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Showing papers on "Modal testing published in 1993"


01 Feb 1993
TL;DR: The Natural Excitation Technique (NExT) is a method of modal testing that allows structures to be tested in their ambient environments as mentioned in this paper, and has been used extensively in the literature.
Abstract: The Natural Excitation Technique (NExT) is a method of modal testing that allows structures to be tested in their ambient environments This report is a compilation of developments and results since 1990, and contains a new theoretical derivation of NExT, as well as a verification using analytically generated data In addition, we compare results from NExT with conventional modal testing for a parked, vertical-axis wind turbine, and, for a rotating turbine, NExT is used to calculate the model parameters as functions of the rotation speed, since substantial damping is derived from the aeroelastic interactions during operation Finally, we compare experimental results calculated using NExT with analytical predictions of damping using aeroelastic theory

659 citations


Proceedings ArticleDOI
19 Apr 1993
TL;DR: In this paper, the authors describe the development of a damage detection and health monitoring approach for large space structures using on-orbit modal identification, which reduces time required for detailed structural inspection.
Abstract: This paper describes the development of a damage detection and health monitoring approach for large space structures using on-orbit modal identification. Several existing model updating methods and model reduction/expansion techniques were evaluated. A new approach to match the order of the undamaged analytical model and the damaged test modes was developed. Two numerical simulation models were initially employed to evaluate practical issues in onorbit modal testing such as measurement noise, number of test modes identified, and limited number of measurements. To experimentally validate the proposed damage detection approach, a ten-bay hexagonal truss with limited instrumentation was tested and analyzed. Damage indicators located the general areas of damage to the truss which reduces time required for detailed structural inspection. Procedures developed for the damage detection are also directly applicable to test/dynamic math model correlation.

85 citations


Journal ArticleDOI
TL;DR: In this paper, the CQC rule for modal combination is extended to include the quasi-static contribution of truncated modes and the effects of input narrowbandedness and cut-off frequency.
Abstract: The CQC rule for modal combination is extended to include the quasi-static contribution of truncated modes and the effects of input narrow-bandedness and cut-off frequency. A simple measure of the error in approximating a high-frequency modal response by its quasi-static contribution is derived. The extended rule is applicable to structures with high-frequency modes and to seismic inputs which may not be regarded as wide band. Numerical examples demonstrate the significance of input bandwidth and cut-off frequency on modal cross-correlation coefficients, and on the error resulting from truncation of high-freqeuncy modes.

67 citations


Journal ArticleDOI
K. N. Koo1, In-Cheol Lee1
TL;DR: In this article, the effects of transverse shear deformation on the modal loss factors as well as the natural frequencies of composite laminated plates have been studied using the finite element method based on the shear deformable plate theory.
Abstract: The effects of transverse shear deformation on the modal loss factors as well as the natural frequencies of composite laminated plates have been studied using the finite element method based on the shear deformable plate theory. The complex modulus of an orthotropic lamina is employed to model damping effect, and a modal approach to the resultant complex eigenvalue problem is introduced to save a considerable amount of computation time. The present result gives a good agreement with the solution of the modal strain energy

37 citations


Journal ArticleDOI
TL;DR: In this article, a newly developed global dynamic model was used to simulate the dynamics of a gear noise rig at NASA Lewis Research Center, where the number of degrees of freedom of the system were reduced by transforming the system equations of motion into modal coordinates.
Abstract: A newly developed global dynamic model was used to simulate the dynamics of a gear noise rig at NASA Lewis Research Center. Experimental results from the test rig were used to verify the analytical model. In this global dynamic model, the number of degrees of freedom of the system are reduced by transforming the system equations of motion into modal coordinates. The vibration of the individual gear-shaft system are coupled through the gear mesh forces. A three-dimensional, axial-lateral coupled, bearing model was used to couple the casing structural vibration to the gear-rotor dynamics. The coupled system of modal equations is solved to predict the resulting vibration at several locations on the test rig. Experimental vibration data was compared to the predicitions of the global dynamic model. There is excellent agreement between the vibration results from analysis and experiment.

33 citations


Journal ArticleDOI
TL;DR: In this paper, a series of experiments has been conducted on a square in-line tube array subjected to water cross-flow, and the results of these analyses are compared to previously published results and to theoretical modal parameter variations with flow velocity, obtained with a slightly extended version of the Lever-Weaver analytical model.

