An elastic/viscoplastic model of the fibre network stress in wet pressing:Part 1

A rheological model of wet fibre networks in compression is required for ca lculating the wet fibre network stress (the so called "structural pressure" ) in hydrodynamic models of wet pressing. The model should incorporate the observed features of wet web compression such as non-linear elasticity and large non-recoverable (plastic) strains that are rate-dependent. The models used so far do not take into account the rate-dependent compression behavi our observed even in experiments in which the flow resistance between the f ibres is insignificant. Recent experimental results indicate that the elast ic strain is independent of the strain rate. The plastic (non-recoverable) strain is, however, clearly sensitive to the strain rate and much larger th an the elastic strain at peak stress. An elastic/viscoplastic model was dev eloped to describe this behaviour and its predictive capability explored us ing data from uniaxial laboratory tests. It captures the observed features of wet fibre network rheology in compression and gives good predictions of the fibre network stress for a wide range of pulse shapes, pulse lengths an d peak pressures. The good agreement between model predictions and experimental results sugge sts that the model can be used to calculate the fibre network stress (i.e. the structural pressure) in hydrodynamic models of wet pressing. Further, i t is consistent with the hypothesis that the rate dependence of the non-rec overable strain, observed even in pressing events in which the flow resista nce between fibres is negligible, is primarily due to visco-plastic effects when the intra-fibre water is squeezed out of the fibre walls.