## 3 Experiment
### Experimental Results  
The torsional strength experiment results are presented in Table 1. The SiC\({}_{\text{f}}\)/Ti composite had a compression strength greater than its tensile strength. Thereby, the torsional strength of the \([45^{\circ}]_{6}\) shaft was slightly greater than that of the \([45^{\circ}]_{10}\) shaft. The specimens were damaged, and the fractures of the specimens were cut, as shown in Figure 9.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Layer Scheme**} & **Torque (M)** & **Experiment** & **Facture Status** \\  & **Direction** & **Value/Nm** & **Facture Status** \\ \hline \([45^{\circ}]_{6}\) & Reverse & \(-11\),\(812\) & Cracks in \(45^{\circ}\) and \(90^{\circ}\) directions \\ \([45^{\circ}]_{10}\) & Positive & \(10\),\(418\) & Cracks in \(90^{\circ}\) direction \\ \hline \hline \end{tabular}
\end{table}
Table 1: Experimental results of the specimens.  
Figure 8: Test system layout: (**a**) test system device; (**b**) measuring equipment; and (**c**) operating end and controller.  
3.2.1. [45\({}^{\circ}\)]\({}_{6}\) Shaft Failure Mode Analysis  
In acoustic emission monitoring of Ti metal matrix composites, 40-60 dB is mostly used for interface cracking, 60-80 dB is used for matrix cracking, and over 90 dB is for fibre fracture [30]. When the torsional load gradually increased from 0 Nm, the interface was first damaged under the tensile stress, and the interface damage signal of 40-60 dB was monitored by acoustic emissions, as shown in Figure 10a. As shown in Figure 11a,b, the interface exhibits obvious debonding. It is shown that, after interface debonding, a flat and regular 45\({}^{\circ}\) crack is formed along the fibre direction. After the crack is generated by the SiC fibre-reinforced part, it rapidly expands to the inner and outer titanium matrix of the shaft in the direction shown by the arrow in Figure 11a, which leads to cracking and causes matrix damage. Then, a relatively large number of matrix damage acoustic emission signals--concurrent with interfacial cracking signals--are produced, and the 60-80 dB acoustic emission signal is shown in Figure 10a.  
Figure 10: Acoustic emission monitoring experiment results: (a) [45\({}^{\circ}\)]\({}_{6}\) shaft results; (b) [45\({}^{\circ}\)]\({}_{10}\) shaft results.  
Figure 9: Damage mode and fracture morphology: (a) [45\({}^{\circ}\)]\({}_{6}\) shaft damage mode; (b) [45\({}^{\circ}\)]\({}_{6}\) shaft fracture morphology; (c) [45\({}^{\circ}\)]\({}_{10}\) shaft damage mode; and (d) [45\({}^{\circ}\)]\({}_{10}\) shaft fracture morphology.  
The 90\({}^{\circ}\) region observation results are illustrated in Figure 11c,d; a large amount of shear plastic deformation of the titanium matrix occurred along the 90\({}^{\circ}\) direction under shear stress. A large number of fibres were also sheared on the fracture, and fibre splitting, C coating dehiscence, and local delamination could occur. W wire cleaning, SiC deposition, and C coating deposition were realized in the same reaction vessel during the preparation of the SiC product. The C coating was between the SiC fibre and matrix, which ensured that the fibre and sputtered matrix have close bonding and no gaps. This improves the performance of the material. Under reverse-torque loading, the fibre was compressed, and the SiC fibre and C coating were separated under compression loading. C coatings were brittle and broke in multiple places under compression loading, resulting in C coating dehiscence and local delamination. In addition, many fibre fracture signals \(>\) 90 dB finally appear in Figure 10a, which clearly indicates that the SiC fibres were sheared in large quantities, driven by the titanium matrix.