Showing papers on "Modal testing published in 1990"

Modal Sensors/Actuators

Abstract: A piezoelectric laminate theory that uses the piezoelectric phenomenon to effect distributed control and sensing of structural vibration of a flexible plate has been used to develop a class of distributed sensor/actuators, that of modal sensors/actuators. The one-dimensional modal sensors/actuator equations are first derived theoretically and then examined experimentally. These modal equations indicate that distributed piezoelectric sensors/actuators can be adopted to measure/excite specific modes of one-dimensional plates and beams. If constructed correctly, actuator/observer spillover will not be present in systems adopting these types of sensors/actuators. A mode 1 and a mode 2 sensor for a one-dimensional cantilever plate were constructed and tested to examine the applicability of the modal sensors/actuators. A modal coordinate analyzer which allows us to measure any specific modal coordinate on-line real-time is proposed. Finally, a way to create a special two-dimensional modal sensor is presented.

Identification of modal properties of bridges

Abstract: Modal stiffness and damping are determined from experimental test data for a single‐span laboratory bridge model and a full‐scale three‐span highway bridge. Steady‐state vibration tests are conducted on both structures using an electro‐hydraulic actuator. Accelerations are recorded at selected locations on the structures. For the laboratory model, the modal properties are calculated from experimental data for the as‐built model and after one flange is cut. Modal properties for the highway bridge are determined during a 40,000‐cycle fatigue test of the in situ structure. Changes in stiffness and mode shapes are related to the observed deterioration of the structures. Mode shapes are the best indicators of the deterioration.

Submatrix approach to stiffness matrix correction using modal test data

Abstract: An efficient method six stiffness matrix correction to match modal testing data is presented. Significant reduction of unknown parameters is achieved by grouping the elements of the same stiffness characteristics and representing them as a multiplication of a scaling factor and a submatrix. A concise formulation to identify the scaling factors is found by utilizing a least squares solution. The formulation also incorporates a capability of reducing finite element mass and stiffness matrices to test degrees-of-freedom for a direct analysis/experiment correlation

The effect of fluid–structural coupling on sound waves in an enclosure—Experimental part

Abstract: The acoustic properties of individual normal modes in a rectangular panel–cavity system were investigated experimentally. The system consisted of a heavy five‐sided concrete box closed on the sixth side with a test panel. Experimental evidence in this paper reveals the dependence of the modal decay time of sound waves in the enclosure upon the modal characteristics of the test panel, such as the modal coupling factors, resonance frequencies, and modal decay times. The results of experimental measurements are used to verify some previous theoretical predictions. The agreement between measured and calculated enclosure modal decay times shows that modal coupling theory can be used to evaluate the sound wave behavior in those enclosures in which the coupling between fluid‐borne and structure‐borne acoustical waves is modal and where the classical model based upon the locally reactive boundary assumption does not apply. The phenomenon of maximum sound energy absorption by the panel was also observed in the exp...

A scanning laser doppler vibrometer for modal testing

Abstract: Accelerometers are widely used to sense structural response in modal testing. The mass loading and local effects due to accelerometers are not always negligible. The laser Doppler velocimeter/vibrometer (LDV) is a noncontact optical sensing tool for accurately measuring point velocities. The noncontact nature of the instrument makes it particularly attractive for use on lightweight structures where measurement interaction must be minimized. Real-time scanning LDV's have recently been introduced to measure fluid flow velocity profiles rapidly. In this paper, the development of a real-time scanning LDV for structural applications is described. The instrument can be used to simultaneously measure the velocity response at a series of locations on a vibrating structure. Standard modal analysis techniques can then be applied to extract the usual modal data, e.g., natural frequencies, damping and mode shapes. The special case of beam vibration is considered in this paper though the technique can be readily extended to generic planar measurements. The measurement technique has been validated through modal testing of a simple beam structure. Comparisons between theoretical and LDV measured mode shapes and natural frequencies are presented. 20 refs.

