Fatigue initiation in C35 steel: Influence of loading and defect

The aim of this work is to study the influence of both defect and tension and torsion loading on stress number of cycles (S–N) 25 curves for C35 steel. A spherical artificial defect has been machined at the surface of gauge length of fatigue samples. The crack initiation mechanisms have been identified based on several observations on Scanning Electron Microscope (SEM) at different stage of fatigue life. The initiation crack length definition is proposed for defect free and defective material. For defect free material, torsion loading allows relatively earlier initiation compared to tension loading. In the case of defective material, it is observed that, for the both types of loading, initiation fatigue life is not negligible by comparison to total fatigue life. It is also concluded that defects are much more deleterious to fatigue life in the range of high cycle fatigue regime. However, for the limited fatigue lives (between 104 and 105 cycles), the defect free and defective material S–N curves are relatively comparable.