Fatigue life variability due to variations in interference fit of steel bushings in 7075-T651 aluminum lugs

Fatigue of metallic components is a concern in rotary wing aircraft. There exists a potential for loss of an aircraft if a component fatigue life does not take into account all factors than can cause life variation. Much literature was located dealing with the impact of large interference fits and cold working. Literature covering small changes was lacking. This study focused on small changes in interference such as those that could come from a tolerance on a drawing. Testing was completed on 7075-T651 Aluminum alloy lugs with steel bushings of varying interference fit. Testing three different levels of interference fit revealed three different S–N curves even though the variation from fit to fit was small. Significant improvements can be achieved in the S–N curve simply by small changes in interference. From a safety point of view these changes should be looked at as potential reductions.

► The influence of interference fits of 0.03%, 0.15%, and 0.25% on the fatigue life, are shown.
► Finite element analysis was used to calculate the residual stress due to the interference fit.
► An empirical equation is given that relates the interference fit to number of cycles to failure.