Technical note High‐cycle fatigue crack initiation and propagation behaviour of high‐strength sprin steel wires

An attempt has been made to characterize high‐cycle fatigue behaviour of high‐strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction‐tempered ultra‐high‐strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation. The fatigue strength of the steel wires between 106 and 109 cycles was determined at a load ratio R = −1. The experimental results show that fatigue rupture can occur beyond 107 cycles. For Cr–V spring wire, the stress–life (S–N ) curve becomes horizontal at a maximum stress of 800 MPa after 106 cycles, but the S–N curve of the Cr–Si steel continues to drop at a high number of cycles (>106 cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high‐cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high‐cycle regime for the steels studied in this investigation.