Low Cycle Fatigue Response and Cyclic Life Prediction Model of Ultra-Pure 26Cr2Ni4MoV Steel Under Strain- and Stress-Controlled Loading

The low cycle fatigue (LCF) behavior and stress–strain responses of ultra-pure 26Cr2Ni4MoV steel were studied under symmetric strain/stress-controlled cycling (R = −1) at room temperature. The characteristic features of cyclic stages—saturation, stable descent, and rapid descent—were identified in both loading modes. Additionally, tension–compression asymmetry (TCA) significantly affected cyclic resistance, with ratcheting strain in stress-controlled cycling reducing the fatigue life. In contrast, strain-controlled conditions exhibit minimal cyclic asymmetry, indicating a negligible mean stress effect on fatigue life. Significant cyclic softening was observed in both control modes. Quantitative analysis using the cyclic softening factor ( ${\mathrm{SF}}_{\mathrm{\epsilon c}/\mathrm{\sigma c}}$) showed it was independent of strain amplitude but correlated with stress amplitude. Furthermore, a novel energy-based Manson–Coffin (EBMC) model was proposed for unified fatigue life prediction, incorporating strain energy and mean stress effects. The EBMC model demonstrated significantly improved prediction accuracy and validation compared to the hysteresis loop strain energy, Smith–Watson–Topper, and Basquin models.