PSI - Issue 51

Abdelhak Nehila et al. / Procedia Structural Integrity 51 (2023) 152–159 A.Nehila and W. Li / Structural Integrity Procedia 00 (2022) 000–000

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By substituting the parameters K f = 1.87 and R = ‒1 into Eq. (19), the predicted S-N curve of the notched specimens is given in Fig. 4. As shown in Fig. 4, the predicted S-N curve is in good tendency with the experimental data. Fig. 5 presents a comparison between the predicted and the experimental data, where a good agreement can be seen according to the factor-of-three lines. Thus, the life prediction approaches for the notch specimens and the relevant illustrations on surface failure mechanisms in this work are reasonable.

700

Surface induced failure, R = -1 Predicted curve

600

300 Stress amplitude σ a , MPa 400 500

200

10 3

10 4

10 5

10 6

10 7

10 8

10 9

Number of cycles to failure N f , cycles

Fig. 4. Predicted S-N curve.

10 6

Surface induced failure

10 4 Predicted lives, N pr 10 5

Factor of three

10 3

10 3

10 4

10 5

10 6

Experimental life, N f

Fig. 5. Comparison between predicted and experimental fatigue lives.

4. Conclusion 1. The VHCF tests of notch specimens for the carburized 17CrNi steel were performed under R = ‒1 with the stress concentration factor K t = 1.89 and the fatigue life ranges from 10 4 -10 8 cycles. In the fatigue life of about 10 6 cycles, the S-N curve shows a turning point, which represents the traditional fatigue limit. The fatigue limit is about 200 MPa. 2. The only failure mode occurred for the notch specimen is the surface failure. This is due to notch effect that leads to local stress concentration on the root of the notch, then induces fatigue failure. 3. Based on the small crack theory, the calculation of fatigue notch factor ( K f ) shown that K f is slightly less than the theoretical stress concentration factor ( K t ). Then K f was used to study notch effect and stress ratio effect on fatigue life.