Effects of notch and surface roughness on long life fatigue behaviour in high strength steels

Abstract

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, α, of 1.16, 1.51 and 2.0, and specimens with roughened surface of the maximum height, R z, of approximately 10μm, 16μm and 19μm. The smooth specimen and the notched specimen with α = 1.16 showed subsurface fracture in long life region. However, subsurface fracture was not seen in the notched specimens of α = 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 α = 1.16. Furthermore, it was indicated that the high strength steel studied had a very high sensitivity to notch. All specimens with roughened surface exhibited subsurface fracture, where the transition stress at which fracture mode changed decreased in the specimens with rougher surface. In the surface fracture region, fatigue strength decreased with increasing surface roughness, while in the subsurface fracture region, it did not depend on surface roughness. The √area parameter model gave a reasonable estimation of the transition stress of the surface-roughened specimens.