A Method for local Damping Identification

Kurzfassung

The increase of computer power enables the analysis of detailed effects using refined numerical models based on the Finite Element Method. But even though refined models for stiffness and inertia properties can be obtained, damping modelling is mostly disregarded or included on a modal basis which does not reflect the governing mechanisms for damping or their location in a structure. For accurate response prediction of lightweight structures, especially when they are exposed to broadband excitation, proper damping modelling is required. More accurate response predictions will improve the accuracy of dynamic loads analysis which is the baseline for detailed structural design and sizing of structural components. The work presented here is intended to support damping modelling as it is required for accurate response analysis or dynamic loads analysis. It addresses the identification of local damping sources and their relative contribution to the global damping of an assembled structure. As input, measured dynamic responses are required, e.g. in the form of frequency response functions. As output, a map of damping distribution over the structure is obtained. These results uncover areas of interest at which damping could be introduced into the structural model, for instance in terms of local physical damper elements. The approach for local damping identification is inspired by methods used in acoustics which analyze the reverberation time in an acoustic cavity. The method presented here uses a hybrid time-frequency domain processing for the estimation of structural damping and spatial distribution of damping. The new analysis strategy allows for estimation of damping over a wide frequency range. At low frequencies, where the modal density is low, the identified damping matches the values obtained with the classical modal analysis tools. At high frequencies, where modal analysis is no longer possible due to high modal density, the presented method is able to estimate equivalent damping values for pre-defined frequency bands. Thus, a variation in damping with frequency can be provided based on experimental observations even in the higher frequency range typically not covered by experimental modal analysis. It is also possible to use these damping ratios for improved accuracy of numerical simulations compared to other approaches such as structural damping or Rayleigh damping. Numerical examples are presented in order to compare and validate the results obtained. Finally, experimental data from a fuselage-like structure is presented.