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conservation research work, as they can be used to pull apart and thereby
measure the strength of yarns, adhesive joints, plastics, etc. They can also be
adapted to crush materials such as stone. In tensile testing of paper, a strip is
clamped between two jaws and is pulled apart as the jaws start to separate. A
load cell measures the stress on the paper. As the paper stretches, the stress
increases until the paper fibres start to slip over one another or the paper starts
to tear, in which case the stress will decrease. The graph of stress against
elongation can be used to calculate several useful parameters relating to the
strength and elasticity of the paper.
When paper breaks under tension, the break involves a combination of
fibres breaking and bonds between fibres being disrupted. In very strong
fibres, the paper breaks mostly by failure of the fibre-to-fibre bonds, while in
weak papers the break occurs by failure of the fibres themselves. If the jaws
holding the paper strip are very close together, the fibre strength dominates
the results obtained as the contribution from fibre-to-fibre bonding is min-
imised. This is called zero span tensile testing.
In all the mechanical tests described in this section, relative humidity must
be controlled carefully as the properties measured change significantly.
International and national standards will give the required RH history for sam-
ples to be tested. As there is a hysteresis effect with water content and RH of
paper, it may be important that the desired RH for testing is approached from
one of either the dry side or the damp side. Of course, different results will be
obtained if properties differ between the CD and MD, but also some proper-
ties, e.g.gloss, colour, roughness, etc., may differ from one side of the sheet to
the other. However, a major problem with all the mechanical tests is variabil-
ity of results. Not only is there non-uniformity in the paper, but test results
which involve breakage of the sample have a scatter in values which is larger
than is desirable because of the catastrophic failure mechanism. Because of
the scatter, a large number of samples have to be tested to obtain a result that
is statistically useful. Testing 15–30 strips is usual. This consumes a large
amount of sample and this type of test is not so useful for irregular papers or
those in short supply such as those from objects.
The ideal mechanical test would be one in which the properties related to
the use of the paper were tested. Although weak papers have low tensile strength
it may be of more interest to measure how they behave when folded. A test
that measures the tensile strength after one fold has been proposed.
If folding is of paramount importance, the MIT folding endurance test is of
great value. In this test, the test strip is held vertically under tension. The upper
jaw that holds the paper is fixed but the lower jaw swings like a pendulum,
with the centre of rotation being the point where the paper enters the jaws. The
tips of the jaws are curved in a defined manner. A counter measures the num-
ber of folds required before the paper breaks. A strong paper will endure over