Fatigue Limit and Crack Propagation Behavior of F690 Marine Steel inAir under Different Stress Conditions

The high frequency fatigue test and fatigue crack growth rate test based on DC voltage drop were conducted on F690 steel in air under different stress conditions. Combined with fracture surface analysis, effects of stress amplitude and stress ratio on fatigue properties and initiation and growth of cracks of F690 steel were studied and compared. The results show that the conditional fatigue limit of F690 steel at stress ratios of -1, 0.1, 0.2, 0.3 was 313, 264, 250, 246 MPa, respectively, which decreased with increasing stress ratio. The fatigue cracks in F690 steel propagated with band mechanism and the mode of fracture was transgranular fracture. As the stress amplitude increased, the fatigue crack spacing and secondary crack size increased, and the fatigue crack growth rate also increased. With increasing stress ratio, the fatigue crack growth rate of F690 steel increased and changed significantly at the middle and later stages of crack growth, and the fatigue crack growth threshold decreased.