# Fiber Bragg Gratings Based Cyclic Strain Measuring of Weld Toes of Cruciform Joints
## 5 Discussion  
The final fatigue fracture surface of the specimen is shown in Figure 16, where the fatigue zone and the instantaneous fracture zone can be clearly observed. Three obvious cracks are visible in the fatigue zone, which are labeled as crack 1, crack 2 and crack 3, respectively. All the three cracks have crack fronts with semi-elliptical shape, whose centers correspond to the crack initiating positions. Relating these three positions with the weld toe geometry captured before testing, it can be seen that three cracks were all initiated in the positions with a larger SCF. In another word, three cracks all initiated near the testing points, which indicates that the cracks initiation depends greatly on the local stress and strain state at the weld toe. Considering the weld toe geometry characteristics discussed in the study above, it can be drawn that geometric points such as convex vertices of the arc segment along the weld toe curve are prone to fatigue damage and should be paid more attention to in engineering application.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Phase**} & **No. of** & **Begin** & **End** & \multicolumn{4}{c}{**Strain (\(\mu\epsilon\))**} & \multicolumn{4}{c}{**Stress (MPa)**} \\ \cline{3-11}  & **Testing Point** & **Cycle** & **Cycle** & \(\epsilon_{\text{max}}\) & \(\epsilon_{\text{min}}\) & \(\Delta\epsilon_{\text{max}}\) & \(\Delta\epsilon_{\text{min}}\) & \(\sigma_{\text{max}}\) & \(\sigma_{\text{min}}\) & \(\Delta\sigma_{\text{max}}\) & \(\Delta\sigma_{\text{min}}\) \\ \hline \multirow{3}{*}{Strain decline} & 1\# & 1 & 57,155 & 1354 & 423 & 1356 & 981 & 283 & 78 & 286 & 202 \\  & 2\# & 1 & 67,036 & 1687 & 453 & 1696 & 1081 & 346 & 87 & 353 & 221 \\  & 3\# & 1 & 80,472 & 1951 & 569 & 1970 & 1110 & 398 & 108 & 407 & 232 \\  & 1\# & 5 In addition, Figure 16 shows that the size and brightness of the three cracks are significantly distinct, reflecting the degrees of oxidation of the cracked surfaces are different from each other, which can indirectly indicate that the initiation time of the three cracks is unsynchronized. Specifically, crack3 is firstly initiated at testing point 3#, and then crack2 at testing point 2# and at last, crack 1 at testing point 1#. The end parts (300,000 cycles-end) of the strain time history of the three testing points were extracted, and the strain range curves were drawn in Figure 17. It shows a trend of stable-rise and rapid-drop. Among the three curves, testing point 3# with the largest SCF first exhibits a mutation, according to which mechanical explanation for the unsynchronized initiation of weld toe cracks can be proposed. When the specimen is subjected to cyclic loads, the site with the largest _SCF_ on the weld toe (for example, testing point 3#) firstly satisfies the crack initiation condition, the first crack initiates and propagates at both the longitudinal direction and the thickness direction simultaneously. As the crack surface becomes larger, the actual load-carrying sectional area of the specimen gradually decreases. Consequently, stresses and strains at another site with the second largest _SCF_ (for example, testing point 2#) increases continuously till it satisfies the crack initiation condition, too. Then, a new crack initiates at this site. This process repeats again and again until the specimen cannot resist the load anymore, resulting in final fracture. Therefore, in some way, the irregular shape of the weld toe directly affects the stress and strain state at the weld toe. If weld defects are not considered, it will further determine multiple cracks phenomenon at the weld toe and the unsynchronized characteristics of these cracks.  
Figure 16: Comparison of cracks position and testing points.  
Figure 17: Strain range curve of the final phase.