# Internal Damage Analysis of Braided Composites Embedded in Carbon Nanotube Yarn
## 3 Results and Discussion  
Different damage information needs to be monitored in the experiment. By analyzing the change in the resistance data of a carbon nanosensor in the material after damage, the existence and type of damage can be determined, and the location of damage can be inferred. All the specimens used in the experiment were prepared by the Institute of Composite Materials, Tianjin University of Technology. Five 3D braided composite specimens with CNT yarn were selected to study the effect of CNT yarn on the internal damage of 3D braided composites. The experimental specimens were rectangular plates with a size of 250 mm \(\times\) 25 mm \(\times\) 4 mm. The relevant parameters of the test pieces are shown in Table 1, and the specimen shape is shown in Figure 7.  
Figure 5: Sketch of SVD of a four-block matrix. Figure 6: Flow chart of the SVD algorithm of the four-block matrix. Specimen 5 was a healthy 3D braided composite material. Specim 1 had a shock damage caused by an Instron Dynatup9250 HV-type load-down impact testing machine produced by the YingSiTe Company. The experiment used a hemispherical hammer with a diameter of 12.7 mm and a drop weight of 6.5 kg. Two damage points were set in Specimens 2 and 3 to fully verify the damage location accuracy. Specim 2 had two circular hole-like damages with a diameter of 5 mm. Damage 3-1 in Specim 3 was a 1D linear damage with a length of 12.8 mm and a horizontal 52.3 degree. The projection length of this damage was 10.13 mm in the longitudinal direction and 7.83 mm in the transverse direction. Damage 3-2 was 1D linear damage with a length of 10.3 mm and a horizontal 46.7 degree. The projection of the damaged straight line was 7.06 mm in the horizontal direction and 7.23 mm in the vertical direction. Specim 4 was a composite specimen with a small defect inside. The data of the four specimens were sampled, and the 2D wavelet denoising method was used to denoise the strain signal. The data before and after noise reduction are shown in Figure 8. The data obtained by the sensor of Specim 1 in Figure 8a had two abnormal mutations, indicating that two lesions were detected. Figure 8b is similar to Figure 8a, and both damages in Specim 2 were detected before and after noise reduction. Figure 8c shows no significant anomalies in the data before noise reduction, indicating that the minor defects in Specim 3 were not detected in the data before noise reduction. An anomaly in the processed data can be seen on the noise reduction line in Figure 8d. Figure 8d indicates that an anomaly was detected before and after denoising. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **1** & **2** & **3** & **4** & **5** \\ \hline Density (g/cm\({}^{3}\)) & 1.5394 & 1.5486 & 1.4912 & 1.5185 & 1.5389 \\ \hline Fiber volume content (\%) & 58.84 & 62.38 & 45.40 & 56.65 & 60.30 \\ \hline Braided angle (\({}^{\circ}\)) & 26.75 & 32.63 & 23.94 & 25.1 & 16.36 \\ \hline Row to the carbon nanotube yarn interval distance (mm) & 1.4 & 1.75 & 2.1 & 2.45 & 1.05 \\ \hline Column to the carbon nanotube yarn interval distance (mm) & 1.24 & 1.55 & 1.86 & 2.17 & 0.93 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Material number and parameters. Figure 7: The 3D six-directional braided composite specimens. The 2D image of the healthy Specim 5 was used as a template, and the corresponding data matrix was divided into m sub-regions. The feature data of the data matrix after the noise reduction of the tested damaged specimen were decomposed, and the internal damage position of the specimen was measured using the method in Figure 6.  
The maximum and minimum rows of the resistance values detected in the definition test were greater than the set thresholds \(l_{\text{max}}\) and \(l_{\text{min}}\), and the maximum and minimum columns were \(c_{\text{max}}\) and \(c_{\text{min}}\), respectively. The damage position coordinates are denoted by \(O_{n}(x,y)\), as follows:  
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