Effect of Repeated Annealing on the Fatigue Life in Extremely Low Cycle Fatigue

The effect of repeated annealing during the course of a fatigue test on a fatigue behavior was studied on two kinds of low-carbon steel having fine-and coarse-grained microstructures, paying special attention to the relationship between the fracture modes and fatigue life in the extremely low-cycle fatigue regime (Nf≦l0²). Results show that in a coarse-grained steel, (i) repeated annealing has the beneficial effect of repairing the damage inside the material; (ii) in this case, final fracture occurs in a surface fracture mode instead of internal fracture mode which normally appears in extremely low cycle fatigue regime. Such a change occurs after strain cycles obtained by the extrapolation from the fatigue life data for a surface fracture mode at low plastic strain range; and (iii) these findings can be clearly explained by the concept of “the competition of two failure limit lines which correspond to each fracture mode”.