## 3 Results
### Numerical Simulation Validation with Experimental Results  
During the computational examination, the crack route is also traced as a force vs. COD trace curve, as illustrated in Figure 27a, and compared to the empirically observed curve. The actual and simulated curves have proven to have a high degree of congruence. The arrow mark is shown in Figure 27a. indicates the maximum load attained during the application of load about the COD opening. It is rather intriguing that the calculated curve has proportionally greater load levels than the observed data points. Due to the failure to account for the change in stiffness that happened during fatigue pre-cracking during the experimental assessment of fracture toughness, a discrepancy was observed. To examine the critical impact of defect sizes on specimen fatigue performance, [PERSON]'s parameter, the square root of defect projected area acquired from RBF specimen fracture surfaces, was utilized to characterize SLM sample defect sizes, and numerical analysis was performed. The defect size distribution features of SLM Ti6 Al4V specimens in 0\({}^{\circ}\), 45\({}^{\circ}\), and 90\({}^{\circ}\) construction directions are shown in Figure 27b, based on the Weibull distribution with three parameters. The majority of 0\({}^{\circ}\) samples (square root of area) were less than 100 m, which was much smaller than the values for 45\({}^{\circ}\) and 90\({}^{\circ}\) samples, which were 154 \(\upmu\)m and 141.2 \(\upmu\)m, respectively. Based on the Weibull distribution function, 99.99% of SLM defects (square root of area) were predicted to be smaller than 147.4 \(\upmu\)m for 0\({}^{\circ}\) samples, 186.9 m for 45\({}^{\circ}\) samples, and 169.2 \(\upmu\)m for 90\({}^{\circ}\) samples. For LM 45\({}^{\circ}\) and 90\({}^{\circ}\) Ti6 Al4V samples, the fatigue performance was worsened and the fatigue limits were lowered when the size of the defects was larger. Figure 27: (**a**) Experimental and simulated Force vs. COD curve for Ti6 Al4V alloy. Reprinted with permission from ref. [84]. 2020, [PERSON], (**b**) Weibull probability density lines and parameters of the SLM Ti6 Al4V specimen defect sizes obtained from RBF sample fracture surfaces. Reprinted with permission from ref. [67]. 2021, Elsevier. * FEA vs. [PERSON]'s model If the porosity is considered a notch, [PERSON]'s approach can determine local stress and strain. [PERSON]'s method was utilized to determine local stress and strain in three pore geometries. Figure 28 compares the FE model findings to [PERSON]'s in terms of computed local strain amplitude under applied stress amplitudes between 200 MPa and 560 MPa with applied stress ratios of 0.1, \(-\)0.2, and \(-\)0.5. Local strain from the FE model matched [PERSON]'s technique for lower plastic strains. [PERSON]'s approach was more conservative for greater plastic stresses than the FE model. [PERSON]'s technique allows complicated algorithms to better approximate non-linear material behavior. Figure 29a shows combined experimental vs. FASTRAN predicated fatigue life for both materials under all loading conditions. For the multiaxial loading conditions, the implemented crack growth-based approach resulted in accurate estimates of the fatigue life of notched AM specimens [85], as compared to the experimental results. Figure 28: Calculated local strain amplitudes: FEA vs. [PERSON]’s method. Dotted lines indicate \(\pm\)10% error. Reprinted with permission from ref. [77], 208, Elsevier. Figure 29: (**a**) notched axial, torsion, in-phase, and 90\({}^{\circ}\) out-of-phase fatigue test lives vs. FASTRAN life estimation results for LPBF Ti-6 Al-4V and 17-4 PH alloys with as-built (AB), machined (M) surface conditions. Dashed lines indicate factors 1/3 and 3 scatter bands [75], (**b**) Comparison of the elastic modulus as calculated from the FE analyses based on the CAD model and the experimental values. Reprinted with permission from ref. [79]. 2020, [PERSON].