Relative severity investigation in a severe load spectrum

A severe spectrum accounting for the load variability has been proposed for durability analysis and test for military aircraft structures in USA JSSG-2006. To assess the relative severity of this spectrum, a large quantity of measured Nz exceedances were analyzed statistically and the result showed the exceedance under the specified Nz follows the normal distribution. So the 50% and 90% Nz exceedance curves were constructed and the corresponding average and severe low–high–low programmed block spectra were developed. The fatigue tests of the LC4 aluminum alloy coupons with ∅8 mm centered hole were carried out for those two spectra, and the dominant crack growth (a, t) data of each coupons were measured with the quantitative fractography. Assuming the crack initiation life follows the log–normal distribution, the median lives were estimated. The mean tested life under the average spectrum was found to be 1.5 times as that under the severe spectrum and the life ratio was in good agreement with the load scatter factor, 1.5, which was proposed in UK DEFENCE STANDARD 00970.