# Flexible DEM Model Development and Parameter Calibration for Rape Stem
###### Abstract  
The discrete element method (DEM) is an effective technical tool for simulating the dynamic behavior of granular materials in agricultural engineering. However, most of the agricultural materials, such as rape stems, are flexible bodies, and it is difficult to simulate their elastic-plastic characteristics such as bending and failure using the rigid discrete element model. In this research, a flexible DEM model for rape stems was developed and the related parameters were calibrated. The proposed model consists of sequentially arranged rigid units, which were bonded together by the Hertz-Mindlin bonding contact model in the EDEM. The range of the contact parameters of a rape stem was first determined by bench test. The rape stem repose angle test was carried out as an evaluation indicator for the calibration of the DEM contact parameters. The significant factors affecting the repose angle in the contact parameters were discovered using the Plackett-Burman simulation test, and the optimal combination of these parameters was determined based on the response surface simulation test. The rape stem repose angle simulation result was 26.55\({}^{\circ}\) with a relative error of 2.2% for the physical tests. The rape-stem flexible DEM model's bonding parameters were calibrated based on a three-point bending physical test and a Box-Behnken simulation test. The test results show that a rape stem's maximum damage force obtained from the constructed model was 16.76 N, and the relative error of the measured values from the physical tests was 3.5%. The flexible DEM model could demonstrate the deformation and fracture of rape stems under an external force and can be used for the simulation of harvesting processes such as cutting, conveying, and threshing. rape stem; discrete element method; flexible DEM model; parameter calibration +
Footnote †: journal: Applied Sciences  
and breaking of rape stems during harvesting are difficult to describe when employing a rigid model. To address the problem of a simulative analysis of flexible bodies such as crop stems, various models have been proposed. [PERSON] constructed a flexible fiber wheat straw DEM model that includes a mega-particle comprising smaller particles and bonded by four beam shaped bonds. The related parameters were calibrated by the three-point bending test and uniaxial compression test of wheat straw. The regression meta-model was established between the particle contact Young's modulus and the bond Young's modulus and Poisson's ratio to predict the Young's modulus of flexible wheat straw [12]. [PERSON] developed a flexible wheat straw model based on the Hertz-Mindlin with bonding model using the discrete element method. A single-factor sensitivity analysis and calibration of the bonding parameters were performed using a three-point bending test. The small difference between the simulated and measured elastic modulus was less than 4.2% [13]. [PERSON] developed a segmented wheat stem for a DEM simulation with a number of realistic bending parameters. The effects of plastic deformation and damage could be incorporated into the model. Stem bending measurements showed that the model could predict the force-deformation relationship under different numbers of segments, stem lengths, support distances, and different degrees of plastic deformation [14]. [PERSON] carried out a bolting-stage rape crop-stalk-chopping model and the parameters were calibrated. A number of spheres were bonded, replacing the whole stalk. The error between the simulation and the physical test value of the bending force until breaking was less than 4. 21% [15]. In this research, a flexible rape stem DEM model was developed based on rigid rape stem units and bonding keys. The basic range of rape stem interaction parameters was tested by the developed test bench. Repose angle physical and simulation experiments were carried out to determine the significant factors and optimal parameter combinations in the collision parameters.