## 2 Materials and Methods
### In-Situ Gaseous \(H_{2}\) Setup Design and Testing  
#### 2.2.1 Setup Design  
The setup designed in this work consists of a hollow specimen in combination with adapters that mount the specimen in an existing tensile tester, thus making it a simple, accessible test methodology. The loading frame used in this research was a Zwick Z100 electromechanical universal test machine equipped with a 100 kN load cell, combined with a Zwick digi-clip extensometer (gauge length 20 mm) and TestXpert II software. The specimen geometry is shown in Figure 3. The cylindrical specimen is machined from the pipe wall of the X60 pipeline steel such that the longitudinal direction of the specimen and the pipeline align. The hole into the specimen was reamed after drilling to create a smoother surface. Because of the orientation in which the samples were machined from the pipe wall, the girth weld region retains its orientation across the circumference of the specimen. If the specimen is considered as a miniature pipeline, the WM thus retains its original function as a girth weld. This results in equivalent loading directions of the weld between the test and actual operating conditions. A blind hole is machined through the specimen from one side, which is the containment volume for the hydrogen gas. The blind hole, the containment volume of the hydrogen gas, is dimensioned to prevent interference between the local stress fields around the edge of the hole and of the outer thread. Thus, failure outside of the gauge length is prevented. In the specimens that included a girth weld, a notch with a radius of 4 mm and a depth of 1 mm was machined to promote fracture in the weld region.  
Figure 3: Technical drawing of the sample geometry. Dimensions are given in mm.  
Figure 2: (a) Microstructure of the base metal (BM) consisting of polygonal ferrite with small pearlite islands (black). (b) Microstructure the weld metal (WM) consisting mainly of acicular ferrite with Widmanstätten ferrite at the prior austenite grain boundaries as indicated.  
Notches create zones of high hydrostatic tensile stress, high stress triaxialities and strain localisation, which increase HE susceptibility locally and adversely affect fracture behaviour [25; 40; 41]. By using a blunt notch, a reduction in the cross-section is achieved that raises the stress in the WM to compensate for the higher strength levels than the BM without creating regions of excessive localisation of stress and strain that could significantly influence HE behaviour. The notch design is further discussed in Section 2.3.2. Two aluminium 7075-T6 adapters were designed to connect the specimen to the test setup. An overview of the setup is shown in Figure 4a with a close-up of the bottom adapter in Figure 4b. The specimen connects to the adapters through the threaded holes. The bottom adapter also connects the specimen to the tubing which supplies and evacuates gas inside the specimen. One side connects to N\({}_{2}\) and H\({}_{2}\) bottles and a pressure gauge, which are used to load the system up to the testing pressure. The other side connects to a release valve that can be used if a test is prematurely aborted and a vacuum pump that is used to evacuate the system of any gases. The connection between the specimen and the bottom adapter is sealed by an O-ring placed around an aluminium insert to ensure proper spacing. A segment of the entire system, including the specimen, can be closed off to create a constant volume of approximately 6 ml so that only a limited amount of gas is released upon fracture of the specimen. This volume is covered by the 'ound engineering practice' regulations stated by the EU's Pressure Equipment Directive for up to 200 bar [38]. This means that the setup can be operated without requiring specialised facilities.