## 2 Experimental
### Tension-Torsion Cyclic Loading Parameters  
The tension-torsion cyclic loading test rig was employed to carry out tension-torsion cyclic loading tests of steel wires (see Figures 1 and 2 in Reference [3]) in different environmental media. Before each test, loading wires were aligned with respect to the cyclic loaded wire to ensure consistent contact. During the test, the cyclic loaded wire was applied to the cyclic tension and torsion employing the electric cylinder and step motor, respectively. The constant contact load between contacting wires was applied using the counterweights. Contacting regions of steel wires were immersed in the plastic container with different environmental solutions to realize the corrosive environment. The cyclic tension and tangential force were recorded by the tension sensor and tangential force sensor, respectively. The cyclic torque and torsion angle of cyclic loaded wire are recorded employing the torque sensor and angular displacement sensor, respectively. The relative displacement between contacting wires is determined by scaling-down of the cyclic loaded wire deformation measured by the electric cylinder. Distinct crossing angles between steel wires can be realized by changing the inclination angle of loading wire fixture. Test parameters are as follows: fretting amplitude of \(\pm\)60 \(\upmu\)m, frequency of 5 Hz, tensile stress amplitude of 510 MPa and tensile stress ratio of 0.5. Environmental media and other test parameters can be found in Table 2. The white light interferometer is employed to investigate wear morphologies and depths of steel wires. Cross-section morphologies of cyclic loaded wires were investigated using X-ray computed tomography (CT). Fractures of cyclic loaded wires were observe employing the scanning electron microscope (SEM).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Environmental Media**} & \multicolumn{5}{c}{**Tension Angle,**} & \multicolumn{2}{c}{**Contact Load,**} & \multicolumn{2}{c}{**Torsion Angle,**} & \multicolumn{2}{c}{**Loading Cycles,**} \\  & \(\beta\) (\({}^{\circ}\)) & \(\Gamma_{n}\) (N)** & \(\theta\) (\({}^{\circ}\)) & \(N\) (\(\times\)10\({}^{\circ}\)) \\ \hline Air; deionized water; & & & & & & & \\ acid, neutral and & 90 & 40 & \(\pm\)2 & 2 \\ alkaline solutions & & & 40 & \(\pm\)2 & 1.5, 2, 2.5, 3 \\  & 90 & 30, 40, 50, 60 & \(\pm\)2 & 2 \\ Acid solution & 90 & 40 & \(\pm\)1, \(\pm\)1.5, \(\pm\)2, \(\pm\)2.5 & 2 \\  & 90, 70, 45, 26 & 40 & \(\pm\)2 & 2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Environmental media and tension–torsion cyclic loading test parameters.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**PH**} & \multicolumn{5}{c}{**Ion Content (mg/L)**} \\  & **H\({}^{+}\)** & **Na\({}^{+}\)** & **K\({}^{+}\)** & **Mg\({}^{2+}\)** & **Ca\({}^{2+}\)** & **Cl\({}^{-}\)** & **OH\({}^{-}\)** & **HCO\({}_{3}^{-}\)** & **SO\({}_{4}^{2-}\)** \\ \hline
3.5 & 1.28 & 92.42 & 11.73 & 364.74 & 676.55 & 28.36 & / & 18.92 & 3283.81 \\
7.5 & / & 141.16 & 11.73 & 49.33 & 80.56 & 89.33 & / & 18.92 & 551.38 \\
9.5 & / & 35.63 & 11.73 & 36.45 & 54.43 & 89.33 & 3.33 & 18.92 & 232.95 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical compositions of solutions.