Morphology of Step-Wise S-N Curves Depending on Notch and Surface Roughness in High Strength Steel

It has been indicated that subsurface fracture occurred at stress levels below the conventional fatigue limit in long life region more than 10

7

cycles, particularly in high strength steels (Dahlberg [

1

], Naito

et al.

[

2

], Nakajima

et al.

[

3

]). A notable feature of subsurface fracture is the presence of a fish-eye with an inclusion near its centre, and the formation mechanism of fish-eye has not been fully understood. High strength steels usually possess very high sensitivity to notch and surface defect. Therefore, it is very important to understand the effects of notch and surface roughness on crack initiation and associated

S-N

characteristics. In the present study, rotary bending fatigue tests have been conducted in laboratory air at ambient temperature using notched specimens with different stress concentration factors and specimens with roughened surfaces in high strength steel. The effects of notch and surface roughness on step-wise

S-N

characteristics, particularly on the transition stress, were discussed.