## 4 Experimental Verification of Eddy Current Magneto-Optical Phase Imaging of CFRP Crack Defects  
### Line Scan Detection  
The experimental sample is a 100 mm long, 100 mm wide, and 2 mm thick T300 woven carbon fiber board, which is processed with a crack about 7 mm long, 1 mm wide, and 2 mm deep. Figure 8 shows the test results using the line scanning method analyzed above. In the experiment, 300 kHz 30 mA, 600 kHz 15 mA, and 900 kHz 10 mA excitation signals were used, respectively. The results show that the change law of the magnetic field phase is consistent with the simulation results, and only a small excitation current is required when the frequency is increased. As shown in Figure 8, two black vertical dashed lines indicate both ends of the defect. When the detection mechanism is far from the defect, the detected magnetic field phase is relatively stable. As the detection mechanism gradually approaches the defect, the detected magnetic field phase begins to decrease and then increases. When the detection mechanism reaches the left edge of the defect, the detected magnetic field phase shows a rapid increase trend. When the detection mechanism reaches the center of the defect, the detection value reaches its peak. Then, as the detection mechanism moves away from the center of the defect to the right of the defect, the magnetic field phase begins to decrease rapidly. When the detection mechanism leaves the right edge of the defect, the detected magnetic field phase still maintains a certain downward trend, followed by an upward trend. When the detection mechanism gradually moves away from the defect, it returns to a stable state. This series of phenomena reflects the rationality of the experimental process.  
Figure 8: The normalized B\(z\) phase by line scan.  
Figure 7: The normalized B\(z\) phase by the line scan with different measuring distances: (**a**) 4 mm and (**b**) 6 mm.  
### Line Scan Detection  
Figure 9a is a phase image of the above experimental sample scanned with a 600 kHz, 30 mA excitation signal. The red box marked in the figure is the actual position of the defect. As with line scanning, the magnetic field phase in the middle of the defect is larger, and the magnetic field phase on both sides of the defect is smaller. The length of the defect can be roughly determined from the figure, but the width is wider than the actual defect.  
Considering that this is the first time using the ECMO method to detect CFRP materials, the size selection should be large, combined with laboratory processing conditions, so a crack defect about 10 mm long, 1 mm wide, and 1 mm deep is machined on the back of the sample with the same size as in the above experiment.  
Figure 9b is the c-scan phase image of the defect area under the excitation signal of 600 kHz and 30 mA. The internal defect can also be detected from the results, but the phase change is smaller than for the surface defect.  
From these two actual inspection images, it can be seen that the imaging law of detecting CFRP material defects using the ECMO method is consistent with the previous simulation analysis results. At the same time, the edge of the defect can be roughly determined. In addition, subsequent algorithms can be used to reconstruct defects.