## 3 Results and Discussion
### Taguchi Analysis

Table 3: S/N response table for the safety factor of the autofrettaged cylinder without machining under a working pressure of 400 MPa. Figure 5: Surface plot of S/N versus Young’s modulus, yield strength, and tangent modulus for the case without machining. A similar outcome to the autofrettage process without machining is evident in the case of the process involving internal or external machining, as depicted in Figures 7 and 8. The distinction between them becomes apparent at the maximum value that the S/N ratio can attain. According to the delta values in Table 4, the most influential parameter on the response for the autofrettaged cylinder with only internal machining is \(S_{y}\). It is clearly seen from the graphical data (Figure 6) that the average S/N ratio increases as \(S_{y}\) and \(T\) increase. This means that the safety factor of the autofrettaged cylinder with only internal machining increases as \(S_{y}\) and \(T\) increase. However, the safety factor decreases as \(E\) values increase. The calculated S/N ratio values (average) for the safety factor of the autofrettaged cylinder with only external machining are provided in Table 5 and presented graphically in Figure 9.  
According to the Delta values in Table 5, the most influential parameter on the response for the autofrettaged cylinder with only external machining is \(S_{y}\). As observed from the graphical data (Figure 9), the average S/N ratio increases with the increase in \(S_{y}\) and \(T\). This implies that the safety factor of the autofrettaged cylinder with only external machining increases with increasing \(S_{y}\) and \(T\). However, the safety factor decreases with increasing \(E\) values. According to the analysis results obtained using Taguchi's robust process design, in order to achieve the maximum safety factor in all three scenarios, the values of \(S_{y}\), \(E\), and \(T\) should be 0.7 GPa, 100 GPa, and 20 GPa, respectively. \begin{table}
\begin{tabular}{c c c c} \hline
**Level** & \(S_{y}\) & \(E\) & \(T\) \\ \hline
1 & 1.022 & 1.284 & 1.262 \\ \hline
2 & 1.277 & 1.276 & 1.276 \\ \hline
3 & 1.530 & 1.269 & 1.291 \\ \hline Delta & 0.507 & 0.015 & 0.030 \\ \hline Rank & 1 & 3 & 2 \\ \hline \end{tabular}
\end{table} 
Table 4: S/N response table for the safety factor of the autofrettaged cylinder with only internal machining under working pressure of 400 MPa. Figure 6: Main effects plots for S/N ratios of the safety factor of the autofrettaged cylinder with only internal machining under working pressure of 400 MPa. Figure 8: Surface plot of S/N versus Young’s modulus, yield strength and tangent modulus for the case with only external machining. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Level** & \(\mathbf{S_{y}}\) & \(\mathbf{E}\) & \(\mathbf{T}\) \\ \hline
1 & 1.079 & 1.388 & 1.335 \\ \hline
2 & 1.369 & 1.361 & 1.366 \\ \hline
3 & 1.656 & 1.356 & 1.404 \\ \hline Delta & 0.576 & 0.032 & 0.069 \\ \hline Rank & 1 & 3 & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: S/N response table for the safety factor of the autofretaged cylinder with only external machining under working pressure of 400 MPa. Figure 7: Surface plot of S/N versus Young’s modulus, yield strength, and tangent modulus for the case with only internal machining.