ISBN 9783843951876

978-3-8439-5187-6, Reihe Luftfahrt

Maren Scheel
Experimental Nonlinear Modal Analysis - Method development with particular focus on nonlinear damping

143 Seiten, Dissertation Universität Stuttgart (2022), Softcover, A5

Zusammenfassung / Abstract

The concept of nonlinear modes is useful for the dynamical characterization of nonlinear mechanical systems. The purpose of this thesis is to develop a method for Experimental Nonlinear Modal Analysis that is suited for non-conservative nonlinearities. The developed method includes two steps: First, a nonlinear mode according to the Extended Periodic Motion Concept is isolated in an experiment by appropriate forcing. Then, modal frequency, damping ratio and deflection shape including higher harmonic components are extracted from the measured data as function of the vibration level. Two different excitation schemes for applying the excitation at a single location are developed in this thesis, namely phase-resonant forcing and positive velocity feedback.

The robustness of the proposed excitation schemes and accuracy of the extracted modal properties are first assessed by means of simulated experiments. The proposed method is then validated experimentally by a series of test rigs. The experimental results confirm the hypothesis that single-point excitation is sufficient to track the modal properties even in the presence of large frequency shifts, high and nonlinear damping, and changes of the mode shape.

Using a single nonlinear-modal oscillator with the identified modal properties, near-resonant frequency responses are predicted. These predictions agree well with measured reference curves, indicating the modal properties' relevance and accuracy. The central limitation of the identified modal quantities is that they only characterize the system in the regime near isolated resonances and for (essentially) sinusoidal excitation.

Finally, the proposed method is directly compared to an existing nonlinear system identification method that yields data-driven models. This comparison highlights that the non-parametric character of the modal approach is a strength when predicting dynamic, near-resonant vibrations under harmonic excitation.