*3.4. Fracture Surfaces*

Fracture surfaces of different samples that underwent a similar number of cycles to failure (~5 <sup>×</sup> <sup>10</sup><sup>4</sup> cycles) were investigated, and different initiation and propagation mechanisms were observed depending on the type of loading and surface treatment.

For instance, under rotating–bending, as shown in Figure 5a–c, all samples exhibited multiple crack initiation spots. These spots corresponded to surface initiation sites for all the tested conditions, and one example of the RB\_20 min TC sample is displayed in Figure 6a. After initiation, these cracks could grow simultaneously, culminating in ductile fracture of the sample. This crack propagation behavior was the same for samples both with and without SMAT. Comparatively, under tension–compression loading, for the polished condition, not so many initiation spots were visible, and the main section reduction resulted from the propagation of a single crack (Figure 5d). After SMAT, the TC\_60 min RT sample exhibited multiple initiation sites (Figure 5e), but only one main crack (initiation zone highlighted in red) propagated substantially into the core of the sample. It seems that this crack initiated at the surface (as shown in Figure 6b) and that, after a certain number of cycles, the crack propagation path changed between the SMAT-affected sub-surface and the core of the specimen, leading to a water drop-shaped zone on the fracture surface (Figure 5e). Higher-resolution fracture surface analyses are needed to gain a better insight into the crack propagation behavior in both the RB and TC samples, because it is more reasonable to address fatigue damage assessment by estimating fatigue crack growth rates.

**Figure 5.** Typical fracture surfaces of (**a**) ground sample tested in rotating–bending, (**b**) sample tested in rotating–bending after 20 min of SMAT at room temperature, (**c**) sample tested in rotating–bending after 20 min of cryogenic SMAT, (**d**) polished samples tested in tension–compression, and (**e**) sample tested in tension–compression after 60 min of SMAT at room temperature.
