Detection of Non-Linearities in Swept-Sine Measurements

Kurzfassung

There is a constant urge from the European aircraft industry to reduce testing time of prototypes without loss of accuracy in the data. As a consequence, substantial changes concerning the testing strategy for modal survey tests, called ground vibration tests (GVTs) in connection to aircrafts, were proposed and applied during the last couple of years. Basic idea is to combine the benefits of the very reliable but time consuming phase resonance method (sine dwell) and the use of phase separation techniques on data stemming from swept-sine excitation. The idea of the current paper is to extend this concept introducing a combined approach also for linearity checks. Plotting modal parameters like the eigenfrequency as a function of the excitation level is the way to explore non-linear phenomena in GVTs for time being. Such plots are also called {em impedance plots}. Traditionally, these are produced using the phase resonance method by variation of the level of excitation energy, i.e. by harmonically exciting one single normal mode. The contemporary GVT strategy includes extensive broad-band excitation at various load introduction points. The authors recommend the use of swept-sine excitation which provide a high excitation level compared to random excitation and short testing time compared to stepped-sine excitation. The modes of the structure are identified from the resulting frequency response functions by means of phase separation techniques in parallel to the on-going measurement. The goal to extract as much information as possible from these data can be complemented by computing impedance plots for modes which have been identified at various excitation levels during the swept-sine testing depending on the excitation point and the exciter force . The paper explains in detail how the excitation level can be determined in case of swept-sine excitation. It gives examples of impedance plots from numerical simulations and complex large aerospace structures. The impedance plots are compared with curves stemming from phase resonance techniques. The achieved results are very encouraging.