Showing papers on "Modal testing published in 1986"

A comparison of several methods for the calculation of vibration mode shape derivatives

Abstract: Four methods for the calculation of derivatives of vibration mode shapes (eigenvectors) with respect to design parameters are reviewed and compared. These methods (finite difference method, Nelson's method, modal method and a modified modal method) are implemented in a general-purpose commercial finite element program and applied to a cantilever beam and a stiffened cylinder with a cutout. A beam tip mass, a beam root height and specific dimensions of the cylinder model comprise the design variables. Data are presented showing the amount of central processor time used to compute the first four eigenvector derivatives for each example problem; errors and rapidity of convergence of the approximate derivative to the exact derivative are taken into account. Nelson's method proved to be most reliable and efficient.

Modal analysis for random vibration of hysteretic frames

Abstract: An algorithm is presented for the direct stiffness formulation of hysteretic frames which is suitable for random vibration analysis under zero mean or non-zero mean random excitation. A modal transformation is used to reduce the number of active degrees of freedom. First and second order response statistics are presented for two example frames using stochastic equivalent linearization and Monte Carlo simulation. The modal transformation stabilizes the stationary response calculation algorithm by eliminating superfluous higher modes. The iterations for transient response calculations under non-zero mean excitations are destabilized by elimination of too many modes. The modal transformation allows reasonable response estimates to be obtained with significant reductions in computing time. The expected lower modes dominate the response, but inclusion of several higher modes is necessary to provide reasonable response estimates.

Real-time flutter identification with close mode resolution

Abstract: Real-time flutter prediction including close modes can be effectively estimated from turbulence or on-board excitation with an Extended Kalman Filter (EKF) approach. A physically based model form enables prediction of the damping rate as well as damping, giving a time to instability estimate with its variance. The approach is recursive and can operate asynchronously to drop data outliers and hence is quite robust. Its speed is reasonable for on-line application but can also be used effectively as an off-line analysis tool for application to any modal testing situation.

Modal characteristics of a stiffened composite cylinder with open and closed end conditions

Abstract: The structural and acoustic modal frequencies and the loss factors of a composite fuselage model under free-free end and support-end conditions are evaluated. The fuselage model is a composite filament wound, cylindrical structure composed of carbon fibers embedded in an epoxy resin and has a ply orientation of +,-45, +,-32, 0, -,+32, and -,+45. A computer aided test system was utilized to obtain resonance frequencies, modal damping, and mode shape coefficients. Sound pressure and phase survey data are utilized to estimate acoustic modal freuencies and reverberation time was measured to calculate the acoustic loss factors. The derived structural and acoustic modal parameters are compared to analytical data from a computer prediction model. The data reveal that the acoustic modal frequencies and mode shape correlate well with predicted data, and there is fair agreement between structural modal frequencies and mode shape for the two data sets.

Measuring the Torsional Modal Frequencies of a 900 MW Turbogenerator

Abstract: Transient torques exerted on the rotors of turbine generators can produce high stresses. This paper describes an investigation into the torsional properties of the shaft of a 1500 rpm steam turbine generator rated 900 MW and includes 1. prediction, by theoretical model of the shaft and rotor structure, of its undamped natural vibrations and natural modes; 2. determination, by measurements and subsequent modal analysis, of the complex natural vibrations (free and damped) and natural modes. For the measurements the rotor was torqued by shock torques applied by the turning gear, as well as by harmonic sinusoidal torques of variable frequency applied by a shaker. Vibrations were measured by accelerometers on the shaft and on some selected turbine blades. Two modal analysis computer programs were used to process the signals. The results obtained by theoretical predictions and by measured modal analysis verify that the theoretical characterization of the rotor structure is indeed valid for calculating the values of the natural frequencies. However, the damping factors measured by the two modal programs differ significantly.

Coupling of the boundary element method and modal analysis for structural acoustic problems.

Abstract: The boundary element acoustic analysis is coupled with structural modal analysis for predicting sound fields generated by vibrating structures. The dynamic equation of the structure is represented by modal parameters obtained by structural analysis. The acoustic equation of the fields is described using the boundary element analysis of the Helmholtz integral equation. These two equations are solved simultaneously. Numerical results are shown to demonstrate the accuracy of our method in comparison with experimental ones. The sound transmission through a cavity-backed plate is studied as the case of an interior problem. As the exterior problem, sound radiation is examined from a plate excited by a hammer.

Studies of modal identification performance using hybrid data

Abstract: Modal identification accuracy can be affected significantly in practice by high modal density and nonlinearities. It may be difficult to determine the true number of modes, and many of the identified mode shapes may be questionable. Results can vary considerably using different analysis techniques. This paper introduces a new method for studying modal identification performance under such circumstances. A difficult set of test data from an erectable truss structure was modified numerically by adding two artificial modes with known parameters. These 'hybrid data' were then analyzed with the Polyreference technique and the Eigensystem Realization Algorithm (ERA). Identification performance is studied by comparing the identified modal parameters for the artificial modes with their known values.

