PSI - Issue 68

Mirko Teschke et al. / Procedia Structural Integrity 68 (2025) 936–941 M. Teschke and F. Walther / Structural Integrity Procedia 00 (2025) 000–000

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LOF defects and rarely GP, the HIP changes the mechanism to GP and selected MS failures. This shows that the LOF defects can be closed by the HIP and do not reopen under cyclic stress, even at higher test temperatures. In Fig. 5b, the data points are color-coded according to the defect size. This shows that the defect size has a very large influence on the fatigue strength, as lower defect sizes lead to a significant increase in fatigue life. This explains the large differences in the fatigue strength of the three material states, as these can be correlated with the defect size of the fracture-inducing defect. It demonstrates that the S-N diagram is unsuitable for inhomogeneous materials, as it cannot consider the influence of defects on fatigue behavior.

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b)

Fig. 5: a) S-N (Woehler) curves with defect types and b) additional color-coded defect size.

The Shiozawa representation was therefore chosen. Fig. 6a shows the cyclic SIF according to Eq. (1) calculated vs. the number of cycles to failure normalized by the defect size. With the help of this representation, the entire defect dependent fatigue behavior of the three material states can be described using one straight line. This representation is valid for the three material states and the resulting defect size distribution as well as the different failure mechanisms. The equation can be described quantitatively using Eq. (2).

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b)

Fig. 6: a) Shiozawa curve and b) defect-based (artificial) S-N curves for surface defects.