Tear and Tensile testing.

In Science this week, we took a break from powerpoint and had a practical session in the labs looking at paper strength tests.

For tear testing (which seems a little redundant to me as you have to first tear the paper to see how it tears…) a small rectangle of paper is specially prepared in a jigged guilotine, which also cuts 2 identical notches into the longest side. This is then placed into the apparatus like this:

Set-up of the apparatus

This particular test is using a pendulum based apparatus, otherwise known as a Elmendorf test, which measure the internal tearing resistance of paper; edge tearing (which we did not experiment with) tests the strength needed to initiate a tear.  The pendulum is released which pulls the paper apart, thus tearing it. The results of the Elmendorf test are measured in millinewtons which are read off the scale on the pendulum when it has come to a stand still, or when the arm has completed it’s movement.

Elmendorf Apparatus, notice the scale and the arm on the left-hand side.

The second test we performed was tensile strength testing of paper. A strip of paper of a predetermined size is placed into the machine using tw0 pressurised clamps on either end of the strip. The top clamp then slowly moves away untill the paper snaps/tears, and the results are transferred to a computer.

Tensile Testing

The machine is calibrated so that the clamps start at exactly the right distance apart to take a relatively taught strip of paper, and can be adjusted so that the speed in which the clamps move apart is fast or slow. Throughout the experiment, up until the paper breaks, readings are being fed to the computer, which creates a graph.

Paper in the clasps.

Paper pulled apart.

The tensile test will be useful to me as I can incorporate it into the research for my final project, as not only does it test the tensile strength of paper, but also the strength of adhesives in a very basic way. Two pieces of paper/parchment/other material are stuck together, and then pulled apart by the apparatus, determining how strong the adhesive is in comparison to the materials it is adhering. However, to get any kind of useable data, I would need to do around 30 tests, which would not be possible as I would be using 5 different adhesives with 5 different materials at least!

Here’s a link to the tensile test machine in action.

References:
Caulfield, D & Gunderson, D. 1988. Paper testing and strength characteristics. TAPPI proceedings of the 1988 paper preservation symposium: 1988 October 19-21.Washington, DC. Atlanta, GA: TAPPI Press, pp. 31-40.

 

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