Non-linear phenomena in ground vibration testing of large aircraft

Kurzfassung

Ground vibration tests (GVTs) on aircraft prototypes are mainly performed to identify experimentally the structural dynamic behaviour in terms of a modal model. This assumes linear dynamic behaviour of the structure. However, it is often observed in practice of ground vibration testing that structures do not behave in a perfectly linear manner. Non-linearities can be determined for example by free play junctions, hydraulic systems in control surfaces, or friction. The paper compiles measured typical non-linear phenomena from various GVTs on large aircraft. The standard procedure in GVTs is nowadays the application of the Harmonic Balance method which linearizes the dynamic behaviour in the level of excitation resulting in the dependendcy of modal parameters on the excitation level. This makes the experimental data compatible to all procedures of post-processing of the measured results, e.g. updating of the finite element model or flutter calculations. The procedure of the Harmonic Balance requires the harmonic excitation of the structure which is usually performed during phase resonance testing. The non-linear behaviour is investigated in terms of linearity plots in which the eigenfrequency of a mode is plotted as a function of the excitation level. This level can be measured by various means. Common measures are the amplitude of the structure at a selected point, in most cases the maximum displacement, and the input force. The paper shows measured linearity plots for some typical non-linear phenomena. The second part of the paper analytical linearity plots are investigated in order to characterise and identify the type of non-linearity from measured linearity plots. The analytical linearity plots are created using a single-degree-of-freedom (SDOF)system with different models of non-linearities. The analytical linearity plots are discussed and an assessment of the linearity plots with respect to the application boundaries is given. The analytical linearity plots are used to interpret the experimental linearity plots stemming from various GVTs on different aircraft prototypes. The observability of non-linear stiffness and non-linear damping characteristics is finally assessed.