Influence and characterisation of weak non-linearities in swept-sine modal testing

Abstract

The European aircraft industry is constantly calling for a reduction in the testing time of new aircraft without diminishing the accuracy of the data. As a consequence, substantial changes in the testing strategy of ground vibration tests (GVTs), were proposed and applied during the last couple of years (Fargette et al., Tasks for improvements in ground vibration testing of large aircraft, in: Proc. of IFASD, CASA/AIAE, 2001, pp. 121–133; G. Gloth, et al., Sound and Vibration 35 (11) (2001) 14–18). The basic idea is to combine the benefits of the very reliable but time-consuming phase resonance method (sine dwell) with the application of phase separation techniques to data stemming from broad-band excitation at various excitation points. The authors recommend the use of swept-sine excitation which provides a high excitation level compared to random excitation and a short testing time compared to stepped-sine excitation. The modes of the structure are identified from the resulting frequency response functions (FRFs) by means of phase separation techniques in parallel to the on-going measurement. In the practice of modal testing it is often observed that structures do not behave in a perfectly linear manner. This article investigates the influence of non-linear structural dynamics behaviour on the measurement results. An expansion of the test concept of swept-sine excitation is proposed which introduces a combined approach for linearity checks as well. The plotting of modal parameters like the eigenfrequency as a function of the excitation level is the manner in which non-linear phenomena in GVTs are presently explored. Traditionally these linearity functions are produced by harmonically exciting one single normal mode on different levels of excitation. The goal to extract as much information as possible from the FRF data can be complemented by computing plots of amplitude-dependent modal parameters 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 article expounds how the excitation level can be determined in the case of swept-sine excitation. It gives examples of linearity plots from numerical simulations and complex, large aerospace structures. The plots are compared with curves that stem from the phase resonance testing.