Analysis of swept sine-runs during vibration tests of large aircraft

Kurzfassung

Ground vibration tests (GVTs) in Europe nowadays combine the benefits of the very reliable but time consuming phase resonance method and the application of phase separation techniques evaluating frequency response functions (FRF). FRFs of a test structure can be determined by a variety of means. Applied excitation signal waveforms include harmonic signals like stepped-sine excitation, periodic signals like multi-sine excitation, and transient signals like impulse, random, or swept-sine excitation. The current paper focuses on slow swept-sine excitation which is a good trade-off between magnitude of excitation level needed for large aircraft and short testing time. {em harmonic} stepped-sine excitation provides of course the highest possible spectral excitation level for a given shaker and excitation point. But a very long testing time is necessary in this case to achieve a good frequency resolution. In contrast, {em non-harmonic}waveforms are fast testing techniques with a good frequency resolution. Swept-sine excitation provides the highest level of all non-harmonic waveforms. However, recent ground vibration tests brought up that reliable modal data from swept-sine tests runs depend on a proper data processing. The paper elucidates the strategy of modal analysis based on swept-sine excitation. The standards for the application of slowly swept sinusoids defined by the international organisation for standardisation in ISO 7626 part 2 are critically reviewed. The theoretical background of swept-sine testing is expounded with particular emphasis to the transition through structural resonances. The effect of different standard procedures of data processing like tracking filter, FFT, and data reduction via averaging are investigated with respect to their influence on the FRFs and modal parameters. Particular emphasis is given to FRF distortions evoked by unsuitable data processing. All data processing methods are applied on numerical examples in order to investigate and demonstrate their capabilities. Their practical usefulness is demonstrated on test data taken from recent GVTs on large aircraft structures. The revision of ISO 7626 part 2 is suggested regarding the application of slow swept-sine excitation. Recommendations about the proper estimation of frequency response functions from slowly swept-sine excitation are given in order to enable the optimisation on swept-sine application for future ground vibration tests.