Joint damping and nonlinearity in dynamics of space structures

Abstract: Analysis of the effect of linear joint characteristics on the vibration of a free-free, three-joint beam model shows that increasing joint damping increases resonant frequencies and modal damping, but only to the point at which the joint gets «locked up» by damping. The maximum amount of passive modal damping obtainable from the joints is greater for low-stiffness joints and for modal vibrations where large numbers of joints are actively participating. A joint participation factor is defined to study this phenomenon. Analysis of the nonlinear three-joint model, with cubic spring at the joints, shows classical single-degree-of-freedom nonlinear response behavior at each resonance of the multiple-degree-of-freedom

Detection and Location of Structural Cracks using FRF Measurements

Abstract: This is follow-on work to an IMAC paper given last year [1] where it was shown that modal testing can be used to detect “faults” in mechanical structures. By “faults”, we mean any of the following occurrences: • failure of the structural material, e.g. cracking, breaking, or delamination.

Development and implementation of a shuttle modal inspection system

Abstract: One of the major tasks between Space Shuttle flights is inspection of the orbiter subsystems such as control surfaces, vertical tail, and wings. To date, inspection techniques have consisted primarily of visual and X-ray methods, which are not only time consuming but not as comprehensive as desired. Previous shuttle component and orbiter ground modal testing revealed that orbiter component damage could be identified using standard modal test methods. As a result, NASA has procured a dedicated shuttle modal inspection system (SMIS) that will be used for subsystem inspection. This paper presents a background on the use of modal testing to detect shuttle component damage, details on this new implementation, and an early use of the SMIS capability to investigate a potential orbiter vibration problem.

Frequency Response of Nonproportionally Damped, Lumped Parameter, Linear Dynamic Systems

Abstract: Modal analysis is used to transform the governing equations of motion to the respective modal coupled equations of motion. The Laplace transform is performed to transfer to the frequency domain. The modal coupling is then analyzed through coupling terms. A criterion in the form of nonproportionality indices is developed, in order to measure the extent of the modal coupling and to predict the error introduced by neglecting this coupling either partially or completely. An attempt to interpret the frequency spectrum of the modal coupling is also made

Modal sampling method for the vibration study of systems of high modal density

Abstract: This paper presents a method to predict the vibration response of a group of modes, using a sample of modes of the group. The accuracy of the prediction can be improved when increasing the size of the sample. Numerical calculations made on a beam, driven in longitudinal motion, show the ability of the method to reconstitute the energy response of a group of modes including resonant modes, with a sample of a number of modes close to that in the effective damping bandwidth.

Modal analysis of UH 60A instrumented rotor blades

Abstract: The dynamic characteristics of instrumented and production UH-60A Black Hawk main rotor blades were measured, and the results were validated with NASTRAN finite element models. The blades tested included pressure and strain-gage instrumented blades, which are part of the NASA Airloads Flight Research Phase of the Modern Technology Rotor Program. The dynamic similarity of the blades was required for accurate data collection in this program. Therefore, a nonrotating blade modal analysis was performed on the first 10 free-free modes to measure blade similarities. The results showed small differences between the modal frequencies of instrumented and production blades and a close correlation with the NASTRAN models. This type of modal testing and analysis is recommended as a standard procedure for future instrumented blade flight testing.

Identification of structural system parameters from time domain data. Identification of global modal parameters of a structural system by the improved state variable method.

Abstract: The paper presents a method of identifying the global modal parameters of a mechanical structure from the free decay responses, which is based on the state variable method expressed in the form of a difference state equation. A recursive algorithm is proposed for calculation of the state transition matrix. The modal shapes of a large number of stations for each mode are obtained by making use of the data acquisition system with a few channels, which results from the uniformity of the normalized modal shapes. The generalized mathematical model of the method is related to the other time domain methods. As illustrative examples, the method is applied to the identification of global modal parameters of a free-free straight pipe and a "T"-shaped plate structure. The identified results are in good agreement with the analytical ones.

Identification of a nonlinear beam. Proposition of an identification technique.

