## 3 Results and Discussion
### Optimization of Corn Straw Epidermal Bonding Parameters  
A straw with a diameter of about 28.5 mm in the middle section of corn straw is selected, and the inner flesh is removed to make a cylinder specimen with a length of 100 mm, and a model of the same size is built in the discrete element for the virtual bending damage test, as shown in Figure 9.  
In order to ensure the reliability of the parameter range in the parameter calibration process and avoid the adverse effects caused by the parameter taking values beyond the range, the response surface test is designed according to the central combination design principle. With \(F_{1}\) as the response value and \(X_{1}\) (normal stiffness), \(X_{2}\) (shear stiffness), \(X_{3}\) (normal critical force), and \(X_{4}\) (shear critical force) as independent variables, the bending damage simulation test and analysis of corn straw are carried out, and the test factors and levels are shown in Table 3, and the bending damage force test scheme and results are shown in Table 4.  
Figure 8: CFD\(-\)DEM coupled simulation cutting and throwing test. Figure 9: Straw skin bending test. (**a**) Epidermal discrete element bending test; (**b**) epidermal physical bending test. Design\(-\)Expert software is applied to the bending damage force test results for quadratic regression fitting analysis. As shown in Table 5, the regression model \(p=0.0017<0.01\), which confirmed that the model was significant within the 95% confidence interval. Moreover, the coefficient of determination (\(R^{2}\)) and the adjusted coefficient of determination (\(R^{2}_{adj}\)) are 0.9453 and 0.9163 close to 1, respectively, indicating that the calculated model is in good agreement with the experimental data. The squared terms of normal stiffness \(X_{1}\), shear stiffness \(X_{2}\), and shear stiffness \(X_{2}\) have a highly significant effect on the bending damage force, and the squared terms of normal contact stiffness \(X_{1}\) and shear critical force \(X_{3}\) have a significant effect on the bending damage force. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Serial Number** & \(\mathbf{X_{1}}\)**/(N\(\cdot\)m\({}^{-1}\))** & \(\mathbf{X_{2}}\)**/(N\(\cdot\)m\({}^{-1}\))** & \(\mathbf{X_{3}}\)**/MPa** & \(\mathbf{X_{4}}\)**/MPa** & **Destructive Power (\(\mathbf{F_{1}}\))/N** \\ \hline
1 & \(5.2\times 10^{9}\) & \(7.4\times 10^{8}\) & 210 & 40 & 119.2 \\
2 & \(7.8\times 10^{9}\) & \(4.4\times 10^{8}\) & 430 & 40 &  
Under the condition that the model is guaranteed to be significant and the misfit term is not significant, the quadratic regression model is optimized and adjusted to eliminate the insignificant terms normal critical force \(X_{3}\) and shear critical force \(X_{4}\) to establish the regression equation of straw skin parameters and bending damage force as follows:  
\[F_{1}=-428.01+6.06\times 10^{-8}X_{1}+1.26\times 10^{-7}X_{2}-4.17\times 10^{-18 }X_{1}^{2}-8.59\times 10^{-16}X_{3}^{2} \tag{2}\]  
Figure 10 shows the residual diagnostic plot of the quadratic model, and Figure 10a demonstrates that the dispersion points are close to a straight line, indicating that the model is sufficient to describe the relationship between the independent variables and the straw bark damage force. Figure 10b shows the corresponding relationship between the residual error and the equation. The more scattered the discrete points, the more irregular they are, indicating that the prediction effect of this equation is better. Figure 10c shows the comparison between the predicted and experimental values of straw epidermal damage force. The linear distribution indicates that the model fits well and the model can analyze and predict the straw epidermal damage force. Figure 10: Residual diagnostic map of the secondary model of straw outer bark. (**a**) Normal plot of residuals; (**b**) residuals vs. predicted; and (**c**) predicted vs. actual.