Showing papers on "Modal testing published in 1975"

Estimating Damping of Real Structures

Abstract: Described in this paper is a method to obtain equivalent modal viscous damping ratios from stiffness and energy dissipation properties of structural components. Specifically, modal viscous damping ratios are computed as the weighted sum of component energy ratios (ratio of energy dissipated to peak potential energy per cycle of motion) weighted by the ratios of component to total peak potential energies. Component energy ratios may be estimated from vibration studies of the individual components or similar structures made exclusively of the component materials. Component potential energies may be computed from component stiffness matrices and modal deflections. A great simplification is that total peak potential energy may be computed as half the natural circular frequency squared for mode shapes normalized such that the generalized mass equals unity. Modal viscous damping ratios are computed for four example structures all having reinforced concrete frames with gypsum wallboard partitions. Damping ratios predicted and measured vary from 2% to 5%.

Modal Analysis of Cable Networks

Abstract: The modal response method applied to the linear and nonlinear dynamic analysis of cable networks. Three sample networks are analyzed and comparisions with the solutions obtained by the dynamic form of the displacement method of structural analysis are made. It is found that for many cable networks the modal response method is a satisfactory analysis procedure; however, there are structures for which its use will not lead to any efficient use of computer time.

Substructure Energy Method for Prediction of Space Shuttle Modal Damping

Abstract: The results of this program demonstrate the validity of a dissipative energy approach for predicting the damping of a four-component Space Shuttle model by means of modal parameters obtained from tests of the individual components. A relationship between modal damping energy per cycle and peak strain (or kinetic) energy is first determined empirically from test data for each component. Undamped analytical models of each component are also developed, and combined into a system model from which are obtained modal kinetic (or strain) energies for its respective modes. These data are then used with the empirical damping curves to apportion the proper amount of damping energy to each component in a combined system mode, and thereby allow a prediction of damping ratio.

An automatic data system for vibration modal tuning and evaluation

Abstract: A digitally based automatic modal tuning and analysis system developed to provide an operational capability beginning at 0.1 hertz is described. The elements of the system, which provides unique control features, maximum operator visibility, and rapid data reduction and documentation, are briefly described; and the operational flow is discussed to illustrate the full range of capabilities and the flexibility of application. The successful application of the system to a modal survey of the Skylab payload is described. Information about the Skylab test article, coincident-quadrature analysis of modal response data, orthogonality, and damping calculations is included in the appendixes. Recommendations for future application of the system are also made.