Measurement of short span stress-strain curves of paper

A new method is reported for measuring the short span stress-strain curve of paper using a zero/short span tester with additional instrumentation to measure load and displacement during a test. The method is to subtract the zero-span displacement-load curve from the short-span displacement-load curve, so as to obtain the displacement-load curve of the free span. An analysis of the short span test shows that if the sample is well-bonded and the free span is small compared to the fibre length, then the stress-strain behaviour of the free span is independent of span. The new method was used to examine the effect of refining on the short span stress-strain curve of a paper made from an unbleached, never-dried commercial Scandinavian softwood kraft. The breaking strain increased from 12 % for sheets made from the unrefined pulp to 23 % for sheets made from pulp refined to 6,000 PFI revolutions.