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

Abstract

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 large a planing surface or ski that rides within a confined channel in which this venting is substantially reduced or eliminated, Mirbod et al (2008). Introduction Whereas the flow inside porous media and the mechanical behavior of the solid phase of porous materials have been extensively studied in the past, relatively little attention has been paid to highly compressible soft porous materials because these materials are traditionally thought to be structurally fragile and support little weight unless highly compressed. In the past such materials were largely used in pillows. This view was dramatically changed as a result of the theoretical investigation by Feng and Weinbaum (2000), hereafter referred to as FW email address: andre@ccny.cuny.edu