## 5 Parametric Analysis on the Assessment Results
### Effect of Material Properties  
When evaluating welded joints, BS 7910 suggested using the tensile properties of materials with lower strength, which is a more conservative approach. In Option 2, uniaxial tensile stress-strain curves of materials with lower strength should also be used. The girth weld joints are generally matched with equal strength or high strength, considering the effect of strength matching; as such, the bearing capacity of the pipe section depends mainly on the material properties of the base metal [28]. Even if the same grade of steel pipe is used for the pipeline, the performance parameters are not completely consistent.  
Therefore, in order to verify the effect of the X65 steel performance parameters on the evaluation results, the maximum and mean values of the tensile strength and yield ratio were selected to analyze their effect on the evaluation results. The final calculation results and the corresponding true stress-strain curves are shown in Table 1 and Figure 9, respectively.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Tensile Strength** & **Yield Strength** & **Yield Ratio** & **Hardening Coefficient** & **Power Hardening Index**\(n\) \\ \hline \multirow{3}{*}{535} & 450 & 0.842 & 1.33 & 20.98 \\ \cline{2-5}  & 462.5 & 0.865 & 1.27 & 24.60 \\ \cline{2-5}  & 475 & 0.888 & 1.21 & 29.72 \\ \hline \multirow{3}{*}{647.5} & 545 & 0.842 & 0.93 & 21.06 \\ \cline{2-5}  & 560 & 0.865 & 0.88 & 24.67 \\ \cline{2-5}  & 575 & 0.888 & 0.83 & 29.77 \\ \hline \multirow{3}{*}{760} & 640 & 0.842 & 0.64 & 21.11 \\ \cline{2-5}  & 657.5 & 0.865 & 0.60 & 24.72 \\ \cline{2-5}  & 675 & 0.888 & 0.56 & 29.80 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material performance parameters.  
Figure 8: Effects of wall thickness and misalignment on the crack evaluation of pipe girth welds. (a) \(\sigma_{D}=0.9\sigma_{Y}\left(\mathbf{b}\right)B=11\) mm.  
From the above section, it can be seen that, the thinner the wall thickness, the higher the risk of cracks in the pipe; accordingly, the \(\mathcal{O}813\times 11\) mm pipeline girth weld crack risk is the highest. Consequently, in this section, the results of the \(\mathcal{O}813\times 11\) mm pipeline girth weld with 25 mm \(\times\) 2.5 mm cracks with different material properties under an axial load of 0.9 times the minimum yield strength are analyzed (Figure 10).  
According to the results shown in Figure 10, before the same tensile strength, the lower the yield strength, the higher the corresponding cutoff value, and the wider the envelope range of the failure assessment curve. The increase range is concentrated mainly in the plastic deformation area. In addition, the higher the yield strength, the higher the fracture ratio of the evaluation point, which indicates that a too high yield ratio is not conducive to pipeline safety.  
Figure 10: \(Cont\).  
Figure 9: True stress-strain curves of pipeline steels based on material properties.  
According to Figure 10c,d, under the same yield ratio, the higher the tensile strength of steel, the wider the envelope range of the corresponding failure evaluation curve. Moreover, the higher the fracture ratio of the evaluation point, the wider the envelope range of the evaluation curve, which can provide a larger safety margin. However, the increase in the fracture ratio of the evaluation point increases the risk. Consequently, whether the increase of the tensile strength of the steel has a positive effect on the evaluation results requires further investigation.