7 Composite Fuselage Impact Testing and Simulation: A Model Calibration Exercise 73 1 2 3 4 5 6 7 8 9 10111213141516 11.57 19.38 31.60 34.13 52.78 74.53 103.20 156.04 223.14 223.23 230.36 230.79 232.33 232.50 234.47 243.86 Model Mode Number Identified Model Frequencies (Hz) Revised Model Frequencies (Hz) 8.78 15.31 36.34 62.21 105.35 116.67 122.28 145.35 157.02 221.27 231.75 309.74 327.53 344.67 358.87 377.25 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Fig. 7.6 Orthogonality results for revised LS-DYNA model and test -50 0 50 200 210 220 230 240 250 260 270 Input Number 3 276 0 50 100 150 200 10-6 10-4 10-2 100 276 Mag. (g/v) Test Model 0 50 100 150 200 -200 -100 0 100 200 276 Phase (deg) Fig. 7.7 Left plot shows a front view of the inner surface of the fuselage with floor node 276 marked and on the right is the corresponding frequency response comparison modes. Figure 7.7 shows a front view of the fuselage inner surface with the location of floor node 276 marked and the corresponding analytical and test frequency response functions. Note that in the critical range (circled in red) the target modes are not clearly excited and therefore are difficult to identify. As an example of needed corrections, the second target mode, which had the highest orthogonality value, was used as a reference to estimate changes required to get the frequencies to agree. Specifically, to get a frequency reduction from 105.4 to 74.5 Hz one would need to reduce the modulus to about 71 % of its original value. In fact, to test this assumption, the Mat 58 unidirectional tape and fabric material properties EA, EB, GAB, GBC, and GCA were all adjusted using optimization to minimize the error between test and analysis. Although a modulus reduction as initially suggested will greatly improve frequency agreement for the target modes, other non-critical modes were adversely affected.
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