2.5.4. Parameter Optimization and Validation

The Design-Expert software's Optimization module optimized the regression model with the experimental values of the stacking angle as the objective. The steepest ascent test determined the parameter optimization range, and a two-sample *t*-test was performed on the simulation and physical results. The reliability of the optimal combination of parameters was verified by checking whether there was a significant difference between the simulation and physical results.

#### **3. Results and Discussion**

#### *3.1. Measurement Results of the Required Parameters for the DEM Simulation*

Based on the physical parameter measurement results of the ramie stalk's phloem and xylem, combined with an analysis of the bounce-back process of the phloem and xylem using HiSpec Control Software, the experimental values of the restitution coefficient were obtained. The friction coefficient was calculated by recording the sliding and rolling angle, and the restitution coefficient and friction coefficient results are shown in Table 3.


**Table 3.** Parameters required for the discrete element simulation.
