## 3 Results and Discussion  
### Pore Density Evaluation of the Model Pressure Vessel  
The results of the defect analysis via computer tomography are displayed in Figure 3. Distinct differences regarding pore distribution and the number of pores were encountered in the two model pressure vessels. Although both vessels were produced in the same run, we encountered differences in pore distribution. A possible reason for this is the significant number of irregularly distributed accompanying samples on the installation space, which caused different laser dwell times. The effect of pores on the total fatigue lifetime of a component was investigated in [20]. It was hypothesized that crack growth increases abruptly when a pore is reached, whereas crack growth is retarded subsequently because the crack must be initiated again. Therefore, the number of cycles until re-initiation of crack growth determines whether further crack growth is accelerated or retarded. In this context, pore distribution in the component is decisive \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **Temperatures** & **Number of FCG\({}_{\text{Air}}\) Tests (R = 0.1, f = 20 Hz)** \\ \hline AM316L-XY & RT & 2 \\ AM316L-ZY & RT & 2 \\ AM316L-ZX & RT & 2 \\
316L (conventional) & RT/650 \({}^{\circ}\)C & 2/1 \\ Crofer22 H, PA & RT & 1 \\ HiperFer 17 Cr2, PA & 650 \({}^{\circ}\)C & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Detailed test matrix of the fatigue crack growth (FCG) experiments. (near or far from the surface, see Figure 4). For this purpose, the impact of pores in an exemplary component section (transition region from the cylinder wall to the torispherical head) under internal pressure loading was evaluated first. This region is characterized by high tensile stresses at the inner surface and compressive stresses at the outer surface. Therefore, three simple axially symmetric models were created using the measurement results from computer tomography (Figure 5). Model I is a non-porous basic model for comparison, and model II features a pore near the surface, i.e., in a critical design area of high tensile stress. Model III  
Figure 4: Working hypothesis for crack growth in structures with pores [20]. Figure 3: Evaluation of pore distribution and size in the two additively manufactured model pressure vessels [20]. presents a pore near the outer surface, i.e., under compressive stress. The stress distributions in the component wall through the pore positions are illustrated in Figure 6.  
An increase in stress at the pore edges only occurred if the pore was located in a critical design area of high stress concentration. In model II, representing a near-surface pore, significantly higher stresses occurred at the pore edges in comparison to the pore-free model I. In the pore-free component (Figure 6a), the highest stresses appeared at the inner surface of the intersection. If there was a pore nearby (Figure 6b), the stresses at the pore edge were higher than at the inner surface. Therefore, the crack initiated from the pore. In the next step, the crack grew towards the highest stresses, which apart from the pore appeared at the inner surface. Only when the crack reached the inner surface did the crack growth direction turn from the pore towards the outer surface and the crack would grow into the depth of the wall [20]. The analysis showed that pores acted as notches in critical design regions and thus may act as crack initiators. A crack retarding effect, as originally assumed, did not exist. In this context, it should be noted that the pores were idealized as spheres of equivalent volume in the FEM analysis. In reality, the surfaces of the detected pores were irregular, sometimes sharp-edged, which may have had an effect on crack initiation properties. Figure 5: Pore simulation based on computer tomography results [20].