Influence of the load ratio on the threshold stress intensity factor range for heavily drawn steel wires

The fatigue behavior of heavily drawn steel wires with a diameter of 175 μm and total strain of 3.5 has been studied. A new approach is used to estimate the threshold stress intensity factor range for crack growth (ΔKth,R) which is based on fatigue crack initiations at internal non-metallic inclusions. At these internal crack initiations a characteristic area is formed around the inclusion (in literature this region is called optical dark area, granularly bright facet or facet). This area corresponds to the short crack growth regime. The transition between short and long crack growth, which can be seen clearly by fractography, is used to estimate ΔKth,R.ΔKth,R is estimated for different stress amplitudes (σa) and load ratios (R). It is observed that ΔKth,R is independent of σa and that the fatigue crack growth is Kmax controlled for R < 0.15 and ΔK controlled for R > 0.15. The two asymptotes of the (ΔK, Kmax) graph, ΔKth∗ and Kmax∗, are calculated to be 3.82 ± 0.09 and 4.47 ± 0.16 MPa √m respectively.