Analysis of Stress/Strain Concentration at Nonmetallic Inclusion and S-N Curve of Ultra-long Fatigue Failure of High-strength Steels.

Abstract

It has been reported that, according to some rotating bending fatigue tests, the S-N curve for high-strength steels consists of two straight lines. One corresponds to the line for short life at high stress level due to fracture from a surface origin, and another corresponds to the line for ultra-long fatigue life at low stress level due to fracture from internal nonmetallic inclusions. In this study, stress/strain concentration around inclusions is analyzed by FEM to seek a possible mechanism for this particular S-N curve behavior of ultra-long fatigue failure of high-strength steels for rotating bending fatigue test. A special focus is put on the growth of plastic zone around an internal inclusion and near surface with increasing applied nominal stress. Considering the different scale of plastic zone near surface and around an internal inclusion, it can be concluded that, in case of rotating bending fatigue test, fracture is likely to occur from a surface origin at high stress level and from an internal inclusion at low stress level. However, this characteristic depends on several factors such as specimen size, residual stress, cyclic yield stress, size and location of the crucial inclusion.