Effects of the shape of small flaws and damage due to a focused ion beam on the fatigue strength characteristics of annealed medium‑carbon steel

Fatigue tests and hardness tests were conducted in order to clarify the effects of the shapes of small flaws and damage caused by a focused ion beam (FIB) on the fatigue strength characteristics of annealed medium‑carbon steel. The notch root radius of the flaw at the branch point dominates the type of the fatigue limit and was found to be approximately 50 μm, a value that was significantly smaller than the 500 μm of two-dimensional notches in steel. The effect of the FIB-damaged layer on fatigue crack initiation from the FIB-milled notch in the steel was negligible. Moreover, the present study indicated that the effect of the FIB-damaged layer on the fatigue crack initiation at the FIB-milled notch could be evaluated simply by using a hardness test for the other materials and FIB conditions.