*2.5. Discrete Element Simulation Test*

In order to achieve a simulation model of the ramie stalk that matched its actual situation and ensured the reliability and authenticity of the model, this study used the Design-Expert 10.0.1 software to carry out tests, such as Plackett–Burman design, steepest ascent test design, and response surface design, to determine the key factors affecting the stacking angle in the simulation parameters of the ramie phloem and xylem, as well as the significant factor levels and parameter optimization of the ramie stalk phloem and xylem. Based on the fitting of the simulation stacking angle and the physical stacking angle of the ramie stalk phloem and xylem, the linear fitting method in MATLAB was used to compare the boundary pixel slope of the simulation model stacking angle and the actual material stacking angle to verify the accuracy of the model.

#### 2.5.1. Plackett–Burman Design

To quickly screen the key factors affecting the response value of the stacking angle in the simulation parameters of ramie stalk phloem and xylem, this study used Design-Expert software to conduct a Plackett–Burman test analysis, taking the stacking angle of the phloem and xylem as the response value, using a 6-factor 2-level test method. The levels were represented in coded form, with a total of 13 groups of tests, each repeated twice, to compare the influence of each factor on the stacking angle of the phloem and xylem. The experimental plan is shown in Table 2.


#### **Table 2.** Plackett–Burman test program.

A first-order polynomial linear model was used for the statistical modeling, as shown in Equation (4). The significance of each factor was obtained through variance analysis, and the significant influencing factors were selected.

$$
\Omega = \sigma\_0 + \sum\_{k=1}^6 \sigma\_k X\_k \tag{4}
$$

Here, Ω represents the stacking angle, ◦; *σ*<sup>0</sup> is the intercept of the model; *σ<sup>k</sup>* is the linear coefficient; and *Xk* refers to the coded level of the independent variable.
