Abstract: To assess the safe-life for a fleet, and account for the effects of load spectrum variability, the fatigue life scatter including the load spectrum scatter and structural property scatter is separated. It is assumed that the fatigue life under the specified load spectrum follows the log-normal distribution, and that the load spectra damage also follows the log-normal. The fatigue life scatter is separable, and then the accurate distribution function of the fatigue life is deduced with the conditioned probability model. The expectation is unchanged, and the variation is equal to the sum of the structural variation and the load spectrum variation. Therefore the safe-life of a fleet can be determined by the small scatter factor under a severe spectrum, and the spectrum severity and the life reliability are discussed.

Key words: scatter, structure, load spectrum, scatter factor, safe-life, reliability