Evolution of force chain in vibration compression of alfalfa

Abstract: Natural forage has been widely compressed into the blocks ideal for long-term food storage in modern agriculture. The mechanical behavior of forage during compression can be directly related to the density of blocks and the compression energy consumption. In this study, a systematic investigation was made on the stress transfer, force chain structure, and evolution of crushed alfalfa in the process of vibration compression using a discrete element method (DEM), in order to clarify the mechanical behavior of forage and the mechanism of vibration compression. A DEM model of crushed alfalfa was also established to determine the stress change during compression. A physical experiment was conducted to verify the DEM model. It was found that the variation trend of compressive force after simulation was consistent with the experimental data. There was no significant difference between the simulated and experimental data at the significance level of α = 0.05. A simulation analysis was performed on the force chain structure and evolution during compression using the DEM model. The research results were as follows. The distribution and evolution of the force chain in compressed alfalfa were different in the process of compression with/without vibration. Once alfalfa was compressed without vibration, the axial force chain was transmitted from the top to the bottom, and the distribution density of the strong chain decreased layer by layer as well. The number of strong contact points was 39 258 in alfalfa block. Once alfalfa was compressed by vibration, the axial force chain was transmitted from the bottom to the top in the early stage of compression, and the upper and lower ends were transmitted to the middle layer at the later stage. At the end of compression, the force chain in the block was distributed evenly, where the number of strong contact points was 38 079. The more strong the contact points were, the greater the compression force was, and the more difficult the material was to be compressed. Once the crushed alfalfa was compressed whether with or without vibration, a strong chain appeared around the center, and the outward transmission was gradually enhanced, as the compression stroke increased in the radial direction. At the same time, the distribution density and the strength of the strong chain generated in the material during vibration compression were less than those without vibration. Furthermore, there was a small porosity in the upper and lower layers at the end of the compression stroke without vibration, and a large porosity in the middle layer, where the absolute difference was 3.65 pecentage points between the porosity of the sparse and dense layers. Once alfalfa was compressed by vibration, the porosity of each layer basically maintained a decreasing trend from the top to the bottom, where the absolute difference of porosity was 2.71 pecentage points between the sparse and dense layers. It infers that the vibration was conducive to homogenizing the block density. The mechanical properties of biomass in the vibration compression were also analyzed at the microscopic scale. The finding can also provide a new way to explore the mechanism of vibration compression in forage production.