Launch vehicles may encounter a variety of mixed sine and random vibration environments during powered flight. The random vibration is typically driven by turbulent boundary layers, shock waves, and other aerodynamic flow effects. The sine vibration may be due to a thrust oscillation for the case of a solid motor. Furthermore, the thrust oscillation frequency and amplitude may each vary with time. Both the sine and random environments may thus be nonstationary.
Avionics components must be designed and tested to withstand the composite vibration environment. A single power spectral density specification which envelops the complete environment is usually desired for simplicity. Furthermore, the power spectral density specification is assumed to have a corresponding time history which is both stationary and Gaussian.
Traditional specification derivation methods involve assuming piecewise stationary flight data and making a maximum envelope from the piecewise segments. A more discerning method is to use the fatigue damage spectrum method to derive a stationary power spectral density which yields the equivalent fatigue damage of the composite nonstationary flight data. This paper demonstrates this fatigue damage enveloping method. swept_sine_fds.pdf
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Matlab script: Vibrationdata Signal Analysis Package
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– Tom Irvine