The earliest spectral method assumes the vibration response is narrow-band, meaning the structure vibrates primarily at a single dominant resonant frequency. Under this assumption, the stress peaks follow a classic Rayleigh distribution. While highly accurate for simple structures, the Bendat model overestimates damage when applied to broad-band systems, making it overly conservative. 2. The Dirlik Approximation
: Combine the input excitation PSD with the FRF to obtain the output stress response spectrum ( 2. Characterize the Random Process vibration fatigue by spectral methods pdf better