In the previous paper, the fatigue strength data of notched specimens of steels had been analyzed by distinguishing the fatigue limit for crack initiation, σ
w1, and that for fracture, σ
w2, with the aid of Nishitani's critical notch radius ρ
0, at the branch point,
i.e., at the critical point where a non-propagating crack appears.
In this study,
S-
N (P-N, S-T) data of rotary bending tests of sharp (notch radius ρ_??_ρ
0), deep (notch depth
t>>√ρ
t/5), notched specimens of steels were extracted from JSMS Data Book and analyzed by fitting a bi-linear type
S-
N curve comprising of inclined and horizontal straight lines on a Log-Log diagram, using Nishijima's method.
By taking the stress intensity factor
K of the circumferential crack with notch depth
t instead of the stress σ in
S-
N curves, and by using the following correction factor κ for low strength steels,
i.e.κ=σ
B/σ
C for tensile strength σ
B<σ
C=940MPa,
κ=1 for σ
B≥σ
C,
S-
N Data of sharp, deep notched specimens of both low and high stength steels could be fitted by a bi-linear type curve on a Log (
K/κ)-Log
N diagram (Fig. 4(e)), with the coefficient of variation 14.0%, while the coefficient of variation for the Log (σ/σ
B)-Log
N diagram (Fig. 3(c)) of corresponding smooth specimens was 11.8%.
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