Effect of surface roughness on step-wise S–N characteristics in high strength steel

Abstract Rotary bending fatigue tests have been performed in laboratory air at ambient temperature using specimens with different surface roughness of very high strength steel (Ni–Cr–Mo steel). The effects of surface roughness on crack initiation mechanism and associated step-wise S – N characteristics were studied. Regardless of surface roughness, step-wise S – N curves were seen. In short life regime where cracks initiated at the specimen surface, fatigue life decreased with increasing surface roughness because surface roughness acted as a small notch. On the contrary, in long life regime where subsurface crack initiation occurred, surface roughness exerted no influence on fatigue life because cracks initiated at an inclusion and grew inside the specimens. A fish-eye was always observed in subsurface crack initiation, which had the same morphological features independent of surface roughness. The transition stress, i.e. the conventional fatigue limit, at which the crack initiation site changed from surface to subsurface was significantly affected by surface roughness and the specimens with rougher surface showed lower transition stress. This was also attributed to the notch effect of surface roughness, which caused crack initiation at the specimen surface more readily even at lower applied stress levels. The transition stress could be predicted reasonably by using the area parameter model in which surface roughness was assumed as a small defect.