13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Furthermore, different approaches were applied to evaluate the fatigue strengths at the investigated load ratios: Because at load ratios R > 0 the database is insufficient for statistical evaluation of fatigue strength, a pragmatic approach is used. As the flat slope of the S-N curves (Fig. 3) is a convenient precondition for assessing the fatigue limit, the fatigue strength is defined as the highest stress amplitude where specimens still not failed. Hence, the following fatigue strengths were determined: 350 MPa for R = 0.1, 230 MPa for R = 0.5 and 145 MPa for R = 0.7. For R = -1, the arcsin√P-method [21] was applied for calculating the fatigue strength, taking all data points in the vicinity of the run-out specimens into account. The resulting fatigue limit for R = -1 is a stress amplitude σa of 495 MPa. 3.2 Fractographiy-based evaluation of fatigue life In the next step, crack initiating defects were identified and analyzed using SEM and EDX. Up to about 107 cycles, most fractures initiated at persistent slip bands (PSBs) or at surface inclusions. Both HCF fracture types were observed for fatigue lives up to 4·107 cycles at all investigated load ratios. A more detailed investigation of the fracture surfaces is given in [19]. Figure 4. Fracture surfaces of typical fish-eye cracks: (a, b) σa = 550 MPa, R = -1, Nf = 4.66·10 7; (c, d) σ a = 370 MPa, R = 0.1, Nf = 1.24·10 8; (e, f) σ a = 250 MPa, R = 0.5, Nf = 1.21·10 9; (g) σ a = 155 MPa, R = 0.7, Nf = 2.35·105; (h) σ a = 150 MPa, R = 0.7, Nf = 2.28·10 7. In the VHCF-regime, i.e. at fatigue lives above approximately 4·107 cycles, fish-eye cracks, which are characteristic for VHCF failure of high strength steels [5], are found in all fracture surfaces (Fig. 4). The frequently observed “optical dark areas” (ODAs) [22-25] are difficult to identify in this steel.
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