Temperature effects on fatigue performance of cold expanded holes in 7050-T7451 aluminum alloy

Cold expansion of fastener holes has been successfully used for many years to impart beneficial compressive residual stresses. These residual stresses serve to extend crack initiation lives and to slow down growth of small cracks. The 7050 aluminum alloy is being used in supersonic aircraft structures that are subject to high temperatures. The purpose of this investigation is to identify possible problematic stress relaxation around expanded fastener holes in this material. Three tests have been used to assess the effects of thermal exposure on cold expanded fastener holes in 6.4 mm thick 7050-T7451 aluminum plate. The three tests are an initial evaluation of tensile strength, an open-hole uniaxial fatigue test and a modified stress relaxation test. The tensile testing and fatigue testing were performed at ambient conditions on material in the as-received condition and also in a thermally aged condition. Thermal aging consisted of a 5000 h exposure in a laboratory oven set to 104 C. Stress relaxation testing was performed at temperatures of 71, 82, 93 and 104 C on a screw driven test frame fitted with an oven. The results of the tests suggest that stress relaxation is present but is not critical at the temperatures tested. The trend in the stress relaxation testing shows that temperatures slightly above 104 C may be detrimental to residual stress fields. (C) 2002 Elsevier Science Ltd. All rights reserved.