Showing papers on "Modal testing published in 1983"

Dynamic Modeling of Structures from Measured Complex Modes

Abstract: A technique is presented to use a set of identified complex modes together with an analytical mathematical model of a structure under test to compute improved mass, stiffness and damping matrices. A set of identified normal modes, computed from the measured complex modes, is used in the mass orthogonality equation to compute an improved mass matrix. This eliminates possible errors that may result from using approximated complex modes as normal modes. The improved mass matrix, the measured complex modes and the higher analytical modes are then used to compute the improved stiffness and damping matrices. The number of degrees-of-freedom of the improved model is limited to equal the number of elements in the measured modal vectors. A simulated experiment shows considerable improvements, in the system's analytical dynamic model, over the frequency range of the given measured modal information.

Experimental Modal Analysis, Structural Modifications and FEM Analysis on a Desktop Computer

Abstract: This article discusses two popular parts of modern day structural dynamics technology; the experimental portion which is referred to as experimental modal analysis or modal testing, and the analytical portion, which is referred to as Finite Element Analysis (FEA) or Finite Element Modeling (FEM). It discusses how experimental and analytical methods are used to solve noise and vibration problems and the importance of using modal parameters to link testing and analysis. Finally, it shows how structural modification techniques are used as a complement to both methods and how all of the tools may be combined on an inexpensive desktop computer. The article concludes with an example showing how experimental modal analysis, structural dynamics modification and finite element analysis were used to analyze the dynamic properties of a test structure. Most noise, vibration or failure problems in mechanical structures and systems are caused by excessive dynamic behavior. This behavior results from complex interactions between applied forces and the mass-elastic properties of the structure. Currently, many companies are actively using Finite Element Modeling (FEM) techniques for structural dynamic analysis. In recent years, however, the implementation of the Fast Fourier Transform (FFT) in low cost computer-based signal analyzers has provided the environmental testing laboratory with a fast and more powerful tool for acquisition and analysis of vibration data. As a result, more and more companies are beginning to use dynamic testing to complement their FEM structural analysis activities. It is this interaction between the experimentalist and the analytical engineer that is so important. Both are able to communicate and reinforce one another to solve troublesome noise and vibration problems that are certain to arise in the life cycle of a product. It is the intent of this article to report on how virtually any design or testing engineer can take advantage of recent advances in computer technology and software developments which allow him to easily use a system capable of offering advanced experimental methods as well as analytical methods in the same basic framework. Before doing so however, it is worthwhile to review some historical facts about FEM and experimental modal analysis, and the associated problems that still confront most of us today.

Modal identification of a rotating-blade system

Abstract: A new testing technique and the Ibrahim time-domain (ITD) modal identification algorithm have been combined, resulting in a capability to estimate modal parameters for rotating-blade systems. This capability has been evaluated on the Sandia two-meter, vertical-axis wind turbine. Variation in modal frequencies as a function of rotation speed has been experimentally determined from 0 rpm (parked) to 800 rpm. Excitation of the rotating turbine was provided by a scheme which suddenly released a pretensioned cable, thus plucking the turbine as it rotated. The structural response was obtained by passing the signals through slip rings. Using the measured free-decay responses as input data for the ITD algorithm, the modes of the rotating turbine were determined at seven rotation speeds. The measured modal parameters were compared with analytical results obtained from a finite element analysis and with experimental results obtained from a complex exponential identification algorithm.

Modal test and analysis correlation

Abstract: Modal testing plays a significant role in experimentally measuring the modal parameters used directly for response analysis of spacecraft or indirectly to update a mathematical model for use in response analysis. Developments in modal testing and correlation of results to analytical models are discussed. Several procedures to semi-automate correlation of resonant frequency and mode shape test data to the model have been shown to be valid for hypothetical problems but have not been successful in practice. Improving the correlation of frequencies and mode shapes does not necessarily improve the correlation of modal force coefficients. The paper discusses other parameters, such as kinetic energy and strain energy, which may be more significant in the correlation process. Because the most demanding requirement of a modal test is in determining dynamic loads, the authors asssume that objective, minimally discussing theoretical developments, summarizing their results, and giving sample data.

Nonlinear effect on modal data analysis method

Abstract: How the presence of nonlinearities in structural test data can be detected when using modern linear modal data analysis methods is discussed. The extent to which linear algorithms can provide useful information on nonlinear systems was discussed. The modal data analysis method successfully identified the frequency components (but not damping) of the true solution of a nonlinear systems. It was found that the approximate modal parameters of the linear system can be identified by the modal method when low levels of nonlinearities are present. The modal method successfully identified the modal parameters of a linear mode in the presence of a nonlinear response.

Fast modal response method for structures

Abstract: On recommande une analyse de la reponse d'une structure qui utilise a la fois les modes naturels et les matrices de masse et de rigidite du systeme pour ameliorer la convergence

Generalized modal shock spectra within indeterminate interface

Abstract: It is noted that the generalized modal shock spectra method had limitations on the degree of structural static determinacy of the spacecraft-to-launch-vehicle interface. The 'interface modes' is used here to remove these practical limitations. The governing differential equations are derived and then shown to be valid either for integrating the modal models of two or more substructures that have been previously obtained separately or for removing a substructure from a previously available system modal model. Emphasis is placed on the integration problems in the context of the generalized modal shock spectra approach. A minimum of information the modal response of a previously obtained transient analysis of the unloaded launch vehicle is exploited in order to define an idealized modal impulse function and derive approximate explicit expressions for an estimate of the bounds of the spacecraft response. In addition, a numerical model is solved in order to compare the present results with the transient solution.

A NASTRAN DMAP procedure for calculation of base excitation modal participation factors

Abstract: This paper presents a technique for calculating the modal participation factors for base excitation problems using a DMAP alter to the NASTRAN real eigenvalue analysis Rigid Format. The DMAP program automates the generation of the seismic mass to add to the degrees of freedom representing the shaker input directions and calculates the modal participation factors. These are shown in the paper to be a good measure of the maximum acceleration expected at any point on the structure when the subsequent frequency response analysis is run.