56 S. B. Cooper et al. Fig. 5.22 3D and 2D Acceleration Surface Method results between 6.5 and 8.2 Hz at the pylon connection In this case, narrow band sine-sweep test was conducted around the first of mode (6.5–8.2 Hz) of the plane assembly at high level of excitation this mode was observed as the mode that activates the connection between the left pylon and the left wing of the plane. The acceleration surface was computed using acceleration data measured at the bottom surface of the wing and the pylon for the selected mode, the velocity and displacement vectors were obtained by integrating the acceleration vectors of the selected points. These measured points were selected to visualize the nonlinear behaviour caused by the connection. To visualise the form of elastic nonlinearities in this connection, a cross section along the axis of the zero velocity value of the acceleration surface plot in was plotted and presented on the second plot of Fig. 5.22. 5.6 Conclusion This paper has presented a case study on investigating the nonlinearities observed during the experimental campaign of a demo aircraft structure designed to understand the side effects of nonlinearities caused by bolted joints and multibody assemblies. The overall aim of the paper was to demonstrate the application of a selected number of techniques for experimental identification of the demo aircraft structure with nonlinear features incorporated in the design. The aim was achieved by three different types of experimental test, the type of test included Random excitation test which was used for the linear identification. The second test was based on sine-sweep and stepped sine excitation test, results obtained from this test were used to detect and ascertain the existence of nonlinearity in the measured time response envelop. The overall results obtained from this investigation has demonstrated the presence of a bilinear type of nonlinearity in the structure and it is therefore important to include such nonlinear phenomena in the finite element model of the structure. Starting from the validated linear Finite Element model, future activities will aim at finding simplified yet accurate ways of introducing the experimentally characterized nonlinear behavior in the aircraft and pylon models. References 1. Noël, J.P., Renson, L., Kerschen, G.: Complex dynamics of a nonlinear aerospace structure: experimental identification and modal interactions. J. Sound Vib. 333(12), 2588–2607 (2014) 2. Czaplewski, D.A., et al.: A soft landing waveform for actuation of a single pole single throw ohmic RF MEMs switch. J. Micromech. Sys. 15, 1586–1594 (2006) 3. Segalman, D.J., et al.: Handbook on dynamics of jointed structures, pp. 1–532. Sandia National Laboratories National Technical Information Service, Albuquerque, NM (2009) 4. T. Dossogne, et al.: Nonlinear ground vibration identification of an F-16 Aircraft—Part II understanding nonlinear behaviour in aerospace structures using sine-sweep testing. In: International Forum on Aeroelasticity and Structural Dynamics, Bristol (2015) 5. Fuellekrug, U., Goege, D.: Identification of weak non-linearities within complex aerospace structures. Aerosp. Sci. Technol. 23(1), 53–62 (2012) 6. Platten, M.F., Wright, J.R., Cooper, J.E., Dimitriadis, G.: Identification of a nonlinear wing structure using an extended modal model. J. Aircr. 46(5), 1614–1626 (2009)
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