# Nondestructive Surface Crack Detection of Laser-Repaired Components by Laser Scanning Thermography
## 4 Scanning Thermography for Crack Detection of Repair Component
### Experimental Results and Analysis

Table 4: Comparison of two post-processing algorithms. Figure 14: Temperature field of laser scanning in four directions and extraction results of crack morphology: **(a)**\(0^{\circ}\) direction; **(b)**\(45^{\circ}\) direction; **(c)**\(90^{\circ}\) direction; **(d)**\(135^{\circ}\) direction. \(0^{\circ}\) and \(90^{\circ}\) is adopted, the horizontal and numerical crack detection can be achieved, but the detection effect along oblique crack is not good. When the scanning combination of \(0^{\circ}\), \(45^{\circ}\), \(90^{\circ}\) and \(135^{\circ}\) is adopted, better detection effect can be obtained, and the resulting image contains more crack details. When six or more scanning directions are used for detection, the detection effect is not significantly improved and the detection efficiency is low. Therefore, it is recommended to use the scanning combination of \(0^{\circ}\), \(45^{\circ}\), \(90^{\circ}\) and \(135^{\circ}\) to ensure the detection effect and efficiency. Thermography method has good detection results in different repair areas. As shown in Figure 16, in the detection of the other two areas, the edge detection algorithm based on Prewitt operator is used to process the temperature change image sequence by combining the four-direction scanning combination mode. Clear crack shape and geometric distribution can be obtained. Therefore, it can be seen that laser scanning thermography has good applicability in the detection of different areas. By using laser scanning thermography in three areas, we can obtain the crack shape and geometric distribution in each part of the repair layer, as shown in Figure 17.  
Figure 16: Detection results of two small areas: (**a**) results of individual scans along each direction and their superimposed results in Area-2; (**b**) results of individual scans along each direction and their superimposed results in Area-3. Figure 15: Detection results of six different scanning direction combination schemes. From the detection results shown in Figure 17, obvious cracks exist in the tested area with a dendritic distribution. The main cracks are distributed along the direction of 0\({}^{\circ}\), and the angle between the secondary cracks and the main cracks is mostly 45-90\({}^{\circ}\). By observing Area-1 with a 50\(\times\) microscope, a clear crack shape and geometric distribution can be obtained as shown in Figure 18a, which is in good agreement with the results of thermographic detection as shown in Figure 18b. Based on the laser spot scanning thermography, a crack with a minimum width of 10.07 \(\upmu\)m can be detected, which proves that the thermography method has good applicability and high detection sensitivity for the detection of irregular surface cracks. Figure 17: The crack detection results of each part of the repair layer. Figure 18: Comparison of laser scanning thermography detection result and microscope observations result of surface cracks in Area-1: **(a)** microscopic observation of surface cracks (50\(\times\)); **(b)** surface crack detection result based on thermography; **(c)** cracks width measurement in area A under microscope; **(d)** cracks width measurement in area B under microscope.