Tensile Set of Rubber and Thermoplastic Elastomers ASTM D412
Tensile set testing of a rubber or thermoplastic elastomer evaluates the residual elongation of a test sample after being stretched and allowed to relax in the specified manner. This elongation consists of both permanent and recoverable components - thus the time for stretching and recovery are important factors, and both are specified by D 412.
Test samples are typically die cut from large sheets.
Two marks are precisely placed on each test sample at the ends of the gauge length of the dumbbell shaped specimens. The initial length between these marks is measured for each sample. The samples are carefully mounted in the test fixture to provide uniform alignment and position. The fixture stretches the samples to the specified elongation and holds them for 10 minutes. The samples are then released from the fixture and allowed to relax for 10 minutes. After the relaxation period, the final distance between the marks are measured.
ASTM D 412 specifies a dumbbell shaped specimen. The specification describes 6 options for the sample dimensions, but the preferred sample is "Die C". Die C has an overall length of 115mm (4.5 inches) with a narrow section 33mm (1.31 inches) long. This provides a gauge length (benchmark) 25mm (1 inch) long and a gauge width of 6mm (0.25 inch).
Tensile set is expressed in percent and is calculated by dividing the change in length of the test sample by the initial length.
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