Heavily drawn steel wires, used in applications such as radial tires, have to withstand a high number of fatigue cycles, typically > 10 8 . Such steel wires have diameters as small as 175ยตm. High cycle fatigue failure in such wires is initiated by either surface defects or internal non-metallic inclusions. The former leads to shorter fatigue lives than the latter.
A methodology has been developed to measure the fatigue threshold Stress Intensity Factor (SIF) for long cracks in these heavily drawn steel wires. Then, the SIF's of the non-metallic inclusions, at which the high cycle fatigue cracks initiate, were compared to the threshold value for long crack growth. It was shown that the defect size that should be used to calculate the SIF is larger than the inclusion size itself. This is validated using Scanning Electron Microscopy in combination with a Focused Ion Beam, proving that the region around non-metallic inclusions is characterized by alterations in the microstructure. In this intermediate region between the inclusion and the start of the long cracks, the crack growth rate is very slow and can be described by a Paris' law like relationship. Hence, a bi-linear Paris law has been proposed, enabling to predict the fatigue life of heavily drawn steel wires containing any size of inclusions.