## 2 Eddy Current Magneto-Optical Imaging System Settings
### Detection of Crack Defects  
The AC/DC module of COMSOL Multiphysics 5.5 was used to perform finite element simulation on the woven carbon fiber plate. The purpose is to study the effect of crack defects on the surface vertical magnetic field of the sample under EC excitation. The schematic diagram of the simulation model is shown in Figure 3. During measurement, the magneto-optical sensor is placed parallel to the test piece, and the probe light is vertically incident on the magneto-optical sensor. Its Faraday rotation angle is only affected by the magnetic field perpendicular to the surface of the test piece. The sample size is 100 mm long, 100 mm wide, and 2 mm thick, and the size of the sample defect is 10 mm long, 0.4 mm wide, and 2 mm deep. The detection point is 5 mm next to the excitation coil center point. Since the magnetic field at the detection point is mainly affected by the EC between the center of the coil and the detection point, the simulation takes the middle point of these two points to represent the position point of the detection device. We made the lower left corner of the specimen surface the coordinate origin (0, 0, 0), and the coil lifting distance is 0.5 mm. The coil moves from 32.5 mm to 67.5 mm in steps of 1 mm. The coil has 1 mm inner diameter, 2.6 mm outer diameter, 0.8 mm height, 0.05 mm wire diameter, and 140 turns; excitation signal frequency set to 100 kHz, 300 kHz, 600 kHz, and 1 MHz; and current set to 30 mA. Due to the simplification of the distribution of electrical conductivity in the braided structure, the conductivity tensor in Equation (3) above can be simplified into a diagonal form, with the value on the main diagonal set to (10,000, 10,000, 100) S/m. The first step is to simulate with a non-defective sample before defect scanning and obtain the corresponding reference value to normalize the scanning results. Next, the excitation coil is placed on the position (50, 50, 0.5) above the sample without defects. Figure 4 shows the line scan of the phase of the magnetic induction intensity Bz in a vertical direction at z = 0.5, y = 50, and x from 30 to 70. The phase of the vertical magnetic field in the area near the coil on the outside of the coil is affected by the eddy current and first becomes larger and then smaller. Some EC magnetic testing methods are used to detect the change of the superimposed magnetic field inside the excitation coil [20]. Only the superimposed magnetic field outside the excitation coil is considered here. The magnitude of the excitation magnetic field outside the coil is several orders of magnitude smaller than the magnitude of the excitation magnetic field inside the coil, but the difference in the magnitude of the eddy current is not large, so the superimposed magnetic field outside the coil is more affected by the EC [20]. When performing line scanning according to Figure 3, the distribution of EC in the sample is shown in Figure 5. In the figure, blue is the excitation current and red is the EC. When the excitation coil approaches the defect, the EC at the edge of the defect will increase, with the distribution shown in Figure 5a. When the excitation coil moves on the defect, the EC distribution is shown in Figure 5b. At this point, the magnetic field direction of the EC at the detection point is opposite that of Figure 5a. The eddy current distribution when the excitation coil continues to move to the edge of the defect is shown in Figure 5c. At this point, the magnetic field direction generated by the EC at the detection position is consistent with that in Figure 5a, but the extended EC will have a certain counteraction effect, so the impact should be less than that in Figure 5a. There are EC magnetic fields in opposite directions and exciting magnetic fields in the same direction at different positions during scanning, which makes the effect of the EC magnetic fields on the phase of the superimposed magnetic fields opposite.