Structureborne noise in aircraft: Modal tests

Abstract: As part of an investigation to develop measurement techniques for structureborne noise, three modal surveys have been conducted on an OV-10A aircraft and the results have been presented The purpose of the modal surveys was to identify suitable locations for mounting accelerometer and strain gages in subsequent tests in which transfer functions relating wing vibration to interior noise were to be determined These surveys are as follows:(1) wing/fuselage modal survey utilizing one shaker under the right wing; (2) complete wing modal survey utilizing two shakers, one under each wing; and (3) fuselage side panel modal survey utilizing a small instrumented hammer The predominant frequencies and damping ratios for each analysis were listed in tables The primary mode shapes at the lower frequencies and at frequencies near the expected engine driving frequencies have been shown for each survey

動吸振器による構造物の振動制御 : 第1報,理論的方法

Abstract: In order to effectively suppress a number of resonance peaks occurring in a machine structure, this paper proposes a new vibration controlling method which combines the modal analysis using the finite element method with a design method of dynamic absorbers enlarged to control the vibration of multi-degree-of-freedom systems. This method is essential in order to determine the optimum mounting position of the dynamic absorbers and the equivalent masses in these positions with the aid of modal analysis. If the equivalent masses are obtained, the established design method can be applied to the design of each dynamic absorber. The effectiveness of the method is demonstrated by suppressing the first five resonance peaks of a rectangular plate using the four optimal designed dynamic absorbers.

Fatigue Lifetime Estimation of Structures Subjected to Dynamic Loading

Abstract: Because fatigue is a typical dynamic phenomenon, a procedure is developed to estimate the lifetime of a structure, based on its dynamic characteristics. By means of modal analysis, a structure can be described by its modal parameters: mode shapes, natural frequencies, and damping values. Using a finite-element model, the modal stress-strain distribution of part of or an entire structure can be calculated. When the applied dynamic forces are known, combination of the modal strains yields the strain response of the structure. A stress-strain simulation controlled by Neuber's rule accounts for stress concentrations. A counting algorithm based on closed hysteresis loops divides the stress-strain history into single reversals. An appropriate damage rule is used to find the damage of one loading sequence. Finally, the lifetime is found from the number of loading sequences that can be applied to satisfy the failure criterion.

Adaptation of Modal Testing Procedure to Shaker Tables

Abstract: This article presents a technique for adapting current modal testing procedures to shaker table tests commonly used for qualification tests of nuclear and space related equipment. It involves a modification of the measured frequency response function of the total acceleration, in order to make the assumptions of the modal testing systems applicable to this type of tests. Experimental results on a simple structure are given and demonstrate the simplicity and applicability of this method.

Step relaxation method for modal test implemented with frequency-domain preprocessing

Abstract: The theory and practical aspects of an implementation of the step relaxation method of modal test that in- cludes preprocessing of data in the frequency domain is presented The method is demonstrated using a test of a continuous longeron space mast that has several low-frequency closely spaced modes N the step relaxation method of modal test of structures, the input excitation is achieved by applying a single force to deform the structure statically and then releasing the force suddenly Data acquired are the force history (for some im- plementations), and the ensuing structural response at a number of points measured before, during, and after release as the structure vibrates freely and damps to a motionless state Data may also be obtained from successive reapplica- tions of the force at the same and different locations and/or directions The modal parameters are extracted from the input and output data using a parameter identification strategy Several strategies for parameter identification with various time- and frequency-domain techniques are possible1"4 This paper concentrates on the category of implementatio n where the structure's frequency-response function is first con- structed in the frequency domain from the measured data, then inverted to the unit impulse response form in the time do- main, and then the modal parameters are identified by fitting an analytic form of the unit impulse response function to the corresponding measurement-based function using the complex exponentials method This type of implementatio n of step relaxation is of in- terest because it is compatible with commercially available spectral analysis equipment, modal analysis software, and well-known spectral analysis data preprocessing techniques (prescreening of data at the Fourier transform level, frequency-domain averaging, etc) Reference 4 reports on this implementation at an earlier stage of development and understanding The mathematical relationships, variables, and the fre- quency-response function are first established Then several procedures for construction of the frequency-response func- tion are discussed and demonstrated using test data from a lightweight flexible structure that is representative of the type for which step relaxation is well suited The use of the polyreference implementation of the complex exponentials method of the Structural Dynamics Research Corporation's Modal-Plus software5 is also illustrated with the represen- tative data

Modified US Army U-8F ground vibration test

Abstract: The Dryden Flight Research Facility of NASA Ames Research Center conducted a ground vibration test on a modified U.S. Army U-8F airplane. Modifications included new engines, propellers, and engine-mounted truss assemblies. The ground vibration test was conducted using sine dwell, single-point random, and impact excitations. The test was performed to determine modal frequencies, mode shapes, and structural damping coefficients of the airframe and propeller with full and empty fuel tanks. The data presented include frequency response plots, rigid-body and structural modal frequencies, and mode shapes.