Safe-life analysis accounting for the loading spectra variability

The scatter of the variable load spectra is considered separately from the scatter of the aircraft structure to assess the safe-life of a fleet. Suppose the fatigue life under the specified load spectrum follows the log-normal distribution with a constant coefficient of variation, and the load spectra damage follows the log-normal distribution with a constant standard deviation, the theoretical distribution function of fatigue life of the fleet that accounts for the variability of the structure and the load spectrum is derived with the conditional probability model. Monte-Carlo simulation shows that the distribution of the fleet fatigue life can be described approximately by a log-normal distribution function, and the log expectation is the log mean life under variable load spectra and the variation equals the sum of the structural variation and the load spectra variation. The scatter factor method is then adopted to assess the safe-life. The scatter factor of the individual aircraft with definite load spectrum, as well as the scatter factor of the in-service fleet that takes the actual load scatter into consideration are listed.