Joint Identification through Hybrid Models Improved by Correlations

Abstract

In mechanical systems coupled with joints, accurate prediction of the joint characteristics is extremely important. Despite years of research, a lot is yet to be learnt about the joints' interface dynamics. The problem becomes even more difficult when the interface Degrees-of-Freedom (DoF) are inaccessible for Frequency Response Function (FRF) measurements. This is, for example, the case of bladed-disk systems with dove-tail or fir-tree type joints. Therefore, an FRF based expansion method called System Equivalent Model Mixing (SEMM) is used to obtain expanded interface dynamics. The method uses numerical and experimental sub-models of each component and their assembly to produce the respective expanded or hybrid sub-models. By applying substructure decoupling to these sub-models, the joint can be identified. However, the joint can be noisy due to expansion and measurement errors which propagate to the hybrid sub-models. In this paper, a correlation based approach is proposed in the SEMM method wherein the quality of the expanded sub-models is improved. In this new approach, several expanded models are generated systematically using different combinations of the experimental FRFs and computing a parameter, Frequency Response Assurance Criteria (FRAC), to evaluate quality of the contribution of the different measurements. The lowest correlated channels or FRFs can be filtered out based on a certain threshold value of FRAC. Using the improved hybrid sub-models, the joint identification also shows a remarkable improvement. The test object for the method is an assembly of disk and one blade with a dove-tail joint.