## 4 Results and Discussion
### Agreement of the Cohesive Model Behavior and Hydrogen Embrittlement Micro-Mechanisms  
The micro-mechanisms causing crack growth in the steel bar when hydrogen-free and when hydrogen-embrittled are not at all in contradiction with the cohesive model. In the absence of hydrogen, the crack front behaves as a stress concentrator in the regime SSY, but transforms into a plastic strain concentrator which spreads ahead of the crack once the SSY regime vanishes. In support of this statement, Figure 9a shows a macroscopic image of the fracture surface of the JFT3-Air specimen (a\({}_{0}=1100\) um), and two higher-magnification details given in Figure 9c,d. According to Figure 9a, the crack grows from the fatigue front and penetrates the resistant ligament, narrowing toward its center and ultimately causing specimen collapse. As noted in the VIC-2D image sequence and indicated in Figure 7, the onset of crack growth occurred quite close to the maximum tensile load and was preceded by a strong blunting of the fatigue front. This is substantiated by Figure 9c and entails high levels of local plastic deformation, which give rise to a quasi-ductile fracture morphology, with a significant amount of relatively large dimples converted into transverse secondary cracks. Figure 9d captures the ductile morphology that characterizes the crack growth prior to the final failure of specimen by plastic instability; a crack length of approx. 450 mm being estimated by considering an equivalent straight propagation front. In contrast, in the J-integral tests performed in the aggressive solution, the crack front absorbs and concentrates diffusible hydrogen, which subsequently induce mechanisms like hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) to operate on a microscopic scale by increasing the deformability of the microstructural matrix of steel. The former increases the dislocation mobility, which intensifies and localizes the plastic deformation that favors the steel embrittlement, and the latter weakens the cohesion between the martensitic matrix components [25; 35; 36]. The material thus altered forms the cohesive zone which, in order to reach a certain level of deformation, would require less stress than that under a hydrogen-free condition, and as a consequence, needs lower cohesive strength. Figure 9: Macroscopic image of the fracture surfaces of specimens: **(a)** JFT3-Air; **(b)** JFT1-FIP; and higher-magnification details, respectively showing: **(c)** the crack blunting in JFT3-Air; **(d)** the ductile crack propagation of approx. 450 mm in JFT3-Air; **(e)** the crack initiation and propagation of approx. 950 μm in JFT1-FIP; **(f)** the quasi-ductile collapse in JFT1-FIP (a\({}_{0}\) initial crack size, including the 0.5 mm notch depth). In sustaining the above, the macroscopic image given in Figure 9b and two details of it (Figure 9e,f) belonging to JFT1-FIP (a\({}_{0}\) = 850 \(\upmu\)m) were selected to observe and evaluate the hydrogen embrittlement effect on the respective morphologies and fracture micro-mechanisms. Figure 9e illustrates the transition from fatigue to the onset of assisted crack followed by stable progress (of approx. 950 mm) until the specimen collapses beyond the maxim load (Figure 8), the value of which is 30% smaller than that of a similar, in-air broken specimen (Tables 3 and 4). This is due to the continuous absorption of hydrogen through the crack tip, which embrittles its front and causes the early initiation of cracking, almost simultaneously with the limit of the SSY regime. Figure 9e captures the environmentally assisted crack initiation of quasi-cleavage morphology indicating either the martensite packets rupture or decohesion and small, secondary cracks probably formed at previous austenite grain borders. This morphology, which characterizes almost the entire crack length of approx. 950 mm, estimated by considering an equivalent straight propagation front, is due to the combined action of the HELP and HEDE fracture mechanisms [18, 20, 26].