Background <p>Fatigue crack nucleation in as-cast AA6082 remains insufficiently resolved at the microstructural scale, particularly regarding how localized strain and slip-system interactions at grain boundaries and triple junctions govern nucleation.</p> Objective <p>This study aims to document the local conditions that lead to crack nucleation, to identify microstructural regions of strain localization, and to evaluate their role in the earliest stages of fatigue damage.</p> Methods <p>Full-field digital image correlation (DIC) was employed under cyclic loading to quantify localized deformation, while complementary electron backscatter diffraction (EBSD) provided crystallographic context through grain orientation, Schmid-factor mapping and local strain incompatibilities. The analysis incorporated 3D representations and angular metrics to evaluate geometric (in)compatibility among potentially active slip systems across neighboring grains, with emphasis on triple junctions.</p> Results <p>Regions of elevated local deformation frequently coincided with grains of high Schmid factor; however, fatigue crack nucleation occurred only when <i>geometric compatibility</i> was lacking among the possible active slip systems. An extensive study of a fatigue nucleation site reveals that the possible active slip systems in the surrounding grains at the triple junction <i>lack geometric compatibility</i>.</p> Conclusions <p>By combining DIC and EBSD, the proposed method establishes a practical framework for describing microscale deformation behavior, clarifies the mechanisms of early crack nucleation in as-cast AA6082, and provides a generalizable approach for other alloys. The approach also enables a statistical perspective based on the probability of critical, geometrically incompatible configurations, informing microstructure-aware fatigue design.</p>