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

The split-sleeve process is a standard life enhancement procedure. Advances in material science are producing new materials (e.g., 7050 and 2297) that are replacing legacy alloys (2024 and 7075). However, coldwork technology has not kept up with material advances. Coldworking the new aluminum alloys, particularly in the short-transverse grain orientation, with the split-sleeve process creates cracks at holes. The split-mandrel process is a simpler, faster, more cost effective coldworking process. Process tooling and life-enhancement results are similar to those of the split-sleeve process. More important, the process is currently being used to successfully coldwork 7050 and similar alloys, in the short-transverse orientation. Similar behavior to split-sleeve coldworking between the two materials indicates that split-mandrel coldworking should effectively coldwork 2297 and other high strength aluminum alloys. West Coast Industries proposes the first comprehensive process characterization of the split-mandrel process. During Phase I, Air Force and industry expectations will be determined, process parameters will be developed, a large quantity of holes will be coldworked and examined for the presence of holes, and modifications for computer models will be developed to differentiate between the two coldworking processes.