Modal Analysis: Controlled Nonlinearities

Nonlinear techniques are the subject of ongoing structural testing research. This article deals with the construction, testing and correlation of a structure with a nonlinearity evaluation. 

A structure was developed to provide a realistic test bed for high-channel-count linear and nonlinear modal analysis. The goal was to model and construct a linear structure to serve as a platform to introduce controlled nonlinearities. These nonlinearities are designed to mimic airframe nonlinear structures including ailerons, engine mounts, payload and bomb mounts including wingtip stores. The model was constructed and modeled with several pretest analyses performed to identify a minimum sensor configuration that could correlate the modes of interest. Impact testing validated the sensor configuration prior to testing with 144 input channels and multiple sources. Correlation was accomplished and resulted in an updated FE model. Once this baseline was completed the introduction of controlled nonlinearities allowed application of the existing and emerging nonlinear modal analysis tools.

The experimental modal software and the 144 input/6 output modal system come from m+p international. The Linear/Nonlinear Modal Analysis group at University of Illinois provided facilities and nonlinear modal methods and test support.