Multiaxial fatigue resistance of shot peened high-strength aluminum alloys

This paper is aimed at investigating multiaxial fatigue of shot peened Al-7075-T6 alloy. Plain axi-symmetric specimens were subjected to combined in-phase tension and torsion loading, under nominal load ratio R = 0.05 and biaxiality ratio λ = τa/σa = 2. The results from multi-axial tests are discussed together with those obtained under pure tension and pure torsion loading. Fatigue crack initiation sites have been investigated through scanning electron microscopy fractography and the role of surface roughness on fatigue resistance has been analyzed. The initial and the stabilized residual stress profiles were used to discuss the improvement in the fatigue response in the hypothesis of crack initiation and early crack propagation as fatigue controlling parameters. For this purpose, several multiaxial fatigue criteria were used to account for the residual stress field. The Crossland and the Findley fatigue criteria incorporating stabilized residual stresses as mean stresses were found to predict the fatigue strength with a relative error lower than 15% within the entire fatigue life interval, for all the material variants and the loading conditions considered.