Influence of bolted items on modal analysis performed on a car body

Paper in proceeding, 2008

When performing Modal Analysis testing on a BIP (Body-in-Prime), some bolted items are included to better take into account their inuence on body stiffness. However, their contribution to the stiffness is not relevant in the frequency range accessible for modal analysis (usually up to 70 Hz on a BIP). On the other hand, these bolted items increase the dispersion between results obtained for nominally identical test objects. The question which arises is whether the items should be included in the BIP definition to perform modal analysis and, in this case, which is their inuence on the results? MIMO (Multi-Input-Multi-Output) measurements were carried out over three, nominally identical, BIPs. Several configurations were measured for each BIP, starting from the complete body the bolted items were progressively removed. A version of LMS PolyMAX method was programmed by Matlab to analyze the measured data. Conclusions about bolted items inuence are drawn based on the study of stabilization diagrams and modal parameters. The poles selection by the stabilization diagrams is one of PolyMAX method keys. The method understanding obtained from programming allows studying the ins and outs of poles selection. Polynomial order plays an important role in physical poles identification, especially for closely spaced modes. Results are shown to highlight its relevance.