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Session: Session 3: Fatigue life enhancement methods and repair solutions I
Session starts: Monday 26 June, 15:30
Presentation starts: 15:30
Room: Theatre room: plenary

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Jacob Warner (United States Air Force)
Dallen Andrew (Hill Engineering LLC)

Abstract:
For over 15 years, the A-10 Aircraft Structural Integrity Program (ASIP) team within the US Air Force (USAF) has facilitated the advancement of fatigue crack growth analytical methods for residual stresses at cold expanded holes through numerous research studies and test programs. These many years of research and test validation have consistently shown that fatigue crack growth analyses that incorporate residual stress can dramatically improve aircraft availability by extending inspection intervals, simultaneously increasing safety by accurately predicting scenarios that legacy methods (such as using a reduced initial crack size) have shown to overpredict. In a breakthrough this last year, the USAF has released new guidance on analytical methods to account for residual stresses in fatigue crack growth analyses in the form of a Structures Bulletin, permitting the use of residual stresses in a Damage Tolerance Analysis (DTA) to justify an extension (or elimination) of recurring inspections. The development and completion of this bulletin was a culmination of many contributions from A-10 ASIP, the Engineered Residual Stress Implementation (ERSI) working group, and Air Force Research Laboratories partners. Leveraging the guidance provided in the new structures bulletin, the A-10 ASIP team has now analyzed more than 10 fatigue critical locations, explicitly incorporating residual stresses in fatigue crack growth analyses with multi-point fracture mechanics models. Analyses are validated by spectrum crack growth test data. A review of major milestones that have contributed to the introduction of residual stresses in A-10 DTAs is provided, including advancements in applying residual stresses, multi-point fracture mechanics, shortfalls of reduced initial crack size methods, and test validation. Test validation results are compared with multi-point and traditional two-point linear elastic fracture mechanics predictions. For comparison purposes, reduced initial crack size predictions are also shown. Including residual stresses directly in DTAs empowers more accurate predictions and inspection intervals while simultaneously increasing aircraft safety and airworthiness.