A suggested method to describe the creep behaviour of paperboard in edgewise compression works for paperboard in the same way as for other polymers. The relation between stress, strain and time maybe determined by a simple equation involving two factors, a power function of time and a factor describing the non-linear behaviour of the stress-strain curve.
For engineering purposes, it is an advantage to be able to design a given product in terms of stress and strain in isochronous curves. In some applications the critical design criteria maybe governed by a critical strain. For such applications isometric curves, i.e. stress versus time at different strains, are used.
The strain at break in compression of the investigated paperboards was found to be independent of time. The strain to break in creep was equal to the strain at break for stress-strain testing under strain control. By inserting the strain at break in creep in the creep equation, the isometric curve at break could be derived which by definition gives the relation between stress and lifetime. The data in this investigation indicate that a rather small number of specimens and a reasonably short creep time are sufficient to provide a good prediction of the long-term creep behaviour. In other words, the creep of paper follows the fundamental behaviour of the material already established at short times, and described by the creep equation.