31 citations


Journal ArticleDOI
Chong-Won Lee1, Young-Don Joh1
TL;DR: In this paper, various excitation methods are proposed for the effective use of complex modal testing for rotating machinery, based on the input/output relationships for complex signals, for the direct or indirect assessment of frequency response and coherence functions between complex inputs and outputs.

29 citations


Journal ArticleDOI
TL;DR: In this article, a manufacturing technique for embedding piezoelectric material in composite laminates while maintaining the structure strength and the material's effectiveness was developed, and an ultrasonic C-scan test was applied to screen out the specimen with possible delamination along the interface of the pieziolectric materials and glass fiber layer.

29 citations



Journal ArticleDOI
Yang Gyu Jei1, Chong Won Lee1
TL;DR: In this article, the modal characteristics of asymmetrical rotor systems associated with rotor rotation are clarified by using the complex modal displacement, and the mode shapes associated with forward and backward modes are discussed.

16 citations


Journal ArticleDOI
TL;DR: In this article, methods for calculating the eigensensitivity, frequency response function sensitivity and its modified new formulation are presented, and the implementation of these sensitivity analyses to the practice of finite element model improvement using vibration test data, which is one of the major applications of experimental modal testing is discussed.

Journal ArticleDOI
TL;DR: In this article, a new modal testing theory is proposed to separate the rotor vibration into positive and negative frequency regions, and a method to identify the directivity of modes such as forward and backward is suggested using the frequency response function obtained by the proposed method.
Abstract: Since all dynamic characteristics of rotor systems are closely related with rotor rotations, the directivity of modes is very important in rotor dynamics. But the classical modal testing theories which have been applied to nonrotating structures were often applied to rotor systems. Whereas the dynamic characteristics in negative frequency region have no meanings in nonrotating stuctures, the dynamic characteristics of rotor systems in the negative frequency region which are different from those in the positive frequency region have significant physical meanings. Here a new modal testing theory is proposed to separate the rotor vibration into positive and negative frequency regions. In particular, the amplitude and directivity variations of frequency response functions in positive and negative frequency regions are discussed when using complex modal displacement. And a method to identify the directivity of modes such as forward and backward is suggested using the frequency response function obtained by the proposed modal testing theory. The whirl directions of forced responses related with the directivity of modes are also discussed. In addition, even if the anisotropy in bearings and the effects of gyroscopic moments are permitted, the relations between the right and left eigenvectors of general damped anisotropic rotor systems are evaluated under some practical conditions. If the relations between the right and left eigenvectors are given, the necessity of additional modal testing to identify the adjoint modal parameters is relaxed.

01 Aug 1993
TL;DR: In this paper, experimental modal data for aluminum, titanium, and fiberglass plates heated at uniform, non-uniform, and transient heating conditions are presented, showing the effect of heat on each plate's modal characteristics.
Abstract: High surface temperatures and temperature gradients can affect the vibratory characteristics and stability of aircraft structures. Aircraft designers are relying more on finite-element model analysis methods to ensure sufficient vehicle structural dynamic stability throughout the desired flight envelope. Analysis codes that predict these thermal effects must be correlated and verified with experimental data. Experimental modal data for aluminum, titanium, and fiberglass plates heated at uniform, nonuniform, and transient heating conditions are presented. The data show the effect of heat on each plate's modal characteristics, a comparison of predicted and measured plate vibration frequencies, the measured modal damping, and the effect of modeling material property changes and thermal stresses on the accuracy of the analytical results at nonuniform and transient heating conditions.

Journal ArticleDOI
TL;DR: Two examples are presented showing that reduced models generated as described will be better or at least as good as those produced by the popular modal cost analysis scheme of Skelton and Hughes (1980).
Abstract: A reduced model of a stable mechanical system with proportional damping represented in modal coordinates is developed, based on a procedure reported by Commault and Guerin (1978). In this model, model error is calculated a priori and is used to determine the dominant modes. Two examples are presented showing that reduced models generated as described will be better or at least as good as those produced by the popular modal cost analysis scheme of Skelton and Hughes (1980).

