most of the modes compared weD, and thus, for the main part of the identified modes the modal estimates most be considered as reliable modal parameters.

Abstract: Output-only modal testing of an object. Vibrations are excited in said object and measured by a number of vibration sensitive detectors. From the data of the measurements, a spectral density matrix function is determined. From this density matrix, auto spectral densities for the individual modes are identified performing a decomposition based on the Singular Value Decomposition technique. From the auto spectral densities of the individual modes, natural frequencies and damping ratios for the modes can be estimated, and from the singular vectors of the Singular Value Decomposition, the modes shapes can be estimated.

Abstract: This study addresses model-based structural damage simulation and identification of the Tsing Ma Suspension Bridge through modal sensitivity analysis. For this purpose, a precise three- dimensional finite element model has been developed with the attributes: (1) the spatial configuration of the original structure remains in the model; (2) the geometric stiffness of cables and hangers has been accurately accounted for in the model; (3) the mass and stiffness contribution of individual structural members is independently described in the model, so damage to any structural member can be directly and precisely simulated. The model was validated using the measured modal data obtained at different erection stages and after the bridge completion. It is then used as a baseline for structural damage simulation and modal sensitivity analysis. Due to the intensive distribution of natural frequencies of the bridge, modal assurance criterion (MAC) is first utilized to check the correlation of mode pairs between the damaged and intact structure. Ten damage cases are simulated and the sensitivities of various modal parameters including natural frequency, mode shape and modal flexibility to different types of damage are evaluated. The goal of this study is to analytically determine which modal parameter is most sensitive to damage for a large-scale suspension bridge. The analysis results show that, in most cases, the frequency sensitivity to damage is low, while the modal flexibility method clearly indicates the damage locations only using a few lowest frequency modes.

Abstract: This paper presents the use of piezoceramic transducers, Lead Zirconia Titanate (PZT) actuators and Polyvinylidene Fluoride (PVDF) sensors, for experimental modal testing of a simply supported plate. A series of rectangular-shaped PVDF films are evenly distributed over the plate and act as the sensing devices instead of traditional acceleration sensors. Pairs of rectangular PZT patches are bounded to opposite sides of the plate to form actuators and when they are excited by 180° out-of-phase stimulation, they will cause pure bending. The theoretical formulation of frequency response functions (FRFs) of the piezoceramic transducers is developed. The mode shape functions of the PZT actuator and the PVDF sensor are identified, respectively. Experiments are performed to obtain a column of FRF matrix. The structural modal parameters, including natural frequencies, modal damping ratios and mode shapes, can then be extracted by a modal parameter extraction method. Results show the modal parameters can be properly obtained and physically interpreted. This paper presents the concepts of smart structural testing (SST) with the use of piezoceramic transducers and leads to the applications of smart structures to health monitoring of structural systems.

Abstract: There is a current trend towards ever more slender concrete floor structures, which is resulting in more frequent problems with their vibration serviceability. Predictive methods for vibration serviceability must consider not only the structures themselves, but also the non-structural elements which are attached to them, as these may have a significant effect on the dynamic characteristics of the floor structural system. As there has been very little past research in this area, this thesis describes an investigation into the effects of raised access floors on the vibration serviceability of long-span concrete floors. The development of a new modal testing facility based on electrodynamic shaker excitation, which was capable of producing high quality estimates of the modal properties of full-scale floor structures, is described. This was subsequently utilised to determine the modal properties of three full-scale floor structures, before and after the installation of various configurations of raised access floors. The response of these structures to controlled pedestrian excitation was also measured. Realistic finite element models of all structures were developed and updated using the results from the experimental work. These were subsequently utilised for investigation of the experimentally measured effects of the raised access floors. It was found that raised access floors had only minor effects on the modal properties of the long-span concrete floors. Reductions in natural frequencies due to the increased mass were, to some extent, offset by the slight increases in stiffness following the installation of the access floors. Modal damping ratios increased for some modes of vibration, but these changes were rather unpredictable and hence they were too unreliable to be used in design. The response of the structures under controlled pedestrian excitation reduced following the installation of various configurations of raised access floors. The reduction appeared to be greater for relatively deep access floors (500 - 600 mm) than for relatively shallow access floors (150 - 200 mm). Therefore, it is recommended that the effects of access floors may be included in vibration serviceability analyses by applying a reduction factor to predicted responses calculated by assuming a bare floor. The proposed reduction factors are 0.9 for access floors where the finished floor height is less than 500 mm and 0.8 for access floors where the finished floor height is 500 mm or greater.

Abstract: For classically damped structures, modal mass, stiffness and damping can be defined directly from formulas that relate the full mass, stiffness and damping matrices to the transfer function matrix. The modal mass, stiffness, and damping definitions are derived in a previous paper [1], and are restated here for convenience. The transfer function is defined over the complex Laplace plane, as a function of the variable ) j s ( ω + σ = . Experimentally, the values of a transfer function are measured only along the ω j -axis in the s-plane, that is for ) j s ( ω = . These values are referred to as the Frequency Response Function (FRF).

Abstract: An experimental investigation into the dynamics of an inflated thin film polyimide torus has been performed. This paper describes an effort to define modal test procedures for extremely lightweight, flexible inflated structures. An impact hammer was modified to excite the global modes of the structure, while avoiding local excitation. Coherence measurements are examined to describe the nonlinearity in the structure. Nonlinearities include shell wrinkling and material nonlinearities such as film modulus variations with frequency and temperature. The baseline goal for the modal test is to determine the first six bending modes of the structure.

Abstract: Precision inflatable space structures promise revolutionary change in spacecraft design over the next decade. Given the variety of previous applications and the potential of future concepts, development of advanced design tools is underway. Validation of these models requires experimental analysis of both ground and orbiting test articles. The present study examined the suitability of piezoelectric polymer materials (namely PVDF) to excite an inflated structure for modal testing. Experiments were undertaken using both a conventional electrodynamic shaker and a small sheet of PVDF bonded to an inflated torus. In addition, a linear finite element model of an inflated torus is compared to the experimental results. The results demonstrate the potential for PVDF to excite inflatable ground test articles.

Abstract: Analysis of the rigid solar array, SA-3, design showed that the pointing control system stability margin requirements would be violated because of the modal characteristics of the SA-3 fundamental bending modes. A damping of 1.5% of critical of the SA-3 fundamental bending modes, at the HST system level, is needed to meet the stability margin requirements. A damper, consisting of a titanium flexure and viscoelastic damping material, has been designed, built, and tested, and is an integral part of the SA-3 mast. The viscoelastic material properties are temperature-sensitive, and direct complex stiffness testing was performed to characterize the frequency- and temperature-dependent behavior of the damper. Fixed-base modal testing showed that the optimized damper is expected to provide modal damping of at least 2.25% of critical (fixed base and extrapolated to zero excitation force levels) over the temperature range of 0/spl deg/C to 25/spl deg/C.