## 4 Results and Discussion
### Agreement of the Cohesive Model with the Experimental Data  
The value of the cohesive resistance Y is the only degree of freedom that allows the theoretical predictions derived from the cohesive model to match the experimental results. However, as shown by the curves in Figure 5, Y operates as a scale factor regarding the predictions and therefore the shape of the theoretical load-CMOD and J-CMOD curves must be similar to those of the experimental ones up to cracking initiation, as actually occurs. The cohesive resistance Y has to be the value of the scale factor that makes the curves of both origins match. Figure 7: Remote stress and J-integral vs. CMOD curves of the six tests performed in-air with precracked SENT specimens (a\({}_{0}\) is the initial crack size, including the 0.5 mm notch depth). Hence, two options were taken into account to find the cohesive resistance from the two curves given in each plot of Figures 7 and 8. The options consist of identifying the maximum remote stress soY and the slope dJ/dwM of the linear part of the J-integral-CMOD curve, respectively, with the theoretical values given by Equations (13) and (14). This double identification leads to the following values YJ and Ys for the cohesive resistance:  
\[\mathrm{Y_{J}}=\frac{1}{1-1.03\ \frac{a}{W}\ \mathrm{d}\omega_{M}} \tag{16}\]  
\[\mathrm{Y_{\sigma}}=\frac{\sigma_{\mathrm{soi}}}{\left(1-\frac{a}{W}\right)^{ 2}} \tag{17}\]  
The values of Ys are derived from the instability stage of the loading process according to the theoretical model. Nevertheless, in the tests instability and cracking could interfere to the detriment of the reliability of Ys. As a consequence, the values of YJ were those used to determine the cohesive resistance of the bar steel from the JTF tests. The condition of the material constant, implicit in the cohesive model for the resistance Y in the absence of hydrogen uptake, means that its experimental values resulting from the JFT-Air tests should not differ from each other much more than expected in a series of tests for measuring any constant characterizing the fracture behavior of a material. In the case of  
Figure 8: Remote stress and J-integral vs. CMOD curves for the six tests performed in the FIP medium with SENT specimens (a\({}_{0}\) is the initial crack size, including the 0.5 mm notch depth). the fracture toughness K\({}_{\text{Ic}}\), a standard deviation of less than 3% of the mean value is almost unattainable [34]. In addition to this, the mean value should be consistent with the physical meaning attributed to the cohesive resistance in the model; that is, the yield stress of an ideally plastic material. The mean value of the 0.2% yield stress and tensile strength, or the tensile strength, are often adopted as ideal yield stress in engineering applications based on theoretical models assuming no hardening strain. The tensile strength of the steel bar is 1138 MPa, as indicated in Table 2.  
Table 3 shows the cohesive resistance values obtained from the J-integral fracture tests of the bar steel performed in-air. The standard deviation of the series of values obtained for Y\({}_{\text{J}}\), of only 7% of the mean value, is not higher than those expected in fracture mechanics testing, despite having used fatigue and assisted cracking to generate the initial crack. The mean value of 1122 MPa hardly differs from the tensile strength of the bar steel of 1138 MPa. Therefore, all three aforementioned conditions for the experimental data to agree with the cohesive model are satisfied by the bar steel when hydrogen-free. Table 4 gathers the cohesive resistance values obtained from each JFT-FIP test of the bar steel performed in the FIP medium. Again, the standard deviation of the series of values obtained for Y\({}_{\text{J}}\), of 8% of the mean value is not higher than those expected in fracture mechanics testing. However, the mean value of the series strongly differs from the tensile strength of the bar steel, to the point of almost not exceeding 60% of it.