Using Reduced Order Models to Predict Modal Coupling in Jointed Structures.

Abstract

Finite element models can be used to capture the nonlinear behavior exhibited by bolted joints in structures. The nonlinearity leads to a shift in the frequency and damping of modes of vibration as a function of vibration amplitude. However, to model this accurately, a high-resolution mesh is often required at the interfaces which can cause the model to be prohibitively expensive to run. As a result, reduced order models (ROMs) are used to decrease the size of the model, while preserving the global behavior. ROMs are often calibrated to reproduce the behavior of individual modes, including the amplitude dependent damping and frequency backbone curves. Recent studies have shown that these nonlinear modes can couple when simultaneously excited such that the excitation of a particular mode may influence the frequency and damping of another. The objective of this research is to evaluate the ability of various ROM strategies for jointed structures to capture this modal coupling. The new Multi-mode Quasi-Static Modal Analysis approach is used to predict the modal coupling on various ROMs that are meant to describe a full-fidelity finite element model of a 2D cantilever beam. The full-fidelity model is taken as the truth model and time integration is used to further validate the predictions. The ROMs considered include an industry standard “spidering” ROM with physical Iwan elements in place of the joints and a System-Level Characteristic Constraint (S-CC) ROM, where the S-CC modes are constrained with Iwan elements.