High-cycle fatigue crack initiation and propagation behaviour of high-strength spring steel wires

An attempt has been made to characterize high-cycle fatigue behaviour of hi gh-strength spring steel wire by means of an ultrasonic fatigue test and an alytical techniques. Two kinds of induction-tempered ultra-high-strength sp ring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa w ere used in this investigation. The fatigue strength of the steel wires between 10(6) and 10(9) cycles was determined at a load ratio R = -1. The experimental results show that fatig ue rupture can occur beyond 10(7) cycles. For Cr-V spring wire, the stress- life (S-N) curve becomes horizontal at a maximum stress of 800 MPa after 10 (6-) cycles, but the S-N curve of the Cr-Si steel continues to drop at a hi gh number of cycles (>10(6) cycles) and does not exhibit a fatigue limit, w hich 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 t echniques were developed to better understand and predict high-cycle fatigu e 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.