Evaluation of identification methods for joints with inaccessible interfaces using frequency-based substructuring

Recently, there has been significant interest in mechanical joint identification through dual substructure decoupling. In this method, each component of a complex mechanical system is handled as an independent substructure. The joint is then identified by subtracting the measured dynamics of the connected subsystems from those of the assembled system. A major challenge with this approach is the need to measure both translational and rotational Frequency Response Functions (FRFs) at the interface between subsystems (interface DoFs). This is often impractical due to the limited space available to place instrumentation. To address this challenge, several techniques available in the literature can be used to derive FRFs at the interface between components using the available measurements. This paper presents a comparative study of three joint identification methods that use two state-of-the-art techniques for obtaining the interface FRFs required for the decoupling process, based on available measurements: Virtual Point Transformation (VPT) and System Equivalent Model Mixing (SEMM). In particular, the advantages and disadvantages of the three methods that combine decoupling with VPT, SEMM, or both, are discussed. Special attention is given to the error propagation associated with each technique. The study is conducted using experimental data from a laboratory benchmark. The results show that the direct VPT approach and the one combining SEMM with VPT give similar results, suggesting that the SEMM is able to accurately reconstruct the FRFs at the boundary DoFs.