Dynamic Compaction of Soft Compressible Porous Materials: Experiments on Air-Solid Phase Interaction

An experiment has been designed to examine the generation of dynamic lift forces during compaction of soft highly compressible fibrous materials in a piston/cylinder apparatus. Novel experimental techniques have been developed to simultaneously measure and separate out the dynamic forces acting on the compressing piston by the air and solid phases, respectively. The measurements clearly demonstrate that the excess pore pressure builds up inside the porous material and reaches its maximum before there is any significant rise in the solid-phase force. The air pressure subsequently decays due to air venting into the ambient environment at the edges of the testing apparatus as the solid phase force rapidly rises. We also examine the internal stresses and strains within the solid phase that develop as a result of friction forces along the sidewalls of the cylinder and show that these lead to important hysteresis effects when the loads on the piston are gradually removed. The experiments are performed on a polyester material with a trace of silk, which has recently been proposed for a soft porous track wherein a high-speed train is supported by a large planing surface or ski that rides within a confined channel in which this venting is substantially reduced or eliminated (Mirbod et al, 2009). Copyright © 2009 Begell House, Inc.