Fatigue Life Enhancement of a D16at Aluminum Alloy for Aircraft Components with Fastener Holes

D16AT aircraft Al-alloy was evaluated. The friction stir hole expansion (FSHE), solid mandrel cold working and symmetric cold expansion (SCE) methods were compared. The results are summarized on the basis of fatigue tests, S–N curves, X-ray diffraction, and microstructural analysis. Under the high-cycle fatigue, SCE provides more than 66 times longer fatigue life as compared to the solid mandrel method and more than 82 times greater fatigue life in comparison with the FSHE method. Through X-ray diffraction analysis it was found that the higher efficiency of the SCE method was due to symmetric distribution (with respect to the plate middle plane) of residual hoop stresses around the hole. On the other hand, the solid mandrel cold working method causes a significant gradient of the residual stress distribution through the plate thickness, which is a precondition for nucleation and propagation of corner fatigue cracks. The FSHE method efficiency was established to be primarily dependent on generated heat and equivalent plastic strains. The combination of these factors determines the beneficial microeffect of the microstructure modifying in the vicinity of the hole and a useful macroeffect due to residual compressive stresses. It was concluded that SCE method should be used for prestressing of fastener holes in the most loaded components in D16AT aircraft structures – wings and fuselage, while the FSHE one can be applied to processing of fastener holes in less loaded aircraft components.