Non Linear Vibration Control Of Long Flexible Structures Employing Inter Modal Energy Transfer Modal Damping

Author : Giovanni Ferrari
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (979 download)

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Download or read book Experimental Investigation of Vibration Damping in Linear and Nonlinear Vibration written by Giovanni Ferrari and published by . This book was released on 2017 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "The main tool for the dynamic analysis of continuous structures is today modal analysis, which applies to lightly dampened structures undergoing small amplitude vibrations. Vibration energy in real structures is always partially lost because of internal material dissipation, Coulomb friction, interaction with fluids and even in presence of passive and active dampers. This dissipation is integrated in modal analysis by the use of a modal damping ratio for each normal vibration mode. A linear viscous or equivalent damping model underlies the use of modal damping ratios and preserves mode superposition in the study. The use of modal damping in linear vibrations is also practically profitable, as real structures tend to show small damping values that vary with the natural modes. However, the estimation of modal damping ratios is impossible a priori, and experimental measurement is necessary. Modern design practices are imposing the use of thin lightweight structures, often in contact with fluids, which easily undergo large amplitude vibrations; stricter safety regulations, in turn, may require the estimate of vibration amplitudes closer to resonance. In these cases, modal parameters remain valid with very good approximation, freeing the industry from the adoption of more complex theories. Modal damping, however, cannot be extended to nonlinear large amplitude vibration: as soon as vibration amplitudes exceed the characteristic dimension of the system, the prediction of vibration amplitude fails altogether. Even under the assumption of stationary vibrations, experimentally determined damping values appear dependent not only on the normal mode of vibration, but also on the vibration amplitude. Provided the availability of suitable know-how, it was decided to undertake a wide experimental characterization of the trend of damping values of thin walled shells and plates during large amplitude forced vibrations, bridging the lack of relevant literature. In order to give general character to this experimental activity, several cases were taken into account: metallic materials, composites and rubbers, in presence and in absence of fluid-structure interaction. The increasingly common smart materials and smart structures, composites for which vibration damping is tuned according to the designer's wish, feature active or semi-active damping. In presence of fluids and active elements, damping can grow to large values or can instead destabilize the system by feeding, not removing, energy. The study of modal damping in case of active vibration amplitude mitigation was therefore included as well. Through this wide range of experiments, a better comprehension of nonlinear damping was sought empirically. Since no other means were available for geometrically nonlinear vibrations, modal damping was applied unchanged to match with experimental results the vibration amplitude predicted by nonlinear models at one specific resonance. As expected, the values had to be changed to reproduce the vibration amplitude given by different force levels of stepped-sine excitation. The dependence on excitation amplitude seemed to follow an auspicious trend across a wide range of cases. Furthermore, different expressions of damping in the nonlinear field were used with respect to the traditional viscous (modal) damping formulation. This is particularly suitable for specific materials (e.g. elastomers showing a viscoelastic behavior) or configurations. Relevant experimental results, however, were not more general than the ones obtained employing modal damping, which indicates the great complexity of the problem. Actually, some evidence seemed to suggest that the equivalent values of damping in the nonlinear field are also influenced by the energy transfers between vibration modes. While damping has been seldom or never investigated previously in case of internal resonances, an estimate of its values in this situation led to additional insight." --