Ultrasonic very-high-cycle fatigue of PBF-LBed AlSi10Mg: effects of specimen size, control volume and defect statistics

The specimen-size effect in very-high-cycle fatigue (VHCF) of additively manufactured (AM) metals arises from the statistical nature of defect-controlled crack initiation. In this work, the ultrasonic VHCF behavior of powder bed fusion–laser beam–fabricated (PBF-LBed) AlSi10Mg was examined using axially loaded specimens with different gauge sizes, representing distinct control volumes. A pronounced negative size effect on VHCF strength was observed, despite identical nominal stress amplitudes. Fatigue cracks consistently initiate from internal and subsurface lack-of-fusion defects surrounded by fine granular areas (FGAs), indicating that the size effect results from statistical scaling of critical defect populations rather than a change in initiation mechanism. Weibull-based probabilistic S–N curves accurately describe fatigue life scatter, and a control-volume-based weakest-link model provides physically consistent and conservative descriptions of size-dependent VHCF behavior. These results demonstrate that the VHCF size effect in PBF-LBed AlSi10Mg is a statistical-mechanical phenomenon governed by defect populations within the stressed volume, offering a mechanics-informed basis for fatigue life assessment and size extrapolation of AM components.