# Nondestructive Surface Crack Detection of Laser-Repaired Components by Laser Scanning Thermography
## 4 Scanning Thermography for Crack Detection of Repair Component
### Experimental Setup System and Detection Scheme  
The system adopted in the experiment is shown in Figures 11 and 12a, which mainly includes laser transmitter, image acquisition device, infrared camera and tested components [18]. VarioCam HR infrared camera produced by Infratec in Germany is used, with a resolution of 1024 \(\times\) 768 pixels, the measurable temperature range is \(-\)40-1200 \({}^{\circ}\)C, the measurement accuracy is \(\pm\)1 \({}^{\circ}\)C or \(\pm\)1%, the temperature resolution (30 \({}^{\circ}\)C) is up to 0.02 K, and the maximum thermal image acquisition frequency is 30 Hz.  
Fiber Laser Marking Machine (XQFL-G50 W) is used as the laser transmitter, and its maximum laser power is 50 W. Based on the parameter analyses in the simulation, we set the laser scanning speed in the experiment to be 15 mm/s, and the laser power 3 W to ensure no damage to the specimen surface. Set the image acquisition frequency to be 30 Hz to obtain enough thermal image data and reduce the loss of information.  
Figure 10: \(T_{G}\) at different thermal image acquisition frequencies (\(v\) = 10 mm/s, \(P\) = 1200 mW and acquisition time is 2 s): (**a**) \(f\) = 10 Hz; (**b**) \(f\) = 20 Hz; (**c**) \(f\) = 50 Hz; (**d**) \(f\) = 100 Hz.  
The edge area of the plate-like specimen shown in Figure 12b is repaired using LENS technique. The repair layer is made of iron-based alloy, and the surface cracks are formed in the process of manufacturing. The repair layer is divided into three main areas: Area-1, Area-2 and Area-3, where the area of Area-1 is 8.4 \(\times\) 2.2 cm\({}^{2}\). Area-1 was detected by a combination of various scanning modes. As shown in Figure 12c, assuming that the direction perpendicular to the molten pool is 0\({}^{\circ}\), and the angle between the scanning direction and the 0\({}^{\circ}\) direction is \(\theta\), The scanning combination modes are shown in Table 3. The line spacing of the laser scanning path is set to be 3 mm to improve the detection efficiency.  
Figure 11: Crack detection system based on laser scanning thermography [10; 37].  
Figure 12: Experimental system: (**a**) laser thermography system; (**b**) laser-repaired component; (**c**) Setting of laser scanning direction.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Combinations** & \(\mathbf{\theta}\) \\ \hline
**Single angle** & One of 0\({}^{\circ}\), 20\({}^{\circ}\), 40\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), 80\({}^{\circ}\), 90\({}^{\circ}\), 100\({}^{\circ}\), 120\({}^{\circ}\), 135\({}^{\circ}\), 140\({}^{\circ}\), 160\({}^{\circ}\) \\
**2 angles** & 0\({}^{\circ}\), 90\({}^{\circ}\) \\
**3 angles** & 0\({}^{\circ}\), 60\({}^{\circ}\), 120\({}^{\circ}\) \\
**4 angles** & 0\({}^{\circ}\), 45\({}^{\circ}\), 90\({}^{\circ}\), 135\({}^{\circ}\) \\
**6 angles** & 0\({}^{\circ}\), 20\({}^{\circ}\), 90\({}^{\circ}\), 120\({}^{\circ}\),140\({}^{\circ}\) \\
**9 angles** & 0\({}^{\circ}\), 20\({}^{\circ}\), 40\({}^{\circ}\), 60\({}^{\circ}\), 80\({}^{\circ}\), 100\({}^{\circ}\), 120\({}^{\circ}\), 60\({}^{\circ}\), 40\({}^{\circ}\), 160\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Detecting Area-1 using different laser scanning combinations.  
The other two areas (Area-2 and Area-3) are detected by scanning in the four directions of 0\({}^{\circ}\), 45\({}^{\circ}\), 90\({}^{\circ}\) and 135\({}^{\circ}\). To ensure the detection quality, the line spacing of area-2 and area-3 is set to be 2 mm and 1 mm to achieve more precise scanning.