Numerical investigation of extremely low-cycle fatigue behavior in cylindrical steel specimens

In extremely low-cycle fatigue (ELCF) tests using cylindrical specimens, localized necking deformation occurs, resulting in a non-uniform stress and strain state in the gauge section. Consequently, fatigue life is affected by gauge length. This effect impacts not only the fracture life but also the fatigue life up to the onset of strength degradation, which can be defined as the damage-initiation fatigue life. In this study, finite element analysis (FEA) was employed to examine the stress and strain state within a cylindrical specimen under ELCF conditions and investigate the influence of gauge length on the damage-initiation fatigue life. FEA was conducted based on two existing ELCF test methods using 490 N/mm 2 grade steel with different gauge lengths and strain amplitudes ranging from 3% to 12%. Two material constitutive models capable of accurately reproducing the stress–strain behavior under constant-strain-amplitude cyclic loading were employed. At the specimen center—where the fracture initiates under ELCF conditions—the mean strain shifted toward the tensile side as the number of cycles increased and the strain amplitude exceeded the controlled strain amplitude measured between the gauge points. Furthermore, the shift in mean strain and the increase in strain amplitude were more pronounced in specimens with longer gauge lengths. The influence of gauge length on fatigue life can generally be assessed by evaluating the damage-initiation fatigue life using the rain-flow counting method and Miner's linear cumulative damage rule applied to the strain history at the specimen center. • ELCF life in cylindrical specimens under constant strain amplitude loading is studied. • Two gauge length settings are compared using experiment-based FEA. • Two material constitutive models are evaluated: UCDC and UCR models. • Strong influence of gauge length on strain distribution is observed. • Fatigue life is evaluated using rain-flow counting method and Miner's rule.