Effect of some aircraft loading program modifications on the fatigue life of open hole specimens

Flight simulation and program tests were performed with different 2024-T3 sheet specimens containing a central hole. The effect of the peak-load frequency on the damage sum and flight number was investigated. Gust spectrum test results were compared with those of other authors. For sufficiently close spectra with similar log-linear gust load distribution but different GAG cycle distribution, the relative Miner rule yielded satisfactory results. In the case of truncation levels $\text{S}{}_{\max }\text{= 1.84σ}{}_{\mathrm{m}}$ and $\text{S}{}_{\max }\text{= 1.685σ}{}_{\mathrm{m}}$, the investigated change of the frequency of stress cycles at the highest loading level (for cycle numbers smaller than those following from the log-linear gust distribution) had a weak effect on the damage sum, while the effect of decrease of the frequency of the lowest stress amplitude $\text{σ}{}_{\mathrm{a}}\text{= 0.222σ}{}_{\mathrm{m}}$ (MiniTWIST instead of TWIST) was considerable in some cases. It was found that in particular loading program cases, rare load peaks may have not only a beneficial but also a detrimental effect on the number of simulated flights. Truncation had a detrimental effect but the increase of the number of overload peaks above the number of cycles at the truncation level associated with the log-linear gust distribution had also a detrimental effect.