ASTM D395 specifies two test methods for testing the compressive properties of rubber. Test Method A governs the compression set under constant force while Test Method B governs the compression set under a constant deflection. Method A uses a spring to push down on the sample and maintain constant force. Test Method B uses screws to tighten down on the sample and maintain constant deflection. The fixtures for Method B is usually inserted into an Environmental chamber for 1-3 days at elevated temperature. Most test periods last between 8-24 hours. In some cases, a shortened test period can be used such as 30 minutes.
Test Method A uses a specially designed fixture which is referenced in our PDF. The procedure requires circular rubber disks to be used according to the official ASTM D395 specification. A calibrated spring is used to apply constant force to the sample over the designated time period.
Test Method B uses a compression fixture which is held together by large diameter screws. There are two sample types. The Universal Grip fixture accepts samples that are either 1.14" in diameter and .49" thick, or smaller specimens that are .51" in diameter and .24" thick. The fixture translates a compression deflection of about 25% onto the sample. Universal Grip stocks standard sized Method B fixtures. Custom fixtures for different sized thicknesses and numbers of cavities can be selected.
The fixture ships with 2 sets of 6 spacers with height of 4.5mm and 9.5mm
The compression jig for Method A uses a spring and two guide rods to evenly load a rubber sample. The specimen is compressed evenly for an extended period of time.
The compression fixture for Method B is quite simple and can be placed in an Environmental Chamber for proper conditioning and testing. Testing at low temperatures is specified in ASTM D1229.
Sample calculations for ASTM D395 are given as a percentage. Essentially, the test is run and sample height is measured to find out how much it has bounced back since the compression. The calculation is specified in such a way that it describes the amount of height that was permanently lost. Thus, if a sample only returns to 80% of its original height, the compression set is given as 20%. Please read the official specification for more information on the calculation values.
Rubber Compression Set
Test Method A Compression Fixture
Test Method B Compression Fixture