## 2 Material and Testing Methods
### J-Integral Fracture Testing Procedures in Air and in FIP Medium  
The J-integral value was determined as a function of the measured CMOD by applying the procedures given by BS-8571 [8] on SENT specimens tested in-air and in the FIP medium, which consists of an aqueous thiocyanate solution as a strong hydrogen promoter and is recommended by UNE-EN ISO 15630-3 [16] for accelerated laboratory testing. Figure 1b shows the main geometry of the tested SENT specimens with the width (W = 5 mm), thickness (B = 0.5 mm), and details the position of the 0.5 mm deep V-notch, made by electrical discharge machining on one of its lateral faces, of 2.5 mm. The notch sharpness eases the fatigue precracking process in air of the 10 tested SENTs, with the fatigue loading consisting of several, successive blocks of harmonic tensile load cycles, with constant but gradually reduced amplitude between them. A single, clip-on strain gauge extensometer of 12.5 mm gauge length, \(\pm\)5 mm travel (Instron 2620-601 model, of 0.005 mm accuracy) attached to the specimen on the notched side face through elastic bands (Figure 2a) allowed the crack depth to be roughly controlled during the fatigue precracking of 10 specimens. Their subsequent slow strain rate fracture tests were performed on an Instron 8501 servo-hydraulic machine (Instron, 825 University Ave., Norwood, MA 02062-2643, USA), equipped with a load cell of 50 kN static (load measurement accuracy: \(\pm\)0.5 N), at 0.001 mm/min constant crosshead speed. Four of them were subjected to J-integral fracture testing in-air (JFT-Air) and the other six in the FIP medium, at 50 \({}^{\circ}\)C (JFT-FIP). Two of these latter specimens were also tested in-air after interrupting the test in the FIP medium following the onset of assisted-cracking. In between the tests, the specimens were moderately heated at 250 \({}^{\circ}\)C for 30 min to ensure \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline
**C** & **Mn** & **Si** & **P** & **S** & **Cu** & **Ni** & **Cr** & **Mo** & **Ti** & **V** & **Fe** \\ \hline
0.45 & 0.65 & 1.77 & 0.017 & 0.004 & 0.01 & 0.06 & 0.15 & 0.02 & 0.03 & 0.003 & Bal \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of the bar steel. Figure 1: Images showing the: (**a**) microstructure of the steel bar; (**b**) geometry of the SENT specimen and test arrangement. \begin{table}
\begin{tabular}{c c c c} \hline
**Elastic Modulus,** & **Yield Strength,** & **Tensile Strength** & **Maximum Uniform** & **Strain-Hardening** \\ (GPa) & **(MPa)** & **(MPa)** & **Elongation** (\%)** & **Exponent** \\ \hline
220 & 988 & 1138 & 8 & 8.2 \\ \hline \end{tabular}
\end{table} 
Table 2: Mechanical properties the bar steel. their complete dehydrogenation. Therefore, four JFT-Air tests were performed with fatigue precracked specimens and two with hydrogen assisted precracked specimens. All the tests were instrumented with a pair of virtual extensometers (Figure 2b,c) provided by the VIC-2D (Correlated Solutions, Inc., South Carolina, SC 29063, USA), a digital image correlation system that assures in-plane measurement of displacements, with 0.01% accuracy. In this view, the system compares temporal sequences of digital images acquired during mechanical loading of the fine speckle-painted specimens in the area of interest [27]. Three of the JFT-Air tests were additionally instrumented with the resistive extensometer used for control fatigue precracking in order to verify the compatibility of the CMOD values measured by both procedures, with no significant deviation being observed. Figure 3a illustrates the CMOD determination with the VIC-2D system through the crack opening displacements V\({}_{1}\) and V\({}_{2}\) of the pair of virtual extensometers, respectively mounted on the speckle painted surface of SENT specimen at h\({}_{1}\) and h\({}_{2}\) depths.