RUBBER COMPRESSION SET
(ASTM D 395)

Model CU-RC  (low carbon steel)
Model WTF-RC  (17-4PH stainless)

ASTM D 395 (Reference 1) governs the testing of rubber and similar materials intended for use in applications in which the material will be subjected to compressive stresses in either air or a liquid medium.  There are two test methods covered in the standard, viz., Method A – Compression Set under Constant Force, and B – Compression Set under Constant Deflection.  Unless otherwise stated, Test Method B is to be used. 

A typical Test Method B test fixture is shown in the following photograph.  It consists of three plates, with three spacers between each pair of plates.  The spacers maintain a fixed deflection of the specimens when the four bolts are tightened.  The plates are 5.5” long for Type 1 specimens, and 4.0” long for Type 2 Specimens.  In both cases the plates are 1.5” wide, and ½” thick.

Type 1 specimens are 1.14” in diameter and 0.49” thick.  Type 2 specimens are 0.51” in diameter and 0.24” thick.  For Type 1 specimens the square spacer bars are 0,375” thick.  For Type 2 specimens the square spacer bars are 0.1770” thick.  This induces approximately 25 percent compression in each specimen.

Rubber Compression Set Test Fixture – ASTM D 395, Test Method B – Compression Set Under Constant Deflection

One specimen is placed in each of the four openings of the fixture and the screws tightened against the stops.  The fixture is then placed in an oven at the required temperature for the specified time.  A suggested time is either 22 or 70 hours.  The specimens are then immediately removed from the fixture and allowed to cool for 30 minutes, at which time thickness measurements are taken.  The compression set is then calculated.

Source of Additional Information:

1)     1)  ASTM Standard D 395-03 (2003) "Standard Test Methods Rubber Property – Compression Set," American Society for Testing and Materials, West Conshohocken, Pennsylvania (first issued in 1934).