Mechanical Testing of Rice Straw and Calibration of a Bonded-Particle Discrete Element Model for Shearing

The mechanical responses of five rice cultivars were measured to support the calibration of a bonded-particle discrete element model for shearing. Geometric measurements and shear, three-point bending, radial compression, and axial tensile tests were conducted. Mean maximum shear force increased from the upper to the lower internode in all five cultivars. A hollow model was constructed using the measured geometry of Shengnong 4 middle-internode specimens, and their experimental mean maximum shear force of 72.072 N was used as the calibration target. Plackett–Burman screening identified the straw–straw coefficient of restitution and normal stiffness per unit area as significant factors. These factors were further investigated using a face-centered central composite design. The reduced quadratic model yielded an R2 of 0.9987. With the restitution coefficient fixed at 0.100, the selected normal stiffness per unit area was 1.12493 × 1012 N m−3. The response-surface prediction and DEM confirmation result were 67.3856 and 62.7892 N, with absolute relative errors of 6.50% and 12.88%, respectively, against the experimental reference. The measurements and calibrated parameters provide a basis for analyzing rice-straw cutting loads and straw–cutter interactions under the specified conditions.