The effect of stress ratio R on the threshold stress intensity range ΔKth for fatigue crack growth has been studied in several steels. ΔKth decreased with increasing R, and the relation was generally expressed as
ΔKth=(1-α·R)·ΔKth0 for R≤Rc
=(1-α·Rc)·ΔKth0=const. for R>Rc
where Rc depends only on the material, and α andΔKth0 are functions of the specimen geometry as well as of materials. The effective value of the threshold stress intensity range ΔKeffth was 2 to 4 MPa√m for all materials and stress ratios examined. The constant value of ΔKth for R>Rc in the above equation was also 2 to 4MPa√m, which is nearly equal to ΔKeffth. The crack closure is concluded to be mainly responsible for the effects of stress ratio and material on ΔKth in steels.