Flexible DEM-Based Analysis of Rice Straw Shear Fracture Mechanisms and Comminution Parameter Optimization for Whole-Feed Combine Harvesters

High-moisture rice straw processed by whole-feed combine harvesters often exhibits high cutting resistance and uneven particle size distribution after comminution. To address these issues, a straw comminution device integrated with the straw outlet of the threshing and cleaning system was developed, and the shear fracture mechanism and operating parameters were investigated. The geometric characteristics, density, contact properties, and bending properties of rice straw cultivars Yongyou 7301 and Kenuigeng 1 were measured. A hollow flexible straw discrete element model was established using the Hertz–Mindlin with Bonding contact model, and its parameters were calibrated and validated through quasi-static shear cutting tests. The effects of shear cutting angle on maximum cutting force, bond failure evolution, and load transfer behavior were analyzed at shear angles of 30°, 45°, and 60°. Device-scale DEM simulations combined with field experiments were further conducted to optimize the guide plate angle and rotor speed. The results showed that the maximum cutting force under quasi-static single-stalk cutting conditions initially decreased and then increased with increasing shear angle. At a shear angle of 45°, the maximum cutting force was 78 N, representing a 44.8% reduction compared with that at 30°. Meanwhile, the fracture zone expanded along the blade sliding direction and stress concentration was alleviated. The DEM model effectively characterized the fracture behavior of rice straw, with an average relative error of 11.07% between simulated and experimental cutting forces. The optimized operating parameters under the tested conditions were a shear angle of 45°, guide plate angle of 55°, and rotor speed of 2500 r/min, resulting in average chopped lengths of 17.3 mm in simulation and 20.5 mm in field experiments, with a comminution qualification rate of 95.26%. These findings provide theoretical support for improving the fracture characteristics and chopping performance of straw comminution systems.