Issue 74

P. Zuliani et alii, Fracture and Structural Integrity, 74 (2025) 385-414; DOI: 10.3221/IGF-ESIS.74.24

process). Since there are only a few articles on specimens produced by AM and most of the articles are carried out at ultrasonic frequency, the articles have been classified by considering the material type. In authors’ opinion, this is the best and clearest classification to discuss notch effect by considering the available literature results. Fig. 1 shows that there are eight articles on steels, four on aluminium alloys, three on titanium alloys, and only one on an INCONEL superalloy. As a consequence, the section dedicated to the INCONEL alloy has been included in the same chapter as the titanium alloys, under the title “Other Metallic Materials”.

Figure 1: Published articles for each material type about the notch effect in the VHCF fatigue.

Steels The first article about the notch effect in the VHCF cycle regime was published by Akiniwa et. al. [8] and it investigates the notch sensitivity of a bearing steel. The authors have analysed the fatigue behaviour of two different batches of SUJ2 bearing steel (100 Cr6). Both of them were normalised, quenched, tempered and polished with emery paper. The difference is that the first batch was also electro-polished to remove residual stresses (type A), while the second one was not electro-polished (type B). The fatigue tests were all conducted at 20 kHz with a stress ratio of R=-1. The analysis has been conducted using different geometries: smooth, circumferentially notched (Fig. 2(a)) and drill hole (Fig. 2(b)) The first geometry was tested using both type A and type B materials.

(a) (b) Figure 2: Geometries of the specimens used by [8]: (a) notched specimens (b) drill hole specimens.

The Scanning Electron Microscopy (SEM) showed that the type A specimens broke mainly near the surface, while type B specimens failed by internal defects. Different behaviour has been attributed to the presence of compressive residual stresses in type B specimens. On the contrary, for all notched and drill-hole specimens the fracture started near the surface because the stress in this point is high and because only type A material was used. One important result of this study is that the notch fatigue factor K f shows a big decrease with the increase of the number of cycles. In particular, the circumferentially notched specimens show a reduction of -5.5 %, while the drill holed the reduction is -1.47 %. The different sensitivity to the number of cycles is schematically represented in Fig. 3(a), while the experimental results digitized from [8] are reported in Fig. 3 (b). This aspect can be explained by the fact that in smooth specimens the crack nucleation mechanism changes by passing from HCF to VHCF regime, while in notched specimens there is surface or near surface crack initiation also in the

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