On 29-01-2013

Pulling a rubber band will lead to rubber deformation, it will get longer while its section will get reduced, and it will endly break.
To qualify a rubber mechanically, it is necessary to measure the task needed to get its rupture.
Rubber test sample extension will be registered at the break out level.
It is necessary to use a traction device (dynometer) to measure this strength. Modern equipment are provided with an extensometer which, by following optically or mechanically 2 marks, can measure at any time the size of the rubber test sample during the extension pulling stage and when breaking point arrives.
Rubber test sample is defined by the NF T 46-006 norm. It has a bone shape or a ring with 4 sizes for the bone shaped sample (H1 to H4).
H2 size rubber sample

The most commonly used rubber test sample is a bone shaped rubber sample (H2 size) which is cut off in a 2 mm thickness rubber sheet. Its central part shows a rectangular section of 4 x 2 mm with a 25 mm length. It is the narrow part of the test sample, and rupture will occur in this area. The 2 marks (20 mm off) for the extension measure are also in this area.
Ring shaped test sample has a size of Ø 44,6 x Ø 52,6 mm with a 4 mm thickness. During its extension, it is rotating so that no particular zone will be affected.
Traction speed is 500 mm per minute. Traction resistance is measured in MPa, and extension in %. With an H2 rubber test sample, for 100 % extension, the distance between the 2 markings will have reached 40 mm.
We use in our laboratory bone shaped test samples on an electronic traction testing machine equipped with an optical extensometer. So, the traction resistance measure cannot be altered by clips following mechanically the 2 markings. All our rubber compounds are tested before being used for production.
Most performing rubbers can reach a 40 MPa traction resistance. Depending on mixtures and on hardness, rubbers are usually between 10 and 30 MPa, while silicons are between 4 and 10 MPa. This difference between rubber and silicon will be reduced while temperatures are increasing.
Hardness is related to extension rupture. However, mixture adaptations will have elongation influence on the results. Some values : a 95 Shore A rubber : 100 up to 150 %. a 60-70 Shore A : 300 up to 500 %. a 40 Shore A : 500 up to 700 %. Higher values (800 up to 1.000 %) can be obtained for special applications with specially developed compound mixtures.

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