## 3 Results and Discussion
### Effects of Fretting and Torsional Parameters on Dissipation Energy Characteristics
#### 3.2.4 Wear Coefficient  
Table 9 shows that as loading cycles increase, the wear coefficient increases rapidly at first, then increases slowly and finally stabilizes. At the initial stage of test, the protection film is removed from the contact surface of cyclic loaded wire attributed to roles of fretting wear and corrosion, which causes the fretting wear between steel matrixes and thereby accelerates the wear of cyclic loaded wire. Then the third body (debris and corrosion products) participates in the regulation and load-bearing, which causes a slow increase in the wear coefficient. Finally, the stable wear coefficient is attributed to dynamic balance between generation and dissolution of debris and corrosion products.  
The coefficient of cyclic wear increases at first and then decreases with increasing contact load. As depicted in Section 3.2.3, the wear volume increases with increasing contact load and thus causes an increase in the wear coefficient. However, a further increase of contact load accelerates the adhesion at the fretting contact surface, which indicates that the acid solution is difficult to enter the contact surface, and relative slip between contact surfaces is smaller. Therefore, both decreases in fretting corrosion and relative slip reduce the wear coefficient.  
An increase of torsion angle induces an overall increase in the wear coefficient. The tangential force and torque both increase with increasing torsion angle, which induces more severe plastic deformation and more irreversible work absorbed by the internal friction attributed to shear stress. Therefore, the acid solution enters the fretting contact surface more easily and thereby causes an increase in the wear coefficient.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Loading Cycles, \(N\) (\(10^{4}\))**} & \multicolumn{4}{c}{**Contact Load, \(F_{n}\) (N)**} \\  & **1.5** & **2** & **2.5** & **3** & **30** & **40** & **50** & **60** \\ \hline Max. length, \(L\) (\(\mu\)m) & 395 & 553 & 584 & 601 & 458 & 553 & 580 & 588 \\ Max. width, \(w\) (\(\mu\)m) & 245 & 327 & 353 & 358 & 250 & 327 & 333 & 347 \\ Max. wear depth, \(h\) (\(\mu\)m) & 11.89 & 16.03 & 18.17 & 20.06 & 10.38 & 16.03 & 18.27 & 19.54 \\ Scar area, \(s\) (\(\mu\)m\({}^{2}\)) & 82,074 & 145,460 & 170,246 & 183,284 & 97,692 & 145,460 & 159,324 & 166,888 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Dimensions of wear scars of cyclic loaded wires at different loading cycles and contact loads.  
Figure 12: Top and front views of wear scars of cyclic loaded wires at different tension–torsion cyclic loading parameters.(**a,b**) Effects of loading cycles; (**c**) and Figure 7c in reference [3] effect of contact load; (**d**) and Figure 7d in reference [3] effect of torsion angle; (**e**) and Figure 7b in reference [3] effect of crossing angle.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Torsion Angle, \(\theta\) (\({}^{\circ}\))**} & \multicolumn{4}{c}{**Crossing Angle, \(\beta\) (\({}^{\circ}\))**} \\  & \(\pm\)**1** & \(\pm\)**1.5** & \(\pm\)**2** & \(\pm\)**2.5** & **90** & **70** & **45** & **26** \\ \hline Max. length, \(L\) (\(\mu\)m) & 401 & 549 & 553 & 567 & 553 & 659 & 687 & 778 \\ Max. width, \(w\) (\(\mu\)m) & 210 & 269 & 327 & 334 & 327 & 303 & 298 & 229 \\ Max. wear depth, \(h\) (\(\mu\)m) & 9.21 & 11.65 & 16.03 & 19.95 & 16.03 & 10.13 & 8.49 & 7.21 \\ Scar area, \(s\) (\(\mu\)m\({}^{2}\)) & 70,073 & 110,783 & 145,460 & 157,927 & 145,460 & 154,131 & 161,380 & 120,787 \\ \hline \hline \end{tabular}
\end{table}
Table 8: Dimensions of wear scars of cyclic loaded wires at different torsion angles and crossing angles.  
The coefficient of cyclic wear increases with increasing crossing angle, which coincides with results in [31]. An increase of crossing angle induces easier failure of grain texture due to longitudinal fibrous grain [32], and increased maximum torque indicating larger shear failure.