## 3 Results
### Fatigue Properties
In this respect, the range of probable fatigue life is computed for defective and non-defective construction specimens by varying the internal flaw size for a constant aspect ratio using the FASTRAN software. Figure 10b depicts the effects of void size on the fatigue life of defective and non-defective constructions. Acceptable fatigue curves developed for various builds of AM Inconel 718 suggest that understanding the statistical distribution of the defect size allows for a reasonable prediction of the scatter in the fatigue life of the AM material using appropriate fatigue analysis software such as FASTRAN. Importantly, the change in fatigue curves relative to the initial defect size also relies on the stress ratio, as seen in Figure 10[70]. The fluctuation in fatigue life for a 100 \(\upmu\)m change in initial flaw size for the test at R = \(-\)1 is not as substantial as that for a 12 \(\upmu\)m change in the initial flaw size for the test at R = 0.1. This  
Figure 8: In-situ fatigue and RBF samples: (**a,d**) 90\({}^{\circ}\) samples; (**b,e**) 45\({}^{\circ}\) samples; (**c,f**) 0\({}^{\circ}\) samples; (**g,h**) geometries of RBF bar and in-situ fatigue samples. Reprinted with permission from ref. [67]. 2021, Elsevier. can be explained by the effect of plasticity-induced crack closure on fatigue life, which is more noticeable for the lower R-value. In Figure 11, the estimated fatigue life at higher stress levels for as-built specimens is cautious. This uncertainty of fatigue damage may be attributable to the unique processes of fracture formation under various stress amplitudes, but the exact cause is unclear. For example, the size of the defect that acts as the starting site might vary depending on the degree of stress. In addition, fatigue failure may be caused by several occurrences of fracture initiation under larger stress amplitudes, which can alter the total crack development rate and the resultant predicted fatigue curve. To make fatigue curves for the non-defective build specimen with as-built surface polish, however, a single crack initiation site with constant geometry was tested at different stress levels. Figure 9: S\(-\)N curves of titanium scaffolds with various relative densities compared to experimental results for the rhombic dodecahedron, diamond, and truncated cuboctahedron structures. Reprinted with permission from ref. [57]. 2015, Elsevier. To estimate the fatigue life in Equation (2) and the failure plane, [PERSON] modification of the FS model is applied. The FS model was used because it gives accurate predictions for many kinds of metals, including AM metals, under a limited number of loading situations. The FS technique is a strain-equivalent model based on mode II/III failure. Where k is the correction factor relating the shear stresses seen in a pure torsion test to the highest shear strains observed in a tension-compression test. The parameter k measures the material's sensitivity to normal stresses, as seen in Figure 12.  
Using an equation, this parameter may be determined based on the fatigue life. As defined by Equation (2), the SWT model specifies the strain energy density as the damage parameter. The damage parameter takes normal strain and stress on the critical plane into  
Figure 11: Experimental data and predicted fatigue lives for average curve, lower and upper bounds of L-PBF Inconel 718 in as-built surface condition. Reprinted with permission from ref. [70]. 2018, Wiley. Figure 10: **(a)** Effective stress intensity factor against crack growth rate for large cracks with a small-crack estimate for L\(-\)PBF Inconel 718; **(b)** Effect of void size on fatigue life of **(A)** defective and **(B)** non-defective builds L\(-\)PBF Inconel 718. Reprinted with permission from ref. [70]. 2018, Wiley. account. This model is suitable for materials or circumstances that exhibit mode I fatigue failure mode.