Compression set tests, described in ASTM D-395, are of two main types: Method A, compression set under constant load; and Method B, compression set at constant deflection of 25%.
In Method A, specimens of standard dimension are compressed between parallel steel plates under a stress of 400 psi. The test assembly is then conditioned for a selected time at a selected test temperature (such as 22 hrs at 158°F (70°C)), after which the specimens are allowed to recover at room temperature for 30 minutes. Compression set is the difference between the original thickness of the specimen and its thickness after the test, expressed as a percentage of the original thickness.
In determining compression set by Method B, the specimen is compressed to 75% of its original thickness. Then, the test assembly is conditioned for the specified time at the selected test temperature. Compression set determined by Method B is the difference between the original thickness of the specimen and its thickness after testing, as a percentage of the deflection employed.
Although the conditioning time and temperature are specified in the ASTM standard, other times and temperatures can be used.
Compression set is particularly applicable to compounds used in machinery, motor mountings, and vibration damping. Compression set tests are intended to measure the ability of elastomeric vulcanizates to retain elastic properties during the prolonged action of compression stresses. The actual stressing in service may involve (1) the maintenance of a definite deflection, (2) the constant application of a known load, or (3) the rapidly repeated deformation and recovery from compression forces.
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