# Nondestructive Surface Crack Detection of Laser-Repaired Components by Laser Scanning Thermography
## 3 Simulation Analysis of Factors Affecting Detection Effect
### Thermographic Process Parameters  
The selection of laser parameters directly determines the quality of surface crack detection, so the optimization of thermographic process parameters is crucial. Based on the simulation model shown in Figure 4, the influences of laser effective power \(P\), scanning speed \(v\) and thermal image acquisition frequency \(f\) on the detection results are studied. A nickel-based alloy is selected as the repair material, the crack width \(L\) is 40 \(\upmu\)m, and the laser spot radius is 1 mm. The setting of parameters to be discussed is shown in Table 2.  
Figure 7: Crack detection for four kinds of materials with different thermomechanical properties: (**a**) superposition result of \(\Delta T\) of each frame under laser excitation; (**b**) gradient results of temperature change field \(T_{G}(T_{G}\geq 0\) ); (**c**) \(T_{G}\) along the crack centerline; (**d**) \(T_{G}\) along the scan path of the laser center point. Figure 8 shows the calculation results of \(T_{G}\) field under different laser effective powers, and the results of the display range with fixed temperature are shown in Figure 8a. With the increase in laser effective power, the heat input per unit time of the model increases, and the thermal resistance effect of the crack on the heat wave increases, so does the amplitude of the temperature gradient. Figure 8c shows that there is a linear relationship between the amplitude of \(T_{G}\) and the effective power \(P\) of the heat source. The increase in effective laser effective power will increase the heat affected zone, accordingly, for materials with low thermal conductivity, the increase in laser effective power is generally used to expand the heat affected zone and improve the quality of detection. However, excessively high laser effective power will cause damage to the specimen surface, so the appropriate laser effective power should be selected under the premise of considering the material properties. Laser scanning speed is one of the important parameters which determines the detection efficiency and quality. With the increase in laser scanning speed \(v\), the following phenomena can be observed:  
1. Due to the rapid movement of the laser, the heat input to the model decreases, and the amplitude of the surface temperature field decreases;
2. The trailing phenomenon is more significant in the laser scanning process (Figure 9a), the temperature field contour gradually becomes flat, the heat-affected zone decreases, and the crack characteristics are weakened;
3. When the heat source scans through the crack, the maximum thermal gradient on the specimen surface decreases due to the weakening of the thermal blocking phenomenon, as shown in Figure 9c. \begin{table}
\begin{tabular}{c c c c} \hline  & \(P\) **(mW)** & \(v\) **(mm/s)** & \(f\) **(Hz)** \\ \hline
1 & 1200 & 5 & 10 \\
2 & 1400 & 10 & 20 \\
3 & 1800 & 20 & 50 \\
4 & 2400 & 40 & 100 \\ \hline \end{tabular}
\end{table} 
Table 2: The setting of heat laser parameters and thermal image acquisition parameters. Figure 8: Crack detection at different laser effective power (\(v=10\) mm/s, \(f=50\) Hz): **(a)** superposition results of \(\Delta T\); **(b)** linear fitting relationship between \(P\) and the maximum of \(T_{G}\); **(c)** the distribution of \(T_{G}\) along crack Center (\(T_{G}\geq~{}0\) ). When selecting the laser scanning speed, the detection quality should be mainly considered, which is reflected by the thermal gradient \(T_{G}\). Secondly, the detection time should also be considered, which is determined by the scanning speed \(v\) and the scanning line spacing. On the one hand, when the scanning speed of laser is too low, especially below 10 mm/s, the scanning time is too long and the amount of data obtained is large, the time for processing costs longer time. Besides, the thermal diffusion range will become limited, which may lead to an incomplete detection of defects.