OBTAINING MODAL MODELS OF A NONLINEAR STRUCTURE USING FORCE-CONTROLLED FREQUENCY RESPONSE FUNCTIONS

Date

2024-7-08

Author

Gürbüz, Muhammed Fatih

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Identification of physical and modal parameters of an engineering structure is important in the field of structural dynamics in order to establish input and output (i.e. forcing and displacement) relationships of the structure. Linear structure identification is a rather well-established field thanks to the research performed on it over several decades. Modal analysis is the most widespread approach used in this area. On the other hand, nonlinear system identification is significantly more challenging and there has been great interest in it over the recent years. There are several different and novel identification approaches proposed by different researchers in the nonlinear structural dynamics field. Response-controlled stepped sine testing (RCT) is one of these methods which is implemented by keeping the response amplitude of a degree of freedom constant over different excitation frequencies. It offers important advantages such as not needing to know nonlinearity infected coordinates and the type of nonlinearities in the structure, and provides a modal model for the whole nonlinear structure. However, most of the experimental methods and testing hardware are compatible with constant amplitude forcing tests rather than constant response tests. This thesis aims to introduce a new approach that extends the use of the response-controlled nonlinear structural identification method to the structural response data acquired with constant amplitude forcing tests. The proposed approach first forms a harmonic force surface (HFS) which is obtained from the measured response through constant force amplitude tests. The constant-response frequency response functions (FRFs) are obtained from this surface data and the modal parameters of the nonlinear structure are identified. The proposed method is validated using numerical case studies, experimental data shared by other researchers as well as the experiments performed as a part of this thesis. This study has the potential to extend the applicability of nonlinear structural identification with response-controlled testing by making it compatible with constant amplitude force data and testing equipment.

Subject Keywords

Nonlinear System Identification, Nonlinear Experimental Modal Analysis, Response-Controlled Stepped-Sine Testing, Force-Controlled Stepped-Sine Testing, Harmonic Force Surface

URI

https://hdl.handle.net/11511/110158

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Graduate School of Natural and Applied Sciences, Thesis