Test Method A of ASTM D412 is a very popular testing standard. It applies to tensile testing of vulcanized thermal set rubber materials and thermoplastic elastomers. Test Method A applies specifically to dumbbell shaped specimens and straight specimens. One confusing issue in planning this test is deciding which specimen geometry to use because the specimen table in the standard offers several geometries without clearly describing where to apply each.
Straight specimens can be used but are discouraged because, with these types of materials, they will almost always break prematurely along the edges of the grips. You should use straight specimens only if you are unable to make dogbone geometries through the use of a cutting die or other means.
Anisotropy or grain directionality may be an issue with these kinds of materials. Therefore your specimen size should be chosen such that you are able to orient and cut your specimens lengthwise in the grain direction.
There are two tables in Figure 2 of the standard that define the detailed dimensions of dogbone specimens for ASTM D412. One has the detailed dimensions of specimens in metric units. The other has detailed dimensions of specimens in English units. The metric and English units specimens are not identical so choose one or the other.
Determine the Force Capacity needed in the Test Machine:
Specimen thickness should be in the range of 1.3 mm (0.05 in) to 3.3 mm (0.13 in) if possible.
Dimension W in the tables is the width of the specimen in the gage length section where it will break. Use the thickness and width to determine the cross sectional area in the gage length region. Multiply that by the nominal tensile strength to calculate an estimate of the force at which the specimen will break. Alter your specimen width and thickness to an appropriate cross-sectional area such that your test machine has the force capacity necessary to break your test specimen. We recommend about 25% extra force capacity in case there is some variability in the strength of your specimen material.
Determine the Vertical Travel and Vertical Space needed in the Test Machine:
Sections 10.2 and 10.3 and Section 11.1.2 of the standard describe how to identify and mark the gage length of your test specimen. For Dies C and D the gage length shall be 25 mm (0.98 in). For all of the other dies the gage length shall be 50 mm (1.97 in).
The vertical test space analysis for ASTM D412 testing is complicated. First, determine the nominal elongation of your specimen material. Multiple that by the initial gage length to determine the travel you need for your extensometer. Add about 20% more because the neck-down portion of your specimen will also be elongated by the test machine. Your test machine must have at least this much crosshead travel capacity plus a little more. Your extensometer will only measure the gage length elongation but must also travel upward enough to accommodate the elongation of the specimen material in the lower neck-down area. One other factor to take into consideration is that the vertical space from the base of your test machine to the crosshead must also have enough space to fit the grips, load cell and any connector fixtures, in addition to the travel described above.
At TestResources, we know elastomer tensile testing. We can provide all of the test equipment and accessories you need for ASTM D412 based elastomer tensile testing including the test machine, long-travel extensometer, appropriate grips and specimen cutting dies.Visit the links below for some additional information and contact us with any questions, one of our experienced application engineers will be happy to help you.
