Maize straw motion law in subsoiling operation using discrete element method

Abstract: Sowing stubbles with straw in ridges have seriously caused the entanglement, blockage, and resistance to the subsoiler and subsequent machine tool when returning corn straws to the field in the northeast areas of China. The purpose of this study was to establish a discrete element model of subsoiler-soil-straw-stubble for the motion behavior of straw during subsoiling operation, thereby exploring the movement and mechanical characteristics of straw under different conditions. The soil bin was also built as a common ridge in Northeast China. Among them, the straw was assumed as a rigid body, whereas, the breakable adhesive stubble was an unbreakable rigid body. The simulation and test data were then selected to investigate the motion mechanism of straws. Specifically, the tracking movement of straw was obtained as the simulation data. A high-speed camera was also utilized to record the movement data of straw in the field test. As such, the large variation in the movement of straw was better simulated during subsoiling operation, particularly in the complex field environment. Four influencing factors were achieved to represent the straw plucking away from the ridge in the operation of subsoiling, including the distance between the straw and the center of the ridge, the angle between straw and machine, the state of stubble (cutting in the middle of stubble, cutting on one side of stubble, picking up of stubble and no stubble), and the interaction between straws. Correspondingly, the displacement and torque of straws on ridges were obtained to determine the disturbance-specific resistance (the ratio of straw disturbance moment to subsoiler resistance) under the action of the subsoiler. The simulation results show that the distance between the straw and the center of the ridge presented the greatest influence on the horizontal and lateral movement of straws, where the horizontal-lateral displacement of straw decreased with the increase of the angle between the straw and machine. More importantly, there was the largest displacement in the forward direction of straw, when the angle of straw was 45°. The primary and secondary order of stubble state affecting straw displacement was as follows: stubble pick up, stubble side cutting, no stubble, stubble middle cutting. Especially, there was the greatest influence of the interaction between the straws on the forward direction of straws, when the distance between the straw and the center of the ridge was 60 mm. Once more than 60 mm, the displacement tended to increase in the forward direction of straws. In addition, the torque of straws was calculated to explore the rotation of straw in simulation tests. It was found that the overall trend was as follows: the peak value was generated after the stable operation, and then tended to be stable. Anyway, there was a great influence of stubble on the torque of straw. The errors of the total displacement, horizontal-lateral displacement and forward displacement obtained by the simulation model with the test values were 0.36% -9.67%, 0.16% -12.31%, and 0.56% -10.11%, respectively. The error of straw torque was also 0.16% to 11.06%. The error between the test and simulation value was within the allowable range, indicating a similar changing trend. Consequently, the test verified the rationality and feasibility of discrete element simulation. The finding can greatly contribute to understanding the mechanism of straw during subsoiling, particularly to the reasonable design of subsoil machinery in modern agriculture.