BS ISO 3384-1:2011 pdf download – Rubber, vulcanized or thermoplastic — Determination of stress relaxation in compression Part 1 : Testing at constant temperature
A test piece of vulcanized or thermoplastic rubber is compressed to a constant deformation and maintained at a predetermined test temperature. The decrease in counterforce is then measured.
In method A, the compression is applied and all counterforce measurements are made at the test temperature. In method B, the compression is applied and all counterforce measurements are made at a standard laboratory temperature.
The test pieces are stored at the test temperature. The test can be conducted in a gaseous or a liquid environment. The two methods, A and B, of carrying out the measurement do not give the same values of stress relaxation, and comparison of values obtained from the two methods should be avoided.
The method selected for use depends on the purpose of the test. Thus, for fundamental studies and in applications where sealing at elevated temperatures is a problem, method A might be preferred, and in applications where temperature cycling from normal to an elevated temperature is a problem, method B might be preferred. NOTE Other methods can be used for specific purposes, such as applying the compression at standard laboratory temperature and making all counterforce measurements at a different temperature.
5.1 Compression device, consisting of two parallel, flat, highly polished plates made from chromium- plated or stainless steel or another corrosion-resistant material, between the faces of which the test pieces are compressed. Flatness, surface roughness, parallelism and rigidity of the plates are all important. The surfaces of the compression plates shall be ground and polished. Compression plates shall be flat and parallel and shall not undergo any distortion when the test load is applied.
NOTE A finish to the surface giving a roughness profile Ra (see ISO 4287) of not worse than 0,4 µm has been found to be suitable. Such a roughness profile Ra can be obtained by grinding or polishing. When the apparatus is assembled without a test piece, the gap between the plates shall not vary by more than 0,01 mm. When the test assembly is subjected to the test load with a test piece between the plates, neither compression plate shall bend by more than 0,01 mm.
The plates shall be of sufficient size to ensure that the whole of the compressed test piece is within the area of the plates and can expand freely laterally.
For ring test pieces, the plates shall have holes of at least 2 mm diameter drilled through their centre portions to allow equalization of pressure and circulation of fluid inside the ring-shaped test piece. It shall be possible to connect the compression device to suitable equipment for compressing the test piece to the specified compression at the specified speed and for measuring the counterforce exerted by the compressed test piece with an accuracy of 1 % of the measured value. The device shall be capable of setting the compression and maintaining it during the whole duration of the test, and it shall be possible to keep the device in an oven at the specified test temperature. Care shall be taken to ensure that there is no loss of heat from the test piece, for example by conduction through metal parts which are connected with the outside of the oven.
5.2 Counterforce-measuring device, capable of measuring compression forces in the desired range with an accuracy of 1 % of the measured value. The device may be a continuous-measurement system which monitors the test piece during the whole duration of the test, making it possible to measure the change in the counterforce with time on a continuous basis. The deformation of the test piece shall be kept within 0,01 mm for the duration of the test. Alternatively, a compression-testing machine may be used to measure the counterforce at prescribed time intervals. In this case, the force necessary to cause a slight increase in the compression of the test piece is measured. This additional compression shall be as small as possible and in no case greater than a force of 1 N for balance-type machines, or greater than 0,05 mm for stress/strain-type machines, applied in either case without overshoot. The whole of the force exerted by the test piece as a result of the extra compression shall act on the force-measuring device. It shall also be possible to repeat the compression to within 0,01 mm from one measurement to another.
5.3 Test environment
5.3.1 For tests in gaseous media, an air oven complying with the requirements of ISO 188 shall be used. An oven meeting the requirements specified for one of the ovens used in ISO 188:2011, method A, is recommended. NOTE If the testing is done in nitrogen, oxidative ageing will be eliminated and the result will be due to thermal ageing only. This may be used to simulate conditions where the product is not exposed to air, such as seals used in oil or steam.
5.3.2 For tests in liquids, the compression device shall be totally immersed in a liquid in a bath, or a closed vessel for volatile or toxic fluids, such that free circulation of the liquid can take place through the holes in the compression plates. The liquid shall be maintained at the specified temperature by proper control of a heater and circulation of the liquid in the bath or, alternatively, by placing the liquid bath and compression device within an air oven as specified above.