The influence of high R ratio on notched fatigue behaviour of 1045 steel with three different heat treatments

Three 1045 steels with hardness levels of R c=10, 37, and 50 were tested under cyclic axial load control conditions using mildly notched specimens, K t=1.65, at high R ratios of 0.8 and 0.9. The notched ultimate tensile strengths, S un, for the three steels were greater than the unnotched ultimate tensile strengths, S u. This allowed values of nominal maximum stress, S max, and mean stress, S m, to exceed S u in most tests. Thus, fatigue limits based on S max were higher than S u in 5 of the 6 test conditions. S– N f curves were very flat in 5 of the 6 test conditions with appreciable scatter. With S max and S m> S u in most tests, usual S– N f fatigue life models involving S u could not be used. Replacing S u with S un, allowed calculations, but these were completely inaccurate. Local strain-life, ε– N f, models were also completely inaccurate for these high R ratios. R c=10 specimens failed by cyclic creep/ratcheting from internal microvoid coelescence and not from fatigue. R c=37 specimens failed by fatigue from surface thumbnail cracks, but were influenced by cyclic creep/ratcheting. R c=50 specimens failed by brittle fracture from minute surface fatigue cracking without cyclic creep/ratcheting. In design situations at long life, usual S– N f models with these materials and high R ratios would restrict S max to levels well below the experimental fatigue limits resulting in very conservative results.