## 3 Results and Discussion
### Delamination Factors and Optimization Process
#### 3.2.1 Optimization of Fd and Fa  
The ANOVA of the experimental data suggested the linear model for both Fd and Fa responses. The ANOVA results for these responses are shown in Tables 4 and 5 for Fd and Fa, respectively. The main effects, including the feed rate and rotational speed, were significant but there was no significant interaction effect between all parameters for both Fd and Fa. The angle of layers was significant for Fd, while it was not significant for Fa. The fitted equations in terms of actual factors for predicting the Fd and Fa were achieved as Equations (5) and (6), respectively, for each angle of the layers' types. Due to the significance of the main effects, the main effect plots were illustrated in Figures 6 and 7 for Fd and Fa, respectively. As can be seen in Figures 6a,b and 7a,b, the delamination values were decreased at the lower feed rate and higher rotational speed, respectively. At higher rotational speed values, the tool friction increases, and a softening of the composite matrix occurs at the drilling site because of the heat generated by tool friction; therefore, removal of the matrix can be more accessible. At higher feed rates, contact between the specimen and the tool increases, violent fracture is produced, and finally, more delamination will happen. These results are in accordance with the literature [4; 5; 18; 21; 28; 31; 32]. Moreover, the angle of 15 degrees between the layers had better conditions with lower Fd values in comparison with the 45 degrees, while this parameter does not affect Fa. These results can be due to Fa being a two-directional response. On the other hand, if the thrust force develops delamination in one direction, the delamination in another direction will decrease; therefore, the total delamination area will not change significantly. In contrast, Fd is a one-directional response, and the lower angle between the layers can more effectively protect the composite from higher directional delamination. Therefore, the lower angle showed a significantly lower Fd value. \[F_{d}= \begin{cases}For\;type\;45:0.184+0.00003A-9.65\times 10^{-6}B\\ For\;type\;15:0.144+0.00003A-9.65\times 10^{-6}B\end{cases}  \tag{5}\]  
\[F_{a}= \begin{cases}For\;type\;45:1.29+0.00018A-0.000026B\\ For\;type\;15:1.28+0.00018A-0.000026B\end{cases}  \tag{6}\] \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source** & **Sum of Squares** & **df** & **Mean Square** & **F-Value** & **\(p\)-Value** & \\ \hline Model & \(1.100\times 10^{5}\) & 8 & 13,752.76 & 212.04 & \textless{}0.0001 & **Significant** \\ A-Feed rate & 41,900.33 & 1 & 41,900.33 & 646.02 & \textless{}0.0001 & \\ B-Rotational speed & 58,203.65 & 1 & 58,203.65 & 897.39 & \textless{}0.0001 & \\ C-Angle of layers & 57.64 & 1 & 57.64 & 0.8887 & 0.3516 & \\ AB & 6048.95 & 1 & 6048.95 & 93.26 & \textless{}0.0001 & \\ AC & 121.50 & 1 & 121.50 & 1.87 & 0.1789 & \\ BC & 1.17 & 1 & 1.17 & 0.0180 & 0.8938 & \\ A\({}^{2}\) & 24.30 & 1 & 24.30 & 0.3747 & 0.5440 & \\ B\({}^{2}\) & 3447.61 & 1 & 3447.61 & 53.16 & \textless{}0.0001 & \\ Residual & 2529.49 & 39 & 64.86 & & & \\ Lack of Fit & 442.62 & 9 & 49.18 & 0.7070 & 0.6978 & **not significant** \\ Pure Error & 2086.87 & 30 & 69.56 & & & \\ Cor Total & \(1.126\times 10^{5}\) & 47 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: ANOVA for first response (\(F_{\text{max}}\)). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Source** & **Sum of Squares** & **df** & **Mean Square** & **F-Value** & **\(p\)-Value** & \\ \hline Model & 0.0471 & 3 & 0.0157 & 62.63 & \(<\)0.0001 & **significant** \\ A-Feed rate & 0.0081 & 1 & 0.0081 & 32.32 & \(<\)0.0001 & **significant** \\ B-Rotational speed & 0.0201 & 1 & 0.0201 & 80.27 & \(<\)0.0001 & \\ C-Angle of layers & 0.0189 & 1 & 0.0189 & 75.31 & \(<\)0.0001 & \\ Residual & 0.0110 & 44 & 0.0003 & & & \\ Lack of Fit & 0.0046 & 14 & 0.0003 & 1.51 & 0.1656 & **not significant** \\ Pure Error & 0.0065 & 30 & 0.0002 & & & \\ Cor Total & 0.0581 & 47 & & & & \\ \hline \hline \end{tabular}
\end{table}