Experimental Study On The Structure Changes Of The Two Dimensional Granular System With Applied Force

Granular material is a kind of soft condensed matter which is gathered by a large number of particles, and the relation between its microstructure and macroscopic mechanical properties is very complex. At the same time, the force situation and sliding channel are very complicated when particle system is squeezed. To the two closely issues, this paper presents a two-dimensional particle container. We use both experimental and theoretical methods to analyze its complex mechanical behavior,and better results are obtained. The full content includs the following two lists:First, the lateral stress distribution of the two-dimensional vertically stacked lattice of granular material under pressure in the vertical direction has been experimentally investigated. The steering behavior of vertical pressure in granular system is discussed and analyzed in details based on the experimental results. The results show that in the process of slow compression, the vertical pressure increases slowly in a nonlinear form at first and gradually transformed into a linear increase. This phenomenon correspondes to the dynamic processes of friction-slip-extrusion.This kind of behavior is more significant for particle system with same size. In the initial stage of the pressing, the vertical force of the stepping motor is mainly used to overcome the friction between the particles and the sliding friction between the particles and the wall. As the friction in granular system relates to the geometry of the particulate deposits, the material of particles, the roughness of the wall surface and other relevant factors, so the front-end of vertical pressure displays nonlinear characteristics continuing to squeeze and pushing forward, a force chain is formed among particles through self-organization. The vertical force is mainly used to overcome the elastic pressing force between the particles and that the force to the wall, so the later vertical pressure performes linear growth. For the system of particles with an established packstructure, the vertical pressure applied in the vertical direction steers along the force chain between particles and the value of horizontal pressure is different at the different stacking height. That is, the pressure in the middle is greater than that at the top and bottom. The saturated value of steering coefficient decreases with the stacking angle. The expression of steering coefficients varying with stacking angle have been obtained through the careful analysis of the geometrical structure and the force situation of granular pile, and the theoretical value agrees with the experimental results well.Then, the mechanical response and structure changes of two-dimensional lattice pile system of granular material in the process of being pressed are experimentally investigated. The structure collapse and self-organization behavior of the particle system are observed when squeezed. Based on the experimental results, the relationship between the horizontal pressure distribution, the sliding channel or the reorganization structure and the stacking angle as well as the boundary friction factor is discussed. According to the theory of statics, different force mechanisms about the boundary particle and the central one are discussed. Studies have shown that smooth wall is easier to become a sliding channel for particles than rough wall, so the force chain formed by the particles is unstable, which means it will easily lead to structural collapse. The friction factor on side wall will not have a significant impact to the stress distribution at different heights. In fact, the recombination of force chain is the main factor leading to stress redistribution at different heights.