Abstract: The basic procedures of the proposed technique are (1) measuring periodic responses of the beam to periodic external forces, (2) determining modal coordinates using the responses and based on the modal functions of the corresponding linear beam, and (3) determining the modal equations, including nonlinear terms, using the principle of harmonic balance

モード解析に関する研究 : 第9報, 多点加振に対応した最尤法に基づく曲線適合方法の提案-その1

Abstract: Experimental modal analysis is widely used in many fields and many curve fitting techniques which estimate modal parameters have been developed. This paper presents a new multireference curve-fitting technique which is based on the maximum likelihood method. The proposed method utilizes the frequency response function (FRF) as input data and obtains modal parameters by iterative calculation. In this paper, initial values for the iteration are supposed to be obtained from the modal parameters estimated by the polyreference method. It turns out that modal parameters converge to the same value regardless of initial values. Therefore, the analyst can get rid of troublesome judgment on selecting physical roots in the polyreference method. The FRF constructed by the result of the proposed method agrees better with the experimental FRF than that by the polyreference method. Moreover, modal parameters estimated by the proposed method are more reliable from a statistical standpoint.

Vibration and wear in high speed rotating machinery

Abstract: I - Particle and Wear Debris Analysis.- Particles in Lubricating Oils - Sources and Consequences -.- Condition Monitoring of Machine Elements: Particle Separation and Investigation.- Condition Monitoring of Machine Elements - Measuring Methods for Particle Amount and Size.- Lubricants - Indicators for Oil System Damage and Malfunction.- Early Failure Detection by Spectrometric Methods.- Magnetic Methods of Condition Monitoring.- II - Erosion, Fretting Wear And Vibrations Fundamentals.- Cavitation Noise in Centrifugal Pumps.- Load Flow Phenomena and Excitation Forces in Centrifugal Pumps.- Estimation of Erosion Rates and Implosion Pressures in Cavitating Pumps and Cavitation Test Device.- Vibrations and Fretting Wear.- Wear Testing of Materials for Machines.- Wear Analysis of Vanes Pumps Running with Thickened High Water Based Fluids.- Machinery Vibration Measurements and Analysis.- Critical Speeds of Continuous Shaft-Disc Systems.- A Theoretical Study into the Effects of Damped Flexible Foundations on the Dynamic Behaviour of a 66 OMW LP Steam Turbine.- A General Method for Rotordynamic Analysis.- Vibration of Rotor-Bearing Assemblies.- Modal Testing Techniques.- Applications and Advances of Modal Testing.- Modal Testing of Machinery Components.- Solid Particle Erosion in Electric Power Plants.- III - Rotor/Bearing Behaviour.- On Bearing Deformation and Temperature Distribution in Dynamically-Loaded Engine Bearings.- The Control of Rotor Vibration Using Squeeze-Film Dampers.- Flow Efficiency and Excitation in Turbine Stages.- An Electroviscous Damper.- Dynamic Behaviour of Hydrostatic Radial Bearings.- Stability of Rotating Machines Running in Variable Stiffness Bearings.- Gas Bearing Design and Instabilities.- Identification of Stiffness, Damping and Inertia Coefficients of Annular Turbulent Seals.- Identification of Modal Parameters of an Elastic Rotor With Oil Film Bearings.- The Effect of Translational and Conical Bearing Misalignment on the Response and Stability of a Non-Linear Rotor Bearing System.- Stability of Flexible Rotor Supported on Journal Bearings.- Non Linear Effects in Lubricated Bearings.- Foundation Effects in Rotor Dynamic Behaviour.- A Method for Dynamics Calculations of Rotor-Bearing-Foundation Systems.- Dynamic Coefficients for Fluid Film Journal Bearings.- Destabilization of Rotors from Friction in Internal Joints.- IV - Expert Systems and Life Assessment Procedures.- A Survey of Dynamic Loading Conditions on Gear Teeth Contacts.- Remaining Life Evalutation for Steam Turbine Blades.- Remaining Life Evaluation for Steam Turbine Rotors.- A Measurement System for Computer Aided Monitoring of Bearings.- Ten Crucial Concepts Behind Trustworthy Fault Detection in Machine Condition Monitoring.- Some Requirements for Automated Data-Reduction Systems Suitable for Processing Vibration Measurements Taken on High-Speed Rotating Machinery for the Support of Operational and Maintenance Decisions.- An Intelligent Knowledge Based System for Fault Diagnosis in Turbomachinery.- The Use of Rotordynamics Simulation as an Aid for Fault Diagnosis in Turbomachinery.- "Fail-Safe" Concept and Reliability in High-Speed Bearing Arrangements for Aerospace Turbomachinery.- Machinery Diagnostics: Examples of Application to Thermoelectric Plants.- Analyse Fonctionnelle de la Microgeometrie d'un Contact E.H.D.. Critere d'Endommagement en Fatigue Superficielle.

