In the case of tensile tests, the test machine exerts a tension load or force which pulls tensile test samples apart. In the case of plastics tensile testing, the test sample is pulled apart to measure tensile strength and other properties including stiffness and yield strength. There are several common industry standards that provide agreed upon methods of plastics tensile tests. ASTM D638 and ISO 527-2 both feature similar but different standardized test sample geometry and dimensions. These tests require tensile grips that are expected to grab the sample and to adjust as it thins out during the test process. These accessories are different than compression fixtures. 

In compression tests, the test machine exerts a pushing or compressive load or force to squish the test sample until it breaks or squishes. Compression tests of a polymer structural foam material is covered by ASTM D1621 which specifies the type of compression plates and deflectometer used. The test sample is placed between compression test platens until the cellular structure fails or ruptures.

A universal test machine can perform either or both tension and compression tests. The crosshead can be used to pull or compress the test sample which is located between the baseplate and the moving head.

The tensile test fixtures, or grips, and strain sensors (known as extensometer), cannot perform compression tests. Also the tensile grips are specially matched to the cover the exact test specimen geometry and dimensions. The compression test platens and deflectometer are also capable of only performing a compression test, and so both sets of accessories are needed in this case.