ASTM D412 is used to characterize the tensile properties of rubber and elastomers. There are a myriad of different polymers with highly elastic behavior including ethylene-vinly acetates, fluoroelastomers, and polyacrylic rubbers. When elastomeric material is manufactured it can be either a thermoplastic which exists in a solid state, or it could be a thermoset, which requires vulcanization. Polymer thermosets become vulcanized once sulfur or another additive is combined with the thermoset, usually at elevated temperatures. Once the chemical combination is cooled, the vulcanized elastomer acquires enhanced mechanical characteristics.
Specimens for this procedure are usually of the "dogbone" variety or lesser referred to as the "barbell" shape. The exact required geometry for these samples can be found in the appropriate ASTM publication, or with a bit of hunting around on the Google machine. As with most material testing applications, the geometry of the cross-sectional area is the main focus. Rubbers can stretch to 3 or 4 times their original length which means that consideration must be made towards the total crosshead travel (stroke) of the tensile tester.
There are several grips and fixtures which can test elastomeric polymers. These materials tend to thin out during the test due to their weak intermolecular bonds, so self tightening grips work best. These include those of the roller variety, as well as the scissor grips. Pneumatic grips also work well but are more expensive and less foolproof.
A successful ASTM D412 test can yield several data sets for material scientists to parse. The basic ones include Ultimate Tensile Strength and Elongation at Break. However there are other analyses that can be gleaned as well, including Young's Modulus, Offset Yield, and the Modulus of Elasticity which is also commonly referred to as the Elongation Modulus.