The effect of dynamic loading on the fatigue scatter factor for Al 7050

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 aft tail hanger position of the CF188 aircraft was examined in this study. The base load spectra with the dynamic content were 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. For all of the spectra examined, the scatter in the crack initiation life was found to increase for specimens taken further from the centre of the block from which they were machined. There was also a slight increase in the mean fatigue life for specimens taken from outer sections of the block. In addition, the crack growth rate was related by a power law to the crack initiation life for all spectra independent of the small cycle content.