PSI - Issue 35

Vera Friederici et al. / Procedia Structural Integrity 35 (2022) 106–114 V. Friederici et al. / Structural Integrity Procedia 00 (2019) 000–000

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Table 3. Paris-law coefficients C and m and calculated threshold values K th for all three heat treatment conditions and different stress ratios. Core Transition Raceway

C 2.877

2.670

3.393

R=0.1

M 8.198E-03

1.491E-02

1.510E-02

K th

7.9

7.6

3.3

C 2.629

2.762

3.940

R=0.3

M 1.999E-02

1.631E-02

8.475E-03

K th

6.3

5.5

3.1

C 2.693

3.053

4.561

R=0.5

M 1.951E-02

1.003E-02

3.736E-03

K th

4.2

4.1

3.1

C 1.40E-03

5.45E-04

1.54E-04

R=-1, prediction R=-1, experiment

M 2.786

3.001

4.186

C 5.68E-04

- -

- -

M 3.033

The obtained SN-curves of both fatigue tests (Figure 2) show the influence of the hardness on the material. The harder the material the higher the endurance limit and the steeper the slope during service life. The results of the rotating bending test show that the orientation of the specimen sampling in the outer bearing ring is not significantly noticeable, so that the specimens of one heat treatment condition can be considered as one data series (Figure 2a). In contrary the results of the push-pull experiment on raceway condition display evident influence of sample orientation (Figure 2b). The reasons for this are the test-related different highly loaded volumes and the elongated expansion of some non-metallic inclusions in the hot-rolling direction of the bearing ring. During rotation-bending only the surface of the specimen is highly loaded whereas in the push-pull samples the whole volume of the cylindrical part of the sample is highly loaded. Since the deformable inclusions have an elongated extension in rolling direction the influence of orientation is more prominent in a larger highly loaded volume.

Fig. 2. (a) SN-curve resulting from rotation-bending experiments of core, transition and raceway condition of samples. (b) SN-curves from push-pull experiments from core and raceway condition of samples. In the displayed SN-curves the 50 % fracture probability is calculated on basis of a Basquin-line. In the region of fatigue strength Weibull distributions were used.