Notch Sensitivity of the Fatigue Limit in High-Strength Steel

The fatigue limit of a tempered martensitic steel was evaluated in specimens with stress concentration sources, which were introduced with a small drill and focused ion beam (FIB). In a previous study using ferrite-pearlite steel, the fatigue limit of a specimen with a FIB notch was almost the same as the fatigue limit of a specimen with a drill hole, and the non-propagating cracks were found around both shapes of stress concentration sources. However, the fatigue limit of the specimen with a FIB notch was about 100 MPa lower than the fatigue limit of the specimen with a drill hole in tempered martensitic steel. Additionally, the non-propagating crack was observed only in the specimen with a FIB notch. The stress concentration source shapes in both materials are the same, then the difference in fatigue limit stems from the material property. This indicates that there is applicable range where stress concentration source is regarded as a pre-crack, and the range depends on material. At first, the reason for the difference in fatigue limit was discussed in terms of the non-propagating crack. In this part, we discussed non propagating crack phenomenon around drill hole. Secondly, the effect of hardness which caused the difference in fatigue limit was discussed by using an analytical result of Dugdale model. Finally, we inferred the notch sensitivity from the propagation by deformation at crack tip. After that, we concluded that the propagation by fracture at crack tip is also important factor in analyzing notch sensitivity.