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

The high and very cycle fatigue (VHCF) properties of ultrahigh-strength Ck45M steel processed by thermomechanical rolling integrated direct quenching were investigated. S–N tests with smooth small drilled holes containing specimens as well near-threshold crack growth measurements performed up to 2 × 1010 cycles using ultrasonic-fatigue testing technique. strength is mainly determined the size nonmetallic inclusions. For surface defects larger than 80 μm, limit can be correlated a constant threshold-stress intensity factor. -parameter model adequately predicts for internal sizes between 30 μm. VHCF failures from smaller occur at stress amplitudes below predicted limit. long-crack threshold in ambient air close effective In optically dark areas interior inclusions, cracks grow mean propagation rates 10−15 m/cycles.