Abstract
A four-point bending stiffness tester was used to study the creep behaviour of corrugated board panels under constant and cyclic relative humidity. By substituting one linerboard in a single-wall corrugated board with steel ribbons, creep tests on linerboard under compression and under tension were also carried out. The results were evaluated using isochronous stress-strain curves relating to both constant and cyclic relative humidities. The isochronous curves showed a straight line relationship at low stress levels, for both constant and cyclic relative humidities. This means that the linerboard could be described as a linear viscoelastic material in both tension and compression at constant relative humidity.
The initial slope of the isochronous curves, the creep stiffness index, was lower under the cyclic relative humidity than at the constant relative humidity equal to the highest used in the cyclic creep experiments, a phenomenon called “mechano-sorptive creep”. It is worth noting that, as opposed to many other investigations, mechano-sorptive creep was demonstrated for paper subjected to comparatively low strain levels. No difference was observed between creep stiffness indices in tension and compression under a constant climate, but under a cyclic relative humidity the compression creep stiffness index was found to be lower i.e. the creep rate was higher in compression. A freely dried paper was found to have a lower compression creep stiffness index both at constant and under cyclic relative humidity than a paper dried under restraint. In machine-made sheets, the compression creep stiffness index was higher in the MD than in the CD both at constant and under cyclic relative humidities. For panels of corrugated board, isochronous bending moment-curvature curves were used to describe creep behaviour under pure bending. The results are discussed in relation to a model previously proposed (1)to describe the cyclic creep behaviour of paper.
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