## 6 Discussion
The principle of this approach consists of considering the constants \(C_{1}\) and \(C_{2}\) in Equation (11) to be functions of a constraint; for example, Q, T/\(\sigma\), or A\({}_{2}\). \[J(\Delta a)=C_{1}(\Delta a)^{C_{2}} \tag{11}\]  
Owing to its simplicity, even though it is primarily intended for small-scale yielding, we chose the T/\(\sigma\) parameter to construct a constraint-dependent R-curve for the SENT specimens. In this case, Equation (11) changes to Equation (12):  
\[J(\Delta a,\Phi)=C_{1}(\Phi)[\Delta a]^{C_{2}(\Phi)} \tag{12}\]  
where \(\Phi=T/\sigma\). For determining the two functions \(C_{1}(\Phi)\) and \(C_{2}(\Phi)\), it is necessary to know the J integral for two crack extensions: \(\Delta a_{1}\) and \(\Delta a_{2}\). Because J(\(\Delta a\),\(\Phi\)) for a given crack extension \(\Delta a\) also depends on the parameter \(\Phi\), the mathematical expression for J(\(\Phi\)) can be obtained  
Figure 8: J-R curves of cracked SENT and CT specimens [18]. by a curve fitting the experimental data. The author of [19] recommends that at least three data points should be used for a linear curve fitting. Unfortunately, we have only one real pair of data points (experimental CT R-curve); however, we believe that the results of [PERSON] and [PERSON] [18], even though obtained by means of a virtual testing framework, can be qualified as experimental data points after transforming the R-curves in Figure 8 to R-curves considering crack blunting. These curves can be seen in Figure 9 for the CT specimens and in Figure 10 for the SENT specimens. The transformation of the R-curves was carried out in such a way that the graphical curves in Figure 8 were digitalized and then the magnitudes of the crack extensions were increased by a blunting amount, i.e., \(\mathrm{J}/4\sigma_{\mathrm{fs}}\). The flow stress \(\sigma\)fs considered was the same as that determined experimentally in this work, i.e., \(\sigma_{\mathrm{fs}}=513\) MPa. The points in the diagrams in Figures 9 and 10 represent the digital readings of the curves in Figure 8, with the \(\Delta\)a values being increased by \(\mathrm{J}/4\sigma_{\mathrm{fs}}\). In both figures, there are two straight lines: the blunting line and the \(0.2\) mm offset line, both determining fracture parameters, namely \(\mathrm{J}_{\mathrm{in}}\) (the point of intersection of the blunting  
Figure 10: SENT R-curve transformed from Figure 8.  
Figure 9: CT R-curve transformed from Figure 8.  
line with the R-curve) and J\({}_{0.2}\) (the point of intersection of the 0.2 mm offset line with the R-curve). Following the procedure in [19], for the two crack extensions \(\Delta a_{1}\) and \(\Delta a_{2}\), it can be determined from Equation (12) that  
\[J_{\Delta a_{1}}(\Phi)=C_{1}(\Phi)(\Delta a_{1})^{C_{2}(\Phi)} \tag{13}\]  
Figure 8.0  
By solving Equations (13) and (14) for C\({}_{1}\)(\(\Phi\)) and C\({}_{2}\)(\(\Phi\)), we can obtain  
Figure 8.1  
Figure 8.2  
It is advantageous to choose \(\Delta a_{2}=1\) mm, for then the function C\({}_{1}\)(\(\Phi\)) is equal to \(J_{\Delta a_{2}}(\Phi)\). Let us further choose \(\Delta a_{1}=0.2\) mm. Now we need to know the dependence of the parameter \(\Phi=T/\sigma\) on the relative initial crack length a\({}_{0}\)/w for CT specimens as well as for SENT specimens. For this purpose, we can use the Compendium of T-stress Solutions published by [PERSON] et al. [20]. There are three sources here for CT specimens and four sources for SENT specimens loaded in tension. We have chosen the polynomial (17) for the CT specimens and polynomial the (18) for the SENT specimens, taking into account the ratio of the length of the SENT specimen to its width. Figure 8.3  
\[ \begin{array}{l}\frac{T}{\sigma}=0.639-27.332\big{(}\frac{a}{w}\big{)}+233.1 6\big{(}\frac{a}{w}\big{)}^{2}-981.98\big{(}\frac{a}{w}\big{)}^{3}+2162.0\big{(} \frac{a}{w}\big{)}^{4}-\\ 2386.7\big{(}\frac{a}{w}\big{)}^{5}+1057.8\big{(}\frac{a}{w}\big{)}^{6}\end{array}