# Fiber Bragg Gratings Based Cyclic Strain Measuring of Weld Toes of Cruciform Joints
## 3 Test Configuration
### Testing Points  
The weld toe of the specimen is not an ideal line, resulting in the stress and strain state along the weld toe is variable under the same testing load, which finally affects the initiation sites of the weld toe cracks. It is necessary to determine the testing points where FBG will be fixed before testing. Based on some previous investigations, the weld toe can be simply described as a curve composed of some small arc segments, where convex vertices and intersectional vertices are the positions of discontinuity [41]. Together with obvious weld defect points, convex vertices and intersectional vertices can be defined as the characteristic points on the weld toe curve, which are potential positions \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(\alpha_{s}\) (MPa) & \(\alpha_{b}\) (MPa) & \(\delta\) (\%) & E (MPa) & \(\mu\) & \(\mathbf{K^{\prime}}\) (MPa) & \(\mathbf{n^{\prime}}\) \\ \hline
385 & 540 & 25.5 & \(2.06\times 10^{5}\) & 0.3 & 1281 & 0.2069 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic mechanical parameters of the specimen. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**C** & **Si** & **Mn** & **P** & **S** & **Al** \\ \hline
0.16 & 0.35 & 1.34 & 0.16 & 0.06 & 0.0004 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical compositions of the specimens (%). Figure 5: Cruciform welded joint: (**a**) Geometry of the specimen (mm); (**b**) 3D model of the specimen. to form micro-cracks, as shown in Figure 6. A series of morphologies of the characteristic weld sections were obtained by a CCD (Charge Coupled Device) digital microscope (LY, Qingyang district, Chengdu, China). Then the geometry parameters of those weld sections were measured based on digital image technology, and weld toe SCFs of corresponding sections were calculated by Equation (5). The measuring results and calculation results were shown in Table 3, where L is the distance from the left end surface to the corresponding section. Due to the full penetration of the specimen, the weld un-penetration size c is zero. Table 3 shows that SCF at different weld toe characteristic points ranges from 1.90 to 2.06. Three points with largest SCF values were chosen as the main testing points (1#, 2#, 3#), at which strain time histories were recorded and analyzed. Another testing point (0#) was configured on the surface of the thin plate, which was used to verify the testing load. It is noteworthy that uniaxial tensile load was employed in the test (as shown in Figure 7), so only the axial strain needed to be measured. Therefore, FBG sensors at testing points 1#, 2#, 3# and 4# were fixed along the loading direction. Temperature compensation was realized by the testing point (T). In order to get rid of extra stress of the specimen, this FBG sensor was placed exactly in the middle of the thick plate and along the weld direction, which was vertical to the loading direction. All the five FBG sensors are single-mode optical fibers (SMF) with different grating Bragg wavelengths, as shown in Table 4. Each FBG sensor was connected to the respective channel of the demodulator. Detail arrangements are shown in Figure 7, where \"L-\" in Table 4 is defined as the distance between the testing point and the left end surface. It should also be noted that, before fabricating the FBG sensors, the surface of the weld of the specimen should be cleaned with alcohol so as to get rid of stains on it. Due to good flexural resistance, the FBG can be fabricated across the characteristic points. In detail, one end of the FBG sensor was fixed on the surface of the thin plate, while the other end was fixed on the surface of the weld. When fabricating, make sure that the grating section of the FBG sensor was firmly attached on the surface of the weld toe. The fixing between FBG sensors and test points is realized by commercial adhesive (Structural Epoxy Adhesives DG-4). In order to prevent the FBG from breaking during the testing,