Mechanism of Fatigue Life Improvement due to Fine Particle Shot Peening in High Strength Aluminum Alloy

Shot peening is widely used in the aerospace industry in order to improve fatigue property of structural high strength aluminum alloy. However, the optimum shot peening condition has not been revealed, considering the effect of the shot peened surface roughness and the residual stress profile on fatigue life. In this study, fatigue life improvement of 7050-T7451 due to the shot peening with the fine particles whose diameter was less than 0.1 mm was studied and compared with the fatigue life of the shot peened material with the conventional steel shot of 0.6 mm in diameter. The fine particle shot peened (FPSP) specimens endured about ten times longer fatigue life than the virgin specimens and several times longer than the conventional shot peened specimens. The fatigue crack in FPSP specimens initiated at the subsurface layer or the inside of the specimen, while the fatigue crack initiation site in the conventional shot peened specimen or the virgin specimen located on the specimen surface. It was revealed that the fatigue improvement effect of FPSP mainly resulted from the transition of the crack initiation site due to the shallow compressive residual stress layer near the specimen surface, considering the arrested small cracks detected on the fractured specimen side surface K analysis based on the compressive residual stress profiles.