How to Perform a Tensile Test to ISO 37

Determining the Tensile Properties of Vulcanized or Thermoplastic Rubber

Written by Meredith Bernstein

ISO 37 is used across a wide range of industries to measure the tensile stress-strain properties of vulcanized and thermoplastic rubbers. It is similar but not equivalent to ASTM D412, and most companies will test to one standard or the other depending on where in the world they are located. ISO 37 is commonly used by companies engaged in raw material manufacturing, development of elastomer technology, and manufacturing of rubber-based consumer products and medical supplies, such as rubber gloves. ISO 37 is also a very common standard used by the energy and automotive sectors, particularly tire manufacturers. This guide is designed to introduce you to the basic elements of an ISO 37 tensile test, including an overview of the equipment, software, and samples needed. However, anyone planning to conduct ISO 37 testing should not consider this guide an adequate substitute for reading the full standard.

ISO 37

What Does it Measure?

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.

Materials Testing System

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.

ASTM D412 test setup

ISO 37 Test Setup

1. Instron 6800

2. Bluehill Universal Dashboard (2490-696)

3. 2580 Series Load Cell

4. 5 kN Pneumatic Side-Action Grips (2712-045)

5. 2603-080 Long Travel Extensometer

 

 

 

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.

2712-045 grips
5 kN Model | 2712-045
2613-002
5 kN Model | 2713-002

Software 

Bluehill Universal comes with a free application module upon purchase. Inside each application module are pre-configured test methods and supporting documentation to assist with testing to various standards, including ISO 37 for those who choose the Elastomer module. Two separate methods are included, one for ISO 37 dumbbell specimens and one for ISO 37 o-ring specimens.

Strain Measurement

Both contacting and non-contacting extensometers meet the accuracy requirements of ISO 37. Instron's AVE2 is a highly versatile non-contacting solution and can be purchased with a 6mm Extra Long FoV Lens which is ideal for measuring elastomers. The AVE2 is the best solution for measuring delicate specimens as it makes no contact with the specimen and can be used for testing high-elongation elastomers.

Another excellent solution is Instron's XL Long Travel Extensometer, which is specifically designed for measuring strain in highly extensible materials such as elastomers. 

ISO 37 extensometers

Helpful Tips and Tricks

  • 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.

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