| Pressure screening effect is one of famous static mechanical properties of granular materials. It says that the bottom stress of a silo will approach to saturation rather than increase linearly as the filling mass increases. We experimentally measure the static stress of granular chains at the bottom in 2D and 3D silos with a precise and reproducible method.The relation, between the filling mass and the apparent mass converted from the bottom stress, is investigated on various chain lengths. Our measurements reconfirm that the scaling behavior of the stress saturation curves is in accord with the theoretical expectation of the Janssen model. In 3D silo, the saturation mass is displayed as a nonmonotonic function of the chain length, and the position of the peak is related to the rigidity of granular chains. It is shown that the volume fraction approximates the saturation with the exponential law as the length of chain increases. In 2D silo, huge fluctuation in stress measurement appears, which is suppressed in 3D silo since the friction acting on a layer of the grain is averaged over a large number of grains. One possible reason is that the reorganization of the force chains occurs due to the limit of the dimension. After each reorganization, it will take a duration before the system reaches a new equilibrium state.Finally, we preliminary measure the bottom stress of granular chains for two different packing structures, and argue that the entanglement and elasticity might be responsible for the results.Flexible and semi-rigid granular chains which are linked by flexible nylon string and rigid rods are used respectively in 3D and 2D silos. There is essential difference between the two connecting backbones. For the semi-rigid granular chains, the packing characteristic might depends on the entanglement, whereas the flexible granular chains relies on the elasticity of the string. Therefore the entanglement and elasticity might be responsible for the different conclusions. |
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