PSI - Issue 31

Vera Friederici et al. / Procedia Structural Integrity 31 (2021) 8–14

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V. Friederici et al. / Structural Integrity Procedia 00 (2019) 000–000

To be able to use this approach, our data had to be fitted with a Paris equation using an equal m for every R ratio. Normally, seen in Fig. 3, m varies slightly with R. The resulting crack propagation curve for R = -1 was then calculated using the fatigue crack driving force parameter K * (Kujawski, D., 2001; Dinda, S., 2004): ∆ � ∗ �� � ��� � �� ��� � � �� ���� ∗ � � ��� � � ∗ ∆ ����� ��� � � ∆ � � � (4) The value α is determined from the slope of the log-log plot of K max vs. Δ K + (positive part of the applied stress intensity factor) for a given da / dN = constant. Using this approach crack propagation curves for different R ratios can be predicted. In Fig. 5 the predicted curves for R = 0 and R = -1 for the hardened material are shown. The tendency that the curves move to higher Δ K values with decreasing R corresponds well with the expectation.

Fig. 5. Prediction of crack propagation curves for R = 0 and R = -1 for the hardened material.

Fig. 6. S-N-curve for the core and hardened material.