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Stress relaxation tests on rubber materials are useful for determining rubber properties. Swedish firm Elastocon that develops testing equipment has introduced a new ALE (Aeration and Liquid Exchange) test , which it believes will be big within the automotive industry in the future.
What is a stress relaxation test?
In the early days, relaxation tests were mostly used in scientific projects, though over the years they have gained popularity in industrial applications and in different product standards, such as sealing rings for pipes and in the automotive industry.
| Continuous testing | Discontinuous testing |
| Less manual work, measurement will continue throughout the test after it has started. | More manual work; need to manually perform measurements at certain points during the test. |
| Logging automatically and continuously which means it is possible to obtain measured values from any given point from the test after it is finished. | No extra data is saved, only the manually taken measurements. Not possible to add extra evaluation points after the test is finalised. |
| No physical movement of the rigs after the start of the test. | Every time a measurement is performed the rig/jig is moved, and it’s proven that each time a rig is moved the result might be affected due to vibrations that occur during the movement. |
| Most customers around the world request for this type of testing; several large companies have it as a standard policy. | Few customers request for this type of test; it’s within some large companies’ internal standard testing policy. |
| Possible to automatically run tests according to ISO 3384-1, ISO 3384-2, ISO 6914 and other technical equivalent standards. | No automatic testing is possible; requires a lot of manual work. |
| Possible to run automatic tests with either stable or cycling temperatures. | The temperature will not be stable throughout the whole test; measurements will as default take place in ambient temperatures (may be performed within a special temperature chamber). |
| Testing either in compression or tension, air or liquid conditions. | Testing in compression, air or liquid conditions (liquid might be rather messy during the measuring stage). |
Stress relaxation is the behaviour of rubber where if a constant strain is applied to rubber, the force needed to maintain this strain is not constant. It decreases with time, due to chemical or physical properties, with both occurring simultaneously under normal conditions.
At low or room temperature, as well as for a short time, the physical process dominates stress relaxation, whilst for testing over a long period of time or at high temperatures the chemical process is the dominant feature.
There are two different ways of testing stress relaxation: continuous and discontinuous method.
Why change the liquid?
The traditional way to test stress relaxation in liquids is in a closed container where both the liquid and the material sample are aged simultaneously. For some long term tests, it is quite common to change the liquid after a certain time interval. This has been the only way to conduct the test in liquids.
During the ageing process, most often in elevated temperatures, both the material and the liquid will degenerate/age and lose its initial features. In a closed container, the amount of oxygen is depleted after some time. These parameters cause the sample to be stored in a degenerated environment that is too often far from the actual environment where the material is going to be used. Hence, the test results will not show the actual values to be expected while using the product in the environment it is intended for.
As already mentioned above, this has until now been handled by changing the liquid during certain time intervals, with the risk of jeopardising the test results when handling the liquid change. This is not the ideal way to test in liquids.
Let’s take an example, where the sealing is used in an oil system in a vehicle. In the vehicle, the oil will pass through the rubber and it will introduce air, together with oxygen present during the movements. Then, from time to time, some additional oil will be added to the system and at some point more or less all the oil in the system will be changed. This means that in a vehicle, there will be a mixture of new oil, old and degenerated oil and oxygen in the air.
In a closed container, there will be no air and only degenerated oil, and when the liquid is changed, there will be new oil and air that will degenerate, thus ending up with the same degenerated liquid. Traditionally, there will be a cycle (from new to degenerated) of the liquid that is not as stable as the liquid passing through the rubber in the actual product usage environment.
The effect of the test result will also have to be considered while moving the rig for the liquid exchange. All the movements will have a negative effect on the test results.
ALE-testing unit developed
Elastocon has developed a method where both aeration and liquid exchange are automatically introduced during the ageing process during a relaxation test. This means that an optional aeration can be introduced in the container as well as liquid exchange, without the rig being moved, thus doing away with the risk of any movements affecting the test results.
The new system, that is an addition to Elastocon’s existing stress relaxation system for continuous measurements, is called ALE (Aeration and Liquid Exchange) test. This system has a new rig with a propeller for mixing in the air in the liquid as well as stirring the liquid in the container to certify an even distribution within the container.
New liquid can be pumped in at the bottom and out in the upper part of the container. Both in/outflows are controlled via a new control box that has both a PLC touchscreen and flowmeter.
A continuous test system can be used to measure automatically the same test that is performed with a discontinuous test system, and with several advantages. Studies made have shown that both HNBR and FKM, two common materials that are used in O-rings for sealing in vehicles, will have different results while testing in the new equipment, compared to the traditional equipment. This result is interesting especially for manufacturers manufacturing materials used in environments with both air and liquids present.
Features:
- Non-volatile liquids only
- Only for stress relaxation in compression
- On the new bracket on the rig additional samples can be aged in the exact same environment while the relaxation test is performed. Samples that can be used for tensile testing, hardness testing or anything else required. Either five small dumbbell shaped samples (eg. ISO 37-2) or O-rings, or three bigger O-rings, can be mounted on the bracket at the same time
- Option to use the liquid exchange or not, and to set the speed for the same
- Option to use the aeration or not, and to set the air flow for the same
- Option to use the stirrer or not
- The new rig is designed to be used with Elastocon’s optional cell ovens for stress relaxation tests, also together with the two ovens with cycling temperatures as options (up to 200°C)