Fatigue Life Enhancement of Fastener Holes Manufactured from High-Strength Aluminum Alloys

Split sleeve cold working has been used for many years to enhance fatigue and crack growth life of holes. Holes cold worked in high strength aluminum alloys such as 7050 and 2297, however, have shown a tendency to crack in the short transverse grain direction, as reported by Burt and Minarecioglu. They showed that these cracks are the result of exceeding the alloy's ultimate shear strength, and that reducing the amount of applied expansion is not sufficient, in all cases, to eliminate these cracks. This investigation will use 3D post-yield finite element models to characterize and understand the mechanisms that cause these "static shear fractures" in 7050 and 2297 aluminum. The models contain all aspects of the split sleeve cold working method, including the sleeve, the sleeve split, and segmented nosecap. OEM experience and StressWave's preliminary investigation have demonstrated that elimination of SSF's will be problematic, however, if not impossible using split sleeve cold working, and a new technique will be required in order to achieve the desired benefit. A new process has produced comparable fatigue and crack growth performance to the split sleeve method in conventional aerospace alloys. Process mechanics are different than the legacy methods and can be tailored to mitigate SSF's. Testing by Boeing, Alcoa, and others have verified effectiveness.