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

New high-strength materials such as 7050 and 2297 aluminum are providing considerable life enhancement while production innovations allow components to be produced from single billets. This results in fastener holes that are normal to the short-transverse grain orientation, leading to problems with established coldworking procedures. Coldworking fastener holes is a common fatigue life enhancement process in the aerospace industry. However the established split-sleeve coldwork process produces cracks in new high-strength aluminum applications, and therefore cannot be used with the new materials. The Air Force is currently searching for a new coldworking process; West Coast Industries proposed to advance the split mandrel coldwork system. The process is similar to the legacy split sleeve system, but it can coldwork newer aluminum alloys without causing cracks. Phase I findings proved the feasibility of the split mandrel process for this application by successfully coldworking approximately 1,000 holes without any identified cracks. Using Phase I results as a foundation, Phase II efforts will optimize the process specification and create new and redesigned process tooling. At the end of a successful Phase II project, WCI will present the Air Force with a complete and thorough coldwork procedure.