# Additively Manufactured Inconel 718 Low-Cycle Fatigue Performance
## 4 Conclusions  
This study investigated the low-cycle fatigue performance of Inconel 718 material produced by L-PBF. Specimens were fabricated in a vertical orientation and evaluated in both as-printed and fully machined surface conditions at room temperature, 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F),  
Figure 24: L-PBF Inconel 718 number of crack initiation sites versus cycles to failure. \begin{table}
\begin{tabular}{c c c c c} \hline
**Specimen ID** & **Surface Condition** & **Temperature** & **Nif (Cycles)** & **\% of Maximum Life** & **\# of Crack Initiation Sites** \\ \hline
01-0075 MB & Fully Machined & 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) & 2868 & 49.8\% & 9 \\
01-007C0L & Fully Machined & 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) & 5760 & N/A & 6 \\
01-0075 MK & As-Printed & 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) & 3817 & 95.4\% & 9 \\
01-0075 MD & As-Printed & 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) & 4002 & N/A & 7 \\
01-007C0C & Fully Machined & 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) & 5192 & 41.7\% & 5 \\
01-007C0Q & Fully Machined & 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) & 12,463 & N/A & 2 \\
01-0075 MR & As-Printed & 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) & 2166 & 57.7\% & 13 \\
01-0075 ML & As-Printed & 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) & 3753 & N/A & 11 \\ \hline \end{tabular}
\end{table} 
Table 5: L-PBF Inconel 718 fatigue performance summary. and an elevated temperature, 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F). Based on the results obtained, the following conclusions can be drawn:  
1. Crack Initiation Sites: All crack initiation sites were observed to occur at the surface of the specimens, regardless of the test temperature or surface condition. However, a distinct difference was noted between the two surface conditions: as-printed specimens predominantly exhibited crack initiation at surface defects, while fully machined specimens displayed cleavage crack initiation. Additionally, many crack initiation sites were found to be connected to adjacent sites via shear lips, which merged to form larger cracks that propagated through the material. Table 5 indicates that a larger number of initiation sites results in a shorter fatigue life. 2. Zone II Crack Propagation: The zone II crack propagation region exhibited a mix of fatigue striations and ductile tearing. Notably, the 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) specimens showed a higher amount of ductile tearing within the zone II region, indicating more frequent disruptions in crack propagation. In contrast, the 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) specimens predominantly displayed fatigue striations with minimal ductile tearing. The final fracture for all specimens occurred along a shear lip consisting of ductile dimple formation, highlighting the ductile nature of the final failure mode. 3. Correlation Between Crack Initiation Sites and Fatigue Life: There was a strong correlation between the number of crack initiation sites and the final fatigue life of the specimens. Specimens with fewer crack initiation sites generally exhibited longer fatigue lives. This is attributed to the reduced number of opportunities for rapid crack growth along shear planes, allowing individual cracks to propagate over a longer period via traditional fatigue mechanisms such as fatigue striations and ductile tearing. 4. Impact of Temperature on Crack Initiation: The 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) as-printed surface specimens had 2 to 6 times more crack initiation sites compared to the fully machined specimens. In contrast, the 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) as-printed specimens had a similar number of crack initiation sites compared to their fully machined counterparts. This indicates that elevated temperatures exacerbate the impact of surface defects on crack initiation in as-printed specimens. 5.