Notch fatigue crack initiation in untreated and heat-treated additively manufactured AlSi10Mg aluminium alloy under in-phase bending-torsion with varying stress ratios

• Fatigue behaviour of UT and HT L-PBF AlSi10Mg notched hollow bars is investigated. • Low-temperature stress relief reduces life scatter but does not affect crack initiation life. • Crack initiation is governed by B/T ratios and predicted by principal stress field. • Crack initiation life is predicted using the BCM model combined with TCD and ESED. • Fatigue failure is dominated by surface and near-surface manufacturing-related defects. The present paper investigates the fatigue behaviour of hollow round bars with a lateral hole fabricated by additive manufacturing using an AlSi10Mg aluminium alloy and subjected to in-phase bending-torsion loading with varying stress ratios. Both untreated and low-temperature stress-relieved material conditions were investigated. After fatigue testing, the fracture surfaces were examined by scanning electron microscopy to identify the dominant fatigue failure mechanisms. Fatigue failure was predominantly caused by surface and near-surface manufacturing-related defects. The heat-treated condition exhibited reduced scatter in fatigue life, but the fatigue crack initiation life was not significantly affected by the heat treatment. The crack initiation sites, crack directions in the early stage of growth, and crack locations in the hole wall were governed by the B/T ratio and were successfully predicted numerically based on the principal stress field in the vicinity of the notch. Crack initiation life was also successfully predicted using the Basquin-Coffin-Manson model combined with the Theory of Critical Distances to account for the notch effect, and the equivalent strain energy density concept for notch plasticity correction.