Tension set can be used either with constant strain (here called method A) or constant load (method B), meaning you either keep it at a certain elongation for a set duration or use a certain weight.

Related Standards

ISO 2285

Introduction

Tension set testing with a constant load has been used in Sweden long before international standardized tests of rubber materials became dominating. Tension set can be used either with constant strain (here called method A) or constant load (method B), meaning you either keep it at a certain elongation for a set duration or use a certain weight. The first method is similar to compression set, in that it uses a set elongation and then measure how well the material springs back. This is either done at lab temperature or elevated temperatures. Tension set using constant load is usually performed in lab temperature and gives information about the elongation, creep and tension set at said load after a certain time.

Method A requires a rig that can keep the samples at the set strain but is otherwise very accessible. Method B can probably be considered even more accessible, since all that is required is customizable weights and clamps to hang onto the sample bodies and some way to measure the elongation. This is most easily done with calipers. A table of comparison can be found below.

Performance

For both methods, the shapes of the samples are standardized and can be punched out of a sheet of rubber using a cutting die. For method A, a flat strip with a width of 2 to 10 mm, preferably 6 mm, can be used. But more commonly a dumbbell shape is used with wider grip areas flank a narrow reference area of 2 × 2 mm with a length of 50 mm. Ring shapes can also be used. For method B, a longer shape is preferable, with a narrow reference area 4 × 2 mm and 100 mm long. This makes it easier to measure the small differences corresponding to the creep.

To perform tension set measurements according to method A, mark the samples with a distance of 25 to 50 mm and elongate them to the desired strain. This strain can vary depending on the material, but ideally one of the standard 15, 20, 25, 50, 75, 100, 200 or 300% of initial length. The initial length is marked L1 and 10 to 20 minutes after straining the sample L2 is measured. This is the strained reference length. If the sample is to be tested at elevated temperatures, it is now put in an oven (or freezer). To measure the final tension set if lab temperature value was used, the strain is released and the sample is left to recover for 20 to 30 minutes and then measured for L3. If a temperature other than lab temperature was used, there are a few variations of the order of removing strain and temperature before measuring. For method A, remove straining device from oven and release strain. For method B, the device is removed from the over but 30 minutes pass before the samples are released from strain. Then a further 30 minutes pass before measuring the tension set length. In the final method C, the samples are released from strain inside the oven for 30 minutes before they are removed from the heat to recover for 30 minutes more.

When the samples are prepared for method B, they are marked at a distance of 90 mm over the thin area and then hanged under load and measured again after 30 seconds. This initial elongation is named E1 and is expressed as a percent increased length. After 60 minutes under load, another measurement is taken which gives the creep, E2. After offloading the sample and allowing it to recover for 10 minutes the final measurement is taken and the tension set, E3, is produced.

In depth considerations

According to the ISO-standard ISO 2285, the test times used should be 24, 72 or 168 hours which can give a wide variety of information. Similarly to compression set, when testing tension set for short times (24 h) and at elevated temperatures it can give information about how well vulcanized the material is.

Conclusion

Tension set is a very similar method to compression set but the sample is more exposed to oxygen. A large difference is the shape of the samples, leading to a difference in result. Similarly to compression set, which gives information suitable for sealing materials, tension set gives information more relevant for materials for use in tension.

Figure 1. Elastocon Tension Set Rig EV 04.

Elastocon performs tension set measurements in our test laboratory in Brämhult, Sweden as well as sell rigs for tension set according to method A. We also sell a kit of software and calipers for data entry for both methods.

Read more about Elastocon's Tension Set Rig, EV 04, here.

For more information, please contact us at This email address is being protected from spambots. You need JavaScript enabled to view it.

And for further reading, please find the referenced standards at:

ISO 2285: https://www.iso.org/standard/77425.html

Kim Bini