Defect sensitivity of high-strength steel 42CrMo4: The role of crack initiation and non-propagation defining the fatigue limit

• Distinction between crack- and notch-like defects is of utmost importance. • Notch-fatigue concepts can be applied to predict crack initiation at small defects. • The critical notch-root radius is a material constant even for small defects. • The critical notch-root radius for quenched-tempered 42CrMo4 steel is around 80 µm. In the defect-based fatigue framework, there are still open questions regarding the role of crack initiation. The fatigue limit of small defects is commonly explained through crack non-propagation, although direct evidence is not always given to support this. In the present study, the defect sensitivity of quenched and tempered 42CrMo4 steel was systematically investigated using specimens containing artificially introduced defects of various sizes and shapes. The fatigue limit is defined by crack non-propagation for defects with a notch-root radius below a critical value of ρ c ≈ 80 µm. On the other hand, crack initiation determines the fatigue limit of blunter, notch-like defects. It is shown that classical notch-fatigue concepts can be applied to predict crack initiation at small defects. For the sake of transferability of experimental results and the development of physics-based fatigue models, distinguishing between the two fatigue limit mechanisms is critical for the safe operation of machine components.