Crack initiation and endurance limit of a hard steel under multiaxial cyclic loads

The endurance limit and the mechanisms of fatigue crack initiation in the high-cycle regime were investigated using round specimens of the bearing steel SAE 52100 in a bainitic condition under longitudinal forces, torsional moments and combinations of these loads. Three specimen types were examined: smooth specimens and specimens with circumferential notches with radii of 1.0 and 0.2 mm. The surfaces of the specimens including the notches were ground resulting in compressive residual stresses in the near-surface region. The influence of mean and multiaxial stresses on the endurance limit can be understood by consideration of crack initiation mechanisms and micromechanics. Crack initiation occurred at oxides, carbonitrides and at the surface. The oxides had little adhesion to the bainitic matrix and acted like pores. The carbonitrides were well bonded to the matrix and caused stress concentrations due to their higher elastic modulus when compared to that of the matrix. The mechanisms of crack initiation could be related to the load type: loads with rotating principal stresses cause more damage for nitrides than for oxides. Increasing maximum stresses are more dangerous for nitrides than for oxides, and damage the surface more than the nitrides. Normal stresses produce more damage for oxides than shear stresses. The endurance limits were calculated by means of an extended weakest-link model which combines volume and surface crack initiation with individual fatigue criteria. For volume crack initiation, the criterion of Dang Van was used. For the correct description of the surface crack initiation, a criterion proposed by Bomas, Mayr and Linkewitz was applied. With this concept, a prediction of the endurance limit is possible. The influence of the notch geometry on the endurance limit is well characterized.