Modal Testing Techniques

Abstract: This is the first of a set of three papers which describe the technology of modal testing and its application to vibration problems in rotating machinery. Modal testing embraces the processes involved in the construction of a mathematical model of a structure from test data and over the past 10–15 years, the technology has been developed and used extensively on structures of all types. Although there are particular problems associated with the vibration testing of structures which are actually rotating, these are now being addressed and overcome and there are many instances where the non-rotating components of machines must be investigated, or rotating components tested in a stationary condition, and these will be included in the following sections. This paper summarises the processes and techniques involved in modal testing.

Static load cycle testing of a low-aspect-ratio four-inch wall, TRG-type structure, TRG-5-4 (1. 0, 0. 56)

Abstract: This report is the second in a series of test reports that details the quasi-static cyclic testing of low height-to-length aspect ratio reinforced concrete structures The test structures were designed according to the recommendations of a technical review group for the US Nuclear Regulatory Commission sponsored Seismic Category I Structures Program The structure tested and reported here had 4-in-thick shear and end walls, and the elastic deformation was dominated by shear The background of the program and previous results are given for completeness Details of the geometry, material property tests, construction history, ultrasonic testing, and modal testing to find the undamaged dynamic characteristics of the structures are given Next, the static test procedure and results in terms of stiffness and load deformation behavior are given Finally, results are shown relative to other known results, and conclusions are presented 33 refs, 140 figs, 13 tabs

Structural modification and vibration reanalysis

Abstract: An exact reanalytical method of the normal modal parameters (the natural frequencies, normal modes, modal masses and stiffnesses) of the modified structure is presented. These parameters will be conveniently obtained by this method which only needs the normal modal parameters of the original (unmodified) structure and the modified magnitudes of the structure as input data. This method has been extended into substructure synthesis and reanalysis. It is not only suited for reanalysis of the complete modal models, but also for the incomplete modal models. The examples of numerical calculation are given to demonstrate the method to be effectual. The results of the examples show that the method is attractive.

Identification of the damping matrix from incomplete modal testing results

Abstract: In modeling structures the damping matrix is the most difficult to represent. This is even more difficult in complicated structures that are not lightly damped. The work presented here applies a method of modeling the damping matrix of a structure from incomplete experimental data combined with a reasonable representation of the mass and stiffness matrices developed by finite element methods and reduced by standard model reduction techniques. The approach taken here is to use the reduced mass and stiffness matrices and the experimentally obtained eigenvalues and eigenvectors. It uses linear or weighted least squares or a pseudo-inverse approach (depending on the number of the equations that are available) to solve for the damping matrix. The results are illustrated through several examples. As an indication of the accuracy of the method, fictitious examples where the damping matrix is originally known are considered. The proposed method identifies the exact viscous or hysteretic damping matrix by only using a partial set of the system's eigenvalues and eigenvectors. The damping matrix is assumed to be real, symmetric and positive semidefinite.

Static load cycle testing of a very low-aspect-ratio six-inch wall TRG-type structure TRG-6-6 (0. 27, 0. 50)

Abstract: This test report is the third for a series of tests carried out by the Los Alamos National Laboratory under the sponsorship of the United States Nuclear Regulatory Commission's Division of Engineering. This research program has a Technical Review Group that recommended test geometries and sizes for the tests. The quasi-static load cycle testing of a totally shear-dominated structure (bending deformation negligible) made of 6-inch-thick reinforced concrete walls is reported herein. The background of the program and the results that led to this series of experiments is first reviewed for continuity. Next, the geometry of the test structure, the design parameters, and the construction of the structure, including the material property tests, are reported. Both modal analysis and modal testing were done to verify the undamaged dynamic properties of the structure. Finally, the results of the quasi-static cyclic testing are reported in detail. Results are compared with other investigations and with the American Concrete Institute (ACI) 349-85 code predictions. 25 refs., 32 figs., 6 tabs.

