**1. Introduction**

Ramie is a culturally significant and traditional economic crop in China. It is widely cultivated in the Yangtze River Basin and southern China, with the planting area and fiber production accounting for over 95% of the global output [1]. Ramie is a perennial herbaceous plant belonging to the family Urticaceae [2], with its entire body being highly valuable. Its fiber exhibits excellent properties, such as strong moisture absorption, great breathability, antibacterial properties, and biodegradability [3–5], which makes it widely useful in textiles, medicine, military, agriculture, and ecofriendly packaging industries. However, the fiber of ramie must be decorticated and processed before use in the textile industry [6]. The ramie decorticator is the main processing equipment for decorticating

**Citation:** Hu, Y.; Xiang, W.; Duan, Y.; Yan, B.; Ma, L.; Liu, J.; Lyu, J. Calibration of Ramie Stalk Contact Parameters Based on the Discrete Element Method. *Agriculture* **2023**, *13*, 1070. https://doi.org/10.3390/ agriculture13051070

Academic Editor: Filipe Neves Dos Santos

Received: 7 April 2023 Revised: 18 April 2023 Accepted: 27 April 2023 Published: 17 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

ramie fiber. The high-speed rotating rollers can crush and eject the xylem through bending and beating the ramie stalk frequently while at the same time keeping the bast fiber intact, thereby obtaining pure raw ramie fiber [7]. However, the movement of each component of the ramie stalk is complex during decorticating, and traditional research methods are unable to simulate and analyze this movement state, which has so far hindered the optimization of decorticating equipment.

Simulation technology is widely used to optimize agricultural machines due to the rapid development of computational speed [8,9]. The EDEM simulation software based on the discrete element method can accurately analyze the movement law of agricultural materials in agricultural machinery. In agricultural machinery simulation operations, agricultural materials can be modeled as particles or clusters. The EDEM software can record the real-time movement trajectory and mechanical behavior of agricultural materials and conduct in-depth research on the interaction mechanism between materials and machinery; this can guide the optimization of the machinery design [10,11]. However, it is crucial to input the accurate physical and contact parameters of the materials to establish a discrete element model, which can faithfully reproduce the characteristic properties of the material and adapt to the real-world operating conditions of the machinery.

The DEM simulation modeling requires the input of intrinsic physical and contact parameters [12]. Due to the individual differences in materials, errors in physical tests, and differences in model constructions, obtaining accurate discrete element parameters through physical tests is difficult. Therefore, the calibration of physical tests and virtual simulations must be conducted to ensure consistency between the simulation and physical results. A stacking angle test can effectively calibrate discrete element parameters [13]. Various stacking angle measurement methods have been developed for different material characteristics, such as the injection, tilted box, cylinder lifting, and extraction of partition methods [14–17]. These methods are widely used to calibrate discrete element simulation parameters in materials such as soil, fertilizer, seeds, and biomass stalks. Xiang et al. [18] built a soil simulation model based on soil stacking tests of physical measurements and EDEM-software-recommended parameters. They used the stacking angle as the response value and completed the calibration and optimization of soil simulation's physical parameters through the Plackett–Burman, steepest ascent, and Box–Behnken tests. Liu et al. [19] obtained the stacking angle of a wheat grain heap through physical and simulation tests under the response value of different parameter combinations, which were finally based on response surface optimization, and they calibrated the discrete element simulation parameters of wheat. Xiao et al. [20] explored the influence of compound fertilizer characteristic parameters on the stacking angle, and they determined the rolling friction coefficients of the three types of granular fertilizers under two particle modeling methods. Shi et al. [21] measured the interval values of the contact parameters of fallen jujube through stacking angle tests. They used EDEM software to establish a simulation test of a fallen jujube stacking angle. They used Plackett–Burman, steepest ascent, and central composite design tests to obtain specific values of simulation parameters from the interval values. Dai et al. [22] used 3D scanning technology to construct a discrete element model of lily bulbs, and they calibrated the contact parameters between the lily bulbs and Q235 steel through bench tests and simulation parameter tests, and then established a regression model for the relative errors of the parameters and optimized the response surface to calibrate the discrete element contact parameters of the lily bulbs. Zhang et al. [17] determined the contact parameters between corn stalks and shredder blades, as well as corn stalks themselves. They used the extraction of the partition method to calibrate the contact parameters for corn stalk discrete element simulation. The research on discrete element simulation modeling and the parameter calibration of agricultural materials mainly focuses on spherical and quasi-spherical particles, such as soil, crop seeds, and fertilizers, and large spherical particles, such as fruits and plant bulbs. Unlike traditional spherical and quasi-spherical particle materials, the ramie stalk has a cylindrical shape in its xylem, and its phloem is strip-shaped after

being decorticated from the xylem. Few scholars have studied the calibration of the contact parameters of discrete element models on the ramie stalk.

To establish a discrete element simulation model of ramie stalks, this study established the discrete element models of the phloem and xylem on ramie stalk through the physical and simulation tests. Based on the measurement results of the phloem and xylem of ramie stalks, the stacking angle of the phloem and xylem particle mixture was taken as the response value. This study employed Plackett–Burman, steepest ascent, and response surface tests to complete the calibration and optimization of the contact parameters of the phloem and xylem on ramie stalks. This can provide a basic model and technical support for simulating the decorticating process of ramie fiber.

#### **2. Materials and Methods**

#### *2.1. Measurement of the Physical Parameters of Ramie Stalk Phloem and Xylem*

We selected the "Zhongzhu No. 1" variety of ramie as the experimental object to measure the structural dimensions, density, shear modulus, and other parameters of the ramie stalk's phloem and xylem. The "Zhongzhu No. 1" variety of ramie was planted at the Shijihu test base of Bast Fiber Crops of the Chinese Academy of Agricultural Sciences Institute in Yuanjiang City, Hunan Province.
