328 P. A. Kerrian Table 34.2 Extracted FB modal frequencies for method 1 from two sets of test data Method 1 Mode FEM FB freq. (Hz) Description FB freq. (Hz) FB freq. (Hz)a 1 55.74 1st bending 49.60 49.55 2 55.75 1st bending 50.07 50.4 3 520.55 2nd bending 461.35 461.24 4 521.3 2nd bending 465.90 465.0 5 857.04 Torsion 544.9 543.78 6 702.88 Plunge 606.27b 660.8 7 1380.3 Breathing mode 8 1380.21 Breathing mode 9 1442.5 3rd bending 1250.92 1258.87 10 1452.81 3rd bending 1257.13 1258.94 A Spurious 1st bending 271.57 B Spurious torsion 568.57 C Spurious plunge 617.96 aAlternative set of test data bStrongly coupled with torsion Fig. 34.7 CMIF of FB FRFs from alternative test data set with only flange and cylinder response DOFs and cylinder impacts as references on the cylinder and flange and impacts on the cylinder. The effects of the frequency shifts associated with the roving impact hammer are seen in this figure because the FBCM was not able to remove the additional nonphysical modes around 270 Hz. Table 34.2 also summarizes the FB modes extracted from the FB FRFs. All of the target modes are clearly visible in the extracted mode set, with the exception of the plunge mode because it was not well excited by the two lateral impacts at the top of the cylinder. However, additional spurious modes were extracted below 1000 Hz that are not real. The process was repeated with an additional data set that was collected with the TA suspended with a softer bungee suspension system. Impacts were made in the X and Z directions at node 101, in the Y direction at node 104, and in the X and Y directions at node 131. Base impacts were reduced to nine vertical and five lateral. Figure 34.7 shows the CMIF of the FB FRFs for all cylinder references and response DOFs on the cylinder and flange. The softer suspension system and reduced number of base impacts helped to reduce the frequency shifts of the first bending mode and eliminated spurious modes below 1000 Hz. The extracted FB modes from the alternative data set are contained in the last column of Table 34.2. Also, the vertical impact at node 101 enabled a clean extraction of the plunge mode. Figure 34.8 contains plots of the FB mode shapes extracted.
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