Defect-interaction-driven fatigue-life scatter in additively manufactured metals: a mechanistic framework

Fatigue-life scatter in additively manufactured (AM) metallic components remains a major obstacle to their reliable use in structural applications. This study presents a mechanistic framework for investigating the origins of fatigue-life scatter through the construction of physically representative fatigue-critical crack geometries from the size, morphology, spatial distribution, and interaction of process-induced defects. The framework incorporates defect-interaction mechanisms derived from experimentally observed defect-linking phenomena in both radial and circumferential directions, enabling fatigue-critical crack configurations to be identified from X-ray microtomography (XCT) scans of intact specimens. Three distinct crack-formation scenarios were identified: (a) isolated surface defects, (b) radial interaction between surface and subsurface defects, and (c) circumferential interaction between neighbouring surface defects. Among these, circumferential defect interaction was found to be the most detrimental configuration, producing the shortest fatigue lives and the largest fatigue-life scatter. A stress-dependent scatter width following a power-law relationship was identified, establishing a mechanistic link between applied stress and defect-controlled fatigue-life variability. The results demonstrate that fatigue-life scatter in AM metals is neither purely stochastic nor an intrinsic material property, but emerges from defect configurations that govern the formation of the initial fatigue-critical crack. The findings establish a mechanistic basis for identifying failure-critical defect interactions and suggest that process optimisation strategies should target not only defect size and density, but also the spatial arrangement of defects that governs fatigue-critical crack formation and fatigue-life variability. The proposed framework supports the development of physics-informed defect-acceptance criteria, reliability-based fatigue design methodologies, and failure-prevention strategies for structural AM applications.