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The effect of stress ratio R on the threshold stress intensity range ΔK_<th> for fatigue crack growth has been studied in several steels. ΔK_<th> decreased with increasing R, and the relation was generally expressed as ΔK_<th>=(1-α・ R)・ΔK_<th0> for R≤R_c =(1-α・R_c)・ΔK_<th0>=const. for R>R_c where R_c depends only on the material, and α and ΔK_<th0> are functions of the specimen geometry as well as of materials. The effective value of the threshold stress intensity range ΔK_<effth> was 2 to 4 MPa√<m> for all materials and stress ratios examined. The constant value of ΔK_<th> for R>R_c in the above equation was also 2 to 4 Mpa√<m> which is nearly equal to ΔK_<effth>. The crack closure is concluded to be mainly responsible for the effects of stress ratio and material on ΔK_<th> in steels.
