USING FULL-SCALE FATIGUE TESTING CRACK GROWTH RESULTS TO LIFE AIRCRAFT FLEETS

摘要

The ability to understand and predict fatigue lives remains a key technical factor affecting modern aircraft fleets which are required to operate safely up to their design lives and sometimes beyond. An essential element of the airframe design and certification process is the full-scale fatigue test, where an airframe is subjected to realistic variable amplitude loads which are predicted to be representative of those that occur over the life-of-type. The results from these fatigue tests are then applied to the lifing of aircraft fleet whose usage may sometime vary considerably from the test spectrum. Therefore a tool capable of interpolating between fleet and test spectra is required. Experience has shown that traditional fatigue prediction tools do not always perform well in predicting the lives of modern highly optimised airframes. This paper summarises some recent semi-empirical models that appear to be capable of producing more accurate fatigue life predictions for many full-scale fatigue test results. Instead of relying on constant amplitude crack growth data the models use variable amplitude data obtained experimentally under more directly relevant conditions. The models are founded on the observation that for cracks in typical aircraft structural details, as a first approximation the lead cracks grow such that the log of the crack depth is proportional to number of cycles or spectrum loading blocks. The implications of this exponential growth are discussed leading to a exponential CG model that provides an alternative and more accurate means of assessing variable amplitude fatigue life, particularly of highly stressed structures.