## 6 Discussion  
A simulation of stable crack growth in CT and SENT specimens was carried out. The calculations performed on one of the CT specimens were used as a calibration for finding the GTN model parameters. It was found from these calculations that there was no need to consider the nucleation of new voids (parameter \(f_{N}\) = 0). The calibrated model was used to calculate the J-R curve for the clamped SENT specimens. The results were processed by regression analysis using the least-square method. A comparison of this regression curve with the experimental R-curve for the CT specimens showed that the SENT R-curve was practically equidistant to the CT R-curve, the shift in the y-direction being AJ \(\sim\) 165 N/mm on average, and its points of intersection with the blunting line and the 0.2 mm offset line provided the following fracture toughness characteristics: J\({}_{\text{in}}\) = 467.6 N/mm and J\({}_{0.2}\) = 718.5 N/mm. As already mentioned, the position of the calculated SENT R-curve above the experimental CT R-curve in the J--\(\Delta\)a diagram was not verified experimentally. This was because the existing standards relating to the determination of fracture toughness in metallic materials using SENT specimens require the minimum thickness to be half the width [16], or even one full width [17]. These standards were set primarily to meet the needs of the steel pipeline industry. The specimens considered in this study (thickness 8.7 mm and width 50 mm) did not meet this requirement. The presented paper attempted to determine the J-R curve for SENT specimens with a thickness smaller than that required by the standards ([16; 17]). This was realized by means of both the GTN method and the experimentally determined J-R curve for CT specimens of the same width, thickness, and nearby relative crack length as those of the SENT specimens. There are a few arguments indicating that the position of the R-curve for the SENT specimens in the J--\(\Delta\)a diagram (Figure 7) is very near to the correct R-curve. The first argument stems from the fact that the J-R curve for the CT specimens calculated by the GTN  
Figure 7: A comparison of the J-R curves for the CT and SENT specimens. model parameters is practically the same as the experimental one. The second argument arises from the work of [PERSON] and [PERSON] [18], in which they used a two-parameter fracture mechanics approach (J--Q) to quantify the level of constraint in a particular component and in fracture test specimens (CT, SENT) of varying constraint levels. By means of a virtual testing framework, they constructed the J-R curves for CT and SENT specimens to determine the J\({}_{0.2}\)-based fracture toughness values. The R-curves obtained are reproduced in Figure 8.  
The material considered was pipeline steel X65, as was the steel used in this paper. As for the geometry of the SENT and CT specimens considered in [18], the width and thickness were the same for both specimens (w = 25.4 mm; B = 12.7 mm), while the relative crack length a/w was 0.2 for the SENT specimens and 0.5 for the CT specimens. As already shown, the width and thickness of the SENT and CT specimens dealt with in this paper were w = 50 mm; B = 8.7 mm, and the relative crack length a/w was 0.3 for the SENT specimen and 0.503 for the CT specimen. The differences in dimensions between the specimens considered in [18] and the specimens considered in this paper cause a certain difference in the crack tip constraint, which can lead to some differences in the respective R-curves. However, the character of the R-curves in Figure 8 indicates that the R-curve for the SENT specimens in Figure 7 is very likely correct. In order to support this statement, we will utilize the curves in Figure 8 together with the experimentally determined R-curve for the CT specimens (Figure 4) to construct the R-curve for the SENT specimens by using the approach of non-dimensional T stress, as described by [PERSON] [19].