Optimal extraction of structural characteristics from response measurements

Abstract: When random input data are not or cannot be measured, then only the available output data can be fitted to a time domain autoregressive moving-average (ARMA) model. This estimation process always produces a minimum phase system. This means that only the natural frequency and damping ratio of the system can be identified. The mode shape cannot be determined uniquely. A new approach has been introduced in this paper to overcome these problems when a structure is randomly excited. The selection of the sampling interval for estimating the modal parameters from a randomly excited structure subjected to unmeasurable inputs is also considered. The theoretical basis of the procedure is presented together with simulation results. I. Introduction E XPERIMENTAL modal analysis has become an increasingly important engineering tool during the past 40 years in the aerospace, automotive, and machine tool industries. Modal parameters estimates obtained from experimental modal analysis are being used in the direct solution of vibration and/or acoustic problems, for correlation with output from finite-element programs, and for prediction of changes in system dynamics due to structural changes. In all cases, the quality of the modal parameter estimates is of major concern. Time series methods have been applied to the synthesis of structural systems excited by random forcing functions as well as to the identification of the natural frequencies and the damping ratio. Autoregressive moving-average (ARMA) models have been used to estimate the characteristics of buildings being excited by wind force and the characteristics of the cutting process that participated with random cutting forces.1'2 Recently, there has been a great deal of interest in determining modal parameters from measured response data taken on operating systems (e.g., turbulent flow over an airfoil; road inputs to automobiles, and environmental inputs to proposed large space structures). When random input data are not or cannot be measured, then only the available output data can be fitted to a time domain ARMA model. This estimation process always produces a minimum-phase system. The transfer functions defined this way by the ARMA model are successful in the estimation of magnitudes of the true transfer functions but do not give the correct phase information3'4 except when the true system is minimum phase. In other words, the mode shape cannot be determined uniquely. The objective of this paper is to solve the aforementioned problem and estimate the modal parameters when the input force is an unmeasured white noise sequence. The selection of the sampling interval for estimating the modal parameters is also considered. Emphasis is placed on the optimum design of uniform data sampling intervals when experimental constraints allow only a limited number of discrete time measurements of the output from the continuous system and the parameters of interest are natural frequencies, damping ratio, and time constant of the continuous system.

Modal Testing of Machinery Components

Abstract: In this paper attention is given to the practicalities of conducting tests according to the procedures outlined in the previous paper, with special reference to the components and subassemblies from rotating machines. First considered are problems associated with providing the appropriate excitation and of correctly measuring the resulting transducer signals of force and response. The question of extending these methods to rotating components is briefly discussed here (but is dealt with in more detail in other papers). Next, the modal analysis methods used to extract modal parameters from the measured FRF data are examined with particular reference to the question of their reliability in the presence of typical experimental errors in the data and of the possibility of non-linearity in the actual test structure. The questions of residual effects and of complex modes are discussed before considering the final stage of modelling and the difficulties which may be encountered here in practice.

Experimental identification of modal identity-related parameters of a spacecraft representative structure

Abstract: Modal momentum coefficients and modal identity parameters can, in principle, be identified experimentally using a procedure that entails obtaining estimates of modal momentum coefficients x scale factor for each mode from driven-base test data, then obtaining modal scale factors from tests with a portable exciter, and, finally, calculating the modal momentum coefficients from the various estimates. The estimated modal momenta can further be used with modal identities to assess the relative importance of modes and the completeness of the set of identified modes. The results of laboratory tests aimed at assessing the practicality of the procedure in the case of a complex structure are presented. The test structure, called FLECS, is representativ e of a typical modular spacecraft in terms of mass properties and modal density. The results demonstrate that the procedure successfully identifies the modal parameters.