A Comparison of Fatigue Life Prediction Methodologies for Rotorcraft.
NATIONAL AERONAUTICS AND SPACE ADMIN LANGLEY RESEARCH CENTER HAMPTON VA
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Because of the current U.S. Army requirement that all new rotorcraft be designed to a six nines reliability on fatigue life, this study was undertaken to assess the accuracy of the current safe life philosophy using the nominal stress Palmgrem-Miner linear cumulative damage rule to predict the fatigue life of rotorcraft dynamic components. It has been shown in the past that this methodology can predict fatigue lives that differ from test lives by more than two orders of magnitude. A further objective of this work was to compare the accuracy of this methodology to another safe life method called the local strain approach as well as to a method which predicts fatigue life based solely on crack growth data. Spectrum fatigue tests were run on notched specimens made of 4340 steel using the Felix28 variable amplitude spectrum a shortened form of a standard loading sequence for fixed or semi-rigid helicopter rotors. Two other spectra which resulted from a simple rainflow reconstruction of Felix28 were also tested.