32 T. J. Skousen and R. L. Mayes 3.5 Free Modal Test of Component and Fixture For the base excitation RC component testing, the RC is mounted on a plate fixture and instrumented as seen in Fig. 3.5. The instrumentation on the RC replicated the instrumentation from the MATV testing. The excitation source is a 6DOF shaker that the fixture just fits onto. To determine the loads for the 6DOF test, first a free modal test of the RC on the fixture needed to be performed. Table 3.1 shows the modal frequencies and modal damping measured in the modal test. Some months after the modal test and after conducting some 6DOF base excitation testing on the RC, additional modal testing was conducted to address unit to unit variability when generating test specifications for the methods discussed in this paper. To accomplish this, modal testing was conducted while varying the torque on each set of bolts in the RC by 25% lower and higher than nominal. Each torque value was implemented multiple times. Median modal frequency and damping values from each torque value is listed in Table 3.2. Fig. 3.5 The free modal test setup for the RC on the plate fixture with instrumentation Table 3.1 Modal parameters for the free modal test of the RC on the plate fixture Elastic mode Modal frequency (Hz) Modal damping (%) 1 389.8 0.19 2 1044 0.49 3 1140 0.83 4 1648 0.97 5 1827 0.33 6 2013 0.16 Table 3.2 Modal parameters for the free modal test of the RC on the plate fixture 75%Torque Nominal torque 125% Torque Elastic mode Modal frequency (Hz) Modal damping (%) Modal frequency (Hz) Modal damping (%) Modal frequency (Hz) Modal damping (%) 1 389.9 0.16 389.7 0.13 390.3 0.13 2 1115 0.23 1109 0.25 1099 0.27 3 1156 0.56 1158 0.56 1158 0.53 4 1668 0.53 1673 0.51 1675 0.51 5 1729 0.42 1745 0.41 1769 0.33 6 1995 0.17 1998 0.18 2003 0.20
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