7 Composite Fuselage Impact Testing and Simulation: A Model Calibration Exercise 71 1 2 3 4 5 6 7 8 9 10 10.07 16.88 38.25 73.02 114.87 133.51 142.08 143.97 158.75 224.62 Impact Shape Vibration Shape Frequency (Hz) Impact Shape Contribution 0.67 0.14 0.06 0.04 0.03 0.01 0.01 0.01 0.00 0.00 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Fig. 7.3 Orthogonality of vibration modes and impact shapes Fig. 7.4 Montage of composite fuselage test section undergoing modal testing accelerometers are shown including a view of the bottom clamp support. For testing, the fuselage rested on a 3-in. diameter tube placed on metal plates on the laboratory floor and clamps kept the fuselage from lifting off the floor. For all testing PCB Piezotronics accelerometers were used with nominal sensitivity of 500 mV/g. For excitation, a PCB Piezotronics impact hammer with a hard tip was used with an in-line load cell to measure the input force. Because only nine accelerometers were available, testing was conducted by sections (i.e. left, top, etc.). For each data set, the fuselage was hit with the impact hammer in all three directions. Although most of the accelerometers were placed to measure accelerations normal to the inner surface of the fuselage, two accelerometers were placed to measure the out-of-plane accelerations (i.e. x-direction). Acceleration data were collected from 0 to 256 Hz with a frequency resolution of 0.5 Hz. A total of 34 acceleration measurements were collected. From the pre-test analysis, only two vibration modes were deemed important for impact: the fourth vibration mode at 73 Hz and the eighth mode at 144 Hz. The mode at 144 Hz is primarily the fuselage floor moving in the vertical direction, and the mode at 73 Hz involves motion of the entire cross section.
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