Effects of Strain Rate on the Very High Cycle Fatigue Properties of Low Alloy Steel SFVQ1A

Uni-axial fatigue tests up to 109 cycles were conducted at two cyclic frequencies (10 and 250 Hz) under several stress ratios (R = −1, 0.1, and 0.5) to investigate the effects of strain rate on the very high cycle fatigue properties of SFVQ1A, which is equivalent to ASTM A508 Cl.3. Longer fatigue lives and 5% higher fatigue limit were measured at 250 Hz than those at 10 Hz for R = −1. Under R = 0.1 and R = 0.5, however, the fatigue properties did not differ between 10 and 250 Hz. Observations of fracture surfaces clarified that all fractures under R = −1 and R = 0.1 were caused by a general crack growth process from non-metallic inclusion(s) at the specimen surfaces. In contrast, the entire fracture surface under R = 0.5 was covered with a dimpled pattern. The fatigue mechanism was considered to be due to ductility exhaustion through ratcheting behavior under high mean stress. The longer fatigue lives and larger fatigue limit at 250 Hz under R = −1 was explained by the increase in crack growth resistance at a high strain rate based on crack growth behaviors and the da/dN-ΔK relation. No differences in fatigue properties between different frequencies under R = 0.1 and 0.5 were likely caused by the negligible effect of strain rate compared with the increase in material strength during fatigue loading, which resulted from strain hardening induced by maximum cyclic stress that was larger than yield stress.