The Definitive Guide to ASTM D412 Tensile Testing of Elastomers

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 ASTM D412 for those who choose the Elastomer module.

In order to test properly, specimens must be held perpendicular to the jaw faces and not tilted on an angle. Specimen misalignment can cause major variations in test results, and proper care should be taken to ensure that the specimens are all aligned consistently for each test.

One way of addressing misalignment is by using a jaw face that is close to the same width as the specimen, making it relatively easy to visually adjust alignment. The easiest way to prevent misalignment is to use a specimen alignment device which mounts directly onto the grip bodies. This is a simple bar that provides an adjustable stopping point so operators can easily see that their specimen has been aligned correctly.

Once grips are tightened onto the elastomeric specimens in preparation for running a test, unwanted compressive forces are frequently applied. These forces, although minute, can interfere with test results if not treated properly. It is important that they not be balanced after the specimen is inserted, as this will cause an offset in results. Bluehill Universal materials testing software can be programmed to normalize forces across multiple specimens and remove any slack or compressive force, ensuring consistent results between specimens. Specimen Protect, which is available on the 6800 Series universal testing machines, is designed to prevent damage to the specimen or system during the set-up phase of a test, before a test’s operational limits are defined. When turned on, Specimen Protect automatically adjusts the crosshead to keep any unwanted forces under a certain limit.

Although extensometers are not required by the standard, a strain measurement is recommended for the most accurate and repeatable results. Strain measurement based on crosshead travel can be less accurate due to elongation of the specimen outside the gauge length. The XL Long Travel extensometers are designed to work for elastomeric materials and can also be used through specimen break. The adjustable clamping force on the knife edges reduces premature failure of the specimen. This extensometer can measure strain up to 3,000% from a gauge length of only one inch.

A non-contacting extensometer like the Standard or Advanced Video Extensometer 2 (SVE 2 or AVE 2) is the most ideal for elastomeric materials. It is especially suited for specimens that are fragile or require an environmental chamber for testing. Non-contacting video extensometers offer the added benefit of reducing results variance due to operator interference and increasing efficiency in labs with high throughput needs.

Elastomers tested under ASTM D412 are often destined for future use under non-ambient conditions. Because ambient conditions have a great impact on the tensile properties of elastomers, care must be taken to ensure that test conditions mimic those of the intended end-use application. Test speed, temperature, humidity, specimen dimensions, and pretest conditions all have notable effects on test results and must be controlled in order for the test to produce useful data. To ensure that these elements simulate the material's end-use application, ASTM D412 is frequently performed inside an environmental chamber where heating or cooling (LN2 or CO2) can be used.

Instron's 600 Series environmental chambers enable operators to monitor the temperature inside the chamber throughout the duration of the test. In addition, soaking time and temperature can be set up within Bluehill Universal to ensure that all specimens are conditioned appropriately before the start of the test.

For labs looking to increase their throughput, several modifications to the system setup can be made. Automatic specimen measuring devices, pneumatic grips, and specimen alignment devices all increase test efficiency by reducing the amount of manual input needed from the test operator. However, testing time for ASTM D412 depends on the expected elongation of the specimen and cannot be minimized. For materials with high ductility, the duration of the test consumes the majority of the overall test cycle. In these cases, multi-station test frames may provide the best throughput because the operator can run up to 5 tests simultaneously. 

Fully automated test systems are also available and are designed to incorporate specimen measurement, specimen loading, testing, and removal. These systems can run for hours without requiring any operator interaction. In addition, these systems help reduce variability due to human error.