# Additively Manufactured Inconel 718 Low-Cycle Fatigue Performance
## 3 Results
### Fractography
#### 3.2.1 Crack Initiation  
Crack initiation sites for all specimens were documented and are evaluated within this section. It was noted that all crack initiation sites occurred at the surface of the specimen regardless of test temperature and specimen surface condition. Figures 8 and 9 display representative images of crack initiation sites for the 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) fully machined specimens MB and 0L. It is shown that specimen MB had a total of nine initiation sites while specimen 0L had six initiation sites. From Figure 8, it can be seen that specimen MB's crack initiation \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Specimen ID** & **Surface Condition** & **Temperature** & \(\Delta\epsilon\) (\%) & \(\Delta\sigma\) (MPa) & **E (GPa)** & **Nf (Cycles)** & **\% of Maximum Life** \\ \hline
01-0075 MB & Fully Machined & 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) & 0.479 & 973 & 204 & 2868 & 49.8\% \\
01-006 COL & Fully Machined & 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) & 0.479 & 971 & 204 & 5760 & N/A \\
01-0075 MK & As-Printed & 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) & 0.479 & 922 & 193 & 3817 & 95.4\% \\
01-0075 MD & As-Printed & 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) & 0.479 & 933 & 192 & 4002 & N/A \\
01-006 COR & Fully Machined & 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) & 0.445 & 788 & 178 & 5192 & 41.7\% \\
01-00 GCQ & Fully Machined & 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) & 0.445 & 692 & 159 & 12,463 & N/A \\
01-0075 MR & As-Printed & 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) & 0.445 & 792 & 178 & 2166 & 57.7\% \\
01-0075 ML & As-Printed & 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) & 0.445 & 702 & 164 & 3753 & N/A \\ \hline \end{tabular}
\end{table} 
Table 4: Minimum and maximum fatigue life specimens selected for fractography. Figure 7: Optical images of fracture surfaces. sites occurred opposite the shear lip. It can also be noted in Figure 8D that the cracks were initiated via cleavage planes. Specimen 0L had a fatigue life of 5760 cycles and was tested at the same condition as specimen MB, which had a fatigue life of 2868 cycles. Similar to specimen MB, specimen 0L's crack initiation site occurred due to cleavage plane initiations as shown in Figure 9D. Figures 10 and 11 display representative images of crack initiation sites for the 24 \({}^{\circ}\)C (75 \({}^{\circ}\)F) as-printed specimens MK and MD. It is shown that specimen MK had a total of nine initiation sites while specimen MD had seven initiation sites. Both specimens MK and MD  
Figure 8: Specim MB (24 \({}^{\circ}\)C fully machined) crack initiation fractography analysis. Figure 9: Specimen 0L (24 \({}^{\circ}\)C fully machined) crack initiation fractography analysis. were tested under the same conditions as specimens MB and 0L; however, specimens MK and MD were tested in the as-printed surface conditions. It is shown in Figures 10 and 11 that specimen MK and MD crack initiation sites tended to occur at surface defects, Such as in Figures 10A and 11D. Although specimens MK and MD had additional surface defects compared to MB and 0L, the as-printed specimens had a similar number of crack initiation sites and similar fatigue life compared to the fully machined specimens. As shown in Figures 10A and 11D, the defect sizes on the as-printed surfaces are roughly 25 \(\upmu\)m deep. Figures 12 and 13 display representative images of crack initiation sites for the 538 \({}^{\circ}\)C (1000 \({}^{\circ}\)F) fully machined specimens 0R and 0Q. It is shown that specimen 0R had a total of five initiation sites while specimen 0Q had two initiation sites. Figure 12 shows that specimen 0R had a large shear lip with five crack initiation sites. These crack initiation sites  
Figure 11: Specim MD (24 \({}^{\circ}\)C, as-printed) crack initiation fractography analysis. Figure 10: Specim MK (24 \({}^{\circ}\)C, as-printed) crack initiation fractography analysis. show evidence of cleavage planes and appear to be connected with each other via shear planes to combine and form a larger primary crack front.