## 3 Results and Discussion
### Fractography  
#### 3.3.1 Base Metal Fractography  
Fractographic observations in the SEM support the findings from mechanical testing. The fracture surfaces of the specimens tested in nitrogen and hydrogen look distinctly different on a macroscopic as well as microscopic level. Figure 9a,b show the macroscopic overview of fracture surfaces of notched BM samples tested in 100 bang N\({}_{2}\) and H\({}_{2}\), respectively. The specimen tested in nitrogen shows a cup-cone fracture morphology characteristic of ductile fracture. This changes to a flat and faceted fracture surface for the specimen tested in H\({}_{2}\). The fracture surface area of the specimen tested in H\({}_{2}\) is much larger than that of the N\({}_{2}\) specimen. Larger magnification images of smooth BM specimens are shown in Figure 9c,d as an indication of the change in fracture appearance of specimens fracture in H\({}_{2}\). Figure 9c was taken from a smooth BM sample tested without any gas, which shows ductile micro void coalescence (MVC) fracture which is characteristic for ductile steels. Figure 9d was taken from the surface of a smooth BM sample tested in 100 bang H\({}_{2}\) gas. The fracture morphology of this surface corresponds to quasi-cleavage (QC) which is typical for fracture in hydrogen environments. This fracture micromechanism is characterised by flat cleavage-like facets that are much smaller than in conventional cleavage fracture and irregular ridges that appear on these facets. Secondary cracks on hydrogen fracture surfaces are also observed in this image as well as in the literature [45; 46; 47].  
Fracture was observed to initiate from the inner surface of all specimens that were tested in a hydrogen environment, even though the highest stress concentration of the notch is present on the outside surface and the zone of highest stress triaxiality is present within the sample wall. Figure 9b shows that secondary cracks have formed on the inside surface parallel to the primary fracture surface. Secondary cracking on specimen surfaces that are in contact with hydrogen is frequently observed in the literature [4; 47; 48]. In this case, secondary cracks are suspected to have initiated from pre-existing machining defects from specimen manufacturing. Similar cracking can be observed in a study by [PERSON] [32] in which the inner surface was honed instead of reamed, which creates a smoother surface than present in this study. A high absorbed H concentration near the inside surface facilitates quick crack propagation from these machining defects. In the specimen tested in N\({}_{2}\) in Figure 9a, it can be seen that the secondary cracks undergo necking to a certain extent and do not influence the final fracture behaviour. The cracks appear to be much sharper in the specimen tested in H\({}_{2}\), where they have a smaller crack mouth opening and the primary fracture surface seems to have originated from them. Machining defects that were previously undiscovered might become significant defects when they come into contact with hydrogen gas. The specimen geometry as developed in this work complicates inspection of the inside surface because of the nature of the blind hole. Careful polishing of the surface that is in contact with the hydrogen environment is therefore advised in future work. An alternative to avoid this problem is to utilise specimens featuring sharper notches on the outside surface, or machined notches on the inside surface to enforce cracking a pre-defined location. However, because HE susceptibility is highly sensitive to notch effects, care should be taken when comparing test data between different specimen geometries.