Influence of small defects and nonmetallic inclusions on the high and very high cycle fatigue strength of an ultrahigh‐strength steel

Abstract The high and very high cycle fatigue (VHCF) properties of ultrahigh‐strength Ck45M steel processed by thermomechanical rolling integrated direct quenching were investigated. S–N tests with smooth and small drilled holes containing specimens as well as near‐threshold fatigue crack growth measurements were performed up to 2 × 10 10 cycles using ultrasonic‐fatigue testing technique. The fatigue strength of smooth specimens is mainly determined by the size of nonmetallic inclusions. For surface defects larger than 80 μm, the fatigue limit can be correlated with a constant threshold‐stress intensity factor. The ‐parameter model adequately predicts the fatigue limit for internal defects and for surface defects with sizes between 30 and 80 μm. VHCF failures from smaller surface defects occur at stress amplitudes below the predicted fatigue limit. The long‐crack threshold in ambient air is close to the effective threshold stress intensity factor. In optically dark areas at interior inclusions, cracks grow at mean propagation rates of 10 −15 m/cycles.