Fatigue behaviour of re-worked fastener holes in aluminum alloys

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This proposal is to study fatigue cracking and crack growth behaviour in re-worked fastener holes in aircraft aluminum specimens. The objective is to understand the fatigue cracking mechanisms involved and to develop analytical techniques to model and design repair schemes for aging aircraft.The majority of fatigue cracks in conventional aluminum airframe structures emanate from fastener holes and are detected before reaching critical size. These holes are re-worked, generally by over sizing the hole to remove the fatigue damage; new fasteners are installed before the aircraft is returned to service. Re-working of a hole may also include one or more of the following: cold expansion to impart beneficial compressive residual stresses, interference fit fasteners to improve fatigue life, and interference fit bushings to improve stiffness, restore net section and improve the fatigue life. In this research, a step by step approach will be used to develop a finite element model for reworked fastener details supported by simultaneously measured experimental fatigue data collected on laboratory specimens. Processing variables will be examined to determine which techniques are most beneficial to life extension and which techniques are detrimental to re-worked hole repair schemes. Processing variables will include: edge distance, amount of cold expansion, and amount of interference fit.The benefits to the aerospace industry, both military and civilian, are tremendous.Conventional aluminum airframes continue to be produced in great numbers; for example, Boeing's production of the 737 and 747 remains exceptionally high. Furthermore, the Canadian Forces continues to operate and acquire new versions of aircraft such as the CC-130 (Hercules) and CH-148 (Chinook), and have to extend the lifetime of platforms such as the CF-188 (Hornet). Life extension technologies for this application are vital to ensure the safety and economy of Canada's air transport, search and rescue, and defence systems for many decades to come.