Accuracy Requirements for Damping Estimation in the Calculation of Resonant Response of Complex Structures

Abstract

Great progress has been made in the mathematical modeling of the stiffness and mass properties of complex structures using lumped-and distributed-parameter representations, permitting accurate prediction of natural frequencies and mode shapes for lower- and middle-order modes. However, in calculating the dynamic response to various types of applied loading, it is common practice to assume that energy dissipation occurs as viscous, complex, or simple frequency-dependent damping rather than realistic spatially distributed material, friction, viscoelastic, radiation, or hydrodynamic damping. Because the assumed damping cannot be adequately compared with the realistic damping, conservatism is used and overdesign results despite a sophisticated analysis. On the other hand, great progress has been made in the engineering description of realistic damping mechanisms and their applications in certain cases for simple structures, such as simple oscillators, beams, plates, and cylindrical shells. These are briefly reviewed and suggestions made for application to more complex structures. Special techniques are recommended for highly nonlinear mechanisms: substantial amounts of friction damping, and material damping above the endurance limit. The sensitivity of these mechanisms to fabrication processes and to the number of applied cycles is described, and suggestions made for future studies. Damping inadequacies are compared with other problems in predicting the dynamic response, such as the representation of the applied loading, the determination of possible failure locations, and the characterization of the failure mechanism. The rôle of damping in stability analysis is also reviewed.