# Fracture Assessment of Weld Joints of High-Strength Steel in Pre-Strained Condition
## 4 Results and Discussion  
In this estimation, the fracture toughness of the pre-strained HAZ obtained by the experiments was employed. WES 2808-2003 [13] provides the estimation method for the pre-strained toughness, as shown in Figure 7. The static fracture toughness of the base metal at the reference temperature of \(\mathrm{T}-\Delta\mathrm{T}_{\mathrm{PD}}\) can be used as the fracture toughness in the pre-strain and the dynamic conditions at the service temperature \(T\). The \(\Delta T_{\mathrm{PD}}\) is the temperature shift of the fracture toughness caused by the pre-strain and the dynamic loading. In WES 2808, the temperature shift \(\Delta T_{\mathrm{PD}}\) correlates with the flow stress elevation, \(\Delta\sigma_{\mathrm{f}}^{\mathrm{PD}}=(\Delta\sigma_{\mathrm{Y}}+\Delta \sigma_{\mathrm{T}})/2\), by pre-strain and dynamic loading. \(\Delta\sigma_{\mathrm{Y}}\) and \(\Delta\sigma_{\mathrm{T}}\) are the increase in the yield stress and tensile strength, respectively. The temperature shift \(\Delta T_{\mathrm{PD}}\) is given by Equation (4) [18]. This formula is applicable to the structural steels of 400 to 590 MPa-strength class.  
\[\Delta T_{PD}\left({}^{\circ}\mathrm{C}\right)=\left\{\begin{array}{l}0.4 \Delta\sigma_{f}^{PD}\ :0\leq\sigma_{f}^{PD}\leq 100\ (MPa)\\ 40\ :100\leq\sigma_{f}^{PD}\leq 300\ (MPa)\end{array}\right. \tag{4}\]  
According to WES 2808, the fracture toughness of the pre-strained HAZ of HSB600 steel was predicted from the HAZ toughness, without the pre-strain. First, the flow stress of the pre-strained HAZ was estimated under the assumption that the flow stress is the same as that of the pre-strained base steel. The yield stress and tensile strength, as well as the flow stress \(\sigma_{\mathrm{f}}^{\mathrm{PD}}\), of the pre-strained weld HAZ are summarized in Table 7; these were calculated using Equations (2) and (3), on the basis of the  
Figure 6: Critical CTODs for the 3.0% pre-strained ETCP estimated from 3.0% pre-strained (three-point bend) 3 PB test results.  
Figure 7: Temperature shift of fracture toughness \(\Delta T_{\mathrm{PD}}\) in pre-strained and dynamic conditions.  
stress-strain curve shown in Figure 5. Then, the temperature shift \(\Delta T_{\text{PD}}\) for the pre-strained HAZ was calculated using Equation (4) [13], and the values are shown in Figure 8. The median of the critical CTODs estimated for the 3.0% pre-strained HAZ at \(-20\)\({}^{\circ}\)C and \(0\)\({}^{\circ}\)C are listed in Table 8.  
Then, from the estimated critical CTODs, \(\delta_{\text{cr,ETCP}}\) for the 3.0% pre-strained ETCP with a crack in the HAZ was estimated with the equivalent CTOD ratio \(\beta_{\text{2a}}\). The \(\beta_{\text{2a}}\) used was the same as that listed in Table 6. As shown in Figure 9, the \(\delta_{\text{cr,ETCP}}\) for the 3.0% pre-strained ETCP, derived from the toughness values estimated on the basis of WES 2808, was almost the same as that estimated from the experimental data of critical CTOD, for the 3.0% pre-strained 3 PB specimen (Figure 6).  
\begin{table}
\begin{tabular}{c c c c c} \hline
**BM-2** & **\(\sigma_{\text{Y}}\) (MPa)** & **\(\sigma_{\text{0.2}}\) (MPa)** & **\(\sigma_{\text{T}}\) (MPa)** & **\(\sigma_{\text{f}}\)****PD (MPa)** & **YR (=\(\sigma_{\text{Y}}\)/\(\sigma_{\text{Y}}\))** \\ \hline \(\varepsilon_{\text{pre}}=0\%\) & 605 & 613 & 693 & 649 & 0.87 \\ \(\varepsilon_{\text{pre}}=1.5\%\) & 649 & – & 748 & 699 & 0.87 \\ \(\varepsilon_{\text{pre}}=3.0\%\) & 684 & – & 759 & 722 & 0.90 \\ \hline \end{tabular} \(\sigma_{\text{Y}}\) : Yield stress, \(\sigma_{\text{0.2}}\): 0.2% proof stress, \(\sigma_{\text{T}}\) : Tensile strength, \(\sigma_{\text{f}}^{\text{PD}}:(=\sigma_{\text{Y}}+\sigma_{\text{Y}})/2\).  
\end{table}
Table 7: Mechanical properties of HSB600 steel with and without pre-strain.  
Figure 8: Temperature shift \(\Delta T_{PD}\) for the pre-strained HAZ predicted with the procedure specified in WES 2808.