Parameter calibration of the breakable flexible fiber model for maize stovers with different moisture contents

Mechanical chopping and returning maize stover to the field has been widely promoted in China in recent years. It is of great significance to precisely predict the fundamental parameters and quality during processing. However, modern equipment has been restricted to the unreasonable simplification and low computational efficiency of numerical models for maize stover chopping. In this study, a series of experiments were first conducted to obtain the physical properties of maize stover, including the average stover density and moisture contents. Three-point bending, tensile, and shear tests were then carried out to measure the static and dynamic friction coefficients and the restitution coefficients during collision. The load-displacement correlations were obtained for the maize stover with three moisture contents after three-point bending tests. The results indicated that the stover shared the higher intensity, as the increase of the moisture content. The tensile and shear tests show that the tensile strength of maize stover with a moisture content of 60% ± 5% was measured as 19.92 MPa, and the shear strength was measured as 2.45 MPa. In addition, the maize stover was simplified into the breakable flexible fiber model. Specifically, the multi-segment sphero-cylinder elements were connected by node spheres using the discrete element. The node spheres were utilized to control the twisting, bending, stretching, and compression of the fibers, as well as the transmission of forces and torques. The motion of each node sphere was calculated with Newton’s second law, indicating the realistic simulation of the stover’s physical interactions. The tensile or shear stress fracture criteria were used with Rocky DEM software. The fracture occurred when the normal or tangential stress at the node spheres exceeded the strength limits. Moreover, the steepest ascent and Box-Behnken design were employed to calibrate four model parameters during simulation: elastic ratio, plastic ratio, bending angle limit, and failure ratio. The calibration results show that the optimal rotational speed of the chopping blade shaft was determined to be 2 088.4 r/min, the optimal forward speed of the machine was 3.3 km/h, the optimal sliding cutting angle of the chopping blade was 32.3°, and the ideal arrangement of the blades was a spiral configuration. Therefore, the chopping pass rate was 93.39% under the optimal parameters. Finally, the breakable flexible fiber model was applied to perform the large-scale simulations in a real maize field. A numerical model of a maize field was established within a 2 m × 10 m domain, consisting of upright stubble and stover on the surface. A global simulation of stover chopping and returning to the field was carried out to compare with the field experiments. The chopping pass rate was 92.58% from the field experiment, with a deviation of 0.87% (less than 2%), compared with the simulation. The field test validated the feasibility of the improved model and the accuracy of the numerical model. In conclusion, the breakable flexible fiber model shared the reasonable calibration suitable for the numerical simulations of stover chopping. The finding can also provide a sound basis to optimize the structure and working parameters of key components in stover chopping and returning machinery.