Studies of the Dynamic Compressibility of Water-Saturated Fibre Networks, 2000 Engineering Conference Proceedings

Hannes Vomhoff--The dynamic compressibility of water-saturated fibre networks was investigated. Two types of experiments were performed: (1) the exertion of a defined stress pulse onto a fibre network in order to investigate the compression behaviour; and (2) the sudden relief of an applied stress in order to study the expansion behaviour. All experiments were performed at three temperatures (23, 50 and 80°C). Webs with a grammage of 30 g/m 2 and made from two pulps, softwood bleached kraft and thermo-mechanical pulp, were used. Roll-nip-like stress pulses were used having a length in the range between 20 ms and 200 s and a peak stress between 5 and 20 MPa. The compression behaviour was evaluated in terms of the work required for compressing the fibre network and as stress-strain relationships. The expansion behaviour was characterised in terms of the time-dependent strain increase and the strain obtained after three hundred seconds.

The qualitative compression behaviour was found to be similar under all process conditions investigated. A large reduction in strain was observed during the stress pulse with the minimum strain obtained between the peak stress and the end of the stress pulse. No significant expansion was observed as long as stress was applied to the network. A rapid and considerable expansion was observed only after the stress pulse. Higher peak stresses, higher temperatures and longer stress pulses led to a larger deformation of the web. The stress-strain relationship was approximated quantitatively by a power-law function. The effects of temperature and pulse length were accounted for by shifting the stress-strain curve horizontally by means of a shift factor. The time-dependent expansion was quantitatively approximated by an exponential function.