## 4 Results and Discussion  
### Residual Shear and Normal Stress Analysis  
The residual stresses at the surface in four rotation angles (i.e., \(\sigma_{\varphi}\) = 0, \(\sigma_{\varphi}\) = 90, \(\sigma_{\varphi}\) = 180, and \(\sigma_{\varphi}\) = 270) were measured using an X-ray diffractometer for each machined specimen. The surface residual  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen** & **Tube target** & **Crystallographic plane** & **Diffraction angle (20)** \\ \hline Inconel 718 & Cr & [311] & 150.89\({}^{\circ}\) \\ \hline
**Tube voltage** & **Tube current** & **X-ray slit** & **Exposure time** \\ \hline
30 kV & 1 mA & 2 mm & 90 s \\ \hline \hline \end{tabular}
\end{table}
Table 4: X-ray diffraction parameters of residual stress measurements.  
Figure 4: The geometry of fatigue test specimen (unit: mm).  
normal stresses \(\sigma_{xx}\) and \(\sigma_{yy}\), calculated by Equations (17) and (18), and the residual shear stress, \(\tau_{xy}\), calculated by Equation (19), are summarized in Figure 5.  
From Figure 5, it is observed that the residual normal stresses \(\sigma_{xx}\) were tensile and the values varied within the range of 199-275 MPa, the residual normal stresses \(\sigma_{yy}\) were tensile and the values within ranges of 320-370 MPa, and the residual shear stresses \(\tau_{xy}\) were negative with the value changes in a range of \(-\)265--211.5. The value of residual shear stress calculated by Equation (19) supported the previous statement that the machining process induced the residual shear stress. A similar phenomenon was found by [PERSON] et al. [23]. [PERSON] et al. [24] reported that residual shear stress had been observed on a bearing surface fatigued by rolling contact.  
It is noted that the residual normal stresses \(\sigma_{xx}\) and \(\sigma_{yy}\) decreased slightly as the cutting speed increased. This drop in residual normal stresses was mainly due to the increased chip flow rate. The increased cutting speed resulted in a higher chip flow rate and reduced the contact time between the tool and the workpiece material. Consequently, the amount of thermal energy was taken away by the chip increase [25]. It is supposed that the heat generated in the primary shear zone, tool-chip contact zone, and tool-workpiece friction zone diffused into the workpiece surface decreased. Therefore, the residual normal stress decreased with the increase in cutting speed. The residual shear stresses \(\tau_{xy}\) had no obvious and orderly variation as the cutting speed increased. A similar phenomenon was found by [PERSON] et al. [26] who concluded that the maximum value of tensile residual stress in longitudinal and transverse directions decreased when the cutting speed increased under dry cutting conditions. They explained that this phenomenon was attributed to the reduction of heat generated due to the increased cutting speed. Nevertheless, [PERSON] et al. [27] point out that tensile residual stress increased significantly in longitudinal and transverse directions as the cutting speed increased under a low cutting speed (\(V_{c}\) < 100 m/min). They explained that the low cutting speed (\(V_{c}\) < 100 m/min) would lead to a low material removal rate (MRR). This resulted in a decrease in the thermal dissipation by the chip. Therefore, more heat generation would be transmitted to the surface of the workpiece resulting in a higher tensile residual stress at the surface.