Residual Film–Cotton Stubble–Nail Tooth Interaction Study Based on SPH-FEM Coupling in Residual Film Recycling

In the cotton fields in Xinjiang, residual film is present in the soil for a long period of time, leading to a decrease in the tensile strength of the residual film and increasing the difficulty of recycling. Existing technologies for residual film recovery focus on mechanical properties and ignore the dragging and tearing of residual film by cotton stubble. The effect of cotton straw–root stubble on residual film recovery can only be better determined by appropriate machine operating parameters, which are essential to improving residual film recovery. Through analyses of the pickup device, key parameters were identified, and a model was built by combining the FEM and SPH algorithms to simulate the interaction of nail teeth, residual film, soil and root stubble. The simulation revealed the force change law of residual film in root stubble-containing soil and the influence of root stubble. By simulating the changes in the characteristics of the residual film during the process, the optimum operating parameters for the nail teeth were determined: a forward speed of 1849.57 mm/s, a rotational speed of 5.5 r/s and a soil penetration angle of 30°. Under these optimized conditions, the maximum shear strain, pickup height (maximum deformation) and average peak stress of the residual film were 1293, 363.81 mm and 3.42 MPa, respectively. Subsequently, field trials were conducted to verify the change in the impact of the nail teeth at the optimized speed on the recovery of residual film in plots containing root stubble. The results demonstrated that when the root stubble height was 5–8 cm, the residual film averaged a recovery rate of 89.59%, with a dragging rate of only 4.10% at crossings. In contrast, 8–14 cm stubble plots showed an 82.86% average recovery and an 11.91% dragging rate. In plots with a root stubble height of 5–8 cm, compared with plots with a root stubble height of 8–14 cm, the recovery rate increased by 6.73%, and the dragging rate of residual film on root stubble decreased by 7.81%. The percentage of entangled residual film out of the total unrecovered film was 30.10% lower in the 5–8 cm stubble plots than in the 8–14 cm stubble plots. It was confirmed that the effect of cotton root stubble on residual film recovery could be reduced under appropriate machine operating parameters. This provides strong support and a theoretical and practical basis for future research on the correlation between root stubble and residual film and how to improve the residual film recovery rate.