A B S T R A C T 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 nearsurface 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. A 0 = reference surface d = net diameter of the specimens E = elastic modulus f = load frequency F = longitudinal load, distribution function K t,σ = concentration factor for normal stress K t,τ = concentration factor for shear stress m = exponent of the Weibull distributionCorrespondence: H. Bomas.
Fatigue strength is one of the key properties in the practical use of ultrafine grained steels. Fatigue tests were conducted on notched specimens by conventional electromagnetic resonance fatigue testing machines. The electromagnetic resonance fatigue testing was carried out at 150 Hz up to 10 7 cycles. The investigated steels had different levels of carbon, 0.15 wt%, 0.30 wt% and 0.60 wt% with tensile strengths of 850 MPa, 950 MPa and 1 105 MPa, respectively. With increasing in carbon content, the tensile strength increased and the total elongation decreased. The notched specimens never showed internal fracture and showed a clear fatigue limit. The notch fatigue limit increased with an increase in tensile strength from 850 MPa to 970 MPa, when the carbon content was 0.15 and 0.30 wt% with microstructures consisting of ultrafine ferrite grains and cementite particles. On the other hand, when the carbon content was 0.60 wt%, the notch fatigue limit decreased, though tensile strength increased to 1 105 MPa. Retained pearlite grains were observed in 0.60 wt%C steel in addition to ultrafine ferrite grains and cementite particles. These retained pearlite grains which were coarse and high angle grain boundary poor regions were attributed to the lower notch fatigue limit.
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