User-friendly method to split up the multiple coherence function into noise, nonlinearity and transient components illustrated on ground vibration testing of an F-16 fighting falcon

Purpose: This paper provides a novel method to split up the multiple coherence function into noise, nonlinear distortion, and transient components. Method: The method relies on the nonparametric estimation framework called the Best Linear Approximation (BLA) where vibro-acoustic systems are excited by special so-called multisines (pseudo-random noise) signals. Challange: Testing of multiple-input, multiple-output (MIMO) nonlinear systems is very involved, and it usually requires expert users. It is because the design and processing of experiments, and nonlinear assessment are not trivial questions. Proposal: To cope with these issues, this paper shows the recent results of a research project addressing the questions related to the user-friendly nonlinear (pre-)processing of MIMO experiments of vibrating structures. The key idea is the novel analysis of the (co-)variance estimates of the BLA framework that can be tailored to split up the classical multiple coherence function into noise, nonlinearity, and transient components. Conclusions: Using the suggested approach, a novice user can quickly determine whether the underlying structure is linear or not, and how much is the possible gain using nonlinear modeling tools. Illustration: The proposed approach is demonstrated on, but not limited to, the analysis of ground vibration testing of a decommissioned F-16 fighting falcon.