Journal ArticleDOI
TL;DR: In this article, a global minimization method is presented for determining the approximate location of a crack in a beam-type structure based on vibration data. But this method is limited to the case of a single beam.
Abstract: A global minimization method is presented for determining the approximate location of a crack in a beam-type structure based on vibration data. The exact modal deflection and the corresponding modal inertia force of the cracked structure are first computed using a measured eigencouple (frequency and mode shape) of the structure. A reduced stiffness model is then adopted in the finite element formulation of the cracked structure to simulate the dynamic behavior and compute the approximate modal deflection of the structure subjected to the exact modal inertia force. An error function measuring the difference between the exact and approximate modal deflections of the cracked structure is constructed and minimized in a global minimization process for identifying the cracked element from which the approximate location of the crack in the structure is determined. The applications of the proposed global minimization method for crack location identification are illustrated by means of several examples.


Journal ArticleDOI
TL;DR: In order to obtain useful quantities for the identification of a finite element model, which is the most suited for damage localization and quantification, real modes have been extracted by filtering out the damping effects from the complex modes derived by appropriately curve-fitting the frequency-response functions.
Abstract: Procedures and results are presented relative to the modal identification of an old masonry building from two sets of experimentally determined frequency-response functions. Both tests were performed by exciting the structure with low-intensity forces produced by a vibrodyne, before and after high-intensity shakes comparable to a destructive earthquake, in crder to detect and quantify the damage suffered by the masonry. In spite of non-linearities appearing in the frequency response functions, a significant modal model is obtained in both cases, provided that a non-proportional damping model, yielding complex modes, is adopted. The decreasing of natural frequencies, in addition to the increasing of damping factors between the two low-intensity tests, indicate remarkable structural damage. In order to obtain useful quantities for the identification of a finite element model, which is the most suited for damage localization and quantification, real modes have been extracted by filtering out the damping effects from the complex modes derived by appropriately curve-fitting the frequency-response functions.

Journal ArticleDOI
TL;DR: In this article, the amplitude of the strain frequency response function at resonant points had large changes (around 60% when the crack became a through-thickness crack) when the cracks grew in size.
Abstract: Strain gauges, along with an accelerometer and a linear variable displacement transducer, were used in the modal testing to detect a crack in a tripod tower platform structure model. The experimental results showed that the frequency response function of the strain gauge located near the crack had the most sensitivity to cracking. It was observed that the amplitude of the strain frequency response function at resonant points had large changes (around 60% when the crack became a through-thickness crack) when the crack grew in size. By monitoring the change of modal parameters, especially the amplitude of the strain frequency response function near the critical area, it would be very easy to detect the damage that occurs in offshore structures. A numerical computation of the frequency response functions using finite element method was also performed and compared with the experimental results. A good consistency between these two sets of results has been found. All the calculations required for the experimen...

Proceedings ArticleDOI
01 Jan 1993
TL;DR: In this article, the authors describe the refinements of a previously published method for estimating a full modal damping matrix from complex test modes from multi-input random vibration test data.
Abstract: This paper describes the refinements of a previously published method for estimating a full modal damping matrix from complex test modes. It also documents application of the refined method to a structure where complex test modes were derived by the ERA method from multi-input random vibration test data. A numerical example based on simulated test data is presented to demonstrate the validity of the method. The application using real data was not successful, presumably because of noise in the small phase angles of the measured complex modes. Alternative test and data reduction procedures are suggested as possible remedies to the problem. A careful analysis of measurement and data processing errors should be made to examine basic feasibility before implementing the alternative procedures. The ability to estimate a full modal damping matrix is considered important for the preflight estimation of on-orbit damping, and for the synthesis of structural damping from substructure tests.

Journal ArticleDOI
TL;DR: In this article, a real-mode modal filtering approach is used for estimating the spatial modal parameters with laser Doppler velocimeter measurements, and angular displacements are calculated from the estimated mode shape surfaces, curve-fitted with two dimensional Fourier series.
Abstract: A spatial modal parameter estimation technique, specially adapted for use with laser Doppler velocimeter measurements, is presented. It is based on a real-mode modal filtering concept and, therefore, is applicable to structures with low damping. From the estimated mode shape surfaces, curve-fitted with two dimensional Fourier series, angular displacements are calculated

