PSI - Issue 77

Klusák Jan et al. / Procedia Structural Integrity 77 (2026) 432–439 Author name / Structural Integrity Procedia 00 (2026) 000–000

436

5

4. Critical distance determination 4.1. Theoretical approach To determine the critical distance for materials subjected to cyclic stress, S-N curves of smooth samples and samples with a model notch are used. The principle of critical distance determination is demonstrated in Fig. 4.

σ a

n

σ y

smooth

n ( l

m )

σ y

s

σ a

n

σ a

n

σ a

notched

2

x

N f

l m

N f

Fig. 4. Critical distance determination The nominal stress amplitudes of the smooth a s and notched an specimens correspond to particular number of cycles to failure f . The stress distribution in front of the notch is calculated for the nominal stress amplitude an . Finally, the critical distance is determined from the stress distribution of the notched specimen, such that the average stress over the critical distance is equal to the nominal stress amplitude of the smooth specimen. y ���� ( m )= (1) 4.2. Results The critical distances presented here are based on data measured for smooth and model notch specimens. The data for the smooth specimens were divided between the polished, and as-machined specimens. Each notch radius was considered as the model notch, and the critical distance was determined based on each of them. The values of the critical distances depend on the number of cycles to failure. The dependence of on f is shown in the following graphs in Fig. 5 for the three studied steels and based on the S-N curve for polished smooth samples. In this case, the critical distance lies between zero and 1 mm for S460 NL and S690 QL. Lower values of are obtained for S960QL steel. These values are mostly below 0.1 mm when calculated from sharper notches. For blunter notches (where the notch root radius is equal to or greater than 0.8 mm), the critical distance =0 . This means that (for particular f ) the peak axial stress in the model notch root is lower than the nominal stress amplitude on the S-N curve for smooth samples at the same f . ���� ( =0) ≤ (2)