Fatigue life evaluation based on elastic modulus degradation behavior in LCF testing

This study examined the degradation behavior of the elastic modulus during unloading in both extremely low-cycle fatigue (ELCF) and low-cycle fatigue (LCF) tests, aiming to gain deeper insight into damage progression in structural steel under cyclic loading. Uniaxial tension–compression cyclic loading with constant total strain amplitudes, ranging from 1 % to 9 %, was applied to two types of structural steel: 490 N/mm² grade steel that exhibited cyclic hardening and 550 N/mm² grade steel that exhibited cyclic softening. Finite element analysis (FEA) was also conducted to explore the influence of changes in specimen geometry on the unloading elastic modulus under cyclic loading. The experimental results showed that for both steels, in the LCF regime, the unloading elastic modulus remains relatively constant until it suddenly degrades owing to fatal macrocrack propagation. Contrastingly, in the ELCF regime, the unloading elastic modulus gradually decreases from the first cycle, primarily owing to the necking phenomenon, as confirmed through FEA, followed by rapid degradation when a fatal macrocrack propagates. As the macrocrack nucleates and grows, both the unloading elastic modulus and tensile peak stress (defined as the stress at peak strain in each cycle under tension) decline sharply, reflecting the reduction in the effective resisting cross-sectional area. Thus, the fatigue life, up to the point where rapid macrocrack propagation begins, can be evaluated by tracking either the unloading elastic modulus or tensile peak stress.