Figures 4-6 show that there is no difference between the natural frequency and damping ratio identified with CONCERTO when this is applied to FRF or transmissibility data. CONCLUSION Identification of structural nonlinearities in dynamics is becoming a pressing issue. The reliability of models and their accuracy can only improve if nonlinearities are accounted for. Different methods which aim at extracting the nonlinear stiffness and damping have been proposed which rely on the Frequency Response Function of the system. However, there are practical cases in which the FRF cannot be measured and instead a base-excitation test needs to be conducted (e.g. airworthiness certification of some aerospace items). It remains the question if an established FRF method, implemented in CONCERTO, can be extended and used for transmissibility measurements. The numerical study conducted in this paper shows that the algorithm produces the same results whether it is fed with absolute transmissibility or FRF data. It is important to highlight that for a better physical interpretation of the results, these needs to be plotted as function of the effective displacement of the spring/damper (i.e. the elements supporting the mass or the mount): this displacement is the absolute displacement of the mass in case of force excitation and the relative displacement between the mass and the base when the system is excited at the base. Albeit of important practical value, more work is needed to improve the identification method. Firstly, there is need to assess the validity of the SDOF assumption upon which is based. A different shortfall to tackle is the assumption of amplitude-dependent nonlinearities. Most notably, transmissibility tests are required for vibration isolation devices such as rubber mounts: these present not only amplitude-dependent stiffness but also a marked frequencydependency which need to be extracted. ACKNOWLEDGMENT This research was conducted at the AgustaWestland Bristol University Technology Centre (AWB-UTC) under the programme “Nonlinear Modal Testing” funded by AgustaWestland Helicopters Company. Also, the authors thank Trevor Walton, AgustaWestland, for his support and fruitful discussions on the topic. 489
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