Understanding and Modeling Nonlinear Behaviors in Aerospace Structures using Sine-Sweep Testing

Auteurs: 

T. Dossogne, J. P. Noël, L. Masset, G. Kerschen (Aerospace and Mechanical
Engineering Department, University of Liège)
B. Peeters (Test Division, Siemens Industry Software NV)

During the design cycle of aircraft, the role played by the structural dynamicists consists in constructing a validated numerical model of the aircraft behavior under vibration. Model validation is performed experimentally, by correlating predicted and measured natural frequencies and mode shapes. This process has become standard in the aircraft industry; however, it relies throughout on the assumption of linearity of the structural vibrations, while it must be acknowledged that, today, test engineers are more and more often confronted with nonlinearities during ground vibration measurement campaigns. Nonlinear behavior may result from various physical mechanisms, the most notable being dynamic boundary conditions, such as clearances and impacts. In this paper, a consistent set of techniques is described to locate, characterize and model nonlinearities using typical aircraft vibration data, with the purpose of upgrading the initial linear finite-element (FE) model into a reliable nonlinear one. A constant thread in this set is the analysis of sensor measurements in the phase space. It is first shown that simple approximations in the structural equations of motion can lead to easy-to-interpret visualization plots for nonlinearity location and characterization. The analysis is then rendered quantitative by describing an effective metric for nonlinear FE model updating. The presented tools are illustrated using sine-sweep data measured at multiple forcing amplitudes on a full-scale F-16 aircraft. Nonlinear stiffness and damping elements, modeling a loosened attachment at the aircraft wing tip, are identified and introduced in a linear FE model, leading to accurate response predictions in a strongly nonlinear regime of motion.

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