(Dr. W.S. Johnson, advisor)
"Durability of the Residual Stresses Surrounding Cold Expanded Fastener Holes in 7050-T7451 Aluminum"
Cold expansion of fastener holes has become a widely used technique in airframe manufacturing and repair. The enhanced fatigue performance of the expanded holes is unmistakable and as a result, more aircraft containing expanded holes in critical structural joints are flying than ever before. The expansion process has been used on aging aircraft as well as new production aircraft. The success of these aircraft is at least in part due to the extensive use of cold expansion.
Our advanced military aircraft incorporate the cold expansion technique in their design and are capable of flying at supersonic speeds that create elevated temperatures within the airframe. Additionally, aircraft that spend the daytime on warm blacktop runways can experience high temperatures within the structure. There is concern that these elevated temperatures could be driving residual stress relaxation in the cold expanded material.
The issue of stress relaxation around expanded fastener holes is the main focus of this investigation. In order to explore the durability of residual stresses at elevated temperatures in this alloy, stress relaxation tests, fatigue testing, and residual stress measurement through X-ray and neutron diffraction have been performed. The stress relaxation tests were designed to track the stress relaxation rate as a function of temperature. Ambient condition fatigue testing has been conducted on a set of specimens with expanded holes. A subset of the fatigue specimens was subjected to a 5000-hour aging period at 104 ºC for comparison. Both X-ray and neutron diffraction experiments have been conducted on a single fatigue specimen before and after the 5000-hour aging period to assess changes in the residual stress field. The experimental results are presented and conclusions about material sensitivity to stress relaxation are discussed.