Tensile testing is one of the most common checks in rubber. It answers a simple question: how hard can you pull on this material, and how far will it stretch before it breaks? Here is how the test is done under ASTM D412, and how to read the numbers.
What the test measures
ASTM D412 covers tension testing of vulcanized rubber and thermoplastic elastomers. A dumbbell-shaped specimen is cut from a molded sheet or a finished part. The specimen is clamped at both ends in a tensile tester and pulled at a steady speed until it breaks. A common speed is 500 mm per minute (20 inches per minute).
The machine records force and stretch the whole way. From that curve, the test reports several values:
- Tensile strength: the highest stress the sample reached before it broke, in MPa or psi.
- Ultimate elongation: how far the sample stretched at break, as a percentage of its original length. A result of 400% means it stretched to five times its original length.
- Modulus (stress at a given elongation): the stress at a chosen stretch such as 100% or 300%. It is a measure of how stiff the rubber is while it is stretching.
How to read the results
High tensile strength with high elongation usually points to a tough compound. High modulus means a stiffer compound. The numbers have to be read together, and against the compound type. A silicone, an EPDM and a natural rubber compound have very different typical ranges, so compare like with like.
Why it matters for molded parts
- Batch control. Testing each batch against the same method shows whether the compound was mixed and cured the same way every time.
- Spec compliance. Many customer specifications list minimum tensile strength and elongation values.
- Aging checks. The same test run after heat or fluid aging shows how much strength the rubber loses in service.
Tips for specifying it
- Give the standard (ASTM D412), the test method letter and the specimen type on the drawing or spec.
- State minimum tensile strength and minimum elongation, and the modulus point if it matters.
- If the part will see heat or fluids, ask for results after aging as well.
See our page on ASTM D412 tensile testing and the rest of the rubber testing videos in our Video Library.
Have a part that needs to hold its shape or pass a spec? Send the drawing or sample requirements to estimator@coirubber.com and our engineering team will review it. Coi Rubber has been in business since 2003, is US owned and operated, and our Wuhu, China manufacturing facility is IATF 16949 certified.