Effect of small cycles and load spectrum truncation on the fatigue life scatter in 7050 Al alloy

Abstract

This work examines the effect of dynamic buffet loads (small cycles) on the scatter in the fatigue life of aircraft aluminum. Current life cycle management of fighter airframes assumes, without engineering evidence, that buffet loads cause an increase in the scatter factor used in safe-life calculations. Hence, the role of small cycles in spectra representative of the CF-188 aileron inboard hinge was examined in this study. The base load spectrum with the dynamic content was filtered to remove specified amounts of dynamic damage as determined by the CI89 strain-based cumulative fatigue damage program. The effect of this filtering on the scatter in crack initiation life and total fatigue life of double edge notched fatigue coupons of 7075-T7451 aluminum alloy were examined. The results indicate that inclusion of the dynamic loading caused the distribution of the crack initiation life to become bimodal. Each mode could be described with a log-normal distribution, the standard deviation of which was lower than the standard deviation obtained for the filtered load spectra. There was no evidence that the standard deviation increased with small cycle content, once the true nature of the distribution was taken into account. In fact, the standard deviation associated with the truncated spectrum was considerably larger. The bimodal distribution is positted to be a result of included particles near the surface on the fatigue limit of individual specimens.