11 Unique Isolation Systems to Protect Equipment in Navy Shock Tests 119 Fig. 11.13 Accel Fig. 11.14 SRS Fig. 11.15 FFT both configurations except at the 70–120 Hz region. The deck (1 V) also shows a similar PV rise in the 70–120 Hz region in the vertical direction (Figs. 11.16, 11.17, and 11.18). Response (4FB)—front/back—The acceleration history, SRS and FFT spectrum are very similar to side (3SS) response except less high frequency content. The stiffer isolation system, configuration (2) shows 25.2 G’ s versus 13.8 G’s for configuration (1). The two-rise SRS plots closely match with high PV in the 70–120 Hz region. Similarly, the Fourier spectrum corresponds to the SRS with a spread of acceleration peaks in the high frequency region. Frequencies are widely distributed out to 220 Hz. As with 3SS, the 4FB peak ‘G’ is a very sharp spike in the acceleration-time history—first 20 ms (Figs. 11.19, 11.20, and 11.21). 11.6.2 Correlation of the Fourier Spectrum and SRS Comparison of response measurements shows a close correlation of the Fourier spectrum and SRS with respect to the presence of high frequencies and the levels at which they are exhibited. As the higher frequencies become more prominent in the Fourier spectrum (for example at 70–120 Hz) they show-up as increased PV levels in the same frequency region
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