## 3 Results and Discussion
### Crack Propagation in Conventionally and Additively Manufactured 316L  
In additive manufacturing of components, process defects (like pores, which may lead to crack initiation) in critical design regions cannot be completely avoided. For this reason, the investigation of crack propagation behavior is a prerequisite for the safety of components, for instance, for energy conversion processes. Figure 7 shows a comparison of the crack growth behavior of the additively manufactured material in the different build directions in comparison to conventionally manufactured material. In all additively manufactured specimens, the crack propagation rates per cycle (da/dN) were lower, regardless of build direction, with the only exception of a small \(\Delta\)K range from \(\approx\)23-26 MPa \(\sqrt{\text{m}}\) in the first ZY specimen. The lowest da/dN was measured at the specimen from the ZX direction. For a given value of \(\Delta\)K, the crack propagation rate was up to eight times lower than in the conventionally produced material. Furthermore, the highest \(\Delta\)K value (\(\approx\)11.5 MPa \(\sqrt{\text{m}}\)) for initiation of crack propagation (note that this value is not a threshold value, i.e., crack growth can occur even below this value) of all the additively manufactured specimens was measured in the ZX samples, too. However, this \(\Delta\)K value for initiating crack propagation was about 2.3 MPa \(\sqrt{\text{m}}\) lower compared to conventionally  
Figure 7: Crack growth in LPBF (three different build directions) and conventionally manufactured 316L at ambient temperature. produced material. In the XY and ZY specimens, lower \(\Delta\)K values for crack propagation initiation were encountered (in the XY specimen: \(\Delta\)K \(\approx\)9.6 MPa\(\sqrt{\text{m}}\), in the \(ZY\) specimen: 9.4 MPa\(\sqrt{\text{m}}\)). In both directions, significantly higher porosity was observed (Figure 8). Pores can act as crack initiators (c.f. Section 3.1). It is therefore hypothesized, that pores were responsible for the lower \(\Delta\)K values for initiation of crack propagation. Figure 9a displays the CT specimens after fracture. Figure 9b depicts the crack paths in the LPBF-manufactured samples. In the XY direction specimen, the crack propagated perpendicular to the build direction, while in the ZY orientation, the crack propagated in parallel and in the ZX direction under an angle of 45\({}^{\circ}\) (cf. Figure 10). The lowest da/dN value, accompanied by the highest \(\Delta\)K value to initiate crack propagation, was measured at the ZX sample. The best reproducibility was found in the XY direction specimens (Figure 7). However, only the ZY direction provided a valid fracture path (according to ASTM regulation [21]). Regardless of build direction, the crack path was transversatline (cf. Figures 9b, 10 and 11). Consequently, a weak connection of the individual melt pools can be excluded as a key factor in cracking behavior. Figure 8: Exemplary distribution of pores with reference to build directions (**a**) XY, (**b**) ZY, and (**c**) ZX. Figure 9: (**a**) Cracked compact tension (CT) specimens after FCG experiments and (**b**) longitudinal crack paths (before cracking of the specimens). Figure 12 depicts the comparison of the fracture surfaces of conventionally and LPBF-manufactured specimens of different build directions. The fracture surface of conventionally produced material offered features of ductile fracture dimples. In contrast, a mixed fracture mode with ductile and cleavage areas, along with several pores, was found in the additively manufactured XY and ZY specimens (Figure 12b,c). The same was observed in the case of the ZX sample, but the crack propagated along lattice planes in the grain interiors (Figure 11d). In this specimen, preferred grain orientations into the {001}, {111}, and {110} directions were measured. With a percentage of 13.46%, the {110} direction was the dominating one (Figure 10).