Showing papers on "Modal testing published in 1977"

Errors in response calculations for non‐classically damped structures

Abstract: The classical normal mode method of determining response is extremely useful for practical calculations, but depends upon the damping matrix being orthogonal with respect to the modal vectors. Approximations that allow the method to be used when this condition is not satisfied have been suggested; the simplest approach is to neglect off-diagonal terms in the triple matrix product formed from the damping and modal matrices. In this paper the errors in response caused by this approximation are determined for several simple structures for a wide range of damping parameters and different types of excitation. Based on these results a criterion, relating modal damping and natural frequencies, is formulated; if this is satisfied, the errors in response caused by this diagonalization procedure are within acceptable limits.

Impulse technique for structural frequency response testing

Abstract: Structural frequency-response testing, also known as "modal analysis", is becoming an integral part of the development and testing of a wide range of industrial and consumer products. It is an essential tool for the definition and solution of many types of structural dynamics problems, such as fatigue, vibration, and noise. The Authors discuss one of the most useful techniques for experimental frequency-response testing, based upon excitation of the structure with an impulsive force. Although simple and fast, the nature of the excitation and response signals in the impulse process requires special signal-processing techniques if accurate frequency-response measurements are to be obtained. The application of the impulse technique is discussed and the special problems encountered and methods of dealing with them are reviewed.

Applications of digital modal analysis using transient excitation

Abstract: A method is described for investigating dynamic characteristics of structures and components by digital analysis of their response to transient excitation. The advantages and limitations of the method are briefly discussed. Two applications of the technique are described. In one application the resulting mode shapes are compared with those obtained by double-pulse holography. Difficulties involved in handling the large amounts of data generated and in displaying the mode shapes are discussed. The method seems to have wide application to small structures or components where resonance is thought to be the source of a noise or vibration problem. Order from: BSRA as No. 46,876.

Modal analysis of the NRC pressure suppression experimental facility

Abstract: The 1/5th scale model Mark I pressure suppression facility was experimentally analyzed in order to determine its fundamental modes of vibration. The results of the modal analysis revealed seven apparent modes with frequencies below 100 Hz. In this report each mode is characterized by a description of the motion, the natural frequency, and the response amplitude. The results indicate that the response of the torus to an impulsive load in the vertical direction is dominated by two modes at 12.2 Hz and 59.8 Hz.

A method for experimental modal separation

Abstract: A method is described for the numerical simulation of multiple-shaker modal survey testing using simulated experimental data to optimize the shaker force-amplitude distribution for the purpose of isolating individual modes of vibration. Inertia, damping, stiffness, and model data are stored on magnetic disks, available by direct access to the interactive FORTRAN programs which perform all computations required by this relative force amplitude distribution method.