Investigating Nonlinearities in a Demo Aircraft Structure under Sine Excitation

Developing on the basic idea behind parametric and non-parametric identification of nonlinear systems, another case study\ua0on integrating system identification and finite element modelling of nonlinear structures is presented.\ua0The first step, which is the focus of this paper, involves\ua0using acquired\ua0input and output data to derive an experimental model for both the underlying linear model and nonlinear model of the proposed structure, no information about the system is required and only the applied excitations and corresponding accelerations are implemented in the nonlinear identification step. The\ua0proposed case study\ua0is demonstrated on a nonlinear simple metallic plane assembly with\ua0localized stiffness and damping\ua0nonlinearities; in this case, an updated linear finite element model of the structure is derived and the nonlinearities experimentally characterised

Weakly nonlinear systems: Modeling and experimental methods

The prior chapter presented rigorous theory and methods for nonlinear systems, which is necessary in general because many nonlinear systems exhibit strong modal coupling due to the nonlinearity; this is commonly the case for the geometrically nonlinear structures that were the focus of that chapter. However, one of the most common sources of nonlinearity in built-up structures is the joints, and in many cases, these introduce only a weak stiffness nonlinearity together with a significant damping nonlinearity. In this case, and in many others that are relevant to industry, one can obtain good estimates of the response of the structure using a weakly nonlinear model in which the linear modes of the structure are presumed to be preserved and coupling between modes is neglected. This chapter provides a brief introduction to these concepts.—Chapter Authors: Randall Mayes and Matt Allen