## 4 Results and Discussion  
### Experimental Results of the J-Integral Fracture Tests Made in Air and in FIP Environment  
Figure 6a shows the load-CMOD data of the six J-integral fracture tests conducted in-air (JFT-Air, four of them on fatigue precracked SENT specimens, and the remaining two, JFT5-Air and JFT6-Air, after previous assisted cracking in FIP solution). These last two tests were performed after enough time from the unloading aggressive solution removal and cleaning to ensure that most of the diffusible hydrogen had vanished. The initial crack size \(\mathrm{a}_{0}\) of specimens, including the 0.5 mm notch depth, ranged from 0.95 mm to 2 mm.  
The graph in Figure 6b illustrates the load-CMOD data of the six J-integral fracture tests performed in FIP solution (JFT-FIP), with initial fatigue crack sizes \(\mathrm{a}_{0}\), including the 0.5 mm notch depth, ranging from 0.6 mm to 1 mm.  
The CMOD data employed in both diagrams given in Figure 6 come from Equation (1), which uses the crack opening displacements \(\mathrm{V}_{1}\) and \(\mathrm{V}_{2}\) measured with the pair of virtual extensometers mounted on the speckled painted surface of each of the tested specimens.  
The simple comparison of the maximum loads reached in the JFT-FIP tests with those obtained in the JFT-Air tests, in which the initial crack sizes were almost double that of the former, shows the strong hydrogen-embrittlement power of the FIP solution. The CMOD values registered at the corresponding maximum loads of the JFT-FIP tested specimens are less than a quarter of those found in the JFT-Air tested ones and confirm that hydrogen uptake not only significantly reduces the bearing load and the deformation capacities of the cracked specimens [32; 33].  
The two-axis plots of Figure 7 show the load (in terms of remote stress) and the J-integral vs. CMOD curves for each of the six tests performed in-air, with the J-integral computed according to BS-8571. The red dot added to each plot indicates the crack initiation, which was automatically detected by the VIC-2D (Version 7) software through the alterations experienced by the sequence of the axial displacements at the crack tip.  
Figure 6: Load–CMOD data of the J-integral fracture tests conducted: (**a**) in air; (**b**) in FIP solution.  
On the same terms are given the plots in Figure 8, which illustrate the experimental results of tests conducted in the FIP solution. In contrast with the tests performed in-air, the onset of cracking occurs with the specimen barely indicating the general yielding. In addition, the stress intensity factor K\({}_{\rm I}\) due to the test load at crack initiation is approximately 31 MPam\({}^{1/2}\), the same that would determine the end of the SSY regime of specimen if it is assumed that the limit of ASTM E-399 [34] is also valid in the SENT case. This limit is given as a function of the specimen thickness B and the 0.2% yield stress R\({}_{\rm p0.2}\) of the material, which yields to:  
\[\rm K_{I}\leq R_{\rm p0.2}\sqrt{0.4B}=31\;MPam^{1/2} \tag{15}\]  
The onset of assisted cracking that occurs at the end of SSY regime, after the first signs of general yielding, shows the embrittlement capacity of steel due to the hydrogen absorption, and also the need for a certain level of plastic deformation for effectiveness.