How to Perform a Tensile Test to ISO 37

Written by Meredith Bernstein

ISO 37 determines a material's tensile strength, elongation at break, stress at a given elongation, and yield point. The measurement of stress and strain at yield is only applicable to some thermoplastic rubbers and certain other compounds.

ISO 37 tests are performed on a universal testing machine. System capacity will depend on the strength of the elastomer, however, most tests to this standard will fall in the 1 kN and 5 kN capacity range, making this application perfect for a single column test frame such as Instron's 34SC or 68SC. Many elastomers tested to ISO 37 are high-elongation materials, and may require an Extra Height frame.

Specimens and Grips

ISO 37 includes two different specimen shapes, dumbbell and o-ring, with dumbbell being the most common. Elastomers are high elongation materials that often thin significantly when load is applied, so it is best to use pneumatic side-action grips or self-tightening roller grips for testing. Pneumatic side-action grips maintain constant clamping pressure during testing, even if the specimen thickness is reduced, while self-tightening roller grips use a snubbing type clamping action to increase the holding force as the load on the specimen increases. Self-tightening roller grips can also be used for testing at non-ambient temperatures. When testing o-ring specimens, an appropriate o-ring fixture should be used instead.

• Dumbbell specimens for testing to ISO 37 should always be cut parallel to the grain of the material, unless results comparing machine direction and cross direction are required. In this case, an equal number of each specimen type should be tested.

• ISO 37 states that an extensometer, where used, should meet class D for type 1, 2, and 1A dumb-bell specimens, and class E for type 3 and 4 dumbbell specimens. Given that the standard calls out reporting stress at several different strain values, it is highly recommended to always use an extensometer for testing to ISO 37. Strain measurement based on crosshead travel can be less accurate due to elongation of the specimen outside the gauge length.