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

Under the widespread promotion background of the mechanization of chopping and returning to the field maize straw in China, the precise prediction on the effect from fundamental parameters to the work process and quality is of great significance. However, the unreasonable simplification and low computational efficiency of numerical models for maize straw chopping restrict the development of the modern equipment. In this study, a series of experiments were firstly conducted to obtain the physical properties of maize straw, including the average straw density and moisture contents, three-point bending tests, tensile and shear tests, as well as the measurement of the static and dynamic friction coefficients and the restitution coefficients during collision. The load-displacement correlations for maize straw with three different moisture contents were obtained through three-point bending tests, indicating that the straw will get higher intensity as the increase of its moisture content. Based on the tensile and shear tests, the tensile strength of maize straw 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 straw is simplified into the breakable flexible fiber model composed of multi-segment sphero-cylinder elements connected by node spheres based on the discrete element method. The twisting, bending, stretching, and compression of the fibers, as well as the transmission of forces and torques, are controlled by the node spheres. The motion of each node sphere is calculated with Newton’s second law, ensuring a realistic simulation of the straw’s physical interactions. The tensile or shear stress fracture criteria conditions were used with Rocky DEM software, and fracture occurs when the normal or tangential stress at the node spheres exceeds the strength limits. During the simulation calculations, the steepest ascent method and Box-Behnken design were employed to calibrate four model parameters: elastic ratio, plastic ratio, bending angle limit and failure ratio. Based on the calibration results, 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. Under this set of optimal parameters, the chopping pass rate was 93.39%. Finally, the breakable flexible fiber model was applied to perform large-scale simulations of a real maize field scenario. A numerical model of a maize field was established within a 2 m × 10 m domain, consisting of upright stubble and straw laid on the surface, to conduct a global simulation of straw chopping and returning to the field, and then the simulations were compared with the field experiments. The results showed that the chopping pass rate from the field experiment was 92.58%, with a deviation of 0.87% from the pass rate obtained from the simulation, which was less than 2%, validating the feasibility of the proposed method and the accuracy of the numerical model. In conclusion, this study indicated that the breakable flexible fiber model, with a reasonable calibration method, is suitable for numerical simulations of straw chopping, providing a sound basis for optimizing the structure of key components in straw chopping and returning machinery and for selecting optimal working parameters.