Finite element analysis and experimental study of impact-induced damage in water chestnuts

Water chestnuts are aquatic root vegetables in the Asian areas. However, the collision damage can often occur in the mechanical harvesting and post-harvest processing of water chestnuts. In this study, a finite element simulation was conducted on the impact between harvesting machinery and water chestnuts using Ansys software. The impact damage to water chestnuts was identified to enhance the quality of mechanical harvesting and post-harvest treatment. A principal strain failure model was also adopted to accurately simulate the energy dissipation during collision, rather than the existing elastoplastic model. Initially, the average size of the water chestnut was measured using three-dimensional scanner. An appearance model was also created after scanning the samples. Three sections were then divided into: peel, pulp, and top bud. Uniaxial compression tests were conducted to measure and calculate the intrinsic parameters of the flesh and peel of the water chestnut. A comprehensive model of water chestnut was established after selection, thus culminating in the selection of a linear elastoplastic and principal strain failure coupling model. The parameters were calibrated and verified by slab drop tests at three heights. A systematic analysis was also made on the evolution of stress, energy, and rupture of the coupled model during drop. The screening device of the harvester also impacted the collision damage to water chestnuts. A simplified rod model was developed to investigate the impact of collision on the water chestnuts under various conditions. To this end, a three-factor, three-level and full-factor collision simulation test was conducted in the fall heights of 200, 400, and 600 mm, collision contact surface diameters of 16, 20, and 24 mm, and collision materials of structural steel, PVC, and rubber. A three-factor, three-level full-factor rod collision simulation test was also to assess the damage susceptibility of water chestnut to bruising, softening, and rupture. The analysis of variance (ANOVA) was employed for the empirical formula of damage susceptibility under different materials. The elastic modulus, yield strength, and tangential modulus of water chestnut pulp were measured as 7.916, 0.586, and 4.554 MPa, respectively. The principal strain of failure was determined to be 0.098. The impact damage distribution was simulated with the failure mesh volume and the stress interval volume exceeding 0.60 MPa. The relative errors in the bruise volume, rupture volume, and dissipated energy were determined to be 7.18%, 8.98%, and 6.62%, respectively. Therefore, the model was expected to describe the mechanical properties, rupture behavior, and energy dissipation of water chestnut under impact load. The rod impact test demonstrated that the drop height of the water chestnut exhibited a direct linear relationship with its damage susceptibility. The elastic modulus of the collision contact material also exerted a substantial influence on the damage. The impact of the contact diameter was found to be associated with the nature of the contact material. In the case of collisions with the structural steel, the damage exhibited an initial increase and subsequent decrease with the increasing diameter. Conversely, the damage decreased with the increasing diameter in collisions with the PVC and rubber. The damage to 2, 4 and 6 mm rubber-coated rod teeth was reduced by 9.3%, 13.7%, and 20.9%, respectively, compared with the unwrapped rod teeth. These findings can offer a valuable reference on the quality of water chestnut during mechanical harvesting.