Proceedings ArticleDOI
01 Jan 1993
TL;DR: In this article, passive suspension constraint forces, which are fully correlated and therefore not usable for spectral averaging techniques, are made effectively uncorrelated by perturbing the stiffness of the constraints over multiple sub-tests.
Abstract: Hardware advances in suspension technology for ground tests of large space structures provide near on-orbit boundary conditions for modal testing. Further advances in determining free-body modal properties of constrained large space structures have been made, on the analysis side, by using time domain parameter estimation and perturbing the stiffness of the constraints over multiple sub-tests. In this manner, passive suspension constraint forces, which are fully correlated and therefore not usable for spectral averaging techniques, are made effectively uncorrelated. The technique is demonstrated with simulated test data.



Journal ArticleDOI
TL;DR: In this paper, the results of modal analysis are used to derive a set of modality filters that are applied to operational deflection shape measurements to estimate modal contributions and physical forces for structures subject to multiple, correlated and uncorrelated, external excitations.
Abstract: The results of modal analysis are used to derive a set of modal filters that are applied to operational deflection shape measurements to estimate modal contributions and physical forces for structures subject to multiple, correlated and uncorrelated, external excitations. Singular value decomposition is applied to measured cross spectral density functions to decompose response measurements into contributions associated with each independent external excitation. Generalized and physical forces, coherent to each independent source, are determined from estimated modal parameters. The approach is demonstrated for single and multiple, correlated and uncorrelated, external excitations.

Journal Article
TL;DR: In this article, a method of modal calibration of vibration sensors is presented, which illustrates the realionship between motion and stress of a body resulting from vibration, and allows us to estimate vibration of an aircraft.
Abstract: In the paper a method of modal calibration of vibration sensors is presented. The results of the modal calibration illustrate the realionship between motion and stress of a body resulting from vibration. Such calibration allows us to estimate vibration of an aircraft. The method allows us also to answer the question if the forced vibrations can be estimated at all on the basis of structural excitation during flight.

Journal ArticleDOI
David L. Hunt1, Ralph D. Brillhart1
TL;DR: A wide variety of challenges have been encountered during the past 10 years of aerospace modal testing as discussed by the authors, including single and multiple input random, data acquisition hardware and analysis tools, more high-quality data can be collected in considerably less time than was possible in the past.
Abstract: A wide variety of challenges have been encountered during the past 10 years of aerospace modal testing. New excitation methods have evolved, including single and multiple input random. Enhancements to traditional single and multiple input sine methods have been developed. Data analysis techniques that allow more consistent modal models to be extracted in less time than previously required have also been developed. New data acquisition hardware allows more rapid acquisition of modal data. As a result of these new excitation methods, data acquisition hardware and analysis tools, more high-quality data can be collected in considerably less time than was possible in the past. Modal surveys with 200 to 400 channels of response are becoming more commonplace. During the development and implementation of these new capabilities, many lessons have been learned about how to manage the increased amount of data collected and how to ensure that the quality remains high.

Book ChapterDOI
01 Jan 1993
TL;DR: In this article, the feasibility of improving spatial resolution by incorporating the zeros of cross transfer functions is demonstrated, and the frequency based procedure was developed specifically to support structural health monitoring in an environment that is less than ideal for modal testing.
Abstract: Frequency based localization is a technique which locates structural flaws by monitoring changes in transfer function pole and zero frequencies. This technique has been successfully applied to isolate simulated structural flaws in a modal database representation of a spacecraft using transfer function poles and driving point zeros. However, the number of usable frequencies and, therefore, the achievable spatial resolution, was limited by high modal density and narrow excitation bandwidth. In this paper, the feasibility of improving spatial resolution by incorporating the zeros of cross transfer functions is demonstrated. The frequency based procedure was developed specifically to support structural health monitoring in an environment that is less than ideal for modal testing. For spacecraft on-orbit, testing to detect structural flaws is constrained by the limited availability of sensors and excitation sources, and by the requirement for minimal impact on mission operations. Frequency based localization is well suited for this application, since measurements of input forces and modal frequencies only are required. This greatly reduces the requirements for sensors and data acquisition compared to localization techniques that rely on measurements of mode shapes.


Journal ArticleDOI
TL;DR: In this paper, a strain-energy criterion for modal recognition has been developed, which can be viewed as a measure of the structural deformation in a two-beam structural system.