Smoothing experimental data in dynamic substructuring of built up systems 3.4 Results for subsystem subtraction Starting from the FRFs of coupled systems AB and residual subsystem B (after spring cancellation), the FRFs of the unknown subsystem A is computed using the procedure outlined in Section 2.2. Results at the coupling DoFs are shown in Figs. 17 and 18. As expected, the result is worse than the one obtained for structure addition. A spurious frequency seems to appear around 5 Hz. Having used an extended interface, it can not be explained as an internal resonance of the residual subsystem with interface DoFs grounded [4]. It might be ascribed to inconsistencies between the FRFs of the coupled system and the FRFs of the residual subsystem. Fig. 17 Inertance H55 of the unknown subsystemA: fitted (blue); after decoupling (red) 0 5 10 15 10−2 100 102 Frequency [Hz] Magnitude [(m/s2)/N ] Fig. 18 Inertance H66 of the unknown subsystemA: fitted (blue); after decoupling (red) 0 5 10 15 10−2 100 102 Frequency [Hz] Magnitude [(m/s2)/N ] The obtained results could be improved by selecting a lower number of poles in curve fitting the FRFs of the coupled systemAB. Specifically (compare Figs. 19 and 20 with Figs. 7 and 8) the natural frequency around 5 Hz, probably due to lateral motion of the coupled system, can be neglected. No such lateral motion occurs during tests on the residual subsystemB, thus avoiding a source of inconsistency. Results of the decoupling procedure at the coupling DoFs shown in Figs. 21 and 22 103
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