PREDICTING FATIGUE LOADS USING REGRESSION DIAGNOSTICS

The process of predicting safe retirement times for rotorcraft compone nts essentially involves three steps which include: (1) Generation of a mission spectrum, (2) Acquisition of fatigue loads from a flight loa d survey, (3) Establishment of an S-N curve at a statistically reduced endurance limit (typically 3 sigma) by conducting tests on 6 full sca le specimens (bench testing) in the laboratory, Both the flight testin g and bench testing aspects of this process are very expensive and the end result is that the fatigue life analyst seldom has all the data h e needs and is, thus, forced to make conservative judgments where the data is sparse or unavailable, This paper will propose a method for im proving the flight load acquisition process so that the analyst has im proved sampling as well as an understanding of the causes of fatigue l oads. The concepts of safe life reliability and confidence relating to flight load surveys will be addressed, The method discussed herein ma y also provide some substantive measures which could be used in an ind ividual aircraft monitoring program, The flight loads data presented w ere acquired from The United States Air Force (USAF) Structural Modifi cation Flight Test (SMFT) of the H-53.