PSI - Issue 54

Nikolai Kashaev et al. / Procedia Structural Integrity 54 (2024) 361–368 Kashaev et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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Results of fracture surface analysis indicate both internal (Fig. 5(b)) and surface (Fig. 5(c)) fatigue crack initiation. The fatigue cracks started from lack-of-fusion and porosity defects located either on the surface or under the surface. No correlation between the crack origin and the number of cycles could be identified in contrast with the results reported by Akgun et al., 2021 where the specimens with surface crack initiation showed reduced fatigue lives. All specimens examined exhibited the two typical areas of stable and unstable crack growth in the fracture surface.

Fig. 5. (a) Fatigue test results. (b)-(c) Fractography of fracture surface in the case of (b) internal and (c) surface fatigue crack initiation.

4.2. Application of fatigue-life assessment model The fracture mechanics framework described in Section 3 was applied to calculate the number of cycles to failure for the tested specimens in the load cycle range between 10 4 and 10 6 . In the first step, the fatigue crack growth parameters C , n , and p of Eq. (1) were identified by fitting to the experimentally obtained data from the fatigue crack growth test. Considering the fact of the limited number of available specimens fatigue crack growth tests were focused on the determination of the threshold of the stress intensity factor range Δ ℎ, according to ASTM E 467. The obtained fatigue crack growth data concerning the values of the stress intensity factor range were below 10 MPa  m. In order to achieve an appropriate predictive capability of Eq. 1 with respect to fatigue crack growth in the Paris regime, additional experimental data for fitting the NASGRO equation were used from the study by Syed et al., 2021b, which were obtained for a WAAM-fabricated Ti-6Al-4V structure with a similar microstructure. Fig. 6 shows the experimentally determined data from the two studies and the fit to the NASGRO equation. The fatigue crack growth data calculated using the NASGRO fits well with the experimental data.

Fig. 6. Experimental FCG data obtained using the decreasing  K test in comparison to experimentally determined FCG data obtained by Syed et al., 2021b and the fit to NASGRO equation (Eq. 1).