Detection of Pre-Crack Fatigue Damage in a U.S. Army MH-47E Chinook Aft Rotor Shaft

Accumulated fatigue damage in flight critical helicopter dynamic components prior to crack initiation and detection is currently estimated by fatigue analysis based upon fatigue test results. To more accurately assess the operational fatigue damage and remaining useful life of a critical component prior to the initiation of a detectable crack, a non-destructive evaluation (NDE) process that directly measures accumulated fatigue damage is required. These NDE measurements are useful in monitoring accumulated fatigue damage, assessing probability of failure, determining remaining useful life, establishing inspection intervals, and refining fatigue damage accumulation models at any point in the operational life cycle of structural components. Induced positron analysis (IPA) was evaluated as an NDE technology to investigate the potential for safely extending the useful life of existing CH/MH-47 Chinook aft rotor shafts that are being retired upon reaching the operational flight hour limits as defined by current design life criteria. An IPA survey of the critical areas of a retired MH-47E shaft was conducted to determine if the IPA technology could reliably detect material degradation that would warrant application to a larger population of operational rotor shafts. The IPA NDE results profiled distinct material condition trends in the shaft upper spline tooth root, sidewall elevations, and top, for both the drive and non- drive sides (36 locations per spline tooth). IPA material damage measurements were successfully correlated with the local stress values as predicted by the OEM\´s finite element analysis (FEA) of the shaft spline. Additionally, IPA measurements axially along several teeth indicated no significant material degradation, despite a visible impression on the lower spline. This visible impression has been erroneously judged to be a "wear-step" and has resulted in unnecessary early retirement of serviceable rotor shafts. Based upon the success of the IPA evalu- ation of the initial MH-47E aft rotor shaft, more CH/MH-47 shafts, at varying stages of their operational life cycles, are programmed for IPA assessment.This paper will describe the underlying IPA technology, the initial MH-47E aft rotor shaft application and test results, the correlation of IPA with FEA results, the path ahead for further CH/MH-47 IPA NDE inspections, and the potential benefits of applying IPA NDE to the entire population of CH/MH-47 aft rotor shafts.