A prediction model has been limited to discrete Element method (DEM) parameters of the northeastern loamy soil at varying moisture contents. However, conventional single-point calibration has caused tedious and repetitive operation, due to highly variable soil moisture levels. In this study, an efficient and direct prediction model was constructed to simulate the mechanical behaviors of cohesive wet soils. A comparison was also conducted on the applicability, accuracy, and reliability of the self-developed Computer-Aided Engineering (CAE) software, AgriDEM, and the widely used commercial software, EDEM. Physical experiments and simulations were employed for validations. Initially, physical stacking tests were conducted using the cylinder lifting at multiple moisture levels. A prediction was then constructed for the relationship between soil moisture content and the angle of repose. Subsequently, the Box-Behnken experiment was utilized to calibrate microscopic contact parameters, including the Hertz-Mindlin with Johnson-Kendall-Roberts (JKR) model in EDEM and the cohesive particle contact model in AgriDEM. The macroscopic angle of repose was selected to map the relationships between the angle of repose and microscopic parameters. Finally, a direct prediction model was established to link the moisture content with the parameters. This coupled model was then independently validated under various vertical loads using soil direct shear tests. The internal friction angle and cohesive force were evaluated between simulations and physical experiments. The results demonstrated that there was a highly significant quadratic polynomial relationship between the soil moisture content and the macroscopic angle of repose, providing for a reliable macroscopic target for parameter inversion. High-precision mapping relationships were established between the angle of repose and microscopic parameters—specifically, the JKR surface energy in EDEM and adhesion parameters in AgriDEM. A direct pathway was achieved from moisture content to parameters, indicating continuous parameter prediction under diverse moisture conditions. The direct shear tests validated that the high accuracy of the prediction model was achieved in AgriDEM software. The simulated moisture contents without the initial modeling were highly consistent with the physical measurement in the direct shear test. Both software platforms accurately reproduced the test. The internal friction angle decreased, while the cohesive force increased, as soil moisture content increased. Notably, the AgriDEM software exhibited a significantly higher degree of agreement with the experimental values in key mechanical indicators, compared with the commercial software EDEM. The relative errors for the internal friction angle and cohesive force also ranged of 0.90% to 4.52% and 0.64% to 6.71%, respectively, in AgriDEM, compared with the EDEM (relative errors of 0.80% to 7.19% and 4.78% to 6.49%, respectively). The parameters in the cohesive particle model were introduced for the maximum attraction adjustment and normal adhesion distance during separation. The adhesion force is maintained over an extended distance during particle separation, thereby indicating the plastic deformation, yield, and liquid bridge of wet cohesive soils. In conclusion, the direct prediction model can provide a highly efficient, reliable, and continuous parameter acquisition for the DEM modeling of soils under varying moisture conditions. The exceptional fidelity of AgriDEM software was validated to simulate the mechanical behavior of wet cohesive soils. These findings can offer robust data support for the agricultural engineering software and the adaptive optimization of machinery components in complete operational cycles.

Stubble crushing machine has been widely used in the rotary tillage after wheat harvest. The wheat roots can often be simulated using the discrete element method. It is very necessary to calibrate the mechanical parameters for the better contact performance during crushing. In this study, a series simulations and tests were implemented to verify the wheat root system. The root system of Ningchun 4 wheat plant at mature stage was taken as the research object. The tensile and shear test of single root were carried out. The ultimate stress and elastic modulus of single root were calculated with the different water content (6%, 15% and 27%). The geometric morphology of the roots was characterized in the soil, together with the lateral roots at all levels. Each root was simplified to be straight and evenly distributed. The discrete element model was established for the single and whole root of wheat using the self-developed software, AgriDEM, according to the database of non-spherical particle filling ball modeling function. A flexible root model was established to bend at any position. The 'parallel bonding model' was used as the mechanical model of the connection between adjacent particles inside the roots. The Plackett-Burman and Box-Behnken simulation test were carried out, according to the tensile test and simulation of a single root. The bonding parameters of the 'parallel bonding model' were calibrated and then optimized. Because the soil type was the viscous and compressible loam in the wheat planting area, the 'clay particle model' was used to characterize the soil characteristics. According to the soil accumulation angle test and simulation, the parameters sensitive to the soil accumulation angle were selected by parameter sensitivity analysis. The linear fitting was used to obtain the static friction coefficient, sliding friction coefficient and adhesion coefficient, and the linear relationship between the above parameters, as well as the soil accumulation angle. There was the linear relationship among them. Finally, some parameters were determined for the 'clay particle model' in the soil simulation. The pull-out test of a single root buried in soil was carried out to verify the simulation. The results show that the relative error of the maximum pull-out force of a single root in the simulation and test was within 15%. There were the similar failure modes prone to occur under various test conditions. The high accuracy was also achieved in the single root bonding and soil parameters with the different water contents. There was the similar variation in the average maximum pull-out force after the test and simulation. The relative error between them was also within 15% within the range of theoretical calculation. The accuracy of the whole plant root model was also verified. The finding can provide a strong reference for the simulation analysis of the tillage and soil preparation after wheat harvest.