Development and experimental validation of a flexible model for Vicia villosa stems based on a DEM-MBD coupled calibration framework

The development of machinery for green manure cutting and incorporation has been constrained by the absence of a discrete element method (DEM) model and parameter set capable of representing Vicia villosa (hairy vetch) stems with sufficient fidelity. To address this issue, the characteristic dimensions and physical and mechanical properties of the stems were measured, and a DEM model was developed to capture their structural features and mechanical responses. Contact parameters were obtained experimentally and calibrated using combined physical and simulated angle-of-repose tests. The relative error between the simulated and physical angles of repose was 1.44%, indicating good agreement between the calibrated model and the measurements. A DEM-MBD coupled tensile simulation framework was then developed. Bonding parameters were calibrated against tensile fracture force, tensile stiffness, shear fracture force, and shear stiffness derived from combined physical and simulated tensile and shear tests. The relative errors of the optimised parameter set were 0.27%, 1.64%, 8.71%, and 2.35%, respectively, suggesting that the model reproduced the mechanical behaviour of hairy vetch stems with reasonable accuracy. The failure modes of the stems under tensile and shear loading were then analysed using the developed model. Further validation through bending tests and field experiments yielded relative errors of 5.89%, 6.24%, and 9.53% for the initial bending slope, peak bending force, and green manure removal rate, respectively, supporting the use of the model to analyse stem fracture and to assist in the optimisation of green manure cutting equipment under comparable operating conditions.