Investigation of stress-strain behaviour of wet paper sheets

Abstract: A rise in the production capacity and runnability of paper-web provides an
opportunity to increase papermaking industry profits. However, the increase
of the speed of a paper machine is accompanied by an increased number of
web breaks due to internal stresses, which develops in the paper web,
especially in the drying section. The drying process is not momentary, so
the time-dependent (rheological) properties of the web affect paper
behavior, in particular, the stress relaxation.

In this work, the influence of loading force and such pulp properties as
water content, composition, and degree of beating on the internal stress
development and stress relaxation during drying were investigated. The
mechanical properties of hand-made and industrially produced paper (kraft-
paper and exercise-book paper) were evaluated prior to and during drying in
order to optimize the conditions of the relaxation process. Further, the
determination of stress dynamics in the structure of paper during
relaxation and drying was carried out.

It was determined that the web tension in the drying section should be kept
below the value of the loading force in the effective point on the load-
elongation curve in order to prevent web breakage.
It was shown that the highest relaxation and the lowest increase of the
internal stress was observed when the loading force was equal to one third
of the breaking force (Fb). At loading of 1/10 Fb the highest stress
development during drying took place in the samples with the highest water
content. Relaxation had the highest compensating effect on paper with a
dryness of 87-90 wt.% when the loading force was equal to one third of the
breaking force.

Investigation of the influence of beating degree showed that it was more
important for the unbleached samples than for the bleached ones. Too fine
beating is similar to the chemical effect during bleaching and makes the
mechanical properties of unbleached pulp similar to the mechanical
properties of bleached pulp. It was conformed that the drying rate affected
the development of the internal stress. It was shown that the rate of
relaxation and stress development during drying were always higher for the
unbleached samples than for the bleached ones.