PREDICTING FATIGUE LOADS USING REGRESSION DIAGNOSTICS

The process of predicting safe retirement times for rotorcraft components essentially involves three steps which include: (1) Generation of a mission spectrum, (2) Acquisition of fatigue loads from a flight load survey, (3) Establishment of an S-N curve at a statistically reduced endurance limit (typically 3 sigma) by conducting tests on 6 full scale specimens (bench testing) in the laboratory. Both the flight testing 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 he needs and is, thus, forced to make conservative judgements where the data is sparse or unavailable. This paper will propose a method for improving the flight load acquisition process so that the analyst has improved sampling as well as an understanding of the causes of fatigue loads. The concepts of Safe Life Reliability and Confidence relating to flight load surveys will be addressed. The method discussed herein may also provide some substantive measures which could be used in an individual aircraft monitoring program. The flight loads data presented were acquired from The United States Air Force (USAF) Structural Modification Flight Test (SMFT) of The H-53.