The shape of small surface cracks and their growth rate data are reported for the smooth axial specimens of annealed S40C steel subjected to alternating two-load level push-pull fatigue. Alternation of high and low load cycling caused different appearances on the fracture surface, from which the crack shape at each loading period can be determined. Surface cracks, after their surface length exceeding 0.2mm, were approximately semi-elliptical and kept the ratio of the depth to half the surface length to be 0.7∼0.9 during growth. The crack growth rate in the depth direction was well correlated down to 10-10m/cycle with the stress intensity range ΔK. On the other hand, by using the strain-based intensity range ΔKε, the growth rate at both the elastic and the fully-plastic regions was well represented by a single relation. From the quantitative comparison between ΔJ, proposed by Dowling, and E(ΔKε)2, it is shown that E(ΔKε)2 gives a rough approximation of ΔJ and hence ΔKε may be used as a correlation parameter for fatigue crack growth even under fully-plastic conditions.
