In the beginning of 1980, UK suggested the test method stress relaxation to ISO for measuring ageing in rubber. ISO started the work to prepare an ISO standard which got the number ISO 3384.

The idea of stress relaxation testing is to measure the reduction in counterforce of a compressed test piece, this shows the degradation of the polymer chains in the rubber material.

A manual compression tester made by Wallace in UKAt that time the only existing instrument was a manual compression tester made by Wallace in UK. In a test jig the test piece was compressed to 25 %. To measure the force a slight further compression was done so the test piece got free from the spacers, so the force could be measured. This was done in 23 °C and the test jig was then placed in an ageing oven at an elevated temperature. The test jig was then taken out from the ageing oven and cooled to 23 °C and the counterforce was measured again. This procedure was repeated at intervals to be able to plot a curve.

The first stress relaxation test rig with a load cell At SP we thought this was an interesting ageing test but using this discontinuous method added too many errors. We started to design a test rig with a load cell so we could measure the force continuously, to avoid all this handling which caused variations in the results.

Then came customers who wanted to buy this instrument, but it was not yet ready to be sold commercially. I got permission from SP to further develop the relaxation system on my free time. I had been using pen recorders for the recording of the force and that produced a lot of paper, so we needed a computer software to record both force and temperature. Then I needed a better cell oven for the ageing with better temperature precision. We had started to use Wallace cell ovens, but they had to narrow and long cells. The test rig also needed to be more rigid and include a temperature sensor.

Now I have one of the rigs in the Elastocon museum.

Owner history: SP (now RISE) in Borås

Göran Spetz