Issue 74

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

Figure 16: S-N curves in the VHCF regime of TC21 titanium alloy. Experimental data digitized from [22]

Figure 17: S-N curves in the VHCF regime of TC21 titanium alloy for equivalent stress. Experimental data digitized from [22]

(a) The fatigue tests were conducted at frequency of 20 kHz and three different stress ratios: R=-1, R=0.1 and R=0.5. The experimental data of [24]have been digitized and S-N curves are reported in Fig. 18 (a) and present three completely different behaviours depending on the stress ratio. For R=-1 a fatigue limit is present at 10 7 cycles, corresponding to a stress of 279 MPa. For R=0.1, S-N curve presents the typical step-wise curve of the VHCF regime. Finally, for R=0.5 there is a continuous decrease among all fatigue life up to 10 10 cycles. For smooth specimens, the different trends in the VHCF regime were attributed to the competition between the surface and interior fatigue failure life [25–27]. According to the Authors, the reason should be different in this case because interior crack initiation was not detected in notched specimens. Indeed, Yang et al. found two types of failure modes by using SEM and EDS: 1. Surface crack initiation with a single initiation point for all the specimens tested at R=-1; 2. Subsurface crack initiation from interior inclusions for some of the specimens tested at R=0.1 and R=0.5.

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