Effects of Small Surface Defect on Fatigue Limit of Spring Steel

Abstract

Effects of small surface defect on bending fatigue limit of spring steel (JIS-SUP9A) was investigated. Bending fatigue tests were carried out under stress ratio of R = -2, -1, 0 and 0.4 for smooth specimens and specimens having a semi-circular surface slit with the depth of 30, 60 and 400 μm. The fatigue test results revealed that the maximum defect size which does not affect the fatigue limit shows stress ratio dependency. Further, the prediction of fatigue limit of smooth specimen and threshold stress intensity factor range at arbitrary stress ratio became practical. Fatigue limits of the specimens having a slit with depths between 30 μm and 400μm were in good agreement with the values predicted by the new fatigue limit prediction equation.