Showing papers on "Modal testing published in 1980"

Differential and Difference Sensitivities of Natural Frequencies and Mode Shapes of Mechanical Structures

Abstract: A method, based upon the theory of the adjoint structures, is formulated for calculating the derivatives of natural frequencies and normalized mode shapes with respect to structural parameter changes in terms of local mass, stiffness, or damping, starting with data obtained by experimental processing techniques such as modal analysis. The method applies for statically or kinematically undeterminate structures, which is not the case for most classical methods of sensitivity analysis. The method is extended to obtain large-change sensitivities and frequency response sensitivities to structural nonparameter changes (e.g., the addition of a damped vibration absorber). Two examples demonstrate the procedure and the usefulness of the sensitivity analysis. HE dynamic analysis of complex mechanical equipment and the prediction of the dynamic behavior of a modified mechanical structure has turned out to be a difficult way of reaching the objectives of such analysis. However, modal analysis techniques and computer-interfaced testing equip- ment have contributed to a solution of those problems. Furthermore, modal analysis results (complex modal displacements, natural frequencies, damping values) may be used in system synthesis methods1>2 to predict mathematically the effect of structural changes on the dynamic behavior. The objective of this paper is to present a * 'sensitivity analysis" using experimental data, obtained by modal analysis, for computational assessment of the most effective parameter change in order to obtain a desired dynamic behavior. The sensitivities give the influence of the building element parameters on the natural frequencies and mode shapes of the mechanical structures. They provide us with an answer to the question of where to change, e.g., to obtain a maximum shift of a specific natural frequency or to reduce most effectively the modal displacements in certain points for a specific mode. The assumption of linear and statically or kinematically determinate structures simplifies the calculation of the sen- sitivities.3"5 Van Belle6'7 developed a more general method, called the theory of adjoint structures, for the sensitivity analysis of mechanical structures, yielding equations still valuable for statically or kinematically undeterminate structures. In this paper, the method based upon the theory of adjoint structures is extended to obtain sensitivities in the case of viscously damped systems and expressions for the sensitivities are derived, using finite instead of infinitesimal changes.

Improvement of analytical dynamic models using modal test data

Abstract: A method developed to determine maximum changes in analytical mass and stiffness matrices to make them consistent with a set of measured normal modes and natural frequencies is presented. The corrected model will be an improved base for studies of physical changes, boundary condition changes, and for prediction of forced responses. The method features efficient procedures not requiring solutions of the eigenvalue problem, and the ability to have more degrees of freedom than the test data. In addition, modal displacements are obtained for all analytical degrees of freedom, and the frequency dependence of the coordinate transformations is properly treated.

Active damping of modal vibrations by force apportioning. [for spacecraft structures

Abstract: Force apportioning, a method of active structural damping based on that used in modal vibration testing of isolating modes by multiple shaker excitation, was analyzed and numerically simulated. A distribution of as few forces as possible on the structure is chosen so as to maximally affect selected vibration modes while minimally exciting all other modes. The accuracy of numerical simulations of active damping, active damping of higher-frequency modes, and studies of imperfection sensitivity are discussed. The computer programs developed are described and possible refinements of the research are examined.

Active damping of modal vibrations by force apportioning

Abstract: Force apportioning, a method of active structural damping based on that used in modal vibration testing of isolating modes by multiple shaker excitation, was analyzed and numerically simulated. A distribution of as few forces as possible on the structure is chosen so as to maximally affect selected vibration modes while minimally exciting all other modes. The accuracy of numerical simulations of active damping, active damping of higher-frequency modes, and studies of imperfection sensitivity are discussed. The computer programs developed are described and possible refinements of the research are examined.

Marine riser vibration response determined by modal analysis

Abstract: Although marine-riser response calculations are usually based on finite-element or finite-difference methods, Maurer Engineering Inc. and the University of Texas outline a modal analysis method as an alternative approach to calculating marine-riser time-dependent stresses. Instead of representing the derivatives by finite differences in setting up a matrix of algebraic equations, the solution is expressed directly in terms of natural modes of vibration. These natural vibration modes and corresponding natural periods can be determined either numerically or from closed-form solutions. An example problem shows that five natural vibration modes give acceptable convergence and engineering accuracy. Dynamic-response calculations are, therefore, greatly simplified because only a limited number of eigenvalues and eigenfunctions are needed. In addition, the time-dependent part of the solution can be determined from elementary single-degree-of-freedom-type equations.

A study on higher mode vibration of ships

Abstract: An experimental study has been performed in order to confirm the validity of the modal synthesis technique derived in the previous paper (1st report). The coupled vibration of the vertical vibration of main hull girder and the longitudinal vibration of superstructure located on aftership has been tested using the aluminum model ship of seven meter length, and the results are compared with theoretical calculation by the modal synthesis technique mentioned above. Results of the calculation show good agreement with those of the experiment. (Author)

Modal Analysis for Aircraft Response to Runway Surface Roughness.

Abstract: : This report develops one and three degree-of-freedom linear vibration models for the prediction of aircraft response to runway surface roughness. The equations of motion are integrated in principal coordinates using modal analysis. The modal parameters required are natural frequency, damping ratio, and mode shape for each degree of freedom. Comparison of results is made with the TAXI code that has a nonlinear strut model. Results are presented for asymmetric motion due to spall profiles in the runway. (Author)