PSI - Issue 77

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

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Smooth specimens were prepared and tested in two conditions: as-machined and polished. Polished smooth samples are usually used to measure fatigue life. However, to determine the critical distance, relation between smooth and notched samples is involved, and the notched samples were only machined, without grinding and polishing. The importance of achieving the same (or comparable) surface quality will be demonstrated. Figure 2 shows centers of the smooth specimens with the as-machined (a) and the polished (b) surfaces.

(a)

(b)

Fig. 2. Smooth specimens as-machined (a) and polished (b)

3. Materials tested Three kinds of high-strength steel (S460NL, S690QL and S960QL) were tested. Samples were cut and machined from sheets measuring 20 × 2000 × 12,000 mm. The following tensile properties are listed in Tab. 2: 0.2% Yield Strength 02 (MPa), Ultimate Tensile Strength σ (MPa) and Elongation at Fracture (%). Table 2. Tensile properties of tested materials Material 02 (MPa) σ (MPa) (%) S960QL 1040.03 1082.77 18.45 S690QL 817.10 874.82 18.08 S460NL 490.82 625.41 27.46 The fatigue properties (S-N curves) were measured in the laboratory of very high cycle fatigue at the Institute of Physics of Materials, CAS. The fatigue testing machine operated at an ultrasonic frequency of 20 kHz. The length of the specimens was precisely adjusted to match the resonant frequency of longitudinal vibrations. The fatigue curves shown in Figures 3(a), 3(b), and 3(c) demonstrate the impact of notches, as well as the effect of the surface quality of the smooth specimens. The data measured for eight sets of specimens (smooth and notched) are distinguished by color points, the lines represent the Basquin's approximations of the experimental data. Samples that did not break even after 10 billion cycles and are considered runouts. The influence of the surface of the smooth geometry specimens is clear. Polishing increases resistance of the material to fatigue crack initiation. Especially note the significant difference between the curves for smooth polished and 3 mm notched specimens in case of S960QL steel. A further decrease in fatigue life is expected as notch radii decrease in all cases.