Developing an accelerated flight load spectrum based on the n_z-N curves of a fleet

Fatigue load spectra are essential for fatigue analyses and full-scale fatigue testing (FSFT) of aircraft structures. Because of the long test period, an accelerated load spectrum is required to reduce time consumption and cost. This paper presents a new approach for developing an accelerated flight load spectrum. Using the measured n_z-N curves per flight for a Fokker F27 fleet, 55 individual load spectra were reconstructed, the damage of which was calculated using the damage curve method, and verification was conducted using fatigue tests on 2024-T3 aluminum alloy centered-hole specimens. A lognormal distribution function was used to model the variation in the load spectrum damage per flight for the fleet. Assuming that the structural property variation follows a lognormal distribution, the fleet life is found to follow a lognormal distribution with the mean equal to that including the load spectrum variation and the variance equal to the sum of the variance in structural properties and load spectra. Then, we obtained the damage severity of the accelerated load spectrum that meets the reliability requirements for fleet life. The n_z-N curve of the accelerated spectrum was subsequently determined based on statistical analysis and probability extrapolation of the exceedance number. Finally, an accelerated flight load spectrum was developed based on the n_z-N curve, the damage severity of which was determined by fatigue analysis and testing.