This study investigated ultra-low cycle fatigue (ULCF) characteristics of SN490 steel across specimen shapes and loading histories through axial tension-compression tests with different notch radii. Plastic strain histories were obtained by digital image correlation (DIC). Despite the wide radius range, the test results indicated limited shape sensitivity. A modified Manson-Coffin law incorporating a strain-based function was proposed to predict ULCF life and introduced in finite element method (FEM) as a damage index based on accumulated plastic strain. Additionally, local displacement extracted from DIC–FEM differences was discussed as a complementary indicator of fracture progression in the ULCF regime.
