Effects of Notch and Surface Roughness on Long Life Fatigue Behaviour in High Strength Steels

摘要

In order to evaluate the effects of notch and surface roughness on fatigue fracture morphology of high strength steels in long life region, cantilever-type rotary bending fatigue tests have been performed using notched specimens with different stress concentration factors, a, of 1.16, 1.51 and 2.0, and specimens with roughened surface of the maximum height, R_v, of approximately 10 mu m, 16 mu m and 19 mu m. The smooth specimen and the notched specimen with alpha=1.16 showed subsurface fracture in long life region. However, subsurface fracture was not seen in the notched specimens of alpha= 1.51 and 2.0 because of much higher maximum stress at the notch root. In the surface fracture region, fatigue strength decreased with increasing stress concentration factor, while in the subsurface fracture region, there was no discernible difference in fatigue strength between the smooth specimen and the notched specimen with alpha=l.16. Furthermore, it was indicated that the high strength steel studied was very sensitive to notch. All specimens with roughened-surface exhibited subsurface fracture where the transition stress at which fracture mode changed decreased with increasing surface roughness. Although fatigue strength decreased with increasing surface roughness in the surface fracture region, it did not depend on surface roughness in the subsurface fracture region. The area parameter model gave a reasonable estimation of the transition stress of the surface-roughened specimens.