Notch effect on S-N curve

This actually means that if a notch sensitivity factor is introduced also for the fatigue finite life that, usually indicated as Idf, it will always be Idf kf reaching eventually the unity at about 10 cycles. Heywood [18] has shown that Idf never becomes exactly equal to one, but it approaches the unity the more the alloy is soft and ductile. He, then, introduced a notch sensitivity factor 4 at 1,000 cycles analogous to the q index already seen in Sect. 7.2 for the fatigue limit, so that it is 

Notch effect on the entire S-N curve for a high strength steel type 4,340 (modified from 

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