PSI - Issue 42

Kamila Kozáková et al. / Procedia Structural Integrity 42 (2022) 270–275 K. Koza´kova´ et al. / Structural Integrity Procedia 00 (2019) 000–000

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The micrographs in Fig. 7 revealed di ff erent way of machining of particular notches. The surfaces of the notches with radii of 0.1, 0.2 and 0.4 mm have relatively smooth surfaces, whereas on the surfaces of notches with radii of 0.8, 1.5, and 3 mm, there are steps caused by the knife tool during machining. It was found that notches with larger radii were machined by knives with a smaller radius than final required notch radius. For comparison, the surface of smooth specimen without notch is shown in see Fig. 9. The knives radii used for cutting the notches are listed in Tab. 4. Probably for technological reasons, the larger notches were machined by small knife tool.

Table 4: Knife tool radii

notch radius r [mm] knife tool radius [mm]

0.1 0.1

0.2 0.2

0.4 0.4

0.8 0.1

1.5 0.1

3

0.1

Specimens machined with the corresponding knives that resulted in smooth notch surfaces have a clear trend. As the radius of the notch increases, the lifetime of specimens increases. Specimens with larger radii (0.8, 1.5, 3 mm) have a similar trend, but due to the manufacturing methods and notch surface morphology, the two trends do not follow each other. The roughness of the larger notches (0.8, 1.5, 3 mm) caused a shorter lifetime.

5.1. The influence of stress concentrations

The method of notch machining using smaller knife is common in practice. However, the steps on notch surfaces work here as additional stress concentrators and they are decisive for earlier crack nucleation. It is apparent that the process of machining and selection of a tool can strongly influence final life time of a sample as well as a notched component. The stress increase is illustrated in Fig. 8. The actual axial stress σ y is much higher than the designer takes into account and premature failure of the component may occur. Further, the role of stress concentrations is stronger with the greater number of cycles. Therefore, in the area of gigacycle fatigue, the small details (such as stress concentrations caused by machining or by inclusions or microstructural defects, etc.) are decisive. For this reason, the slope of S - N curve measured for the samples with notch r = 1.5 mm is steeper (life is shorter in the VHCF region for specimens with stress concentrations caused by machining). The steeper slope could also be expected for notched specimens with a notch radius of 3 mm, but it was not proven, probably due to the small number of samples used for experiments. New experimental data could significantly change the slope of a curve.

Fig. 8: Stress increased caused by way of machining

5.2. Fracture surfaces

Fracture surfaces were observed by SEMmicroscope. Before experiment, smooth specimens were polished and this process lead to reduction of surface roughness, see Fig. 9. Crack initiated from internal inhomogeneities in the case of smooth specimens, see white arrow in Fig. 10. On the other hand, multiple surface crack initiation was observed in the cases of all notched specimens, see the arrows in Fig. 11. The stress concentrator (notch), or the stress concentration caused by the machining tool, or both lead to crack initiation.