42 J.H. Gordis Fig. 4.4 Driving point FRF The first ABC set is the baseline configuration, in this case, the free-free beam with no ABC pins. This impedance QZ .1/ ABC is shown in Fig. 4.5. The heavy green curve is QZ .1/ ABC while the multiple, fainter curves are the diagonal terms of hZ .1/ ABCi. The three vertical red lines indicate the mode frequencies for the ABC set, as predicted by the FE model. As can be seen (and reflecting the values shown in Table 4.1), the frequency agreement is excellent for modes #1 and #2, with some difference for mode #3. The thin vertical green line close to 0 frequency is the user-defined rigid-body mode cutoff. When ABC peaks are selected using the threshold tool, any peaks with a frequency below this cutoff are automatically discarded. The first four mode shapes calculated from the FE model are shown in Fig. 4.6, and are the standard mode shapes for a free-free beam.
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