The loading-path dependency of fatigue life improvement in SMAT-processed AA7075: From torsion to combined tension–torsion

This study evaluates the multiaxial fatigue behavior of an AA7075 aluminum alloy processed by Surface Mechanical Attrition Treatment (SMAT). Fatigue performance was systematically characterized under pure torsion, uniaxial tension-compression, and combined loading conditions. Specimens with identical geometry were used to eliminate biases caused by size effects. A significant “performance paradox” was identified: while SMAT increased fatigue life tenfold and fivefold under torsion and uniaxial loading, respectively, it had a detrimental effect under combined tension–torsion loading. This study demonstrates that the effectiveness of gradient reinforcement is strongly path-dependent by applying post-SMAT electropolishing and annealing treatments to decouple surface and microstructural effects. Combining fractographic analysis with the Findley critical plane criterion reveals that, under multiaxial loading paths, the combined effect of elevated normal stress σ n,max , activation of SMAT-induced surface micro-defects, and accelerated relaxation of compressive residual stresses increases the local damage parameter beyond that of the untreated material. This ultimately leads to premature failure. These findings provide a comprehensive framework for understanding how multiaxiality of loading paths governs the effectiveness of surface-engineered gradient nanostructures.